Long-term atmospheric CO2 enrichment impact on soil biophysical properties and root nodule biophysics in chickpea (Cicer arietinum L.)

• Long-term CO2 enrichment was studied on soil and legume biophysics in chickpea.
• CO2 enrichment increased lability of C in rhizosphere.
• Extracellular enzymes increased substantially with enriched atmospheric CO2.
• Root and nodule morphology improved in enriched CO2 environment.
• Larger symbiotic N-fixation stabilized nodule and soil C:N ratio.

Impact of atmospheric CO2 enrichment on soil aggregation, carbon and other nutrient availability and soil enzyme activities in relation to root and nodule biophysics in chickpea (Cicer arietinum L.; Pusa-1105 genotype) was studied in an open top chamber experiment at IARI, New Delhi, India, continuing since 2003. Soil samples were collected at the end of the crop growing seasons of 2010–11 and 2011–12, and analyzed. Root growth and nodulation were also studied in these seasons. Soil C and P pools, and associated enzyme activities responded differently to CO2 enrichment, while total soil N did not change. Soil labile C fractions viz., water soluble carbohydrate (WSC) and microbial biomass C (MBC) significantly increased, although recalcitrant C fraction declined marginally. The soil-CO2 flux increased by 28%. Dehydrogenase and fluorescein diacetate hydrolysis activity in soil increased by 44% and 67% respectively; and the β-glucosidase activity enhanced by 20% under enriched CO2 condition. The CO2 enrichment induced root growth and N2-fixation by root nodules, which were evidenced by increase in legheamoglobin content and nitrogenase activity. Nodules were bulky and had higher starch and soluble sugar contents under enriched atmospheric CO2 condition, allowing for greater N2-fixation. The rhizosphere C:N ratio, however, remained unaffected. It could be possible that larger partitioning of C to roots along with greater N2-fixation by nodules in chickpea might stabilize the net C:N ratio in the soil. Moreover, the increased soil biological activity under CO2 enrichment resulted in marginal depletion of soil recalcitrant C with increase in labile C pools. These are likely to offset the stability of soil C pools in a legume-based agroecosystems under the enriched CO2 condition in the semi-arid climate.