Deep soil CO2 flux with strong temperature dependence contributes considerably to soil-atmosphere carbon flux

Abstract

The diffusion of CO2 produced by carbon mineralization in vertical profiles is an important CO2 emission process. However, because of the relatively slow rate of change in the subsoil environment compared with the surface soil, carbon mineralization and diffusion are often ignored, and the mechanisms that transfer deep carbon to the soil-atmosphere interface are still unclear. We studied vertical differences in CO2 flux and its driving mechanism in layers situated from 0 to 200 cm in Robinia pseudoacacia plantations with different stand ages on the Loess Plateau under a typical temperate continental semi-arid monsoon climate. The results showed that (1) in the 0–200 cm layer, CO2 flux showed a bimodal seasonal trend, and total CO2 flux of Robinia pseudoacacia forests with a 10 years stand age fluctuated more actively among the months (Standard Deviation: 3.28%–9.57%). (2) Dynamic evaluation of contribution rate showed a stable (21.81–24.42%) contribution of deep CO2 flux to soil atmosphere interface. (3) Temperature sensitivity of CO2 flux (expressed as Q10) significantly increased with soil depth, moisture and soil CO2 flux presented stronger quadratic function relation in deep layers than shallow layers. There was a significant positive correlation between deep CO2 flux and soil organic carbon (SOC), but there was a weak negative correlation in the shallow layers. The CO2 flux in the shallow layers was mainly regulated by soil organic carbon, whereas that in the deep layers was more regulated by soil temperature.(4) Compared with the traditional hydrothermal two-factor model, the new T&M&C (temperature & moisture & organic carbon) model can improve the model precision by 20–25%, especially in the prediction of CO2 flux in deep layers. Overall, this study is significant for understanding the stability and dynamic changes in the soil carbon pool, and the mechanism of deep organic carbon diffusion to the soil-atmosphere interface in the Loess Plateau.