The fate of nitrogen in grain cropping systems: A meta-analysis of 15N experiments

Background/Question/Methods

Intensively managed grain farms are saturated with large inputs of nitrogen (N) fertilizer, which leads to N losses and environmental degradation.  Long-term N mass balance data has demonstrated that agroecosystems with organic N sources, and diversified rotations that include cover crops, sequester more carbon (C) and N than conventionally-managed systems.  We investigated the fate of N additions to temperate grain agroecosystems using a meta-analysis of more than 200 field-scale studies that followed the stable isotope, ¹⁵N, 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 C and N, such as organic N sources and diversified crop rotations and analyzed the following response variables: ¹⁵N recovery in crops, total recovery of ¹⁵N in crops and soil, and crop yield.  
Results/Conclusions

More of the literature (94%) emphasized crop recovery of ¹⁵N than total ¹⁵N recovery in crops and soil (58%), which is the more ecologically appropriate indicator for assessing N losses. There were 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 ¹⁵N 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 studies neglect significant ecological processes that occur over longer time scales.  Field-scale mass balance calculations using the ¹⁵N dataset show that, on average, 43 kg N ha^-1 yr^-1 was unaccounted for at the end of one growing season. Results from this comprehensive assessment of ¹⁵N experiments in temperate grain cropping systems confirm current generalizations about the fate of N (e.g., that the majority (~65%) of plant N comes from soil pools); point to knowledge gaps in the literature; and can inform the development of policies to mitigate non-point source pollution.  N management practices that most effectively increase N retention are not currently being promoted and are rare on the landscape in the United States.  To help achieve environmental sustainability, policies should target management practices that re-couple C and N cycles in agricultural fields.