Medium- and short-term available organic matter, microbial biomass, and enzyme activities in soils under Pinus sylvestris L. and Robinia pseudoacacia L. in a sandy soil in NE Saxony, Germany

Total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities in a sandy soil under pine (Pinus sylvestris L.) and black locust (Robinia pseudoacacia L.) stands were investigated in a field study near Riesa, NE Germany. Samples of the organic layers (Oi and Oe-Oa) and the mineral soil (0-5, 5-10, 10-20, and 10-30cm) were taken in fall 1999 and analyzed for their contents of organic C and total N, hot-water-extractable organic C and N (HWC and HWN), KCl-extractable organic C and N (C-org(KCl) and N-org(KCl) NH4+-N and NO3--N, microbial-biomass C and N, and activities ot beta-glucosidase and L-asparaginase. With exception of the HWC, all investigated C and N pools showed a clear response to tilling, which was most pronounced in the Oi horizon. Compared to soils under pine, those under black locust had higher contents of medium- and short-term available C (HWC, C-org(kcl)) and N (HWN, N-org(KCl)), mineral N (NH4+-N, NO3--N), microbial-biomass C and N, and enzyme activities in the uppermost horizons of the soil. The strong depth gradient found for all studied parameters was most pronounced in soils under black locust. Microbial-biomass C and N and enzyme activities were closely related to the amounts of readily mineralizable organic C (HWC and C-org(KCl)). However, the presented results implicate a faster C and N turnover in the top-soil layers under black locust caused by higher N-input rates by symbiotic N-2 fixation.