The Impact of Water Content on Sources of Heterotrophic Soil Respiration

Heterotrophic respiration (R_H) is a major flux of CO₂ from forest ecosystems and represents a large source of uncertainty in estimating net ecosystem productivity (NEP) using regional soil respiration (R_S) models. R_H from leaf litter (R_HL) may contribute greatly to annual R_H estimates, but its contribution may be misrepresented due to the logistical and technical challenges associated with chamber-based field measurements of R_HL. The purpose of this study was to evaluate the sensitivity of sources of R_H (mineral soil-derived heterotrophic respiration [R_HM] and leaf litter-derived heterotrophic respiration [R_HL]) of a loblolly pine plantation (Pinus taeda L.) to varying soil and litter water content over the course of a dry down event. Additionally, we investigated whether fertilization influenced R_HL and R_HM to understand how forest nutrient management may impact forest soil carbon (C) dynamics. R_HL was measured under dry conditions and at field capacity to evaluate water content controls on R_HL, determine the duration of increased CO₂ release following wetting, and evaluate the potential contribution to total R_H. We also measured R_HM inside collars that excluded plant roots and litter inputs, from field capacity until near-zero R_HM rates were attained. We found that R_HL was more sensitive to water content than R_HM, and increased linearly with increasing litter water content (R² = 0.89). The contribution of R_HL to R_H was greatest immediately following the wetting event, and decreased rapidly to near-zero rates between 3 and 10 days. R_HM also had a strong relationship with soil water content (R² = 0.62), but took between 200 and 233 days to attain near-zero R_HM rates. Fertilization had no effect on R_HM (p = 0.657), but significantly suppressed R_HL rates after the wetting event (p < 0.009). These results demonstrate that there is great temporal variability in both CO₂ released and the water content of differing sources of R_H, and forest fertilization may largely impact forest floor C stocks. This variability may not be captured reliably using conventional weekly to monthly chamber-based field sampling efforts and could lead to over- or underestimation of R_H. In the context of climate change, changes in the frequency and intensity of wetting and drying events will likely alter R_HL and its contribution to R_S. Separate consideration of R_H sources and controls, along with increased field sampling frequency using chamber-based methodology under a broader range of specific environmental conditions, are likely needed to reduce variability in R_H estimates and improve the accuracy of forest NEP predictions.