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 sub arctic/alpine dwarf shrub heath and in a high-altitude fellfield. Net miner alization was determined by using the buried-bag technique, which was combi ned with fumigation-extraction to recover microbial nutrients and estimate microbial nutrient immobilization. The measurements were carried out in unp erturbed plots and in plots, which had been subjected to elevated temperatu re and fertilizer addition for 5 years before the experiment started. Durin g the growing season, the microbes in the unperturbed plots immobilized the major part of the mineralized nutrients, which resulted in low net mineral ization. This occurred also during winter, when we assumed that nutrients s hould be released during microbial die-back. However, we found no evidence for a large microbial winter die-back and the surviving microbial biomass e ven immobilized extra nutrients. Soil temperature enhancement by ca. 2 degrees C in general increased, of te nded to increase, net mineralization. However, there was not necessarily an y strong correlation between net mineralization and temperature because tem perature-induced increase in gross mineralized nutrients could either lead to nutrient immobilization in the microbes or the nutrients could be releas ed 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 mine ralization, but a poor predictor of net mineralization. (C) 1999 Elsevier S cience B.V.