U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA.
Ecology. 2011 Jul;92(7):1481-91. doi: 10.1890/10-1642.1.
Wide natural gradients of soil nitrogen (N) can be used to examine fundamental relationships between plant-soil-microbial N cycling and hydrologic N loss, and to test N-saturation theory as a general framework for understanding ecosystem N dynamics. We characterized plant production, N uptake and return in litterfall, soil gross and net N mineralization rates, and hydrologic N losses of nine Douglas-fir (Pseudotsuga menziesii) forests across a wide soil N gradient in the Oregon Coast Range (U.S.A.). Surface mineral soil N (0-10 cm) ranged nearly three-fold from 0.29% to 0.78% N, and in contrast to predictions of N-saturation theory, was linearly related to 10-fold variation in net N mineralization, from 8 to 82 kg N.ha(-1) x yr(-1). Net N mineralization was unrelated to soil C:N, soil texture, precipitation, and temperature differences among sites. Net nitrification was negatively related to soil pH, and accounted for <20% of net N mineralization at low-N sites, increasing to 85-100% of net N mineralization at intermediate- and high-N sites. The ratio of net: gross N mineralization and nitrification increased along the gradient, indicating progressive saturation of microbial N demands at high soil N. Aboveground N uptake by plants increased asymptotically with net N mineralization to a peak of approximately 35 kg N.ha(-1) x yr(-1). Aboveground net primary production per unit net N mineralization varied inversely with soil N, suggesting progressive saturation of plant N demands at high soil N. Hydrologic N losses were dominated by dissolved organic N at low-N sites, with increased nitrate loss causing a shift to dominance by nitrate at high-N sites, particularly where net nitrification exceeded plant N demands. With the exception of N mineralization patterns, our results broadly support the application of the N-saturation model developed from studies of anthropogenic N deposition to understand N cycling and saturation of plant and microbial sinks along natural soil N gradients. This convergence of behavior in unpolluted and polluted forest N cycles suggests that where future reductions in deposition to polluted sites do occur, symptoms of N saturation are most likely to persist where soil N content remains elevated.
土壤氮(N)的广泛自然梯度可用于检验植物-土壤-微生物 N 循环与水文 N 损失之间的基本关系,并检验 N 饱和理论作为理解生态系统 N 动态的一般框架。我们在美国俄勒冈海岸山脉的一个广泛的土壤 N 梯度上,描述了 9 个道格拉斯冷杉(Pseudotsuga menziesii)森林的植物生产力、N 吸收和凋落物中的归还、土壤总 N 和净 N 矿化率以及水文 N 损失。表层矿质土壤 N(0-10 cm)的范围从 0.29%到 0.78%N,几乎相差三倍,与 N 饱和理论的预测相反,与净 N 矿化率的 10 倍变化呈线性相关,范围从 8 到 82 kg N.ha(-1) x yr(-1)。净 N 矿化与土壤 C:N、土壤质地、降水以及各地点之间的温度差异无关。净硝化与土壤 pH 呈负相关,在低 N 地点,净硝化仅占净 N 矿化的<20%,在中高 N 地点增加到 85-100%。净:总 N 矿化和硝化的比率沿梯度增加,表明在高土壤 N 下微生物 N 需求逐渐饱和。植物地上 N 吸收随净 N 矿化呈渐近增加,达到约 35 kg N.ha(-1) x yr(-1)的峰值。单位净 N 矿化的地上净初级生产力与土壤 N 呈反比变化,表明在高土壤 N 下植物 N 需求逐渐饱和。水文 N 损失在低 N 地点主要由溶解有机 N 主导,随着硝酸盐损失增加,高 N 地点的主导因素由硝酸盐主导,特别是净硝化超过植物 N 需求的地方。除了 N 矿化模式外,我们的结果广泛支持从人为 N 沉积研究中开发的 N 饱和模型在理解自然土壤 N 梯度上的 N 循环和植物及微生物汇的饱和的应用。未受污染和受污染森林 N 循环中的这种行为趋同表明,在未来减少对污染地点的沉积发生的情况下,只要土壤 N 含量仍然升高,N 饱和的症状最有可能持续存在。