Le Roux X, Bardy M, Loiseau P, Louault F
U.M.R. 5557 Ecologie Microbienne; U.S.C. INRA 1196, bat. 741, CNRS-Université Lyon I, 43 boulevard du 11 novembre 1918, 69622, Villeurbanne, France.
Oecologia. 2003 Nov;137(3):417-25. doi: 10.1007/s00442-003-1367-4. Epub 2003 Sep 4.
Stimulation of nitrification and denitrification by long term (from years to decades) grazing has commonly been reported in different grassland ecosystems. However, grazing generally induces important changes in plant species composition, and whether changes in nitrification and denitrification are primarily due to changes in vegetation composition has never been tested. We compared soil nitrification- and denitrification-enzyme activities (NEA and DEA, respectively) between semi-natural grassland sites experiencing intensive (IG) and light (LG) grazing/mowing regimes for 13 years. Mean NEA and DEA (i.e. observed from random soil sampling) were higher in IG than LG sites. The NEA/DEA ratio was higher in IG than LG sites, indicating a higher stimulation of nitrification. Marked changes in plant species composition were observed in response to the grazing/mowing regime. In particular, the specific phytomass volume of Elymus repens was lower in IG than LG sites, whereas the specific volume of Lolium perenne was higher in IG than LG sites. In contrast, the specific volume of Holcus lanatus, Poa trivialis and Arrhenatherum elatius were not significantly different between treatments. Soils sampled beneath grass tussocks of the last three species exhibited higher DEA, NEA and NEA/DEA ratio in IG than LG sites. For a given grazing regime, plant species did not affect significantly soil DEA, NEA and NEA/DEA ratio. The modification of plant species composition is thus not the primary factor driving changes in nitrification and denitrification in semi-natural grassland ecosystems experiencing long term intensive grazing. Factors such as trampling, N returned in animal excreta, and/or modification of N uptake and C exudation by frequently defoliated plants could be responsible for the enhanced microbial activities.
长期（数年至数十年）放牧对硝化作用和反硝化作用的刺激在不同草地生态系统中普遍有报道。然而，放牧通常会引起植物物种组成的重要变化，而硝化作用和反硝化作用的变化是否主要归因于植被组成的变化从未得到过验证。我们比较了在经历了13年高强度（IG）和轻度（LG）放牧/割草制度的半天然草地站点之间的土壤硝化酶和反硝化酶活性（分别为NEA和DEA）。IG站点的平均NEA和DEA（即从随机土壤采样中观察到的）高于LG站点。IG站点的NEA/DEA比率高于LG站点，表明硝化作用受到更高的刺激。观察到植物物种组成因放牧/割草制度而发生显著变化。特别是，IG站点中偃麦草的单位植物量体积低于LG站点，而IG站点中黑麦草的单位体积高于LG站点。相比之下，绒毛草、普通早熟禾和燕麦草的单位体积在不同处理之间没有显著差异。在最后三种物种的草丛下采集的土壤在IG站点中的DEA、NEA和NEA/DEA比率高于LG站点。对于给定的放牧制度，植物物种对土壤DEA、NEA和NEA/DEA比率没有显著影响。因此，在经历长期高强度放牧的半天然草地生态系统中，植物物种组成的改变不是驱动硝化作用和反硝化作用变化的主要因素。诸如践踏、动物粪便中返回的氮和/或频繁落叶植物对氮吸收和碳渗出的改变等因素可能是微生物活动增强的原因。
