UC Riverside

Drying-rewetting cycles are ubiquitous across natural and managed ecosystems. These cycles are known to mobilize carbon (C) in soils producing dramatic pulses in microbial respiration. While many factors contribute to these pulses, how the drying-rewetting history of soils affects these emissions remains unclear, especially in irrigated soils where soil moisture fluctuations are much more repetitive than natural, seasonally influenced soils, and where the volume of soil that water dominates is on much smaller spatiotemporal scales. To understand the controls of repeated wet-up and dry-down effects on agricultural soils, in the first study, we took a systems-level approach to examine the cross section of a furrow irrigated orchard in a semi-arid Mediterranean climate to delineate the C dynamics in the two contrasting regions of water availability: in furrows used to deliver water to tree crops where soils are temporarily but repeatedly inundated versus in berms where trees are planted and soils only receive water during infrequent rain events. Overall, our findings show that the high degree of heterogeneity in soil moisture across the landscape of a furrow-irrigated field results in contrasting gaseous release of C as CO2, microbial selectivity of substrates, and dominant mechanisms for C stabilization. In the same heterogenous landscape, we characterized the temporal variation in composition and concentration of soil C pools over the course of a year. We found that within this timeframe, soils that are typically dry but have large amount of litter and root inputs did not vary significantly in total C concentrations but exhibited increase in the active C pool after seasonal rain events. We also found that sustained anoxic conditions that dominate furrow soils throughout the year limit decomposition, regardless of season. The seasonal trends captured in this study can inform ecological models of semi-arid irrigated soils to better predict C sequestration. Lastly, in the third study, we estimated the potential changes to CO2 flux and soluble C chemistry if a legacy furrow-irrigated soil was converted to precision irrigation, hypothesizing that large releases of C would occur when lower-volume irrigation method is adopted. However, we found that low-volume irrigation leads to water limitations that inhibit CO2 pulse release from berm soils.