Effects of coupled water and nitrogen on soil organic carbon fractions and enzymes in a drip-irrigated cotton field

Objective The effects of irrigation and nitrogen application on soil organic carbon, nitrogen content, and organic carbon pool stability in drip-irrigated cotton fields were studied to provide a theoretical basis for achieving efficient cotton production and sustainable development in Xinjiang.

Methods A two-factor three-level complete random field experiment was conducted using cotton as the test material. The three irrigation volumes (W) were low (360 mm), medium (480 mm), and high (600 mm), and the three nitrogen rates (N) were low (0 kg/hm²), medium (300 kg/hm²), and high (450 kg/hm²), giving a total of 9 treatment combinations. After harvesting the cotton, we collected 0– 20 cm soil samples to measure soil organic carbon (SOC), water soluble organic carbon (WSOC), microbial biomass carbon (MBC), easily oxidized organic carbon (EOC), stable organic carbon (NOC), soil urease (URE), β- Glucosidase (BG), and N-acetyl-β-D-glucosidase (NAG) activity. The soil organic matter decomposition rate and soil carbon pool management index (CPMI) were measured using the net nylon bag method at the in-situ landfill on the field.

Results SCompared with W600N450, W480N300 increased soil organic carbon by 8.9% and 10.9%, C/N by 16.2% and 16.3%, EOC and NOC by 11.8%–15.4% and 14.3%–20.8%, while WSOC decreased by 35.5% and 21.8% in 2015 and 2016, respectively. It was shown that reasonable water and nitrogen management was beneficial to increasing soil organic carbon and stable organic carbon content, reducing water-soluble organic carbon, and improving soil carbon pool stability. Compared with W480, W360 (P<0.05) reduced MBC, while W600 reduced WSOC and NOC. The EOC content in W360 (P<0.05) decreased by 2.1% and 5.3% in two years, while that in W600 decreased by 4.1% and 7.6%, respectively. Compared with N300, the N0 treatment reduced MBC by 41.8%, while N450 reduced NOC. The EOC in the N0 treatment decreased by 20.2% and 16.7% in two years, while that in N450 decreased by 3.8% and 2.4%, respectively. The results showed that the effect of nitrogen application rate on soil organic carbon was greater than the irrigation level. Simultaneously, excessive irrigation and nitrogen application reduced stable organic carbon and increased microbial biomass carbon, which was not conducive to soil organic carbon accumulation. High irrigation (600 mm) and nitrogen (450 kg/hm²) (P<0.05) increased soil enzyme activity and decreased soil carbon management index. The activities of urease, β-glucosidase, and N-acetyl-β-D-glucosidase in W480N300 were 1.0%, 22.4%, and 32.6% higher than in W360N0. The correlation analysis showed that CPMI was positively correlate with active organic carbon (MBC, EOC) in different degrees and negatively correlate with NOC.

Conclusion Under drip irrigation conditions in Xinjiang, excessive irrigation and nitrogen application rates reduce soil organic carbon, stable organic carbon, soil carbon pool activity, and carbon pool management index. In contrast, they increase microbial biomass carbon and enzyme activity, which is not conducive to maintaining soil organic carbon stability. In this study, 480 mm irrigation combined with 300 kg/hm² nitrogen application rate is conducive to promoting soil organic carbon accumulation and improving soil organic carbon activity in drip-irrigated cotton fields.