IN-SITU MINERALIZATION OF NITROGEN AND PHOSPHORUS OF ARCTIC SOILS AFTER PERTURBATIONS SIMULATING CLIMATE-CHANGE

Seasonal net nitrogen (N) and phosphorus (P) mineralization was investigated at Abisko, Swedish Lapland in soils of a subarctic heath and in soils of a colder (by about 4-degrees-C), high altitude fellfield by (a) using in situ soil incubation in soils which had been shaded or subjected to two levels of increased temperature, combined with (b) reciprocal transplantation of soils between the two sites. Proportionally large and significant net seasonal mineralization of N, in contrast to non-significant P mineralization, was found in untransplanted and transplanted fellfield soil. In contrast, P was mineralized in proportionally large amounts, in contrast to low N mineralization, in the transplanted and untransplanted heath soil. The differences indicate that P was strongly immobilized in relation to N at the fellfield and that N was more strongly immobilized than P in the heath soil. The immobilization in both soils remained high even after a temperature change of 4-5-degrees-C experienced by transplanted soils. Air temperature increases of up to 4-5-degrees-C in greenhouses resulted in a soil temperature increase of 1-2-degrees-C and did not cause any extra increase of net N and P mineralization. The results suggest that soil temperature increases of up to 2-degrees-C, which are likely to occur by the end of the next century as an effect of a predicted 4-5-degrees-C rise in air temperature, have only small effects on net mineralization in at least two characteristic tundra soils. These effects are probably smaller than the natural fluctuation of plant available nutrients from site to site, even within the same plant community. A further soil temperature increase of up to 4-5-degrees-C may enhance decomposition and gross mineralization, but the rate of net mineralization, and hence the change of nutrient availability to the plants, depends on the extent of microbial immobilization of the extra nutrients released.