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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:180-185 (2008)
DOI: 10.2136/sssaj2005.0279
© 2008 Soil Science Society of America
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SOIL & WATER MANAGEMENT & CONSERVATION

Evaluating Soil Management Using Particulate and Chemically Labile Soil Organic Matter Fractions

Steven B. Mirskya,*, Les E. Lanyona and Brian A. Needelmanb

a The Pennsylvania State Univ., 116 ASI Bldg., University Park, PA 16802
b Dep. of Environmental Science and Technology, Univ. of Maryland, College Park, MD 20742

* Corresponding author (smirsky{at}psu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Particulate organic matter (POM), an established soil quality indicator, is too costly for routine testing by analytical labs. Chemical oxidation of labile soil organic matter is less costly and may prove to be an equally effective indicator. The objectives of this study were to test the relationship between POM and chemically labile organic matter (CLOM) and to evaluate the effects of soil management on POM and CLOM. The study was conducted within a long-term crop rotation x fertility treatment study in central Pennsylvania. Crop rotation sequences were continuous corn (Zea mays L.), corn–soybean [Glycine max (L.) Merr.], 4 yr of corn followed by 4 yr of alfalfa hay (Medicago sativa L.), and corn–oat (Avena sativa L.)–winter wheat (Triticum aestivum L.)–2 yr of red clover hay (Trifolium pratense L.). Fertilizer treatments were mineral fertilizer, N-based liquid dairy manure, and P-based liquid dairy manure. A significant linear relationship between POM-C and CLOM-C treatment means was observed (r2 = 0.74). Both POM-C and CLOM-C concentrations were greatest for manure-based fertility treatments and for crop rotations receiving the most frequent applications of manure. Only CLOM-C, however, distinguished between the N-based and P-based manure treatments. Further development of CLOM-C as a soil quality indicator may yield a reliable, cost-effective soil quality management tool.

Abbreviations: CA, 4 yr of corn followed by 4 yr of alfalfa • CC, continuous corn • CLOM, chemically labile organic matter • COWH, corn–oat–wheat–2 yr of red clover hay • CS, corn–soybean • HRE, Hunter Rotation Experiment • MF, mineral fertilizer • POM, particulate organic matter • SOM, soil organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil organic matter (SOM) is an established indicator of soil quality (National Research Council, 1993; Karlen and Cambardella, 1996; Reeves, 1997). The significance of SOM is based on its role in soil structural stability, water infiltration, permeability, water holding capacity, biological activity, and nutrient storage and release. Soil organic matter trends may be evaluated on a total-organic-C basis; however, total SOM-C tends to respond slowly to management changes. Farmers attempting to take measures to increase the quality of their soils need more sensitive metrics to monitor short-term management-induced changes. Methods that measure labile SOM have been proposed as soil quality indicators sensitive to short-term management changes. To assist farmers and land managers in evaluating the effects of particular management strategies, inexpensive and reliable monitoring methods are needed to increase adoption of such services by analytical testing laboratories.

Particulate organic matter (POM), or macroorganic matter, the sand-sized fraction of SOM (50–2000 µm), is an estimate of labile SOM. The residence time of POM has been estimated to range from 5 to 20 yr (Carter, 1996). Particulate organic matter is dominated by undecomposed plant residues that retain recognizable cell structures, but also includes fungal hyphae, seeds, spores, and faunal skeletons (Gregorich and Janzen, 1996). Particulate organic matter has been found to be more sensitive to soil management than total SOM (Tiessen and Stewart, 1983; Beare et al., 1994; Wander et al., 1994; Six et al., 1998, 1999). Particulate organic matter is isolated by chemical or sonic disassociation of soil aggregates, followed by sieving at the sand particle size (Gregorich and Ellert, 1993).

Organic amendments, such as swine manure, can increase POM compared with treatments without these amendments (Adams, 1980). In a long-term rotation experiment, larger labile SOM concentrations were observed in continuous wheat, hay rotations, and wheat–fallow treatments that received manure than increased fallow periods in the rotation that received reduced manure rates (Bremer et al., 1994; Janzen et al., 1992). Particulate organic matter shows high sensitivity to management changes from perennial to annual cropping systems, and from conventional to reduced tillage systems (Tiessen and Stewart, 1983; Cambardella and Elliott, 1992; Besnard et al., 1996; Hussain et al., 1999). While POM is an effective indicator of soil quality (Cambardella and Elliott, 1992), the method is cost prohibitive because POM analysis is labor intensive and requires expensive dry combustion with a C analyzer at 900°C.

Soil analytical labs may be more receptive to offering labile SOM testing as a component of their services if the method were more economical and easily repeatable. Chemically labile organic matter (CLOM) may offer a more economical alternative to physical techniques (Lefroy et al., 1993; Blair et al., 1995; Shang and Tiessen, 1997; Bell et al., 1998; Armstrong et al., 1999; Weil et al., 2003). Chemical fractionation is more affordable because the methodology requires less labor and a C analyzer is not necessary for analysis. To be adopted, however, chemical fractionation methods must isolate a labile fraction that has a similar response to soil management practices as accepted physical fractionation techniques.

Typically, chemical fractionation of SOM involves the reaction between SOM and a strong oxidizing agent. Attempts to simulate microbial decomposition in a way that reflects the in situ enzymatic digestion of labile SOM prompted the use of KMnO4 as an oxidizing agent (Loginow et al., 1987). The chemical oxidation of labile SOM or CLOM with KMnO4 is destructive and little is known of the composition of this fraction; however, there have been several accounts relating management to CLOM (Lefroy et al., 1993; Blair et al., 1995; Conteh et al., 1997; Shang and Tiessen, 1997; Bell et al., 1998; Armstrong et al., 1999; Murage et al., 2000; Weil et al., 2003). On a cracking clay soil in Australia, CLOM was successfully used to distinguish the effects of different forages on labile SOM (Armstrong et al., 1999). Blair et al. (1995) also observed the sensitivity of CLOM across a wide range of treatments at several experiment stations, including comparisons between cultivated and uncultivated sites and addition of a pasture sequence into a continuous wheat cropping system.

If CLOM proves to be an adequate alternative to POM as a soil quality indicator of labile SOM, then it may be easier to adopt due to its affordability and ease of implementation. The objectives of this study were to test the relationship between POM-C and CLOM-C and to evaluate whether POM-C and CLOM-C exhibit similar sensitivities to management in a long-term crop rotation and fertility experiment.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
The Hunter Rotation Experiment (HRE), a crop rotation x fertility treatment study, has been in progress since 1969 on Hagerstown silt loam soils (fine, mixed, mesic Typic Hapludalfs) at The Pennsylvania State University's R.E. Larson Research Center in Rock Springs, PA. The split-plot experimental design (fertility effects as main plots, crop rotation treatments as subplots) consists of 192 plots with a 5.76-m width and a 12.8-m length that are under conventional tillage (moldboard plow followed by disking and cultimulching for all annual crops and for forage establishment) in the spring. Each crop in each rotation is grown every year, and all treatments are replicated four times. The full planting factorial allows for soil sampling in the initial corn phase of each rotation every year. There are currently four crop rotation sequences: (i) continuous corn (CC); (ii) a 2-yr rotation of corn and soybean (CS); (iii) an 8-yr rotation of 4 yr of corn followed by 4 yr of alfalfa hay (CA); and (iv) a 5-yr rotation of corn–oat–winter wheat–2 yr of red clover hay (COWH). Perennial crops (i.e., alfalfa and red clover hay crops) received no tillage except during the establishment year. Rotations CC and CS have been in operation since 1969, while rotations CA and COWH were established in 1990 from previous sequences. The crop rotation treatments are in factorial combination with three fertility treatments: (i) mineral N–P–K fertilizer (MF); (ii) liquid dairy manure (47.4–8.8–30.8 and 82.3 g kg–1 dry wt. N–P–K and solids, respectively) based on the N requirements of each crop (since 1990; previously managed as mineral fertilizer); and (iii) liquid dairy manure based on P removal requirements of each crop (since 1982; previously managed as mineral fertilizer). The manure analysis is an average from 1982 through 1996; the same manure source was used throughout the study period. All treatments are limed to maintain a soil pH of 7 to plow depth. Mineral fertilizer applications, applied in the dry form, include NH4NO3, triple super phosphate (calcium monophosphate), and KCl. Fertilizer rates are determined based on annual soil tests. Fertilizer applications were applied in the spring before primary tillage, with mineral fertilizers surface broadcast by hand and liquid dairy manure sprayed on plots using a gasoline-powered trash pump. The quantity of manure added is different for the fertility treatments and rotation combinations (Table 1 ), with only non-legume crops in the rotation sequences receiving manure. Due to variability associated with the timing of manure applications and soil test results, manure applications are applied based on the previous year's manure nutrient characterization. In return, manure is sampled concurrently with field application to assess realized fertilizer additions. Manure was analyzed by The Pennsylvania State University's Agricultural Analytical Services Laboratory for total N by the Kjeldahl reaction, and total P and K by microwave-assisted acid digestion (Wolf and Beegle, 1995; Bremner, 1996).


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  		Table 1. The rate and number of manure applications for the fertility treatments in the Hunter Rotation Experiment for 1969 through the summer of 1996 (Bucher, 1999).

 
Soil Sampling
Soil samples from the HRE were collected from first-year corn or continuous corn plots in June of 1996, 1997, and 1998. The samples consisted of 20 to 25 cores (2 by 15 cm), which were composited. Samples were air dried and stored in brown paper bags. Samples were ground (<2 mm) using a large mechanical mortar and pestle soil grinder (Model F257, Nasco-Asplin, Fort Atkinson, WI).

Labile Soil Organic Matter
The method for separation of POM was adapted from Six et al. (1998) and Gregorich and Ellert (1993). Briefly, 25 g of the prepared soil sample was dispersed for 18 h in 100 mL of sodium hexametaphosphate solution [5 g L–1 (NaPO3)6] on a reciprocating shaker (180 rpm). The suspension was poured onto a 53-µm sieve and rinsed with distilled water until the clay- and silt-size fractions were completely removed (effluent clear of sediment). The retained sand-size fraction was transferred into a glass beaker and dried at 50°C. The dried sand-size fraction was ground for 2 min with a Spex Mixer/Mill 8000 (Spex SamplePrep, Metuchen, NJ). The organic C content of the ground material was determined by dry combustion with a Shimadzu carbon analyzer (TOC-5000, Shimadzu Corp., Kyoto, Japan) at 900°C with a solid sample module (SSM-5000A).

The CLOM analysis was conducted using 0.02 mol L–1 KMnO4 concentrations as described in Weil et al. (2003). Early researchers used a KMnO4 concentration of 0.333 mol L–1 (Blair et al., 1995; Armstrong et al., 1999). Weil et al. (2003) suggested that 0.02 mol L–1 KMnO4 was more sensitive, easier to manage, and less costly than 0.333 mol L–1 KMnO4. Weil et al. (2003) found the higher concentration (0.333 mol L–1 KMnO4) more hazardous to work with and found that it measures a larger fraction of SOM than just the labile fraction. They suggested the lower concentration (0.02 mol L–1 KMnO4) as a simpler, more repeatable, and faster alternative that better measures the labile SOM fraction. Several other researchers have also found CLOM analysis using a lower concentration of KMnO4 to be more sensitive to management than the 0.333 mol L–1 KMnO4 concentration (Shang and Tiessen, 1997; Bell et al., 1998).

Stock solutions of 0.02 mol L–1 KMnO4 and 0.1 mol L–1 CaCl2 (6.32 g of KMnO4 and 29.404 g CaCl2 diluted to 2 L with deionized H2O) were prepared. Additionally, standards ranging from 0 to 0.02 mol L–1 KMnO4 were prepared. Soil samples weighing 5 g were added to 50-mL plastic screw-top centrifuge tubes. The soil samples were reacted with 20 mL of the KMnO4–CaCl2 solution. The soil suspension was shaken on a reciprocating shaker for 2 min (180 rpm) and then allowed to settle for 10 min. A 200-µL aliquot of the solution supernatant was diluted to 10 mL and absorbance was measured on a split-beam spectrophotometer (550 nm). The amount of C oxidized after digestion was determined by the assumed reaction of the reduced KMnO4 (following Weil et al., 2003):

Formula 1[1]
where CLOM-C is in milligrams per kilogram, 0.02 (mol L–1) is the concentration of the initial KMnO4 solution, a is the intercept of the standard curve, b is the slope of the standard curve, z is the absorbance of an unknown, 9 (g C [0.75 mol] mol–1) is the amount of C oxidized by 1 mol of MnO4 changing from Mn7+ to Mn2+, 0.025 (L) is the volume of KMnO4 solution reacted, and x (kg) is the soil used (soil sample containing approximately 15 mg of C). Standard curve parameters were calculated from five standard samples separately for each day of sample analysis.

Statistical Analysis
Linear regression relationships were developed between POM-C and CLOM-C for the HRE and the farm results using the SAS statistical software package (SAS Institute, 1999). The Kolmogorov–Smirnov goodness of fit procedure (SAS Institute, 1999) was used to test the normality of the POM-C and CLOM-C data. The POM-C data were non-normal and were therefore transformed using a logarithmic transformation for the HRE data. For the HRE, management effects on the POM and CLOM fractions were evaluated by analysis of variance statistics using a split-plot design, with fertility as the main plot, crop rotation treatments the subplot, and year as a repeated measure. The multiple comparisons Tukey's honestly significant difference test was used for means separation for the HRE, with a P value of 0.05. Analysis of variance and means separation tests were completed using SAS (SAS Institute, 1999); all results are reported as least square means.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The concentrations of POM-C for the full data set had a mean of 3.14 g kg–1, a range of 1.22 to 5.86 g kg–1, and a standard deviation of 0.83 g kg–1. The CLOM-C concentrations were significantly smaller than POM-C, with a mean of 0.538 g kg–1, a range of 0.415 to 0.681 g kg–1, and a standard deviation of 0.051 g kg–1. Weil et al. (2003) found a similar range in CLOM-C yields (0.36–0.70 g CLOM-C kg–1 soil) in two separate studies located at a research farm in North Dakota and across farms in the Mid-Atlantic region. There was a strong linear relationship between POM-C and CLOM-C (r2 = 0.74, P < 0.001) across the treatment means of the HRE (Fig. 1 ). The proportional relationship between POM-C and CLOM-C means demonstrates that, in general, both indicators responded similarly to soil and crop management.


Figure 1
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  		Fig. 1. The relationship between particulate organic matter (POM) and chemically labile organic matter (CLOM) C concentrations in the Hunter Rotation Experiment (data reported are mean values across years and replicates; MF = mineral fertilizer, CC = continuous corn).

 
Management Effects on Particulate Organic Matter Carbon and Chemically Labile Organic Matter Carbon
The year, fertility treatment, and crop rotation main effects were statistically significant sources of variation for POM-C (Table 2 ). The POM-C in Year 1 was greater than POM-C in Year 3, while Year 2 was intermediate (Table 3 ). The POM-C of the MF treatment was lower (16–21%) than the manure-based fertility treatments (Table 3). There were no significant differences in POM-C between the two manure treatments. The POM-C of the CC treatment was greater (16–25%) than that of the other more diverse and longer crop rotations (Table 3). There were no significant differences in POM-C between the CS, CA, and COWH crop rotations.


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  		Table 2. Analysis of variance of particulate organic matter (POM)-C and chemically labile organic matter (CLOM)-C.

 

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  		Table 3. Mean particulate organic matter (POM)-C within the Hunter Rotation Experiment during a 3-yr period (1996–1998) for year, fertility, and crop rotation treatments. The POM-C main effects of year, fertility, and crop rotation were significant in the Hunter Rotation Experiment; there were no significant interactions.

 
The year, fertility treatment, and rotation main effects and the fertility treatment x rotation interaction were significant sources of variation for CLOM-C (Table 2). The CLOM-C in Year 1 was not significantly different from Year 3, but was greater than Year 2 (Table 4 ). There was no significant difference between the CLOM-C content for Years 2 and 3. We attributed the fertility x crop rotation treatment interaction to differences observed in CLOM-C trends from the manure treatments in the CC crop rotation compared with the other crop rotations. The CLOM-C concentrations averaged across fertility treatments were lower in the CS than in the CC, CA, and COWH crop rotations (Table 5 ). The CLOM-C of the N-based manure treatment in CC was greater than the other rotations. In contrast, there was no significant difference among rotations with the P-based manure treatment. The CLOM-C was greater in the N-based manure treatment for CC than for the P-based manure treatment (Table 5). In the CS rotation, CLOM-C was greater in both the N-based and P-based manure treatments than in the MF treatment. In the CA rotation, CLOM-C for the P-based manure treatment was intermediate between the MF and N-based manure treatments. The CLOM-C was not different among the fertility treatments for the most diverse rotation, COWH.


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  		Table 4. The chemically labile organic matter (CLOM)-C within the Hunter Rotation Experiment during a 3-yr period (1996–1998) (fertility and crop rotation are pooled).

 

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  		Table 5. The fertility and crop rotation interaction for the chemically labile organic matter (CLOM)-C of the Hunter Rotation Experiment (years 1996–1998 are pooled).

 
Year Effects
Reeves (1997) has indicated that SOM decomposition rates and response to organic amendments are strongly regulated by climatic conditions. We therefore attribute the observed fluctuations in labile SOM across years for both soil quality indicators to inherent variability in climatic effects on crop production and biomass and varying rates of SOM cycling. It is also possible that these differences represent a long-term decline in labile SOM; however, this is unlikely because it would not follow typical SOM trends in agroecosystems. With the onset of tillage, there is typically a significant loss in SOM, with a disproportionate amount coming from the labile SOM fraction (Tiessen and Stewart, 1983; Balesdent et al., 1998; Guggenberger et al., 1995; Gregorich et al., 1997). After 3 to 7 yr, the very labile C has been oxidized and the majority of C left is either chemically or physically protected (Adams, 1980; Tiessen and Stewart, 1983). Therefore, in fields under long-term conventional tillage, it is unlikely for POM-C, with a residence time of 5 to 20 yr, to be undergoing a long-term decline, especially in soils receiving high amounts of manure (Carter, 1996).

Management Effects
The labile SOM indicators were able to distinguish soil C changes due to the use of manure as a fertility source and the frequency of its additions, with both POM-C and CLOM-C highest in the manure-based fertility treatments and rotations receiving the most frequent applications of manure. As crop diversity or length of rotation increased, however, fertility treatment effects on POM-C and CLOM-C decreased. Although the responses of POM-C and CLOM-C to management effects were similar, we were able to distinguish more treatment effects with CLOM-C (Tables 4 and 5) than POM-C (Table 6 ). While there were differences in POM-C only between the manure-based and mineral-based fertility treatments in the continuous corn rotation, CLOM-C also distinguished fertility treatment effects in the CS and CA rotations (Tables 5 and 6). The two labile SOM indicators also responded differently to the quantity of manure per application. The N-based manure treatments were expected to have greater labile SOM than the P-based manure treatments; however, this was only distinguished with CLOM-C in the CC treatment (Table 5).


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  		Table 6. The fertility x crop rotation interaction for the particulate organic matter (POM)-C of the Hunter Rotation Experiment (years 1996–1998 are pooled; P = 0.0632).

 
While we did see a significant difference for the main effects between the continuous corn treatment and the remaining crop rotations with POM-C, this does not indicate that continuous corn production will increase POM-C for all fertility treatments. We were not able to discern differences in POM-C among the rotations for the mineral fertilizer treatment (Table 6). The only differences associated with rotations were in the N-based manure treatment (CC > CS, CA, and COWH). Since manure is not applied to legume crops in the rotations, we attribute the significant differences between the CC treatment and the crop rotations for the N-based manure treatment to the frequency of manure additions (annual applications vs. 5 or 6 out of 10 yr) (Tables 3 and 6) rather than to specific crop rotation treatments. The lack of differences across rotations in the P-based manure treatment for both soil quality indicators suggests that POM-C and CLOM-C measurements were not sensitive enough to detect variations in frequency of application at the P-based rates (Table 1).

Several previous examinations also have not observed an effect of crop rotation on labile soil organic matter (Haynes, 2000; Liebig et al., 2002). On medium-textured soils in northeastern Ohio, Bronick and Lal (2005) did not observe a difference in mean POM-C concentrations measured on a range of crop rotations including continuous corn, a no-till crop rotation of corn and hay, and a diversified pasture crop production system (2 yr pasture, 2 yr corn, and 2 yr wheat). Across eastern Canada, 16 replicated field experiments were conducted including six crop rotation treatments, three fertility treatments (several manure sources vs. mineral fertilizers), and seven tillage treatments (Bolinder and Angers, 1999). A sensitivity index was generated by dividing the indicator value of the specified treatment by that of the conventional-management control treatment. The POM-C means were less sensitive to rotation effects (1.18) than to fertility (1.72) or tillage effects (1.30).

Increasing tillage frequency has been demonstrated to reduce labile soil organic matter (Tiessen and Stewart, 1983; Cambardella and Elliott, 1992; Besnard et al., 1996; Hussain et al., 1999). Therefore, we speculate that the lack of tillage effects observed between the CC and CA and COWH treatments within the mineral fertilizer treatment, despite the greater tillage frequency in the CC treatment, are a function of high amounts of low-quality (high C/N) crop residue returned in the CC rotation. Plant residue with high C/N ratios and phenolic acids have been identified as important mechanisms in sequestering C within stable soil aggregates (Martens 2000a,b). Martens (2000a,b) demonstrated significantly higher C accumulation and sequestration in stable soil aggregates from corn and other cereal grains (high C/N and phenolic acids) compared with soybean residue (low C/N and phenolic acids). Conversely, we speculate that the lower POM-C and CLOM-C in the CS rotation receiving mineral fertilizers are related to soybean residue composition and the rate of residue return relative to corn residues.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We conclude that CLOM fractionation is a promising labile SOM indicator to monitor farm management effects on soil quality that may balance the demands of routine testing and the limitations of farmer's budgets. Both soil quality indicators clearly showed sensitivity to fertility source, however, differences from tillage and crop rotation were limited. Therefore, further testing will be required to evaluate CLOM across a wide range of soil management practices. Additionally, experiments that isolate management treatments such as tillage, crop rotation, and fertility management would elucidate the CLOM response to management.


   ACKNOWLEDGMENTS
 
We greatly appreciate the invaluable assistance provided by Douglas B. Beegle. This paper is dedicated to the memory of Les E. Lanyon.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication August 23, 2005.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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