Phosphorus (P) is at the heart of modern agriculture, which helps to sustain crop production. Many of the key processes centered around P solubilisation in the rhizosphere govern both plant uptake or potential leaching. However, the processes of controlling P solubilisation in the rhizosphere are of varying efficiency and now subject to constant changes under the varying soil conditions due to the climate change. In turn it is thought to have a potential onward effect of drying and rewetting (DRW) on the rhizosphere, which is one of the central components of this thesis. Thus, the focus of this PhD is to assess the P solubilisation processes in the rhizosphere with respect to the immediate (i-DRW) and delayed (d-DRW) DRW effect.
Alongside this, the thesis is also focussed in assessing the behaviour of different types of phosphate fertiliser and their potential efficiency in the rhizosphere, particularly widely available new low-grade phosphate rock (PR) for potential crop benefit. The low-grade PR material is newly emerging into the market and can be used as a potentially more sustainable type of P fertiliser over time, as a substitute higher-grade PR.
The overarching hypothesis tested was “P solubilisation in the rhizosphere can be affected by climate driven changes of soil associated with immediate and delayed effects of DRW events and root-zone acidification, under variable P amendments, with a particular consideration of the effects of low-grade phosphate rock”. To approach this, eight sub-hypotheses were tested, each being part of four different controlled experiments in the greenhouse using soils from four distinct agricultural regions of China. All laboratory experiments of P solubilisation (and potential leaching) in the rhizosphere were undertaken in three steps, which were dealt with chapters 3, 4 and 5.
Overall, the key findings indicate that the P solubilisation in the rhizosphere (with corresponding plant productivity) is highly dependent on soil type, in particular the biological and hydrological characteristics. The results show substantially different levels of soil P solubilisation, related to exudative processes of the rhizosphere. Rhizosphere manipulation by inducing soil acidity via chemicals and legumes, had significant effects on soil P solubilsation and plant growth response (chapter 3). The P solubilisation processes also performed better on the soil with recent history of a series of DRW events, than soil with constant moisture treatment (chapter 5). This data supports the practical implications in the crop productivity over arid agricultural regions, especially with soils having low P availability used in these experiments. However, despite the assumptions of an increase in P solubilisation associated with i-DRW, the results of parallel study (chapter 4) did not prove to have a statistically significant difference across all treatments when soils were treated with frequent DRW events immediately.
With regards DRW induced P leaching, the concentration of all P forms in leachate water varied in each cycle against soil type and the amount of P leaching following DRW was also proportionate with the rate of new P application. Concentration of P in the leachates of soils receiving different P sources varied from one i-DRW cycle to another, in respect of frequency and rate of DRW. Taken together, the evidence from the thesis led to the acceptance of the overarching hypothesis of this study.