Microbial Responses to Forest Management in the Western Gulf Coastal Plain, USA

Microbial communities are integral components of the biogeochemistry, fertility, and structure of forest soils, and land management practices that alter the microbial environment may influence the long-term sustainability and productivity of forestlands. In 1989 the Long-term Soil Productivity (LTSP) program was initiated to address the National Forest Management Act's concerns over possible losses in soil productivity due to soil disturbance from forest management on National Forest lands. The LTSP program is a network of 62 sites across the USA and Canada that employs the same 3X3 replicated (X3) factorial experimental design consisting of three harvest intensities (bole only, whole tree, whole tree + forest floor removal) with three soil compaction intensities (none, intermediate, severe) and plots that were split for herbicide when the experiment was initiated. Our purpose was to determine the impact of forest harvest intensity, soil compaction, and their interaction on soil microbial biomass C and N (SMB-C, -N) and soil total nitrogen (TN) and soil organic carbon (SOC) storage in a Pinus taeda L. forest 15-years post-treatment at the Davy Crockett National Forest LTSP site in eastern Texas, USA. We quantified SMB-C and -N using the chloroform fumigation extraction method, and TN and SOC by dry combustion. Soils are loamy sand and were sampled 5X during 2011-2012. In each split-plot, five samples were obtained between two living P. taeda stems to a depth of 10-cm and pooled in the field. Because soil compaction, harvest by soil compaction interaction, and herbicide had no effect on the measured soil properties, results are based on repeated measures ANOVA using harvest and time. Both SMB-C and -N were impacted by harvest and varied with time, and SMB-C had a harvest by time interaction. Generally, both microbial indices decreased in the order: bole only >whole tree > whole tree + forest floor. Soil TN and SOC were both higher in the bole only treatment compared to the more severe harvest treatments; however, while TN was significantly impacted by harvest and varied over time, SOC varied only with time. Temporal variations in SMB-C and -N, TN, and SOC were correlated with temperature, precipitation, and volumetric soil moisture. Data suggest that forest harvest practices that minimize removal of above-ground biomass will favor soil N retention and the maintenance of the SMB pool. Since N limits tree growth in the sandy soils of the western Gulf Coastal Plain, and because SMB plays a key role in N mineralization, harvest practices that favor N retention and SMB will ensure the productivity of future rotations.