Long‐Term Effects of Land Use Conversion on Soil Microbial Biomass and Stoichiometric Indices in Eastern Mediterranean Karst Ecosystems (1981–2018)

ABSTRACT The conversion of forests to agricultural land, primarily driven by human activities, is a key factor in land degradation. Although the impacts of land degradation on ecosystem functions are well documented, the relationship between land degradation and carbon and nitrogen content, as well as certain biochemical indices, has not been thoroughly investigated. To fill this knowledge gap, this study examined the responses of microbial biomass, microbial respiration, and activity to soil organic carbon, soil depth, and land use/land cover change (LULCC), from forest to agriculture. The research was conducted in three different land use types (Forest = F, Agriculture = A, and Converted = C) with two soil depths (0–30 and 30–60 cm) under uniform climatic and topographical conditions in the Northeastern Mediterranean Karst ecosystems along the southern coast of Turkey. Analysis of a total of 180 soil samples showed that there are significant differences in the chemical and biochemical properties of soils according to land use type and soil depth. In particular, soil organic carbon, total nitrogen, microbial carbon, microbial nitrogen, and microbial respiration were higher in the topsoil (0–30 cm depth) of the forest areas. Agricultural areas, in contrast, exhibit significantly lower levels of SOC (10%), TN (10%), C mic (26%), N mic (17%), and MR (6%) compared to converted areas, placing them in the same group based on Tukey's HSD test. Chemical and biochemical soil properties and stoichiometric indices in the study areas varied significantly with soil depth and exhibited a decreasing pattern with depth, except in the case of CaCO 3 (%), C mic /N mic , and q CO 2 . Our findings also reveal that stoichiometric indices, such as C mic /N mic and q Mic, are highly sensitive indicators of land‐use change. These findings indicate that human‐induced land degradation negatively impacts carbon and nitrogen storage as well as sensitive microbial indicators in the topsoil.