Soil C and N storage and microbial biomass in US southern pine forests: Influence of forest management

• Forest harvest intensity impacts on Gulf Coastal Plain biogeochemistry at 15 years.
• SOC, TN, root mass, microbial biomass, and litter were studied 15 years post-harvest.
• Soil TN and microbial biomass were lower in the most intensely harvested plots.
• Harvest intensity had no effect on litter, root mass and SOC 15 years post-treatment.
• Retaining harvest residue favors microbial biomass and TN vital to forest growth.

Land management practices have strong potential to modify the biogeochemistry of forest soils, with implications for the long-term sustainability and productivity of forestlands. The Long-Term Soil Productivity (LTSP) program, a network of 62 sites across the USA and Canada, was initiated to address concerns over possible losses of soil productivity due to soil disturbance from forest management. Network sites employ an experimental design consisting of three harvest intensities (bole only, whole tree, whole tree + forest floor removal) in combination with three soil compaction intensities (none, intermediate, severe). Our purpose was to determine the impact of forest harvest intensity, soil compaction, and their interaction on soil organic carbon (SOC) and total nitrogen (TN) storage, and on soil microbial biomass C and N (MBC and MBN, respectively) in a Pinus taeda L. forest 15-years post-treatment at the Groveton LTSP site in eastern Texas, USA. Soils were sampled (0–10 cm) five times during 2011–2012, and we quantified SOC and TN by dry combustion, and MBC and MBN by chloroform fumigation extraction. SOC and TN 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 seasonally, SOC varied only with season. MBC and MBN were impacted by harvest intensity and varied seasonally, and SMB-N had a harvest by time interaction. Generally, both microbial indices decreased in the order: bole only >whole tree > whole tree + forest floor. Temporal variations in MBN and TN were correlated with temperature. Soil compaction and the harvest intensity × soil compaction interaction had no effect on the measured soil properties. Since N limits tree growth in forest ecosystems, and because soil microbial biomass plays a key role in N mineralization, data suggest that harvest practices that minimize removal of litter and slash will favor soil N retention, maintain the size of the soil microbial biomass pool, and maximize the potential productivity of future rotations.