Understanding the Phosphorus Cycling Strategies of Beech Forest Ecosystems along a Natural Soil Phosphorus Gradient: Observational Evidence and Modeling Challenges

Phosphorus (P) is a macro nutrient affecting the terrestrial ecosystem productivity and future carbon (C) balance, and is known as the main constraint for C sequestration in tropical ecosystems, such as Amazon rainforests and Eucalyptus forests. However, in temperate and boreal forests, nitrogen (N) is usually considered as the main limiting nutrient for plant growth and the role of P cycling is less highlighted. Through a comprehensive investigation of soil and biological properties of five European beech forest sites along a wide range of natural soil P stocks, Lang et al. (2017) have concluded that P availability can shape plant-microorganism-soil interactions in forest ecosystem and the P cycling strategy switches from an “acquiring strategy” to a “recycling strategy” as the soil P availability decreases. The switch from a P-rich “acquiring strategy” to a P-poor “recycling strategy” is significantly concurrent with decreasing forest floor turnover rates (from 1/5 to 1/40 per year), increasing organic C to P ratios (from 110 to 984; A horizons), and increasing proportions of fine-root biomass in the forest floor (from 10% to 80%), as well as some less significant ecosystem properties, such as the total soil organic C and N, resorption of leaf P before senescence, and P concentrations in leaf litter and fine roots. However, none of the vegetative traits (foliar N and P contents, tree height, basal area, and tree volume) changed systematically along the soil P gradient, indicating that the shift in soil P cycling strategies allows for satisfying plant P demand.

This observational evidence imposes great challenges on the modeling of forest ecosystems, especially on the modeling of soil processes and plant-soil interactions. In conventional models, several of these features cannot be reproduced, due to 1) fixed first-order kinetics of decomposition, 2) lack of mechanisms for soil C storage/stabilization, 3) lack of explicit microbial dynamics and processes, 4) fixed stoichiometry in plant and soil, 5) lack of dynamic plant C allocation to roots and root exudation, 6) ignorance of C cost for nutrient mobilization.

In this presentation, we would like to propose modeling solutions to these challenges based on a novel modeling framework of QUINCY (Thum et al. 2019) and JSM (Yu et al. 2020). The new model allows separation of the sink (photosynthesis) and source (nutrients and water availabilities) in plant growth and applies a dynamic C allocation to maximize assimilation of limiting resources. It is vertical- and microbial-explicit in the soil processes, and mechanistically describes the effects of microbial dynamics on soil C decomposition and stabilization as well as nutrient mobilization. The preliminary results have shown increasing C investments from plant-microbe to soil P mobilization with decreasing soil P availability. It allows us to reproduce the observed patterns in litter turnover, root allocation, soil C storage,  and soil stoichiometry, indicating the important role of C-nutrient interactions in the P cycling modeling. We hope with the advances of modeling P cycling strategies in temperate forest ecosystems, we could also better model P cycling in the tropics and better project future C sequestration globally.