Water use efficiency of annual corn and perennial grasses

Background/Question/Methods

Water use efficiency (WUE), carbon gained per unit water lost, is a fundamental plant and ecosystem function that regulates plant productivity, global hydrology and carbon cycles. We used ecosystem (EWUE) and intrinsic (iWUE) WUEs derived from eddy covariance and leaf carbon isotope discrimination, respectively, to examine WUE of annual (corn) and perennial (mixed-prairie, switchgrass and brome grass) grasslands in response to (i) land use history and (ii) varying species composition following land conversion in southwest Michigan. Three fields that had been managed under the USDA Conservation Reserve Program (CRP) and three that had been in conventional agriculture (AGR) corn/soybean rotation were each converted to no-till soybean in 2009; then they were planted to no-till corn, switchgrass and mixed-prairie in the following year. One CRP field was left undisturbed to serve as a reference site. We report here WUE of the annual and perennial grasses through 2013 from all fields.

Results/Conclusions

The iWUE of all C3 perennial grass species varied little suggesting that stomata regulate net carbon assimilation at the leaf level. Corn had consistently greater EWUE at the CRP than at the AGR fields, and generally followed the amount and distribution of precipitation. EWUE of switchgrass was greater at the CRP than at the AGR lands in the first two years of conversion and vice versa in the last two years. The EWUE of mixed-prairie was consistently greater at the AGR than at the CRP fields, except in the first year of conversion when they were similar. EWUE of perennial grasses at the AGR fields increased through the years but remained almost constant at the CRP lands – including the reference site. This was attributed to changes in plant composition of the perennial grasses at the CRP and AGR sites through the years. EWUE showed a significant positive relationship (R² = 0.56, p < 0.001, n = 29) with the increase in % C4 plant species. Therefore the response of WUE to external drivers (e.g., CO₂ fertilization) in grasslands should consider changes in plant community composition (C4 vs. C3) over time.