In-soil trophic interactions between plants, bacteria and nematodes : Potential for increasing plant availability of organic phosphorus

Mezeli, Malika and Haygarth, Philip and George, Timothy and Neilson, Roy and Blackwell, Martin (2021) In-soil trophic interactions between plants, bacteria and nematodes : Potential for increasing plant availability of organic phosphorus. PhD thesis, Lancaster University.

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Abstract

Significant disparities exist between phosphorus (P) fertilizer applications and plant P uptake, in part induced by the in-soil conversion of inorganic P (Pi) to organic P forms (Po), which are not readily plant available. In-soil trophic interactions have been implicated in increased plant access to Po, specifically interactions between bacteria and bacterivorous nematodes. However, the existing data remains inconclusive and lack detail in arable systems, which are of increased economic importance. This work aimed to (1) assess the impact of nematodes as an additional trophic-level on Po use by plants in arable systems and (2) further understand the mechanisms of in-soil trophic interactions in improving plant acquisition of Po. To address these aims, criteria based meta-analysis, glasshouse plant growth trials, in vitro and in soil, and long term experimental (LTE) platforms were used. Results supported the conclusion that nematodes did not improve plant acquisition of Po per se. Time, soil P concentration and soil biological community composition had significant impacts on plant response. Although these factors were identified in two contrasting studies, their impacts on plant responses were stochastic. However, complexity, which describes the average number of trophic links per species, proved to be useful when understanding these data. For example, data from the meta-analysis and the plant growth trials demonstrated it was not simply the additive effect of the number of species or the addition of a specific species which resulted in predictable plant P responses. Both studies employed complexity as a system descriptor, which framed an understanding of these data where they evaded predictability, specifically when considering the temporal nature of these relationships, as complexity inherently includes a temporal element. The LTEs provided empirical data to support such assertions and yielded characteristics indicative of stability in the biotic component of systems previously considered disturbed. This exposed the question of the nature of disturbance (whether natural or anthropogenic). Additionally, it highlighted the importance of which successional time-point was being studied and the impact this has on the data captured. For example, nematode community analysis in the arable site assumed to represent disturbed land, showed characteristics of an undisturbed system, therefore it is not enough to assume an arable system is ‘disturbed’ or that an arable system after 150 years of continuous treatment would replicate results from a site under continuous management of a different temporal scale. Results allowed for the assessment of the experimental approaches used to interrogate complex systems and suggestions are made for a more pragmatic approach for the future. Although simple experimental systems exploring discrete mechanisms should not be abandoned, extrapolation of such data and predictions to more complex systems must involve the abandonment of the linear reductionist model and undergo transformative inclusion into one of complexity.