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)(μ C mic g −1 soil) was determined with the substrate-induced respiration (SIR) method and microbial activity by measuring the respiratory CO 2(μCO 2 g −1 soil h −1) output. C mic ranged from < 20 to 350 μ g −1 soil (0–15 cm) 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 μ g as compared to 80 and 100 μg g −1 soil for the Eucalyptus stands that had been protected from litter removal for 10 and 15yr, respectively. C micc in the mixed forests (with or without legume) was around 140 μg 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 CO 2-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 recovery is better in a diverse, site-adapted monsoon forest.