The number of moist days determines respiration in drying and rewetting cycles

Abstract

The respiration flush upon rapid rewetting of dry soil has been studied extensively, but with inconsistent differences in cumulative respiration between drying and rewetting (DRW) treatments and the constantly moist (CM) control. Therefore, factors influencing respiration in DRW require further research. Three incubation experiments were carried out to assess the relationship between cumulative respiration and the number of moist or dry days in two soils of similar texture but different long-term management: permanent pasture and wheat-fallow soil with organic C content of 39 and 19 g kg⁻¹. In the first experiment, the CM and DRW treatments had the same total length (10 days) with different proportions of moist and dry days in the DRW treatments. The second and third experiment had DRW cycles in which dry and moist periods were of equal length with one cycle in experiment 2 and two cycles in experiment 3. Total cumulative respiration per gram TOC was greater in wheat than in pasture soil. Total cumulative respiration in DRW treatments was lower than in CM except when the moist period was more than 50% of the total length in pasture soil in experiment 1. In the first experiment, cumulative respiration in the dry period was not influenced by the number of dry days, but cumulative respirations in the moist period and in the total DRW cycle were positively correlated with the number of moist days. In both second and third experiment, total cumulative respiration increased with increasing number of days, with a greater increase in CM than in DRW treatments. When subjected to two DRW cycles in the third experiment, cumulative respiration in the dry period was not correlated with the number of dry days in the first cycle but negatively correlated in the second DRW cycle. We conclude that cumulative respiration in DRW cycles is strongly influenced by the number or proportion of moist days. When dry and moist periods are of equal length, the reduction in cumulative respiration in DRW compared to CM becomes greater with increasing DRW cycle length.