Influence of source and quality of plant residues on emissions of N₂O and CO₂ from a fertile, acidic Black Vertisol

Abstract

Few studies have compared emissions of nitrous oxide (N₂O), the potent greenhouse gas associated with decomposition of both below-ground (root) and above-ground (shoot) residues. We report a laboratory incubation experiment to evaluate effects of root and shoot residues from wheat, canola, soybean, and sorghum, incorporated into a naturally fertile acidic Black Vertisol, on N₂O and carbon dioxide (CO₂) emissions. The residue-amended Vertisol samples were incubated at 25 °C and 70 % water-filled pore space (WFPS) to facilitate denitrification activity for a total period of 56 days. The incubated soils were periodically sampled for N₂O, CO₂, mineral N, and dissolved organic carbon (DOC). In general, shoot residues emitted more CO₂ than roots, while N₂O emissions were 50-70 % higher in cereal root residues than those in shoots. Surprisingly, the highest N₂O emissions were associated with soils amended with the more inert high C/N ratio residues (wheat and sorghum roots), and to some extent, lowest emissions were associated with low C/N ratio (more labile) residues, particularly during the early stages of incubation (0-22 days). During this stage, there was a significant (p < 0.01) and negative correlation between N₂O emissions and microbial respiration (CO₂ efflux) and a positive (p < 0.001) correlation between microbial respiration and DOC. These results suggest that residue decomposition linked to N immobilization reduced N₂O emissions during this early stage. Only, later in the study (23-56 days), did the high %N, low C/N ratio residues of soybean shoot and canola roots release at least twice as much N₂O as the majority of the other treatments. We concluded that the unexpected patterns of N₂O emissions were a result of the initially high mineral N content of the incubated soils and that root residues are likely to contribute substantially to emissions from cropping soils.