Elevated CO2 reduces rate of decomposition of rice and wheat residues in soil

The production and quality of belowground roots and plant are likely to be affected by the increase in atmospheric CO2 level with subsequent changes in their decomposition rates in soil. However, the quality of residues has received very little attention, particularly in rice and wheat residues. The present experiment was laid out to study the decomposition of residues of rice (R) and wheat (W) grown in a Typic Haplustept soil under ambient (A) and elevated (E) CO2 conditions maintained in a phytotron. The decomposition of RA and WA was carried out in ambient atmospheric CO2 conditions, while that of RE and WE was done in elevated CO2 condition. Ambient CO2 grown rice and wheat residues were found to decompose at a faster rate compared to the corresponding elevated CO2 grown residues. The amount of residues left over after 150 days of decomposition was comparatively higher in the elevated CO2 grown residues indicating their slow rate of decomposition. However, ambient and elevated CO2 grown wheat residues did not differ significantly with respect to the amount remaining at later stages of decomposition. The RA and RE decomposed to 81% and 77% of their initial amount after 150 days of decomposition, while WA and WE decomposed to 73% and 71% of their initial amounts, while the C loss from RA, RE, WA and WE were 83%, 79%, 76% and 73%, respectively. Ambient atmospheric CO2 grown residues exhibiting narrow C:N ratios decomposed to a faster rate than the elevated CO2 grown residues. Overall, total organic carbon (TOC) content was significantly higher in WA treated soil than in WE treated soil. Net N mineralization (Nmin), microbial biomass carbon (MBC) and Nmin:MBC were greater in soil amended with ambient CO2 grown residues than in elevated CO2 grown residues. Rice residues as compared to wheat residues decomposed at a faster rate thereby releasing higher amount of N in soil. In near future the residues produced under higher concentration of atmospheric CO2 need to be handled carefully as these are decomposed with difficulty due to wide C:N ratios. This has direct implications on N cycling in soil and therefore N fertilization needs to be modified when crop residues are incorporated in soil for optimum crop production. Though lower decomposability of elevated CO2 grown residues might cause more C sequestration in soil, N limitation might adversely affect the plant C sequestration in future.