Plant Trait Diversity Buffers Variability in Denitrification Potential over Changes in Season and Soil Conditions

BackgroundDenitrification is an important ecosystem service that removes nitrogen (N) from N-polluted watersheds, buffering soil, stream, and river water quality from excess N by returning N to the atmosphere before it reaches lakes or oceans and leads to eutrophication. The denitrification enzyme activity (DEA) assay is widely used for measuring denitrification potential. Because DEA is a function of enzyme levels in soils, most ecologists studying denitrification have assumed that DEA is less sensitive to ambient levels of nitrate (NO3−) and soil carbon and thus, less variable over time than field measurements. In addition, plant diversity has been shown to have strong effects on microbial communities and belowground processes and could potentially alter the functional capacity of denitrifiers. Here, we examined three questions: (1) Does DEA vary through the growing season? (2) If so, can we predict DEA variability with environmental variables? (3) Does plant functional diversity affect DEA variability?
Methodology/Principal FindingsThe study site is a restored wetland in North Carolina, US with native wetland herbs planted in monocultures or mixes of four or eight species. We found that denitrification potentials for soils collected in July 2006 were significantly greater than for soils collected in May and late August 2006 (p<0.0001). Similarly, microbial biomass standardized DEA rates were significantly greater in July than May and August (p<0.0001). Of the soil variables measured—soil moisture, organic matter, total inorganic nitrogen, and microbial biomass—none consistently explained the pattern observed in DEA through time. There was no significant relationship between DEA and plant species richness or functional diversity. However, the seasonal variance in microbial biomass standardized DEA rates was significantly inversely related to plant species functional diversity (p<0.01).
Conclusions/SignificanceThese findings suggest that higher plant functional diversity may support a more constant level of DEA through time, buffering the ecosystem from changes in season and soil conditions.

背景：
反硝化作用（Denitrification）是一项重要的生态系统服务，可从受氮污染的流域中去除氮（N），在氮元素抵达湖泊或海洋引发富营养化之前将其返还至大气，从而缓冲土壤、溪流与河流水质免受过量氮的影响。反硝化酶活性（Denitrification Enzyme Activity, DEA）测定法被广泛用于衡量反硝化潜力。由于DEA与土壤中的酶水平相关，大多数研究反硝化作用的生态学家均认为，DEA对环境中硝酸盐（NO₃⁻）与土壤碳的水平敏感性较低，因此其随时间的波动幅度也较野外实测值更小。此外，已有研究表明植物多样性对微生物群落与地下过程具有显著影响，并可能改变反硝化微生物的功能能力。本研究围绕三个问题展开：（1）反硝化酶活性是否会在生长季内发生变化？（2）若确实存在变化，能否通过环境变量预测DEA的波动？（3）植物功能多样性是否会影响DEA的波动？

研究方法与主要发现：
本研究的样地位于美国北卡罗来纳州的一处恢复湿地，其中原生湿地草本植物以单一种植或4种、8种混合的方式进行种植。研究发现，2006年7月采集的土壤样本的反硝化潜力显著高于2006年5月与8月下旬采集的样本（p<0.0001）。同理，经微生物生物量标准化后的DEA速率在7月也显著高于5月与8月（p<0.0001）。在本次测定的土壤变量——土壤含水量、有机质、总无机氮与微生物生物量——中，没有任何一项能够持续解释DEA随时间的变化模式。DEA与植物物种丰富度或功能多样性之间均未发现显著关联。然而，经微生物生物量标准化后的DEA速率的季节波动与植物物种功能多样性呈显著负相关（p<0.01）。

结论与意义：
本研究结果表明，更高的植物功能多样性或许能够维持DEA在更长时间内处于更稳定的水平，从而缓冲生态系统免受季节与土壤条件变化的影响。