J. R. Bettany

Assessments of management-induced changes in soil organic matter depend on the methods used to calculate the quantities of organic C and N stored in soils. Chemical analyses in the laboratory indicate the concentrations of elements in soils, but the thickness and bulk density of the soil layers in the field must be considered to estimate the quantities of elements per unit area. Conventional methods that calculate organic matter storage as the product of concentration, bulk density and thickness do not fully account for variations in soil mass. Comparisons between the quantities of organic C, N, P and S in Gray Luvisol soils under native aspen forest and various cropping systems were hampered by differences in the mass of soil under consideration. The influence of these differences was eliminated by calculating the masses of C, N, P and S in an "equivalent soil mass" (i.e. the mass of soil in a standard or reference surface layer). Reassessment of previously published data also indicated that estimates of organic matter storage depended on soil mass. Appraisals of organic matter depletion or accumulation usually were different for comparisons among element masses in an equivalent soil mass than for comparisons among element masses in genetic horizons or in fixed sampling depths. Unless soil erosion or deposition had altered the mass of topsoil per unit area, comparisons among unequal soil masses were unjustified and erroneous. For management-induced changes in soil organic matter and nutrient storage to be assessed reliably, the masses of soil being compared must be equivalent. Key words: Soil carbon, soil nitrogen, soil phosphorus, soil sulfur, carbon cycle, carbon storage, bulk density effects, Gray Luvisol, soil erosion

Abstract Cultivation has substantially reduced the organic matter contents of many prairie soils. This study attempts to quantify the losses of C, N, and P from three prairie soils of different textures during cultivation. For this purpose cultivated and adjacent uncultivated soils (2 Cryoborolls and 1 Cryorthent) were sampled and their C, N, and P contents as well as their bulk densities and horizon depths were compared. Reductions of about 35% in the C concentration were observed in clay and silt loam soils after 60 to 70 years of cultivation. At the same time reductions in N concentrations were greatly influenced by the presence or absence of legume [alfalfa, ( Medicago sativa L.)] crops grown in the fields and losses varied between 18 and 34%. Phosphorus concentrations were reduced by 12% and all P losses were accounted for by the organic fraction. During a similar period of cultivation a lighter textured sandy loam had experienced greater reductions in C, N, and P concentrations of 46, 46, and 29%, respectively. In this soil P was lost from both the organic and inorganic fractions. Prolonged cultivation of 90 years did not result in a decrease in the rates of losses of C, N, and P on the silt loam soil. Conversion of concentration data to area based total C, N, and P budgets resulted in a decrease in the differences seen between cultivated and uncultivated soils. This was caused by an increase of soil bulk densities under cultivation and by an increase in the standard deviations of the data due to variability of horizon depths in cultivated fields.

Abstract Organo‐mineral complexes in various size fractions from the surface horizons of two Chernozemic soils (Typic Argiboroll and Udic Haploboroll) were separated without chemical pretreatment by ultrasonic dispersion in water, followed by sieving and centrifugation. The organic carbon (C), nitrogen (N), and sulfur (S) composition in the size fractions and the degree of polycondensation of humic materials extracted by an alkaline pyrophosphate technique were compared. Fifty‐five to 58% of the organic C was in the clay fraction, with greatest absolute amounts in the coarse clay (2–0.2 µm). Carbon/nitrogen ratios narrowed as particle size decreased. The organic matter separated from the coarse‐clay and fine‐silt fractions (5–2 µm) was dominated by conventional humic acids (HA‐A), which based on their strong adsorption at 280 nm and resistance to acid hydrolysis, were considered strongly aromatic and recalcitrant in soil. In contrast, the organic matter associated with the fine clay (<0.2 µm) was largely fulvic acids (FA‐A, FA‐B) and humic acids (HA‐B) that were less aromatic than conventional humic acids and contained considerable amounts of hydrolyzable N. The fine‐ and coarse‐clay fractions (<2 µm) contained >70% of the total soil S, >80% of the HI‐reducible S, and >64% of the carbohydrate C. In relation to C and N, S was preferentially associated with the fine‐clay fractions. The C/S and N/S ratios decreased substantially from maximum values in the fine‐silt fraction (approximately 120:1 and 10:1, respectively) to minimum values in fine clay (approximately 33:1 and 4.5:1, respectively). The distinct differences between the humus of the coarse clay‐fine silt and the fine‐clay fractions indicate that size fractionation following ultrasonic dispersion in water is a promising method of isolating stable and labile forms of soil organic matter. The data also support earlier hypotheses on the nature of soil S that were based on studies of chemical separation of organic matter from complete soils.

Abstract Both sulfur and nitrogen deficiencies have been observed in large acreages of western Canadian soils. These deficiencies prompted an investigation of the interactive effects of N and S on rapeseed ( Brassica napus L.), a S‐sensitive crop that is extensively grown on these soils. Four rates of N (0, 50, 100, 300 mg N kg −1 soil) and four rates of S (0, 5, 15, 40 mg S kg −1 soil) were applied in all combinations to rapeseed grown in a pot culture experiment. Maximum seed yield responses to N and S were observed only when the availability of N and S was in approximate balance. Excessive N applications relative to S availability severely suppressed seed production. This effect was attributed to the accumulation of toxic levels of N metabolites. Excessive S applications relative to N availability produced excessive accumulation of S in the plant tissue. The leaves, and, to some extent, the stems, were the predominant sites of excess nutrient accumulation. Seed nutrient concentrations remained relatively stable over all fertilizer treatments. In most plant parts, the applications of one element reduced the concentrations of the other element by a “dilution” effect. The optimum ratio of available N to available S in the soil was estimated to be 7 to 1. Ratios below 7 resulted in inefficient utilization of the assimilated S, while ratios exceeding 7 resulted in reduced seed yields.