Integrated and managed grasses shape soil pore networks and carbon processes in tropical agroecosystems: Insights from synchrotron X-ray tomography

Integrating well-managed grass into agricultural systems is a management target for enhancing soil carbon (C) storage in tropical agroecosystems. Yet, the mechanisms behind physical protection of soil C are not sufficiently lucid. Here we analyzed pore structure and particulate organic matter (POM) characteristics in structurally intact soil using synchrotron X-ray computed micro-tomography (μCT). We combined these analyses with bulk measurements and CO₂ respiration data to explore the interactions between pore structure and soil C characteristics in a mid-term experiment in the Southern Amazon, Brazil. The farming systems were: (i) crop succession (CS) with annual production of soybeans followed by corn; (ii) integrated crop-livestock (ICL) with soybeans and then corn intercropped with palisade grass; and (iii) well-managed pasture (MP) with continuous monoculture of palisade grass. Soils of ICL and MP exhibited higher soil C (18–27 %) and N (27–29 %) contents, along with increased microbial biomass C (25–45 %) compared to CS. Additionally, ICL and MP showed higher μCT-based porosity (26–30 %) than CS and a greater volume of pores in the 70–150 μm range, which are regarded as optimal microbial habitats. Image-determined POM fractions in ICL and MP were 71 % and 51 % higher compared to CS. The spatial distribution patterns of soil pores and POM influenced the magnitude of soil C decomposition. Greater distances to medium pores and to POM fragments in MP are likely associated with lower soil C losses via CO₂ emission, suggesting more effective soil C protection. We surmise that the intricacies of pore networks and their association with the spatial distribution of POM dictate C accrual in soils with greater presence of well-managed grass, thus providing the basis for target-oriented development of management strategies to rebuild soil C in Brazilian agriculture.