Tracy Benjamin F, Frank Douglas A
Biological Research Laboratories, Syracuse University, Syracuse, NY 13244, USA, , , , , , US.
Oecologia. 1998 May;114(4):556-562. doi: 10.1007/s004420050480.
Microorganisms are largely responsible for soil nutrient cycling and energy flow in terrestrial ecosystems. Although soil microorganisms are affected by topography and grazing, little is known about how these two variables may interact to influence microbial processes. Even less is known about how these variables influence microorganisms in systems that contain large populations of free-roaming ungulates. In this study, we compared microbial biomass size and activity, as measured by in situ net N mineralization, inside and outside 35- to 40-year exclosures across a topographic gradient in northern Yellowstone National Park. The objective was to determine the relative effect of topography and large grazers on microbial biomass and nitrogen mineralization. Microbial C and N varied by almost an order of magnitude across sites. Topographic depressions that contained high plant biomass and fine-textured soils supported the greatest microbial biomass. We found that plant biomass accurately predicted microbial biomass across our sites suggesting that carbon inputs from plants constrained microbial biomass. Chronic grazing neither depleted soil C nor reduced microbial biomass. We hypothesize that microbial populations in grazed grasslands are sustained mainly by inputs of labile C from dung deposition and increased root turnover or root exudation beneath grazed plants. Mineral N fluxes were affected more by grazing than topography. Net N mineralization rates were highest in grazed grassland and increased from dry, unproductive to mesic, highly productive communities. Overall, our results indicate that topography mainly influences microbial biomass size, while mineral N fluxes (microbial activity) are affected more by grazing in this grassland ecosystem.
微生物在很大程度上负责陆地生态系统中的土壤养分循环和能量流动。尽管土壤微生物会受到地形和放牧的影响，但对于这两个变量如何相互作用以影响微生物过程，我们知之甚少。对于这些变量如何影响包含大量自由放养有蹄类动物的系统中的微生物，我们了解得更少。在本研究中，我们比较了黄石国家公园北部沿地形梯度设置的35至40年围栏内外的微生物生物量大小和活性（通过原位净氮矿化来衡量）。目的是确定地形和大型食草动物对微生物生物量和氮矿化的相对影响。不同地点的微生物碳和氮变化幅度几乎达一个数量级。含有高植物生物量和质地细腻土壤的地形凹陷处支持了最大的微生物生物量。我们发现植物生物量能准确预测我们研究地点的微生物生物量，这表明植物的碳输入限制了微生物生物量。长期放牧既没有消耗土壤碳，也没有减少微生物生物量。我们推测，放牧草地中的微生物种群主要由粪便沉积中的易分解碳输入以及放牧植物下根系周转或根系分泌物增加来维持。矿质氮通量受放牧的影响大于地形。净氮矿化率在放牧草地中最高，并且从干旱、生产力低的群落向湿润、生产力高的群落增加。总体而言，我们的结果表明，在这个草地生态系统中，地形主要影响微生物生物量大小，而矿质氮通量（微生物活性）受放牧的影响更大。
