Microbial biomass C,N and P in two arctic soils and responses to addition of NPK fertilizer and sugar: implications for plant nutrient uptake

The soil microbial carbon (C), nitrogen (N) and phosphorus (P) pools were quantified in the organic horizon of soils from an arctic/alpine low-altitude heath and a high-altitude fellfield by the fumigation-extraction method before and after factorial addition of sugar, NPK fertilizer and benomyl, a fungicide. In unamended soil, microbial C, N and P made up 3.3–3.6%, 6.1–7.3% and 34.7% of the total soil C, N and P content, respectively. The inorganic extractable N pool was below 0.1% and the inorganic extractable P content slightly less than 1% of the total soil pool sizes. Benomyl addition in spring and summer did not affect microbial C or nutrient content analysed in the autumn. Sugar amendments increased microbial C by 15 and 37% in the two soils, respectively, but did not affect the microbial nutrient content, whereas inorganic N and P either declined significantly or tended to decline. The increased microbial C indicates that the microbial biomass also increased but without a proportional enhancement of N and P uptake. NPK addition did not affect the amount of microbial C but almost doubled the microbial N pool and more than doubled the P pool. A separate study has shown that CO₂ evolution increased by more than 50% after sugar amendment and by about 30% after NPK and NK additions to one of the soils. Hence, the microbial biomass did not increase in response to NPK addition, but the microbes immobilized large amounts of the added nutrients and, judging by the increased CO₂ evolution, their activity increased. We conclude: (1) that microbial biomass production in these soils is stimulated by labile carbon and that the microbial activity is stimulated by both labile C and by nutrients (N); (2) that the microbial biomass is a strong sink for nutrients and that the microbial community probably can withdraw substantial amounts of nutrients from the inorganic, plant-available pool, at least periodically; (3) that temporary declines in microbial populations are likely to release a flush of inorganic nutrients to the soil, particularly P of which the microbial biomass contained more than one third of the total soil pool; and (4) that the mobilization-immobilization cycles of nutrients coupled to the population dynamics of soil organisms can be a significant regulating factor for the nutrient supply to the primary producers, which are usually strongly nutrient-limited in arctic ecosystems.