A New Conceptual Model of Nitrogen Saturation Based on Experimental Nitrogen Addition to an Oak Forest
A New Conceptual Model of Nitrogen Saturation Based on Experimental Nitrogen Addition to an Oak Forest
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DOI：
10.1007/s10021-011-9432-z
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发表时间：
2011-06-01
期刊：
影响因子：
3.7
通讯作者：
Goodale, Christine L.
中科院分区：
文献类型：
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作者：
Lovett, Gary M.;Goodale, Christine L.
森林中氮（N）饱和的主导概念模型预测了关键氮循环指标的时间模式，即当最初氮受限的森林逐渐富含氮时的情况。我们展示了在美国纽约州东南部一片橡树林中进行的一项长期氮添加实验的结果，这些结果在几个方面与概念模型的预测不符。与概念模型的预测相反，我们研究中施氮林分的叶片氮浓度比对照林分的水平高出约20%，然后基本保持不变，而且从施氮林分的氮淋失在实验开始后几乎立即增加，在硝化作用增强之前就已发生。施氮林分土壤溶液中的氮浓度峰值比对照林分高出150多倍。潜在的净氮矿化没有显著变化。施氮林分中的树木死亡率增加，但树木死亡似乎不是硝酸盐过量淋失的主要原因。基于这些结果以及其他近期研究的结果，我们提出了一个新的氮饱和过程概念模型，该模型侧重于氮的质量平衡，而非氮循环指标的时间动态。质量平衡的特征是来自大气沉降和施肥的氮输入、植被和土壤中的内部汇以及淋失和气体损失的输出。该概念模型的要点是：（1）添加的氮可同时流向系统中的所有汇和损失；（2）添加氮的去向以及氮流动的时间模式取决于汇的强度以及控制它们的因素；（3）氮向各个汇的移动决定了氮饱和在生态系统中的表现形式。我们区分了容量氮饱和（其中植被和土壤中的汇为零或负值）和动力氮饱和（其中汇为正值但低于氮输入速率）。植被和土壤中的汇强度有两个组成部分，一个是由于系统中的碳（C）积累，另一个是由于库的化学计量（碳氮比）变化。进一步量化氮汇的大小和控制因素的工作将有助于更好地预测不同类型森林生态系统中氮饱和的动态。
The dominant conceptual model of nitrogen (N) saturation in forests predicts the temporal patterns of key N cycling indicators as an initially N-limited forest is progressively enriched in N. We present the results from a long-term N addition experiment in an oak forest in southeastern New York State, USA, which do not conform to the predictions of the conceptual model in several ways. In contrast to the predictions of the conceptual model, the foliar N concentrations in the N-treated stands of our study increased to about 20% above the levels in the control stands and then remained essentially constant, and nitrogen leaching from the treated stands increased almost immediately after the start of the experiment, prior to the onset of elevated nitrification. Concentrations of N in soil solution of the N-treated stands peaked at over 150-fold greater than the concentrations in the control stands. There were no significant changes in potential net N mineralization. Tree mortality increased in the treated stands, but the tree mortality did not appear to be the primary cause of the excess nitrate leaching. Based on these results and those of other recent studies, we present a new conceptual model of the N saturation process focused on the mass balance of N rather than the temporal dynamics of N cycling indicators. The mass balance is characterized by inputs of N from atmospheric deposition and fertilization, internal sinks in the vegetation and soils, and outputs to leaching and gaseous losses. The key points of the conceptual model are (1) added N can flow simultaneously to all sinks and losses in the system, (2) the fate of the added N and the temporal patterns of flow of N depend on the strength of the sinks and the factors that control them, and (3) the movement of N to the various sinks determines how N saturation is manifested in the ecosystem. We distinguish capacity N saturation, in which the sinks in the vegetation and soil are zero or negative, from kinetic N saturation, in which the sinks are positive but lower than the N input rate. The sink strengths in the vegetation and soil have two components, one due to carbon (C) accumulation in the system and the other due to change in the stoichiometry (C:N ratio) of the pool. Further work quantifying the magnitudes and controlling factors for the N sinks will allow better prediction of the dynamics of N saturation in different types of forested ecosystems.