Long- and short-term precipitation effects on soil CO2 efflux and total belowground carbon allocation

Soil CO2 efflux (E-soil). the main pathway of C movement from the biosphere to the atmosphere, is critical to the terrestrial C cycle but how precipitation and soil moisture influence E-soil remains poorly understood. Here, we irrigated a longleaf pine wiregrass savanna for six years; this increased soil moisture by 41.2%. We tested how an altered precipitation regime affected total belowground carbon allocation (TBCA), root growth, soil carbon, and E-soil. We used two methods to quantify E-soil: daytime biweekly manual measurements and automated continuous measurements for one year. we hypothesized that the low-frequency manual method would miss both short- and long-term (i.e., subdaily to annual, respectively) effects of soil moisture on E-soil while the high-frequency data from the automated method would allow the effects of soil moisture to be discerned. Root growth was significantly higher in irrigated plots, particularly at 0-20 cm depth. Irrigated annual E-soil was significantly greater than that of the control when estimated with the continuous measurements but not when estimated from biweekly measurements. The difference in annual E-soil estimates is likely due to (1) the delayed increase in E-soil following irrigation pulses of soil moisture (i.e., variation that the biweekly manual measurements missed) and (2) the diel timing of biweekly manual measurements (they were completed early to mid-day before peak efflux). With irrigation, estimates of TBCA increased almost two-fold with automated measurements but only 36% with intermittent measurements. Relative to controls, irrigated treatments stored almost 2 Mg C ha(-1) year(-1) more in soils and 0.26 Mg C ha(-1) year(-1) more in roots. High-frequency measurements of E-soil were essential to estimate total belowground carbon allocation. With irrigation, soil carbon pools were not at steady-state, so shifts in soil carbon storage must be considered in TBCA estimates. Published by Elsevier B.V.