• Authors:
    • MacPherson, J. I.
    • Grant, B.
    • Smith, W.
    • Pennock, D. J.
    • Desjardins, R. L.
    • Edwards, G. C.
    • Pattey, E.
  • Source: Agricultural and Forest Meteorology
  • Volume: 142
  • Issue: 2-4
  • Year: 2007
  • Summary: The importance of constraining the global budget of nitrous oxide (N2O) has been well established. The current global estimate of the contribution of N2O to total anthropogenic greenhouse gas emissions from agriculture is about 69%. Considerable progress has been made over the past few years in developing tools for quantifying the emissions from agricultural sources, at the local and field scale (i.e., chamber and tower-based measurements) as well as at the landscape and regional levels (i.e., aircraft-based measurement and modelling). However, aggregating these emissions over space and time remains a challenge because of the high degree of temporal and spatial variability. Emissions of N2O in temperate climate are largely event driven, e.g., in Eastern Canada, large emissions are observed right after snowmelt. The average emissions during the snowmelt period vary considerably, reflecting the influence of many controlling factors. Cumulative emissions reported here range from 0.05 kg N2O-N ha-1 in Western Canada to 1.26 kg N2O-N ha-1in Eastern Canada, values that reflect differences in climatic zones and fertilizer management practices. This paper describes the tools for refining the global N2O budget and provides examples of measurements at various scales. Tower-based and aircraft measurement platforms provide good data for quantifying the variability associated with the measurements. Chamber-based methods lack the temporal and spatial resolution required to follow the event driven nature of N2O fluxes but provide valuable information for evaluating management practices. The model DeNitrification and DeComposition is an example of a technique to estimate N2O emissions when no data is available.
  • Authors:
    • VanGessel, M. J.
    • Scott, B. A.
  • Source: Weed Technology
  • Volume: 21
  • Issue: 1
  • Year: 2007
  • Summary: In November 2004, a 29-question survey was mailed to Delaware soybean growers to determine grower perceptions of glyphosate-resistant (GR) horseweed and if glyphosate applications, GR soybean usage, and management practices had been altered in lieu of the presence of resistance. A total of 213 valid responses were received. Ninety-eight percent of respondents reported planting GR soybean at some point in the last 5 yr, with 90% reporting having planted GR soybean 3 or more years. The presence of GR horseweed on-farm was reported by 38% of the respondents and 95% of those growers with GR horseweed on-farm reported implementing one or more changes in GR soybean management. The most frequent change (66% of growers) due to resistant horseweed was the application of another herbicide with a different mode of action before planting. Forty-eight percent of growers with resistance on-farm reported a $5 to $17/ha increase to manage for GR horseweed, with 28% reporting a greater than $17/ha increase. Regardless of experience with GR horseweed, approximately 80% responded that it was worthwhile to incur additional costs now to preserve glyphosate for future use. Soybean grower reliance on glyphosate has not decreased in light of GR horseweed in Delaware. Misconceptions of timing for the selection of GR horseweed biotypes and the future availability of new herbicides with different modes of action exist within the farming community.
  • Authors:
    • Valentini, R.
    • Tubaf, Z.
    • Sutton, M.
    • Manca, G.
    • Stefani, P.
    • Skiba, U.
    • Rees, R. M.
    • Baronti, S.
    • Raschi, A.
    • Neftel, A.
    • Nagy, Z.
    • Martin, C.
    • Kasper, G.
    • Jones, M.
    • Horvath, L.
    • Hensen, A.
    • Fuhrer, J.
    • Flechard, C.
    • Domingues, R.
    • Czobel, S.
    • Clifton-Brown, J.
    • Ceschia, E.
    • Campbell, C.
    • Amman, C.
    • Ambus, P.
    • Pilegaard, K.
    • Allard, V.
    • Soussana, J. F.
  • Source: Agriculture, Ecosystems & Environment
  • Volume: 121
  • Issue: 1-2
  • Year: 2007
  • Summary: The full greenhouse gas balance of nine contrasted grassland sites covering a major climatic gradient over Europe was measured during two complete years. The sites include a wide range of management regimes (rotational grazing, continuous grazing and mowing), the three main types of managed grasslands across Europe (sown, intensive permanent and semi-natural grassland) and contrasted nitrogen fertilizer supplies. At all sites, the net ecosystem exchange (NEE) of CO2 was assessed using the eddy covariance technique. N2O emissions were monitored using various techniques (GC-cuvette systems, automated chambers and tunable diode laser) and CH4 emissions resulting from enteric fermentation of the grazing cattle were measured in situ at four sites using the SF6 tracer method. Averaged over the two measurement years, net ecosystem exchange (NEE) results show that the nine grassland plots displayed a net sink for atmospheric CO2 of -240 +/- 70 g C m(-2) year(-1) (mean confidence interval at p > 0.95). Because of organic C exports (from cut and removed herbage) being usually greater than C imports (from manure spreading), the average C storage (net biome productivity, NBP) in the grassland plots was estimated at -104 +/- 73 g cm(-2) year(-1) that is 43% of the atmospheric CO2 sink. On average of the 2 years, the grassland plots displayed annual N2O and CH4 (from enteric fermentation by grazing cattle) emissions, in CO2-C equivalents, of 14 +/- 4.7 and 32 +/- 6.8 g CO2-C equiv. m(-2) year(-1), respectively. Hence, when expressed in CO2-C equivalents, emissions of N2O and CH4 resulted in a 19% offset of the NEE sink activity. An attributed GHG balance has been calculated by subtracting from the NBP: (i) N2O and CH4 emissions occurring within the grassland plot and (ii) off-site emissions of CO2 and CH4 as a result of the digestion and enteric fermentation by cattle of the cut herbage. On average of the nine sites, the attributed GHG balance was not significantly different from zero (-85 +/- 77 g CO2-C equiv. m(-2) year(-1)).
  • Authors:
    • McConkey, B. G.
    • Angers, D. A.
    • Gregorich, E. G.
    • VandenBygaart, A. J.
  • Source: Canadian Journal of Soil Science
  • Volume: 87
  • Issue: 4
  • Year: 2007
  • Summary: Accurate predictions of changes in soil organic matter are difficult, at least in part, because of the lack of precision in measurements of soil organic carbon (SOC). This lack of precision is mostly due to the spatial variability in SOC that occurs with depth through the profile and laterally across the soil surface. The objective of this study was to assess the lateral and vertical variability of SOC in several pedologically distinct agricultural soils across Canada. Our goal was to determine the effect of different sampling methods on the precision of SOC measurements, namely: the effect of sampling either by fixed depth or by genetic soil horizon, the influence of compositing samples from different depth increments, and the number of cores required for a minimum detectable difference. Soils were sampled in increments down to 60 cm using a 4 x 3 m grid at six sites: two each from Ontario (Gleysol and Melanic Brunisol), Quebec (Humic Gleysol and Humo Ferric Podzol) and Saskatchewan (Dark Brown Chernozem). At four of the six sites, sampling by genetic soil horizon appeared to increase the precision of SOC measurements, but only when the surface 30 cm of the soil profile was considered. At the other two sites (soil types: Gleysol and Melanic Brunisol) sampling by fixed depth increments was more effective for increasing the precision of SOC measurements than sampling by genetic horizon. The effect of compositing samples from different depth increments had little influence on the precision of SOC measurements for all six soil types. These results suggest that sampling more than two depth increments per soil core has limited advantages for increasing statistical power to detect change in SOC. The high background SOC levels in the Gleysol soil would require a large number of soil cores in order to detect a small change in SOC such as that which would occur in a typical monitoring project. The Chernozem soils had lower spatial variability in SOC than the soil types in eastern Canada. Determining a statistically significant change in SOC of 5 Mg ha(-1) would be difficult with the sampling design used in this study.
  • Authors:
    • Venterea, R. T.
  • Source: Global Change Biology
  • Volume: 13
  • Issue: 8
  • Year: 2007
  • Summary: Nitrite (NO2-) can accumulate during nitrification in soil following fertilizer application. While the role of NO2- as a substrate regulating nitrous oxide (N2O) production is recognized, kinetic data are not available that allow for estimating N2O production or soil-to-atmosphere fluxes as a function of NO2- levels under aerobic conditions. The current study investigated these kinetics as influenced by soil physical and biochemical factors in soils from cultivated and uncultivated fields in Minnesota, USA. A linear response of N2O production rate (PN2O)toNO2- was observed at concentrations below 60 ugNg-1 soil in both nonsterile and sterilized soils. Rate coefficients (Kp) relating PN2O to NO2- varied over two orders of magnitude and were correlated with pH, total nitrogen, and soluble and total carbon (C). Total C explained 84% of the variance in Kp across all samples. Abiotic processes accounted for 31-75% of total N2O production. Biological reduction of NO2- was enhanced as oxygen (O2) levels were decreased from above ambient to 5%, consistent with nitrifier denitrification. In contrast, nitrate (NO3-)-reduction, and the reduction of N2O itself, were only stimulated at O2 levels below 5%. Greater temperature sensitivity was observed for biological compared with chemical N2O production. Steady-state model simulations predict that NO2 - levels often found after fertilizer applications have the potential to generate substantial N2O fluxes even at ambient O2. This potential derives in part from the production of N2O under conditions not favorable for N2O reduction, in contrast to N2O generated from NO3- reduction. These results have implications with regard to improved management to minimize agricultural N2O emissions and improved emissions assessments.
  • Authors:
    • Worth, D.
    • Desjardins, R. L.
    • Dyer, J. A.
    • VergĂ©, X. P. C.
  • Source: Agricultural Systems
  • Volume: 94
  • Issue: 3
  • Year: 2007
  • Summary: In order to demonstrate the impact of an increase in production efficiency on greenhouse gas (GHG) emissions, it is important to estimate the combined methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) emissions per unit of production. In this study, we calculated the GHG emissions from the Canadian dairy industry in 2001 as a fraction of the milk production and per dairy animal. Five regions were defined according to the importance of the dairy industry. N2O and CO2 emissions are directly linked with areas allocated to the dairy crop complex which includes only the crop areas used to feed dairy cattle. The dairy crop complex was scaled down from sector-wide crop areas using the ratios of dairy diet to national crop production of each crop type. Both fertilizer application and on-farm energy consumption were similarly scaled down from sector-wide estimates to the dairy crop complex in each region. The Intergovernmental Panel on Climate Change (IPCC) methodology, adapted for Canadian conditions, was used to calculate CH4 and N2O emissions. Most of the CO2 emission estimates were derived from a Fossil Fuel for Farm Fieldwork Energy and Emissions model except for the energy used to manufacture fertilizers. Methane was estimated to be the main source of GHG, totalling 5.75 Tg CO2 eq with around 80% coming from enteric fermentation and 20% coming from manure management. Nitrous oxide emissions were equal to 3.17 Tg CO2 eq and carbon dioxide emissions were equal to 1.45 Tg. The GHG emissions per animal were 4.55 Mg CO2 eq. On an intensity basis, average GHG emissions were 1.0 kg CO2 eq/kg milk. Methane emissions per kg of milk were estimated at 19.3 l CH4/kg milk which is in agreement with Canadian field measurements.
  • Authors:
    • Voroney, P.
    • Kay, B.
    • Warland, J.
    • von Bertoldi, P.
    • Parkin, G.
    • Wagner-Riddle, C.
    • Jayasundara, S.
  • Source: Nutrient Cycling in Agroecosystems
  • Volume: 79
  • Issue: 2
  • Year: 2007
  • Summary: Best management practices are recommended for improving fertilizer and soil N uptake efficiency and reducing N losses to the environment. Few year- round studies quantifying the combined effect of several management practices on environmental N losses have been carried out. This study was designed to assess crop productivity, N uptake from fertilizer and soil sources, and N losses, and to relate these variables to the fate of fertilizer 15N in a corn ( Zea mays L.)- soybean ( Glycine max L.)- winter wheat ( Triticum aestivum L.) rotation managed under Best Management ( BM) compared with conventional practices ( CONV). The study was conducted from May 2000 to October 2004 at Elora, Ontario, Canada. Cumulative NO3 leaching loss was reduced by 51% from 133 kg N ha(-1) in CONV to 68 kg N ha(-1) in BM. About 70% of leaching loss occurred in corn years with fertilizer N directly contributing 11 - 16% to leaching in CONV and < 4% in BM. High soil derived N leaching loss in CONV, which occurred mostly ( about 80%) during November to April was attributable to 45 - 69% higher residual soil derived mineral N left at harvest, and on-going N mineralization during the over-winter period. Fertilizer N uptake efficiency ( FNUE) was higher in BM ( 61% of applied) than in CONV ( 35% of applied) over corn and wheat years. Unaccounted gaseous losses of fertilizer N were reduced from 27% of applied in CONV to 8% of applied in BM. Yields were similar between BM and CONV ( for corn: 2000 and 2003, wheat: 2002, soybean: 2004) or higher in BM ( soybean: 2001). Results indicated that the use of judicious N rates in synchrony with plant N demand combined with other BMP ( no- tillage, legume cover crops) improved FNUE by corn and wheat, while reducing both fertilizer and soil N losses without sacrificing yields.
  • Authors:
    • Boast, C. W.
    • Ellsworth, T. R.
    • Mulvaney, R. L.
    • Khan, S. A.
  • Source: Journal of Environmental Quality
  • Volume: 36
  • Issue: 6
  • Year: 2007
  • Summary: Intensive use of N fertilizers in modern agriculture is motivated by the economic value of high grain yields and is generally perceived to sequester soil organic C by increasing the input of crop residues. This perception is at odds with a century of soil organic C data reported herein for Morrow Plots, the world's oldest experimental site under continuous corn (Zea mays L.). After 40 to 50 yr of synthetic fertilization that exceeded grain N removal by 60 to 190%, a net decline occurred in soil C despite increasingly massive residue C incorporation, the decline being more extensive for a corn-soybean (Glycine max L. Merr.) or corn-oats (Avena sativa L.)-hay rotation than for continuous corn and of greater intensity for the profile (0-46 cm) than the surface soil. These findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided. These are important implications for soil C sequestration because the yield-based input of fertilizer N has commonly exceeded grain N removal for corn production on fertile soils since the 1960s. To mitigate the ongoing consequences of soil deterioration, atmospheric CO2 enrichment, and NO3- pollution of ground and surface waters, N fertilization should be managed by site-specific assessment of soil N availability. Current fertilizer N managment practices, if combined with corn stover removal for bioenergy production; exacerbate soil C loss.
  • Authors:
    • Kucharik, C. J.
  • Source: Soil Science Society of America Journal
  • Volume: 71
  • Issue: 2
  • Year: 2007
  • Summary: Conservation Reserve Program (CRP) prairie restorations can sequester soil C and N, but the varied effects of soil order and ecosystem age are uncertain. Soil bulk density (Db) (0-20 cm) and soil organic C (SOC) and total N at 0 to 5, 5 to 10, and 10 to 25 cm were measured at 39 paired CRP-crop sites in Wisconsin to quantify SOC and N stock changes as a function of prairie age (4-16 yr) and soil order (Alfisols and Mollisols). Several important outcomes were found regarding land conversion to CRP: (i) soil Db decreased on Alfisols (-0.12 {+/-} 0.11 g cm-3, P < 0.0001) but not Mollisols; (ii) SOC sequestration rates were not significantly different between Mollisols (49.7 {+/-} 64 g C m-2 yr-1) and Alfisols (43.9 {+/-} 86 g C m-2 yr-1), but were only detectable (P < 0.05) in the upper 5 cm; (iii) whole SOC and N to a depth of 25 cm did not change significantly; (iv) the annual average SOC sequestration rate declined (P < 0.05) as prairie age increased (from 72 {+/-} 105 to 13 {+/-} 25 g C m-2 yr-1 for youngest to oldest age groupings); and (v) short-term SOC and N increases could be lost with time. These data suggest that there may be a discontinuity between the intensity of continuing management that is needed for sustained, long-term SOC increases in planted prairies and the resources that the CRP has available to achieve this level of ecosystem functioning.
  • Authors:
    • Field,C. B.
    • Lobell, D. B.
  • Source: Environmental Research Letters
  • Volume: 2
  • Issue: 1
  • Year: 2007
  • Summary: Changes in the global production of major crops are important drivers of food prices, food security and land use decisions. Average global yields for these commodities are determined by the performance of crops in millions of fields distributed across a range of management, soil and climate regimes. Despite the complexity of global food supply, here we show that simple measures of growing season temperatures and precipitation - spatial averages based on the locations of each crop - explain similar to 30% or more of year-to-year variations in global average yields for the world's six most widely grown crops. For wheat, maize and barley, there is a clearly negative response of global yields to increased temperatures. Based on these sensitivities and observed climate trends, we estimate that warming since 1981 has resulted in annual combined losses of these three crops representing roughly 40 Mt or $5 billion per year, as of 2002. While these impacts are small relative to the technological yield gains over the same period, the results demonstrate already occurring negative impacts of climate trends on crop yields at the global scale.