Soils with a history of 2,4-dichlorophenoxyacetic acid (2,4-D) treatment at field application rates and control soils with no prior exposure to 2,4-D were amended with 2,4-D in the laboratory. Before and during these treatments, the populations of 2,4-D-degrading bacteria were monitored by most-probable-number (MPN) enumeration and hybridization analyses, using probes for the tfd genes of plasmid pJP4, which encode enzymes for 2,4-D degradation. Data obtained by these alternate methods were compared. Several months after the most recent field application of 2,4-D (approximately 1 ppm), soils with a 42-year history of 2,4-D treatment did not have significantly higher numbers of 2,4-D-degrading organisms than did control soils with no prior history of treatment. In response to laboratory amendments with 2,4-D, both the previously treated soils and those with no prior history of exposure exhibited a dramatic increase in the number of 2,4-D-metabolizing organisms. The MPN data indicate a 4- to 5-log population increase after one amendment with 250 ppm of 2,4-D and ultimately a 6- to 7-log increase after four additional amendments, each with 400 ppm of 2,4-D. Similarly, when total bacterial DNA from the soil microbial community of these samples was analyzed by using a probe for the tfdA gene (2,4-D monoxygenase) or the tfdB gene (2,4-dichlorophenol hydroxylase) a dramatic increase in the level of hybridization was observed in both soils.(ABSTRACT TRUNCATED AT 250 WORDS)
In western Canada cereal straw is sometimes burned to facilitate seedbed preparation. Evidence on the consequence of this practice is limited. This study was initiated to determine the long- and short-term effects of burning cereal straw on wheat (Triticum aestivum L.) yields and soil properties. It was carried out on chernozemic soils at Melfort (orthic Black), Indian Head (orthic Black), and Swift Current (orthic Brown). Heat from burning straw barely penetrated the soil below 1 cm with maximum temperatures reaching between 338 and 422°C on the straw mulch itself. Between 32 and 76% of the straw weight and 27 and 73% of the N were lost in the burn, but no P was lost. Bacterial and fungal populations decreased immediately and substantially only in the top 2.5-cm of soil upon burning, with the fall burn being more detrimental than spring burn. Repeated burnings in the field permanently diminished the bacterial population by more than 50% but the fungi appeared to recover. Soil respiration measurements also confirmed a permanent reduction in total biological activity from repeated burning. However, for single burns, soil respiration rates increased temporarily to rates considerably above those in unburnt soils. Burning immediately increased the exchangeable NH4-N and the bicarbonate-P content, but there was no buildup of nutrients in the soil profile over the years. Long-term burning reduced total soil N and C and potentially mineralizable N in the 0- to 15-cm soil segment but did not affect humic and fulvic acids. Its effect on the soil surface tension was not conclusive, but burning seemed to increase the susceptibility of the soil to water erosion, reduced the permeability of the Melfort soil, and increased the compaction of the Indian Head soil. Burning stubble for up to 20 consecutive years had no significant effect on grain yields, perhaps because of the high initial fertility of the test soils. However, there are sufficient danger signals to indicate that this practice should be discouraged on the prairies.
Abstract Statistical comparison of the changes in aggregate size distributions (ASD) with time among treatments in a given soil or among soils is difficult. The objective of this study was to compare the use of fractal dimension ( D ), geometric mean diameter (GMD), and alpha (α) parameters as indices to characterize the temporal changes in ASD of a soil subsequent to the incorporation of an oily waste sludge. Soil samples for dry and wet aggregate size distribution (DASD and WASD, respectively) measurements were collected from a loamy sand during a 4‐yr period following sludge incorporation. Sludge incorporation produced a continuous oil content, i.e., 0, 1.0, 1.5, 3.0, and 7.0%, variable in the soil (by mass). The wind erodible fraction (WEF; <0.84 mm) in the sludge treated plots decreased from 80 to 38% but increased with time to 78% 4 yr later. The reduction in WEF was reflected as gains in larger size fractions which decreased with time. The 4.76‐ to 8‐mm water stable aggregates in the sludge treated plots increased from 10 to 80% but decreased with time to 40% 4 yr later. An opposite trend was observed for smaller size aggregates. After sludge incorporation, the D for DASD and WASD decreased with increasing oil content and increased with time. An opposite trend was observed for GMD and α. Initial oil content and time of sampling accounted for 91% of the variability in DASD characterized by GMD, 89% by D , and 60% by α. Initial oil content and time accounted for 78% of the variability in WASD characterized by D and 67% by GMD. Our results indicate that the temporal changes in DASD can be characterized equally well by GMD and D and the changes in WASD by D .
The influence of five crop rotations and the rotation phases (i.e., rotation-yr) on some soil organic matter characteristics was investigated in a long-term (23 yr) study carried out on an Orthic Dark Brown Chernozemic soil at Scott, Saskatchewan. The cropping systems included different cropping frequencies and crop types (cereals, oilseeds, and legume-hay). Soil samples were taken from the 0- to 7.5- and 7.5- to 15-cm depths in mid-September 1988, 2 wk after harvest of the grain crops (i.e., 2 mo after hay harvest and plowdown). Most effects of rotations, and rotation phases, on soil biological characteristics assessed, were significant primarily in the top 7.5-cm soil depth. Increasing the cropping frequency did not increase soil organic matter. Excessive preseeding tillage of stubble plots may have masked any potential advantage provided by frequent cropping. Including alfalfa (Medicago sativa L.) hay crops in rotation with grain crops decreased soil organic matter in the fallow and grain crop rotation phases of rotations. This was likely due to increased moisture stress depressing associated cereal production in this semiarid environment. As expected, rotation phase did not influence soil organic C, but alfalfa under-seeded into barley (Hordeum vulgare L.) increased soil organic nitrogen. We believe this was due to crop residue inputs from the seedling alfalfa. Microbial biomass C and N, C mineralization, the specific respiratory activity (ratio of CO 2 -C respired/microbial biomass C) and hydrolyzable amino acids were also greater in the rotation phases in which barley was underseeded with alfalfa. Carbon mineralization and specific respiratory activity were directly related to estimated crop residue-C returned to soil, but not residue-N. However, both were increased by including alfalfa in the rotation. Carbon mineralization and specific respiratory activity were more sensitive indexes of soil organic matter quality than biomass C and N per se. Hydrolyzable amino acids and amino sugars responded to the treatments in a manner similar to total soil organic N. Relative molar distribution of amino acids was unaffected by crop rotation or rotation phase. Potentially mineralizable N in this soil was low compared to other Canadian prairie soils, even though the total soil organic N of the Scott soil was relatively high. We concluded that (i) all soil biochemical characteristics studied are useful for assessing soil quality changes; (ii) when studying soil changes, thin (0- to 7.5-cm) soil slices are more likely to reveal treatment effects than thicker slices; (iii) all rotation phases should be analyzed whenever forage legumes are constituents of crop rotations. Key words: C mineralization; microbial biomass, amino acids, N mineralization, specific respiratory activity
Nitrogen use on the Canadian Prairies has gradually increased in recent years, and producers have switched from ammonium nitrate to urea and anhydrous ammonia. Ammoniacal N is known to acidify soil, and some segments of society question the efficacy of using anhydrous ammonia, suggesting it might adversely affect soil productivity. A 10-yr experiment conducted on a moderately acid, medium-textured, Orthic Dark Brown Chernozem at Scott, Saskatchewan, enabled us to assess the impact of annual rates of N (0–180 kg ha −1 ), applied as urea or anhydrous ammonia, on yields of hard red spring wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and canola (Brassica campestris L.). The soil was gradually acidified by the fertilizers, with the effect being greatest for anhydrous ammonia. Yields of cereals reflected the positive influence of added nutrients in the case of urea, but because of soil acidification, response to anhydrous ammonia decreased in the later years. Yield of canola (only grown in two later years, when the soil was severely acidified) was negatively related to N rate and was lower for anhydrous ammonia than for urea. Acidification increased soil solution Mn, and this probably contributed to the suppressed yield responses. Grain protein was positively related to rate of N, except when yield increases caused a dilution effect. Different responses to the two N sources may have been due to loss of N from the urea via volatilization, and, thus, urea acidified the soil to a lesser extent than anhydrous ammonia. We concluded that if producers use recommended rates of N, there should be little deleterious influence of acid-forming N fertilizers on cereal production; however, more care will be necessary when growing canola. Key words: Nitrogen rate, canola, wheat, soil acidity, Mn toxicity
Soil organic matter together with physical properties have been proposed as indicators of soil quality. The chapter presents a conceptual multidisciplinary soil environmental model to support the use of indicators in relation to agroecological functions. The basis for the conceptual model is supported with published information and data obtained from long-term crop rotation plots and soil quality benchmark monitoring sites in western and eastern Canada, respectively. Organic carbon dynamics represented an indicator to assess the impact of land use scenarios on carbon sequestration in Canada. Defining functional soil quality indicators helps identify horizontal links between the three facets of sustainable agriculture, and vertical links with policy makers and decision making authorities. Data show that oxygen-nitrogen sources, rate of nitrogen mineralization and the number of nitrifiers were components of the nitrogen cycle that responded to management. These three indicators can be used to evaluate nitrogen mineralization in sustainable agriculture.
