Effects of weed communities with various species numbers on soil features in a subtropical orchard ecosystem

Recent emphasis on species diversification in sustainable agriculture highlights the importance of elucidating how species number and diversity affect soil nutrient processes. Effects of weed species numbers on soil carbon, nitrogen and arbuscular mycorrhizal fungi (AMF) were studied in field experiments during 1998-2001 in a subtropical citrus orchard situated at Changshan County (28°54'N, 118°30'E) in the southwestern Zhejiang Province, PR China. Twelve native weed species were selected for the experiment based on their characteristics of nitrogen fixation, root system type and phenological period. Six kinds of weed communities (groups) of either 0, 1, 2, 4, 8 or 12 weed species were formed by selectively removing the unwanted species. After establishing each artificial weed communities, plant biomass, plant nitrogen, soil organic matter, soil total N, soil microbial C and N, number of AMF spore were measured in May and October 1999-2001. The results demonstrated a strong influence of weed communities with different species numbers on soil N, C and soil microbes because of weed biomass that was returned to soil. Species numbers increasing from 0 to 4 and 8 to 12 enhanced plant biomass significantly that directly affected soil C and N. Microbial biomass C and N increased significantly with species numbers increasing from 0 to 12 in the early growing season but not in the late growing season possibly due to the competition for N and other nutrients between plants and microbes. Results implied that in a community with a few plant species, increasing species numbers plays a determining role in soil C and N by increasing plant biomass, in a species rich community, however species characteristics are important for determining soil C and N. The numbers of AMF spores increased significantly with increasing species number, which may contribute to the mycorrhizal colonization of cultivated plants.