Long-term fate of nitrate fertilizer in agricultural soils is not necessarily related to nitrate leaching from agricultural soils.

Accounting for the fate of inorganic N fertilizer in agricultural systems is critical to sustainable production. Sebilo et al. (1) provide a unique long-term record of NO3 fertilizer fate that demonstrates N molecules from a discrete fertilizer application are transferred to soil organic matter (SOM) and subsequently mineralized over the course of approximately 100 y, during which they contribute to NO3 leaching. The authors conclude “attempts to reduce agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizers” (1). Furthermore, Sebilo et al. suggest that a recent decrease in anthropogenic N inputs to the Mississippi River Basin, without a concomitant decrease in riverine NO3 loads, is consistent with their conclusion. In contrast to the authors’ interpretation, we highlight that short-term changes in agricultural management can lead to large shortterm reductions in NO3 leaching. We attribute the authors’ results to long-term SOM turnover and associated NO3 losses that occur in the absence of high biological N demand. We suggest that changes in agricultural management that produce rapid reductions in total NO3 leaching would produce proportional reductions in NO3 leaching derived from historical fertilizer applications. Diversified cropping systems, conversions to perennial biofuels, and cover crops all provide direct evidence of the short-term potential for decreases in NO3 leaching. In maize–alfalfa cropping systems, NO3 leaching under alfalfa is 75–80% lower than the preceding maize crop (2). Perennial biofuel crops can reduce NO3 leaching by >90% within 4 y of establishment (3). Nonlegume cover crops provide on average a 70% reduction in NO3 leaching during 2to 3-y experiments (4). N labeling studies demonstrate that the proportion of fertilizer transferred to SOM is rapidly distributed among pools with fast and slow turnover rates (e.g., ref. 5). Consistent with ecosystem models, a small portion of the N transferred to slow-turnover SOM-N pools will mineralize every year. In annual cropping systems where plant uptake is not fully synchronized with N mineralization, a portion of the mineralized N will nitrify and leach. As such, we expect strategies that reduce total NO3 leaching will provide proportionally equal reductions in all sources of NO3 leaching independent of whether the NO3 is derived from fastor slow-turnover SOM-N pools. We also question the suggestion that net anthropogenic N inputs (NANI) to the Mississippi River basin are decreasing. None of the cited papers demonstrate a decrease in NANI, and recent analyses show NANI to be relatively static over the last 20 y. With a goal to reduce N losses to the Gulf of Mexico by 45%, we are concerned that readers will misinterpret the authors’ data to suggest short-term agricultural management strategies cannot provide large, rapid reductions in NO3 leaching. Strategies that minimize bare fallows and maximize plant N demand are well known to provide rapid reductions in NO3 leaching (2–4). Although mineralization of historical N fertilizer applications cannot be prevented, the mineralized N can be recycled into SOM, particularly with strategies that aim to restore depleted soil organic carbon stocks.