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

Abstract

Bioenergy has been identified as a key component of climate change mitigation. Therefore, quantifying the net carbon balance of bioenergy feedstocks is crucial for accurate projections of climate mitigation benefits. Switchgrass (Panicum virgatum) has many characteristics of an ideal bioenergy crop with high yields, low maintenance, and deep roots with potential for belowground carbon sequestration. However, the assessments of net annual carbon exchange between switchgrass fields and the atmosphere are rare. Here we present observations of net carbon fluxes in a minimally managed switchgrass field in Virginia (Ameriflux site US‐SB2) over 5 years (3–7 years since establishment). Average annual net ecosystem exchange (NEE) of carbon was near zero (60 g C m−2 year−1) but the net ecosystem carbon balance that includes harvested carbon (HC) was a net source of carbon to the atmosphere (313 g C m−2 year−1). The field alternated between a large and small source of carbon annually, with the interannual variability most strongly correlated with the day of the last frost and the interaction of temperature and precipitation. Overall, the consistent source of carbon to the atmosphere at US‐SB2 differs substantially from other eddy covariance studies that report switchgrass fields to be either neutral or a sink of carbon when accounting for both NEE and HC. This study illustrates that predictions of net carbon climate benefits from bioenergy crops cannot assume that the ecosystem will be a net sink of carbon from the atmosphere. Background climate, management, and land‐use history may determine whether widespread deployment of switchgrass as a bioenergy feedstock results in realized climate change mitigation.

Assessing environmental suitability of Ligusticum chuanxiong based on ecological analyses with chemical and molecular verification

“…This study showed that solar radiation (SRAD) and the minimum temperature of the coldest month (Boi6) were the limiting factors for L. chuanxiong ’s distribution. This finding is consistent with the fact that main distribution zones of L. chuanxiong belong to subtropical humid climate, which is characterized by low temperature, humidity, and little sunlight [ 38 ]. The optimum value of SRAD (5000–11,000 kJ/m 2 per day) was relatively low in China, which suggests that shading excessive sunlight may benefit L. chuanxiong cultivation.…”

“…The optimum value of SRAD (5000–11,000 kJ/m 2 per day) was relatively low in China, which suggests that shading excessive sunlight may benefit L. chuanxiong cultivation. Winters in subtropical humid climate is cool-mild, which means the temperature of coldest month is relatively high [ 38 ]. The coldest month, which arrives during the L. chuanxiong dumping-off period, is associated with a slight increase in the rhizome dry weight, probably explaining the limiting factor above [ 39 ].…”

Combined Carbon and Albedo Climate Forcing From Pine and Switchgrass Grown for Bioenergy

“…The pine tower is located at 37.57342 N, −79.08482 W, elevation 242 m, with a mixture of Clifford loam and Clifford clay loam soils in the fetch. The second tower (US-SB2, Grass hereafter) was installed over a switchgrass field that was harvested annually for bioenergy and began measurements March 26th, 2016 (Ahlswede et al, 2022). The grass tower is located at 37.5605 N, −79.0884 W, elevation 240 m, with a mixture of Clifford clay loam and wintergreen clay loam soils in the fetch.…”

“…In contrast to the pine plantation, the modeled switchgrass field quickly became a net sink of CO 2 after establishment (Figure 1B). One key outlier was the Virginia site where switchgrass was a source in multiple years following establishment (Ahlswede et al, 2022). Minimum NEE was only −355 ± 263 gC m −2 yr −1 .…”