In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide

The effects of elevated CO2 on belowground C input, on decomposition of roots in situ vs. decomposition of disturbed roots, and on soil microbial biomass were investigated in a perennial grass species. Forty ryegrass (Lolium perenne L.) plants were homogeneously C-14-labeled in two controlled environments for 115 d in a continuous (CO2)-C-14 atmosphere at 350 and 700 mu L CO2 L-1 and two soil N levels (low, LN, and high, HN). Thereafter, some of the plants were destructively harvested, Undisturbed root systems of the remaining plants were incubated in situ (IRS) for comparison with a disturbed incubation of the dried and ground roots (DRS) in their original soils. At the start of the incubation, elevated CO2 had increased total C-14-labeled soil C input by 44 and 27% at LN and HN, respectively, compared with input at ambient CO2, After incubation for 230 d, 40% of C-14 soil content was mineralized to (CO2)-C-14 in the disturbed system and 52% in the intact system. Native soil organic matter (SOM) decomposition of the DRS was lower than the SOM decomposition of the IRS, The formation of C-14-labeled soil microbial biomass (C-14-SMB) in the soil with DRS was 130% higher than in the soil with the IRS. Elevated CO2 decreased the decomposition of roots and root-derived products by 10% and increased the size of the C-14-SMB by 28% for both IRS and DRS, whereas the decomposition of SOM was not affected by CO2 at either LN or HN, After plant growth and in situ incubation, the C-14-labeled C in the soil solution showed a highly positive correlation with the amount of C-14-SMB, The ratio between C-14-labeled microorganisms and total (CO2)-C-14 evolved was not affected by elevated CO2, It seems that micrroorganisms adapt to changing soil C input under elevated CO2 and there is no effect on their turnover and behavior.