Residue decomposition and soil carbon priming in three contrasting soils previously exposed to elevated CO 2

Abstract

The effects of elevated atmospheric carbon dioxide (eCO 2 ) on belowground processes are known to occur directly and indirectly via plants. However, the long-term impact of eCO 2 on biochemical properties and processes of agricultural soils in the absence of plants is unclear. The current study investigated whether residue decomposition and the subsequent ‘priming effect’ on soil organic C (SOC) mineralisation were altered in three contrasting soils previously exposed to either ambient CO 2 (aCO 2 ; 390 ppm) or eCO 2 (550 ppm) using free-air CO 2 enrichment (FACE) for 4 years. Surface soils (0–2 cm) of calcisol, luvisol and vertisol were amended (0.5% w w −1 ) with 13 C-labelled field pea (Pisum sativum L. cv. PBA; C:N 20) or wheat (Triticum aestivum cv. Yitpi; C:N 60) residues, and CO 2 derived from soil (CO 2 soil ) and residue (CO 2 residue ) were quantified over the 96-day incubation study. Field pea decomposition was not affected by soil type or CO 2 history, and the decomposition of wheat was similar in all soils previously exposed to aCO 2 . However, wheat decomposition was increased in luvisol (14.4%), decreased in vertisol (26.7%) or not affected by eCO 2 in the calcisol. The relative differences between soils were largely driven by labile N content and the potential to replenish inorganic N via mineralisation. Notably, priming was not influenced by residue type, despite their contrasting N content. In the calcisol, lower basal C mineralisation and C priming under eCO 2 were not explained by lower N concentrations. A greater priming effect in field pea–amended vertisol previously exposed to eCO 2 than aCO 2 was likely due to overcoming the N limitation on microbial C mineralisation in this soil. Overall, the study highlighted that C mineralisation was mainly determined by soil N status, less by CO 2 history and least by residue quality (C:N ratio).