An estimation of microbial death rate and limitations of N or C during wheat straw decomposition

Abstract

The effect of initial microbial biomass size on subsequent straw decomposition was evaluated using labeled wheat straw ( Triticum aestivum L.) which had been leached to remove the readily-available C fraction. The leached, labeled straw was amended with 0.5,10 or 15% (w/w) unlabeled glucose to grow an unlabeled microbial biomass derived from readily-available C during incubation at 23° C. The glucose amendments had no effect on straw decomposition as measured by 14CO 2-C evolution, indicating that the microbial biomass derived from glucose was unable to utilize the nonsoluble portion of straw. Based on the evolution of unlabeled CO 2-C, the glucose-derived microbial biomass was estimated to have a half-life of 5 days with 55% of the C in dead microbial biomass forming new microbial material. The C and N pathways through two microbial pools are postulated to account for the lack of 14CO 2-C evolution in response to glucose amendment and for observed concurrent N immobilization and mineralization. The microbial biomass derived from the readily-available C (biomass pool A) rapidly consumes the available C and dies. Meanwhile, a different microbial biomass is growing on the less readily-available C components of straw (biomass pool B). Its growth is slower but is sustained for longer. The N mineralized from dead biomass pool A is immobilized into the biomass pool B resulting in net immobilization of N. This concept explains the commonly-observed concurrent mineralization and immobilization of N during periods of net immobilization. Evidence of two microbial pools, one C limited and one N limited, was supported by increased CO 2-C evolution following the addition of either glucose or kno 3 after 10 days to straw decomposing in the absence of added mineral N. Furthermore, the addition of glucose at 10 days did not influence the evolution of 14CO 2-C from 14C-labeled straw with or without the addition of N at the start of the experiment.