Abstract
Soil drying-rewetting is known to enhance soil phosphorus leaching, which in part is due to osmotic shock and lysis of microbial cells upon rewetting. However, it is not entirely clear how this may be influenced by the intensity and duration of soil drying. We hypothesized that the intensity and duration of soil drying play important roles in determining the extent of dissolved reactive phosphorus (DRP) leaching resulting from microbial biomass mortality. To test this hypothesis soil sub-samples of a loamy grassland soil were dried (30C or 40C for 2-days or 14-days), rewetted, and the leachate was analysed for DRP. Soil drying at 30C for 2-days and 14-days resulted in leachate DRP concentrations which were 71% and 271% respectively higher than those in leachate from a control moist counterpart. Relatively greater DRP leaching losses occurred from the soil dried at 40C for 2-days and 14-days (143% and 300% respectively). To determine the contribution of the microbial biomass to the DRP in leachate, soil sub-samples were fumigated with chloroform either before or after drying (30C or 40C for 2-days or 14-days). All soil treatments were then either leached with water and analysed for DRP or extracted with 0.5M sodium bicarbonate solution and analysed for microbial biomass phosphorus. Fumigating soil samples before or after drying reduced microbial biomass phosphorus. However, the effect of chloroform fumigation was more pronounced in terms of microbial biomass reduction in the DF (drying followed by fumigation) treatment. Moreover, results revealed that in the DF treatment, soils dried at 30C for 2-days and 14-days had 22% and 13% respectively more microbial biomass phosphorus than their counterparts dried at 40C for 2-days and 14-days respectively. These results suggest that soil drying at higher intensity and for prolonged periods significantly (p<0.05) affect microbial biomass and subsequently increases soil phosphorus leaching following rewetting, due to enhanced contributions from the microbial biomass. These findings, however, need to be verified over a range of soil types under natural field conditions to better assess soil drying-rewetting effects on nutrient leaching.
Highlights
Predicted changes in climate will potentially increase the intensity and frequency of soil drying-rewetting (DRW) cycles (IPCC, 2014), with implications for nutrient dynamics, crop productivity and catchment water quality
dissolved reactive phosphorus (DRP) concentration from the moist fumigated (MF) soil was as much as four times (P < 0.001) more than from the corresponding moist unfumigated (MUF) samples (Figure 1)
The results clearly support our hypothesis that increase in the duration and intensity of drying of soils significantly increases the concentration of DRP in leachate relative to the control from the soil used in this experiment
Summary
Predicted changes in climate will potentially increase the intensity and frequency of soil drying-rewetting (DRW) cycles (IPCC, 2014), with implications for nutrient dynamics (plant availability, sequestration, and leaching), crop productivity and catchment water quality. Gordon et al (2008) reported that DRW induced significant increases in dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved inorganic nitrogen (DIN) in the leachates from two UK grassland soils. These findings support the notion that the changing pattern of climate has the potential to alter soil nutrient dynamics
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