Increased Quantity and Quality of Coarse Soil Organic Matter Fraction at Elevated CO2 in a Grazed Grassland are a Consequence of Enhanced Root Growth Rate and Turnover

Abstract

The aims of this study were to determine whether elevated atmospheric CO 2 concentration modifies plant organic matter (OM) fluxes to the soil and whether any change in the fluxes can modify soil OM accumulation. Measurements were made in a grazed temperate grassland after almost 4 years exposure to elevated atmospheric CO 2 (475 ll l -1 ) using a Free Air CO 2 Enrichment (FACE) facility located in the North Island of New Zealand. Aboveground herbage biomass and leaf litter production were not altered by elevated CO 2 but root growth rate, as measured with the ingrowth core method, and root turnover were strongly stimulated by elevated CO 2 particularly at low soil moisture contents during summer. Consequently, significantly more plant material was returned to the soil under elevated CO 2 leading to an accumulation of coarse (>1 mm) particulate organic matter (POM) but not of finer POM fractions. The accumulating POM exhibited a lower C/N ratio, which was attributed to the higher proportion of legumes in the pasture under elevated CO 2 . Only small changes were detected in the size and activity of the soil microbial biomass in response to the POM accumulation, suggesting that higher organic substrate availability did not stimulate microbial growth and activity despite the apparent lower C/N ratio of accumulating POM. As a result, elevated CO 2 may well lead to an accumulation of OM in grazed grassland soil in the long term.

Specific rhizosphere bacterial and fungal groups respond differently to elevated atmospheric CO2

“…Previous studies have shown that, under elevated CO 2 , especially C 3 plants (as used in our study) increase their total root exudation mainly through the expansion of their root systems (Rogers et al, 1994;Allard et al, 2005), yielding qualitative and quantitative changes in root exudation and other forms of rhizodeposition (Paterson et al, 1996;Hodge and Millard, 1998). Differences in plant exudation patterns between plants species is thought to exert differential selection in the rhizosphere, thereby shaping the size and structure of soil-borne communities (Bardgett et al, 1999;Smalla et al, 2001;Kowalchuk et al, 2002).…”

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

“…Further, in some systems, forbs have been found to be one of the more CO 2 -sensitive plant functional types ). If root growth and turnover are considered, then there was a stimulation in plant biomass at elevated CO 2 that appeared to be strongest under conditions of low soil moisture in New Zealand (Newton et al 1996;Allard et al 2004). …”

Aboveground litter quality changes may drive soil organic carbon increase after shrub encroachment into mountain grasslands

“…In grass, the combination of higher belowground production and lower decay rates suggests that root biomass may be a significant contributor of soil C stocks compared with shrubs. However, higher turnover may also imply an increase in particulate organic matter (Allard et al 2005), which may be less lignified and easily decomposed in the long term.…”