Dynamics Of Soil Microbial Biomass In Degraded Forests

Soil microorganisms are the main mediators of C turnover in soil and are part of the organic C and nutrient pool called microbial biomass (MB). Generally, the microbial biomass represents the sensitive and labile pool. The amount of C contained in the soil microbial biomass mostly ranges from 2 to 3% of the total soil organic C. Microbial biomass provides an early indication of slow changes in organic matter content. The dynamic response of MB to environmental changes makes it a more sensitive index than total organic C for monitoring change. The amount of information on soil microbial biomass in forested ecosystems is limited when compared to agricultural systems. In this paper, soil microbial biomass was studied in a natural forest of Castanopsis kawakamii (NF) and adjacent monoculture plantations of C. kawakamii (CK) and Chinese fir (Cunninghamia lanceolata, CF) in Sanming, Fujian. The results showed that:(1) Quantities of soil microbe in CK and CF decreased significantly when NF has been converted into plantations. Soil bacteria were dominant in total microorganisms. The three forests can be arranged in this sequence with respect to the number of bacteria and fungi: NF>CK>CF, while with respect to the number of actimomycete, the order was CF>NF>CK. The three forests followed the order of NF>CK>CF as to contents of microbial biomass carbon and nitrogen.(2) Soil microbial biomass decreased with increasing soil depth and was significantly different at 0-10cm layer for three forests. Microbial biomass C and N at top 10cm soil in the NF were 1.46 and 1.57 times higher than those in the CK respectively, and 1.83 and 1.84 times higher than in the CF respectively. Microbial biomass in soil profile showed negative relation with soil pH value and bulk density, while positive correlation with contents of soil organic carbon, light fraction and particulate organic carbon and fine root biomass were found. There were no significant difference between soil microbial biomass and DOC, total N, total P, hydrolyzable N and available P.(3) Amount and biomass of soil microbe in the NF, CK and CF showed a unimodal distributionpattern with peaks in July. Most significant difference in soil microbial biomass C and N was in summer when microbial biomass C and N were 1.70 g/kg and 0.26g/kg in the NF,1.25 g/kg and 0.20g/kg in the CK and 1.14g/kg and 0.17g/kg in the CF respectively. No significant correlations between seasonal microbial biomass and monthly amount of litterfall, fine root biomass, DOC were found, whereas it was well related to soil temperature and moisture.(4) Microbial biomass in air-dried soils increased to 2.60~3.84 times following soil rewetting, which indicated the positive effect of soil moisture. The extents of increased microbial biomass among three wet and dry cycles were not different significantly. There was distinct impact of temperature on soil microbial biomass in the laboratory. Soil microbial biomass at 19C in the NF, CK and CF were 1.47, 1.67 and 1.89 times as high as those at 10C respectively. The corresponding values at 28C were 1.19, 1.43 and 1.40 times higher than those at 19C respectively.(5) The response of microbial biomass to environmental changes was more sensitive than total organic C. In addition, there was significant correlation between seasonal change of microbial biomass and soil nutrients and enzyme activity, which made microbial biomass as a better index on soil fertility level.Following the conversion of natural forest to plantation forest, soil microbial biomass C and N decreased rapidly, with maximum happened in the top 0-10cmsoil layer, which might result largely from the disturbance form clear-cutting, slash-burning, soil preparing etc, from the soil erosion prior to canopy closure, and from reduced inputs of both aboveground and belowground detritus Soil microbial biomass C can serve not only as an early indicator of changes in soil organic C pool, but also as an important signal of forest degeneration and soil fertility fall.