Nitrifying Community and N-Cycle Activity Are Reduced By Increasing Forest Harvest Intensity in Surface and Subsurface Soils in the Western Gulf Coastal Plain.

Disturbances such as harvesting timber have the potential to diminish forest productivity by removing limiting nutrients in the harvested biomass and increasing the potential for losses from soil. The Long-Term Soil Productivity (LTSP) program was initiated to address concerns over possible losses of soil productivity due to soil disturbance associated with forest management. We determined the effect of forest harvest intensity (i.e., no-harvest, bole-only harvest, and whole-tree harvest + forest floor removal) on soil microbial biomass carbon (MBC) and nitrogen (MBN), NH₃/NH₄⁺ and NO₃^-- pool sizes, and bacterial and archaeal amoA copy number in surface and subsurface soil of a Pinus taeda L. forest, 18-years post-treatment, at the Groveton LTSP site in eastern Texas. Soils were sampled (0-10, 10-30, 30-60, and 60-100 cm) seasonally during 2014-2015. We quantified microbial biomass by chloroform fumigation, NH₃/NH₄⁺ and NO₃^- with colorimetric assays, and amoA with quantitative polymerase chain reaction methods. MBC and MBN were generally unaffected by harvest method; however, differences were found in respect to soil depth and time. NH₃/NH₄⁺ and NO₃^- concentrations were influenced significantly by treatment differences, soil depth, and time. The ratio of NH₃/NH₄⁺ to NO₃^- was significantly higher for the most intensive harvest treatment suggesting a lower rate of nitrification. The abundance of both bacterial and archaeal amoA were influenced by harvest method, soil depth, and time. Archaeal amoA was significantly more abundant than bacterial amoA, at all depths. Results indicate that while microbial biomass is generally unaffected 18-years post-harvest, NH₃/NH₄⁺ and NO₃^- pool sizes and amoA copy number were better conserved, with respect to the no-harvest treatment, in bole-only plots, which suggest that harvest methods that minimize organic matter removal will favor a higher number of ammonia oxidizing bacteria and archaea, increase soil N availability, and promote the sustainability of forest productivity.