248 



P. L. Gersper et al. 



TABLE 7-7 Rates of Nitrogen Immobilization (g N m ^ cm ' day V 

 in Soils During 1973, Estimated by Different Methods 



♦From Flint (unpubl.). 

 tFrom Benoit (unpubl.). 

 **From Laursen and Miller (unpubl.). 



estimated rate of 0.049 g m'^ cm"' day"'. The mineralization rates obtain- 

 ed in the phytotron experiments are close to those obtained in the field, 

 and both indicate that mineralization exceeds immobilization in the soil 

 during periods of rising temperature. 



The mineralization of phosphorus was not studied in the field. How- 

 ever, simulations of decomposition and of nitrogen and phosphorus re- 

 lease in the soils (Barkley et al. 1978) indicate that anaerobic conditions 

 stimulate the net mineralization of phosphorus. Although anoxic condi- 

 tions reduce the decomposition rate, the decrease in substrate breakdown 

 and gross phosphorus release is outweighed by the decrease in microbial 

 growth and phosphorus uptake per gram of organic matter decomposed. 

 Decreased microbial efficiency due to low temperatures will also increase 

 the release of phosphorus, as well as nitrogen, resulting from a given rate 

 of organic matter decomposition. However, Chapin et al. (1978) have 

 hypothesized that slow decomposition is the major limitation in the Bar- 

 row phosphorus cycle and that the mineralization rate is limited by the 

 microbial recovery rate following periodic population crashes. 



The ratio of mineralization to decomposition may be enhanced by 

 selective degradation of high-nutrient substrates. The constancy of C:N 

 ratios exhibited by the soils over a wide range of decomposition states 

 and depths indicates that high-nitrogen material is not being selectively 

 degraded. However, for phosphorus, C:Po ratios increase with depth, 

 and high C:P„ ratios are generally found in microtopographic units where 

 organic material shows the most advanced states of decomposition (Table 



