Cover crop mixtures: A powerful strategy to reduce post-harvest surplus of soil nitrate and leaching

Cover cropping is a practice with potential to reduce post-harvest surplus of soil nitrate (NO₃⁻), a mobile form of nitrogen naturally susceptible to losses. Increased soil NO₃⁻ leaching occurs when high soil NO₃⁻ coincides with high drainage events – a common occurrence during the non-growing season in cold and humid climates. The potential of cover crops to reduce NO₃⁻ leaching can be affected by the performance of the cover crop and by edaphic-climatic conditions (e.g., freeze-thaw cycles, soil type) and is subjected to interannual variation. Here, we evaluate soil type and crop rotation effects on soil parameters (i.e., NO₃⁻ concentration, temperature, water content) and on NO₃⁻ leaching under freeze-thaw conditions, using a high-frequency weighing lysimeter system. The soil type consisted of silt loam (SIL) and loamy sand (LS) soils and the crop rotation consisted of a rotation with a mixture of leguminous/non-leguminous cover crops (+CC) and no cover crop (-CC). This research was conducted during periods of 2017/2018 and 2018/2019 subjected to significant drainage. Our results indicated that cover crops did not affect drainage but affected soil temperature, with warmer soils observed over the winter when cover crops were used (+CC) than without cover-crops (-CC). Moreover, the cereal-crucifer-legume mixture after spring wheat reduced soil NO₃⁻ content by 80% compared to -CC during 2017/2018. For 2018/2019, due to poor establishment of the cereal-legume mixture under-seeded to corn, no reduction in soil NO₃⁻ content was observed. The reduction in soil NO₃⁻ in +CC plots for 2017/2018 translated to a significant reduction (by 67%) in NO₃⁻ leaching averaged across soil types. Soil drainage, NO₃⁻ content and NO₃⁻ leaching were not affected by soil type, an effect in part influenced by the boundary controls of the lysimeter system which mimicked soil conditions of a non-LS adjacent/reference field. To expand our results, we estimated the potential drainage of a hypothetical excessively drained LS (ED-LS) – a soil naturally subjected to high hydraulic conductivity and low water holding capacity. Overall, our results indicated that cover cropping can reduce NO₃⁻ leaching independent of soil type but the effect can be pronounced for ED-LS.