Modelling soil phosphorus cycle feedbacks in old-growth and regrowing tropical forests in Amazonia

Soil nutrient availability is a key constraint on tropical forest growth. On highly weathered soils, intact forests' carbon-nutrient cycles have developed over pedogenic time scales, leading to tight nutrient cycling of some depleted elements, such as phosphorus. Ecosystem nutrient recycling and plant nutrient limitation are predominant in lowland Amazonia, controlling the forests' response to disturbance and climate change. Deforestation, biomass removal, and fire lead to the loss of carbon and nutrients previously stored in vegetation, potentially enforcing nutrient limitation and reducing carbon storage in regrowing forests.   

Here, we employ the process-based terrestrial biosphere model QUINCY (Thum et al., 2019), coupled with the microbial-explicit soil model JSM (Yu et al., 2020) to simulate carbon and nutrient cycling rates at intact Amazonian forest sites, which span a natural gradient of 30 to 727 mg phosphorus g dry soil^-1, and from 2 to 81% clay. The model QUINCY-JSM accounts for dynamic plant carbon investment in growth and nutrient acquisition, and microbial-explicit growth, turnover, and nutrient cycling. We confront the ecosystems with historical changes in atmospheric CO₂ and climate, and simulate an experimental harvest of the entire forest stand to assess the consequences of that nutrient loss on the regrowing forest stand. Simulations of the Amazon forest sites are in good agreement with forest census data on vegetation carbon dynamics. Biologically-driven nutrient mineralization represents the main source of nutrients for plants, with negligible contribution of inorganic nutrient cycling in highly weathered sites. After experimental deforestation, we find that the inorganic nutrient supply is insufficient and restricts forest growth, leading to lower vegetation biomass equilibrium after deforestation. Our simulations suggest that forest degradation may occur through biomass removal in tropical forests.

Thum, T., Caldararu, S., Engel, J., Kern, M., Pallandt, M., Schnur, R., et al. (2019). A new model of the coupled carbon, nitrogen, and phosphorus cycles in the terrestrial biosphere (QUINCY v1.0; revision 1996). Geoscientific Model Development, 12(11), 4781–4802. https://doi.org/10.5194/gmd-12-4781-2019

Yu, L., Ahrens, B., Wutzler, T., Schrumpf, M., & Zaehle, S. (2020). Jena Soil Model (JSM v1.0; Revision 1934): A microbial soil organic carbon model integrated with nitrogen and phosphorus processes. Geoscientific Model Development, 13(2), 783–803. https://doi.org/10.5194/gmd-13-783-2020