Interannual variability in soil trace gas(CO2, N2O, NO) fluxes and analysis of controllers on regional to global scales

Abstract

Interannual variability in flux rates of biogenic trace gases must be quantified in order to understand the differences between short‐term trends and actual long‐term change in biosphere‐atmosphere interactions. We simulated interannual patterns (1983–1988) of global trace gas fluxes from soils using the NASA Ames Research Center version of the Carnegie‐Ames‐Stanford Approach (CASA) model in a transient simulation mode. This ecosystem model has been calibrated for simulations driven by satellite vegetation index data from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer over the mid‐1980s. The predicted interannual pattern of soil heterotropic CO2 emissions indicates that relatively large increases in global carbon flux from soils occurred about 3 years following the strong El Nino‐ Southern Oscillation event of 1983. Results for the years 1986 and 1987 showed an annual increment of +1 Pg (1015 g) C‐CO2 emitted from soils, which tended to dampen the estimated global increase in net ecosystem production with about a 2‐year lag period relative to plant carbon fixation. Zonal discrimination of model results implies that 80–90% of the yearly positive increments in soil CO2 emission during 1986–1987 were attributable to soil organic matter decomposition in the low latitudes (between 30°N and 30°S). Soils of the northern middle‐latitude zone (between 30° and 60°N) accounted for the residual of these annual increments. Total annual emissions of nitrogen trace gases (N2O and NO) from soils were estimated to vary from 2 to 4% over the time period modeled, a level of variability that is consistent with predicted interannual fluctuations in global soil CO2 fluxes. Interannual variability of precipitation in tropical and subtropical zones (30°N to 20°S) appeared to drive the dynamic inverse relationship between higher annual emissions of NO versus emissions of N2O. Global mean emission rates from natural (heterotrophic) soil sources over the period modeled (1983–1988) were estimated at 57.1 Pg C‐CO2 yr−1, 9.8 Tg (1012 g) N‐NO yr−1, and 9.7 Tg N‐N2O yr−1. Chemical fertilizer contributions to global soil N gas fluxes were estimated at between 1.3 to 7.3 Tg N‐NO yr−1 and 1.2 to 4.0 Tg N‐N2O yr−1.