One of the major challenges of modern ecological research is to understand how the biosphere is responding to human-induced global environmental changes. One of such global change is the increase in atmospheric CO2 since pre-industrial times, currently amounting to 370 ppm. To study the impacts of elevation of CO2, a pot culture experiment was conducted in open top chambers with surface soils collected from a Typic Haplustept (IARI, New Delhi, India). Rice and wheat were grown as test crops at ambient (approx. 370 m mol mol-1) and elevated (600±50 m mol mol-1) levels of atmospheric C02. Wheat and rice residues previously grown with similar atmospheric CO2 concentration were incorporated in soil at the rate of 9 g kg-1 soil in rice and wheat crops, respectively. Destructive samplings were done at four physiological stages of crop growth, namely tillering (Rice) /crown root initiation (wheat), anthesis, grain filling and maturity. Biomass yield, carbon content and its uptake by different plant parts (viz. root, stem, leaf and grain) and various carbon fractions in rhizosphere soil, namely, microbial biomass carbon (MBC), dissolved organic carbon (DOC), carbohydrate carbon (CHC), labile carbon (LBC, 0.33M KMn04 oxidizable) as well as total carbon were determined for both the crops at each of the growth stages.
Average biomass yield of all the plant parts increased due to exposure to elevated C02 in both the crops. The extent of increase were 31.2, 43.9,18.5 and 48.1 per cent for rice and 70.9, 42.4, 20.9 and 35.4 per cent for wheat crop for root, leaf, stem and grain, respectively, over corresponding ambient treatments. The total dry matter yield increased by about 29 per cent for rice and 42 per cent for wheat. Relative preference of dry matter portioning to root was evident in wheat under elevated CO2 as root experienced preferential increase in growth than the aboveground plant parts. In case of rice preferential allocation of biomass to leaf and grain was recorded. Substantial root loss between anthesis /flowering and maturity was observed in both the crops, which indicated probable turnover of older roots during this time. Such loss was more under elevated C02 than ambient condition. Fresh root weight and root volume measurements corroborated such trends.
Carbon content in different plant parts (except grain) either remained unaffected or increased marginally due to increase in atmospheric C02, which suggests absence of dilution effect in spite of increase in biomass yield. This resulted in further significant increase in C uptake data under elevated C02. Such increase was by 35.6 per cent in rice and 42.9 per cent in wheat. Overall data suggested preferential assimilation and partitioning of carbon to underground portion as well as leaf. As a relatively large amount of assimilated C also remains in stem on maturity at elevated CO2 (compared to ambient), its Incorporation might be key to harness possibility of enhanced C sequestration in the long run.
All the active carbon fractions were positively influenced by increase in atmospheric CO2. In case of rice crop, extent of gain was in the order of MBC (41 per cent)>DOC (16.5 per cent)>CHC (9.0 per cent)>LBC (6.8 per cent) over the ambient treatments. In case of wheat crop, the order was slightly altered: DOC (17.4 per cent)>MBC (16.7 per cent)>CHC (7.9 per cent)>LBC (7.5per cent). However, there were no significant changes in total C in soil under any of the crops. The active carbon fractions were generally higher in soil samples at the first stage due to decomposition of residues incorporated. Further, values of DOC decreased with crop maturity, MBC either remained high till anthesis (rice) or peaked at anthesis (wheat), while the values of both CHC and LBC gradually increased towards maturity in both the crops. It seems, DOC primarily was contributed from younger roots as secretion/exudation product; while CHC was probably a degradation consequent of structural carbohydrates.
A relative build up of active carbon pools was noted in rice and wheat rhizosphere soils with relation to their initial status and the degree of increase was higher under elevated CO2 over the ambient treatment. Increases in these fractions were expected to impart short to medium term effect with respect to carbon sequestration in soil.