Effects of elevated CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and N fertilization on plant and soil carbon pools of managed grasslands: a meta-analysis

W M A Sillen,W I J Dieleman

doi:10.5194/bg-9-2247-2012

Abstract

Abstract. Elevated atmospheric CO2 levels and increasing nitrogen deposition both stimulate plant production in terrestrial ecosystems. Moreover, nitrogen deposition could alleviate an increasing nitrogen limitation experienced by plants exposed to elevated CO2 concentrations. However, an increased rate of C flux through the soil compartment as a consequence of elevated CO2 concentrations has been suggested to limit C sequestration in terrestrial ecosystems, questioning the potential for terrestrial C uptake to mitigate increasing atmospheric CO2 concentrations. Our study used data from 77 published studies applying elevated CO2 and/or N fertilization treatment to monitor carbon storage potential in grasslands, and considered the influence of management practices involving biomass removal or irrigation on the elevated CO2 effects. Our results confirmed a positive effect of elevated CO2 levels and nitrogen fertilization on plant growth, but revealed that N availability is essential for the increased C influx under elevated CO2 to propagate into belowground C pools. However, moderate nutrient additions also promoted decomposition processes in elevated CO2, reducing the potential for increased soil C storage. An important role was attributed to the CO2 response of root biomass in soil carbon responses to elevated CO2, since there was a lower potential for increases in soil C content when root biomass increased. Future elevated CO2 concentrations and increasing N deposition might thus increase C storage in plant biomass, but the potential for increased soil C storage is limited.

Highlights

Atmospheric CO2 concentrations have strongly increased since the pre-industrial era (IPCC, 2007), resulting in the contemporary CO2 concentration of about 393 ppm that exceeds all earlier concentrations since the late Tertiary era, when most of the modern plants evolved into their present shapes (Pearson and Palmer, 2000; Crowley and Berner, 2001)
Other findings further demonstrated the regulating role of water availability in plant responses to elevated CO2: root biomass tended to decrease with irrigation compared to nonirrigated systems (Fig. 3, Table 3), root biomass responses to elevated CO2 increased in warmer sites (Table 5), and aboveground biomass responses reduced at sites with higher precipitation rates (Table 5)
This is in accordance with Volk et al (2000), Bunce (2004) and Morgan et al (2004a), all indicating that an increased water use efficiency (WUE) as a consequence of reduced stomatal conductance in elevated CO2 is the major reason for increased plant biomass in higher atmospheric CO2 concentrations

Summary

Introduction

Atmospheric CO2 concentrations have strongly increased since the pre-industrial era (IPCC, 2007), resulting in the contemporary CO2 concentration of about 393 ppm that exceeds all earlier concentrations since the late Tertiary era, when most of the modern plants evolved into their present shapes (Pearson and Palmer, 2000; Crowley and Berner, 2001). The 3.7 billion ha of the Earth’s surface with permanent grasslands have an estimated potential annual C sequestration capacity of 0.01– 0.3 GtC (Lal, 2004), which implies that 4 % of total global emissions of greenhouse gasses could be buffered by grasslands (Soussana and Luscher, 2007). Elevated CO2 tends to increase C allocation to root compartments (Rogers et al, 1994; Luo et al, 2006) as plants need more resources to sustain the enhanced growth (Bryant et al, 1983). Plants tend to increase root exudation in elevated CO2 (Fitter et al, 1997; Drigo et al, 2008; Lukac et al, 2009). As soil organisms tend to be Climited (Zak et al, 1993; Hu et al, 2006), these C inputs

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Conclusion