Elevated CO2 alters the rhizosphere effect on crop residue decomposition

Background and aims: Elevated atmospheric CO 2 (eCO 2 ) can affect microbial decomposition of native soil organic carbon (SOC) via enhanced root exudation and rhizosphere activity. Few studies have examined the effect of eCO 2 on the decomposition of newly-added crop residues, which are important to understand below-ground C changes. A soil microcosm experiment was conducted to examine whether eCO 2 would enhance the rhizosphere effects on the decomposition of crop residues. Methods: White lupin (Lupinus albus L. cv. Kiev) was grown for 34 or 62 days under ambient CO 2 (aCO 2 , 400 μmol mol −1 ) or eCO 2 (800 μmol mol −1 ) in a low-C (2.0 mg g −1 ) soil which was amended with or without dual 13 C and 15 N labelled wheat, field pea or canola crop residues. An isotopic tracing technique was adopted to partition residue-derived CO 2 from total below-ground CO 2 efflux. Two independent groups of data were analysed statistically at either Day 34 or 62. Results: The presence of white lupin increased the decomposition of all residues at Day 34. This positive rhizosphere effect on residue decomposition decreased and was even reversed at Day 62, probably due to depletion of labile C, or microbial N limitation, or rhizosphere acidification. The eCO 2 -induced decomposition depended on residue type at Day 34. Specifically, when compared to aCO 2 , eCO 2 did not affect the decomposition of canola residue, increased that of field pea residue by 13.5% but decreased wheat straw decomposition by 7.4%. However, residue decomposition was, on average, 13% higher under eCO 2 at Day 62, which was correlated positively with the increase in rhizosphere extractable C (P < 0.01). Conclusions: Elevated CO 2 generally increased residue decomposition in the rhizosphere, but this effect was mediated by residue type at Day 34. Enhanced residue decomposition under legumes at eCO 2 may favour C turnover and the release of residue N.