Measurements of soil respiration and simple models dependent on moisture and temperature for an Amazonian southwest tropical forest

Abstract. Soil respiration plays a significant role in the carbon cycle of Amazonian tropical forests, although in situ measurements have only been poorly reported and the dependence of soil moisture and soil temperature also weakly understood. This work investigates the temporal variability of soil respiration using field measurements, which also included soil moisture, soil temperature and litterfall, from April 2003 to January 2004, in a southwest Brazilian tropical rainforest near Ji-Paraná, Rondônia. The experimental design deployed five automatic (static, semi-opened) soil chambers connected to an infra-red CO₂ gas analyzer. The mean half-hourly soil respiration showed a large scattering from 0.6 to 18.9 μmol CO₂ m⁻² s⁻¹ and the average was 8.0±3.4 μmol CO₂ m⁻² s⁻¹. Soil respiration varied seasonally, being lower in the dry season and higher in the wet season, which generally responded positively to the variation of soil moisture and temperature year round. The peak was reached in the dry-to-wet season transition (September), this coincided with increasing sunlight, evapotranspiration and ecosystem productivity. Litterfall processes contributed to meet very favorable conditions for biomass decomposition in early wet season, especially the fresh litter on the forest floor accumulated during the dry season. We attempted to fit three models with the data: the exponential Q₁₀ model, the Reichstein model, and the log-soil moisture model. The models do not contradict the scattering of observations, but poorly explain the variance of the half-hourly data, which is improved when the lag-time days averaging is longer. The observations suggested an optimum range of soil moisture, between 0.115<θ≤0.25 m³ m⁻³, which maximize soil respiration in an approximated non-linear relationship. The Q₁₀ coefficient was overestimated and may lead to erroneous calculation at warmer temperatures, which is of concern for global climate models deploying simple parameterizations under strong climate anomalies in the tropics.