Vegetation–soil–microbial system dynamics during artificial alfalfa ( Medicago sativa L.) grassland degradation in a loess hilly region

Abstract

Alfalfa ( Medicago sativa L.) establishment is an effective strategy of managing desertification in arid regions; however, the course of artificial alfalfa grassland degradation remains poorly understood. Therefore, we investigated the dynamics of vegetation characteristics, soil edaphic factors, and rhizosphere microbial community structure in the course of artificial alfalfa grassland degradation. A space–for–time substitution approach was used to select nine alfalfa stands with different ages (1–50 years old) in the loess hilly region of northwest China. According to the plant diversity of vegetation and important value of alfalfa, the course of grassland degradation could be divided into three stages, artificial grassland (1–10 years), transitional grassland (10–30 years), and natural grassland (>30 years). With an increase in stand age, alfalfa productivity first increased, up to a maximum in the 7-year-old stand, and then decreased. Alfalfa was replaced as the dominant species by Stipa bungeana in the 50-year-old stand. Soil bulk density, total organic carbon, and major nutrient contents were the highest in the artificial grassland. Soil enzyme activity and the relative abundances of potentially beneficial microorganisms (e.g., Mortierella and Glomus) peaked in the transitional grassland. Soil water content and total porosity reached the maximum levels in the natural grassland. The species diversity indices of bacterial and fungal communities first increased and then decreased over time. Both microbial abundance and species diversity in the 0–20-cm soil layer were higher than in the 20–40-cm soil layer. Soil pH and catalase activity predominantly influenced vegetation characteristics, while total and available phosphorus contents were the major edaphic factors shaping rhizosphere microbial community structure. The results indicated that alfalfa establishment altered soil structure considerably, and improved soil fertility in the artificial grassland over the short term. Consequently, soil enzyme activity, microbial diversity, and potentially beneficial microorganisms in the rhizosphere increased during the transitional stage. Following considerable shifts in the soil environmental conditions, alfalfa was no longer the only dominant species and was eventually replaced by S. bungeana, leading to the establishment of a stable natural grassland system.