Soil biogeochemical cycles under climate change: a new model implementation

In the mitigation of climate changes by forest ecosystems, soil plays a key role, storing more than 50% of global carbon stocks. Soil dynamics have a significant influence on the evolution of the forests and, consequently, on their capability to mitigate climate alterations. Some of these dynamics are related to soil nutrient availability for the plants, in particular nitrogen (N) and phosphorous (P), hence linked to biogeochemical cycles that incorporate the transformation processes of organic, inorganic and gaseous compounds of carbon and nutrients, as well as the mechanisms of nutrient losses from the soil. Given the importance of soil biogeochemical cycles under climate change, it's crucial to have tools for a complete and detailed study of biogeochemical processes. For this purpose, the simulation models represent a fundamental tool to simulate future scenarios on the evolution of forest ecosystems and, consequently, on their capability to contrast climate alterations. However, the models show some lacks in the simulation of biogeochemical cycles. Many studies highlight how some processes, especially under climate changes, play a not negligible weight to determine soil nutrient availability for the plants. These mechanisms are not simulated by the models or, in some cases, are reproduced with extremely simplified approaches. An example is constituted by nitrogen transformation between different mineral forms (Dissimilatory Nitrate Reduction to Ammonium, DNRA) (Silver et al., 2001) and by inorganic nitrogen lost in the atmosphere (anammox) (Xi et al., 2016). Other dynamics are linked to root exudates production and to mycorrhizae (Coskun et al.,2017; Phillips et al., 2011), as well as to the effects of these mechanisms on the main biogeochemical processes (Achat et al., 2016). With the aim to overcome the lacks of models to simulate the cycle of nutrients in the soil, a new model, the 3D-CMCC-SOIL, is in implementation. It will integrate the 3D-CMCC BGC (Collalti et al., 2014, 2016; Marconi et al., 2017), a simulation model of forest dynamics. The 3D-CMCC-SOIL implementation has started from a litter and soil scheme constituted by the main organic, inorganic and gaseous pools of carbon, nitrogen and phosphorous and also by nutrient exchange fluxes between the different pools. Based on this structure, a sensitivity analysis is ongoing on the approaches used by the models to simulate the main biogeochemical processes. These mechanisms are represented by the decomposition of organic nutrients (litter and Soil Organic Matter (SOM) decomposition), nitrogen and phosphorous conversion in mineral compounds (mineralization), transformation in recalcitrant organic compounds (immobilization), Symbiotic Biological Nitrogen Fixation (SBNF) and the nutrient losses from the soil due to heterotrophic respiration, plant uptake, denitrification and leaching. The goal of the analysis is to find the best combination between the different approaches that will constitute the basic scheme of 3D-CMCC-SOIL. The scheme will be subsequently integrated by the processes not yet simulated by the models and related to nitrogen transformation between different mineral compounds, root exudates production and mycorrhizae. The new model will also simulate the forest management practices with the aim to study their effects on soil nutrient availability. The 3D-CMCC-SOIL will be a tool for the study of biogeochemical cycles in the soil under climate change and it will contribute to give answer to the questions on the soil nutrient availability as limiting factor for forest growth and also on the role of forest ecosystems in the mitigation of climate change.