Department of Plant Biology, Michigan State University, Plant Biology Laboratories, 612 Wilson Road, Room 368, East Lansing, Michigan 48824, USA.
Ecology. 2013 Jan;94(1):62-71. doi: 10.1890/12-0385.1.
Ecological stoichiometry can explain major trends in how interactions among species change across fertility gradients, but important questions remain. For example, stoichiometry predicts that fertilization should cause plants to reduce carbon allocation to arbuscular mycorrhizal fungi and, consequently, reduce fungal abundance, but responses in the field are highly variable. In a field experiment, we tested three hypotheses that could drive this variation: (1) fungi are nitrogen limited in very nitrogen-poor soils, so fertilization increases their abundance; (2) the N:P ratio of fertilization affects plant carbon allocation to fungi; (3) plant species differences affect fungal response. Our results support all three hypotheses: stoichiometry and species idiosyncrasies jointly determined fungal response to fertilization. We provide field evidence in support of the hypothesis that nitrogen can limit fungal abundance in temperate grasslands. We also show that fungal abundance in soil (hyphal length) differed beneath two dominant plant species: big bluestem (Andropogon gerardii) and smooth brome (Bromus inermis). These grass species also differed in the degree to which they reduced root colonization with fertilization, but these differences in allocation did not lead to differential responses to fertilization in terms of fungal abundance in the soil. This study shows that, while ecological stoichiometry is a useful framework for understanding the effects of eutrophication on this important and widespread species interaction, including these subtleties can increase the predictive power of the theory.
生态化学计量学可以解释物种间相互作用如何随肥力梯度变化的主要趋势，但仍存在一些重要问题。例如，化学计量学预测施肥会导致植物减少对丛枝菌根真菌的碳分配，从而减少真菌丰度，但田间的反应变化很大。在一项田间实验中，我们检验了三种可能导致这种变异性的假设：（1）在非常贫瘠的土壤中，真菌受氮限制，因此施肥会增加它们的丰度；（2）施肥的氮磷比影响植物对真菌的碳分配；（3）植物物种差异影响真菌的反应。我们的结果支持所有三个假设：化学计量学和物种特殊性共同决定了真菌对施肥的反应。我们提供了支持氮可以限制温带草原中真菌丰度的假说的田间证据。我们还表明，土壤中真菌的丰度（菌丝长度）在两种主要植物物种（大须芒草和光滑雀麦）下有所不同。这两种草在受施肥影响时根系定殖的程度也不同，但在分配方面的这些差异并没有导致真菌在土壤中的丰度对施肥产生差异反应。这项研究表明，虽然生态化学计量学是理解富营养化对这种重要且广泛的物种相互作用的影响的有用框架，但包括这些细微差别可以提高理论的预测能力。
