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Title: Variability in carbon dioxide fluxes among six winter wheat paddocks managed under different tillage and grazing practices

Author
item Wagle, Pradeep
item Gowda, Prasanna
item Northup, Brian
item Turner, Kenneth
item Neel, James
item MANJUNATHA, PRIYANKA - Oklahoma State University
item ZHOU, YUTING - Oklahoma State University

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2018
Publication Date: 5/9/2018
Citation: Wagle, P., Gowda, P., Northup, B.K., Turner, K.E., Neel, J.P., Manjunatha, P., Zhou, Y. 2018. Variability in carbon dioxide fluxes among six winter wheat paddocks managed under different tillage and grazing practices. Atmospheric Environment. 185:100-108. https://doi.org/10.1016/j.atmosenv.2018.05.003.
DOI: https://doi.org/10.1016/j.atmosenv.2018.05.003

Interpretive Summary: Understanding the variations in CO2 fluxes in winter wheat (Triticum aestivum L.) paddocks managed under different tillage and grazing practices is crucial to evaluate the role of different management practices on CO2 dynamics and to develop best management practices. Eddy covariance (EC) systems were used to measure CO2 fluxes from grain only, graze-grain, and graze-out winter wheat paddocks managed under conventional till (CT) and no-till (NT) systems during the 2016-2017 growing season. Large variations in magnitudes and seasonal sums of CO2 fluxes [net ecosystem CO2 exchange (NEE), gross primary production (GPP), and ecosystem respiration (ER)] were observed among paddocks. The rate of carbon uptake increased with increasing photosynthetic photon flux density (PPFD), air temperature (Ta), and vapor pressure deficit (VPD) up to certain thresholds, then plateaued or declined as they increased further. Threshold PPFD, Ta, and VPD values for NEE were 1700 µmol m-2 s-1, 22 ºC, and 1.25 kPa, respectively. Across-site analysis showed a strong linear relationship (R2 = 0.76) between percent of canopy cover (Canopy%) and NEE. Leaf area index (LAI), dry biomass weight (DW), and remotely-sensed enhanced vegetation index (EVI) and normalized difference vegetation index (NDVI) explained substantial amounts of variations in GPP and ER. Preliminary results showed that differences in wheat canopies related to paddock management were major drivers for among-site variability in CO2 fluxes. Long-term measurements from clustered and paired EC towers will provide insights into the effects of tillage and different grazing practices on CO2 dynamics in winter wheat.

Technical Abstract: Carbon dioxide (CO2) fluxes from six winter wheat (Triticum aestivum L.) paddocks (grain only, graze-grain, and graze-out) managed under conventional till (CT) and no-till (NT) systems were synthesized for the 2016-2017 growing season to compare the magnitudes and seasonal dynamics of CO2 fluxes and to investigate among-site variability of CO2 fluxes. Large variations in CO2 fluxes were observed among paddocks. Maximum daily (7-day averages) net ecosystem CO2 exchange (NEE) ranged from -3.39 to -8.68 g C m-2, gross primary production (GPP) ranged from 6.71 to 16.35 g C m-2, and ecosystem respiration (ER) ranged from 4.89 to 9.26 g C m-2. Seasonal sums of NEE ranged from -137 to -542 g C m-2. Optimum photosynthetic photon flux density (PPFD), air temperature (Ta), and vapor pressure deficit (VPD) for NEE were approximately 1700 µmol m-2 s-1, 22 ºC, and 1.25 kPa, respectively. Across-site analysis showed percent of canopy cover (Canopy%) was linearly correlated with NEE (R2 = 0.76) and ecosystem light use efficiency (ELUE, R2 = 0.73). Integration of photosynthetically active radiation (PAR) with leaf area index (LAI) and integration of Ta with dry biomass weight (DW) explained 78% and 67% of variations in GPP and ER, respectively. Remotely-sensed enhanced vegetation index (EVI) explained 66% and normalized difference vegetation index (NDVI) explained 69% of the variations in NEE. The EVI × PAR explained 76% of variations in GPP, while NDVI × Ta explained 54% of variations in ER. Results illustrated that differences in wheat canopies related to paddock management, as indicated by differences in DW, LAI, Canopy%, NDVI, and EVI, must be accounted for explaining among-site variability of CO2 fluxes. Long-term measurements from our clustered and paired EC towers will provide insights into the effects of tillage and different grazing practices on CO2 dynamics in winter wheat.