A B S T R A C TTo 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 15 Nlabeled urea and 13 C-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 residueinhibited SOC decomposition per unit maize C mineralized was 0.21 AE 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.

A B S T R A C TDissolved organic matter (DOM) in soils play an essential role in soil physical, chemical and biological processes, but little information is available on the biodegradability of plant-derived DOM and its effect on soil carbon and nitrogen sequestration in field soils. The objectives of this study were to investigate the impacts of crop residue-derived DOM on soil CO 2 and N 2 O emissions, as well as soil carbon and nitrogen sequestration by adding water extracts of maize stalk (i.e., plant-derived DOM) to soils. In this study, wheat was grown in pots under field conditions with treated soils, the soils treatments were: plantderived DOM (PDOM), urea nitrogen (N), PDOM + urea nitrogen (PDOM + N), as well as a control with no additions to soil (CK). Adding plant-derived DOM to soil increased soil CO 2 and N 2 O emissions (P < 0.05). During the wheat growing season, the cumulative CO 2 -C emission from CK, PDOM, N and PDOM + N was 107 AE 1, 157 AE 7, 136 AE 2 and 149 AE 6 g C m À2 , respectively. Meanwhile, the cumulative N 2 O-N emission from CK, PDOM, N and PDOM + N was 188 AE 8, 256 AE 5, 239 AE 10 and 258 AE 7 mg N m À2 , respectively. Compared with N treatment, DOM addition had little effect on soil N sequestration, but it accelerated the decomposition of native soil organic carbon (SOC) and caused a net loss of SOC. The soil C sequestration decreased about 151 AE 67 and 51 AE 45 g C m À2 in PDOM and PDOM + N treatments, respectively. The increased microbial biomass and root biomass were responsible for the greater CO 2 emission in DOMamended soils. Negative correlation between dissolved organic carbon (DOC) content and N 2 O flux suggested that the release of N 2 O was dependent on the supply of DOC. These results indicated that the supply of plant-derived DOM exacerbated soil CO 2 and N 2 O emissions and reduced soil C sequestration. Therefore, agricultural management practices that increase the stability of highly soluble C inputs and/or retard the decomposition of crop residues should be adopted to decrease soil greenhouse gas emission and increase soil C sequestration.2015 Elsevier B.V. All rights reserved.