Decreasing microbial phosphorus limitation increases soil carbon release

Phosphorus (P) limitation to microorganisms is increasingly recognized in soils, but how the limitation mediates the metabolic processes of microbes driving soil carbon (C) release remains unclear. Here, we performed a 60-day incubation experiment adding two 13C-labeled organic C sources (glucose and straw) at five inorganic P addition levels in loess with low available P from the Loess Plateau, China. The nutrient limitations of microbes were quantified by enzymatic vector analysis, associated with soil respiration, microbial metabolic quotient (qCO2), C use efficiency (CUE) and priming effect (PE) at both early (10 days) and late (60 days) stages of incubation. Results showed that reducing microbial P limitation increased CO2 release from soils by 19–26% and from labeled glucose and straw by 12% and 29%, respectively. This indicated that soil P limitation overall constrains rather than promotes microbial C metabolism. A negative relationship between relative C and P limitations at the first 10-day incubation further indicated that added P (decreased P-acquiring enzyme activities) stimulated microbial C metabolism (increased C-acquiring enzyme activities) under enough C source. Whereas a positive relationship at 60-day incubation suggested that high microbial heterotrophic respiration under high P addition alleviate their C limitation. Furthermore, both multiple regression and partial least squares path models indicated that an increase in CO2 release with P and C additions at early incubation was due to two processes, i.e., increasing available P promoted decomposition of native soil organic C due to PE as well as decay of added organic C by increasing qCO2 and decreasing CUE. At late incubation, however, P addition increasing decomposition of native soil C via PE is the dominated control on CO2 release under C limitation. We conclude that microbes are dominant by maintenance rather than growth metabolism in loess with low phosphorus availability, whereas the pathways of the metabolism driving C release depend on soil C availability. Our findings suggest that microbial P limitation has considerable positive effects on soil C sequestration in these ecosystems with low soil P availability.