The Role Of Microbial Processes In Soil Phosphorus Dynamics

Abstract

Historically, several different mechanisms have been proposed to explain soil P transformations, which are strongly influenced by hydrology. Areas in the landscape prone to saturate and produce runoff may, therefore, become important P sources, as well as some strategies for controlling non point source (NPS) P pollution, such as Vegetated Filter Strips (VFSs). Recently, the discovery of numerous microbial processes potentially significant for P transformations has challenged the traditional abiotic perspective of P cycling. Their role has been investigated in this research. Particularly, the potential effects of three processes in P release were evaluated: the decay of soil microbial biomass, the activity of Polyphosphate Accumulating Organisms (PAOs) and dissimilatory iron (Fe) reduction. The experimental approach was divided in two parts. For the first one, undisturbed soil cores from a VFS receiving silage leachate were maintained under flooding and draining cycles with acetate and glucose as carbon (C) sources, followed by an aerobic P enrichment period in order to promote polyphosphate (polyP) storage by PAOs. P release during flooding was dominated by organic forms, suggesting the contribution of the decay of soil microbial biomass. No polyP was found in the soils containing acetate following P enrichment, as revealed by liquid state 31 P Nuclear Magnetic Resonance Spectroscopy (31P-NMR). This indicates that PAOs were indeed inactive, since their presence in the cores was confirmed later in the second part of this research. Fe reduction was observed in the cores containing glucose, supporting the microbial nature of this process, although no concomitant inorganic P (Pi) release occurred. In the second part of this research, the presence of known groups of PAOs and dissimilatory Fe reducers, i.e. Accumulibacter and Geobacteraceae, respectively, was determined in the field site and the soil cores from the first part of this research using Polymerase Chain Reaction (PCR) based techniques. Clone libraries were constructed for Accumulibacter, Geobacteraceae and total bacteria. They were also quantified using quantitative PCR (qPCR). The resulting spatial distribution patterns of Accumulibacter and Geobacteraceae in the study site constitutes important evidence of their potential role in soil P dynamics.