Soil carbon and nitrogen through twenty years of intensive plantation forest management

Abstract

Understanding the effects of silvicultural practices on soil carbon (C) and nitrogen (N) is critical for sustainable forest management. This study investigates how fertilization and vegetation control impact soil C and N concentrations and contents in Pinus taeda plantations across four sites in Georgia, USA. This study considers soil depths of 0–10, 10–30, and 30–50 cm and stand ages of ∼10 and ∼30 years. We hypothesized that vegetation control would reduce soil C and N by limiting belowground inputs from understory vegetation, while fertilization would enhance soil C and N by increasing productivity and suppressing microbial decomposition. Results showed that vegetation control reduced mineral soil C and N by 10 % through 50 cm; however, compensatory accumulation in the forest floor and aboveground biomass led to overall gains in ecosystem C and N. Fertilization increased C and N in the forest floor but did not have an effect on mineral soil through 50 cm. Increasing stand age resulted in insignificant changes in mineral soil C, while mineral soil N increased d by 21 % at the well-drained Piedmont site (Eatonton) and decreased by 36 % at the poorly drained lower coastal plain location (Waycross). Mineral soil N increase in all treatments at Eatonton reflect an unexplained ecosystem-level N input. Intensive management practices such as complete understory vegetation control have repeatedly been demonstrated to deplete mineral soil C and N while fertilization has relatively little effect on soil C or N.