A novel representation of biological nitrogen fixation and competitive dynamics between nitrogen-fixing and non-fixing plants in a land model (GFDL LM4.1-BNF)

Sian Kou-Giesbrecht,Duncan N L Menge,Thomas A Bytnerowicz,Isabel Martínez Cano,Sergey Malyshev,Stephen W Pacala,Elena Shevliakova

doi:10.5194/bg-18-4143-2021

Abstract

Abstract. Representing biological nitrogen fixation (BNF) is an important challenge for coupled carbon (C) and nitrogen (N) land models. Initial representations of BNF in land models applied simplified phenomenological relationships. More recent representations of BNF are mechanistic and include the dynamic response of symbiotic BNF to N limitation of plant growth. However, they generally do not include the competitive dynamics between N-fixing and non-fixing plants, which is a key ecological mechanism that determines ecosystem-scale symbiotic BNF. Furthermore, asymbiotic BNF is generally not included in land models. Here, we present LM4.1-BNF, a novel representation of BNF (asymbiotic and symbiotic) and an updated representation of N cycling in the Geophysical Fluid Dynamics Laboratory Land Model 4.1 (LM4.1). LM4.1-BNF incorporates a mechanistic representation of asymbiotic BNF by soil microbes, a representation of the competitive dynamics between N-fixing and non-fixing plants, and distinct asymbiotic and symbiotic BNF temperature responses derived from corresponding observations. LM4.1-BNF makes reasonable estimations of major carbon (C) and N pools and fluxes and their temporal dynamics, in comparison to the previous version of LM4.1 with N cycling (LM3-SNAP) and to previous representations of BNF in land models generally (phenomenological representations and those without competitive dynamics between N-fixing and non-fixing plants and/or asymbiotic BNF) at a temperate forest site. LM4.1-BNF effectively reproduces asymbiotic BNF rate (13 kgNha-1yr-1) in comparison to observations (11 kgNha-1yr-1). LM4.1-BNF effectively reproduces the temporal dynamics of symbiotic BNF rate: LM4.1-BNF simulates a symbiotic BNF pulse in early succession that reaches 73 kgNha-1yr-1 at 15 years and then declines to ∼0 kgNha-1yr-1 at 300 years, similarly to observed symbiotic BNF, which reaches 75 kgNha-1yr-1 at 17 years and then declines to ∼0 kgNha-1yr-1 in late successional forests. As such, LM4.1-BNF can be applied to project the dynamic response of vegetation to N limitation of plant growth and the degree to which this will constrain the terrestrial C sink under elevated atmospheric CO2 concentration and other global change factors.

Highlights

The terrestrial carbon (C) sink is controlled by the availability of nitrogen (N) for plant growth (Elser et al, 2007; LeBauer and Treseder, 2008; Wright et al, 2018)
Data and Coweeta Hydrological Laboratory (CHL) site data. (c) Simulated soil NH+4 and NO−3 compared to CHL site data. (d) Simulated N mineralization rate and net nitrification rate compared to CHL site data. (e) Simulated N2O and NO emission rates compared to a meta-analysis estimate for temperate forests and simulated dissolved organic N (DON), NH+4, and NO−3 leaching rate compared to CHL site data
This occurs because there is no competitive exclusion of N-fixing plants by non-fixing plants in Land Model 3 (LM3)-Symbiotic Nitrogen Acquisition by Plants (SNAP), which represents a single general plant C pool capable of biological N fixation (BNF) and cannot represent community dynamics (Fig. 4)

Summary

Introduction

The terrestrial carbon (C) sink is controlled by the availability of nitrogen (N) for plant growth (Elser et al, 2007; LeBauer and Treseder, 2008; Wright et al, 2018). S. Kou-Giesbrecht et al.: A novel representation of biological nitrogen fixation plant growth will constrain the terrestrial C sink under elevated atmospheric CO2 concentration is unresolved (Terrer et al, 2019), as there is substantial variation between different land models (Wieder et al, 2015b). The representation of biological N fixation (BNF), the primary natural input of N to terrestrial ecosystems (Fowler et al, 2013; Vitousek et al, 2013), is a key challenge to incorporating N cycling into land models because of its complexity (Davies-Barnard et al, 2020; Meyerholt et al, 2020; Stocker et al, 2016; Thomas et al, 2015; Wieder et al, 2015a). BNF could, as such, be pivotal to overcoming N limitation of plant growth under elevated atmospheric CO2 concentration (Liang et al, 2016; Terrer et al, 2016, 2018)

Results

Discussion

Conclusion