Teague, R. et al. 2019. Impacts of over-seeding bermudagrass pasture with multispecies cover crops o

Agriculture, Ecosystems and Environment 273 (2019) 117–129

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Impacts of over-seeding bermudagrass pasture with multispecies cover crops on soil water availability, microbiology, and nutrient status in North Texas

T

⁎

Richard Teaguea,b, , Paul B. DeLaunea,c, Steven L. Dowhowera a

Texas A&M AgriLife Research Center, Vernon, Texas, USA Department of Ecosystem Science and Management, Texas A&M University, College Station, TX, USA c Department of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Conservation management Soil biota Soil quality Soil restoration

Cover crops over-sown into established perennial bermudagrass (Cynodon dactylon) pastures has been effective in improving forage productivity and net farm profitability in wetter grasslands in US. To assess if the use of cover crops can do so in the relatively dry regions of Southern Great Plains of Texas, we evaluated what positive and negative impacts accrue from sowing winter growing mixed species cover crops into bermudagrass pastures by measuring soil water dynamics, soil nutrient and soil microbial dynamics on three separate commercial farms over three growing seasons. There was no evidence that annual, winter growing cover crops reduced soil moisture to negatively affect the following production of the bermudagrass pasture. This result was obtained in years that had average to above average precipitation and may have been different in drier years. At one of the farm sites the key biological function changes we measured positive changes in soil biological activity and fertility to indicate that positive biological changes had taken place and could after more years add up to greater improved outcomes. However, the lack of positive soil biological responses on the other two farms may indicate that soils initially having low soil organic matter, poor permeability, water holding capacity and fertility would need to develop greater levels of soil organic matter with better management before positive responses using cover crops would yield acceptable improvements. However, they are a likely means of contributing to these desired improvements. Cover crop biomass levels were greater on the drill than Broadcast and Control but the improved biomass with broadcasting to establish the cover crops on all three farm sites has important cost lowering implications even though it was applied at a 33% higher seeding rate. It is likely that improved grazing management of perennial pasture is necessary to improve soil organic carbon and soil biological function in conjunction with the use of cover crops.

1. Introduction The soil functions of carbon sequestration, water infiltration and retention, nutrient acquisition and cycling that provide the ecosystem services underpinning the agricultural industry, are governed by soil biota (Doran and Zeiss, 2000; De Vries et al., 2013; Kallenbach et al., 2016). Widely used agricultural land use practices have reduced the diversity and abundance of soil biota that are fundamental to providing these ecosystem services. The damaging management practices include inappropriate use of tillage, inorganic fertilizers and biocides, and they have negatively affected soil microbial populations and compromised the physical, chemical, and biological properties of soils (Bardgett and McAlister, 1999; Leake et al., 2004; Mikha et al., 2005; Khan et al., 2007; Leigh et al., 2009; Mulvaney et al., 2009; Czarnecki et al., 2013; Lal, 2015; Sanderman et al., 2017). This has led to reduced water ⁎

infiltration, increased nutrient losses via erosion, reduced nutrient concentrations in the remaining soil, and, consequently, reduced nutrient availability to plants (Houghton et al., 1983; Doran and Parkin, 1996; Delgado et al., 2011).

As an alternative to these degrading practices, the use of cover crops, conversion of cropland to permanent grazed perennial pastures, including perennial crops and forages in rotations and other conservation management practices that build soil health and function in North America have resulted in soil health restoration to increase ecosystem services and benefits to producers (Plassart et al., 2008; Delgado et al., 2011). Maintaining high soil microbial biomass and activity enhances soil biological processes, such as soil aggregation, organic matter decomposition and nutrient mineralization and acquisition that enhances water and nutrient availability for crop and forage growth (Coleman and Crossley, 1996; Six et al., 2004). Landuse-

Corresponding author at: Texas A&M AgriLife Research, P.O. Box 1658, Vernon, TX 76385-1658, USA. E-mail address: rteague@ag.tamu.edu (R. Teague).

https://doi.org/10.1016/j.agee.2018.12.013 Received 7 August 2018; Received in revised form 19 December 2018; Accepted 20 December 2018 0167-8809/ © 2018 Elsevier B.V. All rights reserved.

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managed grazing.

Soil microbial parameters are highly effective indicators of changes in soil function and fertility under different land use options (Plassart et al., 2008; De Vries et al., 2013). Processes of carbon loss increased with soil food web properties that correlated with soil C content, such as earthworm biomass and fungal/bacterial energy channel ratio. Soil biological composition, activity and biomass strongly and consistently predicted processes of C and N cycling across land use systems and geographic locations, and they were a better predictor of these processes than land use. De Vries et al., (2013) determined that the soil processes of N cycling influencing soil fertility, such as changes in fungal and bacterial biomass, were explained by changes in soil microbial properties independent of land use. Soil C accumulation and the C/N ratio of cover crop residues accounted for the level of microbial biomass C to reflect changes in soil quality and nutrient dynamics more accurately than SOC changes.

The aim of this paper is to examine if the use of cover crops oversown into established perennial bermudagrass (Cynodon dactylon (L.) Pers) pastures enhances soil microbial composition and activity, organic C, and soil fertility that are the drivers of improving soil ecological function without negatively impacting subsequent soil moisture levels. These management practices have been shown to work in other parts of the country, but in the relatively dry regions of Southern Great Plains of Texas we need to evaluate what positive and negative impacts accrue from sowing winter growing mixed species cover crops into grazed bermudagrass pastures. Specifically, how the cover crops influence soil microbial composition, activity, and biomass, and if they negatively impact soil moisture availability for the bermudagrass growth in the following summer growth period.

It is important to know in this marginal cropping region what possible merits cover crops may have here. It is very important for longterm sustainability that as much of a management unit as possible is kept under permanent perennial grass to maintain soil function as outlined by Plassart et al. (2008) and Acosta-Martinez et al. (2010) while using management that minimizes use of inputs and other practices that are costly and decrease function of the resource base. We hypothesize that fall over-seeding of bermudagrass pastures with mixed species cover crops will: 1) not negatively impact soil moisture availability for the commencement of bermudagrass spring growth in May; 2) improve soil fertility; and 3) increase soil microbial biomass and function. This project fills the gap between research that has been conducted elsewhere and possible application in this region.

induced shifts to more bacterial-dominated microbial communities have been linked to reduced SOC, C sequestration and increased nitrogen (N) losses (Yeates et al., 1997; Bardgett and McAlister, 1999; De Vries et al., 2013). Conversely, fungal-dominated microbial communities, which are common in less intensively managed land use systems, are linked to more conservative nutrient cycling and greater storage of C improving soil fertility and function (De Vries et al., 2013). Soil microbial composition and biomass properties are a better predictor of changes in ecosystem service processes than land use (De Vries et al., 2013) and there is direct evidence that soil microbes produce chemically diverse, stable SOM. Microbial composition and biomass are more important for SOM accumulation than clay mineralogy as microbialderived SOM increases are greatest in soils with higher fungal abundances and more efficient microbial biomass production (Kallenbach et al., 2016).

Management systems that maintain a community of soil organisms similar to those in natural ecosystems have been shown to require fewer artificial inputs because of greater reliance on ecosystem self-regulation (Yeates et al., 1997). The soil communities in natural ecosystems have high soil microbial biomass and are dominated by fungal pathways.

Jointly, this results in enhanced macro- and micro-aggregation, organic matter accumulation, access and provision of nutrients and soil water to plants that otherwise would not be available, and enhancement of nutrient cycling through detritus decomposition (Bardgett and McAlister, 1999). These soil biological processes are greatest in grazed perennial pastures (Yeates et al., 1997; Bardgett and McAlister, 1999) as they are similar to natural ecosystems if managed to exclude practices that compromise soil microbial composition and functions. Such forage production systems comprise self-regulating soil microbial processes that result in accumulation of organic matter to feed soil microbes and plants, thus removing or reducing the need for artificial inputs (Altieri, 1999).

It is widely recognized that winter cover crops increase soil biological processes, soil microbial biomass, soil organic carbon (SOC), soil fertility, fungal populations and activity, and fungal/bacterial ratio (Miller and Dick, 1995; Sainju et al., 2007; Nakamoto et al., 2012; Zhaorigetu et al., 2008; Nishizawa et al., 2010; Delgado et al., 2011; de Vries et al., 2013). In the Texas Rolling Plains and Southern High Plains, to improve the health of their soil, farmers and ranchers are increasingly using conservation management practices of keeping soil covered, using grazed perennial pastures, keeping plants growing throughout the year, and diversifying the crop production with rotations. Taking these steps would allow farmers and ranchers to help reduce erosion while increasing the soil’s ability to provide nutrients and water to the plant at critical times during the growing season.

However, the impact of cover crops on soil moisture availability is a major concern as it is a much warmer and drier area than where these management techniques have been more widely adopted. One potential disadvantage of using cover crops is a possible reduction in soil moisture for the following crop (Dabney et al., 2001).

Soil water content is usually the most limiting factor in crop production within semi-arid environments, and any practices that are perceived to reduce moisture availability will hinder adoption. Previous research evaluating cover crops in the region has indicated that cover crops have limited soil moisture hindering subsequent crop yields (Dozier et al., 2008; Keeling et al., 1996). In contrast, research in this region has documented higher soil moisture availability in conservation tillage systems with cover crops compared to conservation tillage systems without cover crops (Sij et al., 2004). Acosta-Martinez et al.

(2010) have shown that there is great potential for improving soil health, fertility and ecological function establishing perennial forage cover with appropriate management. But the SOC and fertility must be elevated above a threshold level before successful perennial grasses can be established and be productive. The first step then is to use cover crops to elevate SOC and fertility to the point where productive perennial forages can be used to improve soil health. appropriately

2. Materials and methods 2.1. Site descriptions The investigation was conducted on three farms near Muenster in the adjacent Cooke and Montague Counties, North Central Texas (98° 08′ N, 33° 16′ W). The climate is continental with an average of 230 frost-free growing days. Mean annual precipitation is 990 mm and mean annual temperature is 18.1 °C (n = 30 years). It is bimodally distributed with peaks in May (127 mm) and October (108 mm) (n = 30). Mean monthly temperature varies from 6.1 °C in January to 28.4 °C in July (n = 30). Elevation ranges from 230 m to 330 m. Soils at the sites are predominantly loams and clays. The soil series and hydrological soil properties at each farm site are presented in Table 1. The recent histories of management on the three farms in the study are presented below: 1 The Bellows farm (33.841556, -97.488792) was purchased in 1995 and was previously tilled and planted annually to melons or wheat.

In 1996 it was sprigged to bermudagrass and managed for hay production until 2014 using herbicides and inorganic fertilizer inputs. In 2014 hay production was terminated and the area was rotationally grazed by cattle with ± 5 days of grazing followed by at 118

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each farm site and were determined using the NRCS-TX Zone 5 Cover Crop Job Sheet. The mix consisted of Austrian winter peas (Pisum sativum subsp. arvense)(13 kg ha−1), common vetch (Vicia sativa)(5.6 kg ha−1), wooly pod vetch (Vicia villosa ssp. dasycarpa)(2.2 kg ha−1), red clover (Trifolium pratense)(2.5 kg ha−1), triticale (7.8 kg ha−1), winter oats (Avena sativa)(6.7 kg ha−1), cereal rye (Secale cereale) (7.8 kg·ha−1), forage collards (Brassica oleracea hybrid)(0.6 kg·ha−1), and nitro radish (1.1 kg ha−1). The same mix was applied each year at 50 kg ha−1 on the Cover drill treatment and 67 kg ha−1 on the Broadcast treatment.

Table 1 Soil Taxonomy and hydrological properties of soils on each farm site for the bermudagrass cover crop study (USDA Natural Resources Conservation Service, 2009).

Farm

Soil Taxonomy Order

WRB Soil Reference Group

Depth cm

Permeability cm hour−1

Available Water Capacity cm cm−1

Bellows

Alfisol

Nitosol

Leo

Entisol

Fluvisol

Popp

Vertisol

Vertisol

0-36 36-173 0-20 20-200 0-51 51-94

5.0-15.0 1.5-5.0 15.0-51.0 15.0-51.0 < 0.15 < 0.15

0.07-0.11 0.12-0.19 0.07-0.11 0.06-0.10 0.15-0.18 0.15-0.18

2.3. Sampling Prior to initiation of the experiment randomly placed soil cores were collected from each treatment site for soil nutrient analyses at depth increments of 0–15, and 15–30 cm. Soil samples were taken again at 0–15 cm depth after cover crop termination in April for Haney tests.

Ten 16 mm diameter soil cores per plot were composited, dried at 50 °C and ground to pass through a 2-mm sieve. Samples were analyzed for water extractable organic C, N, P (Haney et al., 2012) while the Solvita gel system was used for 24-hour soil CO2 analysis (Haney et al., 2008b).

Soil moisture probes were installed horizontally on 3 subsites per treatment and site, and soil temperature probes were established on 2 of these subsites. A moisture and a temperature probe were placed at the 15 cm depth, while at 45 cm, and 75 cm depths only moisture probes were installed. The Decagon ECH20 10HS moisture probes used assess the soil moisture of 1 L of soil volume around each probe. Data were collected hourly using data loggers.

To determine soil microbial composition, soil samples were collected for living microbial biomass using phospholipid fatty acid (PLFA) analysis at each site with ten randomly located soil cores 16 mm in diameter. They were collected at a depth of 0–10 cm on each treatment plot. Sampling took place on 10 July 2015, 06 June 2016, and 25 April 2017, when growing conditions were good to excellent and soil microbes would be most abundant. Samples were placed in insulated containers with cold packs and shipped over-night to the Ward Laboratory in Nebraska for PLFA analysis.

Vegetation was sampled at 4 subsites per treatment at each farm site in April each year. Estimates of vegetation biomass by species or species group proportions were conducted on 4 × 0.25 m2 quadrats per subsite and were harvested to ground level using the dry-weight-rank method as described by Dowhower et al. (2001). Vegetation group biomass values were averaged for each subsite.

least 45 days of recovery during the summer growing season the area was continuously grazed in the non-growing season. Herbicide and inorganic fertilizer applications were stopped in 2014.

2 Prior to purchase in 1973 the Leo farm (33.456496, -97.384111) was used for cotton and sudangrass for hay. At this time bermudagrass was planted and grazed continuously at a moderate stocking rate and managed using inorganic fertilizers. It was not hayed. In 1982 grazing was switched to adaptive multi-paddock grazing involving moderate grazing by cattle and sheep. Management included using multiple paddocks per herd with short grazing periods (1–3 days) and long recovery periods of 30–45 days in fast growth periods and 60–80 days in slow growth periods. Animal numbers and periods of grazing and recovery were changed proactively based on changing weather and plant and animal needs.

3 Prior to purchase in 2007 the Popp farm (33.702678, -97.238326) was annually cropped for wheat or milo production using tillage and inorganic fertilizers. At this point it was seeded to bermudagrass and managed principally for grazing but was cut for hay when conditions produced forage in excess of grazing requirements. The fertilizer management on the bermudagrass varied each year but in general 112 kg ha−1 of diammonium phosphate (20 kg ha−1 N, 23 kg ha−1 P) and 112 kg ha−1 of urea (52 kg ha−1 N) were applied.

The pasture was managed by a grazing rotation of 2–3 months grazing followed by 2–3 months of rest and cut for hay in some years.

2.2. Treatments Treatments on each farm site included: 1) untreated bermudagrass (Control); 2) cover crop treatment drilled into the established bermudagrass (Cover); and 3) broadcasting the cover crop seed into established bermudagrass (Broadcast) at 33% higher seeding rate. Each treatment consisted of 4 replicate plots that were adjacent to each other. Plots of the control and drilled cover crop treatment were 30 m × 30 m and Broadcast plots were 10 m × 10 m. Initial planting was in November 2014 and repeated in 2015 and 2016 after the first bermudagrass top-killing frost. Due to abnormally wet conditions in fall of 2015 it was not possible to plant at the Popp site with the no-till drill so cover crops were broadcast.

Both the Cover and Broadcast plots were planted immediately after mowing or grazing at high density for 1 or 2 days to remove most of the standing forage. Cattle were removed from the pastures until the bermudagrass was ready for grazing the following spring. In late April prior to this, vegetation sampling to determine cover crop biomass produced by species, and forage nutrient status was conducted.

The cover crop mixture was determined based on soil analysis and protocols developed by the USDA-ARS (Haney et al., 2004, 2006, 2008a, 2008b, Haney and Haney, 2010; Haney et al., 2012) and the Natural Resource Conservation Service (NRCS) Knox City Plant Materials Center. Species included in the cover crop mixes were the same at

2.4. Soil nutrient laboratory analyses For soil nutrient status the Haney et al. (2006) methods were used throughout. Soil samples were air dried and shipped to the USDA-ARS laboratory in Temple, TX for analysis. The soil samples were oven dried at 50 °C for 24 h and ground to pass a 2-mm sieve. Soil samples were then extracted with water and the H3A extractant using 4 g samples at a dilution factor of 10:1, one-part soil and 10 parts extractant. The samples were shaken for 10 min using a reciprocal shaker and centrifuged for 5 min before filtering them through Whatman 2 V filter paper. The H3A extracts were analyzed colorimetrically for NO3−–N, NH4+–N and P on a segmented flow analyzer (Haney et al., 2006).

Water extracts were analyzed for water-extractable organic C (WEOC) and water-extractable N (WEN) containing water extractable organic and inorganic nitrogen, on an Elementar TOC Select Analyzer (Vario TOC Cube, Elementar Analysensysteme GmbH, Langenselbold, Germany), while H3A extracts were additionally analyzed for Al, Fe, Ca and K on an Agilent MP-4200 Microwave Plasma Instrument (Agilent Technologies Inc., Santa Clara, California, USA) as described by Haney et al. (2006).

The Solvita gel system was used for 24-hour soil CO2 analysis following rewetting of dry soil (Haney et al., 2008a; 2008b). The Solvita 1119

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initial oven temperature, 140 °C, was held for 5 min, raised to 210 °C at a rate of 2 °C min−1, then raised from 210 °C to 250 °C at a rate of 5 °C min−1, and held for 12 min.

Identification of peaks was based on comparison of retention times to known standards (Supelco Bacterial Acid Methyl Esters #47080-U, plus MJS Biolynx #MT1208 for 16:1ɷ5). The abundance of individual PLFAs was expressed as μg PLFA g−1 dry soil. Amounts were derived from the relative area under specific peaks, as compared to the 19:0 peak value, which was calibrated according to a standard curve made from a range of concentrations of the 19:0 FAME standard dissolved in hexane.

day CO2-C measurement gives a rapid measure of soil microbial activity. The 40 g soil sample is analyzed with a 24-hour incubation test at 25 °C., the sample is wetted through capillary action by adding 18 ml of DI water to an 8 oz. glass jar (ball jar with a convex bottom) and placed in the jar and then capped. At the end of 24-hour incubation, the CO2 in the jar is be pulled through a LiCor 840 A IRGA, which is a non-dispersive infrared (NDIR) gas analyzer based upon a single path, dual wavelength infrared detection system. The peak height is recorded via the LiCor software and saved in an Excel file. We then use an Excel file with a “peak hunter” macro to find the highest peak for each sample at a constant pressure, the macro then corrects for the ideal gas law and converts the result to CO2-C kg ha−1.

2.6. Statistical analysis 2.5. Phospholipid fatty acid laboratory analyses Analysis of variance (Proc Mixed; SAS, 2012) was used to compare planting treatments, farm sites, and years which were replicated at 2–4 locations within a randomized block design. Only vegetation data was analyzed for the added broadcast treatment to assess this as a viable seeding method. For vegetation, soil chemistry and soil biomass 4 replications were sampled. Seeding treatment, farm and the interaction effects were based on treatment x farm x replication.

For soil moisture, 3 replications at 3 soil depths were monitored and for soil temperature 2 replications were monitored. This environmental data was further divided into cool-season (Nov - Feb), growing season (Mar - Jun), and warm-season (Jul - Oct) and analyzed by farm. For environmental data, planting treatment effects were based on treatment × replication while season and season interaction effects were based on treatment × farm x season × replication. Least square means were compared with these error terms. Abiotic and soil chemistry data were not transformed.

Total vegetation and microbial biomass fractions and microbial ratios with relatively large denominators were more normally distributed.

Broadcast and Cover data were not pooled for statistical analysis.

Broadcast data were only used in the analysis of vegetation responses.

Ratios and indexes that were non-normal were not associated with planting treatment differences and, therefore, were not analyzed additionally. Respiration and vegetation subgroups distribution benefitted from square root transformation. Adjustments for the number of response variables on Type I errors were not considered because the investigational nature of this study was to seek possible, measurable, explicable effects and avoid Type II errors.

Individual fatty acids have been used as signatures for various functional groups of microorganisms (Grayston et al., 2001; Pankhurst et al., 2002; Hamel et al., 2006). Fatty acid methyl esters (FAMEs) were analyzed according to the method of Clapperton and Lacey (Clapperton et al., 2005). The FAME 18:2ɷ6c was taken to indicate fungal biomass (Petersen and Klug, 1994; Frostegård and Bååth, 1996) and FAME 16:1ɷ5 to indicate AM fungi (Spring et al., 2000; Balser et al., 2005).

FAMEs 3OH-12:0, a-12-meth-15:0, i-13-meth-15:0, 15:0, 2OH-14:0, i14-meth-16:0, 16:1ɷ7c, i-15-meth-17:0, 10-methyl- 17:0ɷ8c, 17:0, and 2OH-16:0 were chosen to represent bacterial PLFAs based on the bacterial standards used.

Total soil lipids were extracted in test tubes by shaking approximately 4 g (dry weight equivalent) of frozen soil in 9.5 ml dichloromethane (DMC):methanol (MeOH):citrate buffer (1:2:0.8 v/v) for 2 h. Then, 2.5 ml of DMC and 10 ml of a saturated NaCl solution were added to each tube and shaken for 5 min. Tubes were then centrifuged at 3000 rev min−1 for 10 min. The organic fraction was pipetted into clean vials. Five ml of DCM:MeOH (1:1 v/v) was added to the tubes, which were then shaken for 15 min and centrifuged for 10 min. The organic fractions were combined in the corresponding vials and then dried under a flow of N2 at 37 °C in the fume hood. Samples were dissolved in 2 ml of DCM and stored at −20 °C for less than two weeks.

Lipid-class separation was conducted in silica gel columns. Samples were loaded onto columns and the vials washed twice with a small amount of DCM using a pipette. Care was always taken to keep solvent level above the silica gel. The neutral, glycolipid and phospholipid fractions were eluted by sequential leaching with approximately 2 ml of DCM, 2 ml of acetone and 2 ml of methanol, respectively. The glycolipid fraction was discarded, and the neutral and phospholipids fractions were collected in separate 4 ml vials. These fractions were dried under a flow of N2 at 37 °C in the fume hood. The dried fractions were dissolved in a few ml of MeOH and stored at −20 °C. Fatty acid methyl esters were created through mild acid methanolysis. Neutral and phospholipids fractions were dried under a flow of N2 at 37 °C in the fume hood. Half a Pasteur pipette full of MeOH/H2SO4 (25:1 v/v) was added to the vials, which were placed in an 80 °C oven for 10 min, cooled to room temperature before the addition of approximately 2 ml of hexane with a Pasteur pipette. Vials were vortexed during 30 s and left to settle for 5 min before the lower fraction was discarded. One ml of ultra-pure water was added. Vials were vortexed for 30 s, left still for 5 min before the aqueous fraction was discarded entirely. Ten μl of methyl nonadecanoate fatty acid (19:0; Sigma Aldrich) was added. Samples were dried under a flow of N2 at 37 °C in the fume hood. Vials were washed with 50 ml of hexane using a glass syringe, the samples transferred into 100 ml tapered glass inserts, placed inside a gas chromatograph (GC) vial.

Samples were analyzed using a Varian 3900GC equipped with a CP8400 autosampler and a flame ionization detector (FID). Helium was the carrier gas (30 ml min−1) and the column was a 50-m Varian Capillary Select FAME # cp7420. Sample (2 ml) injection was in 5:1 split mode. The injector was held at 250 °C and the FID at 300 °C. The

3. Results 3.1. Soil moisture and temperature dynamics Following planting of cover crops in the fall of 2014 precipitation was normal until the about the end of their growth period in March 2015. Much greater than normal precipitation persisted from April 2015 to October 2016, followed a period of very close to normal precipitation till October 2017 (Fig. 1). The wet conditions in the fall of 2015 prevented planting with the no-till drill of the Popp site which is a high clay soil, so we broadcast the seed at that site that year. Although considerable differences in precipitation occur among sites during the wet period, moisture was unlikely limiting to growth on any of the farm sites.

The greater than average rains from April 2015 to the end of the summer growing period in fall of 2015 soil moisture in the Control and Cover treatments were essentially the same at each soil depth we measured (P > 0.4; data not presented). Bermudagrass begins growth in this region the first week of May. In both 2016 and 2017 during this critical early growth period, soil moisture in the Control and Cover treatments were indistinguishable at each soil depth we measured (P > 0.4; Fig. 2).

Soil moisture levels of the Cover compared to the Control are presented in more detail in Fig. 3 for the Bellows, Leo and Popp farms in 120

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Fig. 1. Cumulative precipitation at each farm study site compared to the 30-year mean for October through September in the three study years.

3.2. Vegetation impacts

the top soil 0–30 cm, 30–60 cm and 60–90 cm in (a) the winter growth period November to end of February; (b) the spring and early summer growth period March to end of June; and (c) the late summer growth period July to end of October. At the 0–30 cm depth, moisture in the cover crop plots was not less than that in the Control plots except at the Popp site in November to February (P = 0.02; Fig. 3) and May to June (P = 0.007). Soil moisture of the Cover plots was only lower than the Control at the 30–60 cm depth at the Bellows and Leo sites in July to October (P < 0.06), while at the Popp site it was lower at the Cover plots all year (P ≤ 0.07). Importantly, at the 60–90 cm depth there were no differences in soil moisture between the Cover plots and Control plots (P > 0.22).

Soil moisture differences due to treatment were small compared to the recharge following good rainfall events when moisture was depleted. The largest difference between planting treatments by depth was 5.5% compared to about a 20% increase following recharge events in October 2015 averaged over all sites (Fig. 2).

Soil temperature for Cover and Control were not different (P = 0.86) but farm site differences by season were observed (P = 0.002; data not presented). November through February temperatures were similar for the 3 farm sites (P > 0.16) while the Leo site averaged 1.2 and 2.4 °C cooler March through June and 2.4 and 3.2 °C cooler July through October than the Bellows and Popp sites, respectively (all sites differed, P < 0.03).

Herbage biomass was assessed in late April of the 3 years following the peak of growth of the cover crop species and prior to grazing. Total herbage biomass was similar in late April for the 3 years (P > 0.86; Fig. 4) but the Bellows and Leo farms had greater herbage than the Popp farm (P < 0.001).

The focal vegetation group of the study, Cover Crop species, were greater on the Drill than Broadcast and Control (P < 0.001) with Leo greater than Bellows, and Bellows was greater than Popp (P < 0.001; Fig. 5). However, farm × seeding treatment and farm × seeding treatment × year interactions (P > 0.012) reflect the inability to plant the Popp farm for the 2016 growing season as well as poorer stands on the Drill treatment. At Popp, Drill and Broadcast treatments exceeded the Control all 3 years (P < 0.04) while Broadcast exceeded Drill in 2016 when it was not possible to drill due to wet conditions (P = 0.004) but were similar in 2015 and 2017 (P > 0.47; Fig. 5).

Among farms, the dominance of bermudagrass on Bellows was the most obvious difference with Bellows greater than Leo (P = 0.001) and Leo equal Popp (P = 0.49). Also Bellows had more forbs than Leo and Leo more than Popp (P < 0.04). Other grasses were greater on Leo than Popp and Popp more than Bellows (P < 0.001). Interactions of planting treatment × farm and planting treatment × farm × year occur for these vegetation groups as well. But the graphic of vegetation groups by farm and planting treatment provides a good sense of the

Fig. 2. Combined cumulative soil moisture for the Bellows, Leo, and Popp farms at mean soil depths of 15 cm, 45 cm, and 75 cm for cover crop and no cover crop treatments on Bermudagrass in Cooke Co., TX. 121

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Fig. 3. Soil moisture (% by volume ± SE bars) levels of cover crop over-seeded into bermudagrass pasture compared to no-cover crop bermudagrass control at the Bellows, Leo and Popp farms in the soil layers 0–30 cm, 30–60 cm, and 60–90 cm depth intervals in N-F (November to February), M-J (March to June), and J-O (July to October).

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Fig. 4. Overall mean total herbaceous biomass (g m−2 ± SE bars) produced by management category at each cooperator farm site and year. Main effect means with SE bars (Mean of 2015 through 2017).

over-all effects of planting.

Table 2 Pre-treatment Haney Soil Test levels for the bermudagrass cover crop study.

3.3. Soil nutrient impacts Pre-trial soil chemical properties were measured by the Haney Soil Test methodology and presented in Table 2. The Leo site had the highest fertility with all nutrients measured and the Popp site was higher in organic C and organic N than the Bellows site but lower in total N, P and K. This was likely due to the previous management at the Leo site where good grazing management had aimed at and achieved

Farm

Organic C (kg ha−1)

Organic N (kg ha−1)

N(kg ha−1)

P(kg ha−1)

K(kg ha−1)

Solvita CO2 (kg ha−1)

Bellows Leo Popp

202 323 305

32 49 38

36 66 34

33 75 10

39 153 19

128 98 56

Fig. 5. Mean herbaceous biomass standing crop (g m−2 ± SE bars) by plant base and management category on cooperator farms (Mean of 2015 through 2017).

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the Popp site was intermediate at 3.900 mg kg−1, at well above average soil health (Kaur et al., 2005).

Regarding fungal impacts, total fungi were higher at Leo (0.736 mg kg−1), intermediate at Popp (0.512 mg kg−1) (P < 0.0001) and lowest (P < 0.0001) at Bellows (0.322 mg kg−1) (Figs. 7). There were no impacts to using Covers on bermudagrass across all sites (P > 0.33). Both mycorrhizal and saprophytic fungi responded similarly across all sites, and they were no different to that of the Control (P = 0.34). However, at Leo saprophytic fungi were higher under Cover (0.559 mg kg−1) compared to the Control (417 ng g−1) (P < 0.02) but mycorrhizal fungi were not (P = 0.34). At Bellows (0.217 vs 0.245 mg kg−1) and Popp (0.331 v. 0.345 mg kg−1) there were no differences between Cover plots and the Control plots with both mycorrhizal and saprophytic fungi (P > 0.66). Both mycorrhizal and saprophytic fungi were higher at the Leo site than the Popp site (P < 0.0001), and at the Bellows site were lower than at the Popp site (P > 0.001) (Figs. 8 and 9 ).

Regarding fungal to bacterial ratios, the ratios for all 3 farm sites were all > 0.2 indicating slightly above average soil health (Widmer et al., 2001; Kaur et al., 2005). However, they were not different between the Control and the Cover plots (P = 0.81) at any of the sites, indicating reasonable soil health of the untreated Control pasture. The ratio for the Popp site (0.25) was higher than that for the Bellows site (0.22; P < 0.09) but not the Leo site (0.24; P > 0.76).

A high predator/prey ratio (above 0.01) indicates an active soil microbial community with sufficient base level nutrients to support higher trophic levels and predators (Kaur et al., 2005; Strickland et al., 2009). There were no differences in microbial predator to prey ratios between the control (0.029) and the to Cover plots (0.027; P = 0.61).

The ratio on the Cover and Control plots were both > 0.02, indicating high soil health levels on both. The predator/prey ratio at Popp (0.033) was not higher than at Leo (0.027; P > 0.13), but it was higher than on Bellows (0.025; P > 0.57).

Dominance of the bacterial component by gram (+) bacteria is associated with better survival under stressful environmental condition than if gram (-) bacteria dominate (Zelles and Bai, 1993; Kaur et al., 2005). Across all sites, gram (+) bacteria were dominant with gram (+) to gram (-) ratios of 1.20 to 1.40 indicating a balanced bacterial community under relatively low stress. Across all sites, gram (+) actinomycetes were slightly higher with Cover plots (P < 0.10) and were

higher soil organic matter levels.

Soil fertility levels varied by farm over the course of the study as expected because soils, vegetation, and management were initially different (Table 2). Site x Planting treatment interactions occurred for K, N fractions, WEOC, and soil respiration (Table 2.). The Leo farm had the highest level of nutrients, WEOC, and respiration rates while by site analysis indicated these values for Cover were higher than the control (P < 0.06). For Popp, which had the least successful cover crop establishment, these soil measures were similar for Cover and Control (P > 0.35). For the Bellows farm Nmin, K, and respiration were somewhat greater for Cover than Control treatments (P < 0.07).

3.4. Soil biology impacts We used total PLFA extracted at each site as an index of living microbial biomass (Buyer and Sasser, 2012; Goupil and Nkongolo, 2014).

Total microbial biomass varied by farm site (P < 0.05; Table 4) likely associated with different soils, soil chemistry, vegetation, and history of use. Soil microbial group and total live biomass was greatest on Leo, intermediate on Popp, and least on Bellows farm. Microbial groups biomass was similar between planting treatments, except for total microbial biomass (P = 0.091) and non-actinomycete gram (+) bacteria (P = 0.048) that were greater on Cover treatments than Controls.

Though farm × planting treatment interactions were weak for total microbial biomass (P = 0.18) and non-actinomycete gram (+) bacteria (P = 0.11) the patterns were similar to interactions observed for soil K, N, C, and respiration. By farm analysis indicated Leo farm had greater total microbial biomass on the Cover treatment than the Control (P = 0.10) and for the non-actinomycete gram (+) bacteria (P = 0.03). For these groupings planting treatment differences were similar for Bellows (P > 0.16) and Popp farms (P > 0.53).

Over all sites, total soil microbial biomass (mg kg−1) did not show any response to Cover crop overseeding on bermudagrass (P = 0.34; Fig. 6) except at the Leo site (P < 0.02). The high microbial biomass values recorded with the Cover treatment and the Control sites indicates good levels of soil health on the untreated pastures (Kaur et al., 2005). The Leo site (AMP grazing) had the highest values of 5.773 (mg kg−1) indicating excellent soil microbial biomass and the highest microbial activity. The Bellows site had the lowest microbial biomass of 2.500–3.000 mg kg−1indicating slightly above average soil health, and

Fig. 6. Water extractable organic carbon (WEOC; kg CO2-C ha−1 ± SE bars) change resulting from use of a cover crop treatments on bermudagrass in Cooke Co., TX. 124

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Fig. 7. Water extractable organic and inorganic N (WEN; kg ha−1 ± SE bars) change resulting from use of a cover crop treatments on bermudagrass in Cooke Co., TX.

Fig. 8. Total live microbial biomass (mg kg−1 ± SE bars) resulting from use of a cover crop on bermudagrass in Cooke Co., TX.

difference between Covers and controls (P = 0.63). The Bellows site had the lowest microbial diversity (P < 0.001) which is likely associated with it being the site with the highest composition of Bermudagrass. There was no difference in microbial diversity between the Leo and Popp sites (P = 0.32) which both had a greater mix of herbaceous species (Fig. 5).

much higher at Leo than the other sites (P < 0.001). The non-actinomycete Gram (+) bacteria were higher with Cover plots (P < 0.04).

When the saturated fatty acids from bacteria are higher than unsaturated fatty acids and the ratio of monounsaturated acids to polyunsaturated acids is high this is a further indication of less stress (Zelles and Bai, 1993; Kaur et al., 2005). The saturated to unsaturated fatty acid ratios were moderately high while the monounsaturated to polyunsaturated fatty acid ratios were high, indicating low levels of microbial stress associated with healthier and more stable microbial communities.

Microbial diversity indices were moderately-high at Bellows (1.50), and high at Leo (1.59) and Popp (1,62) for all sites. There was no

4. Discussion If cover crops are to be a key element in improving soil health to provide these soil functions, they will need to translate into improved net economic outcomes or they are unlikely to be adopted by farmers. A 125

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Fig. 9. Total fungal biomass (mg kg−1 ± SE bars) resulting from use of a cover crop on bermudagrass in Cooke Co., TX.

improving soil aggregation, and other soil properties (Rawls et al., 2003; Blanco-Canqui et al., 2012; 2013). Lower stored soil moisture July-October in our study was probably from bermudagrass use that is growing during that period.

Bermudagrass is one of the more drought hardy forage grasses (Carrow, 1996) with roots that can grow to a depth of 1.8 m or more depending on soil profile characteristics. However, 76% or more of the root system, is found in the top 30 cm of soil (Doss et al., 1960). Consequently, the cover crop treatments were certainly not limiting to subsequent growth of Control at both the Bellows and Leo sites. At the Popp site the soil moisture at the 60–90 cm depth was not reduced by the cover crop treatment, although cover crops only comprised 25% of the herbaceous biomass at this site. However, soil moisture was reduced somewhat at the other depths and this may have contributed to the lower herbaceous growth at this site as illustrated in Figs. 5–7. A more likely reason is that the lower herbaceous production at the Popp site was due mainly to the considerably lower estimated soil permeability at this site (< 0.15 cm hour−1) compared to that at the Bellows (5.0–15.0 cm hour−1) or Leo (15.0–51.0 cm hour−1) sites (Table 1).

Other possible contributing factors may have been the heavy continuous grazing at the Popp site in year 1 and the late broadcasting of cover crop seed in year 2 due to excessively wet conditions. Due to near record rainfall in May at the Popp site, soil moisture depletion was not as evident as that observed in drier years. Beck et al., (2017) reported that overseeding bermudagrass with alfalfa and clovers did not limit bermudagrass growth early in the season in Arkansas.

When there is no response to cover crops or the following crop it is primarily due to poor weather conditions experienced when each was growing rather than a carry-over effect from one to the other. This is exacerbated if the soils have poor permeability, water holding capacity etc.

The pre-trial soil fertility assessed using the Haney analyses was higher at the Leo site than the other two sites and total N, P and K were lowest at the Popp site. Averaged across sites WEOC was higher on cover crop treatments than the control but this was due entirely the high levels at the Leo site and there was no difference due to cover crops at the Bellows and Popp sites. Grebliunas et al. (2016) concluded that mix species cover crops do not appear to alter soil WEOC quantity and type after three years within a corn silage system. Leo was the only

key question for this, and similarly dry regions, is whether soil moisture availability for the crop following the cover crop is decreased. This would be unacceptable in this region but in other regions it may be a desired outcome. Overseeding cool-season annual grasses and legumes in bermudagrass pastures is common throughout the Southeastern US to sustain cattle production as outlined in a recent review (Rouquette, 2016). Water depletion by cover crops is not as critical in such humid and subhumid climates as compared to our study area, where water depletion by cover crops can be detrimental to subsequent crops as precipitation is limiting (Unger and Vigil, 1998).

We measured no decrease in soil moisture with cover crops compared to the Control at 60–90 cm over the full experimental period, so soil moisture would have been easily accessible to the bermudagrass plants over the study period. At the 30–60 cm depths soil moisture with the cover crops was only less than the control in July to October at the Bellows and Leo sites, but at the Popp site it was lower throughout the year relative to the Control. It should be noted that the herbage total biomass produced during the bermudagrass dormant season was higher for the Control compared to the Cover Crop treatment at the Popp site and the planted cover crop comprised only 25% of the total biomass (Figs. 4 and 5). Hence, it is difficult to draw conclusions based solely upon cover crop herbaceous mass. This reduction in soil moisture could be due to enhanced bermudagrass growth because of enhanced nutrient cycling and microbial activity from the cover crops. It is possible that soil moisture was lower because of cover crops, possibly due to longterm or lingering effect of moisture draw down from spring that was not recharged. Most importantly, our data shows moisture was recharged and not limiting due to cover crops in May when bermudagrass started growing at each location (data not shown). This observation agrees with the assessment of Unger and Vigil (1998) that cover crops deplete soil water supplies while they are growing and conserve water when they are killed. Adoption of cover crops in semi-arid environments is often hindered due to a reduction in subsequent crop yields due to perceived reduction in available water (Unger and Vigil, 1998; Nielsen and Vigil, 2005; Reese et al., 2014; Nielsen et al., 2011). Other studies have shown no crop yield loss despite cover crops reducing soil water content (Holman et al., 2012; Burgess et al., 2014), where precipitation capture and storage may have been improved due to cover crops reducing runoff, increasing infiltration, increasing soil organic C,

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casualties of using management practices that degrade soil function (Bardgett and McAlister, 1999; de Vries et al., 2013).

site where total herbaceous biomass increased due to cover crops over the Control (Fig. 4). Similarly, soil CO2 averaged higher across sites with the cover crop treatments, but this was also due to the much higher values at the Leo site and there was no impact at the Bellows and Popp sites due to cover crop treatment, but CO2 levels were higher at Popp than Bellows. The trend was for measured parameters increasing with increasing cover crop biomass. Haney et al. (2012) reported a strong relationship between WEO C:N ratios and CO2 flush of soils after rewetting. As observed at the Leo site, higher WEOC concentrations were observed with higher CO2 levels.

With all the other nutrients measured, except organic P, nutrient levels were higher with cover crops but only because of the very high levels associated with each at the Leo site following the cover crop treatment. Response to cover crop treatment at both the Bellows and Popp sites did not differ from the Control. Cover crops had no impact on organic P levels. In contrast, studies have shown increased P removal in bermudagrass systems when overseeded with ryegrass or legumes (McLaughlin et al., 2005; Sistani and McLaughlin, 2006; Read et al., 2011).

The soils at Leo were the most fertile at the start of this investigation but differences in fertility among sites were too small to explain the degree to which parameter values at Leo were so much higher than those at the Bellows and Popp sites. The biggest difference among the sites was how much higher the soil permeability was at Leo relative to Bellows and particularly Popp sites (Table 1). The Leo site consisted of a diverse cool-season mixture within the Control treatment, probably due to being an organic beef production system with no recent herbicide applications that are commonly used to control broadleaves (including legumes) in bermudagrass systems. Cool-season legume species can significantly contribute to nitrogen within bermudagrass systems (Morris et al., 1990; Graham and Vance, 2000; Rouquette, 2016; Beck et al., 2017).

Indicators of good soil health of the Control areas in this study include: high microbial biomass; high microbial predator to prey ratios; high bacterial impact factors; high microbial diversity indices; and gram (+) and gram (–) values that indicate low levels of microbial stress. These values are associated with healthier and more stable soil microbial communities probably due to the perennial grass base (Acosta-Martinez et al., 2010). The plant species composition and diversity among the sites is consistent with this as the Leo site had higher plant diversity and Popp site had more diversity than the Bellows site which was strongly dominated by bermudagrass. The Bellows site had the lowest microbial diversity which is likely associated with it being the site with the highest composition of bermudagrass. These differences in plant composition are mirrored in the soil microbial diversity, biomass and composition measured at each location. Bossio et al.

(1998) found that soil type was the most important environmental variable in governing the composition of microbial communities, which may describe the differences among our observed study sites. Regarding response to addition of the cool season cover crops, positive soil biological responses were only recorded at the Leo site with total microbial biomass and saprophytic fungi. Wang et al., (2014) reported higher soil microbial biomass C with lower grazing densities when rye was overseeded in bermudagrass compared to higher grazing intensities in the same system and both low and high grazing intensities in a sorghumsudan system. Furthermore, Dinesh et al. (2009) reported that leguminous cover crops significantly enhanced levels of organic C, N, and microbial activity in soils.

For the cover crops to be of value to farmers they need to substantially improve soil health and function by improving the levels of beneficial fungi and bacteria and other microorganisms. The amount of water entering and remaining in the soil, and not the amount of precipitation, is usually the biggest limiting factor in agricultural systems and the soil aggregation that is determined by how well soil biology influences macro- and micro-aggregation and hence infiltration rate.

This is determined principally by fungi that are usually the first

5. Conclusions The focus of this study was to determine if over-seeding of bermudagrass pastures with mixed species winter growing cover crops would negatively impact soil moisture availability for the commencement of bermudagrass growth in spring, improve soil fertility, and increase soil microbial biomass and function. Results indicate some evidence that the cover crops reduced soil moisture prior to onset of spring growth of the bermudagrass pasture. In April, moisture was lower in cover crop plots, which coincides with peak cover crop growth, but in May, there were no differences in soil moisture between treatments. The performance of the cover crop and the following forage crop is primarily dependent on the rainfall and weather experienced during the growth of each of them. In drier seasons greater differences would likely occur.

The key biological function changes we measured with the Haney test and PLFA indicated positive changes in soil fertility and biological activity at the Leo site. This indicates that the hypothesized positive biological changes had taken place and could, after more years, add up to greater improved outcomes. The lack of positive responses at the Bellows and Popp sites with these elements indicates we need to examine why. Our results may indicate that with soils having low soil organic matter, poor permeability, water holding capacity and fertility, management would need to increase soil organic matter before the use of cover crops would yield acceptable improvements. It is likely that improved grazing management of perennial pasture to improve soil organic carbon would be needed to do this, probably in combination with use of cover crops. The fact that broadcasting to establish the cover crops was as successful as no-till drilling is an important finding as it has important cost lowering implications for farmers.

This was an exploratory investigation conducted on soils representative for the region to determine what positive outcomes or problems might be expected to using cover crops over seeded into perennial pasture. Results emphasize that in dry ecosystems with very variable weather, experiments need to be conducted for periods of eight to ten years to provide reliable information that covers both wetter and drier weather periods (Teague et al., 2004). Increasing soil carbon and soil health is an essential part of increasing the ecosystem services from improved management and full assessment of improved outcomes needs to allow sufficient time for these changes to take place. In these dry regions, progress in improving outcomes takes place in average or good weather years but it either stalls or retrogresses in the drier years, making progress much slower than in less drought prone regions.

Acknowledgements The authors gratefully acknowledge the funding made available from USDA NRCS Grant number: 69-3A75-13-192 and Texas A&M AgriLife Research under project H 8179. We acknowledge the invaluable help of the owners of the Bellows, Leo and Popp farms who allowed us to conduct these investigations on their land and Rhonda Daniels for technical help.

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