Impact of Saline Water Irrigation on Soil Carbon Pool Composition and the Response of Carbon Emissions to Water Regulation

Abstract

In areas with freshwater resource shortages, although saline water irrigation can alleviate agricultural water-use pressure, long-term application may lead to soil salinisation and ecological function degradation, thereby affecting greenhouse gas emissions. To clarify its influence on the soil carbon cycle and greenhouse gas emissions, this study collected farmland soil subjected to long-term saline water irrigation (S1: 1 g/L, S2: 4 g/L, and S3: 8 g/L) and analysed the dynamic changes in CO₂ emissions and carbon and nitrogen components through controlled indoor experiments. The soil moisture gradient was defined as follows: W1, W2, and W3 correspond to 45%, 60%, and 75% of the field water-holding capacity, respectively. The results indicated that soil total carbon and total nitrogen decreased over time and were more strongly affected by salinity than by moisture, with both parameters peaking at a salinity level of S2. Both dissolved organic carbon and microbial biomass (microbial biomass carbon, microbial biomass nitrogen) responded distinctly to moisture and salinity: dissolved organic carbon decreased initially and then increased as the salinity increased (S3 > S1 > S2), but it consistently decreased with increasing soil moisture (W1 > W2 > W3), while microbial biomass carbon and microbial biomass nitrogen rose as the soil moisture increased (W3 > W2 > W1). Microbial biomass nitrogen demonstrated higher salt tolerance than carbon biomass, peaking at S2, whereas microbial biomass carbon declined with rising salinity (S1 > S2 > S3). Soil moisture and salinity significantly influenced CO₂ emissions. CO₂ levels increased with rising soil moisture (W3 > W2 > W1). Moderate salinity promoted CO₂ emissions, whereas high salinity suppressed them (S2 > S1 > S3). Compared to the W3S2 treatment, which showed the maximum value (p < 0.05), CO₂ emissions were reduced by 13.84% in W3S1, 24.85% in W3S3, 33.63% in W1S2, and 20.54% in W2S2. These results recommend controlling irrigation salinity at ≤ 4 g/L and maintaining water content at 60%–75% of the water holding capacity to synergistically sustain soil health and emission reduction benefits.