Khan, Sidra Usman (2020) Soil nutrients mobilization as influenced by climate-change driven soil drying-rewetting or drying-flooding. (PhD thesis), Kingston University, .
Abstract
As phosphorus and micronutrients (e.g. Fe, Mn, Zn, Cu, Co and Ni) are essential elements for all living organisms including plants, their availability in soils is important for sustaining a healthy ecosystem. Climate change driven soil drying-rewetting or drying-flooding processes may promote increased mobilization and potential loss, and thus scarcity of these elements to plants, with potential implications for soil fertility and surface water quality. A series of controlled laboratory experiments were carried out to better understand the influence of soil drying-rewetting on DRP leaching and drying-flooding on mobilisation of phosphorus and micronutrient metals following rewetting/flooding of dried soils. Contribution of microbial biomass phosphorus to DRP leaching: The aim of this experiment was to determine microbial biomass phosphorus contribution to the dissolved reactive phosphorus (DRP) leaching under the influence of drying-rewetting processes. The results showed that soil (Hallsworth-I) drying (30°C or 40°C for 2-days or 14-days) induced reduction in microbial biomass phosphorus could partly be the reason that the DRP concentration in the leachates increased relative to the control. The results suggest that soil drying at higher intensity (40°C) and for prolonged duration (14-days) affect the microbial biomass phosphorus to a greater extent than low intensity-short duration drying (30°C for 2- days), and subsequently causes greater DRP leaching following rewetting of dried soil as observed from the drying-rewetting leaching experiment. Influence of drying-rewetting cycles on DRP leaching: The aim of this experiment was to examine how the intensity and duration of soil drying, rate of soil rewetting and frequency of rewetting cycles influence leaching of soil-borne phosphorus. The results showed that the soil (Hallsworth-I) drying at 25ºC, 30ºC, 35ºC or 40ºC for 2-days or 14-days followed by the first rapid or slow rewetting cycle leached significantly greater DRP concentrations relative to the control moist counterparts. The largest percentage increase in the DRP concentration occurred for the extended drying period (14-days) at 40°C. However, relatively smaller increase in leachate DRP concentration was observed for the shorter drying duration (2-days) at 25°C. The rate at which the dried soil was rewetted also affected DRP leaching, as the leachate oncentrations tended to be higher where the soil was rewetted at the slow rate compared to the rapid rewetting counterparts. The frequency of rewetting cycle also influenced DRP leaching, though without a clear trend as the concentrations showed increasing, decreasing or no change in the second rewetting cycle relative to the first rewetting. However, the DRP leachate concentrations remained higher than the control moist counterparts. In the third rewetting cycle, DRP leachate concentrations tended to decline in most of the treatments, particularly those where the soil was dried for the longer duration (14-days), possibly due to depletion in the easily leachable phosphorus Influence of drying-rewetting cycles on micronutrients leaching: The aim of this experiment was to examine how the intensity and duration of soil drying influence leachate dissolved concentrations of micronutrients (Fe, Mn, Cu, Co, Ni and Zn) following rewetting of dried soils. The results showed that the soil (Hallsworth-I) drying at 30°C or 40°C followed by the first rewetting cycle leached greater concentrations of micronutrients relative to its control moist counterpart. Also, drying soil at 40°C resulted in considerably greater micronutrient leachate concentrations as compared to soil dried at 30°C. The soil dried for 14-days (either at 30ºC or 40ºC) followed by the first rewetting cycle, leached significantly greater dissolved concentrations of Fe, Mn, Cu, Co, Ni and Zn relative to their 2-days drying counterparts. The frequency of rewetting cycles also influenced leachate dissolved metal concentrations, with the concentrations showing varied trends (increased, remained similar or decreased) in the second and third rewetting cycles for those treatments where the soil was dried for 2-days. However, in the treatments where the soil was dried for 14-days, the dissolved leachate concentrations of Fe, Mn, Co, Ni, Cu and Zn significantly decreased in the second and third rewetting cycle relative to the first rewetting counterparts, as observed for DRP, possibly because soluble forms of micronutrients would have been leached. Influence of drying-flooding on mobilisation of phosphorus and micronutrients: The aim of this study was to examine how soil drying followed by extended flooding might influence solubilisation of phosphorus and micronutrients (e.g. Cu, Co, Zn and Ni). The onset of flooding increased dissolved concentrations of phosphorus. The increase in total dissolved phosphorus (TDP) and micronutrients (e.g. Co and Ni) concentrations in the water column coincided with a reduction in redox potential, suggesting reductive dissolution of Fe/Mn oxy/hydroxides minerals. This was further supported by a strong positive correlation between Fe and Mn with TDP or with Co and Ni. The significant positive correlation between total dissolved concentrations of Al and P indicates that non-reductive dissolution of Al-organic matter-P complexes may have also been partly responsible for phosphorus release to the water column. Flooding of soils which were previously dried generally caused greater solubilisation of dissolved concentrations of phosphorus (total dissolved phosphorus, dissolved reactive phosphorus and dissolved unreactive phosphorus) and metals (Mn, Co, Ni and Cu) in water column relative to their moist-flooded counterparts. Crediton dry-flooded soil released higher concentrations of DRP than Hallsworth-II dry-flooded soil. However, most of the phosphorus in the water column of dry-flooded soils was unreactive, with the Hallsworth-II dry-flooded soil releasing higher concentrations of dissolved unreactive phosphorus (DUP). Hallsworth-II dry-flooded soil generally released greater total dissolved metal concentrations of Fe, Mn, Co and Ni in most of the sampling days relative to all other treatments (Hallsworth-II moist-flooded, Crediton moist-flooded and Crediton dried-flooded), possibly due to its greater organic matter (OM) and easily reducible ammonium oxalate extractable Fe-oxide content. The results suggest that soil drying followed by rewetting or flooding have the potential to promote greater mobilisation of soil macro- (e.g. P) and micro-nutrients (e.g. Mn, Co, Ni and Cu) compared to rewetting/flooding of moist soils. Thus, have implications for soil fertility and surface water quality, especially under changing-climate with predicted increase in the intensity and frequency of these climate-extremities rought/floods) for many regions in the UK. Nevertheless, in this study dissolved organic phosphorus (DOP) and dissolved organic carbon (DOC) were not measured. Measuring DOP and DOC would have provided further insights into the processes, particularly the role of microbial biomass-P and transformation between inorganic and organic P. Furthermore, these findings cannot be precisely replicated under natural-field conditions where nutrients mobilised in runoff may be retained in the sub-soil and/or elsewhere in the landscape along the pathway of runoff before they reach catchment waters.
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