The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 μm sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.

Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils

Changes in agricultural management can potentially increase the accumulation rate of soil organic C (SOC), thereby sequestering CO 2 from the atmosphere. This study was conducted to quantify potential soil C sequestration rates for different crops in response to decreasing tillage intensity or enhancing rotation complexity, and to estimate the duration of time over which sequestration may occur. Analyses of C sequestration rates were completed using a global database of 67 long-term agricultural experiments, consisting of 276 paired treatments. Results indicate, on average, that a change from conventional tillage (CT) to no-till (NT) can sequester 57 ± 14 g C m -2 yr -1 , excluding wheat (Triticum aestivum L.)-fallow systems which may not result in SOC accumulation with a change from CT to NT. Enhancing rotation complexity can sequester an average 20 ± 12 g C m -2 yr -1 , excluding a change from continuous corn (Zea mays L.) to corn-soybean (Glycine max L.) which may not result in a significant accumulation of SOC. Carbon sequestration rates, with a change from CT to NT, can be expected to peak in 5 to 10 yr with SOC reaching a new equilibrium in 15 to 20 yr. Following initiation of an enhancement in rotation complexity, SOC may reach a new equilibrium in approximately 40 to 60 yr. Carbon sequestration rates, estimated for a number of individual crops and crop rotations in this study, can be used in spatial modeling analyses to more accurately predict regional, national, and global C sequestration potentials.

Soil organic carbon sequestration rates by tillage and crop rotation : A global data analysis

Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO 2 ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO 2 -C equivalent (CO 2 -C e ) per year (12 % of current anthropogenic CO 2 -C e emissions; 1 Pg = 1 Gt), and total net emissions over the course of a century by 130 Pg CO 2 -C e , without endangering food security, habitat or soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset.

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Sustainable biochar to mitigate global climate change

Review of greenhouse gas emissions from crop production systems and fertilizer management effects

The long-term stabilization of soil organic matter (SOM) in tropical and temperate regions is mediated by soil biota (e.g. fungi, bacte- ria, roots and earthworms), soil structure (e.g. aggregation) and their interactions. On average, soil C turnover was twice as fast in tropical com- pared with temperate regions, but no major differences were observed in SOM quality between the two regions. Probably due to the soil mineralogy dominated by 1:1 clay minerals and oxides in tropical regions, we found a higher aggregate stability, but a lower correlation between C contents and aggregate stability in tropical soils. In addition, a smaller amount of C associated with clay and silt particles was observed in tro- pical versus temperate soils. In both tropical and temperate soils, a general increase in C levels (≈ 325 ± 113 kg C·ha -1 ·yr -1 ) was observed under no-tillage compared with conventional tillage. On average, in temperate soils under no-tillage, compared with conventional tillage, CH4 uptake (≈ 0.42 ± 0.10 kg C-CH4·ha -1 ·yr -1

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Soil organic matter, biota and aggregation in temperate and tropical soils - Effects of no-tillage

Light-fraction (LF) material, comprised largely of incompletely decomposed organic residues, may provide a sensitive indicator of the effects of cropping practices on soil organic matter. The objective of our study was to determine the influence of agronomic variables on soil LF content, and to evaluate the LF as a measure of labile organic matter. Soils from three long-term crop rotation studies in Saskatchewan, Canada, were analyzed for LF content and composition. The experiments, established at Indian Head (Udic Boroll), Melfort (Udic Boroll), and Scott (Typic Boroll), included wheat (Triticum aestivum L.) based rotations varying in fertilizer application, frequency of summer fallow, and cropping sequence. The LF of the surface soil (0–7.5 cm) accounted for 2.0 to 5.4%, 3.3 to 7.1%, and 7.1 to 17.5% of the organic C at Indian Head, Melfort, and Scott, respectively. Within each site, the LF content was generally highest in treatments with continuous cropping or perennial forages and lowest in those with a high frequency of summer fallow. Fertilizer application generally favored LF accumulation. Differences in LF content among sites and treatments were attributed to variable residue inputs and rates of substrate decomposition. The respiration rate and microbial N content of soils was strongly correlated with the LF content, suggesting that the LF is a useful indicator of labile organic matter. Nitrogen mineralization was also correlated with LF content, though the relationship was less consistent, presumably because the high C/N ratio of the LF induced temporary N immobilization. The LF content is a sensitive indicator of the effects of cropping on soil organic matter content and composition but, because of its transient nature, probably reflects primarily short-term effects. LRS Contribution no. 3879163.

Light-fraction organic matter in soils from long-term crop rotations

The effects of crop rotations and various cultural practices on soil organic matter quantity and quality in a Rego, Black Chernozem with a thin A horizon were determined in a long-term study at Ind...

Effect of crop rotations and cultural practices on soil organic matter, microbial biomass and respiration in a thin Black Chernozem

Producers in the Canadian Prairies have begun to extend and diversify their cereal-based rotations by including oilseed and pulse crops, and by managing these newer cropping systems with minimum- and zero-tillage practices. This study examined the implications of these land use changes on non-renewable energy requirements (both direct and indirect), energy output, and energy use efficiency for monoculture cereal, cereal–oilseed, and cereal–oilseed–pulse rotations, each managed using conventional (CT), minimum (MT), and zero (ZT) tillage practices on a thin Black Chernozem in Saskatchewan, Canada. The crop rotations included: spring wheat (Triticum aestivum L.)–spring wheat–winter wheat–fallow (Ws–Ws–Ww–F), spring wheat–spring wheat–flax (Linum usitatissimum L.)–winter wheat (Ws–Ws–Fx–Ww), and spring wheat–flax–winter wheat–field pea (Pisum sativum L.) (Ws–Fx–Ww–P). The findings, based on 12 years of data, showed that non-renewable energy use for the complete cropping systems was largely unaffected by tillage method, but that it differed significantly with crop rotations. Energy requirements were lowest for Ws–Ws–Ww–F (average 6389 MJ ha−1), intermediate for Ws–Fx–Ww–P (11% more), and highest for the Ws–Ws–Fx–Ww (28% more). The substitution of pea for spring wheat in the Ws–Fx–Ww–P versus Ws–Ws–Fx–Ww rotation reduced total energy use by 13%, reflecting the minimal requirement for N fertilizer by pulses due to their ability to biologically fix N, and from the lower fertilizer N rate that was applied to spring wheat grown after the legume. The use of MT and ZT practices provided significant energy savings (compared to CT) in on-farm use of fuel and in machine operation and manufacture for some cropping system components (e.g., summerfallow preparation, spring wheat grown on pea stubble, and for pea grown on cereal stubble), but these savings were often offset by higher energy requirements for herbicides and for N fertilizer with conservation tillage management. Gross energy output averaged 32 315 MJ ha−1 for Ws–Ws–Ww–F, 41 287 MJ ha−1 (or 28% more) for Ws–Ws–Fx–Ww, and 42 961 (or 33% more) for Ws–Fx–Ww–P. Tillage method had little overall influence on energy output for the monoculture cereal and cereal–oilseed–pulse rotations, but it was generally lower with CT than with MT or ZT management for the cereal–oilseed rotation. Energy use efficiency, measured as grain produced per unit of energy input and as the ratio of energy output to energy input, was highest for the cereal–oilseed–pulse rotation (373 and 6.1 kg GJ−1, respectively) and lower, but generally similar, for the cereal–oilseed and monoculture cereal rotations (298 and 5.1 kg GJ−1, respectively). The use of conservation tillage management enhanced overall energy use efficiency for the two mixed rotations, but not for the monoculture cereal rotation. We concluded that adopting diversified crop rotations, together with minimum and zero tillage management practices, will enhance non-renewable energy use efficiency of annual grain production in this sub-humid region.

Effects of tillage method and crop rotation on non-renewable energy use efficiency for a thin Black Chernozem in the Canadian Prairies.

Carbon sequestration was determined for different tillage systems in semiarid to sub-humid climates and coarse to fine-soil texture in Saskatchewan, Canada. Annually cropped rotations sequestered 27–430 kg C ha −1 per year more than crop rotations containing bare fallow. The potential for sequestering soil organic C (SOC) with crop rotations without bare fallow was greater in the sub-humid than in the drier climates. No-tillage (NT) sequestered 67–512 kg C ha −1 per year more than tilled systems. With elimination of both tillage and bare fallow, the SOC increase was approximately 300 kg C ha −1 per year in the semiarid climate regardless of soil texture, and approximately 800 kg C ha −1 per year in the sub-humid climate. Relative annual increase in SOC under no-till was approximately a linear function of clay content across locations. Fine-textured soils have a greater potential for gains in SOC under no-till in Canadian prairie region.

Crop rotation and tillage impact on carbon sequestration in Canadian prairie soils

Disease severity and incidence of fungal species on wheat and pea were evaluated under zero, minimum, and conventional tillage in three rotations with increasing broad-leaved crop diversity from 1991 to 1998 at Indian Head, SK. The objective was to determine whether differences in crop rotation and crop residue at the soil surface would increase crop disease problems. Rotation and tillage had little impact on disease relative to the environment. Reduced tillage did not substantially increase disease severity from leaf spot diseases of wheat or on diseases of field pea. However, the relative importance of root pathogens of wheat was affected; under reduced tillage Bipolaris sorokiniana and Gaeumannomyces graminis decreased, but Fusarium spp. increased on wheat roots. These changes did not affect overall root severity or yield. Rotations had limited impact on wheat disease severity and the prevalence of fungal species, but wheat grown in diversified crop rotations using cereal, pea and flax had consistently...

Effect of tillage and crop rotation on root and foliar diseases of wheat and pea in Saskatchewan from 1991 to 1998: Univariate and multivariate analyses

G. P. Lafond

Papers

Light-fraction organic matter in soils from long-term crop rotations

Effect of crop rotations and cultural practices on soil organic matter, microbial biomass and respiration in a thin Black Chernozem

Effects of tillage method and crop rotation on non-renewable energy use efficiency for a thin Black Chernozem in the Canadian Prairies.

Crop rotation and tillage impact on carbon sequestration in Canadian prairie soils

Effect of tillage and crop rotation on root and foliar diseases of wheat and pea in Saskatchewan from 1991 to 1998: Univariate and multivariate analyses