. 1994. Towards a minimum data set to asseis soil organic matter quatity ii agricultural soils. Can. J. Soil Sci. 74 [367][368][369][370][371][372][373][374][375][376][377][378][379][380][381][382][383][384][385]. Soil quality is a composite measure of both a soil's ability to funct-ion and how welt it functions, relative to a specific use. Soil quality can beassessed using a minimum data set comprisingioil attributes such as texture, organic matter, pH, bulk density, and rooting depth. Soit organic matter has particular significa-nce for soil quality as it can influenie many different soil properties including other attributes of the minimum data set. Asiessment of soil organii matter is a valuable step towards identifying the overall quality of a soil and may be so informative as to be included in minimum data sets used to evaluate the world's soils.In this review, soil organic matter is considered to encompass a set of attributes rather than being a single entity. Included among the attributes and discussed here are total soil organiciarbon and nitrogen, light fraction and macroorganic (particulate) mattei, mineralizable carbon and nitrogen, microbial biomass, soil carbohydrates and enzymes. These attributes are involved in various soil processes, such as those related to nutrient storage, biological activity, and soil structure, and can be used to establish different minimum data sets for the evaluation of soil organic matter quality.Key words: Biological activity, minimum data set, nutrient storage, soil organic matter, soil quality, soil structure (Fig' l). The relationships between cumulative mineralization during 22 wk and 3 or l0 wk, although highly significant (P < 0.0001), were less close for N (Fig. 2) than C (Fie. 1).Mineralizable-C and N may be determined simultaneously in a single incubation, but combined data are rarely reported (Table 3). Under optimal conditions in the laboratory. rates of C mineralization typically range fr-om ] to 30 pg I -' d -' in mineral soils and 150-800 pE g-| d -' in organic layers'Corresoonding rates of N mineralization range from 0'3 to -2.5 is.s-rt-r in mineral soils and from 3.0 to 15 pg g-r d-T in organic layers (Table 3). Carter and Rennie (fSAZ) reported greater rates of C and N mineralization in thin layeis of surface soil under zero tillage compared to conventional tillage because crop residues were concentrated at the surface ofzero-tilled soils. Janzen's (1987b) comparisons of crop rotations indicated that the proportions of the total store of C and N in the mineralizable fractions decreased with increasing frequency of fallow, suggesting that fallow was detrimental to organic matter quality (Table 3). In another study, the amounts of mineralizable C and N were greater in fertilized than unfertilized soils, but the proportions of total soil organic C and N in the mineralizable fractions were similar, because fertilized soils also contained greater amounts of total C and N (Janzen 1987a

ABSTRACTor sink of atmospheric CO 2 depending on the rate of SOC formation and decomposition (Van Breemen andForested ecosystems have been identified as potential C sinks. Feijtel, 1990). It is therefore important to understand depleted (Johnson, 1992

. 2003. Influence of agricultural management on soil organic carbon: A compendium and assessment of Canadian studies. Can. J. Soil Sci. 83: 363-380. To fulfill commitments under the Kyoto Protocol, Canada is required to provide verifiable estimates and uncertainties for soil organic carbon (SOC) stocks, and for changes in those stocks over time. Estimates and uncertainties for agricultural soils can be derived from long-term studies that have measured differences in SOC between different management practices. We compiled published data from long-term studies in Canada to assess the effect of agricultural management on SOC. A total of 62 studies were compiled, in which the difference in SOC was determined for conversion from native land to cropland, and for different tillage, crop rotation and fertilizer management practices. There was a loss of 24 ± 6% of the SOC after native land was converted to agricultural land. No-till (NT) increased the storage of SOC in western Canada by 2.9 ± 1.3 Mg ha -1 ; however, in eastern Canada conversion to NT did not increase SOC. In general, the potential to store SOC when NT was adopted decreased with increasing background levels of SOC. Using no-tillage, reducing summer fallow, including hay in rotation with wheat (Triticum aestivum L.), plowing green manures into the soil, and applying N and organic fertilizers were the practices that tended to show the most consistent increases in SOC storage. By relating treatment SOC levels to those in the control treatments, SOC stock change factors and their levels of uncertainty were derived for use in empirical models, such as the United Nations Intergovernmental Panel on Climate Change (IPCC) Guidelines model for C stock changes. However, we must be careful when attempting to extrapolate research plot data to farmers' fields since the history of soil and crop management has a significant influence on existing and future SOC stocks.

. 2001. Changes in soil carbon under long-term maize in monoculture and legume-based rotation. Can. J. Soil Sci. 81: 21-31. Legume-based cropping systems could help to increase crop productivity and soil organic matter levels, thereby enhancing soil quality, as well as having the additional benefit of sequestering atmospheric C. To evaluate the effects of 35 yr of maize monoculture and legume-based cropping on soil C levels and residue retention, we measured organic C and 13 C natural abundance in soils under: fertilized and unfertilized maize (Zea mays L.), both in monoculture and legume-based [maize-oat (Avena sativa L.)-alfalfa (Medicago sativa L.)-alfalfa] rotations; fertilized and unfertilized systems of continuous grass (Poa pratensis L.); and under forest. Solid state 13 C nuclear magnetic resonance (NMR) was used to chemically characterize the organic matter in plant residues and soils. Soils (70-cm depth) under maize cropping had about 30-40% less C, and those under continuous grass had about 16% less C, than those under adjacent forest. Qualitative differences in crop residues were important in these systems, because quantitative differences in net primary productivity and C inputs in the different agroecosystems did not account for observed differences in total soil C. Cropping sequence (i.e., rotation or monoculture) had a greater effect on soil C levels than application of fertilizer. The difference in soil C levels between rotation and monoculture maize systems was about 20 Mg C ha -1 . The effects of fertilization on soil C were small (~6 Mg C ha -1 ), and differences were observed only in the monoculture system. The NMR results suggest that the chemical composition of organic matter was little affected by the nature of crop residues returned to the soil. The total quantity of maize-derived soil C was different in each system, because the quantity of maize residue returned to the soil was different; hence the maize-derived soil C ranged from 23 Mg ha -1 in the fertilized and 14 Mg ha -1 in the unfertilized monoculture soils (i.e., after 35 maize crops) to 6-7 Mg ha -1 in both the fertilized and unfertilized legume-based rotation soils (i.e., after eight maize crops). The proportion of maize residue C returned to the soil and retained as soil organic C (i.e., Mg maize-derived soil C/Mg maize residue) was about 14% for all maize cropping systems. The quantity of C3-C below the plow layer in legume-based rotation was 40% greater than that in monoculture and about the same as that under either continuous grass or forest. The soil organic matter below the plow layer in soil under the legume-based rotation appeared to be in a more biologically resistant form (i.e., higher aromatic C content) compared with that under monoculture. The retention of maize residue C as soil organic matter was four to five times greater below the plow layer than that within the plow layer. We conclude that residue quality plays a key role in increasing the retention of soil C in agroecosystems and that soils under legumebased...

. 2003. Characterizing organic matter retention for surface soils in eastern Canada using density and particle size fractions. Can. J. Soil Sci. 83: [11][12][13][14][15][16][17][18][19][20][21][22][23]. Interest in the storage of organic matter in terrestrial ecosystems has identified a need to better understand the accumulation and retention of organic C and N in soil. The proportions of C and N associated with clay and silt particles (i.e., "capacity level"), water-stable macro-aggregates (WSA) (>250 µm), particulate (POM) (>53 µm), and light fraction (LF) organic matter, for the 0-to 10-cm soil depth, were assessed at 14 agricultural experimental sites established on Gleysolic, Podzolic, Luvisolic, and Brunisolic soils in the cool, humid region of eastern Canada. Organic C and N in the clay plus silt particles was at or near the capacity level for soils with clay plus silt content < 40%. For soils with >60% clay plus silt, the degree of saturation was 65-70% indicating a potential for further organic C and N retention. The mean proportion of C and N found in the POM was 22 and 27%, while the LF organic matter contained 7 and 5% C and N, respectively. Mean soil WSA content, determined by wet-sieving analysis, was 42% for air-dry soil and 54% for wetted soil, and was significantly (P < 0.05) related to both soil clay plus silt (r = 0.65) and organic C (r = 0.54). Water-stable macro-aggregate C content was proportional to soil organic C (r = 0.96, P < 0.01). At four of the sites, where soil C and N were influenced by management, an increasing level of soil organic C and N was associated with both the clay plus silt particles and the POM fraction until the former was saturated. Once the capacity level was saturated, further organic C and N accumulation was associated with the POM fraction. Although stabilized organic C and N in soil exists as a continuum, both soil particle and particulate fractions provided a practical approach to monitor, quantify and differentiate the storage and retention of C and N in soils of eastern Canada. Dans ceux qui en renferment plus de 60 pour cent, on note un taux de saturation de 65 à 70 pour cent, signe que le maximum n'a pas été atteint pour la rétention de C et de N organiques. En moyenne, on trouve 22 et 27 pour cent de C et de N dans les PMO, alors que la FL n'en contient que 7 et 5 pour cent, respectivement. Le tamisage humide révèle que l'ASE en renferme en moyenne 42 pour cent (sol séché à l'air) ou 54 pour cent (sol humide), concentration significativement (P < 0,05) corrélée au C associé à l'argile et au limon (r = 0,65) ainsi qu'au C organique du sol (r = 0,54). La concentration de C dans les macro-agrégats stables à l'eau est proportionnelle à la quantité de C dans le sol organique (r = 0,96; P < 0,01). La hausse de la concentration de C et de N dans le sol organique a été associée aux particules d'argile et de limon ainsi qu'aux PMO, jusqu'à saturation des premières, à quatre endroits où les pratiques agraires avaient une incidence sur la quantité de C et de N présent...

In presence of vegetation, the CO2–C produced by respiration activity in soils originates from plant C (rhizosphere respiration, Rrh) and from soil C (soil respiration, Rs). Quantitative estimates of the CO2 produced by each source are required in many studies of C dynamics in the soil–plant system. In this study, we (i) used measurements of the 13C value of soil CO2 to separate total soil respiration (Rt) into subcomponents Rrh and Rs in a maize (Zea mays L.) field under undisturbed conditions and (ii) compared these Rrh estimates with values obtained using the root‐exclusion approach. The maximum contribution of Rrh to total respiration was 45%, observed in August. Estimates of Rrh increased from zero 30 d after planting to 2 g CO2–C m−2 d−1 70 d after planting, remained relatively constant at that level in August, and then decreased until the end of the growing season. The total C losses as Rrh were 17% of the crop net assimilation. Estimates of Rs gradually declined from 3.3 g CO2–C m−2 d−1 in late June to 1.4 g CO2–C m−2 d−1 at the end of the season. Losses of soil C represented ≈6% of total soil C. Variable values of δ13C of the soil CO2 in the control plot after Day 250 made the technique less reliable late in the season. However, several observations indicated that the approach has potential to provide quantitative estimates of Rrh and Rs First, the seasonal pattern of the Rrh estimates coincided with that of the plant growth and physiological activity. Second, the cumulated Rrh across the growing season agreed well with published data obtained using 14C labeling techniques. Third, in the maize plot, variation in the estimated Rs was closely correlated with changes in soil temperature with a Q10 of 1.99 Finally, the estimates of Rrh obtained using the isotopic approach agreed well with those obtained using the root exclusion technique.

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ing spatial variations of soil water content using noncontacting electromagnetic inductive methods. Can. J. Soil Sci. 68:715-722.The relationships among the spatial variations of soil water content, soil texture, soil solution electrical conductivity, and bulk soil electrical conductivity were examined for a field characterized by net drainage and low concentrations of dissolved electrolytes. Bulk soil electrical conductivity was measured over various depths at 52loca-tions within a 1.8-ha field using noncontacting electromagnetic inductive meters. Soil water content (0-0.5 m depth) was measured at the same locations using the time domain reflectometry method. Measurements of soil texture and soil solution conductivity were obtained from core samples from-37 of the sampling locations. Soil water content at the site ranged from 0.06 to 0.36 m j m '. Clay content ranged from 2.5 to 44% percent and bulk soil electrical conductivity ranged from 0.0 to 0.21 S m-1. Significant correlation existed among almost all ofthe measured variables. Regression analysis indicated soil solution conductivity had no effect on measured bulk soil electrical conductivity for soil water contents less than 0.25 m3 m-t. Bulk selected to obtain the maximum variation in 0" and texture across the site (Fig. l). RESULTS AND DISCUSSIONSand content in the 0-to 0.5-m depth of the study area varied from 5 to70% and clay content from 2.5 to 44.0% (Table 1)