Organic crop management can decrease labile soil phosphorus and promote mycorrhizal association of crops

A principle concern with organic farming is for the potential depletion of phosphorus reserves in soil. In the Canadian Prairies, many organic farming systems have been shown to be deficient in plant available phosphorus. One of the main objectives of this study was to determine which organic management systems are in danger of depleting phosphorus reserves and if large reserves still exist in these systems in unavailable forms. Another was to determine if the colonization of arbuscular mycorrhizal fungi (AMF) could assist crops in low phosphorus organic systems in taking up phosphorus from the soil. The final objective of this study was to establish whether different management or rotation systems select for specific communities of AMF allowing for the future identification of efficient AMF species for inoculation in the field.;The modified Hedley sequential extraction procedure was used to extract different phosphorus pools from readily plant available, to moderately available, to unavailable phosphorus. The organic systems had lower concentrations of readily plant available phosphorus fractions than conventional but recalcitrant fractions were not significantly different (P < 0.05). The cumulative phosphorus balance of organic and conventional systems demonstrated that organic grain-only systems reduce labile fractions at a low rate due to low yields. However in high yielding systems such as forage-grain, labile phosphorus fractions considerably decreased over 14 years of organic management. Nitrogen limitations considerably affected yields in organic grain-only rotations more than phosphorus limitations in high yielding forage-grain systems. This study demonstrates that only high yield, high phosphorus export organic systems are a concern for developing plant available phosphorus deficiency in the long-term. Labile phosphorus concentrations were generally high in this soil being on average 35 mg kg-1 for conventional and 18 mg kg -1 for organic systems (Modified Kelowna). It is expected that where concentrations are within the agronomic response to phosphorus, depletion of available phosphorus will impart nutrient deficiency in the short-term. According to published values for agronomic response thresholds, the phosphorus concentrations in the organic forage-grain rotations are below this threshold and are considered phosphorus limited whereas the organic grain-only rotation will likely not become limited by phosphorus for another six years.;Colonization and spore populations of AMF were analyzed in order to determine the effect of crop treatment on AMF activity. Colonization levels as percent arbuscules were higher (P < 0.05) in organic than conventional systems; crop rotations did not significantly differ in colonization. The same was true for mycorrhizal spore populations in the forage-grain rotation (P < 0.05). The organic systems had lower labile soil phosphorus than conventional and the forage-grain rotation had lower phosphorus concentrations than the grain-only rotations (P < 0.05); however plant tissue phosphorus concentration was only effected by rotation indicating the increase in AMF colonization in organic systems may have been an important contributor to increased phosphorus uptake in these systems. Correspondence analysis indicated a shift in AMF community structure likely occurred between the prairie and the rest of the treatments and between the organic and conventional grain-only rotations; however, this shift may have been more related to selection of AMF species by crops than phosphorus limitation.;In conclusion, the Glenlea Long-term Rotation study has shown that in a soil with initially high plant available phosphorus there is not necessarily an immediate crisis in long-term organic farming due to phosphorus depletion as the reduced yield of organic systems imparts reduced phosphorus export. For the time scale of 10s of years, in high yielding forage export systems, a challenge will be to utilize the large pool of phosphorus in recalcitrant forms to help provide proper phosphorus nutrition. The colonization of AMF may allow crops to survive in low phosphorus conditions while recalcitrant phosphorus reserves are cycled into labile pools. From this study it appears that AMF communities were altered more by crop rotation than by management and labile phosphorus concentration; however, more work needs to be done in order to determine the AMF species composition with the use of molecular techniques.;The research was carried out at a 14 year-old study at Glenlea, in Southern Manitoba. The site has 3 different 4-year rotations under organic and conventional management: Wheat---Alfalfa---Alfalfa---Flax (forage-grain) with and without manure-compost, Wheat---Pea---Oat---Flax (grain-only), and a restored tall grass prairie planting (prairie). The conventional treatments received synthetic fertilizers as well as pesticides whereas the organic treatments received no inputs other than manure-compost.