Soil microbial biomass during the early establishment of hoop pine plantation: seasonal variation and impacts of site preparation

Site preparation can alter soil physical, chemical and biological properties, which in turn may have significant impacts on tree growth of the following rotation. The major objective of this study was to investigate the impacts of season, soil compaction by mechanical harvesting and soil cultivation on soil microbial biomass carbon (C) and nitrogen (N) during the early stages of a second-rotation (2R) hoop pine (Araucaria cunninghamii) plantation. Soil (0-10 cm depth) was sampled for 14 consecutive months in an established factorial field trial at Yarraman, southeast Queensland. There were three compaction treatments (0 forwarder pass, I forwarder pass and 16 forwarder passes) and two cultivation treatments (zero cultivation and disc plough), which were randomly arranged in three blocks. There was seasonal and spatial variation in soil microbial biomass in mineral soils among the different compaction and cultivation treatments, with soil microbial biomass C (MBC) ranging from 167 to 1149 mug g(-1), microbial biomass N (MBN) from 16 to 257 mug g(-1), and microbial C:N ratios from 4.0 to 14.6. Fluctuations in soil microbial biomass were mainly driven by the seasonal changes in environmental conditions (including rainfall, temperature and soil moisture). The MBC was less variable with season than the MBN. In general, seasonal fluctuations in MBN were similar amongst the treatments with an apparent peak in August (late winter), followed by a sharp decrease to a lowest point in October (mid-spring), and then a second peak in December (early summer). Soil compaction did not significantly affect MBC and MBN, while soil cultivation significantly reduced MBC and MBN only in the 0 forwarder pass for the experimental period except February and March 2001 (summer-early spring) for MBC and January-March 2001 (summer-early spring) for MBN. The potential mechanisms for the diverse impacts of soil compaction and cultivation on soil microbial processes are discussed in relation to the seasonal environmental changes. (C) 2003 Elsevier B.V. All rights reserved.