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Extraction of soil organic mailer (SOM) fractions has been a longstanding approach to elucidating the pivotal roles of SOM in soil processes. Several types of extraction procedures are commonly used, and all provide partial information on SOM function. This report and accompanying papers summarize the information regarding SOM functions in real-world issues that has been gained through physical or chemical fractionations. Each procedure has its strengths and weaknesses; each is capable to some degree of distinguishing labile SOM fractions from nonlabile fractions for studying soil processes, such as the cycling of a specific soil nutrient or anthropogenic compound, and each is based on an agent for SOM stabilization. Physical fractionations capture the effects on SOM dynamics of the spatial arrangement of primary and secondary organomineral particles in soil, but they do not consider chemical agents for SOM stabilization. They appear better suited for C cycling than N cycling. Chemical fractionations cannot consider the spatial arrangement, but their purely organic fractions are suitable for advanced chemical characterization and can be used to elucidate molecular-level interactions between SOM and nutrients or other organic compounds. During all fractionations, the potential exists for sample alteration or mixing of material among fractions. We call for better coordination of research efforts by (i) developing integrated fractionation procedures that include physical, chemical, and/or biological components, and (ii) categorizing fractionations by their most suitable applications, defined by the nutrient, compound, or soil process in question, land use or crop type, crop management strategies, soil type, and possibly other factors. Selecting the most suitable fractionation procedure for a given research application would enable more precise approximation of the functional SOM pool.

Agriculture depends upon soil to serve as a medium for plant growth, as a reservoir of nutrients and water, and as a filter to detoxify chemical inputs. Soil of high [...]

Dairy rotations rely on corn silage, which is estimated to have significant nitrous oxide (N2O) emissions. This study examined whether including legumes within rotations can reduce N2O emissions from the soil surface and dissolved in tile-drainage water. Emissions of N2O were measured from the soil surface and in tile drainage. Cropping systems were: corn–corn (CC), corn + cover crop-corn (C + cc), soybean–corn (SC) and alfalfa–alfalfa (AA) on a clay soil. Liquid dairy manure provided 2-year total N inversely related to legume cropping: 310 (CC), 280 (C + cc), 110 (SC), 50 kg N ha−1 (AA). Losses of N2O via tile drainage were 0.1–0.3% of total emissions. Ratios of N2O-N to NO3−-N in drainage were at least 63% lower than the IPCC default value (0.0075). Reductions of N2O emissions were only observed from established alfalfa in year 2. Compared to the SC treatment, which had the highest emissions in year 2, the AA treatment had 62% lower surface N2O and 88% lower dissolved N2O flux. Alfalfa had low yield in the first year, which led to high yield-scaled N2O emissions; thus, alfalfa may need to be grown 4 years to achieve a similar average yield scaled emission factor as CC. Silage corn had consistently high yield, averaging 317 kg ha−1 yr−1 for N yield, which was 36% higher than AA. As a result, CC had the lowest N2O emissions scaled by N-yield over the 2 years, averaging 2.6% of N-yield, which was 59% lower than AA on average. [ABSTRACT FROM AUTHOR]

Widespread global changes, including rising atmospheric CO2 concentrations, climate warming and loss of biodiversity, are predicted for this century; all of these will affect terrestrial ecosystem processes like plant litter decomposition. Conversely, increased plant litter decomposition can have potential carbon-cycle feedbacks on atmospheric CO2 levels, climate warming and biodiversity. But predicting litter decomposition is difficult because of many interacting factors related to the chemical, physical and biological properties of soil, as well as to climate and agricultural management practices. We applied 13C-labelled plant litter to soil at ten sites spanning a 3500-km transect across the agricultural regions of Canada and measured its decomposition over five years. Despite large differences in soil type and climatic conditions, we found that the kinetics of litter decomposition were similar once the effect of temperature had been removed, indicating no measurable effect of soil properties. A two-pool exponential decay model expressing undecomposed carbon simply as a function of thermal time accurately described kinetics of decomposition. ( R2 = 0.94; RMSE = 0.0508). Soil properties such as texture, cation exchange capacity, pH and moisture, although very different among sites, had minimal discernible influence on decomposition kinetics. Using this kinetic model under different climate change scenarios, we projected that the time required to decompose 50% of the litter (i.e. the labile fractions) would be reduced by 1-4 months, whereas time required to decompose 90% of the litter (including recalcitrant fractions) would be reduced by 1 year in cooler sites to as much as 2 years in warmer sites. These findings confirm quantitatively the sensitivity of litter decomposition to temperature increases and demonstrate how climate change may constrain future soil carbon storage, an effect apparently not influenced by soil properties. [ABSTRACT FROM AUTHOR]

Erosion influences the lateral and vertical distribution of soil in agricultural landscapes. A better understanding of the effects of erosion and redistribution on soil organic carbon ( C) within croplands would improve our knowledge of how management practices may affect global C dynamics. In this study, the vertical and lateral distribution of soil organic C was characterized to evaluate the amounts and timescales of soil organic C movement, deposition and burial over the last 50 years in different agroecosystems across Canada. There was strong evidence that a substantial portion of eroded sediment and soil organic C was deposited as colluvium close to its source area, thereby burying the original topsoil. The deepest aggraded profile was in a potato field and contained over 70 cm of deposited soil indicating an accumulation rate of 152 Mg ha yr−1; aggraded profiles in other sites had soil deposition rates of 40-90 Mg ha−1 yr−1. The largest stock of soil organic C was 463 Mg ha−1 (to 60 cm depth) and soil C deposition ranged from about 2 to 4 Mg ha−1 yr−1 across all sites. A distinct feature observed in the aggraded profiles at every site was the presence of a large increase in soil organic C concentration near the bottom of the A horizon; the concentration of this C was greater than that at the soil surface. Compared to aggraded profiles, the SOC concentration in eroded profiles did not differ with depth, suggesting that dynamic replacement of soil organic C had occurred in eroded soils. A large amount of soil organic C is buried in depositional areas of Canadian croplands; mineralization of this stock of C appears to have been constrained since burial, but it may be vulnerable to future loss by management practices, land use change and a warming climate. [ABSTRACT FROM AUTHOR]

Assessment and monitoring of soil organic matter (SOM) quality are important for determining and developing management practices that will enhance and maintain the productivity of agricultural soils. This requires routine analysis of multiple soil parameters, which can be time-consuming and expensive. Research has suggested that visible near infrared reflectance spectroscopy (VNIRS) may be used as a rapid and cost-efficient tool for SOM quality assessment. In this study, VNIRS (400-2498nm) was used for the first time to simultaneously predict microbial biomass nitrogen (MBN), water-extractable organic N (WEON), light fraction organic matter N (LFOMN), particulate organic matter N (POMN), soil total N (TN), soil organic carbon (SOC) and soil C/N ratio as soil SOM quality indicators in Chernozemic soils of western Canada. The soil samples (n=200) were collected at the 0-15cm depth from a crop rotation experiment conducted at 6 sites in 2010 and 2011. After removal of outliers (five samples) identified by principal components analysis (PCA), 75% of the sample set was randomly selected for calibration (n=146) and the remainder used for validation (n=49). Modified partial least squares regression with cross-validation was used to develop prediction models. The reliability of the models was assessed using the coefficient of determination in validation (R²V) and the ratio of standard deviation of the reference data in the validation set to the standard error of prediction (RPDV). The VNIRS predictions were considered reliable for LFOMN, POMN, TN, and SOC (R²V>0.80, RPDV>2.4), as well as for MBN (R²V=0.74, RPDV=1.93), but less reliable for WEON (R²V=0.67, RPDV=1.70) and soil C/N ratio (R²V=0.54, RPDV=1.45). This study showed that VNIRS has the potential as a non-destructive and cost-efficient tool for rapid determination of SOM quality indicators. [ABSTRACT FROM AUTHOR]