Role of pore structure in driving soil carbon accrual in contrasting land uses in tropical agroecosystems

Incorporating well-managed grasses into crop rotations is a key strategy to enhance soil carbon (C) sequestration in Brazilian agriculture, yet the mechanisms underlying physical C protection remain unclear. We combined X-ray computed microtomography (µCT) with bulk measurements of soil C, microbial biomass carbon (MBC), and CO₂ respiration to investigate interactions between pore architecture and particulate organic matter (POM) in structurally intact soils from a long-term experiment in the southern Amazon, Brazil. The systems evaluated were crop succession (soybean–maize), integrated crop–livestock (soybean–maize intercropped with palisade grass), and well-managed pasture under continuous palisade grass. Soil C contents were higher in pasture and integrated systems than in crop succession, while MBC was lowest in crop succession. Both grass-based systems exhibited greater µCT-based porosity and a higher proportion of medium-sized pores (70–150 μm radius), which are regarded as optimal microbial habitats. Spatial analyses revealed that pasture more effectively protected soil C from short-term mineralization, with greater distances to medium pores and to POM fragments likely associated with lower CO₂ emissions compared with the integrated system. These results demonstrate that soil C accrual in grass-based systems is governed not only by C inputs but also by the spatial organization of pore networks and POM, highlighting the critical role of pore structure in driving C accrual under well-managed grass systems.