Organic Matter Soil Their Relevance Fractions and to Soil Function Michelle Wander CONTENTS History and Purpose of Organic Matter Measurement 68 Importance of SOM and Its Relationship to Management .68 Approaches to Organic Matter Fractionation 71 Types of Organic Matter in Mineral Soils and Their Probable Functions 72 Relationship between Dynamics and Measured Fractions 72 Commonly Described SOM Pools and Related Fractions .73 Fractions Equated with the Biologically Active Pool .73 Fractions Associated with Physically Active and Slow Pools 76 Fractions Associated with Recalcitrant Pools 77 Measures of POM and Their Interpretation 78 POM as an Index .78 Approaches to POM Fractionation and Interpretation of Results 80 Methods Yielding a Single POM Fraction 83 Methods Separating Fresh POM from Resident POM 87 Methods Separating Protected from Nonprotected POM .87 Summary 90 References .90 Improved management of soil organic matter (SOM) in arable soils is essential to sustain agricultural lands and the urban and natural ecosystems with which they interact Humus, which has historically been equated with inherent soil fertility, can be efficiently extracted from mineral soils in alkali The resulting humic and fulvic fractions of SOM continue to be widely studied despite these fractions, which are procedural artifacts existing only in the laboratory that have not proven to be particularly useful guides to adaptive management or contributed notably to our understanding of either SOM dynamics or soil quality The quest continues to understand organic matter's contributions to soil productive capacity, its ability to transform and store matter and energy, and its capacity to regulate water and air movement Successful efforts will identify consistently defined and derived SOM fractions that impart fundamental characteristics to soils This chapter provides an overview of commonly measured SOM fractions and the kinetically or theoretically defined dynamic pools with which they are commonly identified Organic matter of recent origin is most closely associated with biological activity in soils, whereas materials of recent and intermediate age contribute notably to soil's physical status Materials with longer residence times typically comprise the largest reservoirs in soils and exert the greatest influence on the physicochemical reactivity of soils The 67 © 2004 by CRC Press LLC 68 Soil Organic Matter in Sustainable Agriculture characteristics of individual SOM fractions often vary as a result of the techniques used to isolate them or the experimental context Amino sugars, glomalin, and particulate organic matter (POM) fractions have multiple identities In addition to providing information about biologically active SOM, all these fractions provide information about physically active and passive SOM pools The final section of this chapter is devoted to POM, an increasingly popular measure of labile SOM because it responds readily to soil management, often identifying statistically significant trends when measures of total SOM would not POM plays important biological and physical roles in soil Even though POM is most often used as an index of labile SOM, POM fractions include materials that are heterogeneous in age and function Size, density, and energy can be combined in a variety of ways to recover materials that can be associated with active, slow, and recalcitrant pools As is true for humic substances, POM’s value as an index of SOM will not be proven until the relationships between its characteristics and in situ soil processes are clearly demonstrated The utility of POM will be increased by standardizing the approaches used to subdivide its constituents and through better articulation of criteria used to interpret results HISTORY AND PURPOSE OF ORGANIC MATTER MEASUREMENT IMPORTANCE OF SOM AND ITS RELATIONSHIP TO MANAGEMENT As is true for soil science in general, the study of SOM has emphasized its relationship to soil productivity Even in well-fertilized soils, soil productivity is reduced by loss of SOM (Johnston, 1991; Aref and Wander, 1997) Accompanying these losses in productive potential are losses in agroecosystem efficiency Crop response to mineral inputs is increased in soils where organic matter status and biological and physical properties influenced by organic matter are enhanced (Cassman, 1999; Avnimelech, 1986) What exactly “enhanced” means in this context remains a critical question Several studies have suggested that cropping systems that rely on mixed-crop production and organic sources of fertility are better able to maintain or accumulate organic matter and improve its quality than are mono- or bicropped systems that rely on inorganic nutrient sources (Reganold et al., 1987; Wander et al., 1994; Glendining et al., 1997; Liebig and Doran, 1999) Enhancements of SOM status (based on labile fraction characterization) and crop performance are reported for a variety of management practices, including organic (Wander et al., 1994), compost amended (Stone et al., 2001; Willson et al., 2001), pasture (Sbih et al., 2003), mixed-crop and cover cropped (Drury et al., 1991; Collins et al., 1992; Angers and Mehuys, 1988; Stevenson et al., 1998), and no-till systems (Beare et al., 1994b; Dick, 1997; Frey et al., 1999) Competitive crop yields achieved with fewer external inputs are attributed to cropping systems that enhance organic matter characteristics (Liebhardt et al., 1989; Johnston, 1991; Poudel et al., 2001; Nissen and Wander, 2003) Despite our long understanding of the relationship between soil building practices and their benefits to SOM (Russell, 1973), and the general appreciation that SOM underpins ecosystem function in terrestrial systems (Odum, 1969), our ability to quantify or manipulate its characteristics remains quite limited Results from long-term experiments provide critical insights into the influences of management on SOM and its contributions to agricultural sustainability (Rasmussen et al., 1998) Results such as these demonstrate shortfalls in our understanding of SOM’s contributions to soil productivity The general benefits of crop rotation to SOM and soil productivity are suggested by yield trends expressed in Morrow Plots (Wander et al., 2002) In general, differences between the various systems’ yields and SOM levels increase with the complexity, or length, of the crop rotation (Figure 3.1A) If maize yield serves as a bioassay, then results in the three-year rotation (corn–oats–hay, COH) suggests that the productive potential of that soil is higher than that of the soil maintained under the two-year corn–soybean (CS) or continuous corn (CC) rotations Increases in maize yield that result from increased inputs, which include lime, manure, and N, P, K additions and seeding densities adjusted to different rates, are not mirrored by increases in SOM contents (Figure 3.1B) Soils with the highest SOM contents have a history of manure application The yield © 2004 by CRC Press LLC Soil Organic Matter Fractions and Their Relevance to Soil Function 14 30 soil) 10 25 SOC 0-15 cm (g C kg –1 –1 Maize Yield (T ) 35 Continuous Corn Corn-Soybean Corn-Oat-Hay 12 69 20 15 10 0 U M K K S K M P UN P M NP HN P A U M K K S K M P U NP M NP HN P B FIGURE 3.1 Morrow Plots yield (A) and SOM (B) contents in 1997, when all plots were in corn This trial, begun in 1876, is the oldest agricultural experiment in the Northern Hemisphere Since 1967, the plots have included three crop rotations: continuous corn, Zea mays L (CC); corn–soybean, Glycine max (CS), and corn–oats–hay, Avena sativa and Melilotus alba or Trifolium pratense (COH) Before that time, the corn–soybean rotation was a corn–oat system The trial presently compares five fertility regimes, added over the course of the trial: unfertilized controls (U) and combinations of manure (M and MPS, which has a higher seeding density), plots without (UNPK) and with (MNPK) a history of manure amendment that receive inorganic NPK, and plots that had received manure up until 1967 that have subsequently only been amended with the highest P and K rates (HNPK) Since 1967, N has been applied as urea at 200 lb ac–1 in NPH and MNPK plots and 300 lb ac–1 in HNPK plots In NPK and MNPK plots, P as triple superphosphate and K as muriate of potash are applied at 49 and 93 lb ac–1, respectively, when test values are lower than 45 or 336 lb of available P or K, respectively The HNPK plots have received 98 and 186 lb ac–1 P and K, respectively, when test values fall below 112 and 560 values achieved in the higher-input treatments in the CS and COH systems is quite similar even though total SOM levels are not The SOM contents of the CS and CC rotations are quite similar, but the yields differ markedly Soil test levels for pH, P, and K (Figure 3.2) not account for differences in yield achieved in the different rotations or amendment regimes Phosphorus buildup is apparent in CC plots amended with manure every year This is a common problem in plots receiving higher manure application rates A comparatively low seeding rate in the manure-amended plots (M and MPS) likely limits yield in, and associated nutrient removal from, those soils Nutrients are relatively depleted in fertilizer-amended plots producing the highest system yield Plots that receive application rates higher than those recommended by the state are an exception and accumulate both P and K Interestingly, the highest yield is achieved in the COH system even though K test levels are below the reported optimum values Differences in SOM quality, not quantity, and SOM-dependent microbial and physical properties are thought to explain why these three systems differ in their productive potential and the degree to which crops can exploit the soil resource Our understanding of SOM’s specific contributions to soil function has not advanced notably in the past 50 years and remains primarily descriptive in nature (Table 3.1) Cation exchange capacity (CEC), a function of SOM, pH, and mineral characteristics, and percentage of surface residue cover are rare examples wherein quantitative relationships between SOM-dependent characteristics and adaptive management practices are established Soil CEC influences lime and herbicide application rates, whereas residue cover determines eligibility for participation in © 2004 by CRC Press LLC 70 Soil Organic Matter in Sustainable Agriculture 350 80 300 K (lb ac 1) Bray P (lb ac 1) pH 400 100 CC CS COH 60 40 250 200 150 100 20 50 0 U M PS PK PK PK M UN N HN M U M PS PK PK PK M N N HN U M U M PS PK PK PK M UN N N M H Inputs A B C FIGURE 3.2 Inorganic nutrient status of Morrow Plots, pH in 1:1 water, (A) P via Bray P-1 (B), and K extractable in NaOAc (C) Inputs including fertilizers and seed density increase from left to right and include unfertilized controls (U) and combinations of manure (M and MPS, which has a higher seeding density), plots without (UNPK) and with (MNPK) a history of manure amended that receive lime and inorganic NPK additions, and plots that had received manure up until 1967 and have subsequently been amended with lime plus very high fertility rates (HNPK) See Figure 3.1 legend for additional details about amendments Solid horizontal lines indicate recognized optimum test values needed to achieve maximum production TABLE 3.1 Summaries of Physical, Chemical, and Biological Contributions of Organic Matter to Soil Function Summary by Waksman (1938) Summary by Stevenson (1994) Physical Functions Modifies soil color, texture, structure, moisture-holding capacity, and aeration Color, water retention, helps prevent shrinking and drying, combines with clay minerals, improves moisture-retaining properties, stabilizes structure, permits gas exchange Chemical Functions Solubility of minerals; formation of compounds with elements such as Fe, making them more available for plant growth; increases the buffer properties of soils Chelation improves micronutrient availability; buffer action maintains uniform reaction in soil and increases cation exchange Biological Functions Source of energy for microorganisms, making the soil a better medium for the growth of plants; supplies a slow but continuous stream of nutrients for plant growth © 2004 by CRC Press LLC Mineralization provides source of nutrients; combines with xenobiotics, influencing bioavailability and pesticide effectiveness Soil Organic Matter Fractions and Their Relevance to Soil Function 71 conservation programs Despite SOM's importance to food and fiber production, routine methods to quantify its contribution to soil productivity not exist or are not widely agreed on The contribution of SOM to soil N supply is still so poorly described that it is estimated by fertilizer equivalency trials, expected yields, and cropping history or by the preplant soil profile NO3 (PPNT) or presidedress NO3 (PSNT) tests that then serve as a basis for N fertilizer application rates (Magdoff et al., 1984; Dahnke and Johnson, 1990) The need to predict organic N supply by using measures that are not as dynamic as nutrient fluxes (Spycher et al., 1983; Mulvaney et al., 2001) explains the research interest in biologically active SOM fractions However, SOM fractions that are highly labile, varying within a season or a year, might prove to be as difficult to use as indices as are inorganic nutrients Assessments of labile SOM will improve with separation of fractions most closely associated with fresh inputs, which have annual dynamics tightly coupled to edaphic factors, from constituents that reflect the recent (decadal) influence of management Fractions of SOM that reflect management deserve particular attention because they predict trends in soil productivity and the efficiency with which the soil cycles matter and energy The quality and quantity of SOM reserves and the edaphic factors that regulate their dynamics will need to be considered Measures of SOM that effectively predict nutrient supply, soil–water relations, aeration, pesticide immobilization, and trends in carbon sequestration are likely to differ APPROACHES TO ORGANIC MATTER FRACTIONATION According to Waksman (1936), the term humus dates back to the Romans and was used by the ancients in reference to soil and the “fatness of the land,” where fatness connoted fertility Wallerius in 1761 first defined humus in terms of decomposed organic matter In 1808, Thaer, cited in Walksman (1936), wrote, “Humus is the product of living matter, and the source of it.” Even though Walksman cautioned in 1936 that “any attempt to divide humus on the basis of its practical utilization would prove to be largely artificial,” this objective remains a top priority of many wishing to better manage the soil resource Humus classification schemes probably began with Linneaus’s classification of soils in accordance with humus types Archard (1786) was probably the first to attempt to extract humic substances from soils De Saussure (1804) equated the Latin term for soil, humus, with dark material produced from decayed plants Wallerius (1761), cited in Walksman (1936), speculated that chalk and likely salts helped dissolve humus to make it available to plants He advised that alkali be used alternately with dung to satisfy plant demand The perception that alkali-soluble humic materials contributed to soil’s native fertility and the fact that alkali extracts humic materials efficiency, removing typically 20 to 50% and up to 80% in some cases of the organic material from the soil (Stevenson, 1982; Rice, 2001), explain why the study of SOM has focused on humic substances recovered after their dissolution in a dilute base (typically 0.10 N NaOH, or, increasingly, Na4P2O7) Many extraction methods have been vigorously explored, because separation of organic matter from the mineral matrix facilitates chemical characterization of SOM by HPLC, GC-MS, wet chemistry, and elemental analyses These techniques would be impossible to apply to intact soils Separation methods have commonly been judged on their ability to isolate pure, reproducible, and homogenous components (Stevenson, 1994) This quest reflects a historical desire to describe SOM in primarily biochemical terms by using molecular formulae Berzelius proposed the first chemical formulas for two organic matter fractions: crenic (C24H12O16) and apocrenic (C24H6O12) acids (Stevenson, 1994) Crenic acid was isolated from iron- and mudrich waters by treating them with potassium hydroxide followed by acetic acid and then copper acetate Apocrenic acid was obtained by treating coal with nitric acid The often-unstated assumption that legitimate organic matter fractions will be pure in composition with tractable and uniform, or at least consistent, routes of origin has oriented research in only a few directions The classical method for humic substance fractionation is to acidify the organic colloids obtained after dispersion in dilute sodium hydroxide Humic acids (HAs) precipitate in acidified solutions whereas fulvic acids (FA) remain in suspension (Swift, 1996) This method continues to be widely used despite © 2004 by CRC Press LLC 72 Soil Organic Matter in Sustainable Agriculture criticism that the strategy is archaic and does not produce chemically discrete fractions (Russell, 1973) Humic substances are understood to include a continuum of complex biogenic amorphous heterogeneous molecules that are both chemically reactive and refractory in nature and that are ubiquitously formed through random chemical alteration of diverse precursor molecules The average properties of FA and HA are distinct and remarkably uniform across soils (Rice and MacCarthy, 1991; Mahieu et al., 1999) The abundance of C in FAs is lower (40–50%) than that in HAs (53–60%), and the abundance of O in FAs higher (40–50%) than that in HAs (32–38%) This is consistent with the higher exchange capacity of FAs, which is 640–1420 cmol (+) kg–1 FA, compared with 560–890 cmole (+) kg–1 HA (Stevenson, 1994) Reported molecular weight ranges are 3000 Da for HA, 1000 to 3000 Da for FA, and lower (