Objectives Partial replacement of inorganic nitrogen (N) with organic manure is encouraged in China as an effective technique for improving soil fertility and nutrient use efficiency. N-butyl thiophosphamide (NB) and nitrapyrin (CP) promote N use efficiency by blocking urea hydrolysis and inhibiting soil N nitrification, but their impact on soil organic carbon (SOC) transformation remains unclear. Thus, we investigated the influence of NB and CP on soil organic carbon transformation.

Methods Soil batch incubation experiment was performed in plant growth chamber at 25℃ and 35℃. Six treatments were designed, namely: no fertilizer control (CK); single urea (U); urea+cattle manure (UM); urea+cattle manure+NB (UM_NB); urea+cattle manure+CP (UM_CP) and urea+cattle manure+NB+CP (UM_NB+CP). Soil samples were collected at 7, 32 and 81 days after incubation, and the total soil organic carbon, microbial biomass carbon (MBC) and easily oxidized organic carbon (EOOC), soil invertase, cellulase, β-D-glucosidase and polyphenoloxidase were analyzed.

Results At 25℃ and 35℃, the SOC contents in UM_NB, UM_CP and UM_NB+CP were similar with that in UM at all sampling days, and they were all significantly higher (P < 0.05) than that in U. The average EOOC contents in UM_NB (3.26 g/kg), UM_CP (3.34 g/kg) and UM_NB+CP (3.06 g/kg) at 25℃ and 35℃ were 7.6%, 5.4% and 15.4% lower than that in UM (3.53 g/kg), respectively. Similarly, the average MBC contents in UM_NB (279.7 mg/kg), UM_CP (224.0 mg/kg) and UM_NB+CP (211.0 mg/kg) at 25℃ and 35℃ were 7% (UM_NB), 27% (UM_CP) and 40% (UM_NB+CP) lower than that in UM (300.6 mg/kg), respectively. At 25℃, the EOOC contents in UM_NB, UM_CP, UM_NB+CP treatments in the three incubation periods were not significantly different from that in UM (P > 0.05), except on the 32nd day when they were significantly lower than that in UM (P < 0.05). At 35℃, all the EOOC contents in the four manure and inhibitor treatments in the three periods were not significantly different (P > 0.05), only except on the 32nd day when the UM_NB+CP was significantly lower (P < 0.05) than other treatments. For MBC, the content was in order of UM_NB < UM_CP < UM_NB+CP at both 25℃ and 35℃ (P < 0.05). The MBC in UM_CP and UM_NB+CP at 25℃ were significantly lower than that in UM_NB at the 7th and 32nd days, and at 35℃, only in the 7th day they were significantly different, but there was no significant difference in MBC between UM_CP and UM_NB+CP. Compared with urea alone and CK treatments, the β-D-glucosidase and polyphenoloxidase activities were higher, while the cellulase activity were lower (P < 0.05) in the 4 cattle manure containing treatments. The invertase activity was similar (P > 0.05) in the four cattle manure containing treatments across all sampling days. Cellulase activities of treatments with NB or CP addition were inhibited by NB and CP addition.

Conclusions Replacing 40% of urea N by 40% cattle manure N, combined with the addition of urease inhibitor (NB) and nitrification inhibitor (CP) significantly reduces microbial biomass carbon content, decreases cellulase activity, and inhibits the conversion of organic carbon in the soil. Moreover, simultaneous addition of the two inhibitors has stronger inhibitory effect on the conversion of organic carbon. Therefore, urease and nitrification inhibitors should be used circumspectly under combined application of organic fertilizers with chemical fertilizer.