华西雨屏区不同退耕模式细根、草根分解及主要土壤微生物功能群动态

Major Microbial Functional Groups Related to Fine Root and Grass Root Decomposition in Different Models of Conversion of Farmland to Forest in the Rainy Zone of West China

采用原状土芯(intact core)法,探讨了四川洪雅柳江退耕还林模式:光皮桦(Betula luminif-era)与扁穗牛鞭草(Hemarthria compressa)复合模式(HN)、光皮桦人工林(H)、扁穗牛鞭草牧草地(NC)、柳杉(Cryptameria fortunei)人工林(LS)细根、草根分解速率以及其与细根、草根分解密切相关的土壤微生物功能群变化,结果表明:①细根、草根分解速率符合Olson单指数分解模型,年分解常数(k)分别为1.06、0.93、1.32、0.86/年,分解1年后干重损失率分别为65.49%、60.55%、73.32%、57.51%;②在一年的分解期间的各个阶段HN模式的好气固氮细菌、氨化细菌、硝化细菌、纤维素分解菌数量都显著大于其他3种模式(p<0.01);③4种模式固氮细菌与氨化细菌数量呈显著正相关关系,与C/N呈显著负相关关系,氨化细菌数量与硝化细菌数量呈显著正相关,HN、LS模式固氮细菌数量与细根、草根N相对含量呈显著正相关关系(p<0.05),表明细根、草根的分解受到土壤微生物生理类群相互作用的影响,自生固氮在林草复合模式与柳杉人工林生态系统的自肥作用中扮演着重要角色。

Fine root decomposition often plays an important role in maintaining soil fertility and mass cycle in soil ecosystem. As an irreplaceable decomposer in fine root decomposition, soil microorganism is closely linked to the chemical components in fine roots. Many compound models for vegetation restoration were founded in the process of converting farmland to forest （ grass）, which is beneficial to the development of environment and economy. As yet there is a lack of soil microbial functional group variation related with fine root decomposition although only a few reports have been published. Therefore, we placed 560 intact soil cores in nylon bags with 0. 25 mm mesh in Birch （ Betula luminifera） -grass （ Hemarthria compressa） compound model （ HN）, birch （ Betula luminifera） plantation （ H）, grassland （ Hemarthria compressa） （ NC ） and cedar （ Cryptomeria fortunei ） plantation （LS） using intact-core technique for 1 year. Intact soil cores were sampled at 30, 90, 180, 270 and 365 days from the starting date. On each sampling date, we collcted fine roots and soils （grass roots） from each soil core for analysis. Decomposition rates and major microbial functional groups （aerobic azotobacter, ammonifiers, nitrifiers, cellulolytic bacteria） in relation to fine root and grass root decomposition were analyzed. The percentage of initial mass remaining of decomposing roots fitted a single-exponential model. The decomposition constant （k） for the four models were 1.06,0.93,1.32 and 0. 86 per year respectively, with an annual loss of 65.49% ,60. 55% ,73.32% and 57.51%. The numbers of aerobic azotobacter and ammonifiers increased consistently until 90 or 180 days, while nitrifiers and cellulolytic bacteria decreased during 30 days, followed by an increase in four models. The numbers of aerobic azotobacter, ammonifiers, nitrifiers and cellulolytic bacteria in HN model were much higher than the other three models during the decomposition period. Aerobic azotobacter in four models showed significant positive correlation with ammonifiers and negative correlation with C/N ratio, and ammonifiers also had positive correlation with nitrifiers. Aerobic azotobacter in HN and LS models showed positive correlation with relative nitrogen content of fine root and grass root. Our results show that fine roots and grass roots decomposition is strongly influenced by synergic interaction among microbial functional groups. It also suggests that biological nitrogen fixation may have an important role in self-sustaining capability for forest-grass compound model.