Nitrogen and Carbon Cycling in a Grassland Community Ecosystem as Affected by Elevated Atmospheric CO2

H A Torbert,H W Polley,H B Johnson

doi:10.1155/2012/817343

Abstract

Increasing global atmospheric carbon dioxide (CO2) concentration has led to concerns regarding its potential effects on terrestrial ecosystems and the long-term storage of carbon (C) and nitrogen (N) in soil. This study examined responses to elevated CO2in a grass ecosystem invaded with a leguminous shrubAcacia farnesiana(L.) Willd (Huisache). Seedlings ofAcaciaalong with grass species were grown for 13 months at CO2concentrations of 385 (ambient), 690, and 980 μmol mol−1. Elevated CO2increased both C and N inputs from plant growth which would result in higher soil C from litter fall, root turnover, and excretions. Results from the incubation indicated an initial (20 days) decrease in N mineralization which resulted in no change in C mineralization. However, after 40 and 60 days, an increase in both C and N mineralization was observed. These increases would indicate that increases in soil C storage may not occur in grass ecosystems that are invaded withAcaciaover the long term.

Highlights

The rise of CO2 in the atmosphere is well documented [1]; what has not been documented are the sinks for this C, with an estimated unknown sink of 1.4 × 1015 g C yr−1 arising from the global C balance [2]
It has been theorized that the commonly observed increase in plant C : N ratio under elevated CO2 could lead to slower residue decomposition resulting in increased soil C storage and reduction in available N for plant production [8]
The increase in biomass at elevated CO2 required no more water than was consumed by the shrubs grown near ambient CO2 concentration

Summary

Introduction

The rise of CO2 in the atmosphere is well documented [1]; what has not been documented are the sinks for this C, with an estimated unknown sink of 1.4 × 1015 g C yr−1 arising from the global C balance [2]. It has been theorized that the commonly observed increase in plant C : N ratio under elevated CO2 could lead to slower residue decomposition resulting in increased soil C storage and reduction in available N for plant production [8]. Observations from field and laboratory studies indicate that with elevated atmospheric CO2, N may limit the rate of plant residue decomposition and slow the release of N from decomposing plant material [13]. This indicates that understanding N cycling as affected by elevated CO2 is fundamental to understanding the potential for soil C storage on a global scale

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