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Soil Science Society of America Journal 64:177-184 (2000)
© 2000 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Spatial and Temporal Variability of Extractable Lipids as Influenced by Cropping History

H. Dinela and M.C. Nolinb

a Eastern Cereals and Oilseeds Research Centre, Agriculture and Agri-Food Canada, Ottawa, ON, Canada K1A 0C6
b Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, Sainte-Foy, QC, Canada G1V 2J3

dinelh{at}em.agr.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Tillage and crop rotations may alter soil organic matter (SOM) content and quality. Organic matter content can be satisfactorily assessed by determining total organic matter and C content, but easily obtainable, reliable, and highly sensitive indicators of its quality are still lacking to assess its biological, chemical, and physical functions. The spatial and temporal distributions of diethyl ether–(DEE) and chloroform-(CHCl3) extractable lipids and gravimetric DEE/CHCl3 and CHCl3/total extractable lipids (TEL) ratios were evaluated as indicators of organic matter quality in two soils of Agri-Food Canada cropping system fields situated in Québec. Diethyl ether and TEL were significantly (P <= 0.001) more abundant under perennial than annual crops, indicating that the perennial crops were better suppliers of easily biodegradable organic matter. Chloroform-extractable lipids remained relatively constant under both crop rotations, indicating that soil biochemical inertness was relatively the same. Spatial and temporal distributions of these indicators of soil organic matter quality closely correlated with total organic C and clay contents. The DEE/CHCl3 and CHCl3/TEL ratios proposed for assessing organic matter quality were very sensitive (P <= 0.01) in detecting changes in SOM status resulting from crop rotations and tillage practices.

Abbreviations: DEE, diethyl ether • OM, organic matter • SOM, soil organic matter • TEL, total extractable lipids


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
ORGANIC MATTER (OM) and its interactions with other soil constituents are critical factors of soil productivity and overall soil quality (Stewart et al., 1987; Bauer and Black, 1994; Monreal et al., 1997a). Under field conditions, SOM is highly heterogenous and changes continuously. The incorporation into soil of more or less decomposed organic residues triggers several biological processes that involve soil fauna, microbes, and extracellular enzymes that result in biochemical transformations of newly added OM as well as existing SOM.

The biochemical transformations of OM in the soil depend on the types of C molecules. For instance, polysaccharides, proteins, and short-chain aliphatic acids are preferred by microrganisms to long-chain aliphatic and aromatic molecules. Thus, as OM decomposes, proportions of polysaccharides and proteins decrease, while those of long-chain aliphatics increase (Dinel et al., 1990). These transformations of SOM are also corroborated by the abundance of long-chain aliphatics in humic and fulvic acids and humins (Schnitzer and Schulten, 1989).

Knowledge concerning the composition and biochemical transformation of SOM was obtained by extracting SOM with more or less selective agents (Hamblin and Greenland, 1977; Cheshire et al., 1984; Chaney and Swift, 1984; Capriel et al., 1990). The limited selectivity of extractants and the chemical reactions associated with some extracting agents such as NaOH, KOH, and HCl, restricted the use of the information obtained. On the other hand, the use of amphiphilic mixture (Ma'shum et al., 1988) or supercritical n-hexane (Schnitzer et al., 1986), diethyl ether, and chloroform (Dinel et al., 1996a, 1996b, 1998) to extract an unaltered fraction of SOM, although of small quantities, opened new avenues for indicators of SOM quality.

Lipids extractable with DEE and chloroform (CHCl3) are associated with easily biodegradable and biorecalcitrant fractions of SOM, respectively (Dinel et al., 1992, 1996a, 1996b, 1998). For composted organic residues and for fine- and coarse-textured soils, gravimetric ratios of extractable lipids (DEE/CHCl3 and CHCl3/TEL) are highly sensitive indicators of OM biodegradability in compost, as well as under no-till and conventional tillage (Dinel et al., 1998).

The objective of this work was to investigate the temporal and spatial variability of the above proposed indicators under annual and annual–perennial crop conditions in two cropping systems.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Site Selection
The two sites selected for this study were part of a comprehensive research project on the monitoring of soil quality (Wang et al., 1994). Field plots of 5.07 ha (perennials) and 4.26 ha (annuals) were located {approx}30 and 50 km southeast of Montréal in Verchère County, Québec, respectively. This region belongs to the St-Lawrence lowlands terrestrial ecoregion of Canada (Ecological Stratification Working Group, 1995), an almost flat land (0–2% slopes). Mean annual precipitation of this region is {approx}1000 mm. About 1700 effective growing degree days are available during growing season. An intensive soil survey (30 by 30 m) was conducted at the beginning of the study. The two fields are dominated by soils developed on clayed marine deposit of the Champlain Sea, which was slightly reworked by the Proto-St-Lawrence river. Providence soil series are classified as Orthic Humic Gleysols (very-fine clayey, mixed, Mesic Aeric Humaquepts). Surface soil texture was used to stratify perennial and annual experiment fields into four (silty clay loam to heavy clay) and three (silty clay loam to clay) mapping units, respectively (Fig. 1) .



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  		Fig. 1 Soil map and sampling points of perennial and annual fields

 
Soil samples ( ) were collected on a 120 by 30 m rectangular grid. At each grid point, five cores (one in each quadrant of a 2-m radius sampling circle), 0 to 15 cm deep were composited to obtain a representative soil sample of the surface layer. Soil sample were air dried and ground to 2 mm. Sand, silt, and clay contents were obtained by the hydrometer method (Sheldrick and Wang, 1993). Organic C content was obtained by the dry combustion method using a Leco furnace (Leco, St. Joseph, MI) (Tiessen and Moir, 1993). pH was measured in CaCl2 (Hendershot et al., 1993). The textural characteristics, organic C contents, and pH of the surface Ap layers for each mapping unit are shown in Table 1 . Tillage practices and cropping history from 1981 to 1994 are described in Table 2 . The position of the 15 sampling points at the two sites is illustrated in Fig. 1. The cropping history of the two sites may be described as a rotation of annual–perennial crops for Site A and continuous annual crops for Site B (Table 2). A more detailed description of the studied sites was published elsewhere (Nolin et al., 1995).


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  		Table 1 Some characteristics of the Ap1 horizons based on their texture

 

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  		Table 2 Cropping history of the two sites

 
Extraction of Lipids
Six to 8 g of dry soil were sequentially extracted at 6.9 MPa and 100°C with DEE and CHCl3 using an Accelerated Solvent Extractor (ASE 200, Dionex, Sunnyvale, CA). Temperature and pressure used for the extraction were established following preliminary tests comparing Sohxlet and ASE extractions (Monreal et al., 1997a, 1997b; Paré et al., 1999). The extracts from both solvents were dried until dryness on a sand bath maintained at 45°C and weighed to calculate gravimetric extractable lipid ratios DEE/CHCl3 and CHCl3/TEL (DEE + CHCl3). These ratios are proposed as indicators of the biochemical quality of OM (Dinel et al., 1996a, 1996b).

Statistical Analysis
The General Linear Models (GLM) procedure of the Statistical Analysis System, version 6, (SAS Institute, 1990) was performed to separate the effect of crops and time of cultivation on biochemical quality of SOM.

Geostatistical analysis (semivariance) was performed using GS+ version 2.3 (Gamma Design Software, 1995). Experimental variograms were fitted to theoretical models using the average semivariance of four distance values (40, 120, 240, and 360 m) estimated with 15, 36, 27, and 18 pairs, respectively. Surfer version 6.0 for Windows (Golden Software, 1995) was used for spatial interpolation by kriging using a fitted model to evaluate the importance and pattern of the within-field spatial variability of soil quality indicators.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Soil Characteristics
Geostatistical analysis indicated that the surface soil textural characteristics (sand, silt, and clay contents) of both fields showed a strong spatial continuity (unbounded linear semivariogram models) (Fig. 2 and 3) . Along the north direction (y-axis) in both fields, as the clay content increased the silt decreased (perennials P <= 0.001, annuals P <= 0.001). The sand content was similar in the two fields.



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  		Fig. 2 Experimental and fitted variograms of selected soil properties: (a) silt content (perennials), (b) clay content (annuals), (c) sand content (annuals), (d) organic C (annuals), (e) DEE/CHCl3 (perennials, 1989), (f) CHCl3/TEL (perennials, 1989), (g)CHCl3/TEL (annuals, 1994), (h) DEE/CHCl3 (annuals, 1994). DEE, CHCl3, and TEL represent diethyl ether–, chloroform-, and total extractable lipids, respectively

 


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  		Fig. 3 Spatial distribution of sand, silt, clay, and total organic C contents in perennial and annual fields (1989)

 
The pH of Ap horizons indicated that perennial and annual fields were slightly acidic and of similar spatial variability (Table 1). The spatial variability observed in the fields was explained by the presence of a narrow fluvial ridge in the center of the field, which affects the water runoff dynamics and the soil hydrological regime, as well as numerous soil physical and chemical characteristics (Wang et al., 1994).

The organic C content of the Ap horizon was not significantly different between perennial and annual fields, which had average values of 29.6 and 29.0%, respectively (Table 1). In both fields, a spatial relationship existed between organic C and clay contents (perennials P <= 0.01, annuals P <= 0.05) (Fig. 2 and 3). Thus, a high clay content corresponded with a higher level of organic C. This finding confirms the protective action of clay at preserving soil humus (Simard and N'Dayegamiye, 1993) and at reducing SOM turnover by the formation of organo-mineral complexes (Monreal et al., 1997b).

Extractable Lipids
The growth of perennial or annual crops had a well-defined and significantly different (P <= 0.001) impact on the amount of DEE-extractable lipids, an indicator of the presence of easily biodegradable organic matter (Table 3) . In the perennial field, the amount of DEE-extractable lipids was twice the amount extracted from the annual field in 1992 and {approx}22 and 14% higher in 1989 and 1994, respectively (Table 3). The 1992 data may be explained by the growth of white mustard (Brassica hirta Moench) as green manure at the end of the 1991 growing season in addition to four consecutive years of alfalfa (Medicago sativa L.) + timothy (Phleum pratense L.) + brome (Bromus inermis Leysser) crops (Table 2). Thus, under perennial crops, the soil contained more DEE-extractable lipids than under annual crops (Dinel et al., 1990), and their residence time in the soil was longer because the soil was not plowed each year, as in the case of annual crops. A similar trend was also observed for TEL (Fig. 4) . Our data corroborate those reported by Ford and Greenland (1968) and Janzen (1987) for the evaluation of different cropping practices, in which pasture contained more light-fraction materials than continuous wheat (Triticum aestivum L.) and than fallow–wheat.


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  		Table 3 Extractable lipids and their gravimetric ratios of the Ap horizon of the two fields.{dagger}

 


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  		Fig. 4 Spatial and temporal distributions of CHCl3–extractable lipids in perennial and annual fields

 
The distribution of the DEE-extractable lipids in perennial and annual fields illustrates the impact of continuous annual and perennial crops on the biochemical characteristics of SOM. The amounts of DEE-extractable lipids were greater under perennial cropping and not significantly different between years, whereas the DEE-extractable lipids were significantly different (P <= 0.01) between years for annual crops (Table 4) . In perennial fields, the amount of DEE-extractable lipids was higher and the distribution was more heterogeneous than in annual fields (Table 3). Similar results were found for the light-fraction contents, which were generally highest with continuous cropping or perennial forages and lowest in soils with a high frequency of summer fallow (Janzen et al.,1992). Furthermore, the light fraction was more affected than the total organic C or N concentrations in soil (Ford and Greenland, 1968), a situation that was also observed in our study (Table 1).


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  		Table 4 Mean squares and significance for the effect of crops and time of cultivation on extractable lipids.{dagger}*

 
The amounts of DEE-extractable lipids was the highest in 1992, after a late harvest of corn (Zea mays L.) grain and sowing of white mustard as green manure in the previous year. As a result of the late plowing of white mustard and the deficiency of drainage facilities, especially in the northern part of the field, the decomposition of fresh OM was delayed until the following spring. This situation was also confirmed by the TEL values (Table 3). On the contrary, in annual fields, the production in 1991 of barley (Hordeum vulgare L.), which was harvested and residues plowed early, showed that soil microbes had sufficient time to use the easily biodegradable OM, resulting in low yields of highly uniform DEE-extractable lipids (Table 3).

By contrast, the amounts of bioresistant CHCl3–extractable lipids were similar and not significantly different in the two fields between the 1989 and 1994 samplings; values varied between 198 to 250 mg kg-1 under perennials and from 204 to 216 mg kg-1 under annuals (Table 3). These results confirmed a study by Dinel et al. (1996a, 1996b) on the changes of lipids during composting of various types of manures. Their results showed that lipids extractable with CHCl3 were highly bioresistant to decomposition and remained relatively constant throughout the 60-d composting. Furthermore, Dinel et al. (1998) and Monreal et al. (1997a) found in fine- and coarse-textured soils of two catenas under conventional and zero-tillage that CHCl3–extractable lipids were also relatively similar. In addition, the amounts of CHCl3–extractable lipids produced by perennial and annual plants were very similar (Fustec et al., 1985).

From a temporal and spatial point of view, the geostatistical analysis of the CHCl3–extractable lipids showed a clear case of spatial continuity, particularly in annual fields (Fig. 4). Spherical models with sill and a mean range value of 250 m, corresponding with the center of the field were fitted ( ) to the experimental variograms. Spatial patterns depicted by kriging were similar to those shown for clay and organic C contents. In annual fields, they were closely related to the microtopography. This relatively bioresistant fraction of soil organic matter was highly related to pedogenesis processes and therefore represents a better indicator of intrinsic SOM quality.

The gravimetric DEE/CHCl3 and CHCl3/TEL ratios were proposed as indicators of OM maturity for compost (Dinel et al., 1996a, 1996b) and for SOM quality (Dinel et al., 1998). The DEE/CHCl3 ratio is indicative of the easily biodegradable OM and is mainly associated with fresh and relatively undecomposed OM. The CHCl3/TEL ratio represents a more biochemically recalcitrant fraction of SOM and is indicative of the ease by which microbes may or may not physically access readily biodegradable C. Thus, higher values of DEE/CHCl3 lipids combined with lower values of CHCl3/TEL reflect conditions under which the amounts and accessibility of easily biodegradable C compounds contribute to an active microbial activity, whereas lower values of DEE/CHCl3 lipids combined with higher values of CHCl3/TEL indicate that although easily biodegradable OM is present, the remaining easily biodegradable OM is less physically accessible to soil microbes due to biochemical recalcitrance and more hydrophobic molecules (Monreal et al., 1997a).

The DEE/CHCl3 and CHCl3/TEL ratios were highly responsive in differentiating between annual and perennial crops (P <= 0.001). In the perennial field, the extractable lipids and their ratios, except TELs, indicated no significant difference between years, whereas in the annual field, DEE-extractable lipids and DEE/CHCL3 and CHCl3/TEL ratios showed a highly significant difference (P <= 0.001) (Table 4). In our study, the DEE/CHCl3 and CHCl3/TEL ratios for perennial and annual fields indicated that the OM in perennial field is likely to support a higher microbial activity than that in annual fields under comparable field conditions (Table 3). The situation was slightly different in annual fields where easily biodegradable C is present in a lesser amount than in perennial fields, but this C is not physically available, as indicated by the relatively higher CHCl3/TEL ratio (Janzen et al., 1992; N'Dayegamiye and Côté, 1996; N'Dayegamiye et al., 1997).

From a spatial and temporal point of view, the two gravimetric ratios (DEE/CHCl3 and CHCl3/TEL) illustrate the impact of cropping practices on SOM quality (Fig. 5 and 6) . For instance, in annual fields, sampling in 1989 and 1992 was done following corn grain crops, which is known to be very demanding on soil quality (Dinel and Gregorich, 1995; Liang and Mackenzie, 1992; Blevins et al., 1983). The two indicators showed that the SOM was more degraded, as shown by larger areas of higher CHCl3/TEL values and lower values for DEE/CHCl3 (Fig. 2).



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  		Fig. 5 Spatial and temporal distributions of DEE/CHCl3 gravimetric ratio in perennial and annual fields

 


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  		Fig. 6 Spatial and temporal distributions of CHCl3/total extractable lipids (TEL) gravimetric ratio in perennial and annual fields

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The direct assessment of SOM quality is essential to predict the physical, chemical, and biological reactivities of OM under soil management. Extractable lipids and their ratios offer very important insights into biochemical characteristics of SOM. Quantities of DEE-extractable and total extractable lipids were greater in fields where perennial crops were grown than in fields with annuals, indicating a larger supply of easily biodegradable OM for soil microbes, whereas amounts of CHCl3–extractable lipids were relatively the same for both types of crops, indicating that the degree of biochemical inertness of the soil was very similar.

Diethyl ether–extractable lipids and the DEE/CHCl3 ratios serve as indicators of the degree of biodegradability of SOM, whereas the CHCl3–extractable lipids and the CHCl3/TEL ratios indicate the degree of evolution of SOM and its biochemical inertness. The two ratios (DEE/CHCl3 and CHCl3/TEL) originally proposed for assessing OM maturity in compost are sensitive to changes in the SOM status resulting from cropping history and tillage practices.

Received for publication May 25, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
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Right arrow Citing Articles via ISI Web of Science (3)
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Right arrow Articles by Dinel, H.
Right arrow Articles by Nolin, M.C.
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Right arrow Articles by Nolin, M.C.
Agricola
Right arrow Articles by Dinel, H.
Right arrow Articles by Nolin, M.C.

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