Residue decomposition and soil carbon priming in three contrasting soils previously exposed to elevated CO2
The effects of elevated atmospheric carbon dioxide (eCO2) on belowground processes are known to occur directly and indirectly via plants. However, the long-term impact of eCO2 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 CO2 (aCO2; 390 ppm) or eCO2 (550 ppm) using free-air CO2 enrichment (FACE) for 4 years. Surface soils (0–2 cm) of calcisol, luvisol and vertisol were amended (0.5% w w−1) with 13C-labelled field pea (Pisum sativum L. cv. PBA; C:N 20) or wheat (Triticum aestivum cv. Yitpi; C:N 60) residues, and CO2 derived from soil (CO2 soil) and residue (CO2 residue) were quantified over the 96-day incubation study. Field pea decomposition was not affected by soil type or CO2 history, and the decomposition of wheat was similar in all soils previously exposed to aCO2. However, wheat decomposition was increased in luvisol (14.4%), decreased in vertisol (26.7%) or not affected by eCO2 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 eCO2 were not explained by lower N concentrations. A greater priming effect in field pea–amended vertisol previously exposed to eCO2 than aCO2 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 CO2 history and least by residue quality (C:N ratio).
KeywordsCrop residue Priming effect Free-air CO2 enrichment (FACE) Carbon and nitrogen cycling
We are grateful to Kaien Ra for her excellent technical support, Leanne Lisle for performing the IRMS analyses and Patrick Bloye who worked on this project as part of his Honours project. The SoilFACE facility is part of The Australian Grains Free Air CO2 Enrichment (AGFACE) facility, which is jointly operated by The University of Melbourne and DEDJTR with funding from the Grains Research and Development Corporation (GRDC) and the Australian Government Department of Agriculture. We thank the SoilFACE technical team for managing the field experiment and Mahabubur Mollah for the FACE infrastructure.
This research was supported by an Australian Research Council Linkage Project (LP100200757) and was conducted the Department of Economic Development, Jobs, Transport and Resources (DEDJTR), Victoria at Horsham.
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