Long-term drought alters pore structure and biochemical characteristics in soils of short-grass steppe

Recent climatic changes have increased the unpredictability of rainfall events with a heightened probability of droughts, thus influencing the belowground carbon sequestration. Soil structure is shaped by physical, chemical, and biological processes and their interactions. Droughts are linked to the loss of soil structural stability, reduced pore-water connectivity, and organic carbon transport, therefore affecting soil microbial activity. The protection of carbon within the soil matrix is majorly driven by its accessibility to microbial decomposers and is also determined by the abundance of soil pores of a specific size range. In this study, we investigated the effects of drought on soil pore characteristics like pore size distribution, porosity, distances to pores, and biochemical properties such as microbial biomass carbon, ergosterol content, and soil organic carbon. The study site was a Long-term Ecological Research experiment of a short grass steppe ecosystem with treatments of 66% rain exclusion (regarded as drought) and control plots in a randomized complete block design. Dominant plant species include C4 grasses, blue grama (Bouteloua gracilis), buffalo grass (Buchloe dactyloides), and C3 plains prickly pear cactus (Opuntia polyacantha). This study aims to understand the importance of soil structure in interaction with organic matter and microbial activity. Intact soil cores of 5 cm in height by 5 cm in diameter were collected from 5-10 cm of soil depth to derive the soil pore characteristics using the X-ray computed microtomography technique (X-ray µCT, resolution of 18 µm). Bulk and intact soil samples were collected during the fifth year of the treatments in place.

The results demonstrated that drought differentially affected pores of different size ranges, substantially increasing volumes of > 60 µm diameter pores, and decreasing the volumes of 36-60 µm pores, while not affecting <18 µm pores. Drought decreased total volume, number of fragments, and fragment size of soil POM, and markedly decreased microbial biomass,  and enzyme activities. Furthermore, the bulk soil samples were analyzed for base chemical properties such as pH, cation exchange capacity, available phosphorus, exchangeable potassium, magnesium, and calcium We surmise that a 5-year drought in SGS prairie soils,  despite increasing the volume of medium-sized pores and pore connectivity, the lower microbial quotient (qMic), along with higher metabolic quotient (qCO₂) contributes to greater loss of C as CO₂ and slower C accumulation in the soil.