Integrating food-energy cropping systems in temperate climates

Walter Zegada-Lizarazu, Andrea Monti Department of Agricultural and Food Sciences, University of Bologna, Italy

Detecting adequate agronomic strategies and production potential of alternative crops and cropping systems to integrate food and energy production.

Climate change and energy security are the two main challenges that are forcing governments around the world to put on the top of their agendas the search and development of renewable energy alternatives. The agricultural sector, differently for the forestry woody biomass sector and its age-related carbon storage dynamics, is able to provide biomass feedstocks in annual basis so to contribute to the expected REDII zero emissions at the point of biomass conversion in shorts growth cycles. Then the agricultural sector is called to take action to find new solutions capable of guaranteeing large quantities of lignocellulosic biomass, locally available, for energy production purposes in a rational and sustainable manner without negatively affecting the main role of agriculture to supply food/feed, maintain biodiversity, and reduce CO2 emissions.

It is, therefore, necessary to evaluate the most adequate agronomic strategies and production potential of alternative crops and cropping systems that would allow to integrate the production of food and energy without agricultural land competition issues so to ensure reliable pathways for the provision of biomass without detrimental effects on the climate and biodiversity.

Across Europe cereals are the predominantly conventional cultivated crops, with wheat (Triticum sp.) planted in autumn and harvested in summer, and maize (Zea mays) planted in spring and harvested in autumn. Therefore, the soil can remain uncultivated (fallow) for months, allowing the introduction of fast growing crops in between two main crops, namely integrated foodenergy cropping systems, entailing also several environmental benefits in terms of soil cover, enhanced biodiversity, carbon neutrality, and framers’ revenue. One example of such sequential cropping system is the so-called relay planting, where a second crop is planted after the first one has completed the major part of its development (i.e. flowering stage), in that way plant interference, shading, and competition for resources are avoided / minimized. In addition, relay cropping may help to solve current problems of intensified agriculture such as controversies in species sowing time, chemical fertilizer application, and soil erosion and degradation. Relay planting renders also possible to cultivate two crops in a single growing season in areas and /or cropping systems where the growing season is too short or inadequate for two crops in sequence. If carefully planned and managed, the proposed innovative cropping systems could not only increase the system productivity and efficient use of land resources, but also significantly improve the local feedstock availability. However, in order to be successful the selection of the second crop and its agronomic management requires careful evaluation. In fact, an important and little studied aspect of relay planting depends on the correct combinations of sowing time and method with the right species and varieties choice to avoid shading, nutrient competition, or inhibition brought about by phytotoxic compounds produced during the decomposition of the residues of a preceding crop. Moreover, relay planting represents an opportunity for incorporating N2-fixing legume crops into annual cropping systems without sacrificing grain production and enhancing biodiversity. Sunn hemp (Crotalaria juncea L.) is a promising legume crop of tropical origin that could be easily adapted to temperate climates as a summer annual crop. Besides that, sunn hemp can improve the soil fertility through its biological N2 fixation, help control erosion, deter root-knot nematodes, and suppress weeds. In tropical areas, sunn hemp is traditionally grown as a nonwood fiber crop thanks to its high lignocllelusoic content, cultivated in rotation with rice, maize, cotton, sugarcane, tobacco and coffee. Very little information, however, is available on the performance of sunn hemp as a lignocellulosic feedstock for bioenergy purposes and on its adaptability to innovative cropping systems (e.g. relay planting) in temperate climates.

The evaluation of the physiological and productive performance of legume crops like sunn hemp as a new crop could cast light on their suitability to temperate climates

where the growing season is limited to cultivate two crops in sequence during the same growing season. Moreover, sunn hemp demonstrated to be adapted to no-tillage systems, thus to reduced agronomic and energetic inputs to prepare the soil (in terms of economic and CO2 equiv. ha-1 yr-1 cost, equipment, and time that should be committed) and more importantly demonstrated to be adapted to direct sowing, an important factor for the development of relay planting system. In fact, biomass yield of sunn hemp under no tillage systems are within the ranges reported in other studies under different environmental conditions and conventional management practices. Moreover, it was estimated that at the full flowering growth stage sunn hemp could fix 60 to 80 kg N2 ha-1 through root nodulation, constituting a significant N2 contribution to the following crops in rotation, thus enhancing the sustainability of the systems and the farmers’ economy. Therefore, harvesting at the right time is very important not only to ensure the highest yield and the largest N2 contribution possible to the subsequent crops, but also to ensure increased soil organic matter and soil stability. From the energy conversion technologies point of view, sunn hemp harvested between full flowering and beginning of seedpod formation are still physiologically immature crop materials, with relative high moisture (i.e. about 75% in our trials) and mineral contents (i.e. ashes) that would require further desiccation and conversion pre-treatments. Taking into account that sunn hemp is a tropical species, reaching crop senescence in temperate climates is difficult (at least with the current genetic material available). In that sense relay planting with wheat could enlarge as much as possible the growing season and therefore bring the crop to full, or close to, maturity with the beneficial effect on increased/maximized N2 contribution to the system, production of seeds (rich in oil), and reduced moisture and minerals in the lignocellulosic feedstock. In a sunn hemp – wheat relay system, cumulated lignocellulosic biomass yield (sunn hemp biomass + wheat

Figure 3 - Emergence of sunn hemp seedlings after direct sowing.

straw) was about 15 Mg ha-1 yr-1 which is comparable to the productivity of some high yielding perennial grasses (i.e. giant reed, miscanthus), while grain quantity and quality of wheat was maintained close to the standards of the production area.

All these suggests that in temperate climates relay cropping could be a sustainable system to integrate food and dedicated biomass crops production in such a way that the local availability of dedicated lignocellulosic feedstocks is greatly enhanced, while food (wheat grain) production is not penalized. Not to mention the sustained fertility of the soil. To have a better understanding of the factors that lead to the sunn hemp performance close to the standards reported in tropical climates, it is important to consider, besides growth and productivity, the physiological components that are determined genetically and by environmental variables. In general, all physiological parameters seem to corroborate the good growth crop processes, productivity, and therefore suitability of sunn hemp to be used as an annual summer crop in temperate climates. Information, however, is largely missing on the physiological characteristics and plastic response of sunn hemp to changes in the growing conditions, growing season length, expected threshold responses to a combination of factors, cropping systems, etc. Overall it is possible the development of innovative food - energy cropping systems that on one hand could respond to the improved nutrient management and crop rotations obligations to be set by the CAP and, on the other, could increase biomass availability and the efficiency of land use without (or minimum) land competition issues with conventional food crops. The introduction of sunn hemp as a summer leguminous crop within a traditional crop rotation do not appear to be particularly problematic from the agronomic and/or physiological point of views.

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