Links between soil microstructure dynamics and carbon cycling in response to land use and climate change

Land-use systems differ in the balance between organic carbon inputs and microbial mineralization, affecting long-term soil carbon storage. Perennial grasslands maintain continuous root growth without tillage, promoting the accumulation of stable soil microstructure and biopores. In contrast, annual croplands experience fallow periods and periodic plowing, which disturb soil microstructure and accelerate the mineralization of physically protected carbon. However, the strength of soil microstructural regulation on carbon cycling and its responses to climate change remains unclear. Here, we studied five land-use types (two croplands and three grasslands) under ambient and future climate scenarios over five years, starting from the fifth year after establishment. The future climate scenario reflected regional projections of increased temperature and modified precipitation regimes. Using deep-learning-based X-ray CT image segmentation, we found that grasslands consistently contained higher volumes of biopores, particulate organic matter (POM), and decaying roots due to sustained root activity and turnover. Croplands exhibited a higher relative amount of fresh root in spring probably due to the rapid early-season growth of annual species, reduced microbial activity during fallow periods, and lack of year-round root inputs. A typical grassland microstructure fully developed in topsoil (5–10 cm) after 4–5 years. Land-use differences in deep soil (35–40 cm) remained small even after 10 years. Microbial biomass carbon and extractable organic carbon were consistently greater in grasslands, whereas total organic carbon diverged more slowly. The future climate scenario primarily influenced heterotrophic respiration and labile carbon pools through soil moisture, but did not significantly alter topsoil microstructure or carbon pools. POM volume, rather than pore structure, was the key driver of carbon mineralization, as the aeration of these microbial hotspots was not limiting. These findings highlight the potential of microstructure characteristics like root channel density and root degradation indicators to quantify long-term ecosystem development including carbon storage.