Cover crop effects on maize drought stress and yield

Cover crops have been proposed as a tool for adapting cropping systems to drought stress caused by climate change. However, little research has directly evaluated whether cover crops reduce drought stress in the following cash crop. We grew maize in both ambient and imposed drought conditions following four functionally diverse cover crops and a fallow control in a two-site-year study. We looked for evidence that cover crops reduced drought stress by improving cash crop water access or nitrogen (N) status. The study was embedded in an organic cropping systems trial in Pennsylvania, USA. Cover crops and manure fertilizer were incorporated with full inversion tillage. Overall, cover crops neither ameliorated nor exacerbated drought stress in the following maize crop. There was no interaction between cover crop and moisture treatment in a mixed model ANOVA predicting maize kernel yield in either year. Drought reduced yield by 33 % in 2014 and 17 % in 2015. Cereal rye (Secale cereale L. cv. Aroostook) reduced yield relative to cover crop treatments that contained legumes by up to 43 %. There were no other yield differences among cover crop treatments, including the fallow control. Our results are likely influenced by the short legacy of cover cropping (only one or two preceding cover crops) and the use of full inversion tillage. Cover crops may have greater potential to reduce maize drought stress after long-term use, in systems with less soil disturbance, and when residues are retained on the soil surface. Further research is needed to assess the potential for cover crops and other soil-building practices to reduce drought stress by increasing infiltration rates, improving soil water-holding capacity, enhancing cash crop root exploration, and reducing evaporation from the soil surface. Multiple lines of evidence in this study lead to a new hypothesis: cropping systems that rely on cover crops and other organic amendments for N supply may be at risk for N limitation under drought. First, drought reduced mineralization of N from cover crop residues, especially cereal rye. Second, drought reduced maize N status when topsoil drying was more severe. Since cover crop residues and manure were concentrated in the plow layer (top 20 cm) and maize roots extended into the subsoil, topsoil drying may have reduced N mineralization more than crop water uptake. Further research is needed to determine whether and under what circumstances the hypothesized effect meaningfully reduces crop yield.