Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

Abstract

Understanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO 2 exchange (NEE) and its components -gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) -in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic lightresponse curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light-VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to À33.02 AE 1.96 lmol CO 2 m À2 s À1 and the highest daily integrated NEE was À35.89 g CO 2 m À2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO 2 during 5-6 months and total seasonal uptakes were À1128 AE 130 and À1796 AE 217 g CO 2 m À2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year-round measurements over several years are required to assess a long-term source-sink status of the ecosystem.

Four‐year measurement of net ecosystem gas exchange of switchgrass in a Mediterranean climate after long‐term arable land use

“…However, although lower than measured by Zeri et al. (; ~ −51 to −80 Mg CO 2 ha −1 year −1 ), GPP measured at our site (−44.1 to −65.0 Mg CO 2 ha −1 year −1 ; Table ) was substantially higher than recorded in other studies on switchgrass (Skinner & Adler, ; Wagle & Kakani, ).…”

“…As this range was recorded only in 2012, GPP might have been limited by temperatures in the following 3 years. However, although lower than measured by Zeri et al (2011;~À51 to À80 Mg CO 2 ha À1 year À1 ), GPP measured at our site (À44.1 to À65.0 Mg CO 2 ha À1 year À1 ; Table 3) was substantially higher than recorded in other studies on switchgrass (Skinner & Adler, 2010;Wagle & Kakani, 2014).…”

“…Mean NEE and NECB of switchgrass during the postestablishment years (second to fourth year) were −26.9 and −9.5 Mg CO 2 ha −1 year −1 , respectively, with NEE reaching peak monthly values of −8.1, −7.9 and −7.6 Mg CO 2 ha −1 during the 2013, 2014 and 2015 growing seasons, respectively. In the same way, in a semiarid environment in Oklahoma, Wagle and Kakani () measured a peak monthly value of NEE in a young switchgrass stand of −9.9 Mg CO 2 ha −1 . Different from the present study, they estimated that NECB made switchgrass a small C source; nevertheless, their monitoring was limited to the establishment period.…”

Nitrogen fertilizer and landscape position impacts on CO2 and CH4 fluxes from a landscape seeded to switchgrass

“…Seasonal variations in CO 2 fluxes depend on temperature and moisture (Wagle & Kakani, ). Warm and moist summers with associated intense and long duration sunshine caused up to four times higher CO 2 flux than other seasons likely related to high rates of carbon fixation resulting in greater carbon availability below ground, and thus higher biomass and microbial activity in soil (e.g., Smith et al ., ).…”

Estimation and analysis of gross primary production of soybean under various management practices and drought conditions

“…The limitation of canopy CO 2 exchange necessitated the modification of the classical rectangular or nonrectangular hyperbolic light-PPFD equation by introducing the VPD-dependent control of photosynthetic uptake (Gilmanov et al, 2014;Lasslop et al, 2010;Wagle and Kakani, 2014b).…”