Soil pore-size distributions and selected biochemical properties in short-grass steppe under long-term drought

Recent climatic changes have increased the unpredictability of rainfall events with a heightened probability of droughts. Impact of long-term droughts can be especially devastating in dry environments, such as short-grass steppe (SGS) prairies, common in North America's central and southern Great Plains. The goal of the study is to assess the impact of a continuous 5-year drought on soil pore structure, particulate organic matter (POM), and microbial activity in an SGS ecosystem. We collected soil samples (5-10 cm depth) from replicated field plots, which either received ambient precipitation or were subjected to 66 % rain exclusion (referred to hereafter as drought). We used X-ray micro-computed tomography (X-ray μCT) to analyze soil pore and POM characteristics, and measured biochemical properties, such as microbial biomass C, ergosterol content, activities of hydrolytic enzymes, and soil organic C. The results demonstrated that drought differentially affected pores of different size ranges, substantially increasing volumes of >120 μm diameter (Ø) pores, numerically decreasing the volumes of 36-60 μm Ø pores, while not affecting <18 μm Ø pores. Drought decreased the total volume and fragment size of soil POM. The trends of lower microbial quotient (qMic) and higher metabolic quotient (qCO₂) in drought soil suggest a reduction in microbial community size and changes in microbial functioning, e.g., devoting more energy to maintenance needs rather than growth. In addition to water stress, drought also reduced plant inputs and POM, and increased air-filled pore spaces that probably further amplified the stress to the microbial community.