Soil carbon and nitrogen pools and processes in an old-growth conifer forest 13 years after trenching

Abstract

We measured surface soil (0-15cm) C and N pools and processes inside and outside an area that had been trenched 13 years earlier in an old-growth conifer forest (>450 years) to assess the long-term impacts of reduced root inputs on C and N turnover. Trenching, combined with frequent clipping of understory plants, was originally conducted to prevent nutrient uptake by plants, as part of a study of the role of vegetation in ecosystem retention of N. Thirteen years following trenching, the median values of bulk density, pH, total C and N concentrations, annual rates of in situ net N mineralization and nitrification, microbial biomass C and N, microbial respiration, and anaerobically mineralizable N in the trenched plot were all within the 25-75% interquartile range of values found in the replicated, untrenched plots. The trenched plot had higher rates of net N mineralization (41% higher in October, 484% higher in June) and net nitrification (25% higher in October, and lower net NO3- immobilization in June) during laboratory incubation and a 22% higher water content in October. In June, soil water content in the trenched plot was about 8% lower than in the untrenched plots. Our results suggest that soil C and N dynamics in these old-growth forests are relatively resistant to perturbations resulting from major reductions in root input to the soil.