Elucidating the role and contribution of cover crop roots to soil fertility and crop nutrition in organic cropping systems

氮预算在有机种植系统中是一项复杂的任务，因为作物施肥是基于有机肥（动物粪便、覆盖作物、堆肥等），而不是现成的矿物肥料。这些有机肥对后续作物营养的贡献，特别是覆盖作物根系对土壤氮收支的贡献，仍有许多未知因素。事实上，在估算覆盖作物对氮收支和施肥计划的贡献时，一般只考虑覆盖作物的茎部生物量。然而，被覆盖作物的根系被假设对农业土壤的土壤氮供应有很大贡献。以前没有研究解决这些重要和创新的研究问题。

Nitrogen budgeting is a complex task in organic cropping systems, because crop fertilization is based on organic fertilizers (animal manures, cover crops, compost, etc.) rather than readily available mineral fertilizers. Many unknowns remain on the contribution of these organic fertilizers to the nutrition of subsequent crops, and particularly on the contribution of cover crop roots to soil N budget. Indeed, generally, only the shoot biomass of cover crops is considered when estimating the contribution of cover crops to nitrogen budget and fertilization plan. However, cover crop roots are hypothesized to contribute considerably to the soil N supply of agricultural soils. No research has previously addressed these important and innovative research questions. The long-term objective of the proposed research program is to elucidate the mechanisms of contribution of cover crop roots to soil fertility and crop nutrition. The short-term objectives of the proposed research program are 1) to quantify and differentiate the contribution of cover crop shoots and roots to the soil N dynamics; 2) to improve the current conceptual model of transfer of cover crop residue N into various soil organic and inorganic fractions; 3) to elucidate the temporal dynamics of N derived from the decomposition of cover crop roots in the soil-plant continuum; and 4) to synchronize the N supply from decomposing cover crops with N uptake of cash crops grown under organic management. Two field experiments and one incubation study will be conducted to achieve the research objectives. The field experiment #1 will compare the shoot and root characteristics of different cover crop species incorporated at different termination stages, and their contribution to organic corn nutrition. The long-term incubation study will quantify the contribution of cover crop roots and shoots to soil N supply. A second field experiment will be conducted using cover crop material grown in 15N-enriched environment, to address specific unknowns about N partitioning in various soil N fractions. My research will provide a fundamental understanding of cover crop root decomposition and associated soil N supply to plants and to the replenishment of the soil reserve and to plants. This will allow for better estimation of the soil N supply to subsequent crops and, therefore, for better forecasting of the need for other N organic amendments. Knowledge of the actual contribution of cover crop roots will be integrated in farm N budgets, therefore conceptually advancing the cover cropping and nutrient budgeting in organic cropping systems. Moreover, increase in crop yield due to better N management and timing would represent important gains in revenues to farmers. On a broader scale, gathering information on soil N budget may help to reduce N losses from cover crop-based cropping systems like organic farming systems.