Microbial C:N:P stoichiometry and turnover depend on nutrients availability in soil: A 14C, 15N and 33P triple labelling study

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Microbial biomass turnover and the associated recycling of carbon (Cmic), nitrogen (Nmic) and phosphorus (Pmic) depend on their stoichiometric relationships and plays a pivotal role for soil fertility. This study examines the responses of Cmic, Nmic, Pmic, the microbial respiration rate (CO2 efflux), and the total DNA content to C and nutrient addition in forest soils with very low (Low-P) and high P (High-P) contents. Both the Low-P and High-P soils were treated with a low and high level of C, N and P (5% and 200% of Cmic, Nmic and Pmic). Phosphorus (33P) was added before the addition of C (14C) and N (15N) to investigate the potential P limitation. We hypothesized two modes of microbial biomass C and nutrient turnover: 1) maintenance through intracellular metabolisms and/or 2) microbial growth and death through necromass reutilization. In Low-P soil, the 2-day-sooner increase of Cmic and Pmic compared to the increase of CO2 efflux and DNA content after high CN input showed the rapid initial uptake of C and limiting nutrients into microbial cells. It also demonstrated a lag period before microbial growth commenced. In High-P soil, however, the CO2 efflux and DNA content increased simultaneously with increases in microbial biomass, reflecting the microbial capacity for immediate growth. Afterwards, CO2 efflux and DNA content dropped to the level before CNP addition, with a decline of Cmic and Pmic in Low-P soil and a decline of Nmic in High-P soil, suggesting a C and P limitation in Low-P soil and N limitation in High-P soil. Under low CNP addition, the microorganisms in High-P soil are ready to grow, while those in Low-P soil are mainly in maintenance mode. The microorganisms under maintenance in low-P soil can switch to growth/death mode after removing the nutrient limitation. High CNP input caused a non-homeostatic response of Cmic: Nmic: Pmic stoichiometry from 691:105:1 to 33:1:1 in Low-P soil, mainly resulting from a higher storage of the limiting elements (C and P) in microbial biomass. The ratio remained stable under low CNP addition due to the endogenous metabolism of C and nutrient at maintenance. The C and nutrient were turnovered much faster by microorganisms in the growth/death mode, confirming a key principle of ecology: the stronger the limitation by an element, the more efficiently that element is retained within an organism, and the more intensively it is reused. The triple labeling approach linked with Cmic: Nmic: Pmic stoichiometry helped to identify the dominant maintenance and growth/death modes of microbial biomass CNP turnover in nutrient-limited and -unlimited soil.