C. van Kessel

Clear-cut forest harvest impacts on soil quality indicators in the mixedwood forest of Saskatchewan, Canada

Abstract The concept of soil quality is relevant to a range of human-induced disturbances on soil; our objective was to examine the impact of clear-cutting on soil quality conditions in six Mixedwood ( Populus tremuloides-Picea glauca ) stands in Central Saskatchewan, Canada. Soil quality conditions at two short-term (1 to 5 years) and four medium-term (6 to 20 years) clear-cut sites were compared to eight mature Mixedwood sites. All sites had a similar, albeit complex, distribution of soil and parent sediments. No major differences were observed between the short-term clear-cut sites and the mature Mixedwood sites. At the medium-term sites, substantial losses of soil organic carbon (24%), soil nitrogen (27%), and LFH thickness (28%) were observed, along with decreases in the soil surface (0 to 15 cm layer) of exchangeable calcium and magnesium (30%), soluble organic P (15%), and cation exchange capacity and base saturation (20%). The losses were much lower in the 15 to 45 cm layer of the soil. Overall, the levels of these soil components observed at the clear-cut sites were within the natural or undisturbed range as assessed at the mature Mixedwood sites; however the amounts of soil organic carbon and nitrogen at the clear-cut sites were lower than the natural range. Although the absolute levels of loss of soil organic carbon and nitrogen were much lower than in typical agricultural landscapes, the long-term ecological significance of the losses in these forested landscapes remains a concern.

Landscape-scale variability of N mineralization in forest soils

Abstract Our understanding of the controls on N-cycling and availability in forest soils following disturbance is limited. A comparative study was conducted to examine the spatial distribution of soil N in forest soils and assess the effects of site disturbance. Sampling grids were established within a 120 × 120 m representative area at a native site, and at recently (i.e. 4-year-old) burned and clear-cut sites. A three-dimensional classification of landscape form was used to stratify each landscape into distinct landform elements. The spatial distribution of inorganic-N was not related to landform element, irrespective of site disturbance, indicating an absence of topographic control at the scale studied. However, a narrowing of the NH4+-to-NO3− ratio at the clear-cut site compared to the native site suggests that N-cycling was influenced by site disturbance. Similarly, an increase in the size of the microbial biomass at the clear-cut site, coupled with a widening of the microbial biomass C-to-N ratio, suggest that disturbance altered both the size and composition of the microbial biomass. Potential N and C mineralization, and net nitrification in the forest floor and surface mineral horizons representing two distinct landform complexes were studied in a controlled aerobic 8-wk incubation experiment. Accumulation of NH4+ and NO3− differed markedly between sites although the effects of topographic position were generally non-significant. Inorganic-N accumulated principally as NH4+ in soils from the native site due to an extended lag in nitrification. In contrast, NH4+ accumulation in soils from the recently disturbed sites remained limited, whereas NO3− accumulation predominated. Thus, although topography did not markedly influence N distribution at the scale studied, site disturbance had a direct effect on N-cycling processes in these forest soils.

Carbon dioxide evolution from wheat and lentil residues as affected by grinding, added nitrogen, and the absence of soil

SummaryA study was conducted to determine the effects of grinding, added N, and the absence of soil on C mineralization from agricultural plant residues with a high C:N ratio. The evolution of CO2 from ground and unground wheat straw, lentil straw, and lentil green manure, with C:N ratios of 80, 36, and 9, respectively, was determined over a period of 98 days. Treatments with added N were included with the wheat and lentil straw. Although the CO2 evolution was initially much faster from the lentil green manure than from the lentil or wheat straw, by 98 days similar amounts of CO2 had evolved from all residues incubated in soil with no added N. Incubation of plant residues in the absence of soil had little effect on CO2 evolution from the lentil green manure or lentil straw but strongly reduced CO2 evolution from the wheat straw. Grinding did not affect CO2 evolution from the lentil green manure but increased CO2 evolution from the lentil straw with no added N and from the wheat straw. The addition of N increased the rate of CO2 evolution from ground wheat straw between days 4 and 14 but not from unground wheat straw, and only slightly increased the rate of CO2 evolution from lentil straw during the initial decomposition. Over 98 days, the added N reduced the amounts of CO2 evolved from both lentil and wheat straw, due to reduced rates of CO2 evolution after ca. 17 days. The lack of an N response during the early stages of decomposition may be attributed to the low C:N ratio of the soluble straw component and to microbial adaptations to an N deficiency, while the inhibitory effect of N on CO2 evolution during the later stages of decomposition may be attributed to effects of high mineral N concentrations on lignocellulolytic microorganisms and enzymes.

Selection of Rhizobium leguminosarum strains for lentil (Lens culinaris) under growth room and field conditions

Most of the production of lentil (Lens culinaris) on the Great Plains occurs on soils that are free of indigenous Rhizobium leguminosarum. Inoculation is required to increase yields through N2 fixation. A screening program to evaluate the effectiveness of R. leguminosarum strains for lentil was initially carried out under controlled environments followed by an evaluation under field conditions. In two separate growth room experiments, the effectiveness of 185 and 24 different strains of R. leguminosarum were tested for Laird and Eston lentil. Significant differences between strains in number of nodules, shoot weight and nitrogenase activity (acetylene reduction activity, ARA) were found for lentil grown for 5 weeks. When lentil were grown for 7 weeks, significant differences between strains in number of nodules, total plant weight, total N, and % N were observed.Fourteen strains plus Nitragin ‘C’ inoculant were selected for further field testing on Eston and Laird lentil at two locations in 1986 and one site in 1987. Inoculation increased yield up to 135%. Percent Ndfa and total N2 fixed ranged from 0 to 76 and 0 to 105 kg ha-1, respectively. N2-fixing activity was site specific and higher spring soil NO3-levels resulted in lower N2-fixing activity. Depending on site and growing conditions, strains 99A1 and I-ICAR-SYR-Le20 appeared to be superior to the other strains tested. A good agreement was found between the estimates for N2 fixation based upon the 15N-isotope dilution and the classical N difference methods. Number of nodules, dry weight of nodules and ARA of Eston and Laird lentil grown under growth room conditions failed to show positive correlations with total dry matter production, total N or total N2 fixed of field grown lentil. However, total plant weight and total N of lentil grown under growth room conditions were highly correlated with field parameters, and were the most reliable screening parameters for the selection of superior rhizobial strains.

Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NHinf4sup+, total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NOinf3sup-levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NHinf4sup+but not with available soil water, pH, electrical conductivity, NOinf3sup-, or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the δ 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.

Construction and evaluation of a reference electrode assembly for use in monitoring in situ soil redox potentials

Abstract A simple procedure for the construction of inexpensive Ag,AgCl reference electrode assemblies is described. Disposable plastic syringes (60‐cm) were used to form the bodies of the electrode assemblies and small Ag,AgCl internal reference elements were made from silver wire. The internal reference elements were immersed in a solution of AgCl‐saturated KCl, which also served as the salt bridge. Contact between the salt bridge and test solution or soil was made through a ceramic microtensiometer cup. This procedure yields reference electrode assemblies that perform as well as commercially available reference electrodes in terms of their stability and reproducibility. Because of their rugged construction and ease of maintenance, these reference electrode assemblies are ideal for use in the field. Moreover, they proved to be particularly useful when in situ soil redox potentials were monitored for 84 h.

An economical method for the preparation of plant and animal tissue for δ13C analysis

Abstract This study describes an economical and simple apparatus and methodology for preparing plant material for δ13C analysis by the combustion of material in sealed Pyrex tubes at 550°C. The largest difference in δ13C for samples combusted at 550°C in Pyrex tubes compared with those combusted at 800°C in the more expensive Vycor tubes was 0.65 δ units. Pyrex combustion tubes are approximately 7% of the cost of Vycor tubes and are easier to handle. Excellent reproducibility was observed with an average standard deviation for samples combusted at 550°C and 800°C of 0.12 and 0.13 units of δ13C, respectively. During the cryogenic distillation of CO2 from the other gasses generated during the combustion, equilibration and evacuation times were varied between 10 s and 120 s. Sample size was varied between 0.4 mg and 2.2 mg. Variations in evacuation time, equilibrium time, and sample size did not significantly affect the value for δ13C. The total duration for the cryogenic distillation of one sample was appro...

Carbon isotope discrimination and indirect selection for transpiration efficiency at flowering in lentil (Lens culinaris Medikus), spring bread wheat (Triticum aestivum L.) durum wheat (T. turgidum L.), and canola (Brassica napus L.)

SummaryCarbon isotope discrimination has been proposed to indirectly select for transpiration efficiency in several C3 species. To determine the effectiveness of carbon isotope discrimination to indirectly select for transpiration efficiency at flowering we measured: (i) variability for carbon isotope discrimination, (ii) the magnitude of the genotype-by-water regime interaction for carbon isotope discrimination, and (iii) the magnitude of the correlation between carbon isotope discrimination and both transpiration efficiency and dry matter at flowering. Ten lentil (Lens culinaris Medikus) genotypes, ten wheat genotypes (eight spring wheat (Triticum aestivum L.) and two durum wheat (Triticum turgidum L.)), and ten canola (Brassica napus L.) genotypes were grown in a greenhouse at 80, 50 and 30% field capacity. Above ground dry matter was harvested at 80% flowering and dry matter at flowering, water used, and carbon isotope discrimination determined. Genotype variation for carbon isotope discrimination was observed in lentil, spring wheat and canola at each water regime, and when averaged over the three water regimes. The largest range in carbon isotope discrimination among lentil and spring wheat genotypes was observed using the wet regime; whereas, the dry regime provided the largest range for CID in canola genotypes. In all species the genotype-by-water regime interaction for carbon isotope discrimination was nonsignificant. The correlation between carbon isotope discrimination and dry matter at flowering was inconsistent across water regimes and years. In addition, in all three crops, no correlation was observed between carbon isotope discrimination and transpiration efficiency at any of the water regimes, and when averaged over water regimes and years. These results suggests that under the conditions reported here, carbon isotope discrimination cannot be used effectively to indirectly select for transpiration efficiency in lentil, spring wheat, and canola.

Ureide production by N2-fixing and non-N2-fixing leguminous trees

Abstract Xylem-sap and stem and leaf extracts from 35 species, comprising 14 genera, of leguminous trees were analyzed for ureides, nitrate and α-amino acids. Trees were either inoculated with Rhizobium or fertilized with NH 4 NO 3 . The dominant form of soluble N in stem and leaf extracts and xylem sap was 2-amino acids. Certain non-N 2 -fixing species, i.e. Tamarindus indica and Adenantherapavonina , produced significant amounts of ureides. Several N 2 -fixing species. Mimosa scabrella, Sesbania grandiflora. Acacia mearnsii and Gliricida sepium , grown on mineral-N had higher absolute amounts of ureides in both extracts and exudates than did most nodulated species. Nodulated A. meamsii and S. grandiflora , had the highest amounts of ureides in xylem sap. The relative abundance of ureides in stem and leaf extracts was lower than in xylem sap, but was correlated. Results indicated that the presence of ureides, per se , was not a reliable indicator of N 2 -fixing activity. Moreover, the relative abundance of ureides in most of the species tested was too low to use as a presumptive test for, or as a means of, estimating N 2 fixation.

Assessment of reference crops for the quantification of N2 fixation using natural and enriched levels of 15N abundance

Abstract In studies with the 15 N-isotope dilution method for quantifying N 2 fixation, the importance of selecting a reference crop with a similar pattern of soil N uptake as the N 2 -fixing crop has been emphasized. Because temporal variation in the 15 N enrichment of soil inorganic N will be different following addition of 15 N-enriched and natural abundance sources of N, we hypothesized that only a valid reference crop would provide similar estimates of N 2 fixation in both cases. Barley (Hordeum vulgare L.) , flax (Linum usitatissimum L.) , and a non-N 2 -fixing pea (Pisum sativum L.) cv. were grown as reference crops for N 2 -fixing pea under greenhouse conditions. The soil was amended with either 15 N-enriched plant material or natural abundance (NH 4 ) 2 SO 4 . The amount and 15 N enrichment of soil inorganic and total plant N were determined periodically between 6 and 84 days after planting. Barley and non-N 2 -fixing pea assimilated soil inorganic N earlier than N 2 -fixing pea, while flax assimilated soil inorganic N later. The δ 15 N of soil inorganic N increased between days 14 and 36 due to isotopic fractionation during plant N uptake. By day 84 the δ 15 N of plant N was the same as initial soil inorganic N for barley and non-N 2 -fixing pea, while flax had a lower δ 15 N due to incomplete depletion of soil inorganic N. In the 15 N-enriched treatment, barley had a lower 15 N enrichment at the final sampling date than the other reference crops, even though it had obtained a higher proportion of its N earlier. This was attributed to a greater root-induced turnover of soil N in the barley treatment. Estimates of N 2 fixation were only similar in natural abundance and 15 N-enriched soils when barley was used as the reference crop. It was concluded that comparison of estimates of N 2 fixation in natural abundance and 15 N-enriched soils may provide a useful criterion for selecting a valid reference crop.