Soil respiration and microbial biomass in multiple drying and rewetting cycles - Effect of glucose addition

Previous studies showed that the respiration flush upon rapid rewetting of dry soil decreases with increasing number of drying and rewetting (DRW) cycles which is thought to be due to various factors, for example, lower substrate availability or microbial biomass or changes in microbial community structure. To investigate if the reduction of respiration flush with increasing cycle number is due to lack of available substrate upon rewetting, incubation experiments with four DRW cycles were conducted where glucose solutions of different concentrations, low (50 μg C g− 1 soil), medium (250 μg C g− 1 soil) and high (500 μg C g− 1 soil) were added either in all four cycles or only in cycle 1, 2, 3 or 4 (the other cycles rewetted with water). Additional treatments included a constantly moist control (CM, 70% of maximum water-holding capacity) and DRW treatments with only RO water. When dry soil was rewetted only with RO water, the respiration flush upon rewetting was about twice as high in the first two cycles than in cycles 3 and 4. Soil microbial biomass C (MBC) one day after rewetting remained stable until cycle 3 after which it decreased by about 20%. In all glucose treatments, the glucose-induced increase in respiration rate was greater in earlier than later cycles, irrespective of whether glucose was added to one cycle only or in all cycles. But when expressed relative to CM, the respiration rate was greater in the last two cycles. Single glucose addition increased MBC only in the cycle in which it was added whereas repeated glucose addition increased MBC over time. Glucose addition and DRW increased the metabolic quotient, thus reduced C use efficiency. In treatments with glucose added in all four cycles, the metabolic quotient increased with DRW cycle number which indicates lower efficiency in later cycles. It is concluded that lower substrate availability can be one of the reasons for the decrease in respiration flush upon rewetting with cycle number.