Allerton Project: can cover crops recover legacy phosphorus?

BACKGROUND

Can cover crops recover legacy phosphorus?

Cover crops such as buckwheat have been shown to reduce nitrogen loss to water and to reduce soil erosion. © Chris Stoate/GWCT

As one of five research and demonstration farms in Defra’s national Sustainable Intensification research Platform (SIP), we set up an experiment to explore the potential benefits of cover crops to soil and commercial crops. This article describes a PhD project, in partnership with Nottingham University and Rothamsted Research, which investigated one biological aspect of phosphorus cycling in more detail. Phosphate fertiliser is essential for crop growth but is a finite resource. It is available only from a limited number of sources worldwide, many of them at risk of political instability. The price to farmers has increased substantially in recent years. Like other farms across the country, we try to maintain soil phosphate at an index of 2 for optimum use of this resource, although even with regular soil nutrient testing, this is not always easy to manage. Once in the soil, phosphate becomes bound to soil particles and most of the phosphate applied as fertiliser over the years is estimated to have become unavailable to crops in this way. This soil-bound fraction is known as legacy phosphate.

Loss of soil to water is associated with transport of phosphate into watercourses where it is a major cause of eutrophication and deterioration of aquatic ecosystems. From both an environmental and an economic perspective, there is an increasingly recognised need to improve the efficiency of phosphate use on farmland.

One area of interest is the store of phosphate currently locked up in soil. This can be remobilised by phosphatase enzymes produced by soil microorganisms and some plant roots. We were interested in exploring the potential of cover crops grown over the autumn and winter between the harvest of one crop and the drilling of a following spring-sown crop. Cover crops have been shown to reduce nitrogen loss to water and to reduce soil erosion, with implied benefits in terms of phosphate conservation, but we were interested in potential additional benefits associated with the biological activity in the soil.

In 2017, we set up a replicated experiment involving plots of oats, radish, phacelia, vetch and buckwheat, with bare stubble plots lacking cover crops as controls, with three replicates of each. We collected soil samples for laboratory analysis for phosphatase in March, and in the following June when the spring-sown oats crop was actively growing. Through laboratory analysis at Rothamsted Research, this enabled us to assess the presence of phosphatase enzymes at the end of the cover crop growing period, and during the period of peak growth for the following cash crop.

In March, cover crop plots of oats showed significantly greater phosphatase activity than any of the other plots, with phacelia showing intermediate activity. The same relationship was found in the following oats crop, with the plots that had been oats cover crop showing the highest activity, and phacelia intermediate levels (see Figure 1), so cover crop effects were following through potentially to benefit the spring-sown cash crop.

0.2

Acid phosphatase activity (units/ml soil)

0.15

0.1

0.05 bc

a ab ab c d

0 Stubble Vetch Radish Buckwheat Phacelia Oats

Unfortunately, results of laboratory analysis in 2019 reveal that similar findings were not obtained when the experiment was repeated in 2018, and nor were these findings repeated under controlled conditions in pot-based laboratory experiments at Rothamsted. Some cover crops may have a role to play in making legacy phosphate available to crops, reducing the need for imported and purchased fertiliser, but there remains uncertainty over the potential of this approach.

We are currently contributing to the RePhoKUs project, led by Lancaster University, to better understand broad issues associated with the phosphorus cycle at a range of scales. This includes laboratory tests at Sheffield University to assess the rate of depletion of available phosphate from two local soil types, with a view to exploring other options for mobilising legacy phosphate. It is important to note that, like mined rock phosphate, legacy phosphate in soils is also a finite resource and our work with the RePhoKUs project is exploring options for more efficient management of phosphate through the food chain. Figure 1

Soil acid phosphatase activity under different cover crop species sampled in June. 1 unit is the amount of enzyme to hydrolyse 1 µm para-nitrophenyl phosphate min-1. Species similarly superscripted (a, b, c, d) are not significantly different

KEY FINDINGS

Oats and phacelia cover crops were associated with higher phosphatase activity and phosphate mobilisation than other crops. Phosphatase is an enzyme secreted by roots and microorganisms which increases availability of soilbound phosphorus.

These results were not repeated in year 2 or in the lab.

Some cover crops may have the potential to improve phosphate use efficiency, but we don’t yet have the evidence to make a recommendation.

Chris Stoate Sam Reynolds

Once in the soil, phosphate becomes bound to soil particles, known as legacy phosphate, and becomes unavailable to crops. © Paul Maguire/Shutterstock