Forage Composition, AMF Root Colonization and Biological Nitrogen Fixation Distinguish Perennial Forage Systems on Organic Versus Conventional Dairy Farms in Ontario, Canada.

Recent studies exploring soil phosphorus (P) fertility on Canadian organic farms have reported low concentrations of Olsen soil test P (STP), yet the impact on crop productivity is not clear. It has been suggested that soil biological activity, including the presence and/or abundance of arbuscular mycorrhizal fungi (AMF), in organically-managed systems has an increased role in providing the crop with P not measured by the conventional Olsen soil P test. This study used a systems approach to evaluate mixed forage (Medicago sativa (alfalfa) / Phleum pratense (timothy)) productivity, soil biochemical properties, AMF root colonization, and biological nitrogen fixation (BNF) on organic and conventional dairy farms in Ontario, Canada. Forage fields of three long-term (>20 y) organically-managed dairy farms with low Olsen STP (mean of 5.1 mg kg^-1) were compared with adjacent conventional dairy farm fields with long-term histories of synthetic P fertilizer application and higher STP levels (mean of 11.7 mg kg^-1). Total forage yields on organic fields (8.0 Mg DM ha^-1) did not significantly differ from conventional field yields (9.5 Mg DM ha^-1), despite lower Olsen STP and alfalfa tissue P concentrations. The organic systems supported greater proportions of alfalfa in forage stands (65 vs 28%) and legume content was negatively correlated with STP. Mean AMF root colonization of alfalfa was greater in organic systems (37% vs 23%) and was negatively correlated with STP (P = 0.0006). Preliminary results indicate the total BNF was greater under organic management. This study indicates that legume-based forages can produce acceptable yields at low STP values (< 10 mg kg^-1). It is likely that increased AMF root colonization contributed to the forage crops maintaining sufficient P contents and yields. This research supports the importance of legumes, legume BNF, and biological processes linked to soil P cycling in organic dairy production systems.