Intercropping systems combined with reclaimed wastewater and biomass-C inputs enhance soil macroaggregation and carbon persistence in a semiarid agroecosystem of the Brazilian Tropics

Restoring soil structure and organic carbon (C) pools in semiarid environments requires management strategies that balance productivity and water-use efficiency. This study examined how intercropping forage cactus ( Opuntia stricta ) with Sudan grass ( Sorghum sudanense ) or buffel grass ( Cenchrus ciliaris L.), combined with mixed grass mulch under treated wastewater irrigation, affected soil aggregation and C dynamics. Six treatments were tested (CS0, CS8, CS12, CB0, CB8, and CB12), representing two intercropping systems, CS (cactus-Sudan grass) and CB (cactus-buffel grass), with three application rates of mixed grass mulch (0, 8, and 12 Mg ha −1 , corresponding to 0, 3.5, and 5.3 Mg C ha −1 ). Evaluations included aggregate mass proportions (AM), the aggregate stability index (ASI), carbon preservation capacity (CPC), total and easily extractable glomalin-related soil proteins, and stocks of total organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) within macroaggregates, along with the biomass productivity of the intercropping systems. Across the 0–40 cm profile, CB12 showed the strongest structural response, increasing the proportion of extra-large macroaggregates by 84% relative to CB0 and reaching an ASI of 41.8% and a CPC of 5.2 g kg −1 . Within extra-large macroaggregates, CB12 also showed the highest SOC (~77 Mg ha −1 ) and MAOC (~72 Mg ha −1 ) stocks, while its total POC stock across macroaggregate classes reached ~5.0 Mg ha −1 . CS12 and CB12 increased total glomalin-related soil protein contents, while CB12 also increased easily extractable glomalin-related soil protein relative to the corresponding no-mulch treatment. Soil under CS12 produced 194 Mg ha −1 of fresh forage cactus biomass and 17 Mg ha −1 of cactus dry matter. Under treated wastewater irrigation, higher mixed grass mulch inputs enhanced soil aggregation and C retention in cactus-grass intercropping systems, accompanied by greater accumulation of glomalin-related soil proteins and physical protection of SOC within macroaggregates.