半干旱区深层土壤CO_(2)浓度对降雨事件的响应

Response of deep soil CO_(2) concentration to precipitation events in semi-arid areas

降雨是干旱半干旱地区土壤CO_(2)产生、传输或扩散的重要影响因素,并进一步影响土壤和大气中的CO_(2)浓度。目前大量研究集中在地表CO_(2)通量变化与降雨的关系,深层土壤有机碳储量巨大,但深层土壤CO_(2)浓度变化对降雨事件的响应机制尚不清楚。本研究通过对10 cm、50 cm和100 cm处土壤CO_(2)浓度进行原位连续监测,分析不同深度土壤CO_(2)浓度对降雨事件的响应过程及其影响因素。结果表明:试验期间,78%的降雨事件能迅速引起10 cm处土壤CO_(2)浓度发生改变,且随着降雨量增大,土壤CO_(2)浓度发生变化的深度逐渐增加。当降雨量在10~25 mm时,50 cm处土壤CO_(2)浓度在91 h后降低;降雨量>25 mm时,100 cm处土壤CO_(2)浓度在121 h后降低。当土壤由干变湿时,降雨量>25 mm的降雨事件促进10 cm处土壤CO_(2)浓度升高30%后开始降低,而50 cm和100 cm处土壤CO_(2)浓度随水分升高分别降低16.3%和10.9%。在半干旱区,当土壤含水量较低时,降雨可以对10 cm处土壤CO_(2)浓度变化产生短暂的正激发效应,而深层土壤含水量往往高于田间持水量,水分升高会导致该处土壤CO_(2)浓度降低。降雨对不同深度土壤CO_(2)浓度变化的影响存在差异,这在很大程度上取决于土壤含水量状况。

In arid and semi-arid areas,soil moisture strongly influences the balance between respiration and diffusion,altering soil CO_(2)concentration and surface flux.Numerous studies have focused on the relationship between surface soil CO_(2)flux changes and rainfall events.Subsoil carbon constitutes a large fraction of the total carbon stock,but it is unclear how rainfall events influence subsoil CO_(2)concentration dynamics.We continuously monitored CO_(2)concentrations at 10,50,and 100 cm in the soil profile from 2019 to 2021,and analyzed the various responses of subsoil CO_(2)concentration to rainfall events.In this study,soil temperature showed apparent seasonal characteristics.As the air temperature changed,the soil temperature of different depths also changed from 100 cm<50 cm<10 cm to 10 cm<50 cm<100 cm.The soil moisture content of different layers was in the order of 10 cm<100 cm<50 cm,and a significant fluctuation was found at 10 cm.The soil CO_(2)concentration gradually increased with the increase of the depth in the order of 10 cm<50 cm<100 cm,with mean values of 0.66×10^(4),0.87×10^(4),and 1.04×10^(4)μmol·mol^(-1),respectively.On sunny days,the soil CO_(2)concentrations at 10,50,and 100 cm showed apparent diurnal variations and could be expressed as a single-peak curve.However,rainfall events significantly affected the change trends of CO_(2)concentrations.Approximately 78% of the rainfall events quickly altered the soil CO_(2)concentration in 10 cm layer.When the rainfall amount was exceeded 25 mm,the CO_(2)concentration at50 and 100 cm decreased after 91 and 121 hours.When the soil moisture status changed from drying to wetting phases under rainfall events,>25 mm precipitation promoted an increase in soil CO_(2)concentration at 10 cm by 30% which then began to decrease.The soil CO_(2)concentrations at 50 and 100 cm decreased by 16.3% and 10.9%,respectively,with an increase in soil moisture.In arid and semi-arid areas,rainfall negatively affects the changes in soil CO_(2)concentration at 10 cm depth under lower soil moisture content.This is because the decrease in gas diffusivity led to an increase in CO_(2)concentration.Soil CO_(2)concentrations at 50 and 100 cm depths decreased under rainfall events,although the soil moisture was higher than the field capacity.This was caused by the high soil moisture content,which inhibited microbial respiration.The responses of soil CO_(2)concentration at different depths to rainfall differed and largely depended on the soil moisture content.