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

The aims of this study were to determine whether elevated atmospheric CO2 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 CO2 (475 mu l l(-1)) using a Free Air CO2 Enrichment (FACE) facility located in the North Island of New Zealand. Aboveground herbage biomass and leaf litter production were not altered by elevated CO2 but root growth rate, as measured with the ingrowth core method, and root turnover were strongly stimulated by elevated CO2 particularly at low soil moisture contents during summer. Consequently, significantly more plant material was returned to the soil under elevated CO2 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 CO2. 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 CO2 may well lead to an accumulation of OM in grazed grassland soil in the long term.