Mechanistic alterations in soil carbon oxide and nitrous oxide emissions caused by brackish water irrigation

Abstract

Utilizing brackish water for irrigation is a key strategy for mitigating water-use conflicts across industries and promoting sustainable agriculture. Understanding the emission patterns and underlying mechanisms of carbon dioxide (CO2) and nitrous oxide (N2O) under saline conditions is of great practical significance for developing optimized water and salt management strategies that promote water conservation and emission reduction. Therefore, we conducted an indoor incubation experiment using five different salinity gradients (0, 1.0, 3.0, 5.0, and 7.0 g/L) to analyze the effects of salinity gradients on soil microorganisms and related enzyme activities and explain the mechanisms of the impact on soil CO₂ emissions. Additionally, we performed an acetylene inhibition experiment to analyze the effects of salinity on soil N₂O production pathways (autotrophic nitrification, heterotrophic nitrification, and denitrification). Our findings indicated that higher salinity levels inhibited the growth of soil bacteria, fungi, and actinomycetes and reduced microbial carbon metabolism. This led to decreased activities of key soil enzymes, such as cellulase, sucrase, and α-glucosidase, thus suppressing soil CO2 emissions. Higher salinity levels were associated with increased soil N2O emissions. As salinity intensified, the contribution of autotrophic nitrification to N2O emissions decreased from 89.79 % to 64.95 %, whereas that of denitrification significantly increased from 8.61 % to 33.29 % during the 2nd to 4th days of cultivation. When the irrigation water salinity was lower than 3.0 g/L, its effect on the soil microbial community and enzyme activity was relatively minor and did not cause excessive soil CO2 and N2O emissions. These insights will be instrumental in shaping water and salt management practices that align with sustainable agriculture and greenhouse gas reduction goals.