The fate of nitrogen in grain cropping systems: a meta‐analysis of 15N field experiments

Intensively managed grain farms are saturated with large inputs of nitrogen (N) fertilizer, leading to N losses and environmental degradation. Despite decades of research directed toward reducing N losses from agroecosystems, progress has been minimal, and the currently promoted best management practices are not necessarily the most effective. We investigated the fate of N additions to temperate grain agroecosystems using a meta‐analysis of 217 field‐scale studies that followed the stable isotope 15 N in crops and soil. We compared management practices that alter inorganic fertilizer additions, such as application timing or reduced N fertilizer rates, to practices that re‐couple the biogeochemical cycles of carbon (C) and N, such as organic N sources and diversified crop rotations, and analyzed the following response variables: 15 N recovery in crops, total recovery of 15 N in crops and soil, and crop yield. More of the literature (94%) emphasized crop recovery of 15 N than total 15 N recovery in crops and soil (58%), though total recovery is a more ecologically appropriate indicator for assessing N losses. Findings show wide differences in the ability of management practices to improve N use efficiency. Practices that aimed to increase crop uptake of commercial fertilizer had a lower impact on total 15 N recovery (3–21% increase) than practices that re‐coupled C and N cycling (30–42% increase). A majority of studies (66%) were only one growing season long, which poses a particular problem when organic N sources are used because crops recover N from these sources over several years. These short‐term studies neglect significant ecological processes that occur over longer time scales. Field‐scale mass balance calculations using the 15 N data set show that, on average, 43 kg N·ha −1 ·yr −1 was unaccounted for at the end of one growing season out of 114 kg N·ha −1 ·yr −1 , representing ∼38% of the total 15 N applied. This comprehensive assessment of stable‐isotope research on agroecosystem N management can inform the development of policies to mitigate nonpoint source pollution. Nitrogen management practices that most effectively increase N retention are not currently being promoted and are rare on the landscape in the United States.