Functions of Soil Organic Matter and the Effect on Soil Properties [PDF 2.04

GRDC Project No CSO 00029 Residue Management, Soil Organic Carbon and Crop Performance - Functions of Soil Organic Matter and the Effect on Soil Properties Evelyn S. Krull, Jan O. Skjemstad Jeffrey A. Baldock CSIRO Land & Water PMB2 Glen Osmond SA 5064 Dissolved Charcoal material Particulate Humus material 200 100 0 Chemical Shift (ppm) 200 100 0 Chemical Shift (ppm) 200 100 0 200 100 0 Chemical Shift (ppm) Chemical Shift (ppm) Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Grains Research and Development Corporation. No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. The Grains Research and Development Corporation will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication. 2 Contents: page 1. Summary ………………………………………………………………………. 4 2. Introduction: a. What is soil organic matter? …………………………………………… 5 b. Soil quality and the role of SOC ………………………………… …….. 5 c. Do generic critical threshold values exist for SOC? .……………. …….. 7 d. Overview of principal functions of SOM in soils ……………….. …….. 11 3. Soil carbon fractions and SOC analytical methods ……………………………... 13 4. Role of SOM on soil functions a. Physical functions: i. Soil structure and aggregate stability ….……………………….. 21 ii. Water-holding capacity …...……………………………………. 47 iii. Soil Colour ……………………………………………………… 55 b. Chemical functions: i. Cation exchange capacity (CEC) ……………………………….. 59 ii. Buffering capacity (BC) and pH ………………………………... 77 iii. Adsorption and complexation …………………………………... 85 c. Biological functions: i. SOM as a source of energy ……………………………………... 94 ii. SOM as a source of nutrients ……………………………............ 98 iii. Soil resilience and organic matter ………………………………. 106 5. The worth of SOC ………………………………………………………………… 107 6. Conclusion ………………………………………………………………………... 110 Appendix: List of abbreviations ………………………………………………………. 112 References …………………………………………………………………………….. 114 3 SUMMARY Soil organic matter (SOM) and specifically soil organic carbon (SOC) are known to play important roles in the maintenance as well as improvement of many soil properties. While agriculture is the area most concerned with key functions and critical levels of SOC, forestry and grazing as well as groundwater contamination and C sequestration are areas where knowledge about the functions of SOC is vital. This literature review aims to provide a comprehensive assessment of the current state of knowledge of the functions of SOC and its effect on the physical, chemical and biological properties of soil. Particular emphasis of this report, in context with the GRDC project, is placed on the effect of SOC on soil structure (aggregate stability), on cation exchange capacity (CEC) and buffer capacity (BC) of soils and on the soil’s water holding capacity (WHC). Although these properties are discussed separately, it is important to emphasise the dynamic and interactive nature of the soil system and that changes in one property will likely affect other soil properties as well. Thus, functions of SOC almost always affect several different properties and engage in multiple reactions. While this review primarily focuses on the effect of SOC on physical, chemical and biological soil properties, it was vital to include a brief discussion on soil methodology to provide a summary of methods currently used and their respective advantages and shortcomings. Furthermore, the rationale for separating SOM into discrete organic pools by particle size separation is discussed. Specifically, we highlight that total SOC is often not a good indicator for assessing soil properties. Frequently, such properties are affected by specific pools with particular properties. Only by studying these pools separately and in conjunction with a specific function is it possible to understand what the key impacts of a SOC pool are. The last part of the review examines the value of SOC in an ecological sense and reviews the cost and effectiveness of the carbon trading scheme, particularly with respect to mitigation of greenhouse gases. 4 INTRODUCTION What is soil organic matter? The term “Soil organic matter” (SOM) has been used in different ways to describe the organic constituents of soil. In this report, SOM will be used as defined by Baldock and Skjemstad (1999) as “all organic materials found in soils irrespective of origin or state of decomposition”. Since SOM consists of C, H, O, N, P and S, it is difficult to actually measure the SOM content and most analytical methods determine the soil organic carbon (SOC) content and estimate SOM through a conversion factor. The amount of SOC that exists in any given soil is determined by the balance between the rates of organic carbon input (vegetation, roots) and output (CO2 from microbial decomposition). However, soil type, climate, management, mineral composition, topography, soil biota (the so- called soil forming factors) and the interactions between each of these are modifying factors that will affect the total amount of SOC in a profile as well as the distribution of SOC contents with depth. It is important to note that any changes made to the natural status of the soil systems (e.g. conversion to agriculture, deforestation, plantation) will result in different conditions under which SOC enters and exits the system. Therefore, perturbed systems may still be in the process of attaining a new equilibrium C content and any measurements of SOC have to take into account that the soil is in the process of re-estabilishing equlibrium, which could take >50 years (Baldock and Skjemstad, 1999). Soil quality and the role of SOC It is now widely recognised that SOC plays an important role in soil biological (provision of substrate and nutrients for microbes), chemical (buffering and pH changes) and physical (stabilisation of soil structure) properties. In fact, these properties, along with SOC, N and P, are considered critical indicators for the health and quality of the soil. Since Lal’s (1993) initial definition of soil quality as the capacity of soil to produce economic goods and services and to regulate the environment, the term “soil quality” has been refined and expanded by scientists and policy makers to include its importance as an environmental buffer, in protecting watersheds and groundwater from agricultural chemicals and municipal wastes and sequestering carbon that 5 would otherwise contribute to a rise in greenhouse gases and global climate change (Reeves, 1997). Doran and Parkin (1994) and Doran and Safley (1997) initially distinguished between “soil quality” and “soil health” before inclusively using the term “soil health” and defining it as “the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity, promote the quality of air and water environments, and maintain plant, animal and human health”. However, the general perception of a healthy or high-quality soil is one that adequately performs functions, which are important to humans, such as providing a medium for plant growth and biological activity, regulating and partitioning water flow and storage in the environment and serving as an environmental buffer in the formation and destruction of environmentally hazardous compounds. Considering this wide variety of performance indicators, Karlen et al. (2003) and Norfleet et al. (2003) pointed out that soil quality needs to be assessed with regard to what the soil is used for, as a particular soil may be of high quality for one function and may perform poorly for another. In particular, the suitability of soil for sustaining plant growth and biological activity is a function of physical (porosity, water holding capacity, structure and tilth) and chemical properties (nutrient supply capability, pH, salt content), many of which are a function of SOM content (Doran and Safley, 1997). Similarly, Elliott (1997) indicated that SOM was a key indicator of soil health but further suggested that particulate organic matter (POM) could be used as an indirect measure of soil health because of its short turnover time. Swift and Woomer (1993) regarded POM as the “organic fertiliser” property of SOM. In general, increases in SOM are seen as desirable by many farmers as higher levels are viewed as being directly related to better plant nutrition, ease of cultivation, penetration and seedbed preparation, greater aggregate stability, reduced bulk density, improved water holding capacity, enhanced porosity and earlier warming in spring (Carter and Stewart, 1996; Lal, 2002). Reeves (1997) noted that “SOC is the most often reported attribute from long-term agricultural studies and is chosen as the most important indicator of soil quality and agronomic sustainability because of its impact on other physical, chemical and biological indicators of soil quality”. However, Janzen et al. (1992) pointed out that the relationship between soil quality indicators (e.g. SOC) and soil functions does not always comply to a simple relationship increasing linearly with magnitude of the indicator and that therefore “bigger is not necessarily better”. 6 Do generic critical threshold values exist for SOC? SOM concentrations are often cited as major indicators of soil quality. However, only few studies attempt to discuss minimum or maximum threshold values of

Load more