Mineralization and microbial immobilization of N and P in arctic soils in relation to season, temperature and nutrient amendment

In situ summer, winter and annual net N and P mineralization and microbial immobilization of mineralized nutrients were measured in a low-altitude subarctic/alpine dwarf shrub heath and in a high-altitude fellfield. Net mineralization was determined by using the buried-bag technique, which was combined with fumigation-extraction to recover microbial nutrients and estimate microbial nutrient immobilization. The measurements were carried out in unperturbed plots and in plots, which had been subjected to elevated temperature and fertilizer addition for 5 years before the experiment started. During the growing season, the microbes in the unperturbed plots immobilized the major part of the mineralized nutrients, which resulted in low net mineralization. This occurred also during winter, when we assumed that nutrients should be released during microbial die-back. However, we found no evidence for a large microbial winter die-back and the surviving microbial biomass even immobilized extra nutrients. Soil temperature enhancement by ca. 2 degrees C in general increased, of tended to increase, net mineralization. However, there was not necessarily any strong correlation between net mineralization and temperature because temperature-induced increase in gross mineralized nutrients could either lead to nutrient immobilization in the microbes or the nutrients could be released to the soil inorganic pool. Fertilizer additions had no major effect on net nutrient mineralization or nutrient immobilization. However, the lignocellulose index, which has been used as a predictor of substrate quality and usually correlates negatively with decomposition rate, also appeared to be a good predictor of gross mineralization, but a poor predictor of net mineralization. (C) 1999 Elsevier Science B.V.