THE SEASONAL PATTERN OF NITROGEN AND CARBON MINERALIZATION IN FOREST AND PASTURE SOILS IN SOUTHERN ONTARIO

Abstract

Deficiency of N could be an important limitation to the establishment of sugar maple (Acer saccharum Marsh.) on former agricultural soils. Consequently a study of the seasonal pattern in N and C mineralization was conducted with calcareous soils from four similar segments of a land unit that supported maple woodlot, old field, red pine (Pinus resinosa Ait.), planted old field, and pasture, respectively. Net N mineralized was determined after incubation in situ; rate of C mineralization was measured in situ; and both net N and C mineralized were determined after incubation (perfusion) in the laboratory. All soils showed marked seasonal variation in both their content of exchangeable ionic N and the rate at which N was mineralized. The results from in situ and laboratory determinations were similar. Maxima in May-June and in September were separated by a minimum in July. Winter minima occurred in November and late April. The direction of seasonal change was little affected by short-term changes in temperature and soil moisture, and appeared to be explicable as a response to phenological changes in the vegetation. The rate at which C was mineralized in situ reached a maximum in late July and minima in May and September; it was closely correlated with soil temperature (P = 0.001). In the laboratory the rate reached a maximum in June, and a minimum in July, followed by a slow recovery thereafter until the end of November. Both in situ and in the laboratory, N was mineralized most rapidly from the maple woodlot soil. While there were large differences among soils in the rate at which N was mineralized, there were only small differences in the rate of C mineralization. Thus, per unit of C mineralized, the net amounts of N mineralized in the laboratory from pine stand, pasture, and old-field soils were only 0.29, 0.27, and 0.22, respectively, of the amount mineralized from maple woodlot soil; comparable figures from in situ measurement were 0.24, 0.59, and 0.30. These results indicate that a diminished rate of mineralization of N on former agricultural soils vis-à-vis that on uncleared forest soils is associated with little diminution in their level of biological activity. Forest trees grown on such soils can, therefore, be expected to suffer a degree of N deficiency.