Hanna Poffenbarger

Cover crops can be used to provide some of the nitrogen (N) needs of a cash crop to complement mineral fertilizers or manure, but there has yet been limited work to describe corn ( Zea mays L.) yield as a function of cover crop quality and N inputs. We investigated the response of corn yield to gradients of both preceding cover crop C:N ratio and poultry litter (PL) application rates in Beltsville, MD during 2012–2014. To achieve different C:N ratios of the cover crops, hairy vetch ( Vicia villosa Roth. "Groff") and cereal rye ( Secale cereale L. "Aroostook") were seeded in a replacement series of six seeding rate proportions, resulting in shoot C:N ratios of 9.2:1 to 152:1 across years. For each hairy vetch/cereal rye sown proportions, PL was side‐dressed at corn V5–V8 stage in subsurface bands (SSB) at four targeted rates: Zero, P‐based (67 kg plant available nitrogen [PAN] ha −1 ), N‐based (135 kg PAN ha −1 ), and excess N and P (269 kg PAN ha −1 ). We found that corn yield followed a linear‐plateau relationship across these two dimensions. Within the linear region, each unit increase in log‐scaled cover crop C:N ratio resulted in a yield decrease of 2.56 ± 0.26 Mg ha −1 at a given rate of SSB PL. To optimize corn yields, we describe a model where each unit increase in log‐scaled cover crop C:N ratio required an additional 45.9 ± 6.22 kg PAN ha −1 from SSB PL. Yields following winter fallow were typically intermediate to the range of yields observed following the gradient of cover crop C:N ratios. We did not find significant differences in corn yield responses when comparing SSB PL to at‐planting incorporated or broadcast PL; we also found no significant differences between SSB PL and surface‐banded urea ammonium nitrate. Taken together, our approach of modeling yield response across two dimensions can be widely used to guide adaptive N management in subsequent cash crops following winter cover crops, thereby balancing both economic and environmental objectives in cover crop‐based cropping systems.

There is interest in harnessing cropland C storage potential at a large scale to mitigate climate change and improve land productivity. While the effects of soil management practices on C storage have been studied extensively, opportunities to select for C sequestration traits in crop plants remain largely unexplored. This review describes how genetic improvement of major US crops may have altered soil C stocks historically and identifies potential opportunities for plant breeding to increase cropland C stocks. Through quantitative literature review, we find that breeding has led to an increase in aboveground residue C inputs to the soil for corn ( Zea mays L.) and soybeans ( Glycine max (L.) Merr) and a decrease in aboveground residue C inputs for wheat ( Triticum aestivum L. and Triticum turgidum L.). Breeding has largely not altered the root:shoot ratio of these crops. Given that there is limited potential for further major improvements in harvest index, breeding for high grain yields may necessitate increasing aboveground biomass and residue production in the future. Crop root traits and residue quality may influence the stabilization of crop‐derived C in soil, but there is uncertainty regarding historical changes in these traits due to breeding, the magnitude of their effect on soil organic C stocks, and tradeoffs or synergies with breeding for high yield. Nevertheless, root traits such as suberin content, rhizodeposition, mycorrhizal association, and depth emerge as potential targets for more efficient C stabilization. There is also a large opportunity for plant breeding to enhance the performance of cover crops, double crops, perennial grains, and perennial groundcovers, which can increase annual C inputs to the soil by occupying fallow periods. Our review reveals that there are many opportunities for plant genetic improvement to fix more C in cropping systems and enhance its stabilization in the soil to meet the goals of sustainable intensification and cropland C capture.