Elevated CO2 alters the rhizosphere effect on crop residue decomposition

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Elevated atmospheric CO2 (eCO2) can affect microbial decomposition of native soil organic carbon (SOC) via enhanced root exudation and rhizosphere activity. Few studies have examined the effect of eCO2 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 eCO2 would enhance the rhizosphere effects on the decomposition of crop residues. White lupin (Lupinus albus L. cv. Kiev) was grown for 34 or 62 days under ambient CO2 (aCO2, 400 μmol mol−1) or eCO2 (800 μmol mol−1) in a low-C (2.0 mg g−1) soil which was amended with or without dual 13C and 15N labelled wheat, field pea or canola crop residues. An isotopic tracing technique was adopted to partition residue-derived CO2 from total below-ground CO2 efflux. Two independent groups of data were analysed statistically at either Day 34 or 62. 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 eCO2-induced decomposition depended on residue type at Day 34. Specifically, when compared to aCO2, eCO2 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 eCO2 at Day 62, which was correlated positively with the increase in rhizosphere extractable C (P < 0.01). Elevated CO2 generally increased residue decomposition in the rhizosphere, but this effect was mediated by residue type at Day 34. Enhanced residue decomposition under legumes at eCO2 may favour C turnover and the release of residue N.