Long-term saline water irrigation decreased soil organic carbon and inorganic carbon contents

Soil carbon is a key component of ecosystem functions and is crucial to global climate conservation and crop productivity. Saline water irrigation can maintain crop yield inmost world regions of freshwater shortage. However, saline water irrigation may also induce soil salt accumulation, which would result in the change of soil physical or chemical properties. Saline water irrigation’s effect on soil organic carbon (SOC) and soil inorganic carbon (SIC) contents is of little concern. In this study, we irrigated soil with 1 g L-1, 4 g L-1 and 8 g L-1 saline water in a winter-wheat and summer-maize rotation system. After 14 years of irrigation, we sampled soils in a winter wheat and summer maize rotation system, and analyzed soil water, soil salt, SOC, and SIC contents. The results showed that, compared with 1 g L-1 water irrigation, 8 g L-1 saline water irrigation significantly increased soil water and salt contents. Moreover, 8 g L-1 saline water irrigation significantly decreased SOC and SIC contents in the 0-20 cm soil layer (p < 0.05) and mainly decreased SOC content in >1 mm aggregates and wheat-derived SOC content in bulk soil. In comparison, 4 g L-1 saline water had no significant effect on soil water, soil salt, SOC, and SIC contents. These results indicated that a high concentration of saline water irrigation is harmful for soil carbon sequestration, while a low concentration of saline water did not affect soil carbon sequestration. Thus, using no more than 4 g L-1 saline water irrigation for 14 years can maintain soil carbon storage in the water shortage areas.

土壤碳是生态系统功能的核心组成部分，对全球气候保护与作物生产力至关重要。在全球多数淡水匮乏地区，咸水灌溉可维持作物产量。然而，咸水灌溉也可能引发土壤盐分累积，进而改变土壤的物理与化学性质。目前关于咸水灌溉对土壤有机碳（Soil Organic Carbon, SOC）和土壤无机碳（Soil Inorganic Carbon, SIC）含量影响的研究仍较为匮乏。本研究在冬小麦-夏玉米轮作系统中，分别设置1 g·L⁻¹、4 g·L⁻¹及8 g·L⁻¹三个浓度的咸水灌溉处理。经过14年的灌溉处理后，我们对该轮作系统的土壤进行采样，并测定了土壤水分、土壤盐分、SOC及SIC含量。结果表明，与1 g·L⁻¹清水灌溉相比，8 g·L⁻¹咸水灌溉显著提升了土壤水分与盐分含量。此外，8 g·L⁻¹咸水灌溉显著降低了0-20 cm土层的SOC与SIC含量（p < 0.05），且主要降低了>1 mm粒径团聚体中的SOC含量，以及整体土壤中小麦源SOC的含量。相较而言，4 g·L⁻¹咸水灌溉对土壤水分、盐分、SOC及SIC含量均无显著影响。上述结果表明，高浓度咸水灌溉不利于土壤碳固存，而低浓度咸水灌溉则不会影响土壤碳固存。因此，在淡水匮乏地区采用浓度不超过4 g·L⁻¹的咸水进行长达14年的灌溉，可维持土壤碳储量。