Chronic nitrogen additions reduce total soil respiration and microbial respiration in temperate forest soils at the Harvard Forest

Abstract

At the Harvard Forest Long-term Ecological Research Site Chronic Nitrogen Amendment Study, a red pine and a mixed deciduous stand showed immediate changes in soil respiration following nitrogen additions (low N: 5 g N m−2 per year; high N: 15 g N m−2 per year) during the initial year (1988) of the study. In the hardwood stand, soil respiration rates increased after N additions in the first year (control: 482.0 g C m−2 per year; high N: 596.5 g C m−2 per year). This increase is attributed to increased productivity in the hardwood stand compared to the pine stand; N additions are hypothesized to have increased either root or microbial activity, or perhaps both. In the second year, however, respiration in the fertilized hardwood plots was not different from the control plot. In the pine stand, annual soil respiration was 21 and 25% lower, respectively, in low N and high N plots than the control (429.9 g C m−2 per year), with further reductions in the second year.

Weekly measures of soil respiration during summer 2001 showed that after 13 years of continuous nitrogen fertilization, soil respiration in the high N plots during growing season months was suppressed by 41% in both stands. To investigate the possibility that reduced microbial activity contributed to decreased total soil respiration, we incubated root-free soil and measured CO2 fluxes. The pattern in average respiration for incubated soils was similar to that observed from total soil respiration measured in the field. Laboratory respiration rates from the hardwood high N and pine high N soils were 43 and 64%, respectively, lower than rates from control soils. This indicates that nitrogen additions have reduced microbial activity and thus CO2 production in the field. Declines in forest productivity measured at both sites, as well as substantial tree mortality observed at the high N sites, may also lower root activity and rhizodeposition, and are also likely to reduce microbial decomposition by reducing organic matter available to soil microbes.