Relating Net Nitrogen Input in the Mississippi River Basin to Nitrate Flux in the Lower Mississippi River

Abstract

A quantitative understanding of the relationship between terrestrial N inputs and riverine N flux can help guide conservation, policy, and adaptive management efforts aimed at preserving or restoring water quality. The objective of this study was to compare recently published approaches for relating terrestrial N inputs to the Mississippi River basin (MRB) with measured nitrate flux in the lower Mississippi River. Nitrogen inputs to and outputs from the MRB (1951 to 1996) were estimated from state-level annual agricultural production statistics and NOy (inorganic oxides of N) deposition estimates for 20 states that comprise 90% of the MRB. A model with water yield and gross N inputs accounted for 85% of the variation in observed annual nitrate flux in the lower Mississippi River, from 1960 to 1998, but tended to underestimate high nitrate flux and overestimate low nitrate flux. A model that used water yield and net anthropogenic nitrogen inputs (NANI) accounted for 95% of the variation in riverine N flux. The NANI approach accounted for N harvested in crops and assumed that crop harvest in excess of the nutritional needs of the humans and livestock in the basin would be exported from the basin. The U.S. White House Committee on Natural Resources and Environment (CENR) developed a more comprehensive N budget that included estimates of ammonia volatilization, denitrification, and exchanges with soil organic matter. The residual N in the CENR budget was weakly and negatively correlated with observed riverine nitrate flux. The CENR estimates of soil N mineralization and immobilization suggested that there were large (2000 kg N ha-1) net losses of soil organic N between 1951 and 1996. When the CENR N budget was modified by assuming that soil organic N levels have been relatively constant after 1950, and ammonia volatilization losses are redeposited within the basin, the trend of residual N closely matched temporal variation in NANI and was positively correlated with riverine nitrate flux in the lower Mississippi River. Based on results from applying these three modeling approaches, we conclude that although the NANI approach does not address several processes that influence the N cycle, it appears to focus on the terms that can be estimated with reasonable certainty and that are correlated with riverine N flux.

Differences in Phosphorus and Nitrogen Delivery to The Gulf of Mexico from the Mississippi River Basin

“…Pasture/ rangelands include nutrients from nonrecoverable animal manure-i.e., manure from unconfined animals and manure lost during the collection, storage, and treatment of wastes from confined animals, including concentrated animal feeding operations (22). The model assumes that nutrient immobilization and mineralization rates in soils of the MARB are approximately in equilibrium as in prior MARB models (18,23,24).…”

“…Total ammonium deposition in the MARB represents less than 10% of the nitrogen inputs to cropland from commercial and manure fertilizers and biologically fixed N2. Much of the deposition originates from livestock ammonia emissions, which are more than double those from fertilizer (37), with about half of the deposition occurring within 50 km of the emission source (23). Ammonium deposition was not explicitly included in the model inputs to cropland because of uncertainties in agricultural sources and transport distances (i.e., long-range vs local deposition).…”

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…Compared with process-based models, statistical models are predominant for addressing watershed legacy nutrient dynamics. Based on long-term (>30 years) observations, statistical models between annual riverine nitrate flux and previous several years' average net anthropogenic nitrogen inputs (NANI) were developed to address N leaching lag effect in the Mississippi River watershed (previous 2-5 and 6-9 years' average NANI) in 1960-1998(McIsaac et al, 2001) and the Yongan watershed (previous 0-6 years' average NANI) in eastern China in 1980(Chen et al, 2014b. Other several statistical models were directly established between annual riverine nutrient flux and current year's NANI or NAPI, hydroclimatic, and land-use variables in the Yongan watershed and Yangtze watershed in China (Chen et al, 2015b(Chen et al, , 2016c.…”

“…Considering the difference of accumulated legacy nutrient pools across landscapes, Chen et al (2016a,b) further developed statistical models for linking net anthropogenic N or P inputs to agricultural/forest and residential systems with riverine TN or TP flux and then identified contributions of agricultural and residential legacy nutrient sources in Yongan watershed in eastern China in 1980-2010. In these statistical models, relationships between riverine nutrient flux and anthropogenic nutrient inputs were usually expressed as an exponential function, which may be a consequence of anthropogenic nutrient inputs exceeding the capacity of terrestrial and/or aquatic systems to assimilate or retain excessive nutrient (Chen et al, 2015b;McIsaac et al, 2001;Sharpley et al, 2013). ), NANI 0-6 is the average annual NANI during the previous 0-6 years (kg N ha À1 year À1 ), T 0-4 is the average air temperature during the previous 0-4 years (°C), and D % is the drained agricultural area percentage during the previous 0-3 years ), and NANI is the net anthropogenic nitrogen input (kg N km À2 year À1 ) …”

Long-term fate of nitrate fertilizer in agricultural soils

“…Also, due to the long mean residence time of fertilizer N in soils the effects of changes in soil management practices on nitrate loading of the hydrosphere may be considerably delayed. For instance, studies of the Mississippi River Basin have revealed a decrease in anthropogenic N inputs without any concurrent reductions in riverine nitrate loading (41)(42)(43).…”