Drying and rewetting frequency influences cumulative respiration and its distribution over time in two soils with contrasting management

Understanding the factors determining cumulative respiration upon rewetting of dry soil is critical for predicting C efflux from soils. The response of respiration to drying and rewetting may be influenced by land management due to its effect on the soil organic C pool and differ between soil size fractions. An incubation experiment was conducted with soils collected from two plots with a long history of different management (wheat-fallow and permanent pasture). The soils were sieved to 4-10 mm and <2 mm to obtain two size factions. There were five moisture treatments with the same length (48 days). The constantly moist control (CM) was maintained at 50% of WHC throughout. In the drying and rewetting (DRW) treatments, the number of dry and moist days was equal but the number of DRW events ranged from one to four (1-4DRW). Respiration was measured daily, microbial biomass C (MBC) was determined six days after rewetting in each DRW cycle and on day 48 (end of the experiment). The proportion of soil in the 4-10 mm size fraction decreased over time with a greater decrease in pasture than in wheat soil and in the DRW treatments compared to the constantly moist treatment (CM). Cumulative respiration at the end of the experiment was greater in the <2 mm than in the 4-10 mm fraction in both soils and was highest in CM and 1DRW. In wheat soil, cumulative respiration decreased from 1DRW to 3DRW, whereas it decreased only between 2 and 3DRW in pasture soil. In treatments with two to four DRW, the proportion of total cumulative respiration was lowest in the last cycle. In 2DRW, cumulative respiration was smaller in the second than in the first moist period whereas the reverse was true for 3DRW and 4DRW. Cumulative respiration in the second moist period was greater in 3DRW than in 2DRW (8 and 12 prior moist days) whereas cumulative respiration in the third moist period was greater in 4DRW than in 3DRW (12 and 16 prior moist days). At the end of the experiment, the MBC concentration in the 4-10 mm fraction was unaffected by moisture treatment, whereas in the <2 mm fraction, it was greatest in CM and lowest in 4DRW. We conclude that the response of respiration to drying and rewetting is more strongly influenced by management than size fraction. In a given soil, the cumulative respiration upon rewetting is influenced not only by the number of DRW cycles but also the number of moist days prior to rewetting.