The Impact of Water Content on Sources of Heterotrophic Soil Respiration

Heterotrophic respiration (R<inf>H</inf>) is a major flux of CO<inf>2</inf> from forest ecosystems and represents a large source of uncertainty in estimating net ecosystem productivity (NEP) using regional soil respiration (R<inf>S</inf>) models. R<inf>H</inf> from leaf litter (R<inf>HL</inf>) may contribute greatly to annual R<inf>H</inf> estimates, but its contribution may be misrepresented due to the logistical and technical challenges associated with chamber-based field measurements of R<inf>HL</inf>. The purpose of this study was to evaluate the sensitivity of sources of R<inf>H</inf> (mineral soil-derived heterotrophic respiration [R<inf>HM</inf>] and leaf litter-derived heterotrophic respiration [R<inf>HL</inf>]) 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<inf>HL</inf> and R<inf>HM</inf> to understand how forest nutrient management may impact forest soil carbon (C) dynamics. R<inf>HL</inf> was measured under dry conditions and at field capacity to evaluate water content controls on R<inf>HL</inf>, determine the duration of increased CO<inf>2</inf> release following wetting, and evaluate the potential contribution to total R<inf>H</inf>. We also measured R<inf>HM</inf> inside collars that excluded plant roots and litter inputs, from field capacity until near-zero R<inf>HM</inf> rates were attained. We found that R<inf>HL</inf> was more sensitive to water content than R<inf>HM</inf>, and increased linearly with increasing litter water content (R2 = 0.89). The contribution of R<inf>HL</inf> to R<inf>H</inf> was greatest immediately following the wetting event, and decreased rapidly to near-zero rates between 3 and 10 days. R<inf>HM </inf>also had a strong relationship with soil water content (R2 = 0.62), but took between 200 and 233 days to attain near-zero R<inf>HM</inf> rates. Fertilization had no effect on R<inf>HM </inf>(p = 0.657), but significantly suppressed R<inf>HL</inf> rates after the wetting event (p < 0.009). These results demonstrate that there is great temporal variability in both CO<inf>2</inf> released and the water content of differing sources of R<inf>H</inf>, 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<inf>H</inf>. In the context of climate change, changes in the frequency and intensity of wetting and drying events will likely alter R<inf>HL</inf> and its contribution to R<inf>S</inf>. Separate consideration of R<inf>H</inf> 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<inf>H</inf> estimates and improve the accuracy of forest NEP predictions.