Interactive Influence of Crop Residue Quality and Soil Carbon Content on Priming Effect Intensity

Crop residues are the main source of carbon (C) to soil, but residue addition may accelerate the mineralization of native soil organic C (SOC) by promoting the priming (PE). The aim of this study was to investigate the influence of initial SOC content, and quality and quantity of crop residues on the mineralization of residue C and of native SOC in two soils. We incubated 13C-labelled shoots and roots of soybean and sorghum (4, 8 or 12 Mg ha-1) to sandy and clayey soils with high- and low-C contents for 360 days. The mineralization of native SOC was influenced by initial SOC content, residue type, and their interaction. The proportion of initial SOC that was mineralized after 360 days was 1.6 times greater in high-C soils than in low-C soils, which was attributed to a greater proportion of particulate SOC (>53 µm) in the high-C soils. Contrary to expectations, cumulative PE was not systematically greater in high-C than low-C soils. We observed negative PE values in the later stage of incubation in both high-C soils and in the low-C clay soil. Crop residue quality did not appear as a robust predictor of PE as we found significant interaction between residue quality and initial soil C content, refuting our hypothesis that high quality residue would generate greater cumulative PE. Overall, roots decomposed more slowly than shoots; yet they often induced PE intensity similar to shoots. The PE increased with crop residue addition rate, however, the increase was not linear and levelled off at higher rates. Our findings indicate complex interactions among initial soil C content, and residue quality and quantity on PE, which could change in intensity and direction as a function of time during incubation. This would explain inconsistencies in the existing literature on PE following fresh C inputs.