Understanding Biological Contributions to Phosphorus Availability in Organic Dairy Farm Soils
Recent studies exploring soil phosphorus (P) fertility on dairy farms under organic management in Canada have reported low concentrations of Olsen soil test P (STP), yet the impact on crop productivity is not clear. In this thesis, soil biological and biochemical properties relating to plant P availability in perennial forage fields (Medicago sativa / Phleum pratense) of three paired organic and conventional dairy farms in southwestern Ontario, Canada were compared. Molecular approaches (PCR-DGGE and quantitative real-time PCR) were used to explore arbuscular mycorrhizal fungi (AMF) communities in these soils. Finally, an incubation experiment using the radioisotope 33P was conducted to assess biological P fluxes and microbial P uptake in select forage soils along a gradient of STP concentrations. Results of the field study found no significant differences between forage yields on organically versus conventionally managed fields, and forage yield was not correlated with STP. Plant tissue P concentrations supported that plant-available P was greater under conventional management; however organic tissue P concentrations were not lower than critical concentrations. Solution 31P NMR-spectroscopy revealed a greater organic P (Po): inorganic P (Pi) ratio and greater proportions of orthophosphate diesters in NaOH-EDTA soil extracts from organically managed systems. Mean AMF root colonization of alfalfa was also greater under organic management (37%) compared with conventional management (23%). AMF community composition was found to differ between management systems; the data indicated that organic systems may support AMF communities that promote greater host-plant P-use efficiencies. In the 33P incubation experiment gross Po mineralization contributed a mean of 35% of the total isotopically-exchangeable P after 8 days. Significant uptake of solution Pi by the microbial biomass occurred in 8 days, with 11/16 soils having greater than 20% of the initially added 33P in the microbial biomass. The proportion of 33P found in the microbial P pool was negatively related to solution P¬¬i concentrations. This research supports the importance biological processes to soil P dynamics in organic farming systems. These relevant processes need to be considered when assessing P availability in agricultural systems, which would ultimately aid in the greater goal of achieving P-use efficiency in agriculture.