Soil CO2 influx in drylands: A conceptual framework and empirical examination

Abstract

Soil CO2 flux is dominated in most ecosystems by respiration of soil organisms and roots, and is therefore expected to be positive (i.e., efflux) during dark hours. Growing evidence from various ecosystems contradicts this expectation, showing nocturnal soil CO2 influx. Little is known about this puzzling phenomenon and the underlying mechanisms governing it. We attempted to bridge this knowledge gap by proposing the “dryland carbon influx” (DCI) framework. Based on Henry's law, decreases in soil temperature stimulate CO2 dissolution in soil solutions, creating a CO2 concentration gradient between soil air and the atmosphere. This may lead to CO2 diffusion from the atmosphere into the soil. This influx is expected to be stronger at lower temperatures, high soil to atmosphere temperature gradients, and in CaCO3 rich alkaline soils that characterize drylands. The soil abiotic influx is expected to govern the overall CO2 flux when dry conditions and low soil organic matter limit organismal respiration. We examined the underlying conditions predicted by the DCI framework using a literature review and soil CO2 flux measurements from the Negev Desert. We found 24 studies that reported dark soil CO2 influx, almost exclusively from drylands. CO2 influx was recorded mainly in alkaline soils, was positively correlated with pH levels, and was associated predominantly with low soil temperature, decreasing soil temperature or a positive soil to air thermal gradient. CO2 influx was found not only in dry soils but also in poor organic content wet soils. In the Negev Desert, CO2 influx occurred only when (a) soil temperature was higher than air temperature and was decreasing, and (b) soil moisture was 7.76% or lower. Only above this threshold, a positive association between CO2 efflux and microbial biomass was found. Our results largely support the DCI framework, suggesting that soil CO2 influx is prevalent across drylands worldwide, facilitated by the edaphic and climatic conditions that induce abiotic CO2 exchange and restrain biotic respiration. Considering the abiotic contribution to soil CO2 flux, and determining the conditions that regulate it may substantially improve our carbon budget calculations, and estimations of soil biotic respiration.