Nitrous Oxide Emissions From Agriculture In Perspective.

Nitrous oxide (N₂O) is a major greenhouse gas and the most important ozone-depleting emittant of our century. Anthropogenic N₂O emissions are dominantly derived from agricultural soils, and driven by increased nitrogen (N) inputs via fertilizers and manures. Here, we argue that the mitigation of N₂O emissions should be considered within the broader agronomic and environmental context of agroecosystems. We present the results of three studies that illustrate our approach, which may lead to new approaches towards mitigating N₂O emissions.

First, we argue that the supply of other nutrients than N have to be considered too when minimizing N₂O emissions. In an experiment with silage maize, cumulative N₂O emissions were reduced by 58% (p<0.01) when phosphorus (P) availability was increased from moderate to sufficient. This could be linked to faster N uptake during the growing period.

Second, N₂O emissions are often expressed as a function of fertilizer N application rate. However, we argue that, because of the increasing global demand for food, N₂O emissions should be related to yield. Using a literature review, we found that such 'yield-scaled' N₂O emissions lead to management recommendations that are remarkably similar to those of common good agricultural practices: to aim for high N use efficiency at intermediate fertilizer rates.

Third, not only N₂O but also other environmental fluxes should be considered in relation to yield. In a modelling study we show that emissions of N₂O, other greenhouse gases, and NO₃ per kg of plant food, milk, beef/pork/poultry and egg produced differ significantly between products and countries. This indicates how use efficiencies in the food chain can be improved and how we can avoid trade-offs between different emissions.

We conclude that understanding and mitigating N₂O emissions is only possible if we consider those emissions within the agronomic and environmental context in which they exist.