Carbon and water dynamics in co-located winter wheat and canola fields in the U.S. Southern Great Plains

Agricultural and Forest Meteorology 279 (2019) 107714

Contents lists available at ScienceDirect

Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet

Carbon and water dynamics in co-located winter wheat and canola fields in the U.S. Southern Great Plains

T

⁎

Pradeep Waglea, , Prasanna H. Gowdaa, Priyanka Manjunathab, Brian K. Northupa, Alexandre C. Rocatelib, Saleh Taghvaeianc a

USDA, Agricultural Research Service, Grazinglands Research Laboratory, El Reno, OK 73036, USA Oklahoma State University, Department of Plant and Soil Sciences, Stillwater, OK 74078, USA c Oklahoma State University, Department of Biosystems and Agricultural Engineering, Stillwater, OK 74078, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Conventional till Ecosystem light use efficiency Ecosystem water use efficiency Eddy covariance No-till

The magnitudes and seasonal dynamics of net ecosystem exchange (NEE) of carbon dioxide (CO2) and evapotranspiration (ET), measured using the eddy covariance (EC) technique from co-located, paired (conventional till, CT and no-till, NT) winter wheat (Triticum aestivum L.) and canola (Brassica napus L.) fields, were compared during a presumably favorable growing season for both crops. The magnitudes (7-day average) of NEE, gross primary production (GPP), and ET reached approximately −8 g C m−2 d−1, 16 g C m−2 d−1, and 5 mm d−1, respectively, at both CT and NT wheat fields due to uniform canopy stands. The magnitudes (7-day average) of NEE reached −5.20 ± 0.49 and −4.66 ± 0.36 g C m−2 d−1, GPP reached 12.47 ± 1.16 and 10.86 ± 0.97 g C m−2 d−1, and ET reached 4.7 ± 0.42 and 4.28 ± 0.36 mm d−1 at CT and NT canola fields, respectively. Poor recovery of canola stand in the NT field after winter dormancy resulted in smaller magnitudes of fluxes in spring 2017. Wheat had a larger potential as a carbon sink during winter than canola. Larger magnitudes of CO2 fluxes and longer periods as carbon sinks for wheat caused large differences in carbon sequestration potential between wheat and canola. Fluxes showed similar responses to climatic conditions as optimum air temperature (Ta) was ∼22 °C for NEE and GPP, and ∼25 °C for ET, and the fluxes peaked at ∼1.7 kPa vapor pressure deficit (VPD) for both crops. However, the GPP-PPFD (photosynthetic photon flux density) relationship was weaker beyond optimum Ta and VPD in canola than in wheat. Ecosystem light use efficiency (ELUE) and ecosystem water use efficiency (EWUE), determined using different metrics, were higher in wheat than in canola. The results indicate higher adaptability, and water and light use efficiencies of wheat than canola. This study provides an initial baseline on the dynamics of CO2 fluxes, ET, EWUE, and ELUE for canola, and side-by-side comparison of eddy fluxes in two major winter crops grown in the Southern Great Plains.

1. Introduction Wheat ranks third among field crops in the United States in terms of area planted and grain production, after maize (Zea mays L.) and soybean (Glycine max L.) (USDA-ERS, 2018). Wheat occupies ∼25% of the newly converted croplands from grasslands (Lark et al., 2015). It is the dominant crop in the Southern Great Plains (SGP) region of the United States, with the region accounting for ∼30% of nation's wheat production (Lollato and Edwards, 2015). Approximately 9 million ha of contiguous areas of low-precipitation are planted to wheat annually (Fischer et al., 2014). Continuous production of winter wheat is the dominant cropping

⁎

system in the SGP (Edwards et al., 2011; Redmon et al., 1995). However, winter annual grasses, especially Italian ryegrass (Lolium multiflorum Lam.) and annual bromes (Bromus spp.), have invaded fields used for continuous production of wheat (Barnes et al., 2001; White et al., 2006). These weeds are difficult to control, and have been reducing wheat yields through direct competition and wheat quality due to presence of foreign material in wheat grains (Appleby et al., 1976; Justice et al., 1994). However, controlling grassy weeds by applying herbicides to wheat is limited by the cost, treatment efficiency, and grazing restrictions on the use of wheat as pasture (Redmon et al., 1995; Trusler et al., 2007). Thus, crop rotations have been recognized as a successful strategy to control weeds in winter wheat (Daugovish et al.,

Corresponding author. E-mail addresses: [email protected], [email protected] (P. Wagle).

https://doi.org/10.1016/j.agrformet.2019.107714 Received 1 April 2019; Received in revised form 14 August 2019; Accepted 19 August 2019 0168-1923/ Published by Elsevier B.V.

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

1999). Crop rotations can also aid in breaking wheat disease and insect cycles that develop in continuous production of wheat. Recently, interest has grown in the SGP in rotating winter wheat to winter-hardy cultivars of canola (Brassica napus L.) (Duke et al., 2009; Gunstone, 2004; Lofton et al., 2010). Such a rotation was shown to result in significantly higher grain yields and net returns for following wheat crops compared to continuous wheat (Bushong et al., 2012). Both crops are planted and harvested at roughly the same time of year. Thus, the acreage of winter canola may continue to increase in the SGP as part of short-term rotations with winter wheat. Conventional tillage (CT) is the traditional management technique applied to winter wheat in the SGP (Hossain et al., 2004). However, conservation tillage (e.g., no-till, minimum tillage, residue management) has been gaining attraction worldwide in the last decades due to several impacts, including reduced soil erosion, increased soil moisture by enhancing water infiltration and reducing soil evaporation, and enhanced soil quality (Gajri et al., 1992; Hu et al., 2013; Ward et al., 2012; West and Post, 2002). As a result, both CT and no-till (NT) are commonly used in winter wheat and canola production in the region (Hossain et al., 2004). It is necessary to examine carbon and water dynamics of these two economically important crops under different tillage practices to examine their impacts on the dynamics of carbon and water budgets (Angers et al., 1997; Chi et al., 2016; Wagle et al., 2018b). Several studies have reported the magnitudes and seasonal dynamics of CO2 fluxes and/or ET in wheat using eddy covariance (EC) systems (Anthoni et al., 2004; Aouade et al., 2016; Aubinet et al., 2009; Bajgain et al., 2018; Burba and Verma, 2005; Fischer et al., 2014; Gilmanov et al., 2003; Liu et al., 2002; Schmidt et al., 2012; Wagle et al., 2018b). In comparison, few EC studies have been reported on canola for different purposes (Chi et al., 2017; Liu et al., 2012; Prescher et al., 2010). Chi et al. (2017) reported cumulative NEE and ET for winter wheat and spring canola sites in the Inland Pacific Northwest, USA to compare with the outputs of CropSyst model. Liu et al. (2012) measured ET in a canola field over 54 days in Coleambally Irrigation Area, Australia to calibrate and validate the performance of the advection-aridity (AA) and the Katerji and Perrier (KP) methods of ET estimations. Prescher et al. (2010) reported EC-based carbon budgets for a full crop rotation of winter barley (Hordeum vulgare L.), rape seed or canola, winter wheat, and maize near Dresden, Germany. However, those studies lacked details on the magnitudes and seasonal dynamics of CO2 fluxes and ET in canola. Furthermore, concurrent measurements of carbon gain, water loss, and climatic variables offer an opportunity to determine ecosystem water use efficiency (EWUE, the tradeoff between carbon gain and water loss) and ecosystem light use efficiency (ELUE, the efficiency of using absorbed radiation energy to gain carbon), and to characterize ecosystem responses to climatic controls (Barr et al., 2007; Law et al., 2002; Wagle et al., 2016a). However, such information is still limited for winter wheat and is lacking for canola. In addition, comparative studies of CO2 fluxes and ET from two co-located major winter crops (winter wheat and canola) do not exist in the SGP or worldwide. The objectives of this study were to determine and compare: 1) seasonal dynamics and magnitudes of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET); 2) responses of NEE, GPP, and ET to major climatic variables; 3) magnitudes and seasonal dynamics of ELUE and EWUE for co-located, paired (CT and NT) winter wheat and canola fields during a presumably favorable growing season (i.e., one of the best growing seasons). In addition to providing baseline information on the carbon and water dynamics, EWUE, and ELUE of minimally studied canola during a favorable growing season, this comparative study provides greater insights on how wheat and canola respond to the same climatic conditions.

Table 1 Management practices for winter wheat and canola fields managed under conventional till (CT) and no-till (NT) systems during the 2016–2017 growing season. Sites

Date

Activities

Wheat CT (27.5 ha)

9/6/2016 2016 (9/8, 10/8) 10/15/2016 10/16/2016 10/16/2016 3/3/2017 6/12/2017 10/17/2016 10/17/2016 10/18/2016 10/18/2016 3/3/2017 6/10/2017 9/6/2016 9/7/2016 10/4/2016 10/04/2016 11/4/2016 3/3/2017 6/15/2017 8/19/2016 9/19/2016 10/3/2016 10/3/2016 10/27/2016 3/3/2017 6/29/2017

Dry 46-0-0 fertilizer (broadcast) Tillage Seedbed preparation Planting Dry 32-23-0 Fertilizer (in furrow) Herbicide (Chem-Surf 90, Quelex) Harvesting Seedbed preparation UAN 28% Fertilizer (broadcast) Planting Dry 32-23-0 Fertilizer (in furrow) Herbicide (Chem-Surf 90, Quelex) Harvesting Dry 46-0-0 fertilizer (broadcast) Tillage Dry 18-46-00 Fertilizer (in furrow) Planting Glyphosate herbicide Glyphosate herbicide Harvesting Dry 46-0-0 fertilizer (broadcast) Glyphosate herbicide Dry 18-46-00 Fertilizer (in furrow) Planting Glyphosate herbicide Herbicide Harvesting

Wheat NT (18.7 ha)

Canola CT (17.2 ha)

Canola NT (20.5 ha)

2. Materials and methods 2.1. Site description This study included two wheat fields of 27.5 ha (CT) and 18.7 ha (NT) ha, and two canola fields of 17.2 ha (CT) and 20.5 ha (NT) located at the United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Grazinglands Research Laboratory, El Reno, Oklahoma (35°33′29″ N, 98°1′50″ W, and ∼414 m above sea level). Major soil types at the sites were a complex of Renfrow-Kirkland silt loams, Bethany silt loams, and Norge silt loams (USDA-NRCS, 1999). Tillage was initiated a year prior (in 2015) to this experiment. The EC towers were deployed at the beginning of the 2016–2017 growing season (September/October 2016). Wheat (cv. Gallagher at ∼90 kg seeds ha−1) and canola (cv. DKW46-16 at ∼6 kg seed ha−1) were sown at ∼19 cm row spacing. Detailed management practices for the fields during the 2016–2017 growing season are given in Table 1. Fertilizers were applied before planting in all fields. Over the course of the growing season, major management practices were fertilizer applications based on soil tests and herbicide applications based on necessity. Overall, crops were managed for high yield potential using practices common to the region. 2.2. Eddy covariance data collection and processing A 3-D sonic anemometer (CSAT3, Campbell Scientific Inc., Logan, Utah, USA) and an open path infrared gas analyzer (LI-7500-RS, LI-COR Inc., Lincoln, Nebraska, USA) were mounted at a 2.5 m height from the ground surface in a tripod near the center of each field to measure wind components and changes in CO2 and H2O concentrations at 10 Hz frequency. The fields were large enough to have adequate fetch length for the EC systems (more than 100 m to a few hundred meters in all directions). The 30-min values of supporting meteorological variables: air temperature (Ta), relative humidity (RH), net radiation (Rn), photosynthetic photo flux density (PPFD), soil water content (SWC), soil temperature (Ts), and soil heat (G) fluxes were collected in a CR1000 2

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 1. Seasonal evolution of aboveground dry biomass and leaf area index (LAI) for conventional till (CT) and no-till (NT) winter wheat and canola during the 2016–2017 growing season. Bars represent standard errors of the means.

(equivalent to ER). Finally, GPP was computed as a difference between ER and NEE. Details on post-processing of eddy flux data with REddyProc can be found in Wutzler et al. (2018). Negative values of NEE indicated a gain of carbon (sink) and positive values of NEE indicated a loss of carbon (source) by the fields in this study. The gap filled data were used to compute daily and seasonal sums, but only non-gap-filled, good quality flux data (quality flag 0) were used for all other analyses.

data logger (Campbell Scientific, Logan, UT, USA). Relative SWC was computed as (θ – θmin)/(θmax – θmin), where θmin and θmax are minimum and maximum values of soil water content (θ), respectively. The EddyPro software version 6.2.0 (LI-COR Inc., Lincoln, Nebraska, USA) was used to compute 30-min fluxes of NEE and ET. Poor quality fluxes with flag 2 (bad quality) were discarded. Physically unreasonable fluxes: CO2 fluxes beyond ± 50 µmol m−2 s−1, sensible heat (H) fluxes beyond −200 and 500 W m−2, and latent heat (LE) fluxes beyond −200 and 800 W m−2 were removed (Sun et al., 2010; Wagle and Kakani, 2014; Zeeman et al., 2010). In addition, statistical outliers (i.e., beyond ± 3.5 standard deviation) based on two-week running windows were excluded (Wagle et al., 2017; Wagle et al., 2015a). Fluxes during calm conditions (u* <0.1 m s−1) were also removed. Gaps in NEE and ET data were ∼17% and 20% for CT wheat, ∼22% and 25% for NT wheat, ∼17% and 19% for CT canola, and ∼23% and 25% for NT canola, respectively. The REddyProc package (https://www.bgc-jena.mpg.de/bgi/index. php/Services/REddyProcWebRPackage) from the Max Planck Institute for Biogeochemistry, Germany was used to fill gaps in data and to partition NEE into GPP and ecosystem respiration (ER). In REddyProc, three gap-filling methods: look-up tables (LUT), mean diurnal course (MDC), and marginal distribution sampling (MDS) are implemented (Wutzler et al., 2018). The eddy fluxes and meteorological data are gap filled using methods similar to Falge et al. (2001) but also exploiting the covariation of fluxes with the meteorological variables and their temporal autocorrelation based on LUT and MDC methods (Reichstein et al., 2005). For the flux partitioning, we used the most widely used nighttime-based flux partitioning approach (Reichstein et al., 2005) using an Arrhenius-type (Lloyd and Taylor, 1994) temperature response function of nighttime NEE

2.3. Ecosystem water use and light use efficiencies The EWUE was calculated as the ratio of sums of carbon gain (GPP) to water loss (ET), and ELUE was calculated as the ratio of sums of carbon gain (GPP) to photosynthetically active radiation (PAR) (Wagle et al., 2016a). As EWUE was calculated as the ratio of total carbon gain (GPP) to total water loss (ET), and small amount of ET loss occurs during nighttime, we used nighttime ET as well in the calculation of EWUE. The ELUE was computed only for daytime since GPP and PAR are zero during nighttime. We also determined ELUE for daytime NEE, GPP, and ET from the slope of linear regressions of NEE-PPFD, GPP-PPFD, and ET-PPFD relationships, respectively (Wagle et al., 2018a). Similarly, EWUE was determined for NEE and GPP from the slope of linear regressions of NEE-ET and GPP-ET relationships (Wagle et al., 2016b). The EWUE was also determined based on grain yield and seasonal ET to facilitate inter-site comparisons with previous studies. 2.4. Biometric measurements and harvesting for grains Leaf area index (LAI) was measured non-destructively using an LAI2200C plant canopy analyzer (LI-COR Inc., Lincoln, Nebraska, USA). 3

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 2. Dynamics (7-day average) of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) for conventional till (CT) and no-till (NT) winter wheat and canola during the 2016–2017 growing season. Bars represent standard errors of the means.

Aboveground biomass was collected destructively from five randomly located 0.5 × 0.5 m2 quadrats within each field at every two weeks intervals during the active growing season. Dry biomass weights were recorded after drying samples in forced-air oven at 70 °C for a minimum of 48 h. Wheat fields were harvested for grain during June 10–12, 2017, while the CT and NT canola fields were harvested for grain on June 15 and 29, 2017, respectively.

and ET for both crops. To further examine the saturation of NEE to PPFD, we further sorted daytime good quality NEE data for CT winter wheat and CT canola fields for the peak growth (March and April 2017) into increasing PPFD at every 100 µmol m−2 s−1 intervals for high PPFD levels (1400–1500, 1500–1600, 1600–1700, 1700–1800, 1800–1900, and >1900 µmol m−2 s−1). 3. Results and discussion

2.5. Reponses of NEE, GPP, and ET to major climatic variables 3.1. Weather conditions and crop growth Daytime (PPFD >5 µmol m−2 s−1) good quality (non-gap-filled data with quality flag 0) NEE, GPP, and ET from November 2016 to April 2017 (i.e., before the initiation of senescence) were sorted into multiple bins of increasing PPFD, Ta, and VPD to compare responses of NEE, GPP, and ET to those climatic variables for wheat and canola. Optimum values of PPFD, Ta, and VPD were determined for NEE, GPP,

The study sites received 517 mm of well-distributed total precipitation from October 2016 to May 2017, which was slightly lower (∼9%) than the 30-year (1981–2010) mean of 567 mm for the same period. Overall, severe droughts were not observed during the growing season. In addition, January–March 2017 period was slightly warmer as 4

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 3. Soil temperature (Ts) and relative soil water content (SWC) in conventional till (CT) and no-till (NT) canola fields during the 2016–2017 growing season.

the SGP (Oklahoma, Texas, and Kansas) in the past (Patrignani et al., 2014). The grain yield of nearly 5 t ha−1 in this study was substantially higher than the average yields of <2.7 t ha−1 for winter wheat in Oklahoma for the same year (USDA-NASS, 2018). Grain yield approximated 0.91 t ha−1 and 0.97 t ha−1 at CT and NT canola fields, respectively. Canola yields of ∼1.6 and ∼1.9 t ha−1 were reported from plot level studies at nearby Chickasha and Perkins locations, respectively, in 2009 (Bushong et al., 2012). However, yields can be overestimated when extrapolating from plot level studies to larger scales. If harvested on time, the tentative grain yield was estimated at ∼35 bu ac−1 (∼1.96 t ha−1) for the CT field in this study. Grain loss to shattering related to delayed harvesting caused by rains resulted in lower yields of canola during this study. Based on our direct observations of the fields during the entire study period and reported grain yields, the growing conditions were presumably optimal for both crops.

Table 2 Growing season (November 2016 – May 2017) sums of net ecosystem CO2 exchange (NEE, g C m−2), gross primary production (GPP, g C m−2), and evapotranspiration (ET, mm d−1), and growing season average ecosystem light use efficiency (ELUE, computed as the ratio of cumulative GPP to PAR, g C mol−1 PAR) and ecosystem water use efficiency (EWUE, computed as the ratio of cumulative GPP to ET, g C mm−1 ET) for winter wheat and canola fields managed under conventional till (CT) and no-till (NT) systems. Sites

NEE

GPP

ET

ELUE

EWUE

Wheat CT Wheat NT Canola CT Canola NT

−567 −513 −342 −181

1514 1426 1147 959

460 470 413 390

0.24 0.23 0.18 0.15

3.29 3.03 2.78 2.46

compared to the 30-year means. As a result, crop growth and productivity were observed at the level of maximum potential, indicating presumably favorable (i.e., one of the best growing seasons) for wheat and canola. Seasonal dynamics of biomass and LAI were similar in both wheat fields, with maximum aboveground dry biomass of ∼1.3 kg m−2 (equivalent to 13 t ha−1) and LAI of ∼7 m2 m−2 in April (Fig. 1). Biomass and LAI were similar in both canola fields during fall 2016, but were substantially lower in the NT field in spring 2017 due to poor recovery of the crop after winter dormancy. As a result, maximum LAI were 4.75 ± 0.25 and 5.33 ± 0.3 m2 m−2 in December, but maximum dry biomass were 0.82 ± 0.03 and 0.41 ± 0.04 kg m−2 in April at CT and NT canola fields, respectively. Grain yield averaged approximately 4.86 and 3.53 t ha−1 for CT and NT wheat fields, respectively. The recorded grain yields of wheat in this study were higher than the average yield (∼3.0 t ha−1) recorded for

3.2. Magnitudes of NEE, GPP, and ET Seven-day averages daily peak NEE, GPP, and ET were approximately −8 g C m−2 d−1, 16 g C m−2 d−1, and 5 mm d−1 for both CT and NT wheat fields, respectively (Fig. 2). The magnitudes of NEE, GPP, and ET were similar for CT and NT wheat fields due to uniform canopy stands, as reflected by similar biomass and LAI in Fig. 1. We observed daily maximum NEE of approximately −11 g C m−2 d−1, GPP of 18.5 g C m−2 d−1, and ET of 6.2 for wheat fields. The observed NEE of ∼−11 g C m−2 d−1 in this study was similar or slightly higher (absolute values) to those reported (<−10 C m−2 d−1) for winter wheat in Oklahoma (Arora, 2003; Bajgain et al., 2018; Fischer et al., 2007) and slightly smaller than those reported (up to −12 to 5

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 4. Monthly sums of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) for conventional till (CT) and no-till (NT) winter wheat and canola during the 2016–2017 growing season.

−13 g C m−2 d−1) for winter wheat in Germany (Anthoni et al., 2004; Schmidt et al., 2012). However, maximum GPP (∼19 g C m−2 d−1), aboveground biomass (∼1.3 kg m−2, and LAI (>6 m−2 m−2) were similar for winter wheat fields in Germany (Schmidt et al., 2012) and Oklahoma (this study). Grain yields were substantially higher (8–9 t ha−1) for winter wheat in Germany (Anthoni et al., 2004; Schmidt et al., 2012) as compared to ∼5 t ha−1 or less in Oklahoma. The recorded daily ET rates of 6–6.25 mm for wheat in our study were slightly smaller than those reported for rainfed winter wheat (up to 7 mm) in north-central Oklahoma (Burba and Verma, 2005), which may be attributed to higher annual rainfall (1202–1342 mm yr−1) during their study period, compared to ∼925 mm long-term average annual rainfall for our study area. Maximum daily ET was recorded up to 7.3 mm in an irrigated winter wheat field under semi-arid climate in central Morocco (Aouade et al., 2016). In comparison, the maximum ET for winter wheat was only ∼5 mm d−1 in Thuringia, Germany

(Anthoni et al., 2004) though NEE rates and grain yields were higher than in our study. The results indicated greater uptake of carbon and greater water use efficiency of winter wheat in Germany. This can be attributed to lower respiratory (ER) and ET losses due to lower temperature (mean annual temperature of <10 °C as compared to ∼15 °C for our study site) and annual precipitation (∼500 mm as compared to ∼900 mm for our study site) at wheat sites in Germany. For example, ER declined from 12–13 g C m−2 d−1 at around 20 °C to ∼4 g C m−2 d−1 at around 10 °C for wheat in Germany (Schmidt et al., 2012). We observed ER up to ∼10 g C m−2 d−1, while ER reached up to ∼8 g C m−2 d−1 for wheat in Germany (Aubinet et al., 2009). Seven-day averages daily peak NEE were −5.20 ± 0.49 and −4.66 ± 0.36 g C m−2 d−1, GPP were 12.47 ± 1.16 and 10.86 ± 0.97 g C m−2 d−1, and ET were 4.7 ± 0.42 and 4.28 ± 0.36 mm d−1 for CT and NT canola fields, respectively (Fig. 2). We observed daily maximum NEE of −8.6 g C m−2 d−1, GPP of 6

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 5. Response of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) to air temperature. Daytime NEE, GPP, and ET were aggregated in classes of increasing air temperature for the active growth period of November 2016 through April 2017. Bars represent standard errors of the means. Vertical lines represent optimum values of air temperature.

14.6 g C m−2 d−1, and ET of 6.2 mm d−1 for CT canola. The ET rates from 2 to 8 mm d−1 were reported over the EC measurement period of 54 days in an irrigated canola field in Coleambally Irrigation Area (semi-arid climate), Australia (Liu et al., 2012). Weighing lysimeterbased measurement showed ET rates up to ∼10 mm d−1 in an irrigated canola field located in central Spain (semi-arid, temperate Mediterranean climate) (Sánchez et al., 2014). Poor recovery of canola stands in the NT field after winter dormancy caused substantially smaller magnitudes of fluxes during spring 2017. Soil temperature was lower (average of ∼3 °C) throughout the growing season, and relative SWC was higher (average of 0.4), especially from mid-January through harvesting, for NT canola than CT canola (Fig. 3). Note that our measurements of soil moisture at 5-cm depth near the flux tower do not reflect the actual SWC of water logging conditions in micro-depressions in different parts of the NT field during higher

rainfall in February–March. Lower Ts and poor drainage condition could be the reasons for poor stand recovery after winter dormancy in the NT field. Few extension reports have mentioned that canola requires well-drained conditions to perform well. More winter kill problems have been observed in NT conditions under heavy residue by farmers in the region (Rich, 2014). 3.3. Seasonal dynamics and cumulative values of NEE, GPP, and ET Wheat fields behaved as carbon sinks from mid-November through the first week of May (Fig. 2). However, they were near carbon neutral during mid-December to mid-January, with an average NEE <−0.7 g C m−2 d−1, due to cold temperatures and lower solar radiation. The seasonal dynamics of NEE, GPP, and ET were similar for the entire growing season for CT and NT wheat fields due to similar 7

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 6. Response of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) to vapor pressure deficit (VPD). Daytime NEE, GPP, and ET were aggregated in classes of increasing VPD for the active growth period of November 2016 through April 2017. Bars represent standard errors of the means. Vertical lines represent optimum values of VPD.

recovery, as reflected in dry biomass and LAI in Fig. 1, large differences in NEE and GPP occurred from the end of February to the end of April. As a result, CT canola field gained about 50% more carbon compared to NT canola field at the seasonal scale (November 2016 – May 2017), with cumulative NEE of −342 (CT) and −181 (NT) g C m−2, and GPP of 1147 (CT) and 959 (NT) g C m−2 (Table 2). Prescher et al. (2010) reported cumulative NEE of −224 g C m−2 for a year-long rape seed period. In comparison, when accounted for carbon loss of ∼250 g C m−2 at our CT canola field during the summer fallow period, the annual NEE balance of CT canola field was −92 g C m−2. If removal of carbon from harvesting grain (∼1 t ha−1, equivalent to 100 g m−2) was accounted, assuming 58% of carbon in dry grains (Prescher et al., 2010) the loss of carbon is estimated to be 58 g C m−2. If it was harvested on time and the grain yield was ∼2 t ha−1 then the loss of carbon estimate would be 116 g C m−2. Accounting for the loss of carbon from grain harvest and some uncertainties in the measurements,

canopy stands as shown in Fig. 1. The similar rates and seasonal dynamics of fluxes from two wheat fields with uniform canopy stands indicated the capacity of EC systems to properly capture and record CO2 fluxes and ET on an ecosystem level. Both wheat fields were strong sinks of carbon at the seasonal scale (November 2016 – May 2017), with cumulative NEE of −567 (CT) and −513 (NT) g C m−2, and GPP of 1514 (CT) and 1426 (NT) g C m−2 (Table 2). Wheat fields were the largest sinks of carbon during March, with monthly NEE sums of −170 to −180 g C m−2 (Fig. 4). Chi et al. (2017) reported monthly NEE sums up to approximately −300 g C m−2 for winter wheat in the Inland Pacific Northwest, USA. Canola fields behaved as carbon sinks from late October to midDecember, then acted as carbon sources from mid-December through early February (CT)/late February (NT), and were carbon sinks until early May (both fields). The rates of CO2 fluxes were similar until end of January for both canola fields (Fig. 2). Due to differences in winter 8

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 7. Linear relationships of net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) to incident photosynthetic photon flux density (PPFD). Daytime NEE, GPP, and ET were aggregated in classes of increasing PPFD for the active growth period of November 2016 through April 2017.

our results indicate that the annual carbon status of canola, with summer fallow, can be a small sink to small source in this region based on removal of grain yields. The ET rates increased with increasing crop growth and accumulation of biomass in the early growing season, then decreased to low levels (0.5 mm d−1 or less) during mid-December to mid-January due to cold weather and shorter periods of daylight (Fig. 2). The ET rates increased after mid-January through early to mid-May due to rising temperatures and increasing amounts of crop growth. Thereafter, ET rates declined sharply with grain fill and eventual crop senescence. Monthly ET sums for both wheat fields were similar during the growing season, ranging from 25 to 30 mm in December to >110 mm in May (Fig. 4). Chi et al. (2017) also reported monthly ET >110–120 mm in the Inland Pacific Northwest, USA. Seasonal cumulative ET was similar (460–470 mm, Table 2) for both wheat fields due to similar canopy characteristics. Cumulative ET for wheat during the most active growth period (February–May) in 2017 was approximately 80% of cumulative rainfall (457 mm) for the same period, while cumulative ET during the entire growing season (November 2016 – May

Table 3 Average net ecosystem CO2 exchange (NEE) at the given photosynthetic photon flux density (PPFD) for conventional till (CT) wheat and canola fields. Daytime good quality (non-gap-filled) NEE data for the peak growth (March and April 2017) were sorted into increasing PPFD at every 100 µmol m−2 s−1 intervals for high PPFD levels (1400–1500, 1500–1600, 1600–1700, 1700–1800, 1800–1900, and >1900 µmol m−2 s−1). Maximum NEE (absolute values) and the corresponding PPFD values are highlighted in bold. Wheat CT PPFD

NEE

1451 1552 1646 1741 1843 1963

−26.72 −29.92 −32.56 −33.30 −36.63 −31.08

± ± ± ± ± ±

0.95 0.69 0.80 0.91 1.01 1.73

Canola CT PPFD

NEE

1454 1549 1649 1747 1846 1943

−20.51 −21.91 −21.06 −24.19 −23.56 −23.72

± ± ± ± ± ±

0.57 0.55 0.50 0.63 1.0 1.22

9

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 8. Response of gross primary production (GPP) to incident photosynthetic photon flux density (PPFD) for the periods of air temperature (Ta) >22 °C and vapor pressure deficit (VPD) >1.7 kPa for the active growth period of November 2016 through April 2017.

10

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 9. Dynamics (7-day average) of ecosystem water use efficiency (EWUE) and ecosystem light use efficiency (ELUE) for conventional till (CT) and no-till (NT) winter wheat and canola during the 2016–2017 growing season.

beyond VPD of 1.7 kPa, but ET plateaued or did not decline at higher VPD for both crops. The results showed that optimum Ta was ∼22 °C for CO2 fluxes and ∼25 °C for ET in both crops. An optimum Ta range of 17–23°C was reported for wheat growth and development by a review article (Porter and Gawith, 1999). An optimum Ta of 25 °C was reported for wheat photosynthesis by another study (Wardlaw, 1974). An optimum Ta of 25 °C was used to simulate growth and development of canola in Australia (Robertson et al., 1999). Another study reported 20 °C as an optimum Ta for canola growth and development under controlled environmental conditions in Australia (Robertson et al., 2002). Note that optimum Ta and VPD for crop growth or photosynthesis can vary for the same crop due to different environmental conditions, stage of development, genotypes/cultivars, degree of water stress, and so on. For example, optimum Ta for photosynthesis increased by 3 °C when mean growth Ta was increased by 4 °C for winter wheat (Sawada, 1970). Thus, it is difficult to conclusively determine optimum Ta for crop growth and development. However, these results will provide some baseline information regarding optimum Ta for winter wheat and canola. Although partial stomatal closure at higher Ta and VPD decreases leaf stomatal conductance to both CO2 fluxes and ET (Morison, 1985), loss of water (i.e., ET) was less affected than gain of carbon (i.e., NEE or GPP) by stomatal controls in response to increased Ta and VPD as shown in previous studies (Wagle et al., 2018a; Wagle et al., 2015b). Comparison of stomatal conductance in winter wheat at ∼20, 27, and 34 °C of Ta with increasing VPD by 1 kPa showed that stomatal conductance decreased almost linearly from 20 to 27 °C and from 27 to 34 °C under ambient CO2 condition but stomatal conductance did not decline up to ∼27 °C under elevated CO2 condition (Bunce, 2000). The NEE, GPP, and ET had strong linear relationships (R2 >0.90)

2017) was >90% of cumulative rainfall (502 mm) for the same period. Similarly, monthly ET sums for both canola fields were similar during the growing season, ranging from 20–25 mm during December and January to >110 mm in May (Fig. 4). Chi et al. (2017) also reported monthly ET >110 mm for NT canola and >125 mm for CT canola in the Inland Pacific Northwest, USA. Seasonal cumulative ET of canola fields was 413 (CT) and 390 (NT) mm (Table 2). Cumulative ET for canola during the most active growth period (February–May) in 2017 was approximately 65–70% of cumulative rainfall for the same period, while cumulative ET during the entire growing season (November 2016 – May 2017) was approximately 80% of cumulative rainfall (502 mm) for the same period. Similar cumulative ET were noted for CT and NT canola despite large differences in canopy stand and CO2 fluxes during spring 2017, indicating more evaporative loss of water from the NT canola field due to sparse vegetation after winter dormancy. Larger magnitudes of CO2 fluxes and longer periods of sinks of carbon for wheat caused large differences in the potential for carbon sequestration between wheat and canola. Wheat fields showed larger carbon sink potential during winter (December–February) than canola, as cumulative NEE during winter was −187 (CT) and −181 (NT) g C m−2 at wheat fields compared to −29 (CT) and 38 (NT) g C m−2 at canola fields.

3.4. Responses of NEE, GPP, and ET to major climatic variables For both crops, NEE and GPP increased almost linearly with increasing Ta to ∼22 °C and declined thereafter (Fig. 5). Similarly, ET also increased almost linearly with increasing Ta to ∼25 °C and declined thereafter for both crops. The fluxes peaked at ∼1.7 kPa VPD for both crops (Fig. 6). However, CO2 fluxes (NEE and GPP) declined 11

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

Fig. 10. Linear relationships of weekly average values of net ecosystem CO2 exchange (NEE) and gross primary production (GPP) with evapotranspiration (ET) for conventional till (CT) and no-till (NT) winter wheat and canola during the 2016–2017 growing season.

2017), but were smaller than those reported (0.40–0.55 g C mol−1 PAR) for highly productive rainfed and irrigated maize, a C4 crop, in Nebraska (Wagle et al., 2016a). Similarly, peak values of weekly EWUE reached ∼5.2 g C mm−1 ET for wheat and ∼4.3 g C mm−1 ET for canola. Surprisingly, these EWUE magnitudes were comparable to those reported (4.22–5.81 g C mm mm−1 ET) for highly productive rainfed and irrigated maize in Nebraska (Wagle et al., 2016a). Higher EWUE and ELUE values are anticipated in maize (C4 crop) due to greater photosynthetic capacity as compared to the C3 wheat. However, our results indicated that winter wheat also has high EWUE potential in the SGP. This high potential EWUE for winter wheat in the SGP can be possible due to the fact that cool-season crops like winter wheat are at the most productive growth stages in early spring when evaporative demand is low. At the growing season scale, ELUE was 0.24 (CT) and 0.23 (NT) g C mol−1 PAR for wheat fields, and 0.18 (CT) and 0.15 (NT) g C mol−1 PAR for canola. Similarly, growing season average EWUE was 3.29 (CT) and 3.03 (NT) g C mm−1 ET for wheat fields, and 2.78 (CT) and 2.46 (NT) g C mm−1 ET for canola. The slopes of NEE-PPFD, GPP-PPFD, and ET-PPFD relationships represent ELUE for NEE, GPP, and ET, respectively. The ELUE values (i.e., slopes) for NEE, GPP, and ET were substantially higher in wheat than canola (Fig. 7). The slopes of NEE-PPFD, GPP-PPFD, and ET-PPFD relationships were identical (0.019 for NEE, 0.02 for GPP, and 0.0035–0.0037 for ET) for CT and NT wheat due to similar canopy stands and magnitudes of fluxes. In comparison, the slopes of NEE-PPFD and GPP-PPFD relationships were substantially higher for CT canola (0.012 for NEE and 0.014 for GPP) than NT canola (0.007 for NEE and 0.008 for GPP) due to poor recovery of canola after winter dormancy in the NT field. However, the slope of ET-PPFD relationship was similar for CT (0.0026) and NT (0.0025) canola fields. Large differences in

with PPFD for both crops (Fig. 7). It is widely accepted that PPFD is the most significant climatic variable for variations in carbon gain and water loss (Ruimy et al., 1995; Wagle et al., 2016b). However, fluxes can decline at high levels of PPFD due to saturation of stomatal conductance. A study reported that stomatal conductance saturated at ∼1500 μmol m−2 s−1 PPFD for winter wheat grown even under elevated CO2 in Maryland, USA (Bunce, 2000). Further examination of the saturation of NEE to PPFD for the peak growth only (March and April 2017) by binning at every 100 µmol m−2 s−1 intervals for high PPFD levels showed that NEE saturated at ∼1850 µmol m−2 s−1 for winter wheat and ∼1750 µmol m−2 s−1 for canola in our study (Table 3). Since binning data in different classes for a particular climatic variable can minimize the confounding effect of other climatic variables, we further examined the GPP-PPFD relationships at higher than optimum values of Ta (>22 °C) and VPD (>1.7 kPa) reported above (Fig. 8). The scatter plots showed that GPP and PPFD were correlated more in wheat than in canola, indicating greater adaptability of winter wheat to higher Ta and VPD than canola.

3.5. Magnitudes and seasonal dynamics of ELUE and EWUE Seasonal dynamics of ELUE and EWUE followed the growth patterns of winter wheat and canola (Fig. 9). Seasonal dynamics and magnitudes of ELUE and EWUE were similar for CT and NT wheat fields, but differences in canopy stand in spring 2017 for CT and NT canola fields caused large discrepancies in seasonal dynamics and magnitudes of ELUE and EWUE. Peak values of weekly ELUE reached ∼0.4 g C mol−1 PAR for wheat and ∼0.37 g C mol−1 PAR for canola. These ELUE values were higher than the maximum weekly ELUE (∼0.22 g C mol−1 PAR) reported for rainfed soybean at the same location (Wagle et al., 12

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

References

canopy stands and ELUE for NEE and GPP, but similar ELUE for ET between CT and NT canola fields indicated more contribution of evaporative loss to ET due to sparse vegetation after winter dormancy. The slopes of NEE-ET and GPP-ET relationships represent EWUE for NEE and GPP, respectively. Like ELUE values, EWUE values (i.e., slopes) for NEE and GPP were higher in wheat than in canola (Fig. 10). The slopes for NEE-ET relationships were 1.65–1.67 for wheat fields as compared to 1.35–1.39 for canola fields. The slope for GPP-ET relationship was 3.65 ± 0.3 for CT wheat as compared to 3.47 ± 0.24 for CT canola field. Substantial differences in carbon uptake than the use of water and light resulted in higher light and water use efficiencies for winter wheat than canola in the SGP.

Angers, D., et al., 1997. Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada. Soil Tillage Res. 41 (3), 191–201. Anthoni, P.M., Freibauer, A., Kolle, O., Schulze, E.-D., 2004. Winter wheat carbon exchange in Thuringia, Germany. Agricult. For. Meteorol. 121 (1–2), 55–67. Aouade, G., et al., 2016. Combining stable isotopes, Eddy Covariance system and meteorological measurements for partitioning evapotranspiration, of winter wheat, into soil evaporation and plant transpiration in a semi-arid region. Agric. Water Manag. 177, 181–192. Appleby, A., Olson, P., Colbert, D., 1976. Winter wheat yield reduction from interference by Italian ryegrass 1. Agron. J. 68 (3), 463–466. Arora, V.K., 2003. Simulating energy and carbon fluxes over winter wheat using coupled land surface and terrestrial ecosystem models. Agricult. For. Meteorol. 118 (1), 21–47. Aubinet, M., et al., 2009. Carbon sequestration by a crop over a 4-year sugar beet/winter wheat/seed potato/winter wheat rotation cycle. Agricult. For. Meteorol. 149 (3), 407–418. Bajgain, R., et al., 2018. Carbon dioxide and water vapor fluxes in winter wheat and tallgrass prairie in central Oklahoma. Sci. Total Environ. 644, 1511–1524. Barnes, M.A., Peeper, T.F., Epplin, F.M., Krenzer, E.G., 2001. Effects of herbicides on Italian ryegrass (Lolium multiflorum), forage production, and economic returns from dual-purpose winter wheat (Triticum aestivum). Weed Technol. 15 (2), 264–270. Barr, A.G., et al., 2007. Climatic controls on the carbon and water balances of a boreal aspen forest, 1994–2003. Global Change Biol. 13 (3), 561–576. Bunce, J.A., 2000. Responses of stomatal conductance to light, humidity and temperature in winter wheat and barley grown at three concentrations of carbon dioxide in the field. Global Change Biol. 6 (4), 371–382. Burba, G.G., Verma, S.B., 2005. Seasonal and interannual variability in evapotranspiration of native tallgrass prairie and cultivated wheat ecosystems. Agricult. For. Meteorol. 135 (1), 190–201. Bushong, J.A., Griffith, A.P., Peeper, T.F., Epplin, F.M., 2012. Continuous winter wheat versus a winter canola–winter wheat rotation. Agron. J. 104 (2), 324–330. Chi, J., et al., 2017. Carbon and water budgets in multiple wheat-based cropping systems in the Inland Pacific Northwest US: comparison of CropSyst simulations with eddy covariance measurements. Front. Ecol. Evol. 5, 50. Chi, J., et al., 2016. Assessing carbon and water dynamics of no-till and conventional tillage cropping systems in the inland Pacific Northwest US using the eddy covariance method. Agricult. For. Meteorol. 218, 37–49. Daugovish, O., Lyon, D.J., Baltensperger, D.D., 1999. Cropping systems to control winter annual grasses in winter wheat (Triticum aestivum). Weed Technol. 120–126. Duke, J.C., Epplin, F.M., Vitale, J.D., Peeper, T.F., 2009. Canola-wheat rotation versus continuous wheat for the Southern Plains. Selected paper In: Atlanta: Southern Agricultural Economics Association Annual Meeting. Atlanta, Georgia. January. Edwards, J., Carver, B., Horn, G., Payton, M., 2011. Impact of dual-purpose management on wheat grain yield. Crop Sci. 51 (5), 2181–2185. Falge, E., et al., 2001. Gap filling strategies for defensible annual sums of net ecosystem exchange. Agricult. For. Meteorol. 107 (1), 43–69. Fischer, M.L., Billesbach, D.P., Berry, J.A., Riley, W.J., Torn, M.S., 2007. Spatiotemporal variations in growing season exchanges of CO2, H2O, and sensible heat in agricultural fields of the Southern Great Plains. Earth Interact. 11 (17), 1–21. Fischer, R., Byerlee, D., Edmeades, G., 2014. Crop Yields and Global Food Security. ACIAR, Canberra, ACT. Gajri, P., Arora, V., Prihar, S., 1992. Tillage management for efficient water and nitrogen use in wheat following rice. Soil Tillage Res. 24 (2), 167–182. Gilmanov, T.G., et al., 2003. Gross primary production and light response parameters of four Southern Plains ecosystems estimated using long‐term CO2‐flux tower measurements. Global Biogeochem. Cycles 17 (2). Gunstone, F.D., 2004. Rapeseed and Canola Oil: Production, Processing, Properties and Uses. CRC Press. Hossain, I., Epplin, F.M., Horn, G.W., Krenzer Jr, E.G., 2004. Wheat production and management practices used by Oklahoma grain and livestock producers. Oklahoma Agricultural Experimental Station, B-818. Oklahoma State University, Stillwater. Hu, Z., et al., 2013. Soil respiration, nitrification, and denitrification in a wheat farmland soil under different managements. Commun. Soil Sci. Plant Anal. 44 (21), 3092–3102. Justice, G.G., Peeper, T.F., Solie, J.B., Epplin, F.M., 1994. Net returns from Italian ryegrass (Lolium multiflorum) control in winter wheat (Triticum aestivum). Weed Technol. 8 (2), 317–323. Lark, T.J., Salmon, J.M., Gibbs, H.K., 2015. Cropland expansion outpaces agricultural and biofuel policies in the United States. Environ. Res. Lett. 10 (4), 044003. Law, B., et al., 2002. Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation. Agricult. For. Meteorol. 113 (1), 97–120. Liu, C., Zhang, X., Zhang, Y., 2002. Determination of daily evaporation and evapotranspiration of winter wheat and maize by large-scale weighing lysimeter and microlysimeter. Agricult. For. Meteorol. 111 (2), 109–120. Liu, G., Liu, Y., Hafeez, M., Xu, D., Vote, C., 2012. Comparison of two methods to derive time series of actual evapotranspiration using eddy covariance measurements in the southeastern Australia. J. Hydrol. 454, 1–6. Lloyd, J., Taylor, J., 1994. On the temperature dependence of soil respiration. Funct. Ecol. 315–323. Lofton, J., Godsey, C., Zhang, H., 2010. Determining aluminum tolerance and critical soil pH for winter canola production for acidic soils in temperate regions. Agron. J. 102 (1), 327–332. Lollato, R.P., Edwards, J.T., 2015. Maximum attainable wheat yield and resource-use

4. Conclusions Net ecosystem exchange (NEE) of carbon dioxide (CO2) and evapotranspiration (ET) from co-located, paired (conventional till, CT and no-till, NT) winter wheat and canola fields were compared during a presumably favorable growing season (i.e., one of the best growing seasons) for both crops. The magnitudes of CO2 fluxes and ET were larger for wheat than canola. In addition, wheat fields were sinks of carbon for longer periods, including winter months (December–February). As a result, large differences were observed in carbon sequestration potential between wheat and canola though both crops were strong sinks of carbon at the seasonal scale. The fluxes for both crops exhibited similar optimum values of Ta (∼22 °C for NEE and GPP, and ∼25 °C for ET) and VPD (∼1.7 kPa), indicating similar functional responses and well adaptation of both crops in the SGP. However, the GPP-PPFD relationships were weaker (i.e., more scatter) beyond optimum Ta and VPD in canola than in wheat, illustrating greater adaptability of wheat for higher Ta and VPD. Consequently, wheat was more efficient than canola in using water and solar radiation to gain more carbon. This side-by-side comparison of two major winter crops offers more insights into their carbon and water dynamics, and functional responses to climate in the SGP.

Acknowledgments A research grant (Project No. 2013-69002) through the USDANIFA's Agriculture and Food Research Initiative (AFRI) partly supported this study. The authors would like to thank Research Technicians Craig Mittelstaedt, Jeff Weik, and Kory Bollinger for management of the study fields and assistance in data collection. Disclaimer “Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.” “The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual's income is derived from any public assistance program. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA's TARGET Center at (202) 7202600 (voice and TDD). To file a complaint of discrimination, write to USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer.” 13

Agricultural and Forest Meteorology 279 (2019) 107714

P. Wagle, et al.

USDA-NRCS, 1999. Soil Survey of Canadian County, Oklahoma. Supplement Manuscript. USDA-NRCS and Okla. Agric. Exp. Stn., Stillwater, OK. USDA-NASS, 2018. August Wheat Production. https://www.nass.usda.gov/Statistics_by_ State/Oklahoma/Publications/Oklahoma_Crop_Reports/2018/spr-wheat-prod-082018.pdf (verified on March 13, 2019). Wagle, P., Gowda, P.H., Anapalli, S.S., Reddy, K.N., Northup, B.K., 2017. Growing season variability in carbon dioxide exchange of irrigated and rainfed soybean in the southern United States. Sci. Total Environ. 593, 263–273. Wagle, P., Gowda, P.H., Moorhead, J.E., Marek, G.W., Brauer, D.K., 2018a. Net ecosystem exchange of CO2 and H2O fluxes from irrigated grain sorghum and maize in the Texas High Plains. Sci. Total Environ. 637, 163–173. Wagle, P., et al., 2018b. Variability in carbon dioxide fluxes among six winter wheat paddocks managed under different tillage and grazing practices. Atmos. Environ. 185, 100–108. Wagle, P., Gowda, P.H., Xiao, X., Kc, A., 2016a. Parameterizing ecosystem light use efficiency and water use efficiency to estimate maize gross primary production and evapotranspiration using MODIS EVI. Agricult. For. Meteorol. 222, 87–97. Wagle, P., Kakani, V.G., 2014. Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand. GCB Bioenergy 6 (4), 339–350. Wagle, P., Kakani, V.G., Huhnke, R.L., 2015a. Net ecosystem carbon dioxide exchange of dedicated bioenergy feedstocks: switchgrass and high biomass sorghum. Agricult. For. Meteorol. 207, 107–116. Wagle, P., Kakani, V.G., Huhnke, R.L., 2016b. Evapotranspiration and ecosystem water use efficiency of switchgrass and high biomass sorghum. Agron. J. 108 (3), 1007–1019. Wagle, P., et al., 2015b. Biophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States. Agricul. For. Meteorol. 214, 293–305. Ward, P., Flower, K., Cordingley, N., Weeks, C., Micin, S., 2012. Soil water balance with cover crops and conservation agriculture in a Mediterranean climate. Field Crops Res. 132, 33–39. Wardlaw, I., 1974. Temperature control of translocation. Bull. R. Soc. NZ. West, T.O., Post, W.M., 2002. Soil organic carbon sequestration rates by tillage and crop rotation. Soil Sci. Soc. Am. J. 66 (6), 1930–1946. White, A.D., Lyon, D.J., Mallory-Smith, C., Medlin, C.R., Yenish, J.P., 2006. Feral rye (Secale cereale) in agricultural production systems. Weed Technol. 20 (3), 815–823. Wutzler, T., et al., 2018. Basic and extensible post-processing of eddy covariance flux data with REddyProc. Biogeosciences 15 (16), 5015–5030. Zeeman, M.J., et al., 2010. Management and climate impacts on net CO2 fluxes and carbon budgets of three grasslands along an elevational gradient in Switzerland. Agricult. For. Meteorol. 150 (4), 519–530.

efficiency in the southern Great Plains. Crop Sci. 55 (6), 2863–2876. Morison, J.I., 1985. Sensitivity of stomata and water use efficiency to high CO2. Plant, Cell Environ. 8 (6), 467–474. Patrignani, A., Lollato, R.P., Ochsner, T.E., Godsey, C.B., Edwards, J., 2014. Yield gap and production gap of rainfed winter wheat in the southern Great Plains. Agron. J. 106 (4), 1329–1339. Porter, J.R., Gawith, M., 1999. Temperatures and the growth and development of wheat: a review. Eur. J. Agron. 10 (1), 23–36. Prescher, A.-K., Grünwald, T., Bernhofer, C., 2010. Land use regulates carbon budgets in eastern Germany: from NEE to NBP. Agricult. For. Meteorol. 150 (7-8), 1016–1025. Redmon, L.A., Horn, G.W., Krenzer, E.G., Bernardo, D.J., 1995. A review of livestock grazing and wheat grain yield: boom or bust? Agron. J. 87 (2), 137–147. Rich, D., 2014. No-till canola depends on residue management. High Plains/Midwest Agric. J. https://www.hpj.com/archives/no-till-canola-depends-on-residuemanagement/article_d2c9c88a-18b8-598b-b903-a55b62229765.html (verified on March 14, 2018). Reichstein, M., et al., 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biol. 11 (9), 1424–1439. Robertson, M., et al., 2002. Environmental and genotypic control of time to flowering in canola and Indian mustard. Austr. J. Agric. Res. 53 (7), 793–809. Robertson, M., Holland, J., Kirkegaard, J., Smith, C., 1999. Simulating growth and development of canola in Australia. In: Proceedings of the 10th International Rapeseed Congress’. Canberra, ACT. The Regional Institute, Gosford, NSW Available at. Ruimy, A., Jarvis, P., Baldocchi, D., Saugier, B., 1995. CO2 fluxes over plant canopies and solar radiation: a review. Adv. Ecol. Res. 26, 1–68. Sánchez, J., López-Urrea, R., Rubio, E., González-Piqueras, J., Caselles, V., 2014. Assessing crop coefficients of sunflower and canola using two-source energy balance and thermal radiometry. Agric. Water Manag. 137, 23–29. Sawada, S.-I., 1970. An ecophysiological analysis of the difference between the growth rates of young wheat seedlings grown in various seasons. J. Faculty of Sci. 10 (11/ 13), 233–263 University of Tokyo, 3 (Botany). Schmidt, M., Reichenau, T.G., Fiener, P., Schneider, K., 2012. The carbon budget of a winter wheat field: an eddy covariance analysis of seasonal and inter-annual variability. Agricult. For. Meteorol. 165, 114–126. Sun, G., et al., 2010. Energy and water balance of two contrasting loblolly pine plantations on the lower coastal plain of North Carolina, USA. For. Ecol. Manag. 259 (7), 1299–1310. Trusler, C.S., Peeper, T.F., Stone, A.E., 2007. Italian ryegrass (Lolium multiflorum) management options in winter wheat in Oklahoma. Weed Technol. 21 (1), 151–158. USDA-ERS, 2018. Overview. https://www.ers.usda.gov/topics/crops/wheat/ (verified on March 13, 2019).

14