Manipulating cover crop termination time to alter soil moisture dynamics and mitigate soybean water stress

Growing fall cover crops (CCs) before soybeans ( Glycine max L.) is an encouraged practice in the Mid‐Atlantic United States. Previous studies indicated that CC termination time can influence soil moisture during the soybean growing season. However, there is a lack of studies providing comprehensive data on soil moisture dynamics and plant water stress metrics. This study was conducted over 3 site‐years in Pennsylvania on silt loam and silt clay loam soils to evaluate four treatments: early planting brown (soybean planted early into pre‐killed CC, i.e., terminated approximately 2 weeks before soybean planting), early planting green (soybean planted early into living CC, i.e., terminated immediately after soybean planting), late planting brown (soybean planted late into pre‐killed CC), and late planting green (soybean planted late into living CC). In 1 site‐year, late planting green conserved soil water content later in the growing season, compared to planting brown. In all site‐years, soybean grain δ 13 C, an indicator of plant water stress, was higher in the early planting brown (−27.5‰) than in the late planting green (−28.0‰) treatment. δ 13 C was negatively correlated with CC biomass at termination ( r = −0.40) and yield ( r = −0.50). When soybeans were planted early, soybean yield was 7%–70% higher with planting green than planting brown. Late planting green treatment yielded comparably or higher than early planting brown. These findings suggest that delaying CC termination to increase CC biomass can mitigate soybean water stress and translate to yield gains in the rainfed no‐till systems of central and southeast Pennsylvania.