PS 38-46 - Seasonality of dominant abiotic controls and contributions of baseline and peak soil respiration in Sonoran desert grasslands

Abstract: Soil efflux of CO₂ (F_s) is the sum of microbial and root respiration, and is controlled by a host of biotic and abiotic factors; the two dominant abiotic controls are soil moisture and temperature. The Sonoran desert is characterized by two growing seasons, one during the summer and one during the spring. The summer growing season is driven by monsoonal precipitation while the spring growing season is in response to winter precipitation. Abiotic controls of F_s during the summer season are well understood, but less is known about how these variables influence F_s during the spring season. For example, soil moisture may have a weaker influence on F_s when temperatures are low, which would decrease cumulative seasonal F_s compared to when temperatures are high. In addition, the influence of these two variables on the contribution of background F_s (baseline) versus spikes in F_s (peak) might also vary across the year. We hypothesized that (1) soil moisture is the dominant control of F_s in the summer whereas temperature is the dominant control in the spring, and (2) baseline respiration is more important than peak respiration for the cumulative F_s during the summer compared to the spring. We tested these hypotheses using automated soil respiration chambers situated within a rainfall manipulation experiment in southern Arizona. Rainout shelters intercept 100% of ambient precipitation from hydrologically-isolated plots positioned beneath; plots are then irrigated with captured rainwater. Consistent with prior research, we found that soil moisture is the dominant control of F_s during the summer when temperatures are high. However, our results reveal that temperature is also positively related to F_s during the summer and more tightly regulates F_s than previously thought. Overall, we found a positive relationship between F_s and both soil moisture and temperature regardless of the time of year: F_s is positively related to temperature when the soil is wet and is positively related to moisture when the soil is dry. Additionally, the relationship between temperature and F_s switches to being slightly negative during May and June when it is extremely dry and temperatures are high. Currently, we conclude that while temperature regulates the seasonal magnitude of F_s throughout the year, both soil moisture and temperature co-control F_s year-round. Forthcoming analyses will assess the seasonal contributions of baseline versus peak F_s in warm versus cool periods of the year.