MICROBIAL C AND N DYNAMICS DURING MESOPHILIC AND THERMOPHILIC INCUBATIONS OF RYEGRASS

Laboratory studies were conducted to determine C and N dynamics during the decomposition of ryegrass straw under mesophilic and thermophilic conditions. A K-C of 0.61 was developed for the chloroform-fumigation extraction method to estimate microbial biomass C. These estimates sh owed that the C and N requirement of the thermophilic biomass was appr oximately 50% of the mesophilic biomass. There was no relationship bet ween chloroform-fumigation microbial biomass estimates and plating of microorganisms from straw on specific media. Mineralized C was measure d as 185 and 210 g kg(-1) straw in the 25 degrees C and 50 degrees C t reatments, respectively. The efficiency of microbial substrate use, on a total straw basis, was 34 and 28% in the 25 degrees C and 50 degree s C incubations, respectively. The level of soluble C declined more sl owly than total C mineralization at both temperatures, indicating that a portion of the labile C was not readily biodegradable, The addition of N decreased the rate of C mineralization at both temperatures. The reduced N requirement of the thermophiles explains why rapid degradat ion of the high C:N residue occurred without additional N or the need for the addition of a low C:N ratio substrate. Additional inoculum did not affect the decomposition process. We conclude that the promotion of thermophilic biomass activities, through composting for example, ma y prove useful in upgrading agricultural wastes for introduction into sustainable cropping systems.