The Soils and Their Nutrients 247 



tively efficient when evaluated in vitro. Net mineralization might also be 

 due to the microbial utilization of specific fractions of the organic matter 

 containing above-average nutrient concentrations. The additional possi- 

 bility is that tundra microorganisms produce biomass with concentra- 

 tions of organic nitrogen and phosphorus below those found in temper- 

 ate organisms. Some indication of this is given by the nitrogen levels of 

 fungal hyphae, which were around 2% (Laursen 1975), considerably 

 lower than the average levels of 5 to 6% reported from temperate regions 

 (Cochrane 1958). 



The efficiency of fungi (grams of fungal biomass produced per gram 

 substrate degraded) is generally higher than that of bacteria (Alexander 

 1961), and the efficiency of bacteria decreases markedly under anaerobic 

 conditions (Hattori 1973). Therefore, the anaerobic conditions that exist 

 in the soils (Figure 7-6) may enhance mineralization by excluding fungi 

 and decreasing bacterial efficiency. Low temperatures may also decrease 

 microbial efficiency, since the microflora includes species which continue 

 respiration, and therefore substrate degradation, at temperatures below 

 the minimum for growth. Thus the environmental conditions of the soils 

 lead to decomposer populations with average efficiencies lower than 

 those of the same species in better drained and warmer soils. 



Field and laboratory studies of nitrogen transformations support 

 these conclusions. Maximum rates of immobilization are expected to oc- 

 cur during the early growing season, since overwintering, standing dead 

 vegetation and fresh litter with C:N ratios from 30:1 to 60:1 (Flanagan 

 and Veum 1974) have been incorporated into the soil surface, moisture 

 from melting snow is plentiful, and temperatures are rising. Early season 

 rates of nitrogen immobilization have been calculated from changes in 

 the pools of available nitrogen and microbial biomass (Table 7-7). All 

 these methods are indirect but agreement between them is reasonably 

 good. The maximum rate observed was consistently around 0.025 g N 

 m"^ cm"' day"' for the three organic horizons studied: moss, hemic and 

 buried sapric. 



Net rates of nitrogen mineralization in the field have been estimated 

 by observing changes in size of nitrogen pools (Flint and Gersper 1974), 

 in particular the buried sapric horizon of a wet meadow soil at 14 to 20 

 cm depth. Just after thaw there was a sudden increase in exchangeable 

 ammonium in the horizon, which was interpreted as net mineralization. 

 The computed rate, corrected for diffusion, is 0.077 g N m'^ cm"' day"'. 

 Conditions were probably somewhat anoxic during the measurement pe- 

 riod. Moreover, the horizon was cold, about 1 °C after thaw, indicating 

 that nitrogen mineralization can occur at significant rates at very low 

 temperatures. Phytotron experiments under anaerobic conditions 

 showed a maximum mineralization rate of 0.075 g N m"^ cm"' day"' for a 

 hemic horizon at 6°C, while the buried sapric horizon at 2°C gives an 



