The proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

• Published in: Soil biology & biochemistry 2005-03, Vol.37 (3), p.507-515
• Main Authors: Wu, J., Brookes, P.C.
• Format: Article
• Language: English
• Subjects: adenosine triphosphate; Agronomy. Soil science and plant productions; air drying; ATP; Biochemistry and biology; Biological and medical sciences; biomass; C mineralisation; carbon; carbon dioxide; Chemical, physicochemical, biochemical and biological properties; clay loam soils; equations; Fundamental and applied biological sciences. Psychology; gas production (biological); grassland soils; isotope labeling; Microbial biomass C; Microbiology; mineralization; Organic matter; Physics, chemistry, biochemistry and biology of agricultural and forest soils; silt loam soils; Soil air-drying and rewetting; soil microorganisms; Soil organic carbon; soil organic matter; Soil science; soil water content
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2004.07.043

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO 2-C flushes that resulted. To do this, a UK grassland soil was amended with 14C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO 2-C evolved (varying from 83 to 240 μg g −1 soil), compared to the control with, overall, less CO 2-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and 14C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g −1 biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO 2-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO 2-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.