[Soil Microbial Nutrient Limitation Characteristics During Anthropogenic Transition from Desert Steppe to Shrubland]

The practice of planting xerophytic shrubs for vegetation restoration in arid and semi-arid grasslands has become a common model. In fact, it is accelerating the process of regional grassland shrub encroachment. At present, studies on the ecological consequences of anthropogenic shrub encroachment on grassland ecosystems have been gradually carried out. However, the changes in soil microbial nutrient utilization and limitation during the anthropogenic transition from desert grasslands to shrublands have not been thoroughly explored. Therefore, a representative grassland-shrubland mosaic formed by nearly 40 years of shrub introduction in the desert steppe of eastern Ningxia was selected as the research object. Studies on the stoichiometry of soil-microbe-extracellular enzymes, microbial nutrient limitation, and homeostatic characteristics were conducted at both the community scale (desert grassland, grassland edge, shrubland edge, and shrubland) and the microhabitat scale (plant patches, bare patches, and root zones) during the transition from desert grassland to shrubland. The results showed that during the transition from desert grassland to shrubland, at the community scale, the soil carbon to nitrogen ratio (C∶N), soil carbon to phosphorus ratio (C∶P), microbial biomass nitrogen to phosphorus ratio (MBN∶MBP), extracellular enzyme carbon to nitrogen ratio (EEAC∶EEAN), and microbial nitrogen use efficiency (NUE) decreased significantly. In contrast, the microbial biomass carbon to nitrogen ratio (MBC∶MBN), extracellular enzyme carbon to phosphorus ratio (EEAC∶EEAP), extracellular enzyme nitrogen to phosphorus ratio (EEAN∶EEAP), and microbial carbon use efficiency (CUE) increased significantly. Although the above-mentioned indicators varied greatly under different microhabitats, there was no obvious "rhizosphere effect" or "fertile island effect," and they did not show regular changes with the distance from the shrub root zone. The enzyme stoichiometry vector analysis model indicated that after the transition from desert grassland to shrubland, the microbial nutrient limitation shifted from phosphorus limitation to nitrogen limitation, and the relative microbial carbon limitation also increased. However, the elemental homeostatic characteristics changed from strict homeostatic state in desert grassland to weak plasticity in shrubland. The results of redundancy analysis showed that soil microbial biomass carbon, soil C∶N ratio, and soil C∶P ratio could serve as important indicators for predicting microbial nutrient limitation. These findings suggest that the transition from desert grasslands to shrublands has altered the carbon, nitrogen, and phosphorus stoichiometry of soil-microbe-extracellular enzymes, as well as microbial nutrient limitation and homeostatic characteristics. Considering the long-term trends of soil drying and degradation after shrub introduction, microorganisms may further affect the positive feedback relationship among shrubs/herbaceous plants, soil, and microorganisms, intensifying the competitive advantage of shrubs. This may bring complex and unpredictable consequences to the native vegetation and soil of desert grasslands. Therefore, the approach of shrub introduction for vegetation restoration should be treated with caution, and nature-based solutions may be the preferred option for vegetation restoration in arid and semi-arid regions.