This study was conducted to explore the effects of different chemical fertilizer optimization managements on the dynamics of soil organic carbon (SOC) fractions, soil carbon process, and subsequent spring wheat yield in a post-wheat green manure (hairy vetch, Vicia villosa Roth) rotation of saline-alkali soil in the Hetao Irrigation District. The aim was to identify effective fertilizer reduction pathways for synergistically enhancing soil carbon sequestration and productivity.

Based on a long-term filed experiment established in 2015 in Linhe District, Bayannaoer City, four treatments were set up by randomized block design, including CK (nitrogen, phosphorus and potassium fertilizer), K0G (nitrogen and phosphorus fertilizer), P0G (nitrogen and potassium fertilizer), and N0G (phosphorus and potassium fertilizer). SOC fractions and enzyme activities in 0-20 and 20-40 cm soil layers, and spring wheat yield were measured in 2023, and the soil carbon process index for soil carbon transformation intensity was also calculated.

(1) Compared with K0G and P0G treatments in 0-20 cm soil layer, N0G treatment (nitrogen fertilizer reduction) significantly reduced soil pH by 0.17 and 0.11 units, while significantly increasing available phosphorus (AP) content by 27.6% and 96.3%, respectively, and available potassium (AK) content by 26.5% and 46.3%, respectively. Compared with P0G, N0G treatment significantly increased AP and AK contents by 192.7% and 18.2% in 20-40 cm soil layer, respectively. (2) Compared with K0G and P0G treatments, N0G treatment significantly enhanced carbon-related enzyme activities in 0-40 cm soil layer: β-glucosidase (27.1% and 53.6%, respectively), xylanase (82.6% and 65.9%, respectively), and cellobiosidase (46.2% and 60.1%, respectively). (3) Compared with K0G and P0G, N0G treatments significantly increased labile organic carbon (LOC) by 25.7% and 26.3%, respectively, and particulate organic carbon (POC) contents by 50.0% and 48.8% in 0-40 cm, respectively. Compared with K0G, N0G significantly increased mineral-associated organic carbon (MAOC) content by 19.2% in 0-20 cm. SOC fractions showed significant positive correlations with soil nutrients and enzyme activities, but significant negative correlations with soil pH. (4) Spring wheat yield under CK, K0G, P0G and N0G treatments were 8.17, 7.76, 7.95 and 7.95 t·hm^-2, respectively. There was no significant difference between P0G, N0G and CK, except that K0G significantly reduced the yield by 5.1% compared with CK. Soil enzyme activities were the significant predictors of spring wheat yield. (5) N0G treatment exhibited higher carbon process index in both 0-40 cm layers, significantly exceeding K0G and P0G treatments. Spring wheat yield showed a significant positive correlation with the carbon process index in the 0-20 cm soil layer.

N0G (multiple cropping of hairy vetch without nitrogen fertilizer after wheat) effectively reduced soil pH, activated carbon cycle-related enzyme activities, promoted organic carbon labile fractions accumulation, and enhanced carbon process index, ultimately increasing spring wheat yield of a wheat-green manure cropping system in the Hetao Irrigation District. Therefore, this approach was a viable fertilizer reduction strategy for harmonizing soil amelioration (pH reduction, C sequestration) with stable crop productivity in saline-alkali soil.

This study was conducted to explore the responses of saline-alkali soil respiration to straw interlayer and irrigation regime, and to clarify the relationships between soil respiration rate and soil temperature/water content in Hetao Irrigation District.

A randomized block design was set up in 2015 with three treatments: autumn and spring irrigation (I_SA), spring irrigation without autumn irrigation (I_S), and spring irrigation plus straw interlayer without autumn irrigation (SI_S). Soil respiration, soil temperature and moisture content were measured in 2017 and 2018, and the temperature sensitivity of soil respiration rate was also estimated.

(1) Soil temperature and soil moisture content in the 0-20 cm soil layer fluctuated obviously, while it was relatively stable in the 20-40 cm soil layer. SI_S increased the soil temperature in 0-40 cm soil depth and moisture content in 20-40 cm soil depth. (2) The soil respiration rate in 2017 was higher than that in 2018. It was the highest in the flowering stage, followed by budding stage, before spring irrigation, before sowing, and harvest stage. (3) The soil respiration rates of I_S and SI_S were significantly higher than I_SA at the flowering stage (P<0.05). Compared with I_SA, I_S and SI_S increased soil respiration rate by 0.12-0.44 and 0.06-0.42 μmol·m^-2·s^-1, respectively. Compared with the I_S, the soil respiration rate of SI_S decreased by 0.01-0.49 μmol·m^-2·s^-1. These results indicated that the soil respiration rate was increased without autumn irrigation, while it was decreased with straw interlayer. (4) The soil respiration rate positively correlated with soil temperature (P<0.01), while it had no significant correlation with soil moisture content. Soil temperature at the 0-20 and 20-40 cm soil layer explained 40.74%-53.84% and 39.27%-53.46% variation of soil respiration rate, respectively. (5) The temperature sensitivity of soil respiration (Q₁₀) varied within 1.68-1.98 for different treatments, and the Q₁₀ of the 20-40 cm soil depth was higher than that of the 0-20 cm soil depth. Compared with I_SA, I_S and SI_S reduced Q₁₀. However, SI_S increased Q₁₀ compared with I_S.

Overall, the straw interlayer increased the soil temperature in 0-40 cm soil depth and moisture content in 20-40 cm soil depth, reduced soil respiration rate, and increased the temperature sensitivity of soil respiration, which was used as an effective practice for water saving and emission reduction in the Hetao Irrigation District.