Low transient storage and uptake efficiencies in seven agricultural streams: implications for nutrient demand

We used mass load budgets, transient storage modeling, and nutrient spiraling metrics to characterize nitrate (NO3 -), ammonium (NH4 +), and inorganic phosphorus (SRP) demand in seven agricultural streams across the United States and to identify in-stream services that may control these conditions. Retention of one or all nutrients was observed in all but one stream, but demand for all nutrients was low relative to the mass in transport. Transient storage metrics (As/A, Fmed 200, Tstr, and qs) correlated with NO3 retention but not NH4 + or SRP retention, suggesting in-stream services associated with transient storage and stream water residence time could influence reach-scale NO3 demand. However, because the fraction of median reach-scale travel time due to transient storage (Fmed 200) was £1.2% across the sites, only a relatively small demand for NO3 could be generated by transient storage. In contrast, net uptake of nutrients from the water column calculated from nutrient spiraling metrics were not significant at any site because uptake lengths calculated from background nutrient concentrations were statistically insignificant and therefore much longer than the study reaches. These results suggest that low transient storage coupled with high surface water NO3 inputs have resulted in uptake efficiencies that are not sufficient to offset groundwater inputs of N. Nutrient retention has been linked to physical and hydrogeologic elements that drive flow through transient storage areas where residence time and biotic contact are maximized; however, our findings indicate that similar mechanisms are unable to generate a significant nutrient demand in these streams relative to the loads. Low Transient Storage and Uptake Efficiencies in Seven Agricultural Streams: Implications for Nutrient Demand Richard W. Sheibley,* John H. Duff, and Anthony J. Tesoriero It is widely understood that humans have dramatically altered the earth’s ecosystems through alterations of global biogeochemical cycles (Galloway et al., 1995; Vitousek et al., 1997). In particular, nutrient cycles have been modified through production of Nand P-based fertilizers, cultivation of N-fixing crops, animal waste disposal practices, combustion of fossil fuels, and extensive mining practices (Galloway et al., 1995; Vitousek et al., 1997; Tilman et al., 2001). Agricultural practices have been implicated as the biggest driver of nutrient cycling changes in aquatic ecosystems (Tilman et al., 2001; Bernot et al., 2006; Birgand et al., 2007), and some researchers argue that the dramatic increase in reactive N in the hydrosphere is a more important ecological problem than climate change (Tilman et al., 2001). For example, Tilman et al. (2001) project that N use from future agriculture will increase 1.6 times by 2020, with increases in P and irrigation water reaching 1.4and 1.3-fold, respectively, during the same time period. Furthermore, with evidence of legacy groundwater pollution (Tesoriero et al., 2013), nutrients that are currently applied to the landscape may not discharge to streams for decades or more. The consequence of this influx of human-derived N and P are dramatic and have led to degradation of drinking water supplies (Burow et al., 2010), eutrophication of aquatic ecosystems (Rabalais et al., 2002), and contributions to global climate change (Groffman et al., 2000). In agricultural settings only about half of the added fertilizer is captured by crops (Tilman et al., 2001; Puckett et al., 2011), with release of N and P to groundwater, terrestrial, and aquatic ecosystems making up the balance. As a result, the fate and transport of nutrients in agricultural ecosystems has been gaining attention in the past decade (Kemp and Dodds, 2002a; Royer et al., 2004; Bernot et al., 2006; Duff et al., 2008; Mulholland et al., 2008; Puckett et al., 2008, 2011). In particular, there is much interest in how streams process high nutrient loads from agricultural land, and recent studies on agricultural streams have shown that rates of N and P uptake are higher than corresponding reference sites (Bernot et al., 2006; Mulholland et al., 2008). However, the efficiency of NO3 uptake is lower in agricultural streams because of higher N concentrations (Mulholland et al., 2008). Abbreviations: OTIS-P, one-dimensional transport model with inflow and storage; SRP, inorganic phosphorus. Richard W. Sheibley, U.S. Geological Survey, 934 Broadway, Suite 300, Tacoma, WA 98402; John H. Duff, U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94019; Anthony J. Tesoriero, U.S. Geological Survey, 2130 SW 5th Ave., Portland, OR 97201. Assigned to Associate Editor Ying Ouyang. Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.