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Agricultural and industrial activities have greatly altered the global nitrogen (N) cycle and increased the availability of biologically reactive N in many ecosystems. Though N is a limiting nutrient in most terrestrial ecosystems, production responses to enhanced N availability are not likely to pace increases in N deposition. Soils may represent a large and relatively stable sink for anthropogenic N, but few studies have explicitly addressed the influence of soil properties on N retention. We examined the influence of soil organic matter (SOM), particulate organic matter (POM), and soil texture on N retention in a series of soils across an environmental gradient in the western Great Plains of the United States. We applied 2.5 and 50 g N/m2 of 15N-labeled (NH4)2SO4 to coarse- and fine-textured soils in the spring of 1996 and collected soils and plants at four sampling intervals over the following three growing seasons. Within the grassland ecosystems studied, soils were the dominant sink for both the low-N and high-N treatments. Following low-N additions, plots retained a high proportion of N distributed in plant and POM fractions, suggesting tight cycling among pools with rapid and intermediate turnover. High-N plots retained 35% of N additions after three growing seasons following application. On average, recovery of N was greatest in fine-textured soil relative to coarse soils, although this trend was significant only in plots receiving low-N additions. Soil organic carbon (C) content explained a significant proportion of the variability in estimates of N retention, particularly for high-N plots where soil C accounted for up to 40% of the variance in total recovery of the N applications. We conclude that soils are the dominant sink for retention of N applications and that stabilization is controlled in large part by soil organic C. Based upon our results, we estimate the threshold for N retention of pulse N addition in the surface soils of semiarid grasslands systems to be |
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