McIsaac Gregory F, David Mark B, Gertner George Z, Goolsby Donald A
University of Illinois, Department of Natural Resources and Environmental Sciences, W-503 Turner Hall, 1102 S. Goodwin Ave., Urbana, IL 61801, USA.
J Environ Qual. 2002 Sep-Oct;31(5):1610-22. doi: 10.2134/jeq2002.1610.
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.
对陆地氮输入与河流氮通量之间关系的定量理解有助于指导旨在保护或恢复水质的保护、政策及适应性管理工作。本研究的目的是将近期发表的关于密西西比河流域（MRB）陆地氮输入的方法与密西西比河下游实测的硝酸盐通量进行比较。根据州级年度农业生产统计数据以及占MRB 90%的20个州的氮氧化物（NOy，氮的无机氧化物）沉积估算值，估算了1951年至1996年期间MRB的氮输入和输出。一个包含产水量和总氮输入的模型解释了1960年至1998年密西西比河下游观测到的年度硝酸盐通量变化的85%，但往往低估高硝酸盐通量，高估低硝酸盐通量。一个使用产水量和人为净氮输入（NANI）的模型解释了河流氮通量变化的95%。NANI方法考虑了作物收获的氮，并假设流域内作物收获量超过人类和牲畜营养需求的部分将从流域输出。美国白宫自然资源与环境委员会（CENR）制定了一个更全面的氮预算，其中包括氨挥发、反硝化作用以及与土壤有机质交换的估算值。CENR预算中的剩余氮与观测到的河流硝酸盐通量呈弱负相关。CENR对土壤氮矿化和固定的估算表明，1951年至1996年期间土壤有机氮存在大量（2000 kg N ha-1）净损失。当通过假设1950年后土壤有机氮水平相对恒定且氨挥发损失在流域内重新沉积来修正CENR氮预算时，剩余氮的趋势与NANI的时间变化紧密匹配，并且与密西西比河下游的河流硝酸盐通量呈正相关。基于应用这三种建模方法的结果，我们得出结论，尽管NANI方法没有涉及影响氮循环的几个过程，但它似乎关注的是能够以合理确定性估算且与河流氮通量相关的项。
