Heat Units for Describing Carbon Mineralization and Predicting Net Nitrogen Mineralization

Rate constants that describe C and N mineralization vary due to the changing complexity of the C compounds decomposed. Both economic and environmental concerns point to the importance of predicting net N mineralization from plant materials whose decomposition is initially characterized by net N immobilization. This study was conducted to examine the potential of the heat unit concept for describing C mineralization and for predicting the onset of net N mineralization from soil amended with an organic material. Rates of 0 and 59 g kg −1 papermill sludge were added to a soil and incubated at 5, 10, 15, 20, 25, and 30 °C at constant water content in the laboratory. Carbon mineralization rates varied with temperature and apparent substrate complexity, but a nearly identical amount of C (4.7 to 5.3 g kg −1 soil) was mineralized before the beginning of net N mineralization. The cumulative heat input resulting in that amount of evolved C was within the same range (600 to 684 degree days) for the 10 to 25 °C treatments. The number of degree days accumulated until commencement of net N mineralization was also similar for these four temperature treatments. Field microplots were established with 0 and 59 g kg −1 papermill sludge amendment rates. Commencement of net N mineralization in the field occurred at approximately 649 degree days, a value in close agreement with laboratory study results. These data suggest the heat unit concept may be useful for collectively describing mineralization of C fractions with contrasting decomposability, it may allow prediction of the onset of net N mineralization, and it may also serve as a method for linking laboratory and field studies of such biological processes as those studied here.