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Tile drainage and surface runoff are major pathways for pollution of water resources by agricultural nutrients and chemicals. Little is known, however, of the pathways and amounts of carbon entry into water resources from agricultural land. This paper evaluates dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) losses in tile drainage and surface runoff from a Brookston clay loam after more than a half century of monoculture maize ( Zea mays L.), continuous bluegrass sod ( Poa Pratensis L.), and maize-oat ( Avena sativa L.)–alfalfa ( Medicago sativa L.)–alfalfa rotation. Water loss in tile drainage and surface runoff accounted for 27%, 32%, and 18% of annual precipitation (876 mm) for rotation, monoculture maize, and continuous sod, respectively. Tile drainage comprised 66%–89% of water loss from rotation and continuous sod, but only 15% from monoculture maize, with the remaining 85% of water loss from monoculture maize due to surface runoff. On an annualized basis, the measured dissolved C loss was 79 and 83 kg C ha–1 yr–1) from rotation and continuous sod, respectively, while 49 kg C ha–1 yr–1 was lost from monoculture maize. As up to 9% off-gassing loss of CO2 from water samples was measured, total dissolved carbon losses in tile drainage and runoff water were likely greater. For Brookston clay loam soil, leaching into tile drains was the dominant mechanism for dissolved carbon loss from long-term continuous sod and crop rotation, while surface runoff was the dominant mechanism for dissolved carbon loss from long-term monoculture maize.

The objective was to quantify the effect of crop rotations, crop type, life cycle, nitrogen fertilizer, manure application, and fallow on soil hydrophobicity (SH). The SH was measured for a long-term (16 yr) dryland field experiment on a Dark Brown clay loam soil in southern Alberta, Canada. Mean SH was significantly (P ≤ 0.05) greater in rotations with grass, perennial crops, manure application, and continuous cropping; whereas cereal–legume rotations and N fertilizer effects were undetectable. A strong, positive correlation occurred between SH and soil organic carbon concentration (r = 0.73). Soil water repellency should be measured on these plots using water-based methods.

The short-term legacy effects following long-term (17 yr) feedlot manure application on CO2 efflux for a surface soil (clay loam) were studied over 2 yr (2016–2017) on a Dark Brown Chernozem in southern Alberta. The five treatments were stockpiled (SM) or composted (CM) manure with either straw (ST) or wood-chips (WD) bedding applied at 77 Mg·ha−1 (dry wt.) and an unamended control (CON). Surface soil efflux was measured during the growing season of the 2 yr using the dynamic, closed-chamber method. Ancillary measurements (soil water and temperature, total carbon, bulk density) were also obtained. Soil CO2 efflux was similar (P > 0.05) among the four amended treatments in the first (0.63–0.86 g·m−2·h−1) and second (0.40–0.46 g·m−2·h−1) years. However, soil CO2 efflux was significantly greater for amended than unamended treatments by 54–110% in the first year (CON = 0.41 g·m−2·h−1) and by 33–53% in the second year (CON = 0.30 g·m−2·h−1). Soil CO2 efflux was similar for SM and CM in both years and was significantly greater for WD than ST bedding in the first but not second year. Weak positive correlations (r ≤ 0.39) occurred between soil CO2 efflux and total soil C, water-filled pore space (WFPS), and soil temperature. Overall, our findings suggested that legacy effects of manure may persist for 1–2 yr following discontinued applications, but are mostly restricted to greater soil CO2 efflux for amended than unamended soils.

Agri-environmental indicators are being developed in Canada to assess and report on the environmental sustainability of agricultural production activities. Concerns about water quality have led to the development of an indicator called residual soil nitrogen ( RSN ), which reflects annual nitrogen left in the soil profile after crops are harvested. The Canadian agricultural nitrogen budget (CANB) is used to integrate this indicator with the economic Canadian regional agriculture model (CRAM) to assess the implications of national and regional agricultural policy scenarios. CANB has built-in scaling-up and scenario-analysis capabilities, so that its outputs can be generated and mapped at the soil landscapes of Canada (scale 1:1 million) level, as well at provincial and national scales. Using census of agriculture data for 1981 and 1996, the CANB model predicted RSN levels across Canada for 2008. The impacts of CRAM policy scenarios in the year 2008, which include changes in crop areas, animal numbers, fertilization and animal feeding practices, each with a low, medium and high level of adoption, are evaluated in terms of changes in RSN compared to a 2008 ‘business as usual’ scenario. National average RSN values are projected to increase from 21.8 kg N/ha in 1981 to 36.5 kg N/ha in the 2008 ‘business as usual’ scenario. Between 1996 and 2008, the proportion of farmland containing greater than 40 kg of excess N/ha increased from 18% to 34% in response to increased fertilizer use, increased manure application and increases in the area of legume crops. Changes in land use practices, such as decreasing the summerfallow area or switching some annual cropland to perennial forages and forestry, appear to have a minimal effect on RSN levels as compared to the ‘business as usual’ scenario. However, policy scenarios that improve nitrogen fertilization and animal feeding practices were found to significantly decrease RSN levels and thereby reduce the risk of nitrate contamination of the environment.

An on-farm evaluation of controlled drainage and subirrigation systems can raise farmer awareness and acceptance of these innovative technologies. A four hectare field experiment was established on sandy-loam soil near Harrow in Essex County, Ontario. The field was modified to allow the implementation of two water table-management treatments, controlled drainage with subirrigation (CDS) and free outlet tile drainage (DR). Volume of tile drainage, nitrate concentration and loss in the tile drainage water and crop yields were measured. The objectives of the study were to provide on-farm demonstrations of controlled drainage and subirrigation systems, and to determine their effect on crop yields and environmental benefits.The CDS system reduced flow weighted mean nitrate concentration in tile drainage water by 38% and total nitrate loss by 37% compared to the DR system from May 1995 to April 1997. The CDS system increased marketable tomato yields by 11% in 1995. The average marketable tomato yields were 58.4...

The dissipation and loss of atrazine and metolachlor from field plots and an agricultural watershed were compared for scale factors related to dissipation and loss. The watershed and the field plots, located in southwestern Ontario, were planted with corn and cash crops associated in the watershed. The studied areas were plowed by conventional methods. The watershed has well-drained Guelph loam and Embro silt loam and the field plots has a poorly drained Brookston clay loam. Dissipation rates of atrazine and metolachlor in the surface runoff and tile drainage from the field plots were faster than in runoff from the agricultural watershed. The applied atrazine and metolachlor losses in combined surface and subsurface runoff, were respectively 1.81, 0.33, 1.18 and 0.15%, for field plots and the watershed. The ratios between the herbicide input rate and output rate of the watershed and of the field plot are 2.33 and 2.76 for atrazine and metolachlor, respectively. These ratios illustrate that herbicide loss is independent of spatial variations. Leading factors identified from the study to be considered for scale effect are, soil organic carbon, water table depth, soil moisture content, soil textures, antecedent days, and time after application.

To determine the inhibitory effect of O2 on denitrification in pure cultures of Pseudomonas fluorescens, measurements of net NO and N2O production rates, fNO and fN2O, under anaerobic conditions were compared to rates under partially aerobic conditions. Nitrate or NO2− solutions were injected into buffered suspensions and gaseous NO and N2O were sparged by constant flows of N2 carrier gas, or mixtures of N2 with measured small quantities of O2 (ca 6–60 Pa). Apparent Michaelis-Menten parameters were calculated from saturation kinetic plots obtained by monitoring net NO or N2O production following periodic addition of NO3−, NO2− or NO to suspensions of cells cultured aerobically in the absence of NO3−. With NO3− substrate added under anaerobic conditions the apparent KM=3,74± l.80mM, and under partially aerobic conditions KM =2.10 ± 0.54 mM; with NO2− substrate (anaerobic conditions) KM = 5.28±0.76 nM (partially aerobic conditions), KM=6.05± 1.30 μM, and with NO substrate, anaerobic conditions, KM = 7.3 ± 7.4 mu. The addition of O2 resulted in short-term transient effects indicating rapid and reversible inhibition of NO and N2O production. When O2 was present, NO2−, accumulated in greater amounts than observed under completely anaerobic conditions. The transient effects suggest that the reduction of NO2−, NO and N2O are each inhibited to increasingly greater extents by O2.

This study was designed to determine the effects of water and O2 on the speciation of denitrification gases (NO and N2O). Nitric oxide was found to be the principal end product from soil incubated under low-moisture conditions, whereas the relative amount of N2O increased under wetter moisture regimes. The total amount of NO plus N2O produced increased with increasing water content for the Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaguoll), whereas it peaked at 150 g kg(15%) water content with the Fox sandy loam (fine-loamy over sandy or sandy or sandy-skeletal, mixed, mesic, Typic Hapludalf). The decrease in NO plus N2O at higher water contents was probably the result of the subsequent reduction of N2O to N2 in the Fox sandy loam soil. The residence time of the denitrification gases in the soil increased with increasing water content, hence facilitating the subsequent conversions of NO to N2O and N2. The thickness of the water film surrounding the microbes affected both the diffusion of O2 through the water and into the microbes as well as the diffusion of denitrification gases (NO, N2O, and N2) from the microbes into the atmosphere. In the sandy loam soil, O2 content and soil water affected both the amount and species of evolved denitrification gases. Oxygen was more effective in decreasing NO production at lower than at higher water contents. N OXIDE, N2O, and N? are gaseous products of nitrification and denitrification processes and NO and N2O are important atmospheric trace gases. Nitric oxide (and NO2) in the troposhere catalyzes photochemical O3 production, contributes to formation of acid rain and controls the formation of hydroxyl radicals, the major reactant for a number if atmospheric constituents (Parrish et al., 1990). Nitrous oxide is an important greenhouse gas and the major source of stratospheric NO, one of the predominant catalysts for O3 removal. The molar ratio of NO to N2O was found to be > 1 in nitrifier cultures, whereas it has been found to be < 1 for cultures of denitrifiers (Anderson and Levine, 1986). This may be the result of the influence of O2 on denitrification. Nitric oxide production was found to be independent of O2 partial pressure (pO2) with nitrifiers, whereas N2O production was inversely related to pO2 for the denitrifiers. The primary effect of water on denitrification in aerobic and partially aerobic soils is to restrict O2 levels by reducing the air-water interfacial area within air-filled pores (Skopp, 1985) thus producing the anaerobic condition thought to be required for denitriC.F. Drury and W.I. Findlay, Research Station, Agriculture Canada, Harrow, ON, Canada NOR 1GO; and D.J. McKenney, Dep. of Chemistry and Biochemistry, Univ. of Windsor, Windsor, ON, Canada N9B 3P4. Received 21 Feb. 1991. *Corresponding author. Published in Soil Sci. Soc. Am. J. 56:766-770 (1992). fication. Oxygen diffusion rates are some four orders of magnitude slower in water than in the gas phase and hence O2 concentraton at the microbial cell is regulated by water content (Klemedtsson et al., 1988). Relatively small changes in water content in soils may directly influence rates of denitrification. Many field studies have shown that intense denitrification activity often follows heavy rainfall or flooding periods when aeration of the soil is severely restricted (Sexstone et al., 1985). Excess water not only limits O2 diffusion into the soil, but also affects the movement, distribution, and relative proportion of evolved denitrification gases. For example, Mahendrappa and Smith (1967) reported that a 10% increase in water content above field capacity in fully anaerobic soils markedly increased the amount of N2O converted to N2 with little effect on NO 3 disappearance. Water contents above these levels further decreased rates of N2O production. The objective of this study was to investigate the physical effects of water on NO and N2O production under conditions favoring denitrification, using a gasflow column approach (McKenney et al., 1982). With this technique, NO and N2O are rapidly scrubbed from the water and pore space in soil columns by a continuous flow of an inert carrier gas, enabling us to determine the production rates of these denitrification gases. In addition, the combined effect of O2 and soil water content on NO production was investigated. MATERIALS AND METHODS Anaerobic Nitric and Nitrous Oxide Production at Varying Water

The relationships between denitrification, soil microbial biomass and selected soil properties were investigated in 13 soils which varied in physical and chemical properties. It was hypothesized that estimates of biomass C may be directly related to the background (unamended) denitrification rates. Denitrification potential (with C and NO−3 amendments) was also compared to biomass C and the indigenous soil properties. The background and potential denitrification rates were measured in these soils during 75 h of anacrobiosis. Background denitrification was highly correlated with biomass C. organic C content and moisture content at field capacity (−33 kPa). Soil organic C was also highly correlated with microbial biomass C. Nitrate was not limiting for most soils as in all but two soils the residual NO−3 content following incubation exceeded 10mg N kg−1. Hence. NO−3 concentration was not strongly correlated with the background denitrification rate. The denitrification potential was 5 times greater than background denitrification. In our study, C was the factor limiting denitrification. Further, microbial biomass appeared to be a sensitive indicator of both soil C content and background denitrification. There was no significant relationship between any of the other soil chemical and physical properties measured and the denitrification potential.

The fate of added 15 N-labelled (NH 4 ) 2 SO 4 was determined in high and low NH + 4 fixing soils under mineralizing conditions after 30 days of incubation at 15°C. Although soils with a high capacity for NH + 4 fixation had a higher organic 15 N content than low fixing soils, none of the added NH 4 + was recovered in microbial biomass N. The soil internal N cycle is largely comprised of two independent processes, mineralization and immobilization, both under microbial control but physically and/or temporally separate. The connection between these two processes is through the inorganic N phase, with both forms of inorganic N (NH + 4 and NO 3 − ) participating in immobilization depending on availability to the heterotrophic microbial biomass.

The effect of chloroform fumigation on O 2 inhibition of denitrification was evaluated in non-fumigated, fumigated and sterile soil with 0, 1, 2. 3 or 6% added O 2 using the C 2 H 2 inhibition technique. Chloroform fumigation was used to lyse microbes and thereby release nitrate reductase which may decrease O 2 inhibition. With the fumigated treatment, soil samples were exposed to chloroform for 24 h followed by evacuation, whereas a CHCl 3 saturated atmosphere was maintained during the incubation in the sterile soil treatment. Nitrous oxide. CO 2 and O 2 in the headspace of incubation containers were measured over 10d. Denitrifiers apparently recolonized the fumigated soil within 12 h after chloroform removal. Oxygen inhibited denitrification in fumigated and non-fumigated soils. Oxygen consumption and CO 2 production were greater in fumigated than in non-fumigated soil samples as a result of the additional carbon from lysed microbes. Nitrite reductase appeared to be more sensitive to O 2 than nitrate reductase.

The effects of available C from alfalfa and straw on denitrification rates in incubated soil were investigated. Denitrification rates were compared with ground alfalfa and wheat straw at concentrations of 1.0, 10 and 100 g kg −1 soil using the acetylene block method. Similar rates of denitrification were observed with those treatment pairs known to produce similar soluble C concentrations in soil; specifically 1.0 g alfalfa and 10 g straw, or 10 g alfalfa and 100 g straw kg −1 soil. Less than half the applied NO − 3 was recovered as N 2 O with the 100 g kg −1 alfalfa treatment. A subsequent experiment using K 15 No 3 showed that about one half of the added 15 N appeared as 15 NH + 4 with the 100 g kg −1 alfalfa treatment, thus suggesting dissimilatory nitrate reduction (DNR). DNR did not occur when a comparable amount of glucose C was added to the soil. However, DNR occurred when sulfide and glucose C were added together to soil at concentrations comparable to those with the 100 g kg −1 alfalfa treatment. It is suggested that sulfide, under highly reduced conditions, may inhibit denitrification and thereby promote DNR by microorganisms. It appears that DNR becomes more competitive with denitrification as the reducing state of the soil increases.