Nutrient Cycling on Organic Farms across a California Landscape
This project focuses on how landscape heterogeneity and management strategies affect ecosystem functions, and in particular nutrient cycling, and soil microbial communities in organic agroecosystems. We are focusing on farms that grow Roma-type tomatoes for both processing and fresh markets across a gradient of soil organic matter on similar soil types in Yolo Co. This on-farm, participatory research seeks to combine knowledge and innovations from both farmers and researchers to improve nutrient cycling on organic farms. A landscape approach is needed to complement site-specific research on one or a few management practices, since these approaches cannot account for the effect of heterogeneity in either biophysical or social factors on key ecosystem functions. Embedded within this landscape study is an investigation of the plant-soil-microbe interactions that underpin nutrient cycling using high resolution methods, such as plant gene expression, soil organic matter spectroscopy (see Gene Expression in Tomato Roots and FTIR indicators of soil organic matter quality across a landscape of organic management), as well as analysis of microbial activity and community composition in a collaboration with Veronica Acosta-Martinez. This focus will provide insight into the processes behind the patterns we observe.
Preliminary results show that management approaches differ substantially across farms, despite growing the same crop. Some of these differences seem to be related to market orientation (i.e. processing or diversified fresh markets). Soil texture and pH in the 0-15 cm surface layer were similar across the 13 fields, but there was a three-fold range of soil C and N as well as substantial variation in inorganic N and available P that reflected current and historical management practices. Biological measures of nitrogen availability were strongly related to soil organic carbon and nitrogen. These measures were also more strongly related to crop productivity than soil inorganic nitrogen, highlighting the limited utility of traditional measures of soil nitrogen availability (e.g. nitrate) in organic systems. Interestingly, several sites showed high crop productivity with low soil inorganic pools over the duration of the growing season. This may indicate that certain types of management and soil characteristics could support rapid and tight nitrogen cycling, whereby plants obtain sufficient nitrogen for productivity but without as much potential for nitrogen loss.
Activities of C-cycling enzymes increases with inorganic N availability, while activities of N-cycling enzymes increased with C availability. Microbial community composition was overall less variable across fields than enzyme activities, but there were slight community differences that were related to organic amendments (manure vs. composted green waste). Overall, however, the general similarity among fields for particular taxonomic indicators, especially saprophytic fungi, likely reflects the high disturbance and low complexity in this landscape (see Agricultural Intensification Across a California Landscape). The on-farm approach provided a wide range of farming practices and soil characteristics to reveal how microbially-derived ecosystem functions can be effectively manipulated to enhance nutrient cycling capacity.
This project is funded by the USDA Organic Research and Extension Initiative (OREI) program grant, “Researcher and Farmer Innovation to Increase Nutrient Cycling on Organic Farms”. For questions, contact Tim Bowles (Ph.D. candidate) or Louise Jackson.
View the poster, "Nutrient cycling on organic farms across a gradient of soil organic matter in Yolo Co., California: On-farm research to improve nitrogen availability and retention" from the recent CalCAN Summit, describing preliminary results from this project
Bowles, T.M., Acosta-Martinez, V., Calderon, F., and Jackson, L.E. 2014. Soil enzyme activities, microbial communities, and carbon and nitrogen availability in organic agroecosystems across an intensively-managed agricultural landscape. Soil Biology and Biochemistry. View this paper