Mitigating nitrogen pollution by interseeding cover crops into spelt

Abstract

Agricultural intensification has increased the production capacity of agroecosystems by using soluble nitrogen (N) to fertilize crops. However, N saturation produces negative impacts for terrestrial, aquatic, and atmospheric ecosystems. Ecological nutrient management is an approach to agricultural production with the goal of tightening nutrient cycles by increasing the residence time of nutrients. This strategy entails maximizing the timing and diversity of plant cover, minimizing tillage, and using organic sources of nutrients. Cover crop interseeding is an old practice that could achieve nutrient reduction goals by employing cover crops and reduced tillage to improve nitrogen retention and provisioning. However, cover crops are used on only a small fraction of cropland in the United States. As a result, more research is needed to understand the impact of cover crop interseeding and to expand cover crop adoption. To assess cover crop interseeding, we established a cropping systems experiment in central Pennsylvania, USA. This experiment utilized legume-grass cover crop bicultures no-till interseeded under a spelt crop contrasted with similar cover crops planted post-harvest with tillage. Biogeochemical approaches were used to measure agroecosystem N pools and to estimate N fluxes such as N fixation, nitrate leaching, and nitrous oxide emissions. We found that reduced tillage in the interseeded systems maintained smaller soil inorganic N pools (<5 mg N/kg soil) than the tilled systems (p < 0.001). This was associated with a significant 50% decrease in cumulative nitrous oxide emissions during the fall (p < 0.001). However, there was no significant difference in potentially leachable nitrate over the winter despite minimal fall soil nitrate availability in the interseeded systems (p = 0.086). Significantly lower spring cover crop biomass N in the interseeded systems (p = 0.022) may indicate increased susceptibility for spring nitrate leaching. One piece of evidence for this conclusion is that the δ15N of the interseeded legume in the spring shifted closer to the grass δ15N, which suggests the legume N is being mineralized and taken up by the grass. If legume N is being released in this way, it may also be lost through leaching. Spelt yields were not significantly different between the four systems from which we conclude that the interseeded cover crops did not compete with the spelt. In addition, a forage cutting was taken from the interseeded cover crops in the fall providing an additional source of income to the farmer. Spring cover crop biomass N in all systems provides a N credit to the following cash crop, though this was significantly lower in the interseeded systems (p = 0.022). Nitrogen removed in the forage cutting may have contributed to a decreased ability for the interseeded cover crop to provide N to the following cash crop. These results suggest that no-till interseeded cover crops could mitigate the climate impact of agriculture by minimizing nitrous oxide emissions. However, interseeding did not improve nitrate leaching or N provisioning, which suggests that different cover crop species should be selected for these management goals. More research is necessary to optimize these cropping systems for a variety of outcomes. Overall this research illustrates that ecological nutrient management using strategies like no-till cover crop interseeding may be an effective approach to tightening nutrient cycles in agroecosystems.