NITROGEN RETENTION ACROSS A GRADIENT OF 15N ADDITIONS TO AN UNPOLLUTED TEMPERATE FOREST SOIL IN CHILE
NITROGEN RETENTION ACROSS A GRADIENT OF 15N ADDITIONS TO AN UNPOLLUTED TEMPERATE FOREST SOIL IN CHILE
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DOI：
10.1890/04-0415
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发表时间：
2005
期刊：
影响因子：
4.8
通讯作者：
S. Perakis;J. Compton;L. Hedin
中科院分区：
文献类型：
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作者：
S. Perakis;J. Compton;L. Hedin
加速的氮素输入可以驱动森林生态系统中氮素循环、保持和损失的非线性变化。土壤中的氮处理对于理解这些变化至关重要，因为土壤通常是森林中最大的氮汇。为了阐明氮循环在宽梯度的氮供应中发生变化的土壤机制，我们在智利南部未受污染的古老温带森林中添加了9个处理水平的15 NH 415 NO3，处理水平从0.2 kg到640 kg N·ha−1·yr−1。我们在0-25 cm的土壤中回收了大约一半的15 N示踪剂，主要是在表面10 cm处。低到中等的N供应率未能刺激N淋失，这表明大多数未回收的15 N从土壤转移到地面以上的未测量的汇。然而，当氮素输入量大于160 kg·ha-1·yr-1时，土壤硝态氮的溶解损失急剧增加，这对应于土壤氮素有效性升高和土壤15 N持留量下降的阈值。在低氮输入率下，土壤有机质（<5.6 mm）占主导地位，但粗根和颗粒有机质在高氮供应下变得越来越重要。粗根和颗粒有机物一起占38%的回收15 N在土壤中的最高N输入，并可能解释了相当大的一部分“失踪的N”经常在研究中报告的N输入到森林的命运。与预期相反，氮添加并没有刺激总氮循环，潜在的硝化作用，或铵氧化剂人口。我们的研究结果表明，在N保留和损失的非线性直接导致过量的N供应相对于汇，独立的植物-土壤-微生物的反馈。然而，氮的添加确实会导致微生物生物量C：N急剧下降，这是由N饱和理论预测的，并且可以通过降低净N矿化的临界C：N比来增加土壤有机质中的长期N储存。所有测得的汇积累了15 N示踪剂在整个梯度的N供应，这表明短期的非线性N保留导致饱和的吸收动力学，而不是吸收能力，在植物，土壤和微生物池。
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 N·ha−1·yr−1 to an unpolluted old-growth temperate forest in southern Chile. We recovered roughly half of 15N 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 N·ha−1·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.