Structure and Organic Matter Storage in Agricultural Soils

M.R. Carter, B.A. Stewart

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000157475
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Soils comprise the largest pool of terrestrial carbon and therefore are an important component of carbon storage in the biosphere-atmosphere system. Structure and Organic Matter Storage in Agricultural Soils explores the mechanisms and processes involved in the storage and sequestration of carbon in soils. Focusing on agricultural soils - from tropical to semi-arid types - this new book provides an in-depth look at structure, aggregation, and organic matter retention in world soils. The first two sections of the book introduce readers to the basic issues and scientific concepts, including soil structure, underlying mechanisms and processes, and the importance of agroecosystems as carbon regulators. The third section provides detailed discussions of soil aggregation and organic matter storage under various climates, soil types, and soil management practices. The fourth section addresses current strategies for enhancing organic matter storage in soil, modelling techniques, and measurement methods. Throughout the book, the importance of the soil structure-organic matter storage relationship is emphasized. Anyone involved in soil science, agriculture, agronomy, plant science, or greenhouse gas and global change studies should understand this relationship. Structure and Organic Matter Storage in Agricultural Soils provides an ideal source of information not only on the soil structure-storage relationship itself, but also on key research efforts and direct applications related to the storage of organic matter in agricultural soils.

Impact of Climate, Soil Type, and Management

Aggregation and Organic Matter Storage in Cool, Humid Agricultural Soils

D.A. Angers and M.R. Carter

I. Introduction

II. Soil Structure and Aggregation Models

III. Organic Matter Protection and Macroaggregation

IV. Potential for Increasing Organic Matter Retention

A. Perennial Forages

B. Short-Term Rotations and Cover Crops

C. Fertilization and Organic Additions

D. Conservation Tillage

E. Type of Vegetation

V. Conclusions

Acknowledgements

References

I. Introduction

Cool to cold humid climates characterised by long cold winters are mainly found in the northern hemisphere between 40°C to 60°N (Money, 1974). Native vegetation for this climatic region is primarily boreal forest which is dominantly coniferous. In southern parts, the conifers grade into forests of broad-leaved deciduous trees. Soil types developed in the above glaciated ecosystems are predominantly Podzols, Luvisols and Gleysols (Canada Expert Committee on Soil Survey, 1978).

In the native state (usually forest) soils of the cool, humid climatic regions usually show large accumulation of organic matter. Post et al. (1982) and Zinke et al. (1984) estimated the C density of cool temperate forest soils to be about 13 kg C m^-2, which contrasts with a value of approximately 5 kg C m^-2 for dry soils (Eswaran et al., 1993). Generally, the influence of climate on soil C content is reflected in the balance between C inputs from vegetation and C losses via decomposition (Mele and Carter, 1993). This balance can be characterized by the quotient of mean annual temperature and annual precipitation, where a decreasing ratio (i.e. cool wet climate) is associated with slow soil C turnover (Tate, 1992). However, type of vegetation and agricultural system will also greatly influence the content and storage of soil C (Schlesinger, 1990). This underlines the importance of characterizing the organic carbon storage potential for soils in cool humid

Cultivation of virgin soils (native grassland or forest) has usually resulted in a decline in soil organic matter and net release of CO, to the atmosphere (Schlesinger, 1990; Davidson and Ackerman, 1993). For forest soils, this decline in organic matter is related first of all to a change in type of vegetation and subsequent change in C inputs and magnitude of various C pools (e.g. litter); secondly, to increases in soil temperature, associated with a reduction in soil shading and greater incidence of net radiation at the soil surface, which can influence organic C decomposition; and thirdly to changes associated with cultivation such as modification of soil structure, incorporation of organic C into the soil, and increased potential for soil erosion. However, although the above changes are operative, the actual net change in total soil organic C mass can be minor. Martel and Deschênes (1976) reported an average loss of C of 30% upon long-term (> 30 years) cultivation of three Québec forest soils. More recently, in eastern Canada, the comparison of 22 boreal forest soils with adjacent agricultural soils not subject to significant soil redistribution via erosion showed that the mass of soil organic C under the latter system was depleted by an average of 22% (Gregorich et al., 1993). This is generally comparable or somewhat lower than the reported losses of organic matter inventory from North American prairie soils (Mollisols) subject to cultivation (Davidson and Ackerman, 1993).

The role of soil structure modification as expressed by aggregate degradation (breakdown) in the loss of soil organic C upon cultivation is still unclear. The introduction of tillage may cause a decline in organic C through disruption of aggregates and subsequent release of CO,. In contrast, a change in plant productivity associated with arable farming may result in reduced organic C inputs and organic aggregate binding agents, causing a consequent decrease in aggregation.

The main objective of this chapter is to evaluate the potential for managing soil aggregation for the storage and sequestration of organic matter under cool, humid climatic conditions. Theng et al. (1989) emphasized the advantage of characterizing organic matter within well-defined climatic regions. To this end, the chapter will concentrate mainly on studies conducted in Podzolic, Luvisolic and Gleysolic soils of eastern Canada to illustrate short-term dynamics of soil aggregation and organic C storage under various cropping systems.

II. Soil Structure and Aggregation Models

Soil structure can be defined in terms of form and stability (Kay et al., 1988). Structural form refers to the heterogeneous arrangement of solid and void space that exists at a given time, whereas the stability of a soil’s structure is its ability to retain this arrangement when exposed to different stresses (ibid.). Soil aggregates are normally not a random arrangement of primary particles. Primary particles and aggregates of different size are usually arranged in a hierarchical fashion. Tisdall and Oades (1982) presented an aggregation model for Australian grassland soils, which appears to have universal application for soils when organic matter is the main aggregate stabilizing agent (Oades and Waters, 1991). The model suggests that, as proposed by Edwards and Bremner (1967), the building blocks or elementary units are stable microaggregates (<250 μm) which are bound together to form stable macroaggregates (> 250 μm). The cementing or binding agent between the microaggregates is relatively labile organic matter which Tisdall and Oades (1982) called transient and temporary binding agents. These were identified to be fungal hyphae and fine roots (Tisdall and Oades, 1982), although other binding agents are polysaccharides (Angers and Mehuys, 1989; Haynes and Swift, 1990) and hydrophobic aliphatics (Capriel et al., 1990). Binding material within microaggregates is recalcitrant organic matter and inorganic constituents (Tisdall and Oades, 1982).

The hierarchical model appears to be valid for soils in cool, humid climates. Baldock and Kay (1987) found that conventional corn (Zea mays L.) production decreased the proportion of macroaggregates > 1 mm of a silty loam relative to a bromegrass (Bromus inermis Leyss) stand. The decrease in macroaggregates was associated with an increase in stable microaggregates <250 μm. For a clay soil, Angers and Mehuys (1988) found that, under an alfalfa (Medicago saliva L.) stand, aggregates >2 mm were formed mostly at the expense of stable aggregates <1 mm. Ultrasonic energy was necessary to disrupt these aggregates (< 1 mm) into smaller units (Angers and Mehuys, 1990). These results confirm that the hierarchical model holds for these soils but that the size of the building blocks (microaggregates) and the resulting macroaggregates may vary. The model is also very consistent with the fact that rapid management-induced changes in soil aggregation are most often observed in the large macroaggregate size fraction (> 1 mm) (Baldock and Kay, 1987; Angers and Mehuys, 1988; Carter, 1992) which reinforces the hypothesis that labile organic matter is responsible for binding microaggregates into macroaggregates.

III. Organic Matter Protection and Macroaggregation

The hierarchical model described above would suggest a differentiation in the concentration and composition of the soil organic matter between the aggregate size classes. However, experimental results show that the variations in total organic C and N contents with aggregate size are not always clear and consistent. For grassland soils, Dormaar (1984), Elliott (1986), and Cambardella and Elliott (1993) generally found that organic C and N contents decreased with decreasing aggregate size. Conversely, Baldock et al. (1987) working on an Ontario Luvisol observed that both organic C and total carbohydrate contents increased with decreasing aggregate size. Gupta and Germida (1988) and Elliottet al. (1991) found no clear trend in C concentration between aggregate size fractions. It should be pointed out that in most of th...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• About the Editors
• Contributors
• Table of Contents
• Introduction
• Mechanisms and Processes
• Impact of Climate, Soil Type, and Management
• Assessment of Soil Organic Matter Storage
• Index