Downscaled climate models predict warmer temperatures and shifting precipitation patterns for the delta of British Columbia’s Fraser River, one of the most productive agricultural regions in Canada. Shifting precipitation has already resulted in wetter soil conditions during spring and fall when mechanized operations are most critical for production, as well as hotter and drier summers. Soil organic carbon (SOC) plays a key role in buffering farms, particularly on fine textured soils like those found in the delta, from climate impacts, at the same time, mitigates climate forcing emissions. SOC helps increase water infiltration, drainage and water holding capacity which improves a field's potential to support mechanized equipment without damaging wet soils thus increasing the number of workable days (WD) and reduce the need for irrigation. Many farmers in the delta have installed drainage infrastructure but it is unclear if this has improved their adaptive capacity or how drainage has effected the oxidation of SOC. The objective of this study was to better understand these tradeoffs by: 1. Inventorying soil properties of farms with drained fields, drained fields with pumps and undrained fields; 2. Monitoring water dynamics during key periods of the year to determine workable days or water table levels.
In 2015, we sampled and analyzed soil from a total of 25 blueberry and vegetable fields down to 1m. During the wet season (September to April) of 2015-17 we monitored soil moisture, water table and ponding. The number of WD and available water content (AWC) were calculated using pedotransfer functions.
The results of linear mixed effects modelling indicate that there are no significant differences in SOC concentration or AWC among drainage management options or crop type at the 0-15 cm depth. Differences in SOC observed across fields were largely driven by variation in clay concentration. There was also no difference in calculated threshold for optimal workability between fields with and without drainage tile alone. Fields with drainage tile and pumps to remove the water from the system, however, had a 26% improved optimal workability value. This difference resulted in a 14% increase in the number of WD observed in the Spring of 2016. Our results show that drainage management requires a pump system to improve the capacity of farmers to adapt to wetter shoulder seasons. Our analysis did not indicate that there have been mitigation tradeoffs but it is likely that improved drainage will increase SOC losses over the long-term.