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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Neutral and Acidic Sugars in Particle-Size Fractions as Influenced by Climate

^a Inst. of Soil Science and Soil Geography, Univ. of Bayreuth, 95440 Bayreuth, Germany
^b El Macero Drive, 4104 Davis, CA 95616 USA

wulf.amelung{at}uni-bayreuth.de

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Changes in the composition and amount of saccharides have been shown to reflect soil organic matter (SOM) dynamics. The effect of climate on soil monosaccharide pools was investigated in this study. Particle-size fractions were obtained from composite samples taken from the top 10 cm of soil at 18 native grassland sites along mean annual temperature (MAT) and precipitation (MAP) transects from Saskatoon, Canada to southern Texas, USA. Neutral and acidic sugars were determined in the bulk soil, <2-µm (clay), 2- to 20-µm (silt), 20- to 250-µm (fine sand), and 250- to 2000-µm (coarse sand) size separates. As particle size decreased, the concentration of monosaccharides decreased significantly from 297 g kg^-1 soil organic C (SOC) in coarse sand to 174 g kg^-1 SOC in the silt fractions, but increased to 239 g kg^-1 SOC in clay. Ratios of hexoses to pentoses increased with decreasing particle size, indicating that SOM of the finer fractions contained more microbe-derived saccharides, this effect being more pronounced at lower MAT. The concentrations of neutral saccharides decreased in silt and fine-sand fractions as MAT decreased, but increased in all fractions <250 µm as MAP increased. The concentration of acidic sugars in clay and silt was related only to MAP. The results suggest that the moisture regime primarily affected the saccharide concentrations of the finer particle-size fractions, whereas the temperature regime affected primarily the saccharide concentrations of the coarser fractions. Particle-size fractionation was thus a useful tool in decoding the differing effects of MAT and MAP on saccharide dynamics.

Abbreviations: ANOVA, analyses of variance • ara, arabinose • gal, galactose • fuc, fucose • man, mannose • rham, rhamnose • MAT, mean annual temperature • MAP, mean annual precipitation • PC, principal axis component • SOC, soil organic C • SOM, soil organic matter. *,**,*** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
THE COMPLEX BIOLOGICAL, CHEMICAL, AND PHYSICAL PROCESSES involved in SOM turnover can partly be deduced from the dynamics of soil carbohydrates. Plant-derived sugars, especially pentoses, are a source of energy and C for the soil fauna and microorganisms. In turn, the microorganisms resynthesize primarily hexoses and release them to the soil (Cheshire, 1979; Oades, 1984; Murayama, 1984). Both the concentration and composition of the soil's saccharides can therefore be used to assess plant–microbe relationships in SOM dynamics.

In the native grasslands of North America, the above- and belowground litter production was found to be linearly related to the MAP but not to MAT (Sala et al., 1988). In turn, SOM turnover was shown to increase with increasing soil temperature and increasing soil moisture (Parton et al., 1987; Donelly et al., 1990). This suggests that the relative contribution of plants and microbes to SOM, and thus to its saccharides, depends on MAT and MAP. Amelung et al. (1997) showed that soil polysaccharides of the Great Plains are indeed affected by climate; however, the authors only investigated root-free, bulk soil samples and did not differentiate among individual monosaccharides.

According to Skjemstad et al. (1986), SOM stabilization is the essential mechanism that controls SOM decomposition. Stabilization of saccharides may be of particular importance, since they tend to accumulate in older SOM with increasing soil depth (Tsutsuki and Kuwatsuka, 1989), relative even to the chemically more stable lignin (Amelung et al., 1997). Guggenberger et al. (1994) showed that plant-derived sugars were concentrated in sand-sized particulate organic matter, whereas microbe-derived carbohydrates accumulated in the clay, the SOM of which, in North American grasslands, clearly reflected climatic differences (Amelung et al., 1998). Still, little information is available about the effect of climate on the amount and distribution of neutral sugars in particle-size fractions, and equivalent data on acidic sugars are completely lacking. Therefore, it was our objective to investigate how neutral and acidic sugars in particle-size fractions obtained from native grassland topsoils relate to climate.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils
Eighteen sites with moderate range in clay content (170–350 g kg^-1 soil) were selected for sampling in late spring 1994 along temperature and precipitation transects across the native North American prairie (Table 1 ; Amelung et al., 1998). The particle-size distribution of the samples was reported by Amelung et al. (1998); the classification of sites shown here refers to current U.S. soil taxonomy (Soil Survey Staff, 1997). The vegetation resembled the potential natural vegetation of respective Land Resource Areas (USDA-SCS, 1981). Exceptions were the invasion of downy brome (Bromus tectorum L.) at the Arvada, WY; Akron, CO; Holdrege, KS; Hayes, KS; and Manhattan, KS sites, and the invasion of Johnson grass [Sorghum halepense (L.) Pers.] at the Beaumont, TX site. At the Alice, TX site, there had probably been a removal of brush that was followed by a short cultivation period that ended 50 yr ago. Thereafter, the potential vegetation was reseeded. At the Webb County, Texas site, brush displaced the grasses, presumably because of recent extensive grazing. All soils resembling those at the sampling sites have previously been characterized by USDA-SCS (1994) or the Canadian Soil Survey (Acton and Ellis [1978] for the Hoey and Aberdeen, Saskatchewan sites; Ayres et al. [1985] for the Swift Current, Saskatchewan site). Most of them have been monitored at agricultural experiment stations and have well-documented land-use histories.

View this table: [in this window] [in a new window]   Table 1 Soils, climate, and carbohydrate signature of particle-size fractions of the sites under study (ratios have dimension one)§§

Particle-Size Fractionation
Bulk soil samples (<2 mm) were fractionated into clay (<2 µm), silt (2–20 µm), fine sand (20–250 µm), and coarse sand (250–2000 µm) by the following procedure. Thirty grams of fine earth (<2 mm) were treated ultrasonically at 60 J mL^-1 with a probe-type sonicator (Model W 185 F, Heat Systems, Farmingdale, NY) in a 1:5 soil/water ratio to disperse macroaggregates (>250 µm). The output of ultrasonic energy was 48.6 ± 2 W, determined according to North (1976). The 250- to 2000-µm fraction was isolated by wet sieving. A final ultrasonic dispersion treatment of 440 J mL^-1 was used with the <250-µm suspension in a 1:10 soil/water ratio. Centrifugation was used to separate the clay fraction, and wet sieving was used to separate "silt" (USDA fine silt) from "fine sand" (USDA very fine and fine sand) (USDA-NRCS, NSSC, 1996). Preliminary results (Amelung, 1997) suggested that this procedure minimized the redistribution of SOM during the ultrasonic dispersion, but that yields of clay were similar to those obtained from conventional particle-size analysis (USDA-SCS, 1994). All fractions were dried at 40°C and ground for chemical analysis.

Organic Matter Analyses
Subsamples of all size fractions were analyzed for SOC, total N, and total S (data from Amelung et al., 1998). Sugars were determined according to the method of Amelung et al. (1996). Individual neutral sugars and acidic sugars were released from soil by treatment with 4 M trifluoroacetic acid at 105°C for 4 h. The samples were filtered through glass fiber filters and dried with a rotatory evaporator. The residues were dissolved in 3 mL of distilled water and purified by percolating through XAD-7 followed by Dowex 50W X8 cation-exchange resin (Fluka Chemie GmbH, Neu-Ulm, Germany). The resulting hexoses, pentoses, and uronic acids were converted into O-methyloxime trimethylsilyl derivatives, separated on a Hewlett Packard HP 6890 gas chromatograph (Hewlett Packard, Palo Alto, CA), and detected by flame ionization. An HP 5 fused silica capillary column (25 m by 0.2 mm by 0.33 µm film thickness) was used, with N₂ as carrier gas (total flow 60 mL min^-1). Myo-inositol was added as the first internal standard prior to hydrolysis, and 3-O-methylglucose was added as a second internal standard prior to derivatization in order to determine the recovery of the inositol. The analysis was repeated when recovery was lower than 60% of the initial spike level.

After fractionation, an average of 70% of the neutral sugars and 81% of the acidic sugars were recovered relative to their initial concentration determined on the bulk soil samples. Obviously, size fractionation caused some saccharide losses. However, the ratios (galactose [gal] + mannose [man])/(arabinose [ara] + xyl) and (fucose [fuc] + rhamnose [rham])/(ara + xyl) in the fractionated material were 107 and 99%, respectively, of the corresponding values for the bulk soil. Therefore, relative concentrations of these sugars were apparently not influenced by size fractionation.

Statistical Evaluation
Comparisons of sugar ratios and concentrations in the fractions were conducted by one-way analysis of variance (ANOVA) followed the LSD means separation test. For ANOVA, the software package Statistica 5.0 for Windows (Statsoft GmbH, Hamburg, Germany) was used. Additional multivariate statistical analysis was performed with the software package SPSS 5.0 for Windows (SPSS, Munich, Germany). Linear multiple and partial regression analysis were run using the SPSS standard routine; principal component analysis was conducted using factor extraction (eigenvalue >1) after VARIMAX rotation. To investigate the stability of the factor assignment relative to sample size and data errors, analyses were repeated with random elimination of input data.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Neutral and Acidic Sugars of the Size Fractions
The majority of total neutral (53%) and total acidic sugars (60%) were found in the clay fractions. The silt fraction contained approximately one-fifth of the neutral and acidic sugars. Only 11% of both neutral and acidic sugars were present in the fine-sand fraction. Seventeen percent of the neutral and 11% of the acidic sugars still remained in the coarse-sand fractions.

The concentration of neutral sugars (in g kg^-1 SOC) decreased significantly with decreasing particle size in the order: coarse sand > fine sand > silt, but increased again from the silt to the clay fractions (Fig. 1) . Increasing ratios of hexoses to pentoses, such as (fuc + rham)/(ara + xyl) or (gal + man)/(ara + xyl) (Fig. 1), supported earlier evidence that the contribution of microorganisms to the saccharides in soil increased as particle size decreased (Cheshire, 1979; Guggenberger et al., 1994). Thus, the decrease in saccharides from the coarse-sand to silt fractions resulted from an increasing microbial conversion of saccharides. In contrast, the higher sugar contents in the clay fractions may suggest stabilization of microbially synthesized sugars (Martin and Haider, 1986; Christensen, 1996; Amelung et al., 1997).

View larger version (33K): [in this window] [in a new window]   Fig. 1 Average distribution of (a) neutral and (b) acidic sugars in particle-size fractions of 18 native grassland topsoils (0–10 cm). Different letters above the columns indicate that the corresponding sugar concentrations were statistically different at the P < 0.01 probability level. The error bars result from variations in sugar concentrations of a respective fraction across the prairie

The distribution of acidic sugars among the size fractions was similar to that observed for the neutral sugars. Decreasing concentrations of the acidic sugars from coarse sand to silt might be attributed to decomposition of plant-derived sugar acids. In contrast, but in analogy to the neutral sugars, the enrichment of acidic sugars in the clay fraction might reflect an additional microbial release of uronic acids that are, for instance, common in extracellular bacterial gums (Cheshire, 1979). Because carboxylic groups promote interactions between organic matter and minerals (Kaiser et al., 1997), acidic sugars probably bind more effectively to minerals than do neutral sugars. This may be the reason that we found higher proportions of acidic sugars relative to neutral sugars in the clay fractions of the soils.

Amelung et al. (1997) found that the concentration of lignin-derived phenols in SOM decreased with increasing soil depth. Because polysaccharides decreased much less, the authors concluded that microorganisms resynthesized or recycled carbohydrates (or both) during SOM alteration within the soil profiles. Stabilization of saccharides led to decreasing ratios of the lignin-derived phenols to noncellulosic saccharides with soil depth. Based on data from Amelung (1997), the average lignin/saccharide ratio for the SOM of individual size fractions decreased significantly from 0.25 in the coarse sand to 0.05 in the clay fraction (P < 0.001). This supports the hypothesis that saccharides are quantitatively more attached to mineral surfaces than is lignin, and, as a result, are stabilized and preserved in much larger amounts (Martin and Haider, 1986; Stott and Martin, 1990; Amelung et al., 1997). The range of lignin/saccharide ratios among the size fractions was similar to the range observed with depth in similar soils of the Great Plains (0–50 cm; Amelung et al., 1997). Apparently, the heterogeneity of lignin and saccharides among size fractions may be similar in magnitude to the heterogeneity of these compounds among soil depths (0–50 cm).

In the bulk soil, the content of neutral + acidic sugars (in g kg^-1 soil) paralleled changes in bulk SOM (r = 0.96*** for the correlation of sugar contents with SOC, both in g kg^-1 soil; P < 0.001). Therefore, for an understanding of the dynamics of saccharides, it is important to normalize sugar content to SOC, which reflects the sugar concentration in SOM.

Effects of Climate on Saccharide Concentrations
In the bulk soils, the concentration of neutral sugars (in g kg^-1 SOC) increased as MAT decreased (r_MAT|MAP = -0.80***, P < 0.001, MAP partialized) and as MAP increased (r_MAP|MAT = +0.79***, P < 0.001, MAT partialized), confirming that both of the climatic elements influenced the saccharides in soil (Amelung et al., 1997).

In the coarse-sand fraction, which contained significant amounts of particulate SOC, low hexose/pentose ratios indicated that microbial utilization of saccharides was low (Fig. 1). Thus, the SOM of this fraction was too young to be significantly affected by climate (Amelung et al., 1998). As a result, correlations between climatic elements and concentrations of both neutral and acidic saccharides in the coarse-sand fraction were insignificant (r² < 0.1). However, the lack of such correlations indicates that saccharides of plant litter did not change systematically along the climosequence.

In contrast to the coarse-sand fractions, the SOM of the fine-sand fraction consisted of decomposed plant residues (Amelung et al., 1998). For the fine sand and also for the silt fractions, the concentration of neutral sugars was significantly correlated with climatic elements (Table 2) . However, individual correlations with MAT or MAP were insignificant if not partialized for MAP or MAT, respectively. This indicates that MAT and MAP effects on neutral sugars in SOM of these two fractions were interdependent. As MAP increased from 300 to 565 mm in the frigid temperature regime, for instance, the saccharide concentration in the silt and fine sand increased by only as much as 10% of the initial sugar concentration, whereas a similar increase of MAP at the hyperthermic temperature regime almost doubled the neutral sugar contents in the fractions (Table 1).

View larger version (27K): [in this window] [in a new window]   Fig. 2 Relationship of the neutral- and acidic-sugar concentrations in the soil organic matter (SOM) of the clay fractions with mean annual precipitation (MAP)

The ratio of (fuc + rham)/(ara + xyl) in the clay fraction increased with decreasing MAT (Fig. 3) . Partializing this correlation for MAP does not change the correlation coefficient, suggesting that MAP did not significantly affect the (fuc + rham)/(ara + xyl) ratio in the clay fraction. The degree to which microbes affected the saccharide spectrum in the clay fraction (and perhaps in the sand fraction, see above) depended, consequently, only on MAT and possibly other factors governed by MAT (e.g., duration of snow cover and frost periods). Given that no such trend was found for plant residues of the sand fractions, we concluded that cool climates promote the contribution of microbe-derived hexoses to the total sugars in the clay relative to plant-derived pentoses such as arabinose or xylose.

View larger version (21K): [in this window] [in a new window]   Fig. 3 Relationship between the ratios of fucose (fuc) plus rhamnose (rham) to arabinose (ara) plus xylose (xyl) in the clay fraction and mean annual temperature (MAT)

Differences in MAT in the Plains reflect differences in length and severity of winters to a greater extent than differences in summer temperatures. Hence, the apparent low turnover of SOM in the Northern Plains probably reflects longer periods of biological dormancy when the soil is frozen rather than lower decomposition rates during the growing season. When the soils thaw in the spring, microbes begin to degrade plant residues, a process which partly ends in a production of fine-sand-sized SOM from coarse-sand-sized SOM (Amelung et al., 1998). This results in a synthesis of hexoses, of which (fuc + rham) accumulate in the clay, and possibly (gal + man) in the sand fractions. The lower the MAT, the earlier the soils freeze in the following winter, and weakly decomposed litter residues of the fine-sand fractions (Amelung et al., 1998) as well as microbially weakly altered SOM are left for the next year. In the Northern Plains (cryic to mesic temperature regime), there is more opportunity for decomposition if the growing season is longer. The situation is different in the Southern Plains. When soils are hardly if at all frozen in winter (mesic to hyperthermic temperature regime), higher SOM turnover and saccharide mineralization are caused by higher rates rather than by differences in growing season.

Quantification of Precipitation and Temperature Effects on Saccharide Concentrations
Through linear regression analysis the concentrations of neutral sugars (in g kg^-1 SOC of a fraction) in size separates <250 µm and in the bulk soil can be predicted as a function of MAP (in mm) and MAT (in °C): Clay:

(1)

Silt:

(2)

Fine sand:

(3)

Bulk soil:

(4)

Including either pH or texture data in the regression did not improve R, suggesting that neither variable modified the observed saccharide concentrations. The same applies to the relationship of acidic sugars in clay and silt to MAP.

The regression coefficient for MAT in Eq. [1] was not significantly different from zero (P < 0.05). Consequently, effects of MAT on neutral sugars in the clay fraction were neglected. When individual regression coefficients are normalized by the variance of the corresponding variable j, the |ß(j)| value is obtained. It reflects the relative contribution of the variable j to the coefficient of determination (R²) in the regression function. For individual fractions, |ß(MAT)| amounted to 0.20 for clay (not significant), 0.73 for silt and 0.52 for fine sand, whereas |ß(MAP)| ranged from 0.73 (silt) to 0.96 (fine sand), i.e., exceeding, on the average, those of |ß(MAT)|. Consequently, the effect of MAP on the saccharide concentrations in the fractions was stronger than that of MAT (for clay fractions) or as strong as the effect of MAT (for the silt fractions). The opposite trend was obtained for the bulk soils, where the MAT contributed more to R² than did MAP [ß(MAT) = 0.93; ß(MAP) = 0.89]. This can be attributed to high proportions of particulate SOM in the bulk soil that, rich in saccharides (Fig. 1), decreased with increasing MAT (Amelung et al., 1998). The results suggest that with increasing degree of microbial SOM alteration (from coarse-sand to clay fractions) the influence of MAT on sugar concentrations became smaller than that of MAP, whereas the temperature effect on the saccharides' composition (e.g., proportion of microbial to plant-derived sugars) became larger. The following principal axis component (PC) analysis was conducted to test this hypothesis.

Multivariate Evaluation of the Data Structure
Principal component analysis was used to identify interrelations among data. Input data included the concentrations of neutral and acidic sugars as well as hexose/pentose ratios in the fractions, the climate and texture data, and the pH of the bulk soil. After VARIMAX rotation, the program extracted six PCs explaining 83.3% of the data variance.

A given variable was assigned to only one PC, that being the one having the maximum loading. The factor loadings (i.e., the correlation coefficients of the variables with the particular PC to which they were assigned) ranged absolutely from 0.70 to 0.96. When input data were randomly eliminated, individual factor loadings varied. Therefore, they should not be interpreted and are not presented. However, these variations of factor loadings due to changes of samples size remained within the range of 0.70 to 0.96. In other words, the assignments to a particular PC remained stable with respect to variations in sample size. Variables printed in italics (see below) could not be clearly assigned to a particular PC; the corresponding factor loadings ranged from 0.50–0.69. The assignment of variables to a PC became clearer only after VARIMAX rotation, for which it is assumed that a simple structure does not exist within the data. The data that load highest in a particular PC are intercorrelated, and can be regarded to be controlled by the same factor. Only the stable results with respect to variations in sample size are shown below:

• 1st PC (30.7%): Neutral sugars in coarse sand, (gal + man)/(ara + xyl)_coarsesand, (fuc + rham)/(ara + xyl)_coarsesand, (gal + man)/(ara + xyl)_finesand, (fuc + rham)/(ara + xyl)_finesand, neutral sugars in fine sand, (fuc + rham)/(ara + xyl)_silt, pH
  
• 2nd PC (16.7%): MAP, neutral sugars in clay, acidic sugars in clay and silt, neutral sugars in fine sand and silt,
  
• 3rd PC (13.9%): MAT, (fuc + rham)/(ara + xyl)_clay, acidic sugars in fine sand, neutral sugars in silt
  
• 4th PC (9.2%): Content of fine sand and silt, pH
  
• 5th PC (7.4%): Content of clay and coarse sand
  
• 6th PC (5.4%): (gal + man)/(ara + xyl)_clay, (gal + man)/(ara + xyl)_silt
  

The first PC clearly isolated the saccharide concentrations and hexose/pentose ratios of coarse sand. This indicates that this fraction comprises a separate saccharide pool in Mollisols that has higher O-alkyl-C resonances in nuclear magnetic resonance spectra (Baldock et al., 1992), but it is yet little considered in other studies as a unique saccharide pool (review: Christensen, 1996). Acidic sugars in the coarse-sand fraction could not be clearly assigned to a certain PC. Climatic elements never loaded together with sugar data of the coarse-sand fraction even during variations in sample size. This supported the hypothesis that the SOM of the coarse-sand fractions was too young to be affected by climate (Amelung et al., 1998).

Saccharide data for the fine-sand fraction loaded the first PC less clearly, probably because there were confounding effects of climate on the concentration of the neutral (2nd PC) and acidic sugars (3rd PC) in this fraction. The pronounced correlations of MAP with the neutral- and acidic-sugar concentrations in the SOM of clay (Fig. 2) and silt were reflected by the 2nd PC. The 3rd PC showed primarily the influence of MAT on saccharides in the clay and also partly in silt and fine-sand fractions. Only when the data of the three Canadian sites were eliminated from the analyses did the (fuc + rham)/(ara + xyl) ratio not load with MAT in a common PC. The (gal + man)/(ara + xyl) ratios in clay and silt, as well as the (fuc + rham)/(ara + xyl) ratio in silt, were controlled by a single factor (5th or 1st PC, respectively) that was not related to climate, texture, or pH.

The texture data were assigned to PCs (4th and 5th) other than those containing sugar data. Also the pH(CaCl₂) correlated with the 4th and only weakly with the 1st PC. Since individual correlations between the pH and the saccharide data of the 1st PC were insignificant, we suggest that the dynamics of saccharides in soils of the North American prairie were not related to either pH or to texture (within the range of clay contents from 170 - 350 g kg^-1 soil).

In contrast to PC analyses with data from SOC, N (Amelung et al., 1998), and lignin analyses (Amelung, 1997), the sugar data were not clearly grouped into particle-size classes. Instead, climatic elements regulated the assignment of sugar data to different PCs. The MAP described a common PC with the neutral-sugar concentration in the clay fractions as well as with the neutral- and acidic-sugar concentrations in the silt fractions. The MAT was commonly assigned to PCs containing the ratio of (fuc + rham)/(ara + xyl) in the clay fractions and in places with the concentration of neutral sugars in the silt and fine-sand fractions. Consequently, MAT and MAP affected sugars in soils differently. The MAP controlled the sugar concentration in the SOM with a high degree of microbial alteration, represented by the SOM in clay fractions. In contrast, MAT affected the sugar composition in the clay fraction as well as neutral-sugar concentrations in coarser fractions (except the coarse sand).

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Particle-size fractionation into clay, silt, and fine and coarse sand yielded four different saccharide pools in surface soils of native prairie. Climate influenced saccharides in the fractions <250 µm. There was no evidence that site properties such as pH, texture, or a litter-specific saccharide composition contributed to the correlations between saccharide concentrations or (fuc + rham)/(ara + xyl) ratios and climatic elements.

Correlations between saccharide concentrations and climatic elements existed for the fine sand but not for the coarse-sand fraction. Because little if any of the fine-sand-sized SOM was bound to the mineral matrix (Amelung et al., 1998), an interaction of SOM with minerals is not required for climate to alter SOM composition. Therefore, we interpreted climatic effects on saccharides in fractions as being a result of microbial processes:

1. The impact of climate on saccharides in temperate grassland soil became clear only after microbial modification of coarse-sand-sized SOM

2. The warmer the climate, the faster microbes decompose SOM (Trumbore et al., 1996) and fewer saccharides were found in the bulk soils (Amelung et al., 1997). This effect is thought to be due mainly to increasing decomposition of fine sand-sized SOM (Amelung et al., 1998) and its saccharides (Eq. [3]). Sugar concentrations of the clay fractions were not significantly related to MAT. We concluded that MAT influences saccharides in soil mainly before they accumulate in the clay fractions.

3. Moisture promotes the production of polysaccharides by plants and microbes as well as their stabilization, thus the SOM of wet sites contained more saccharides than that formed under drier conditions (Amelung et al., 1997). Particle-size fractionation revealed that this effect was mainly due to an increasing incorporation of saccharides into the SOM of the clay fractions (Eq. [1]).

As MAP increases, primary plant production increases (Sala et al., 1988) and so will the input of plant-derived sugars. The growth of microorganisms is more or less proportional to the root input (Dormaar, 1990), as will be the production of microbially derived saccharides. Therefore, only bulk neutral-sugar contents, but neither the microbe-derived amino sugars (Amelung et al., 1999) nor hexose/pentose ratios (this study), are related to MAP. In contrast, the negative correlation of MAT with (fuc + rham)/(ara + xyl) ratios of the clay fractions suggests that, the warmer the climate, the more efficiently microorganisms mineralize their hexoses before they accumulate in the clay fractions. In order to verify this hypothesis, studies of the age and stabilization kinetics of saccharides in different climatic regions (e.g., using isotope tracer studies) need to be conducted.USDA-NRCS 1996

	ACKNOWLEDGMENTS

 
We thank V.O. Biederbeck, L. Brown, S. Brown, C. Campbell, G.R. Carlson, G. Creinwelge, R.F. Follett, D.W. Fryrear, R. Molina, E. Montemayor, E.G. Pruessner, C. Richardson, G.E. Schuman, E. Skidmore, D. Sweeney, H. Tiessen, C. Thompson, D. Towns, F. Turner, M. Vorhees, R. Vredenburg, and R. Zink for help with site location and sampling. I. Dörfler and Y. Yuan are thanked for contributing to the analyses. The work has been supported by the German Research Foundation (DFG Ze 154 / 22-2).

Received for publication February 13, 1998.

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