Abstract：
Nitrogen (N) is a limiting nutrient for primary productivity in most terrestrial ecosystems, but whether N limitation is strengthening or weakening remains controversial because both N sources and sinks are increasing in magnitude globally. Temperate marshes are exposed to greater amounts of external N inputs than most terrestrial ecosystems and more than in preindustrial times owing to their position downstream of major sources of human-derived N runoff along river mouths and estuaries. Simultaneously, ecosystem N demand may also be increasing owing to other global changes such as rising atmospheric [CO2]. Here, we used interannual variability in external drivers and variables related to exogenous supply of N, along with detailed assessments of plant growth and porewater biogeochemistry, to assess the severity of N-limitation, and to determine its causes, in a 14-year N-addition × elevated CO2 experiment. We found substantial interannual variability in porewater [N], plant growth, and experimental N effects on plant growth, but the magnitude of N pools through time varied independently of the strength of N limitation. Sea level, and secondarily salinity, related closely to interannual variability in growth of the dominant plant functional groups which drove patterns in N limitation and in porewater [N]. Experimental exposure of plants to elevated CO2 and years with high flooding strengthened N limitation for the sedge. Abiotic variables controlled plant growth, which determined the strength of N limitation for each plant species and for ecosystem productivity as a whole. We conclude that in this ecosystem, which has an open N cycle and where N inputs are likely greater than in preindustrial times, plant N demand has increased more than supply.
摘要：
在大多数陆地生态系统中，氮（N）是初级生产力的限制性养分。但是氮限制是增强还是减弱仍然存在争议，因为氮源和汇在全球范围内都在增加。温带沼泽比大多数陆地生态系统暴露于更多的外部氮输入，并且比工业化前时期暴露于更多的外部氮输入，因为它们位于沿河口和河口的人类衍生氮径流的主要来源的下游。同时,由于其他全球变化，如大气[CO2]上升，生态系统的氮需求也可能在增加。这里,我们使用了与外源N供应相关的外部驱动因素和变量的年际变化，以及对植物生长和孔隙水生物地球化学的详细评估，为了评估N限制的严重程度，并确定其原因，在14年的氮添加×二氧化碳升高实验中。我们发现孔隙水[N]的年际变化很大，植物生长,和实验氮素对植物生长的影响，但是N池的大小随时间的变化与N限制的强度无关。海平面,其次是盐度，与主要植物功能群生长的年际变化密切相关，这驱动了氮限制和孔隙水[N]的模式。植物实验性地暴露于高CO2和高洪水泛滥的年份加强了莎草的氮素限制。非生物变量控制植物生长，确定了每种植物物种和整个生态系统生产力的氮限制强度。我们得出结论，在这个生态系统中，它有一个开放的N周期，其中N个输入可能大于工业化前时期，工厂N的需求增加超过了供应。
