Direction and magnitude of the change in water content between two periods influence soil respiration, microbial biomass and nutrient availability which can be modified by intermittent air-drying

Abstract

Soil respiration and nutrient availability are influenced by soil water content and the effect of rewetting of dry soil has been studied extensively. But there are fewer systematic studies about the influence of less dramatic changes in water content on soil respiration and nutrient availability. In this study, two experiments were carried out with sandy loam soil. In both experiments, the soil was incubated at 200 g water kg−1 (W200) for two weeks which is optimal for microbial activity in this soil. Then the soil was either maintained at this water content or dried in a fan-forced oven at < 30 °C to about 10 g kg−1 (air-dried, AD), 40 g kg−1 soil (W40) or 100 g kg−1 soil (W100). Soil was maintained at these water contents for 14 days. In Experiment 1 after 14 days, soil water content was either maintained at W40, W100 or W200, reduced within 2 days or increased within a few minutes on day 16 to W40, W100 or W200 and kept at this water content for another 14 days. In Experiment 2, after the 14 days at AD, W40, W100 or W200, soil was air-dried within two days (in an oven at < 30 C°) after which soil water content was increased within a few minutes to W40, W100 or W200 and then kept at this water content for another 14 days. Soil respiration rate was measured during both 14-day periods, N and P availability and microbial biomass N and P (MBN and MBP) were determined on days 14, 23 and 30. In both experiments, respiration rate in the first 14 days and MBN on day 14 were very low in AD and increased with soil water content whereas available N was higher in AD than the other treatments. In Experiment 1, rewetting AD on day 16 induced a flush of respiration which was about 30% higher when rewet to W200 than rewetting to W100 or W40. Rewetting of W40 to W100 and W200 also induced a flush, but it was smaller than in rewetting of AD. In soil that was at W100 and W200 in the first two weeks, there was no respiration flush after day 16. In the second 2-week period, MBN on day 23 (7 days after the change in water content) was higher than on day 14 only in AD where it increased with water content in the second 2-week period. Available N was higher on day 23 than on day 14 in all treatments, 50% higher in AD and 2–3 fold higher in the other treatments. In Experiment 2 where the soil was dried to AD between day 14 and 16, irrespective of the water content in the first two weeks, rewetting to W100 and W200 induced a respiration flush which was about 30% higher in W200 than W100. The flush was smallest in soil that was at W200 in the first 14 days. MBN increased in AD and W40 treatments with a greater relative increase in AD (10-fold) than in W40 (2 fold). Available N changed little between day 14 and 23 in AD and W200, but increased about 2-fold in W40 and W100. In both experiments, differences in MBP and available P among treatments were small. In conclusion, respiration after a drying and rewetting event was influenced by the water content before drying where the respiration flush and cumulative respiration declined with increasing prior water content. Microbes in previously moist soils were more susceptible to drying and rewetting because MBN remained unchanged or decreased after rewetting. In previously dry soil on the other hand, rewetting induced not only a flush of respiration, but also an increase in MBN.