Consequences of natural vegetation conversion to cropland in semiarid regions: Evidence from soil multifunctionality...

The effects of converting primary land cover for agricultural use on litter composition and soil quality indicators remain insufficiently explored in semiarid regions. Shedding light onto this matter is therefore vital for optimizing soil performance and ecosystem services in these types of regions. To address this, the present study examined a wide range of litter and soil physical, chemical, and biological properties in natural sites (a forest and a rangeland) and in the respective agricultural lands (F-A and R-A, respectively) in northern Iran. In each site, three 1-ha plots were selected and, in each plot, four composite samples of litter and soil, were collected during the summer season. Soil sampling was also conducted in the autumn season to study the seasonal variation of soil microclimate and biological features. Our results showed that litter accumulation in the natural forest and rangeland sites was about 1.6 times greater than in the F-A and R-A sites. This greater accumulation was associated with improvements in soil physical properties (including lower bulk density, higher porosity, and greater stability), along with a 1.5-2-fold increase in the contents of essential nutrients (N, P, K, Ca, and Mg) in the primary sites. Soil microbial biomass carbon and nitrogen were 13-21% and 29-37% lower, respectively, in the agricultural area compared to the natural forest and rangeland sites. Furthermore, the conversion of the surveyed forest and rangeland to agricultural land (F-A and R-A) led to a significant decrease in soil enzyme activities (i.e., urease, acid phosphatase, and arylsulfatase) by approximately 51 %, 55%, and 17%. Land conversion also resulted in a reduction in fine root biomass by 30% and 40%, as well as in coarse root biomass by 64% and 84%, respectively. Likewise, the population of soil organisms decreased significantly, between 30 and 80%, together with the microbial abundance in the agricultural site with a history of rangeland being about 60% lower than in the natural forest. Moreover, most of the targeted soil faunal groups were affected by the season, registering a reduction in their densities during the summer compared to the autumn season. Overall, the higher input of better-quality organic matter in the forest and rangeland areas appears to be a key factor in the improved soil physical traits and fertility indicators in these natural habitats. These features are expected to boost soil biological indicators including microbial activity and biomass, enzyme activities, and the overall density of soil fauna. Conversely, a noticeable decline across all these indicators was observed in agricultural lands. This study highlights the importance of preserving primary vegetation to protect soil quality and biodiversity; as well as, the use of adapted trees and shrub species for potential restoration programs to conserve soil resources.