Simulating ectomycorrhiza in boreal forests: implementing ectomycorrhizal fungi model MYCOFON in CoupModel (v5)

Abstract

Abstract. The symbiosis between plants and Ectomycorrhizal fungi (ECM) is shown to considerably influence the carbon (C) and nitrogen (N) fluxes between the soil, rhizosphere, and plants in boreal forest ecosystems. However, ECM are either neglected or presented as an implicit, undynamic term in most ecosystem models, which can potentially reduce the predictive power of models.In order to investigate the necessity of an explicit consideration of ECM in ecosystem models, we implement the previously developed MYCOFON model into a detailed process-based, soil-plant-atmosphere model, Coup-MYCOFON, which explicitly describes the C and N fluxes between ECM and roots. This new Coup-MYCOFON model approach (ECM explicit) is compared with two simpler model approaches: one containing ECM implicitly as a dynamic uptake of organic N considering the plant roots to represent the ECM (ECM implicit), and the other a static N approach in which plant growth is limited to a fixed N level (nonlim). Parameter uncertainties are quantified using Bayesian calibration in which the model outputs are constrained to current forest growth and soil C / N ratio for four forest sites along a climate and N deposition gradient in Sweden and simulated over a 100-year period.The "nonlim" approach could not describe the soil C / N ratio due to large overestimation of soil N sequestration but simulate the forest growth reasonably well. The ECM "implicit" and "explicit" approaches both describe the soil C / N ratio well but slightly underestimate the forest growth. The implicit approach simulated lower litter production and soil respiration than the explicit approach. The ECM explicit Coup-MYCOFON model provides a more detailed description of internal ecosystem fluxes and feedbacks of C and N between plants, soil, and ECM. Our modeling highlights the need to incorporate ECM and organic N uptake into ecosystem models, and the nonlim approach is not recommended for future long-term soil C and N predictions. We also provide a key set of posterior fungal parameters that can be further investigated and evaluated in future ECM studies.

CoupModel (v6.0): an ecosystem model for coupled phosphorus, nitrogen and carbon dynamics – evaluated against empirical data from a climatic and fertility gradient in Sweden

“…A general structural and technical overview of the CoupModel can be found in Jansson and 155 Karlberg (2011) and Jansson (2012). He et al (2018) introduced an explicit plant-mycorrhizal representation scheme into CoupModel v5.…”

“…Where piavail is a parameter that describes the maximum fraction of Pilab that is available for uptake by plant roots (i.e., not covered by the fungal mantle), and fm, which is an uptake reduction parameter that describes cover by the fungal mantle. m is the mycorrhization degree, see He et al (2018). 410…”

“…The development of managed forests was simulated in daily resolution over a rotation period from stand age class 10 until 10 years after final harvesting to cover the potential nutrient leaching during the regeneration phase as in Gärdenäs et al (2003). The trees in all regions 440 were assumed to be planted in 1961; thus, the period 1961 to 1970 represented the spin-up period.…”

“…Projections of future weather data were generated by the climate change and environmental objective (CLEO) project, using ECHAM5 projections and bias correction of regional climatic data (Personal communication to Thomas Bosshard, SMHI). N-concentration in the deposition was kept constant for each region throughout the simulation as in Svensson et al (2008) and Gärdenäs et al (2003). 460…”

“…availability; and 5) simultaneous uptake of nutrients to roots or mycorrhizae from both organic and inorganic pools. The Coup-CNP model was evaluated using four forest regions 135 situated along a climatic and fertility gradient in Sweden that has earlier been considered by He et al (2018) and Svensson et al (2008).…”