. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soit Sci. 75: 529-538 Assessments of managemenrinduced 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 thickniss and bulk density of the soil layers in the fieid 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 bray Luviscl soils under nitive aspen forest and various cropping systems were hampered by differences in the mass of soil under consideraiion. 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 accumrlation usually were different for cimparisonr u-ong element masses in an equivalent soil mass than for comparisons among element massei in genetic horizons or in frxed sampling depths. Unless soil erosion or deposition had altered the mass of topsoil per unit area, comparisons among unequal soil massei 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. Consider the amount of soil organic C in a soil before and after tillage without any gains or losses of soil or C (Fig. 1).The assumed concentration remains constant at 2Vo or 20 kg C Mg-l^soil. Before tillage the soil has a bulk density of 1.6 Mg --3, such that the 0 to 150 mm layer consists of 2400Mg soil ha-l and contains 48 Mg C ha-l. Then consider the impact of a single tillage operation that decreases soil bulk density to 1.2 Mg m-3 without changing the soil C concentration or causing lateral redishibution of the soil (Fig. 1) Analyses for C, N, S and P were performed on finely ground subsamples (< 150 pm). Methods used to determine total and inorganic C, N, S, P, and sulfate-S are outlined in Roberts et al. (1989). Organic C, N, S, P, and sulfate-S were calculated as differences between the concentrations of total and inorganic elements. Ideally, management-induced changes in organic matter can be assessed from comparisons among similar soils (i'e. identical original thickness, bulk density, texture) with contrasting management histories. Thus, management effects can be inferred simply from changes in element concentrations in the surface horizons, provided that changes in horizon thickness exactly compensate for changes in bulk density, such that soil masses are identical. In practice, however,...

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.

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.

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.

The profiles of a forest (Typic Cryoboralf), grassland (Aridic Haploboroll), and gleyed (Argiaquoll) soil were examined to assess the role of organic matter leaching in determining organic matter composition in genetic horizons of soil profiles. An increasing proportion of NaOH extractable C, N, P, and S was found in the fulvic acid (FA) fraction as depth in the profiles increased, suggesting that fulvic acids produced in biologically active surface horizons have been translocated to B and C horizons by percolating water. The fulvic acid in B and C horizons was highly enriched in N, P, and S relative to the humic acid and fulvic acid in surface horizons, indicating that the organic matter being translocated downward is richer in N, P, and S than fractions not susceptible to leaching. Leaching of nutrient‐rich organic matter thus provides a valid explanation for the observed narrowing of soil organic matter C/N, C/P, and C/S ratios with increasing depth. The lowest HA/FA ratios coincided with the depth of CaCO3 accumulation, revealing a possible role of Ca and other basic cations in precipitation of organic matter from solution. Of the four elements studied, P appeared to be the most susceptible to deep leaching in the organic form. This is believed to reflect the predominance of organic P in the low molecular weight, mobile fraction of soil organic matter. Loss of labile, nutrient‐rich organics from surface horizons by leaching may be important in controlling element turnover rates and nutrient distribution within soil profiles. It may also represent a major nutrient export mechanism.