Priming effect of maize residue and urea N on soil organic matter changes with time

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To investigate the effects of urea nitrogen (N) and crop residues on soil organic carbon (SOC) decomposition, a batch of incubation experiments was carried out for 250 days by incorporating 15N-labeled urea and 13C-labeled maize residue into soil. Adding maize residue alone or adding maize residue together with urea N had a significant priming effect on SOC. Furthermore, the direction of the priming effect changed over the incubation. This effect could be characterized by three stages. The first stage occurred just after maize residue addition when the substrate for microorganisms switched from native SOC to easily available maize C (lasting ∼7 days). The second stage showed a positive effect on the decomposition of native SOC (lasting ∼28–58 days). The third stage showed a negative effect on the decomposition of native SOC. In contrast, adding N alone caused a positive effect over the first 65 days of incubation, followed by a slight negative priming effect. The overall effect of maize residue C and urea N addition on the decomposition of native SOC was dependent on the balance between the inhibitory and stimulatory effects. At the end of the incubation, adding maize residue alone had little effect on the decomposition of native SOC; urea N addition alone increased SOC decomposition by 9.1%, while adding N to soil amended with maize residue decreased SOC decomposition by 9.5%. The amount of residue-inhibited SOC decomposition per unit maize C mineralized was 0.21±0.06 in the Maize+N treatment. Application of urea N significantly increased the mineralization rate of maize residue after 20 days of incubation. The increased N availability, microbial biomass and dissolved organic carbon (DOC) induced by the addition of N were responsible for the higher mineralization rate of maize residue. This indicates that the priming effect induced by maize residues could persist for a long time and involved not only one mechanism but a succession of processes. The response of the priming effect to the addition of maize residue and urea N differed depending on the microbial biomass, substrate C and N availability and the stage of decomposition. Adding N to soil amended with maize residue led to a more efficient use of maize residue at the slow mineralization stage.