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

Abstract

Switchgrass (Panicum virgatum L.) is a perennial lignocellulosic crop that has gained large interest as a feedstock for advanced biofuels. Using an eddy covariance system, we monitored the net ecosystem gas exchange in a 5‐ha rainfed switchgrass crop located in the Po River Valley for four consecutive years after land‐use change from annual food crops. Switchgrass absorbed 58.2 Mg CO2 ha−1 year−1 (GPP—gross primary production), of which 24.5 (42%) were fixed by the ecosystem (NEE—net ecosystem exchange). Cumulated NEE was negative (i.e. C sink) even in the establishment year when biomass and canopy photosynthesis are considerably lower compared to the following years. Taking into account the last 3 years only (postestablishment years), mean NEE was −26.9 Mg CO2 ha−1 year−1. When discounted of the removed switchgrass biomass, ecosystem CO2 absorption was still high and corresponded to −8.4 Mg CO2 ha−1 year−1. The estimation of the life cycle global warming effect made switchgrass an even greater sink (−12.4 Mg CO2 ha−1 year−1), thanks to the credits obtained with fossil fuels displacement. Water use efficiency (WUE), that is the ratio of NEE to the water used by the crop as the flux of transpiration (ET), corresponded to 1.6 mg C g−1 of H2O, meaning that, on average, 170 m3 of water was needed to fix 1 Mg of CO2. Again, considering only the postestablishment years, WUE was 1.7 mg C g−1 of H2O. In the end, about half of annual precipitation was used by the crop every year. We conclude that switchgrass can be a valuable crop to capture significant amount of atmospheric CO2 while preserving water reserves and estimated that its potential large‐scale deployment in the Mediterranean could lead to an annual greenhouse gas emission reduction up to 0.33% for the EU.

A minimally managed switchgrass ecosystem in a humid subtropical climate is a source of carbon to the atmosphere

“…Overview of annual carbon balance, with and without accounting for harvested carbon, for published eddy covariance switchgrass studies and this study (Abraha et al, 2018; Di Virgilio et al, 2018; Eichelmann et al, 2016; Skinner & Adler, 2010; Wagle & Kakani, 2014; Zeri et al, 2011). Width of bars for each site depends on the number of site years…”

“…Previous work has suggested that biofuel crops benefit climate by offsetting CO 2 and increasing belowground carbon storage (Adler et al, 2007; Anderson‐Teixeira et al, 2013). However, there can be large differences in carbon sequestration rates based on species, region, age, and land‐use history (Abraha et al, 2018; Di Virgilio et al, 2018; Eichelmann et al, 2016; Follett et al, 2012; Zeri et al, 2011). At this relatively warm and wet switchgrass site that was converted from a pasture, we found no net carbon sequestration 3–7 years after switchgrass establishment.…”

“…Eichelmann et al (2016) used EC to study a mature switchgrass (sixth and seventh year) field in Ontario, Canada for 2 years and found that switchgrass can alternate between sink and source depending on the climate conditions. Di Virgilio et al (2018) studied switchgrass using EC during the first 4 years of establishment in the Po River Valley in Northern Italy and found switchgrass to be a strong sink each year, even including harvested carbon (HC). Finally, Abraha et al (2018) used EC to study two switchgrass fields for 8 years following establishment in Michigan and found that the switchgrass field's carbon balance depended on the previous land use.…”

Progress in watershed geography in the Yangtze River Basin and the affiliated ecological security perspective in the past 20 years, China

“…In geographical research, which accounts for either part or the entire earth surface as a basic unit (Hou, 1994), many studies focus on specific land or water surface systems (Davis et al, 2003;Di Virgilio et al, 2018;Fu et al, 2019); however, few studies have focused on surface systems where land-water interactions take place (Fu et al, 2015). Watershed physiography regards a basin as its elementary unit, applying qualitative metasynthesis respective of rules associated with material, energy, and information flow among subsystems that include human activities, geographical environments, and water bodies in an interactive land-water interface (Grizzetti et al, 2003;Wei and Sun, 2009;Xie et al, 2010).…”