Abstract
AbstractPerennial cellulosic crops are promoted for their potential contributions to a sustainable energy future. However, a large‐scale perennial bioenergy production requires extensive land use changes through diversion of croplands or conversion of uncultivated lands, with potential implications for local and regional hydrology. To assess the impact of such land use conversions on ecosystem water use, we converted three 22 year‐old Conservation Reserve Program (CRP) grasslands and three 50+ year‐old conventionally tilled corn‐soybean crop fields (AGR) to either no‐till continuous maize (corn) or perennial (switchgrass or restored prairie) bioenergy crops. We also maintained one CRP grassland without conversion. We measured evapotranspiration (ET) rates on all fields for 9 years using eddy covariance methods. Results show that: (a) mean growing‐season ET rates for perennial crops were similar to the ET rate of the corn they replaced at the previously cultivated (AGR) field but ET rates for perennial crops at CRP fields were 5–9% higher than ET rate for corn on former CRP fields; and (b) mean nongrowing season ET rates for perennial fields were 11–15% lower than those for corn fields, regardless of land use history. On an annual basis, mean ET rates for perennial crops tended to be lower (4–7%) than ET rate of the corn that they replaced at AGR fields but ET rates for perennial crops and corn at CRP fields were similar. Over 9 years, mean ET rates for the same crop across land use histories were remarkably similar for corn, whereas for the perennial crops they were 4–10% higher at former CRP than at former AGR fields, mainly due to differences in growing season ET. Over the 9 years and across all fields, ET returned ~60% of the precipitation back to the atmosphere. These findings suggest that large‐scale substitution of perennial bioenergy crops for rainfed corn in mesic landscapes would have little if any (0 to −3%) impact on terrestrial water balances.
Highlights
Bioenergy crops could potentially help mitigate climate change and enhance energy security by displacing conventional fossil fuel sources of liquid transportation fuel and electricity (Chu & Majumdar, 2012; Robertson et al, 2017)
Growing season ET for the same crop across land use histories showed that corn ETgs rates were similar at both fields from 2012 to 2017, higher at the AGR field in 2010 and 2018, but less in 2011 than at the Conservation Reserve Program (CRP) field, with overall mean difference of only 1 ± 13 mm gs−1 (Figure 4)
Over the 9 years of this study, annual ET rates—sum of the growing and nongrowing season rates—for perennials were 4–7% lower than for the corn they replaced at formerly cultivated (AGR) fields but the rates were similar between the perennials and corn at the former CRP fields
Summary
Bioenergy crops could potentially help mitigate climate change and enhance energy security by displacing conventional fossil fuel sources of liquid transportation fuel and electricity (Chu & Majumdar, 2012; Robertson et al, 2017). Field experiments that compare water use in rainfed annual corn and perennial (e.g., switchgrass and restored prairie) bioenergy crops are few and limited to the early- to mid-establishment phases of the perennial crops; these studies have shown that crops on well-drained soils have similar ET rates (e.g., Abraha et al, 2015; Hamilton, Hussain, Bhardwaj, Basso, & Robertson, 2015; Parish, Kendall, Thompson, Stenjem, & Hyndman, 2019). Our study compares three cropping systems over nearly a decade, a period that encompasses considerable climatic variability, and as well between the same crops across two contrasting land use histories (intensive row crops vs. conservation grasslands) to reveal legacy effects of prior land use
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