| Abstract: | Accelerated nitrogen (N) inputs can drive nonlinear changes in N cycling, retention, and loss in forest ecosystems. Nitrogen processing in soils is critical to understanding these changes, since soils typically are the largest N sink in forests. To elucidate soil mechanisms that underlie shifts in N cycling across a wide gradient of N supply, we added 15NH415NO3 at nine treatment levels ranging in geometric sequence from 0.2 kg to 640 kg NA? ha-1A? yr-1 to an unpolluted old-growth temperate forest in southern Chile. We recovered roughly half of tracers in 0-25 cm of soil, primarily in the surface 10 cm. Low to moderate rates of N supply failed to stimulate N leaching, which suggests that most unrecovered 15N was transferred from soils to unmeasured sinks above ground. However, soil solution losses of nitrate increased sharply at inputs > 160 kg NA? ha-1A? yr-1, corresponding to a threshold of elevated soil N availability and declining 15N retention in soil. Soil organic matter (<5.6 mm) dominated tracer retention at low rates of N input, but coarse roots and particulate organic matter became increasingly important at higher N supply. Coarse roots and particulate organic matter together accounted for 38% of recovered 15N in soils at the highest N inputs and may explain a substantial fraction of the ‘missing N‘ often reported in studies of fates of N inputs to forests. Contrary to expectations, N additions did not stimulate gross N cycling, potential nitrification, or ammonium oxidizer populations. Our results indicate that the nonlinearity in N retention and loss resulted directly from excessive N supply relative to sinks, independent of plant-soil-microbial feedbacks. However, N additions did induce a sharp decrease in microbial biomass C:N that is predicted by N saturation theory, and which could increase long-term N storage in soil organic matter by lowering the critical C:N ratio for net N mineralization. All measured sinks accumulated 15N tracers across the full gradient of N supply, suggesting that short-term nonlinearity in N retention resulted from saturation of uptake kinetics, not uptake capacity, in plant, soil, and microbial pools. |
| Genre: | Article |
| ProdID: | 1016242 |
| Citation Author: | Perakis, Steven S.; Compton, J. E.; Hedin, L. O. |
| Citation Contributing Office: | Forest and Rangeland Ecosystem Science Center |
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| Citation End Page: | 105 |
| Citation Issue: | 1 |
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| Citation Language: | English |
| Citation Larger Work Title: | Ecology |
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| Citation Month: | JAN |
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| Citation Number Of Pages: | 10 |
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| Citation Phsyical Description: | p. 96-105 |
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| Citation Search Results Text: | Nitrogen retention across a gradient of 15N additions to an unpolluted temperate forest soil in Chile; 2005; Article; Journal; Ecology; Perakis, Steven S.; Compton, J. E.; Hedin, L. O. |
| Citation Start Page: | 96 |
| Citation Volume: | 86 |
| Citation Year: | 2005 |
| Type: | citation/reference |
| Text: | Nitrogen retention across a gradient of 15N additions to an unpolluted temperate forest soil in Chile; 2005; Article; Journal; Ecology; Perakis, Steven S.; Compton, J. E.; Hedin, L. O. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Sat, 1 Jan 2005 00:00 -0600 |
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