Labile nitrogen, carbon, and phosphorus pools and nitrogen mineralization and immobilization rates at low temperatures in seasonally snow-covered soils

Surface mineral horizons from four ecosystems sampled in the northwestern Italian Alps were incubated at -3 and +3°C to simulate subnivial and early thaw period temperatures for a seasonally snow-covered area. The soil profiles at these sites represent extreme examples of management, grazed meadow (site M) and extensive grazing beneath larch (site L) or naturally disturbed by avalanche and colonized by alder (site A) and the expected forest climax vegetation beneath fir (site F). Changes in labile pools of nitrogen (N) and phosphorus (P) were active at all sites at both temperatures during 14 days of laboratory incubation. Ammonium was the dominant inorganic form of total dissolved N (TDN), being equivalent to 1.8-9.8 g N m-2 within the mineral horizon. Gross rates of ammonification were similar at the two temperatures but significantly (p<0.05) greater in soil from beneath fir than in the other three. Nitrification occurred in all soils and displayed a wide range in rates, from 2 to 85 mg N m-2 day-1, and was least in the two most acid soils, A and F. Immobilization of NH4+ as microbial N was greater in the fir soil than in the other three. Also, the fir soil showed greatest gross ammonification and least accumulation of NO3- and greatest tendency to retain N. This high N retention capacity in the climax ecosystem contrasted with the managed systems characterized by higher nitrification rates and greater potential spring NO3- loss. Dissolved organic N ranged between 30 and 50% of the TDN, while dissolved organic P was greater than 70% of total dissolved P (TDP). The dissolved organic compounds were important components of the labile pool, in equilibrium with a large reserve of organic N, and may significantly contribute to the soil N availability at low temperatures.