Modeling the effects of organic nitrogen uptake by plants on the carbon cycling of boreal forest and tundra ecosystems

Q. Zhuang,Q. Zhu

doi:10.5194/bg-10-7943-2013

Abstract

Abstract. Boreal forest and tundra are the major ecosystems in the northern high latitudes in which a large amount of carbon is stored. These ecosystems are nitrogen-limited due to slow mineralization rate of the soil organic nitrogen. Recently, abundant field studies have found that organic nitrogen is another important nitrogen supply for boreal forest and tundra ecosystems. In this study, we incorporated a mechanism that allowed boreal plants to uptake small molecular amino acids into a process-based biogeochemical model, the Terrestrial Ecosystem Model (TEM), to evaluate the impact of organic nitrogen uptake on ecosystem carbon cycling. The new version of the model was evaluated for both boreal forest and tundra sites. We found that the modeled organic nitrogen uptake accounted for 36–87% of total nitrogen uptake by plants in tundra ecosystems and 26–50% for boreal forests, suggesting that tundra ecosystem might have more relied on the organic form of nitrogen than boreal forests. The simulated monthly gross ecosystem production (GPP) and net ecosystem production (NEP) tended to be larger with the new version of the model since the plant uptake of organic nitrogen alleviated the soil nitrogen limitation especially during the growing season. The sensitivity study indicated that the most important factors controlling the plant uptake of organic nitrogen was the soil amino acid diffusion coefficient (De) in our model, suggesting that the organic nitrogen uptake by plants is likely to be regulated by the edaphic characteristics of diffusion. The model uncertainty due to uncertain parameters associated with organic nitrogen uptake of the tundra ecosystem was larger than the boreal forest ecosystems. This study suggests that considering the organic nitrogen uptake by plants is important to carbon modeling of boreal forest and tundra ecosystems.

Highlights

Terrestrial ecosystems play an important role in cycling carbon between land and the atmosphere through photosynthesis, plant respiration and soil respiration (Heimann et al, 1998; Melillo et al, 1993)
We ran both Terrestrial Ecosystem Model (TEM) and ON-TEM (1) at the US-Brw site in 2001 using parameters calibrated with US-Brw data in 1999; and (2) at the UCI 1989 and UCI 1850 sites using parameters calibrated at the UCI 1998 site to evaluate the goodness of models and parameters
In ON-TEM simulations, organic nitrogen contributed 36– 87 % and 26–50 % of total nitrogen uptake at the tundra and boreal forest sites, respectively, suggesting that the tundra ecosystem likely relies more on the organic form of nitrogen. These results were consistent with the findings of Nasholm et al (1998), who stated that at least 42 % nitrogen uptake was from the organic form of nitrogen for trees in boreal forests

Summary

Introduction

Terrestrial ecosystems play an important role in cycling carbon between land and the atmosphere through photosynthesis, plant respiration and soil respiration (Heimann et al, 1998; Melillo et al, 1993). The high C / N ratio litter slows down N mineralization, decreasing available N in the soil and limiting the plants’ ability to assimilate carbon. The nitrogen limitation effect on carbon cycling has been confirmed by the elevated CO2 studies (Norby et al, 2010; Reich et al, 2006a, b). The feedback between carbon and nitrogen cycles could reduce the CO2 fertilization effect and strongly affect the plant carbon productivity. The nitrogen cycling is critically important to carbon cycling, especially in regions where the N availability is limited (e.g., McGuire et al, 1992; Sokolov et al, 2008)

Methods

Results

Conclusion