NITROGEN MINERALIZATION ALONG AN ALTITUDINAL GRADIENT - INTERACTIONS OF SOIL-TEMPERATURE, MOISTURE, AND SUBSTRATE QUALITY

Net soil N mineralization was measured in situ for 2 consecutive years along a 2000-m altitudinal gradient encompassing six vegetation types in northern California. Both anaerobic and aerobic field incubations showed that soil temperature and moisture strongly controlled N release. In sealed container studies, N mineralization per unit of total Kjeldahl N (tkn) had a Q10 rate of 2 and varied between 5 and 38 g N/kg tkn when moisture was abundant (anaerobic incubation). However, aerobic mineralization rates fell to between 2 and 22 g N/kg tkn because of summer drought. Aerobic rates were greatest at mid-elevations (the mixed-conifer forest), and were reduced by cold temperatures at higher elevations and by soil drought at lower. The lowest rate found for any site was in a pine plantation scalped of topsoil a decade previously, during site preparation. Open-container incubations involving intact, 15-cm soil cores and ion-exchange resins produced mineralization rates correlated with potential growing days, but not with rates from sealed containers. The forest floor accounted for nearly all the net N mobilized on the coldest site, but only one-half to one-third of the N mobilized on warmer sites. Open containers have important advantages in assessing soil N dynamics. © 1990.