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

Abstract

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 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 carbon (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. More of the literature (94%) emphasized crop recovery of 15N than total 15N 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 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-term studies neglect significant ecological processes that occur over longer time scales. Field-scale mass balance calculations using the 15N data set show that, on average, 43 kg N x ha(-1) x yr(-1) was unaccounted for at the end of one growing season out of 114 kg N x ha(-1) x yr(-1), representing approximately 38% of the total 15N 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.

Legume, cropping intensity, and N-fertilization effects on soil attributes and processes from an eight-year-old semiarid wheat system

“…Researchers have cautioned that legume N residue needs to be managed properly in order to increase N synchrony with crop uptake demands (Crews and Peoples 2005;Gardner and Drinkwater 2009), and to improve legume-N retention (Hauggaard-Nielsen et al 2009;Starovoytov et al 2011). Even in semiarid Saskatchewan, a 17-year old LGM (lentil)-W-W rotation led to higher NO 3 -N leaching than a F-W-W control when the N supplying capacity of the soil was underestimated (Campbell et al 2006).…”

Ecosystem Services in Biologically Diversified versus Conventional Farming Systems: Benefits, Externalities, and Trade-Offs

“…Finally, improved nutrient use efficiency of combined cropping systems can produce a cobenefit of reducing soil nitrate loads and thus the potential for nitrate leaching (Zhang and Li 2003). http://www.ecologyandsociety.org/vol17/iss4/art40/ Studying the effect of crop rotation on nutrient management, Gardner et al (2009) conducted a meta-analysis of studies that followed the fate of 15 N isotope tracer in crop biomass and soils (35 responses). They found that, relative to continuous cropping or simplified rotations such as corn/soy, more diverse rotations (including those with cover crops) significantly increased 15 N recovery, i.e., by 17% in the grain cash crop alone and by 30% across all crops and soils.…”

Crop yield, root growth, and nutrient dynamics in a conventional and three organic cropping systems with different levels of external inputs and N re-cycling through fertility building crops

“…Still the results on soil N and especially soil N in deeper layers show that this increased root exploitation of the soil has strongly improved the ability of the system to retain N and avoid that this resource is being lost by leaching, i.e. that a normal function of natural ecosystems has to a significant extent been introduced into a farming system and that the biogeochemical cycles of carbon and N has been recoupled by the fertility building crop strategies (Tonitto et al, 2006;Gardner and Drinkwater, 2009). Further significant improvement in this aspect of arable cropping systems will require much more dramatic changes to the system, e.g.…”

“…This way the reliance on fertility building crops and diversified crop rotations for proper nutrient husbandry is decreased in the organic systems, but this decouples the biogeochemical cycles of carbon and N (i.e. less co-flow of C and N cycles) with possible increase of N losses from the production (Tonitto et al, 2006;Gardner and Drinkwater, 2009). While such a development may in the short term help organic farms to supply the market demand and to be commercially viable, it may in the longer term undermine consumer confidence if organic farming is not seen to deliver the really different, sustainable, and environmentally friendly alternative to conventional farming.…”