Simulated chronic NO 3 − deposition reduces soil respiration in northern hardwood forests

Abstract

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO 2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO 3 − deposition (3 g N m −2 yr −1 ) to four sugar maple-dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO 3 − -amended plots. In all subsequent measurement years, soil respiration rates from NO 3 − -amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO 2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO 3 − additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO 2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO 3 − -amended plots are consistent with this explanation.