Increased nitrogen availability alters soil carbon quality by regulating microbial r-K growth strategy, metabolic efficiency, and biomass in degraded temperate grasslands

Experimental evidence is limited for the role of soil microbial r-K growth strategy, metabolic efficiency, and biomass in regulating soil carbon (C) quality under the atmospheric nitrogen (N) deposition scenario, especially in degraded grasslands. Here, soil samples were collected from four sites with non, moderately, severely, and extremely degraded grasslands in Inner Mongolia, China. The soil was incubated for 200 days with N addition treatment. Soil CO2-C release rates were measured to fit a three-pool model for describing soil C quality by the pool sizes of the fast, slow, and passive C fractions. Soil microbial r-K growth strategy, metabolic efficiency, biomass, and major abiotic factors were measured. In non-degraded grasslands, N addition significantly increased the fast C pool size but decreased the slow C pool size primarily through increases in microbial biomass N. However, in all three degraded grasslands, a consistent pattern was observed among different C pool sizes with N addition through changes in microbial r-K growth strategy, metabolic efficiency, and biomass: the fast C pool size remained relatively stable because of the contrasting effects of increases in r-strategists and enhanced metabolic efficiency; the slow C pool size was increased by the combined positive impacts of microbial metabolic efficiency and microbial biomass N and the passive C pool size decreased unexpectedly, although the metabolic efficiency, microbial biomass N, and r-strategists increased. These findings highlight the important role of changes in microbial r-K growth strategy, metabolic efficiency, and biomass in regulating soil C quality in degraded temperate grasslands.