Assessing carbon and water dynamics of no-till and conventional tillage cropping systems in the inland Pacific Northwest US using the eddy covariance method

Analysis of carbon and water budgets in cropping systems is important for understanding the impacts of different management practices and meteorological conditions, in the context of climate change, on agriculture. We have established a pair of long-term eddy covariance flux towers at the R. J. Cook Agronomy Farm (CAF) near Pullman, Washington, US. The tower sites have similar crop rotation, weather conditions, and management practices except tillage. One site has been under no-till management (CAF-NT) since 1998, while the other site has used conventional tillage practice (CAF-CT) over the same time period. Measurements conducted above a garbanzo bean crop (Cicer arietinum) between October 2012 and September 2013 showed the CAF-NT site was close to CO2 neutral with an annual cumulative net ecosystem exchange of CO2 (NEE) of -20 +/- 38 g Cm-2. By contrast, the CAF-CT site was a CO2 source with an annual NEE of 117 +/- 39 g Cm-2 during the same time period. The annual NEE values at each site were significantly different (p <0.05). When carbon losses via harvest export were taken into account, the net rates of carbon loss from each ecosystem to the atmosphere were 32 +/- 50 and 178 +/- 48 g Cm-2 for CAF-NT and CAF-CT, respectively, indicating CAF-NT was close to carbon neutral and CAF-CT was a net carbon source during the measurement period. Partitioning of NEE into gross primary productivity (GPP) and total ecosystem respiration (R-eco) shows that the annual cumulative GPP of each site did not differ much, but CAF-NT had lower annual cumulative R-eco compared to CAF-CT during the period of October 2012-September 2013. Water budget analyses presented similar magnitudes of annual sums of evapotranspiration (ET), 425 and 416 mm for CAF-NT and CAF-CT, respectively. ET was partitioned into evaporation (E) and transpiration (7) for both sites, and CAF-CT had a larger fraction of annual E compared to CAF-NT during the measurement period. Relationships between carbon and water fluxes were investigated, and for the period of October 2012-September 2013, high correlations were found in GPP vs. T and NEE vs. ET at both sites. In summary, the site deploying no-till practices had lower total ecosystem respiration and evaporation throughout the measurement period, more net carbon uptake and a greater ratio of transpiration over evapotranspiration (T/ET) during the growing season. (C) 2015 The Authors. Published by Elsevier B.V.