EFFECT OF AFFORESTATION ON MICROBIAL BIOMASS AND ACTIVITY IN SOILS OF TROPICAL CHINA

Microbial biomass and activity were studied in soils from a barren erosion area and six afforestation sites (three Eucalyptus exserta forests, mixed forests with and without legumes. secondary monsoon forest) at Xiaoliang, South China. Microbial biomass C (C(mic)) (mu-g C(mic) g-1 soil) was determined with the substrate-induced respiration (SIR) method and microbial activity by measuring the respiratory CO2 (mu-g CO2 g-1 soil h-1) output. C(mic) ranged from < 20 to 350-mu-g g-1 soil (0--15cm) in the barren land and secondary monsoon forest soils. respectively. C(mic) of the "unprotected" Eucalyptus forest (where the aboveground litter is constantly removed by local residents) was about 40-mu-g as compared to 80 and 100-mu-g g-1 soil for the Eucalyptus stands that had been protected from litter removal for 10 and 15 yr, respectively. C(mic) in the mixed forests (with or without legume) was around 140-mu-g g-1 soil. In the 15-25 cm layer, C(mic) contents were between 30 and 70% lower than in the 0-15 cm layer. In most cases, C(mic) contents were lowest in March (end of dry season) and highest in late May, suggesting a considerable immobilization of nutrients in microbial biomass during the first phase of the wet season. Although the site-to-site differences were less pronounced, basal respiration was significantly higher in the mixed than in the Eucalyptus exserta forest. The specific respiration (mg CO2-C g-1 C(mic) h-1) was higher in the less "complex" monospecied forests than in the mixed forests, perhaps indicating a more efficient C turnover in the mixed forest. It may be concluded that afforestation rapidly improved soil microbial properties, and associated with it, soil C and nutrient status. However, if the forests are not protected from litter removal, the improvement may only be transitory. The results indicate that soil recoverv is better in a diverse, site-adapted monsoon forest.