Microbial biomass in relation to C and N mineralization during laboratory incubations

Abstract

Net carbon and nitrogen mineralization rates were determined for an arable soil during 12 weeks at 37†C using an aerobic incubation-leaching technique. The amounts of mineralized C and N were compared to changes in the contents of C and N in microbial biomass (as determined by the chloroform fumigation incubation method; CFIM) during the incubation and to amounts of organic C and N in the leachates. Microorganisms were also followed by direct counting of bacteria, measurements of total hyphal lengths and fluorescein diacetate (FDA)-active hyphae, and by most probable number determinations of protozoa (naked amoebae and flagellates). Numbers of naked amoebae increased nearly 10-fold initially and then decreased between weeks 6 and 12. Bacterial numbers and FDA-active hyphae decreased during the incubation, and the relative composition changed slightly in favour of bacteria. Total hyphal lengths remained almost constant. A total of 105 μg N g' − soil dry wt and 1179 μg C g − soil dry wt was mineralized during the incubation, while the microbial N pool decreased by 42 γm − soil dry wt and the microbial C pool decreased by 225μ g − soil dry wt. Soluble organic matter in the leachates amounted to 16 and 31% of mineralized C and N, respectively. The possibility of measuring C mineralization with less frequent teachings and determinations of N mineralization offers an easy method for assessing changes in labile soil organic matter over time or for comparisons between soils. Through the use of appropriate C-to-N ratios, the N-content in the labile pool can be calculated.