Soil Management Strategies for Improving Microbial Functioning: Learning from Transitions to and within Organic Production.

Many different strategies are used by farmers to transition to, and to improve soil management on organic farms. On-farm research provides a context for examining how plant-soil-microbe interactions and nutrient cycling are affected by a wider set of amendments, farming systems, and times in organic production than typically occur in research station trials. This presentation will consist of three parts: 1) a three-year study on transition of two ranches to organic production by conventional vegetable growers in the Salinas Valley, CA, which showed, with time in transition, higher microbial biomass and arbuscular mycorrhizal fungi, lower soil nitrate pools, adequate crop nutrients, and minor disease and weed problems; 2) a landscape survey of 13 organic tomato farms in the Sacramento Valley, CA, in which three nitrogen (N) management strategies were observed (i.e. N excess, N deficiency, and tightly-coupled plant-soil N cycling), each with a distinctive pattern of potential soil enzyme activities, soil C and N pools, and root gene expression for N assimilation, as measured by quantitative PCR; and 3) some ideas and challenges for better achieving tightly-coupled plant-soil N cycling, as related to microbial communities, crop N availability, and appropriate testing tools for adaptive management, and the tradeoffs that may occur between planning for multiple ecosystem benefits (e.g. C sequestration, N retention, soil structure, water holding capacity, and crop yield).