The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUE _eco ), is widely expected to increase because of the rising atmospheric carbon dioxide concentration ( C _a ). However, little is known about the interactive effects of rising C _a and climate change on WUE _eco . On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUE _eco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.

Land use conversions into and out of agriculture may influence soil–atmosphere greenhouse gas fluxes for many years. We tested the legacy effects of land use on cumulative soil nitrous oxide (N₂O) fluxes for 5 yr following conversion of 22‐yr‐old Conservation Reserve Program (CRP) grasslands and conventionally tilled agricultural fields (AGR) to continuous no‐till corn, switchgrass, and restored prairie. An unconvertedCRPfield served as a reference. We assessed the labile soil C pool of the upper 10 cm in 2009 (the conversion year) and in 2014 using short‐term soil incubations. We also measured in situ soil N₂O fluxes biweekly from 2009 through 2014 using static chambers except when soils were frozen. The labile C pool was approximately twofold higher in soils previously inCRPthan in those formerly in tilled cropland. Five‐year cumulative soil N₂O emissions were approximately threefold higher in the corn system on formerCRPthan on former cropland despite similar fertilization rates (~184 kg N·ha⁻¹·yr⁻¹). The lower cumulative emissions from corn on former cropland were similar to emissions from switchgrass that was fertilized less (~57 kg N·ha⁻¹·yr⁻¹), regardless of former land use, and lowest emissions were observed from the unfertilized restored prairie and reference systems. Findings support the hypothesis that soil labile carbon levels modulate the response of soil N₂O emissions to nitrogen inputs, with soils higher in labile carbon but otherwise similar, in this case reflecting land use history, responding more strongly to added nitrogen.

Over 5 million ha of US Conservation Reserve Program (CRP) grasslands have been converted to annual crops since 2000, driven mainly by demand for corn grain ethanol. Much of the soil carbon sequestered under CRP is lost upon conversion, creating a 'carbon debt' that is presumed to be repaid by future greenhouse gas (GHG) savings from ethanol's substitution for petroleum. Model simulations, extrapolations, and national statistics rather than direct measurements have been used thus far to estimate the long-term global warming impact (GWI) of such conversions. Here we report measured GWIs for three 22-year-old CRP grassland fields and three conventionally tilled agricultural (AGR) fields (11-17 ha) converted to either annual no-till corn or perennial cellulosic (switchgrass or restored prairie) bioenergy crops. We assessed GWIs for each field over eight years using whole-system life cycle analysis (LCA) by measuring: (a) GHG fluxes via eddy covariance and static chamber methodologies, (b) farming operations and agronomic inputs, and (c) the fossil fuel offset by ethanol use. Payback times were much longer than those estimated by prior modeling efforts. After 8 years, cumulative GWIs of switchgrass, restored prairie, and corn at the CRP grasslands were, respectively,-2.6±4.0, 6.9±3.6 and 85.2±5.1 Mg CO 2-equivalent ha-1. The switchgrass system had repaid its carbon debt by year eight and the restored prairie will have likely repaid by year ten; however, the no-till corn system appears likely to require >300 years. The same bioenergy crops grown on former agricultural lands, with no sequestered carbon lost on conversion, repaid their carbon debts within two years. Results indicate that GWI estimates and carbon debt payback times due to conversion of CRP lands to annual bioenergy crops have been substantially underestimated by current models.

Low‐power, open‐path gas sensors enable eddy covariance (EC) flux measurements in remote areas without line power. However, open‐path flux measurements are sensitive to fluctuations in air temperature, pressure, and humidity. Laser‐based, open‐path sensors with the needed sensitivity for trace gases like methane (CH₄) and nitrous oxide (N₂O) are impacted by additional spectroscopic effects. Corrections for these effects, especially those related to temperature fluctuations, often exceed the flux of gases, leading to large uncertainties in the associated fluxes. For example, the density and spectroscopic corrections arising from temperature fluctuations can be one or two orders of magnitude greater than background N₂O fluxes. Consequently, measuring background fluxes with laser‐based, open‐path sensors is extremely challenging, particularly for N₂O and gases with similar high‐precision requirements. We demonstrate a new laser‐based, open‐path N₂O sensor and a general approach applicable to other gases that minimizes temperature‐related corrections for EC flux measurements. The method identifies absorption lines with spectroscopic effects in the opposite direction of density effects from temperature and, thus, density and spectroscopic effects nearly cancel one another. The new open‐path N₂O sensor was tested at a corn (Zea maysL.) field in Southwestern Michigan, United States. The sensor had an optimal precision of 0.1 ppbv at 10 Hz and power consumption of 50 W. Field trials showed that temperature‐related corrections were 6% of density corrections, reducing EC random errors by 20‐fold compared to previously examined lines. Measured open‐path N₂O EC fluxes showed excellent agreement with those made with static chambers (m = 1.0 ± 0.3;r² = .96). More generally, we identified absorption lines for CO₂and CH₄ flux measurements that can reduce the temperature‐related corrections by 10–100 times compared to existing open‐path sensors. The proposed method provides a new direction for future open‐path sensors, facilitating the expansion of accurate EC flux measurements.