Right: Nitrogen loss from agricultural fields during high winds were assessed using profile samples that grapped windblown dust, right of center in the photo. Left: Carbon export through surface runoff was monitored using parshall flumes and event-based water sampling. The image above was taken at the conventional tillage site. 

Carbon and Nitrogen Losses Due to Erosion

Issue: There is a risk that future climate change will deplete organic carbon and organic nitrogen in the soil and therefore reduce overall soil fertility and crop productivity. In addition to atmospheric exchange with the crops, soil carbon and nitrogen are transported by wind and water, which degrades water and air quality. Maintaining soil organic carbon levels and minimizing the negative environmental effects of carbon transport require an understanding of how management can reduce carbon loss. However, few data sets are available that document carbon and nitrogen transport by wind and water in the Palouse region. 

Action taken: Soil carbon and nitrogen transport by wind and water were measured at specific field sites across the region. In the western, arid region, where soil erosion is driven primarily by wind erosion, windblown dust samples were collected from multiple storm events and used to determine nitrogen loss rates. In the wetter, eastern region, soil carbon and nitrogen transport by water were measured from the micro-drainage scale to regional watershed scales. The effect of tillage management strategy on organic carbon transport was assessed at the field scale using both event-based water sampling and process-based computer modeling. The relative loss of dissolved organic carbon through subsurface tile drainage versus through surface runoff was also assessed at a long-term no-tillage site.

Results: Sediment and nitrogen loss associated with windblown dust for 13 high-wind events and eight site years showed that nitrogen loss from windblown dust ranged from 0.1 to 1.9 kilograms per hectare (kg/ha) across the 13 events, with annual losses that could exceed 5 kg/ha. The data indicate that the windblown dust was not enriched in nitrogen as compared with parent soil materials, which suggests that more-simplistic modeling efforts that ignore sorting processes to determine total nitrogen transport by wind are appropriate. The water erosion data indicate that no-tillage practices greatly reduce soil organic carbon transport from 37 kilograms per hect- 7 REACCH Project Impacts Carbon export through surface runoff was monitored using parshall flumes and event-based water sampling. The image above was taken at the conventional tillage site. are per year (kg/ha/yr) in conventional tillage to 6.6 kg/ha/yr in no-tillage. Interestingly, the majority of carbon loss from the notillage field (5.6 kg/ha/yr) was transported as dissolved organic carbon in the subsurface artificial tile drainage flow; we observed very little soil erosion. We observed a general decrease in total carbon export with increasing scale, ranging from 19 kg/ha/yr at the edge of the field to 13 kg/ha/yr from the Palouse River at Hooper, WA, suggesting that much of the carbon that erodes at the field scale deposits within the stream system. The overall magnitude of soil carbon loss by wind and water at rates measured here is small relative to the overall soil carbon pool, estimated at 260,000 kg/ha in the top 1.5 meters of soil. However, process-based modeling indicates that without changes in tillage-management practices, soil carbon transport by erosion will likely increase by an order of magnitude by mid-century, which would negatively affect yields, especially for fields under conventional-tillage practices. Understanding the trends and effects of management on carbon export is an essential first step in the development of carbon budgets and full-scale cropping models capable of evaluating precision-based carbon-loss mitigation strategies.