Abstract
AbstractA potential strategy for mitigating nitrous oxide (N2O) emissions from permanent grasslands is the partial substitution of fertilizer nitrogen (Nfert) with symbiotically fixed nitrogen (Nsymb) from legumes. The input of Nsymb reduces the energy costs of producing fertilizer and provides a supply of nitrogen (N) for plants that is more synchronous to plant demand than occasional fertilizer applications. Legumes have been promoted as a potential N2O mitigation strategy for grasslands, but evidence to support their efficacy is limited, partly due to the difficulty in conducting experiments across the large range of potential combinations of legume proportions and fertilizer N inputs. These experimental constraints can be overcome by biogeochemical models that can vary legume‐fertilizer combinations and subsequently aid the design of targeted experiments. Using two variants each of two biogeochemical models (APSIM and DayCent), we tested the N2O mitigation potential and productivity of full factorial combinations of legume proportions and fertilizer rates for five temperate grassland sites across the globe. Both models showed that replacing fertilizer with legumes reduced N2O emissions without reducing productivity across a broad range of legume‐fertilizer combinations. Although the models were consistent with the relative changes of N2O emissions compared to the baseline scenario (200 kg N ha−1 yr−1; no legumes), they predicted different levels of absolute N2O emissions and thus also of absolute N2O emission reductions; both were greater in DayCent than in APSIM. We recommend confirming these results with experimental studies assessing the effect of clover proportions in the range 30–50% and ≤150 kg N ha−1 yr−1 input as these were identified as best‐bet climate smart agricultural practices.
Highlights
Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) and plays a dominant role in stratospheric ozone depletion (Ravishankara et al, 2009; Smith et al, 2014)
Nitrous Oxide Emissions Nitrous oxide emissions ranged between 0.01 and 1.9 kg N2O‐N ha−1 yr−1 (0.1–0.4% of N fertilizer amounts (Nfert)) across scenarios for APSIM variants and between 1.8 and 9.6 kg N2O‐N ha−1 yr−1 (1.4–5.5% of Nfert) for DayCent model variants averaged across sites (Table 1, Figure 3)
The mitigation option would be ineffective if N2O emissions were independent of the source of N; this finding shows that an important precondition for effectiveness of the mitigation strategy was met
Summary
Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) and plays a dominant role in stratospheric ozone depletion (Ravishankara et al, 2009; Smith et al, 2014). Agriculture contributes 58–84% to global anthropogenic N2O emissions Smith et al, 2008) and is important for N2O mitigation Most agricultural N2O emissions arise from inputs of nitrogen (N) by being too high or out of synchrony with crop or pasture demand, resulting in the applied N being susceptible to losses through leaching or gaseous emissions including N2O (Davidson & Kanter, 2014)
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