Assessing the Impact of Irrigation and Crop Type on Soil Respiration in Agricultural Soils

Identifying the main drivers of soil CO2 emissions in tropical agroecosystems is essential for balancing productivity and climate mitigation. This study evaluated the effects of crop type, irrigation, phenological stage, fertilization, soil cover condition, and season on total soil respiration in a humid marshland system in Rwanda using a two-season field experiment. Five crops (maize, soybean, common bean, Irish potato, and Brachiaria) were grown under irrigated and rainfed conditions, and total soil CO2 emissions were measured across 19 sampling campaigns in both crop-covered and adjacent non-vegetated conditions in all plots using the closed static chamber method. Crop type and growth stage were the dominant drivers of soil CO2 emissions (p < 0.001), while irrigation had no significant direct effect despite increasing yields (p < 0.001). As a result, irrigation reduced yield-scaled CO2 emissions for beans and Irish potato (p < 0.05). Brachiaria showed higher emissions, particularly during the development stage, but its high biomass led to lower emissions per unit yield. Fertilization significantly increased total soil respiration (p < 0.001), and emissions were higher under crop-covered soil than non-vegetated soil conditions (p < 0.001). Season did not significantly affect soil CO2 emissions (p = 0.123), and similar emission patterns were observed across the two cropping seasons. Because the measurements represented total soil respiration, the observed differences reflect the combined contributions of autotrophic (root) and heterotrophic (microbial) respiration and do not distinguish between these individual components. These findings indicate that crop traits, plant developmental stage, vegetation cover, and nutrient inputs are the primary factors associated with variation in total soil CO2 emissions under moisture-sufficient tropical conditions and highlight the importance of biological drivers in regulating carbon dynamics in marshland agroecosystems.