Degradation of 14C‐atrazine [2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine] and 14C‐metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐1‐methylethyl) acetamide] was monitored for 6 and 2 mo, respectively, using sterile and nonsterile soil microcosms. Both chemical and biological degradation were observed for atrazine, metolachlor degraded only biologically. The calculated halflife of atrazine was 3.6 wk in nonsterile surface samples (0–5 cm). At the surface, after 22 wk, bound residues accounted for almost 60% of the recovered radioactivity while 36% was recovered as 14CO2, indicating significant cleavage of the triazine ring. For sterilized surface samples, atrazine degraded chemically with bound residues accounting for 63% of the recovered label and had a calculated halflife of 6.2 wk. Degradation and binding were somewhat lower in soil samples from 20 to 25 cm and deeper subsurface samples (45 and 75 cm) showed almost no degradation and very little binding. Metolachlor degraded only in the surface nonsterile samples; no degradation was observed in subsurface samples or in sterile samples from any depth. Bound residues occurred in high amounts in the surface soil (31%) but declined rapidly with depth, indicating that organic matter is the primary binding site for metolachlor. Very little 14CO2 (<1.6%) was produced from metolachlor in any sample. This study showed that both herbicides degraded slower and sorbed less to the soil with increasing soil depth, especially below 25 cm. Quantifying degradation rates of agricultural chemicals in the vadose zone is important for predicting and preventing groundwater contamination as well as for successful implementation of in‐situ bioremediation of contaminated subsoils.
This study was conducted to evaluate the effects of vegetation, N fertilizers, and lime addition on landfill CH4 oxidation. Columns filled with compacted sandy loam and sparged with synthetic landfill gas were used to simulate a landfill cover. Grass‐topped and bare‐soil columns reduced inlet CH4 by 47 and 37%, respectively, at peak uptake; but the rate for both treatments was about 18% at steady slate. Nitrate and NH4 amendments induced a more rapid onset of CH4 oxidation relative to KCl controls. However, at steady state, NH4 inhibited CH4 oxidation in bare columns but not in grassed columns. Nitrate addition produced no inhibitory effects. Lime addition to the soil consistently enhanced CH4 oxidation. In all treatments, CH4 consumption increased to a peak value, then declined to a lower steady‐state value; and all gassed columns developed a pH gradient. Neither nutrient depletion nor protozoan grazing could explain the decline from peak oxidation levels. Ammonium applied to grassed cover soil can cause transient reductions in CH4 uptake, but there is no evidence that the inhibition persists. The ability of vegetation to mitigate NH4 inhibition indicates that results from bare‐soil tests may not always generalize to vegetated landfill caps.
High temperature, pH, and salt stresses in tropical alkaline soils limit nodulation and dinitrogen fixation by strains of Rhizobium from the root nodules of nitrogen fixing trees (NFTs). This study was conducted to determine the variability among Rhizobium strains isolated from different NFTs in growth response to high temperature, pH, and salt concentrations. Variable response to increases in temperature, pH, and salt concentrations was observed. Rhizobium strain isolated from Albizia lebbek survived at 50 °C, while Rhizobium strains isolated from Sesbania formosa, Acacia farnesiana, and Dalbergia sissoo were well adapted to grow on pH 12.0. All the Rhizobium strains tolerated salt concentrations up to 5.0%. Strains were further characterized with respect to utilization of 27 carbon sources and for their effectiveness in substrate utilization at pH 7.0 and 9.0. Generally higher rates of O2 consumption were observed at pH 7.0 compared with pH 9.0.Key words: Rhizobium, leguminous trees, root nodules, stress tolerance.
This study, conducted in the Piedmont of North Carolina, was initiated to determine how reductions in N fertilization and green‐manuring with crimson clover (Trifolium incarnatum L. cv. Tibbee) would affect populations and activities of soil microorganisms. Four continuous corn (Zea mays L.) treatments were used: no‐till (receiving herbicides and soil insecticides) with 0 or 140 kg N ha−1 as NH4NO3; conventionally tilled, receiving 140 kg N ha−1, but no pesticides; and conventionally tilled with a crimson clover green manure, but no fertilizer or pesticides. Populations were determined using selective media for culturable bacteria, gram‐negative bacteria, fungi, actinomycetes, Bacillus spp., and Pseudomonas spp. Microbial activities were estimated by enzyme assays for acid and alkaline phosphatase, arylsulfatase, and β‐glucosidase. Microbial biomass C was determined by a chloroform fumigation‐extraction procedure and levels of available N were measured after anaerobic incubation. Surface soil (0–7.5 cm) from the no‐till treatment receiving 140 kg N ha−1 contained significantly more fungi than did soil from the unfertilized, no‐till treatment. Microbial biomass C and available N were not affected by N addition, but levels of acid phosphatase and β‐glucosidase were significantly higher in the fertilized soil than in the unfertilized soil. Surface soil from a crimson clover‐corn rotation contained significantly larger populations of Bacillus spp. (260% more), actinomycetes (310% more), and culturable bacteria (120% more) than did soil from the well‐fertilized conventionally tilled, no‐pesticide treatment. Also, microbial biomass, available N, and levels of alkaline phosphatase, arylsulfatase, and β‐glucosidase were significantly higher in surface soil from the crimson clover treatment than the nonmanured soil. Although the soil biological properties changed significantly during the year, seasonal variations were similar across treatments. Microbial numbers and activities were high in the spring and fall and low during the late summer.
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