Zheke Zhong;Franz Makeschi

Institute for Soil Science, Dresden University of Technology, D-01737, Tharandt (Germany);Institute for Soil Science, Dresden University of Technology, D-01737, Tharandt (Germany

subscript;san;mjc;gnr;indices;0

To evaluate the validity of different indices in estimating soil readily mineralizable N, soil microbial biomass (N-mic), soil active N (SAN), soluble organic N (SON), net N mineralization rate (NNR) and gross N mineralization rate (GNR) in mineral soils (0-10 cm) from six forest stands located in central Germany were determined and compared with two sampling times: April and November. Additionally, soil density fractionation was conducted for incubated soils (with addition of (NH4)-N-15-N and glucose, 40 days) to observe the sink of added N-15 in different soil fractions. The study showed that N-mic and NNR in most stands differed significantly (P <= 0.05) between the two sampling times, but not GNR, SAN and SON. In November, no close relationships were found between GNR and other N indices, or between N-mic, SON, and SAN and forest type. However, in April, GNR was significantly correlated (P < 0.05) with N-mic, SAN, and NNR along with N-mic under beech being significantly higher (P <= 0.05) than under conifers. Furthermore, density fractionation revealed that the light fraction (LF, 0.063-2 mm, > 1.7 g cm(-3)) was not correlated with the other N indices. In contrast, results from the incubation study proved that more N-15 was incorporated into the heavy fraction (HF < 0.063 mm, > 1.7 g cm(-3)) than into LF, indicaing that more labile N existed in HF than in LF. These findings suggested that attention should be paid to the differences existing in N status between agricultural and forest soils.