Ian B. Strachan

Impact of nitrogen and environmental conditions on corn as detected by hyperspectral reflectance

Abstract Indices derived from hyperspectral reflectance spectra have the potential to be used as indicators of environmental stress in crops. This study uses canopy-scale, ground-based measurements of hyperspectral reflectance to demonstrate the temporal patterns in corn development under imposed fertility (N rate) and environmental (water availability) stresses. In 1998, two large areas in a 30-ha corn ( Zea mays , L.) field near Ottawa, Canada (45°18′N, 75°44′W) were supplied with 99 and 17 kg N ha −1 , while the balance of the field received the recommended rate of 155 kg N ha −1 . Reflectance measurements were taken nine times using a portable spectroradiometer at georeferenced locations within these areas. Individual reflectance-based indices demonstrated the relative differences between application rates and identified both nitrogen and water stresses at various times in the growing season. No single index was able to describe the status of the corn crop throughout the season. Canonical discriminant analysis provided accurate classification of samples by N rate during early, mid, and late season conditions with overall success rates of 70%, 88%, and 93%, respectively. A shift in importance from green-based derivatives to red-based derivatives was noted from mid to late season and attributed to the natural reduction in green pigments as the crop entered senescence. Canopy-scale photochemical reflectance index (PRI) was shown to be correlated with canopy radiation use efficiency (RUE). Mid-season water stress affected the relationship. Multiple years of data are required to demonstrate robust relationships between hyperspectral indices and corn ecophysiological status because of the interaction between environmental and nutrient stresses. Identifying areas of fields sensitive to weather-induced stresses will allow better management of N application. Timing the collection of hyperspectral image data at early and late vegetative phase could enhance precision agriculture by allowing supplemental nutrient application, identifying stress patterns and aid in yield forecasting.

Measuring nighttime CO2 flux over terrestrial ecosystems using eddy covariance and nocturnal boundary layer methods

The respiration from plants and soil is an important component of the carbon balance of ecosystems. Its measurement is challenging due to the relatively small size of the carbon dioxide fluxes and because these fluxes occur under environmental conditions that are frequently unfavorable for flux measurements. Micrometeorological techniques based on turbulent transfer frequently underestimate CO2 fluxes during nighttime conditions. An approach based on calculating the CO 2 budget in the nocturnal boundary layer (NBL) might be an alternative during light wind conditions to estimate nighttime CO2 fluxes. This study presents typical net CO2 efflux observations measured at night over agricultural crops for several years near Ottawa, Ont. and over an old black spruce stand near Candle Lake, Sask. during the intensive field campaigns of the Boreal Ecosystem Atmosphere Study (BOREAS). We used the eddy covariance technique for windy nights and the NBL budget approach for calm nights. Criteria for screening data into either windy or calm conditions were made using the friction velocity ( u∗) and the standard deviation of the vertical wind speed (σ W). The threshold at which the 30 min turbulent CO2 flux observations were independent of u∗ or σ W and had limited scatter were determined to be in the range 0.075–0.1 m s −1 for u∗ and σ W, and about 1.5 m s −1 for horizontal wind speed (U) for multiple years of corn and soybean data. σW ≥ 0. 4ms −1 was shown to be a good screening threshold over the black spruce canopy. Current methods for handling nocturnal CO2 data involve systematically replacing data during calm conditions where eddy covariance is deficient with those from windy conditions. This can lead to an overestimation of the nocturnal CO2 flux. We suggest a variation in this procedure which, through the screening of entire nights, allows the retaining of an acceptable proportion of calm periods within predominantly windy nights. The NBL budget method requires calm nights for measuring the respiration unless other budget terms are quantified. A good agreement was found between CO2 flux measured using the NBL approach when the NBL was well developed and the eddy covariance technique, when restricted to windy nights. The NBL profiles integrate a larger area than eddy covariance, which means that high-emission spots can be included with this kind of approach. Crown Copyright

Towards standards for measuring greenhouse gas fluxes from agricultural fields using instrumented towers

This is a discussion of the available technology for measuring turbulent fluxes using instrumented towers. This review focuses on the flux measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) for agricultural systems and the development of standards and protocols for measuring them. Agroecosystems present unique challenges because they undergo large leaf area index (LAI) and canopy architecture changes in a relatively short period of time (i.e., months) coupled with the fact that many of the greenhouse gas sources are diffuse. This review examines all aspects of the theory and application of the micrometeorological techniques, with focus on the flux gradient, eddy accumulation and eddy covariance techniques. Instrument placement, sens or response and noise characteristics are also explored. Innovative applications of micrometeorological methods are discussed for closed- and open-path trace gas sensors and commonly used meteorological instrumentation. The use of fast response single-...

Daily steps are low year-round and dip lower in fall/winter: findings from a longitudinal diabetes cohort

BackgroundHigher walking levels lead to lower mortality in type 2 diabetes, but inclement weather may reduce walking. In this patient population, we conducted a longitudinal cohort study to objectively quantify seasonal variations both in walking and in two vascular risk factors associated with activity levels, hemoglobin A1C and blood pressure.MethodsBetween June 2006 and July 2009, volunteer type 2 diabetes patients in Montreal, Quebec, Canada underwent two weeks of pedometer measurement up to four times over a one year follow-up period (i.e. once/season). Pedometer viewing windows were concealed (snap-on cover and tamper proof seal). A1C, blood pressure, and anthropometric parameters were also assessed. Given similarities in measures for spring/summer and fall/winter, and because not all participants completed four assessments, spring and summer values were collapsed as were fall and winter values. Mean within-individual differences (95% confidence intervals) were computed for daily steps, A1C, and systolic and diastolic blood pressure, by subtracting spring/summer values from fall/winter values.ResultsAmong 201 participants, 166 (82.6%) underwent at least one fall/winter and one spring/summer evaluation. Approximately half were women, the mean age was 62.4 years (SD 10.8), and the mean BMI was 30.1 kg/m2 (SD 5.7). Step counts averaged at a sedentary level in fall/winter (mean 4,901 steps/day, SD 2,464) and at a low active level in spring/summer (mean 5,659 steps/day, SD 2,611). There was a -758 (95% CI: -1,037 to -479) mean fall/winter to spring/summer within-individual difference. There were no significant differences in A1C or in anthropometric parameters. Systolic blood pressure was higher in fall/winter (mean 137 mm Hg, SD 16) than spring/summer (133 mm Hg, SD 14) with a mean difference of 4.0 mm Hg (95% CI: 2.3 to 5.7).ConclusionsDaily step counts in type 2 diabetes patients are low, dipping lower during fall/winter. In this medication-treated cohort, A1C was stable year-round but a fall/winter systolic blood pressure increase was detected. Our findings signal a need to develop strategies to help patients increase step counts year-round and prevent both reductions in step counts and increases in blood pressure during the fall and winter.

Carbon release from boreal peatland open water pools: Implication for the contemporary C exchange

While peatland ecosystems overall are long-term net carbon (C) sinks, the open water pools that are characteristic of boreal peatlands have been found to be C sources to the atmosphere. However, the contribution of these pools to the ecosystem level C budget is often ignored even if they cover a significant area of the peatland surface. Here we examine the annual CO2 and CH4 ecosystem-atmosphere exchange, including the release following ice melt, from pools in a boreal maritime peatland, in order to estimate the annual loss of C from these water bodies. Over a 16 month period, dissolved CO2 and CH4 were measured periodically in five pools while continuous measurements of CO2 were made in one pool using a nondispersive infrared (NDIR) sensor. Fluxes were calculated using the thin boundary layer model and the eddy covariance technique (spring release only). We calculated an annual C release from pools of 103.3 g C m−2 yr−1 of which 15% was released during the spring ice melt. This release is the same order of magnitude, but with the opposite sign, as the average net ecosystem carbon balance for pool-free northern peatlands (−22 to −70 g C m−2 yr−1). We discuss the origin of the released C, as the magnitude of the release could have a significant impact on the contemporary C exchange of boreal peatlands.

Narrowband vegetation indexes and detection of disease damage in soybeans

A portable narrowband spectroradiometer was used to detect sclerotinia stem rot infection, caused by the fungus Sclerotinia sclerotiorum in soybeans. Increasing levels of fungal inoculum were used to cause a gradient of disease infection in the field. Canopy reflectance measured in narrowband R/sub 675/-R/sub 685/ and broadband R/sub 635/-R/sub 685/ could estimate 86% of the variation in soybean plants damage measured by a count of early dead plants. Plant damage was also associated with the chlorophyll absorption in reflectance and the normalized pigment chlorophyll vegetation indexes, showing a loss of chlorophyll pigment compared to healthy plants. A new field approach is suggested for the investigation of plant damage with narrowband spectroradiometry.

Carbon dioxide and methane exchange at a cool-temperate freshwater marsh

Freshwater marshes have been shown to be strong sinks for carbon dioxide (CO2) on an annual basis relative to other wetland types; however it is likely that these ecosystems are also strong emitters of methane (CH4), reducing their carbon (C) sequestration potential. Multiyear C balances in these ecosystems are necessary therefore to determine their contribution to the global C cycle. Despite this, the number of multiyear studies in marshes is few, with, to the best of our knowledge, only one other Northern marsh C balance reported. This study presents five years of eddy covariance flux measurements of CO2, and four years of warm-season chamber measurements of CH4 at a cool-temperate Typha angustifolia marsh. Annual average cumulative net ecosystem exchange of CO2 (NEE) at the marsh was −224 ± 54 g C m−2 yr−1 (±SD) over the five-year period, ranging from −126 to −284 g C m−2 yr−1. Enhancement of the ecosystem respiration during warmer spring, autumn and winter periods appeared the strongest determinant of annual NEE totals. Warm season fluxes of CH4 from the Typha vegetation (avg. 1.0 ± 1.2 g C m−2 d−1) were significantly higher than fluxes from the water surface (0.5 ± 0.4 g C m−2 d−1) and unvegetated mats (0.2 ± 0.2 g C m−2 d−1). Air temperature was a primary driver of all CH4 fluxes, while water table was not a significant correlate as water levels were always at or above the vegetative mat surfaces. Weighting by the surface cover proportion of water and vegetation yielded a net ecosystem CH4 emission of 127 ± 19 g C m−2 yr−1. Combining CO2 and CH4, the annual C sink at the Mer Bleue marsh was reduced to −97 ± 57 g C m−2 yr−1, illustrating the importance of accounting for CH4 when generating marsh C budgets.

Spring thaw and growing season N2O emissions from a field planted with edible peas and a cover crop

Nitrous oxide emissions are highly episodic and to accurately quantify them annually, continuous measurements are required. A tower-based micrometeorological measuring system was used on a commercial cattle farm near Co teau-du-Lac, (QC, Canada) during 2003 and 2004 to quantify N2O emissions associated with the production of edible peas. It was equipped with an ultrasonic anemometer and a fast-response closed-path tunable diode laser. Continuous measurements of N2O fluxes were made during the spring thaw following corn cultivation in summer 2002, then during an edible pea growing season, followed by cattle manure application, cover crop planting and through until after the next spring ploughing. The cumulative N2O emissions of 0.7 kg N2O-N ha-1 during the initial snowmelt period following corn harvest were lower than expected. Sustained and small N2O emissions totalling 1.7 kg N2O-N ha-1 were observed during the growing season of the pea crop. Solid cattle manure applied after the pea harvest generated th...

Monitoring crop biomass accumulation using multi-temporal hyperspectral remote sensing data

The estimation of the above-ground dry phytomass accumulation is important for monitoring crop growth, predicting potential yield, and estimating crop residues in the context of the carbon cycle. Hyperspectral remote sensing has been proven to be a very effective tool for the estimation of crop variables such as LAI, pigment and water content; therefore it is reasonable to expect that data from hyperspectral remote sensing can show great potential for monitoring crop biomass accumulation, either directly or indirectly through other variables. The objective of this study is to investigate the relationships between optical indices and either crop dry mass or height using multi-temporal, multi-field hyperspectral data. Using the Compact Airborne Spectrographic Imager (CASI), hyperspectral data were acquired in three deployments during the 2001 growing season over corn, soybean and wheat fields in the former Greenbelt Farm of Agriculture and Agri-Food Canada in Ottawa. High correlation was observed between the measured above-ground crop dry biomass and the other two parameters, crop height and leaf area index (LAI). The vegetation index MTVI2, calculated from hyperspectral images, was used to estimate the accumulated absorbed photo-synthetically active radiation (APAR) for the monitoring of crop biomass production. Both dry biomass and crop height were highly correlated with the accumulated APAR. For all the samples from the three dates, the coefficient of determination (R2) between the estimated APAR and crop dry mass was 0.95, 0.99 and 0.76, and R2 between the estimated APAR and crop height was 0.90, 0.89 and 0.70 for corn, soybean and wheat, respectively. However, further analysis shows that the correlation between biomass increment and the accumulated APAR during a short period of time is much lower for wheat. This demonstrates that apart from APAR, biomass accumulation is affected by other factors as well

Can boreal peatlands with pools be net sinks for CO2

Peatland open-water pools, a common feature on temperate to subarctic peatlands, are sources of carbon (C) to the atmosphere but their contribution to the net ecosystem carbon dioxide exchange (NEE-CO2) is poorly known; there is a question as to whether peatlands with pools are smaller sinks of atmospheric C, or even C-neutral, compared to other peatlands. We present growing season NEE-CO2 measurements using the eddy covariance technique in a peatland with pools. We found the maximum photosynthetic uptake and ecosystem respiration rates at 10 °C to be in the lower range of the published data. The lower total vegetation biomass, due to the presence of pools, reduced CO2 uptake during day and the autotrophic component of ecosystem respiration. The low CO2 uptake combined with reduced CO2 loss resulted in the site being a net sink for CO2 of a similar magnitude as other northern peatlands despite the inclusion of pools.