Improving soil carbon in semiarid agroecosystems: reclaimed water and mulch effects in cactus-sorghum intercropping

In semiarid regions, soil organic carbon (SOC) stocks and soil organic matter (SOM) pools are often low due to limited biomass input and inadequate management. This study evaluated SOC stocks and SOM fractions in a forage cactus–sorghum intercropping system irrigated with treated sewage water under diverse mulch in the northeastern Brazilian semiarid. The experiment followed a randomized split-plot block design with four replicates. Main plots included four irrigation levels (0, 80, 100, and 120 % of sorghum evapotranspiration (ETc)), and split plots comprised two mulch treatments: no mulch (NM) and mulch (WM) with 8 Mg ha −1 of sabi grass, spiny burrgrass, and goosegrass. Soil samples were collected at 0–0.10, 0.10–0.20, and 0.20–0.40 m depths in three sorghum cuts to determine labile SOM fractions: hot water-extractable C (HWEO-C), potassium permanganate-oxidizable C (POX-C), and particulate organic C (POC). In addition, SOC stocks and humic substances (HS), including humin (HU), fulvic acid (FA), and humic acid (HA), were determined at the end of the experiment. Intercropping system productivity was also evaluated. The highest SOC, POC, POX-C, and HWEO-C stocks occurred in 80WM and 100WM treatments, especially in HS, with HU as the dominant component. SOC in the HU fraction exceeded that in native vegetation soils, with threefold increases at 0–0.10 m and six-to sevenfold increases in deeper layers. Soils without irrigation, regardless of mulch, exhibited lower C storage, underscoring the importance of water management. Combining reclaimed water irrigation and mulching enhanced SOC accumulation, particularly in stable humic fractions, boosted carbon sequestration and crop productivity, and fostered sustainable, climate-resilient agriculture in semiarid tropical regions. • Reclaimed water increases soil carbon stocks in the semiarid region. • Mulch and reclaimed water synergistically increase C accumulation in SOM pools. • The combination of reclaimed water and mulch improves forage sorghum yield. • Irrigation at 80–100 % ETc boosted humic C stocks in tropical semiarid soils.