Numerical modelling of soil CO2 dynamics and soil respiration

Abstract

The global carbon cycle and its impacts on climate is a major worldwide concern. Soil respiration, i.e., the ux of CO2 from the soil surface into the atmosphere, is one of the greatest contributors to this cycle, thereby making studies of soil respiration crucial to better manage and mitigate atmospheric CO2 concentrations. Although processes driving soil CO2 dynamics and soil respiration are qualitatively known, their quantification and representation in models is still challenging. The main aim of this study is to test a detailed mechanistic model of soil CO2 dynamics against field data and use the model to understand the role of spatial variability on soil CO2, fluxes and soil respiration. Specifically, (1) we used an existing one-dimensional mechanistic model to reproduce data of soil moisture, soil temperature and air-phase soil CO2 concentrations, and (2) used a two-dimensional model to test whether horizontal variability of soil moisture and soil temperature can explain the hysteresis between soil respiration and soil temperature observed in many field experiments. We used the mechanistic model of Simunek and Suarez (1993), which was solved with finite elements with the software COMSOL Multyphysics. A case study was performed using data collected from a Loblolly Pines at Duke Forest, North Carolina (USA). Soil moisture (θ), soil temperature (Ts) and CO2 dynamics were investigated over 180 days. Given the sharp changes of soil moisture near the surface, due to the layered soil and the presence of most of root water uptake from the first 30 cm of soil, θ resulted to be very difficult to model. Conversely, a good agreement between model and data was achieved for Ts, and modelled CO2 concentrations were reasonably comparable with measured data especially near the surface. This is one of the first studies that try to simulate soil CO2 concentrations measured at high spatial and temporal resolution. Many field observations showed hysteretic cycles between soil respiration and soil temperature as well as soil [CO2] and soil temperature at diffierent depths. Several explanations for these cycles are available, some attributing this hysteresis to experimental procedures and some involving the role of other variables, such as soil moisture and plant photosynthesis. Here we extended the model by Simunek and Suarez (1993) to two dimensions and analyzed the possible role that horizontal variability of soil moisture, soil temperature, and CO2 production can have in generating the hysteresis. Results show that horizontal gradients do not play major role in the hysteresis. Moreover, CO2 production has more inuence that root water uptake on the hysteric cycles.