The nitrogen legacy: emerging evidence of nitrogen accumulation in anthropogenic landscapes

Abstract

Watershed and global-scale nitrogen (N) budgets indicate that the majority of the N surplus in anthropogenic landscapes does not reach the coastal oceans. While there is general consensus that this 'missing' N either exits the landscape via denitrification or is retained within watersheds as nitrate or organic N, the relative magnitudes of these pools and fluxes are subject to considerable uncertainty. Our study, for the first time, provides direct, large-scale evidence of N accumulation in the root zones of agricultural soils that may account for much of the 'missing N' identified in mass balance studies. We analyzed long-term soil data (1957-2010) from 2069 sites throughout the Mississippi River Basin (MRB) to reveal N accumulation in cropland of 25-70 kg ha −1 yr −1 , a total of 3.8±1.8 Mt yr −1 at the watershed scale. We then developed a simple modeling framework to capture N depletion and accumulation dynamics under intensive agriculture. Using the model, we show that the observed accumulation of soil organic N (SON) in the MRB over a 30 year period (142 Tg N) would lead to a biogeochemical lag time of 35 years for 99% of legacy SON, even with complete cessation of fertilizer application. By demonstrating that agricultural soils can act as a net N sink, the present work makes a critical contribution towards the closing of watershed N budgets.

Two centuries of nitrogen dynamics: Legacy sources and sinks in the Mississippi and Susquehanna River Basins

“…nitrogen, the hydrologic legacy corresponds to dissolved N, primarily in the form of nitrate, in the unsaturated zone and groundwater reservoirs, while the biogeochemical N legacy corresponds to the buildup of organic N in the root zones of soils, as has recently been shown in soils across the Mississippi River Basin [Van Meter et al, 2016]. Herein, legacy N is defined as N which remains within a watershed at least 1 year beyond its initial application at the land surface.…”

Fertilizer legacies meet saltwater incursion: challenges and constraints for coastal plain wetland restoration

“…Our results suggest that agricultural legacies in restored wetlands that are vulnerable to increased salinization could have a significant impact on downstream estuaries in all these areas, but differences in duration of agricultural practices, soil types, and hydrology will affect the magnitude and timing of nutrient release. While the potential for legacy P to continue to cause eutrophication problems even after decreased loading has been well recognized in the literature Sharpley et al, 2013), only recently has it been documented at large scales for N (Van Meter et al, 2016). Legacy N could potentially remain in soils for up to 35 years after stopping agricultural practices in the Mississippi basin (Van Meter et al, 2016).…”

Response of chlorophyll a to total nitrogen and total phosphorus concentrations in lotic ecosystems: a systematic review protocol

“…"green" pathways involving primary producers, "brown" pathways involving heterotrophic bacteria and fungi [11,12]); and many interacting environmental factors are also involved, such as land use, flooding, and stream size, affect stressor-response relationships [13][14][15]. Temporal factors also complicate relationships, with legacy (historic) nutrient sources contributing to stressors [3,16,17]. Finally, high spatiotemporal variability of both nutrient concentrations [18] and lotic systems more generally [19] can complicate evaluation of stressor-response relationships in these systems.…”