Grazing summer cover crops mix impact on carbonnitrogen cycling, soil water, and wheat yields

Summer fallow practice is common in the US semiarid Southern Great Plains ecoregion. Its primary objective is to store water to enhance winter wheat production for grain and grazing. Prior research on replacing fallow with legume cover crops (CC) (green manure) reported a yield loss through soil moisture depletion for winter wheat. Recent research has shown the potential of grass CC to enhance soil physical properties compared to legume CC. Cost-benefit analyses of CC make it difficult to justify their adoption. Grazing, however, offers an opportunity to mitigate CC costs. The impact of a grazed CC mix on stored soil water, CN cycling, intercropping, and wheat yields was evaluated. Although CC used soil moisture early in the growing season and nitrate+nitrite-N, they recharged soil moisture by the critical wheat seeding and dormancy-breaking periods. In the summer of 2014, net mineralization in CC increased nitrate+nitrite-N by 10-fold compared to 2013 concentrations, resulting in no significant differences in yields in 2015. Nitrate+nitrite-N was reduced by 28% in 2015 compared to the preceding year due to CC growth and net nitrate+nitrite-N immobilization in residues, resulting in 25% less yield in CC treatments in 2016 compared to no-till (NT) fallow. Grazed and ungrazed CC enhanced organic N by up to four times that in NT fallow and sequestered particulate organic N and C by 7–17% and 6–10%, respectively, in micro-aggregates. Despite reducing biomass by 56%, grazing did not differ from most ungrazed measured soil parameters, offering the potential for mitigating CC production costs.