Labile Organic Matter Fractions as Central Components of the Quality of Agricultural Soils: An Overview
Introduction
Concerns regarding soil degradation and agricultural sustainability have kindled interest in assessment of soil quality. Soil quality is simply defined as the capacity of a soil to function, encompassing its living and dynamic nature (Karlen et al., 1997). A more specific definition is the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health (Carter 1997, Doran 1994). An assessment usually includes measurement of soil quality indicators that, in some way, influence the function for which the assessment is being made. Such indicators can be divided into chemical (e.g., pH, extractable nutrients, salinity), physical (e.g., aggregation, bulk density, hydraulic conductivity), and biological (e.g., microbial biomass C, basal respiration, earthworm numbers).
Soil organic matter is an extremely important attribute of quality since it influences soil physical, chemical, and biological properties and processes. For example, it is a source of energy and nutrients for soil biota, it is a plant nutrient (N, S, and P) source via mineralization, and it affects aggregate stability, trafficability, water retention, and hydraulic properties. As a result, soil organic matter content and quality are now regarded as key factors in the evaluation of the sustainability of management practices (Gregorich 1994, Gregorich 1997a).
Changes in total soil organic matter content in response to alterations in soil management practice are difficult to detect because of the generally high background levels and natural soil variability (Haynes and Beare, 1996). However, soil organic matter is a heterogeneous mixture of materials, ranging from fresh plant and microbial residues to relatively inert humic compounds, with turnover rates measured in millennia (Baldock 2000, Stevenson 1994). Many attempts have been made to identify labile pools of organic matter that are more sensitive to changes in management or environmental conditions than total soil organic matter content. Examples include C and N held in the microbial biomass and particulate organic matter and in water soluble, easily extractable and potentially mineralizable fractions (Gregorich 1997a, Haynes 1996, Janzen 1997, Moore 1997).
The level of our knowledge regarding the significance and applicability of various labile fractions as indicators of soil quality differs greatly. For example, a number of workers have reviewed, in detail, the significance of microbial biomass C and N levels (Carter 1999, Dalal 1998, Smith 1990, Sparling 1997). By contrast, the nature and significance of the non-living, labile organic matter pools are much less well understood. For example, past research on soluble C and N has concentrated on forest soils, and their significance to the quality of agricultural soils has only recently been recognized. The significance of particulate organic matter has been recognized for some time (Gregorich and Janzen, 1996), but that of the mineralizable and extractable fractions is less well-known.
The objective of this chapter is to discuss the nature and significance and interrelationship between these non-living labile organic matter fractions and their value as indicators of the quality of agricultural soils.
Section snippets
Total Soil Organic Matter
Soil organic matter content is generally measured as organic C and⧸or total N content. Although the organic fraction of soils typically accounts for a small, but variable, proportion (typically 5–10%) of soil mass, it exerts far-reaching effects on soil properties. Indeed, soil organic matter has long been suggested as the single most important indicator of soil productivity (Allison 1973, Campbell 1978). This is because organic matter greatly affects chemical, physical, and biological
Particulate Organic Matter
Particulate organic matter (POM) is a transitory pool of organic matter between fresh plant residues and humified organic matter (Gregorich and Janzen, 1996). It is typically enriched in C and nutrients, and although it represents only a small portion of the soil mass, it is an important attribute of soil quality since its short turnover time makes it an important source of C and nutrients.
Dissolved Organic Matter
Dissolved organic matter (DOM) is the organic material present in dissolved form in soil solution. It originates as leachates from plant litter, exudates from soil microflora, and roots and hydrolysis of insoluble soil organic matter. It consists of a wide range of organic compounds. The vast bulk of research into the nature and significance of this fraction has been concentrated in forest soils (Herbert and Bertsch, 1995). In very recent times it has been used as a labile organic matter
Extractable Forms of Organic Matter
Many different chemical extractants have been used in attempts to extract a labile portion of organic matter from soils. For example, many chemical indices of potentially mineralizable soil N have been proposed (Goh 1986, Keeney 1982). These can be divided into three broad groups: (i) weak (hot water, hot 0.01 M CaCl2, hot 1 M or 2 M KCl, 0.01 M NaHCO3), (ii) intermediate (alkaline permanganate, Na2CrO4 plus H3PO4, 1 M NaOH), or (iii) strong (6 N H2SO4, K2Cr2O7-H2SO4) extractants (Goh and
Method of Quantification
Mineralizable C is usually measured by incubating a sample of field-moist soil in a sealed chamber containing an alkali trap. The CO2-C accumulated in the trap is measured by acid titration (Öhlinger 1996a, Zimbilske 1994). The incubation period commonly ranges from 10 to 30 days, and the chamber is opened and the trap periodically replaced to allow gas exchange and thus maintenance of aerobic conditions. The CO2 accumulated in the headspace can also be measured using a CO2 analyzer (a gas
Significance of Labile Organic Matter Fractions
Total soil organic matter content can be considered as a course indicator of soil quality. However, as discussed throughout this chapter, changes in the content of organic C and total N occur only slowly and do not provide an adequate indication of important changes in soil organic matter quality that may be occurring. In order to evaluate such changes, measurement of labile organic matter pools (that make up a relatively small proportion of total organic matter) is required. These pools are
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