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Size and density fractionation of soil organic matter and the physical capacity of soils to protect organic matter
European Journal of Agronomy 7 (1997) 189–199
Size and density fractionation of soil organic matter and the physical capacity of soils to protect organic matter Jan Hassink a ,*, Andrew P. Whitmore, Jaromir Kuba´t b a
Research Institute for Agrobiology and Soil Fertility (AB-DLO), P.O. Box 129, 9750 AC Haren, The Netherlands b Research Institute of Crop Production, Drnovska´ 507, 16106 Praha 6, Ruzyne, Czech Republic Accepted 16 May 1997
Abstract Soil organic matter (SOM) has important chemical, physical and biological functions in the soil. It is difficult to predict the dynamics of SOM because it is very heterogeneous and because its behaviour is affected by soil texture. In this study we used a new size and density fractionation to isolate SOM fractions that differ in stability and we estimated the amount of SOM that can be preserved in different soils. An investigation was carried out into (1) how fast size and density fractions of soil organic matter respond to changes in C input, (2) whether the capacity of soils to preserve C by its association with clay and silt particles is limited and related to soil texture and (3) whether the long term dynamics of soil C can be described with a simple model that makes the assumption that the net rate of decomposition of soil C does not simply depend on soil texture, but on the degree to which the protective capacity of the soil is already occupied. Light and intermediate fractions of the macroorganic matter (.150 mm) respond much faster to changes in C input than smaller size fractions. This shows that the light and intermediate macroorganic matter fractions can be used as early indicators of effects of soil management on changes in SOM. There was a close positive relationship between the proportion of particles ,20 mm in a soil and the amount of C associated with this fraction in the top 10 cm of grassland soils. Arable sandy soils, which contained less C than corresponding grassland soils, had the same amounts of C associated with the fraction ,20 mm, indicating that the amount of C that can become associated with this fraction had reached a maximum. The observed relationship: C in fraction ,20 mm (g/kg soil) = 6.9 + 0.29× % particles ,20 mm can be used as a first estimation for the capacity of a soil to preserve C. The amount of C in macroorganic matter is controlled by soil management, while the amount of C protected by clay and silt particles is controlled mainly by soil texture. The simulations of the changes in C in soil without input of C or with additions of lucerne or chaff were excellent in both sandy and clay soils. The build-up of C in soils receiving farmyard manure (FYM) was not simulated so well. 1997 Elsevier Science B.V. Keywords: Soil organic matter; Size and density fractionation; Physical protection; Capacity; Modelling
1. Introduction Organic matter is a key component of soil which affects its physical, chemical and biological proper* Corresponding author. Tel.: +31 50 5337320; fax: +31 50 5337291; e-mail: [email protected]
ties. Organic matter improves soil structure, increases the water holding capacity and promotes biological transformations such as N-mineralization. Maintenance of sufficiently high levels of organic matter is a prerequisite for sustainable, high, production levels of crops. Agricultural management practices influence the amount of organic matter in soil and cause
1161-0301/97/$17.00 1997 Elsevier Science B.V. All rights reserved PII S1161-0301 (97 )0 0045-2
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changes in the rate of soil organic matter turnover. It often takes years, however, before changes in agricultural management lead to detectable changes in the quantity and quality of soil organic matter (SOM). This makes it difficult to evaluate the effect of different management strategies on SOM dynamics. Several models have been constructed to predict organic matter dynamics. SOM is very heterogeneous and is composed of a series of pools from very active to passive (Schimel et al., 1985). To account for this, models of organic matter dynamics generally include compartments with a rapid turnover rate and a slower turnover rate. A major problem related to predicting SOM dynamics is that, except for the microbial biomass, the different compartments can not be determined directly by chemical or physical fractionation procedures (Paustian et al., 1992). Successful development of techniques for direct measurement of pool sizes would represent a major step towards appropriate verification of models (Bonde et al., 1992). Size and density fractionation show promise for physically dividing SOM into pools differing in composition and biological function (Christensen, 1992). Size fractionation is based on the observation that sand-size organic matter (macroorganic matter; .150 mm) is often more labile than organic C in the clay and silt size fractions (Tiessen and Stewart, 1983; Feller et al., 1991). Density fractionation is based on the observation that during humification parts of SOM become more associated with mineral particles and thus occur in particles of higher density (Barrios et al., 1996). Meijboom et al. (1995) have developed a new and simple density fractionation procedure using silica suspensions and recovered three density fractions in the macroorganic matter: the light fraction consisting of recognizable plant residues, the intermediate fraction of partly humified material and the heavy fraction of amorphous organic material. It was shown that the decomposition rates decrease in the order light, intermediate and heavy macroorganic matter, while the decay rates of C in the clay and silt size fractions were lowest (Hassink, 1995a). Size and density fractionation might enable us to identify labile fractions that respond much faster to changes in organic matter input than total SOM and can serve as sensitive indicators of changes in the SOM content (Janzen et al., 1992; Barrios et al., 1996). It has been recognized that soil texture and soil
structure have a predominant effect on organic matter decomposition. It is generally accepted that there is more physical protection in fine-textured soils than in coarse-textured soils (Jenkinson, 1988; Van Veen and Kuikman, 1990). As a consequence fine-textured soils have higher organic C contents than coarse textured soils when supplied with similar input of material (Jenkinson, 1988; Hassink, 1994). Considerable published evidence indicates that one of the principal factors responsible for physical protection of organic matter in soils is its ability to associate with clay and silt particles (Theng, 1979). Little is known, however, about the capacity of soils to protect organic matter physically. Hassink et al. (1997) suggested that the physical capacity of a soil to preserve organic matter is limited. Hassink (1996) defined the protective capacity as the maximum amount of C that can be associated with clay and silt particles in the soil. He suggested that the degree of saturation of the protective capacity of a soil would affect the preservation of newly added carbon in residues and not soil texture per se. Less of the applied C would be preserved in the soil when all protective sites were occupied than when sites were available to stabilize organic C. This explains why the preservation of applied C is directly related to soil texture when C is applied to fine-textured soils with low organic matter contents (e.g. Amato and Ladd, 1992) and why there is no correlation between soil texture and preservation of applied C in soil when fine-textured soils with higher organic matter contents are used (e.g. Gregorich et al., 1991). Based on the assumptions that (1) the preservation of applied C is controlled by the degree of saturation of the clay and silt size fractions with SOM (instead of soil texture per se) and (2) that the protection of SOM can be described kinetically in the same way as adsorption and desorption, a simple simulation model was developed that predicts the long term dynamics of SOM (Hassink and Whitmore, 1997). Short term changes in SOM may best be studied by focusing on the labile organic matter fractions, while long term dynamics of SOM are probably affected mainly by the capacity of a soil to protect organic C physically. The objectives of the present study are: (1) to determine how fast size and density fractions of SOM respond to changes in input of organic inputs and to identify measurable indicators most sensitive to changes in SOM; (2) to estimate the amount of C that
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can be protected physically by association with clay and silt particles (,20 mm); and (3) to test whether a model that explicitly describes physical protection as a function of the capacity of clay and silt particles to hold organic matter, can simulate the long term dynamics of C in sandy and clay soils in the Netherlands that received either no organic C or annual applications of different organic materials and a clay-loam soil in the Czech Republic that received either no organic C or annual applications of manure.
2. Materials and methods 2.1. Experimental sites To determine how fast size and density fractions of SOM respond to changes in input of organic inputs and to identify indicators most sensitive to changes in SOM (objective 1), we defined and fractionated SOM from sand and clay soils that had received no input and samples of the same soils that had received annual applications of different organic materials. The soils were located in Haren, in the Northern part of the Netherlands. Characteristics of the soils at the begin-
ning of the experiment (1961) are given in Table 1. The soils have been kept bare since 1961 and received (1) no organic C and N or (2) 10 t C per ha/year by the application of either lucerne (2.5% N, 43.6% C), wheat chaff (0.8% N, 42.5% C) or farmyard manure (FYM; 2.9% N, 42.7% C). The residues were applied at the beginning of June and mixed through the top 25 cm of the soil. Soil samples of the top 25 cm were taken at 0.5, 3, 8, 15 and 25 years after the start of the experiment. To estimate the amount of C that can be protected physically by association with clay and silt particles (objective 2), we sampled the top 10 cm of grassland soils which had been under grass for at least 30 years and determined C in size and density fractions. Grassland soils generally have higher C contents than arable soils, because they receive more organic C each year and are not tilled (Jenkinson, 1988; Lugo and Brown, 1993). The grasslands were grazed by dairy cattle and received 400–500 kg fertilizer-N ha/year. Characteristics of the grassland soils are presented in Table 1. In the sandy soil of Tynaarlo we compared the distribution of C between organic matter fractions in soil from a grassland field with soil from an adjacent arable field which had been under a 4 year rotation of winter wheat, sugarbeet, barley and ware potatoes. In the
Table 1 Characteristics of the top 25 cm of the sandy and clay soils at the beginning of the long term experiment in 1961 (objective 1) and the top 10 cm of the grassland soils used for objective 2 Location
Objective 1 Haren Sand Clay Objective 2 Achterberg Burum Zaltbommel 1 Zaltbommel 2 Heino Finsterwolde 1 Finsterwolde 2 Tynaarlo Cranendonck
% particles
pH (K–Cl)
,20 mm
Total Soil C (g/kg)
,2 mm
,20 mm
ND ND
17.0 65.6
7.4 7.0
7.2 15.2
2.7 6.6
0.4 0.4
10.1 22.2
3.0 24.1 25.8 51.1 1.9 8.4 45.8 2.6 1.0
5.8 36.5 42.6 76.0 2.8 13.3 65.9 5.0 3.5
4.9 4.8 5.8 5.4 5.0 5.0 7.1 4.4 5.4
9.7 21.2 19.9 31.0 1.4 11.3 20.3 13.0 3.6
5.8a 11.9a 14.3a 25.2a 14.4a 34.7a 7.1 22.2a 3.3
7.9 8.8 5.7 4.5 3.9 7.7 4.6 5.9 6.7
23.4 41.9 39.9 60.7 19.8 53.7 30.2 41.1 14.2
ND, Not determined. Calculated as total soil C − C in fractions ,20 and .150 mm.
a
C in size fraction (g/kg) 20–150 mm
.150 mm
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sandy soil of Cranendonck we compared the distribution of C among organic matter fractions in a 30 year old grassland field with a field that had been under maize for at least 25 years. We also compared the distribution of organic matter in the top layer (0–20 cm) with deeper soil layers (30–40 and 60–80 cm deep) at this one location. To test our model that explicitly describes physical protection (objective 3), we simulated the long term dynamics of soil C in the sand and the clay soil receiving lucerne, chaff or FYM residues annually since 1961 (see above) and a clay loam from Ruzyne in the Czech Republic receiving 0, 80 or 160 t/ha FYM annually.
referred to as particle size fraction 20–150 mm. The suspension passing the 20 mm sieve was placed at 4°C till it had sedimented (usually after 24 h) and the clear solution was sucked off; this fraction will be referred to as particle size fraction ,20 mm. It was assumed that the particle size fractions 20–150 mm and ,20 mm were derived primarily from microaggregates as microaggregates are resistant to wet-sieving (Tisdall and Oades, 1982). All fractions were dried at 40°C and analyzed for total C and N. In most of the grassland soils (indicated in Table 1), the amount of C in the 20–150 mm fraction was not determined, but calculated as the difference between total soil C and C in the fractions ,20 mm and .150 mm.
2.2. Size and density fractionation of soil organic matter
2.3. Modelling
Dried soil samples taken from the long term experiment on the sand and clay soil that had been stored were rewetted before fractionation. From the other soils field-moist samples were used for fractionation. We determined five fractions: the light, intermediate and heavy fractions of the macroorganic matter fraction (.150 mm) and the size fractions 20–150 mm and ,20 mm. For the density fractionation of the macroorganic matter, samples of 250 g were washed through two sieves (top sieve, mesh size 250 mm; bottom sieve, 150 mm). The soil was pushed through the top sieve, till the water passing the sieve became clear. In this way all macroaggregates were destroyed. The mineral fraction was discarded by decantation. After combining the organic fractions from both sieves, it was further fractionated in silica suspensions with a density of 1.13 and 1.37 g/cm3 as described by Meijboom et al. (1995). The macroorganic matter was separated into three fractions: a light fraction (density ,1.13 g/cm3); an intermediate fraction (density between 1.13 and 1.37 g/cm3) and a heavy fraction (density .1.37 g/cm3). To isolate the finer fractions, samples of 50 g were washed on three sieves (top sieve, mesh size 250 mm; second sieve, 150 mm; bottom sieve, 20 mm). The suspension passing the bottom sieve of 20 mm was collected in a bucket. The soil was pushed through the top sieve again (destroying the macroaggregates), till the water passing the sieve became clear. The material accumulating on the 20 mm sieve will be
Hassink and Whitmore (1997) have suggested a model for the dynamics of the physical protection of organic matter in soil based on a mechanism analogous to adsorption and desorption. Two SOM pools are distinguished: Protected (POM) and Non-protected (NOM) organic matter. NOM is attacked by soil microorganisms relatively easily, whereas POM cannot be decomposed unless it is first released by desorption. Assuming that the capacity of soil to protect organic matter (protective capacity; X) has a maximum value that cannot be exceeded, it is likely that the rate at which NOM becomes protected will slow down as this maximum is approached. The rate of desorption of POM on the other hand, is independent of X. Hassink and Whitmore (1997) found a good relationship between the clay content of a soil and X (maximum amount of SOM that can be protected by the soil). Based on this relationship we estimated X for three different soils: the sand and the clay soil receiving lucerne, chaff or FYM residues annually since 1961 (see above under objective 1) and a clay loam from Ruzyne in the Czech Republic. Composted manure was applied annually to this last soil at the rate of 0, 80 or 160 t/ha containing 0, 8 or 16 t/ha carbon (Nova`k and Apfelthaler, 1971). Additions stopped after 31 years of the experiment and the decrease in soil C was determined for another 5 years. Manure was added to the top 10 cm (80 t/ha) or top 20 cm (160 t/ha) of the experiment and at least in the first year therefore the concentrations of added organic matter were the same. However, because manure
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J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199 Table 2
Amount of C in the light, intermediate and heavy macroorganic matter fractions and the fractions 20–150 mm and , 20 mm in the top 25 cm of the bare sandy and clay soil (Haren) that received no organic C and N 0.5, 3, 8, 15 and 25 years after the start of the experiment (g/kg soil) Fraction
Time (years) 0.5
Sandy soil Light Intermediate Heavy 20–150 mm ,20 mm Clay soil Light Intermediate Heavy 20–150 mm ,20 mm
3
8
15
25
0.06 0.06 0.24 2.70 7.23
0.05 0.08 0.19 3.20 6.30
0.02 0.04 0.14 2.85 6.11
0.01 0.02 0.12 3.18 5.16
0.01 0.04 0.17 3.89 5.12
0.08 0.10 0.35 6.63 15.22
0.07 0.06 0.30 6.02 14.02
0.02 0.04 0.20 5.32 13.23
0.02 0.02 0.23 5.78 11.26
0.04 0.06 0.28 6.38 10.23
was added to the same depth of soil each year, the mass of soil in this depth decreased as the bulk density decreased, and the effective concentration of manure added (per mass of soil) increased each year. Both soils were sampled to 20 cm depth and this means that the apparent concentration of organic matter in the soil receiving 160 t/ha increased more quickly than in the soil receiving 80 t/ha. Corrections for the change in bulk density were made using a relationship derived by Whitmore et al. (1992). Lucerne, chaff and manure contained respectively 8%, 11.2% and 29% lignin (Haan de, 1977); Whitmore and Matus (1996) give a formula for reducing the rate of decomposition of crop residues in relation to their lignin content. The manure added to the Ruzyne experiment was assumed to contain 29% lignin also, but because it was composted we further assumed that 5% would be chemically resistant to attack and not decompose during the course of the experiment (e.g. Jenkinson et al., 1988). 2.4. Chemical analysis Total C in soil, in macroorganic matter fractions and in the particle size fraction 20–150 mm was defined as dichromate-oxidizable C according to Kurmies (Mebius, 1960). Total C in the particle size fraction ,20 mm was determined with a CHN autoanalyzer (Carlo Erba NA 1500).
2.5. Statistical analysis The relationships between the percentage of soil particles ,20 mm and total soil C and C associated with the particle size fractions ,20 mm, 20–150 mm and .150 mm were analyzed with correlation and regression techniques (Genstat, 1987).
3. Results The identification of soil organic matter fractions that are sensitive to changes in input of organic materials (sandy and clay soil of objective 1) At all sampling times, more than 90% of all soil carbon was present in the fractions 20–150 mm and ,20 mm in the treatments where no organic material was applied to the soil (Table 2tablehere>). The amount of C in the heavy macroorganic matter fraction and the 20–150 mm fractions did not change significantly with time, while C in the other fractions decreased with time. The relative changes in the amounts of C in size and density fractions due to the application of chaff, lucerne and FYM are expressed as the amounts of C in the size and density fractions in the chaff, lucerne and FYM treatments, divided by the corresponding amounts in the treatments where no organic inputs were applied 0.5, 3, 8, 15 and 25 years after the start of the treatments. There were no
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The relative changes were larger for the FYM treatment than for the lucerne and chaff treatments. The application of FYM increased the amount of C in the heavy macroorganic matter fraction relatively more than the application of lucerne or chaff (Figs. 1 and 2). 3.1. Relationship between soil texture and C in organic matter fractions in grassland soils (soils of objective 2) Fig. 1. The amounts of C in the light, intermediate and heavy macroorganic matter fractions and the 20–150 mm and ,20 mm fractions in the chaff treatment divided by the corresponding amounts of C in the fractions in the no input treatment 0.5, 3, 8, 15 and 25 years after the start of the experiment. Average of the sandy and clay soil.
differences in the relative changes of C in the fractions between the sand and clay soils and between the lucerne and chaff treatments. The amounts of C in each of the fractions derived from the chaff and FYM treatments divided by the corresponding amounts in the treatments receiving no input were averaged for the sand and clay soils (Figs. 1 and 2). Generally, when organic materials were added to the clay and sand soil pronounced changes took place quickly in the amounts of C in the light and intermediate fractions of the macroorganic matter pool. The heavy fraction of the macroorganic matter pool responded more slowly, while changes in the fractions 20–150 mm and the ,20 mm were relatively slow and small (Fig. 1 and 2). Within a few years of application of organic materials, changes in the light and intermediate fractions of the macroorganic matter fraction could be determined easily. The relative increases in the light and intermediate fractions of the macroorganic matter pool reached a maximum within 8–15 years after the start of input of organic material. At this maximum, the amounts of C in the light and intermediate macroorganic matter fractions were 6–23 times greater in the treatments where organic material was applied than in the treatment where there was no input of organic material. The amount of C in the heavy macroorganic matter fraction and the fractions 20–150 mm and the ,20 mm fraction continued to increase relative to the nil input treatments. After 25 years, the relative increases ranged from 4 to 19 for the heavy macroorganic matter fraction and from 1.3 to 4.4 for the 20–150 mm and the ,20 mm fractions.
We investigated the relationships between soil texture and organic C in different organic matter fractions. For the fraction .150 mm, we present the sum only of the light, intermediate and heavy fractions, because the density fractions did not give any additional information with respect to the capacity of soils to protect C physically. There was a highly significant correlation (r = 0.91) between the clay and silt content of grassland soils and the amount of C associated with this fraction (Table 1; Fig. 3; C associated with the fraction ,20 mm = 6.9 + 0.29× % particles ,20 mm). The amounts of C in the fractions 20–150 mm and .150 mm did not correlate significantly with the clay and silt fraction and varied considerably between soils with similar clay and silt content (Table 1). C in size fractions in soils with similar textures but differing in organic matter input (Tynaarlo and Cranendonck soil of objective 2 and the sandy and clay soil of objective 1) In spite of the fact that the arable field from Tynaarlo, the maize field from Cranendonck and the
Fig. 2. The amounts of C in the light, intermediate and heavy macroorganic matter fractions and the 20–150 mm and ,20 mm fractions in the FYM treatment divided by the corresponding amounts of C in the fractions in the no input treatment 0.5, 3, 8, 15 and 25 years after the start of the experiment. Average of the sandy and clay soil.
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J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199 Table 3 Amounts of C in different size fractions and total soil C in the top 10 cm of the grassland and arable soil in Tynaarlo and the top 20 cm and the soil layers at 30–40 and 60–80 cm depth in the grassland and maize field in Cranendonck (g/kg) Location
Tynaarlo Grassland Arable Cranendonck 0–20 cm grass 30–40 cm grass 60–80 cm grass 0–20 cm maize 30–40 cm maize 60–80 cm maize
C in fraction (treatment)
Total soil C
,20 mm
20–150 mm
.150 mm
11.9 11.8
9.6 5.4
11.6 2.8
36.3 23.8
3.6 3.4 3.4 3.6 3.5 3.1
4.7 4.0 3.6 3.1 2.1 1.7
6.7 3.5 2.8 2.6 1.2 0.9
15.9 12.1 9.7 8.8 7.1 7.2
fractions was generally close to the total amount of soil C (Table 3). The application of chaff, lucerne and FYM to the sand and clay soil for 25 years (long term experiment of objective 1) resulted in a considerable increase in total soil C in comparison with the treatment where no organic residues were applied (Table 4). In the sandy soil, the increase was concentrated in the 20–150 mm fraction, while in the clay soil most of the increase took place in the ,20 mm fraction (Table 4). In the clay soil, the sum of C in the fractions was close to the total amount of soil C; in the sandy soil, the sum of C in the fractions was 40% higher than total soil C for the no input treatment, while in the other treatments the sum of C in the fractions was again close to total soil C (Table 4). No explanation can be offered for the high recovery in the sand soil without input. 3.2. Modelling (sandy and clay soil of objective 1 and Czech soil at Ruzyne)
deeper layers of the grassland and maize field from Cranendonck had much lower total amounts of soil C than the corresponding top layers of the grassland fields, the amounts of C associated with the clay and silt fraction were not less (Table 3). The amounts of C in the fractions 20–150 mm and .150 mm were less in the arable field from Tynaarlo and the maize field from Cranendonck than in the corresponding grassland sites, and decreased with increasing depth at Cranendonck (Table 3). The sum of C in the different
The simulations of the increase in total soil C in soil with addition of lucerne or chaff, or the decrease in C in soil in the treatments without addition were excellent (Figs. 4 and 5) in both the sand and clay soils. The simulated build-up of organic matter in the soils receiving FYM also agreed well with the measurements during the first 13 years. During the second half of the experiment, however, a sharp increase in soil C was measured, which could not be simulated
Table 4 Amounts of C in different size fractions after 25 years of no input or annual applications of chaff, lucerne and FYM (g/kg) and the percentage of increase in soil C (in comparison with the no input treatment) that is associated with the fraction , 20 mm for the sand and clay soil in Haren
Sandy soil No input Chaff Lucerne FYM Clay soil No input Chaff Lucerne FYM
,20 mm
20–150 mm
.150 mm
5.1 7.5 8.5 12.4
3.9 9.4 10.1 21.5
0.2 1.6 1.1 5.5
25.5 32.0 24.1
6.6 16.8 16.5 42.5
10.2 15.4 16.5 26.0
6.4 9.7 10.0 19.2
0.4 1.6 0.9 4.3
53.2 60.8 48.6
14.7 25.9 25.0 52.7
Increase in fraction , 20 mm (% of total increase)
Total soil C
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(Figs. 4 and 5). In all treatments receiving additions, an odd decline in soil C during the last 3 years of the experiment was measured. The build-up and decline of organic C at Ruzyne was simulated well (Fig. 6) and it is particularly interesting to see that simulating the change in bulk density allowed us to reproduce the extra increase in concentration of organic C (in g per C/kg soil) in the plot receiving 160 t/ha 20/cm each year despite the fact that the addition on this plot was nominally the same as that on the plot receiving 80 t/ ha 10/cm.
4. Discussion The first objective was to test how fast different size and density fractions respond to input of organic material. We found that the light and intermediate fractions of the macroorganic matter fraction were much more sensitive to input of organic residues than the other fractions. This is in line with the results of studies in other agricultural systems (Janzen et al., 1992; Barrios et al., 1996). In a previous study Hassink (1995b) found that the C:N ratio as well as the amounts of the light and intermediate macroorganic matter fractions were affected more by the long term differences in residue input than other fractions. The light and intermediate fractions of the macroorganic matter fraction can be used as sensitive indicators of changes in SOM. This enables us to detect effects of
Fig. 3. Relationship between C in the particle size fraction ,20 mm (clay and silt in g/kg soil) and the percentage of soil particles ,20 mm in the top 10 cm of grassland soils, the top 10 cm of an arable field in Tynaarlo, the top 20 cm of a maize field in Cranendonck and the top 25 cm of the FYM treatments of the long term experiment in the sandy and clay soil.
Fig. 4. The dynamics of organic carbon in the sandy soil with no input (O), and annual inputs of chaff (W), lucerne (B) and FYM (X) during a 25 year period. Lines are simulations, points are measured values.
agricultural practices on SOM dynamics within a much shorter period. The second objective was to test whether the amount of C that can become associated with clay and silt particles is limited and to quantify the relationship between soil texture and the maximum capacity of a soil for C to be associated with clay and silt particles. We observed a close positive correlation between the percentage of soil particles ,20 mm and the amount of C associated with these particles. We found that although the top layers of the sandy grassland soils in Tynaarlo and Cranendonck contained more C than the corresponding arable sites and the deeper soil layers, the amounts of C associated with the clay and silt fraction were not different. The increases in soil C were only observed in the larger size fractions. This suggests that the amount of C that can become associated with the clay and silt fraction had reached a maximum in these sandy soils. The hypothesis that soils have a limited capacity to protect C by their association with clay and silt particles is also confirmed by the observation that the relationship between soil texture and the amount of clay and silt associated C in the sandy and clay soil that had received FYM for 25 years was the same as for the grassland soils (Fig. 3). The observation that total soil C did not increase any further with FYM application (both in the sandy and clay soil; Figs. 4 and 5) also suggests that the protective capacity of the soils was saturated after 25 years of FYM application. We assume that the observed relationship between the percentage of soil particles ,20 mm and the amount
J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199
Fig. 5. The dynamics of organic carbon in the clay soil with no input (O), and annual inputs of chaff (W), lucerne (B) and FYM (X) during a 25 year period. Lines are simulations, points are measured values.
of C associated with this fraction can be used as a first estimation for the capacity of a soil to preserve C. Not only soil texture but also clay type may affect the capacity of soils to protect organic C physically. Specific surfaces of clays vary from 2–4 m2/g for quartz (Wilding et al., 1977), 6–39 m2/g for kaolinite (Dixon, 1977), 50–100 m2/g for illite to 800 m2/g for smectite and vermiculite (Robert and Chenu, 1992). Soil dominated by clays with a high specific surface area are expected to adsorp more humic substances than soils dominated by clays with a low specific surface area (Tate and Theng, 1980). Although the dominant clay minerals in Dutch sandy soils are quartz and kaolinite (clay minerals with a low specific surface) and the dominant clay minerals in fine-textured Dutch soils are illite and smectite (clay minerals with a higher specific surface) (Favejee, 1949; Marel, 1949; Breeuwsma, 1990), the clay and silt particles in the sandy soils had higher C contents than the clay and silt particles in fine-textured soils (Hassink et al., 1997). In most cases a decrease in pH favours the bonding of organics to clay particles (Varadachari et al., 1994). This might explain why the amount of clay and silt associated C was greater for the Tynaarlo soil (pH 4.4) than for the Cranendonck soil (pH 5.4; Table 3). The results suggest that exposure of clay particles to free organic matter and aggregation plays an important role in determining the actual protective capacity (X) in soil. Moreover, most clays with a low specific surface area have a relatively high ratio of external to internal surface area, whereas for clays with a high specific surface area the opposite is true (Robert and
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Chenu, 1992). Possibly, many organic compounds do not penetrate the interlayer space and adsorb only on the external surfaces (Tate and Theng, 1980). The third objective was to test whether a simple model that explicitly describes physical protection as a function of the capacity of clay and silt particles to hold organic matter could simulate the dynamics of soil C in different soils with different inputs assuming that the protective capacity (X) was directly related to soil texture as described by Hassink and Whitmore (1997). Except for the second half of the FYM experiment in the sand and clay soil, the results of the model were in line with the experimental values. The unexplained increase in soil C in the FYM experiment in the sand and clay soil might be due to the presence of chemically resistant C in FYM that accumulated during the course of the experiment, or due to the fact that with the application of FYM, soil particles are applied to the soil, possibly leading to a gradual increase in the protective capacity of the soil. In the coarse sandy soils of Tynaarlo and Cranendonck, increases in soil C under grassland were concentrated in the sand size fraction; in the heavier textured sandy soil of the long term experiment, the accumulation of C in the chaff, lucerne and FYM treatments mostly took place in the 20–150 mm fraction in contrast to the treatment with no application, while in the clay soil of the long term experiment the accumulation of C was concentrated in the ,20 mm fraction. The percentage of C accumulating in the ,20 mm fraction was less in the FYM treatment than in the chaff and lucerne treatments (Table 4).
Fig. 6. The dynamics of organic carbon in the clay-loam soil in the Czech Republic receiving 0 (X), 80 (B) or 160 (O) t/ha FYM annually for 31 years and no addition during a subsequent 5 years. Lines are simulations, points are measured values.
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This may be due to the fact that the ,20 mm fraction in the FYM treatments has reached its capacity. The results strengthen the assumption that the degree of saturation of the clay and silt fraction determines the physical protection of applied C and that with this description of physical protection we are able to simulate the long term dynamics of soil C under different conditions. We conclude that the light and intermediate fractions of the macroorganic matter are most sensitive to changes in C input and that the amount of macroorganic matter depends on soil management and is independent of soil texture, while the amount of clay and silt associated C is strongly affected by soil texture and only affected by management as long as the protective capacity of this fraction is not yet saturated. Our results are in agreement with results of Quiroga et al. (1996) who found for Argentinian soils that the amount of C associated with particles ,50 mm was only affected by soil texture and not by soil management, while coarse organic matter was strongly affected by soil management, but not by soil texture. We can conclude that the light and intermediate fractions of the macroorganic matter pool can be used as early indicators of effects of changes in soil management on the amount and quality of SOM. The capacity of soils to preserve SOM can be estimated from the clay and silt content of the soil. The observation that the capacity of soils to preserve SOM is limited has important consequences for the estimation of the long term behaviour of SOM and the estimation of the amounts of SOM that can be stored in soils in a sustainable way. References Amato, M. and Ladd, J.N., 1992. Decomposition of 14C-labelled glucose and legume materials in soil: properties influencing the accumulation of organic residue C and microbial biomass C. Soil Biol. Biochem., 24: 455–464. Barrios, E., Buresh, R.J. and Sprent, J.I., 1996. Organic matter in soil particle size and density fractions from maize and legume cropping systems. Soil Biol. Biochem., 28: 185–193. Bonde, T.A., Christensen, B.T. and Cerri, C.C., 1992. Dynamics of soil organic matter as reflected by natural 13C abundance in particle size fractions of forested and cultivated oxisols. Soil Biol. Biochem., 24: 275–277. Breeuwsma, A., 1990. Mineralogical composition of Dutch soils. In: W.P. Locher and H. de Bakker (Editors), Soil Science of the
Netherlands. Malmberg, Den Bosch, The Netherlands, pp. 103– 107 (in Dutch). Christensen, B.T., 1992. Physical fractionation of soil and organic matter in primary particle size and density separates. Adv. Soil Sci., 20: 1–89. De Haan, S., 1977. Humus, its formation, its relation with the mineral part of the soil, and its significance for soil productivity. In: Soil Organic Matter Studies, Vol. 1. International Atomic Energy Agency, Vienna. pp. 21–30. Dixon, J.B., 1977. Kaolinite and serpentine group minerals. In: R.C. Dinauer (Editor), Minerals in Soil Environments. SSSA, Madison, WI, pp. 357–403. Favejee, J.Ch.L., 1949.The mineralogical composition of the clay fraction of Dutch soils. Landbouwk. Tijdschr., 61: 167–171 (in Dutch). Feller, C., Fritsch, E., Poss, R. and Valentin, C., 1991. Effet de la texture sur le stockage et la dynamique des matieres organiques dans quelques soil ferrugineux et ferrallitiques. Cah. O.R.S.T.O.M. Ser. Pedol., 26: 25–36. Genstat Manual, 1987. A General Statistical Program. Clarendon, Oxford. Gregorich, E.G., Voroney, R.P. and Kachanoski, R.G., 1991. Turnover of carbon through the microbial biomass in soils with different textures. Soil Biol. Biochem., 23: 799–805. Hassink, J., 1994. Effects of soil texture and grassland management on soil organic C and N and rates of C and N mineralization. Soil Biol. Biochem., 26: 1221–1231. Hassink, J., 1995a. Decomposition rate constants of size and density fractions of soil organic matter. Soil Sci. Soc. Am. J., 59: 1631–1635. Hassink, J., 1995b. Density fractions of macroorganic matter and microbial biomass as predictors of C and N mineralization. Soil Biol. Biochem., 27: 1099–1108. Hassink, J., 1996. Preservation of plant residues in soils differing in unsaturated protective capacity. Soil Sci. Soc. Am. J., 60: 487– 491. Hassink, J. and Whitmore, A.P., 1997. A model of the physical protection of organic matter in soils. Soil Sci. Soc. Am. J., 61: 131–139. Hassink, J., Matus, F.J., Chenu, C. and Dalenberg, J.W., 1997. Interactions between soil biota, soil organic matter and soil structure. Adv. Agroecol., in press. Janzen, H.H., Campbell, C.A., Brandt, S.A., Lafond, G.P. and Townley-Smith, L., 1992. Light-fraction organic matter in soils from long term crop rotations. Soil Sci. Soc. Am. J., 56: 1799–1806. Jenkinson, D.S., 1988. Soil organic matter and its dynamics. In: A. Wild (Editor), Russel’s Soil Conditions and Plant Growth, 11th edition. Longman, New York, pp. 564–607. Jenkinson, D.S., Hart, P.B.S., Rayner, J.H. and Parry, L.C., 1988. Modelling the turnover of organic matter in long-term experiments at Rothamsted. Intecol Bull., 15: 1–8. Lugo, A.E. and Brown, S., 1993. Management of tropical soils as sinks or sources of atmospheric carbon. Plant Soil, 149: 27– 41. Mebius, L.J., 1960. A rapid method for the determination of organic carbon in soil. Anal. Chim. Acta, 22: 120–124.
J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199 Meijboom, F.W., Hassink, J. and Van Noordwijk, M., 1995. Density fractionation of soil macroorganic matter using silica suspensions. Soil Biol. Biochem., 27: 1109–1111. Nova`k, B. and Apfelthaler, R., 1971. Effect of manuring and fertilization on carbon and nitrogen transformations in soil. In: Studies about Humus. Transactions of the International Symposium Humus et Planta V. Prague, pp. 73–76. Paustian, K., Parton, W.J. and Persson, J., 1992. Modelling soil organic matter in organic-amended and nitrogen fertilized long-term plots. Soil Sci. Soc. Am. J., 56: 476–488. Quiroga, A.R., Buschaiazzo, D.E. and Peinemann, N., 1996. Soil organic matter particle size fractions in soils of the semiarid argentianian Pampas. Soil Sci., 161: 104–107. Robert, M. and Chenu, C., 1992. Interactions between soil minerals and microorganisms. In: G. Stotzky and J.M. Bollag (Editors), Soil Biochemistry, Vol. 7. Marcel Dekker, New York, pp. 307– 404. Schimel, D.S., Coleman, D.C. and Horton, K.A., 1985. Soil organic matter dynamics in paired rangeland and cropland toposequences in North Dakota. Geoderma, 36: 201–214. Tate, K.R. and Theng, B.K.G., 1980. Organic matter and its interactions with inorganic soil constituents. In: G.K.G. Theng (Editor), Soil with a Variable Charge. N. Z. Soc. Soil Sci., Lower Hutt, New Zealand, pp. 225–249. Theng, B.K.G., 1979. Formation and Properties of Clay-Polymer Complexes. Elsevier, Amsterdam.
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Tiessen, H. and Stewart, J.W.B., 1983. Particle-size fractions and their use in studies of soil organic matter. II. Cultivation effects on organic matter composition in size. Soil Sci. Soc. Am. J., 47: 509–514. Tisdall, J.M. and Oades, J.M., 1982. Organic matter and water stable aggregates in soils. J. Soil Sci., 13: 141–163. Varadachari, Ch., Mondal A.H., Nayak, D.C. and Ghosh, K., 1994. Clay-humus complexation: effect of pH and the nature of bonding. Soil Biol. Biochem., 26: 1145–1149. Van der Marel, H.W., 1949. Mineralogical composition of a heath podzol profile. Soil Sci., 67: 193–207. Van Veen, J.A. and Kuikman, P.J., 1990. Soil structural aspects of decomposition of organic matter by microorganisms. Biogeochemistry, 11: 213–233. Whitmore, A.P. and Matus, F.J., 1996. The decomposition of wheat and clover residues in soil measurements and modelling. Proc. 8th Nitrogen Workshop on Soils, Ghent. Whitmore, A.P., Bradbury, N.J. and Johnson, P.A., 1992. The potential contribution of ploughed grassland to nitrate leaching. Agric. Ecosys. Environ., 39: 221–233. Wilding, L.P., Smeck, N.E. and Drees, L.R., 1977. Silica in soils: quartz, crostobalite, tridymite and opal. In: D.C. Dinauer (Editor), Minerals in Soil Environments. SSSA, Madison, WI, pp. 471–552.
Size and density fractionation of soil organic matter and the physical capacity of soils to protect organic matter Jan Hassink a ,*, Andrew P. Whitmore, Jaromir Kuba´t b a
Research Institute for Agrobiology and Soil Fertility (AB-DLO), P.O. Box 129, 9750 AC Haren, The Netherlands b Research Institute of Crop Production, Drnovska´ 507, 16106 Praha 6, Ruzyne, Czech Republic Accepted 16 May 1997
Abstract Soil organic matter (SOM) has important chemical, physical and biological functions in the soil. It is difficult to predict the dynamics of SOM because it is very heterogeneous and because its behaviour is affected by soil texture. In this study we used a new size and density fractionation to isolate SOM fractions that differ in stability and we estimated the amount of SOM that can be preserved in different soils. An investigation was carried out into (1) how fast size and density fractions of soil organic matter respond to changes in C input, (2) whether the capacity of soils to preserve C by its association with clay and silt particles is limited and related to soil texture and (3) whether the long term dynamics of soil C can be described with a simple model that makes the assumption that the net rate of decomposition of soil C does not simply depend on soil texture, but on the degree to which the protective capacity of the soil is already occupied. Light and intermediate fractions of the macroorganic matter (.150 mm) respond much faster to changes in C input than smaller size fractions. This shows that the light and intermediate macroorganic matter fractions can be used as early indicators of effects of soil management on changes in SOM. There was a close positive relationship between the proportion of particles ,20 mm in a soil and the amount of C associated with this fraction in the top 10 cm of grassland soils. Arable sandy soils, which contained less C than corresponding grassland soils, had the same amounts of C associated with the fraction ,20 mm, indicating that the amount of C that can become associated with this fraction had reached a maximum. The observed relationship: C in fraction ,20 mm (g/kg soil) = 6.9 + 0.29× % particles ,20 mm can be used as a first estimation for the capacity of a soil to preserve C. The amount of C in macroorganic matter is controlled by soil management, while the amount of C protected by clay and silt particles is controlled mainly by soil texture. The simulations of the changes in C in soil without input of C or with additions of lucerne or chaff were excellent in both sandy and clay soils. The build-up of C in soils receiving farmyard manure (FYM) was not simulated so well. 1997 Elsevier Science B.V. Keywords: Soil organic matter; Size and density fractionation; Physical protection; Capacity; Modelling
1. Introduction Organic matter is a key component of soil which affects its physical, chemical and biological proper* Corresponding author. Tel.: +31 50 5337320; fax: +31 50 5337291; e-mail: [email protected]
ties. Organic matter improves soil structure, increases the water holding capacity and promotes biological transformations such as N-mineralization. Maintenance of sufficiently high levels of organic matter is a prerequisite for sustainable, high, production levels of crops. Agricultural management practices influence the amount of organic matter in soil and cause
1161-0301/97/$17.00 1997 Elsevier Science B.V. All rights reserved PII S1161-0301 (97 )0 0045-2
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changes in the rate of soil organic matter turnover. It often takes years, however, before changes in agricultural management lead to detectable changes in the quantity and quality of soil organic matter (SOM). This makes it difficult to evaluate the effect of different management strategies on SOM dynamics. Several models have been constructed to predict organic matter dynamics. SOM is very heterogeneous and is composed of a series of pools from very active to passive (Schimel et al., 1985). To account for this, models of organic matter dynamics generally include compartments with a rapid turnover rate and a slower turnover rate. A major problem related to predicting SOM dynamics is that, except for the microbial biomass, the different compartments can not be determined directly by chemical or physical fractionation procedures (Paustian et al., 1992). Successful development of techniques for direct measurement of pool sizes would represent a major step towards appropriate verification of models (Bonde et al., 1992). Size and density fractionation show promise for physically dividing SOM into pools differing in composition and biological function (Christensen, 1992). Size fractionation is based on the observation that sand-size organic matter (macroorganic matter; .150 mm) is often more labile than organic C in the clay and silt size fractions (Tiessen and Stewart, 1983; Feller et al., 1991). Density fractionation is based on the observation that during humification parts of SOM become more associated with mineral particles and thus occur in particles of higher density (Barrios et al., 1996). Meijboom et al. (1995) have developed a new and simple density fractionation procedure using silica suspensions and recovered three density fractions in the macroorganic matter: the light fraction consisting of recognizable plant residues, the intermediate fraction of partly humified material and the heavy fraction of amorphous organic material. It was shown that the decomposition rates decrease in the order light, intermediate and heavy macroorganic matter, while the decay rates of C in the clay and silt size fractions were lowest (Hassink, 1995a). Size and density fractionation might enable us to identify labile fractions that respond much faster to changes in organic matter input than total SOM and can serve as sensitive indicators of changes in the SOM content (Janzen et al., 1992; Barrios et al., 1996). It has been recognized that soil texture and soil
structure have a predominant effect on organic matter decomposition. It is generally accepted that there is more physical protection in fine-textured soils than in coarse-textured soils (Jenkinson, 1988; Van Veen and Kuikman, 1990). As a consequence fine-textured soils have higher organic C contents than coarse textured soils when supplied with similar input of material (Jenkinson, 1988; Hassink, 1994). Considerable published evidence indicates that one of the principal factors responsible for physical protection of organic matter in soils is its ability to associate with clay and silt particles (Theng, 1979). Little is known, however, about the capacity of soils to protect organic matter physically. Hassink et al. (1997) suggested that the physical capacity of a soil to preserve organic matter is limited. Hassink (1996) defined the protective capacity as the maximum amount of C that can be associated with clay and silt particles in the soil. He suggested that the degree of saturation of the protective capacity of a soil would affect the preservation of newly added carbon in residues and not soil texture per se. Less of the applied C would be preserved in the soil when all protective sites were occupied than when sites were available to stabilize organic C. This explains why the preservation of applied C is directly related to soil texture when C is applied to fine-textured soils with low organic matter contents (e.g. Amato and Ladd, 1992) and why there is no correlation between soil texture and preservation of applied C in soil when fine-textured soils with higher organic matter contents are used (e.g. Gregorich et al., 1991). Based on the assumptions that (1) the preservation of applied C is controlled by the degree of saturation of the clay and silt size fractions with SOM (instead of soil texture per se) and (2) that the protection of SOM can be described kinetically in the same way as adsorption and desorption, a simple simulation model was developed that predicts the long term dynamics of SOM (Hassink and Whitmore, 1997). Short term changes in SOM may best be studied by focusing on the labile organic matter fractions, while long term dynamics of SOM are probably affected mainly by the capacity of a soil to protect organic C physically. The objectives of the present study are: (1) to determine how fast size and density fractions of SOM respond to changes in input of organic inputs and to identify measurable indicators most sensitive to changes in SOM; (2) to estimate the amount of C that
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can be protected physically by association with clay and silt particles (,20 mm); and (3) to test whether a model that explicitly describes physical protection as a function of the capacity of clay and silt particles to hold organic matter, can simulate the long term dynamics of C in sandy and clay soils in the Netherlands that received either no organic C or annual applications of different organic materials and a clay-loam soil in the Czech Republic that received either no organic C or annual applications of manure.
2. Materials and methods 2.1. Experimental sites To determine how fast size and density fractions of SOM respond to changes in input of organic inputs and to identify indicators most sensitive to changes in SOM (objective 1), we defined and fractionated SOM from sand and clay soils that had received no input and samples of the same soils that had received annual applications of different organic materials. The soils were located in Haren, in the Northern part of the Netherlands. Characteristics of the soils at the begin-
ning of the experiment (1961) are given in Table 1. The soils have been kept bare since 1961 and received (1) no organic C and N or (2) 10 t C per ha/year by the application of either lucerne (2.5% N, 43.6% C), wheat chaff (0.8% N, 42.5% C) or farmyard manure (FYM; 2.9% N, 42.7% C). The residues were applied at the beginning of June and mixed through the top 25 cm of the soil. Soil samples of the top 25 cm were taken at 0.5, 3, 8, 15 and 25 years after the start of the experiment. To estimate the amount of C that can be protected physically by association with clay and silt particles (objective 2), we sampled the top 10 cm of grassland soils which had been under grass for at least 30 years and determined C in size and density fractions. Grassland soils generally have higher C contents than arable soils, because they receive more organic C each year and are not tilled (Jenkinson, 1988; Lugo and Brown, 1993). The grasslands were grazed by dairy cattle and received 400–500 kg fertilizer-N ha/year. Characteristics of the grassland soils are presented in Table 1. In the sandy soil of Tynaarlo we compared the distribution of C between organic matter fractions in soil from a grassland field with soil from an adjacent arable field which had been under a 4 year rotation of winter wheat, sugarbeet, barley and ware potatoes. In the
Table 1 Characteristics of the top 25 cm of the sandy and clay soils at the beginning of the long term experiment in 1961 (objective 1) and the top 10 cm of the grassland soils used for objective 2 Location
Objective 1 Haren Sand Clay Objective 2 Achterberg Burum Zaltbommel 1 Zaltbommel 2 Heino Finsterwolde 1 Finsterwolde 2 Tynaarlo Cranendonck
% particles
pH (K–Cl)
,20 mm
Total Soil C (g/kg)
,2 mm
,20 mm
ND ND
17.0 65.6
7.4 7.0
7.2 15.2
2.7 6.6
0.4 0.4
10.1 22.2
3.0 24.1 25.8 51.1 1.9 8.4 45.8 2.6 1.0
5.8 36.5 42.6 76.0 2.8 13.3 65.9 5.0 3.5
4.9 4.8 5.8 5.4 5.0 5.0 7.1 4.4 5.4
9.7 21.2 19.9 31.0 1.4 11.3 20.3 13.0 3.6
5.8a 11.9a 14.3a 25.2a 14.4a 34.7a 7.1 22.2a 3.3
7.9 8.8 5.7 4.5 3.9 7.7 4.6 5.9 6.7
23.4 41.9 39.9 60.7 19.8 53.7 30.2 41.1 14.2
ND, Not determined. Calculated as total soil C − C in fractions ,20 and .150 mm.
a
C in size fraction (g/kg) 20–150 mm
.150 mm
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sandy soil of Cranendonck we compared the distribution of C among organic matter fractions in a 30 year old grassland field with a field that had been under maize for at least 25 years. We also compared the distribution of organic matter in the top layer (0–20 cm) with deeper soil layers (30–40 and 60–80 cm deep) at this one location. To test our model that explicitly describes physical protection (objective 3), we simulated the long term dynamics of soil C in the sand and the clay soil receiving lucerne, chaff or FYM residues annually since 1961 (see above) and a clay loam from Ruzyne in the Czech Republic receiving 0, 80 or 160 t/ha FYM annually.
referred to as particle size fraction 20–150 mm. The suspension passing the 20 mm sieve was placed at 4°C till it had sedimented (usually after 24 h) and the clear solution was sucked off; this fraction will be referred to as particle size fraction ,20 mm. It was assumed that the particle size fractions 20–150 mm and ,20 mm were derived primarily from microaggregates as microaggregates are resistant to wet-sieving (Tisdall and Oades, 1982). All fractions were dried at 40°C and analyzed for total C and N. In most of the grassland soils (indicated in Table 1), the amount of C in the 20–150 mm fraction was not determined, but calculated as the difference between total soil C and C in the fractions ,20 mm and .150 mm.
2.2. Size and density fractionation of soil organic matter
2.3. Modelling
Dried soil samples taken from the long term experiment on the sand and clay soil that had been stored were rewetted before fractionation. From the other soils field-moist samples were used for fractionation. We determined five fractions: the light, intermediate and heavy fractions of the macroorganic matter fraction (.150 mm) and the size fractions 20–150 mm and ,20 mm. For the density fractionation of the macroorganic matter, samples of 250 g were washed through two sieves (top sieve, mesh size 250 mm; bottom sieve, 150 mm). The soil was pushed through the top sieve, till the water passing the sieve became clear. In this way all macroaggregates were destroyed. The mineral fraction was discarded by decantation. After combining the organic fractions from both sieves, it was further fractionated in silica suspensions with a density of 1.13 and 1.37 g/cm3 as described by Meijboom et al. (1995). The macroorganic matter was separated into three fractions: a light fraction (density ,1.13 g/cm3); an intermediate fraction (density between 1.13 and 1.37 g/cm3) and a heavy fraction (density .1.37 g/cm3). To isolate the finer fractions, samples of 50 g were washed on three sieves (top sieve, mesh size 250 mm; second sieve, 150 mm; bottom sieve, 20 mm). The suspension passing the bottom sieve of 20 mm was collected in a bucket. The soil was pushed through the top sieve again (destroying the macroaggregates), till the water passing the sieve became clear. The material accumulating on the 20 mm sieve will be
Hassink and Whitmore (1997) have suggested a model for the dynamics of the physical protection of organic matter in soil based on a mechanism analogous to adsorption and desorption. Two SOM pools are distinguished: Protected (POM) and Non-protected (NOM) organic matter. NOM is attacked by soil microorganisms relatively easily, whereas POM cannot be decomposed unless it is first released by desorption. Assuming that the capacity of soil to protect organic matter (protective capacity; X) has a maximum value that cannot be exceeded, it is likely that the rate at which NOM becomes protected will slow down as this maximum is approached. The rate of desorption of POM on the other hand, is independent of X. Hassink and Whitmore (1997) found a good relationship between the clay content of a soil and X (maximum amount of SOM that can be protected by the soil). Based on this relationship we estimated X for three different soils: the sand and the clay soil receiving lucerne, chaff or FYM residues annually since 1961 (see above under objective 1) and a clay loam from Ruzyne in the Czech Republic. Composted manure was applied annually to this last soil at the rate of 0, 80 or 160 t/ha containing 0, 8 or 16 t/ha carbon (Nova`k and Apfelthaler, 1971). Additions stopped after 31 years of the experiment and the decrease in soil C was determined for another 5 years. Manure was added to the top 10 cm (80 t/ha) or top 20 cm (160 t/ha) of the experiment and at least in the first year therefore the concentrations of added organic matter were the same. However, because manure
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J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199 Table 2
Amount of C in the light, intermediate and heavy macroorganic matter fractions and the fractions 20–150 mm and , 20 mm in the top 25 cm of the bare sandy and clay soil (Haren) that received no organic C and N 0.5, 3, 8, 15 and 25 years after the start of the experiment (g/kg soil) Fraction
Time (years) 0.5
Sandy soil Light Intermediate Heavy 20–150 mm ,20 mm Clay soil Light Intermediate Heavy 20–150 mm ,20 mm
3
8
15
25
0.06 0.06 0.24 2.70 7.23
0.05 0.08 0.19 3.20 6.30
0.02 0.04 0.14 2.85 6.11
0.01 0.02 0.12 3.18 5.16
0.01 0.04 0.17 3.89 5.12
0.08 0.10 0.35 6.63 15.22
0.07 0.06 0.30 6.02 14.02
0.02 0.04 0.20 5.32 13.23
0.02 0.02 0.23 5.78 11.26
0.04 0.06 0.28 6.38 10.23
was added to the same depth of soil each year, the mass of soil in this depth decreased as the bulk density decreased, and the effective concentration of manure added (per mass of soil) increased each year. Both soils were sampled to 20 cm depth and this means that the apparent concentration of organic matter in the soil receiving 160 t/ha increased more quickly than in the soil receiving 80 t/ha. Corrections for the change in bulk density were made using a relationship derived by Whitmore et al. (1992). Lucerne, chaff and manure contained respectively 8%, 11.2% and 29% lignin (Haan de, 1977); Whitmore and Matus (1996) give a formula for reducing the rate of decomposition of crop residues in relation to their lignin content. The manure added to the Ruzyne experiment was assumed to contain 29% lignin also, but because it was composted we further assumed that 5% would be chemically resistant to attack and not decompose during the course of the experiment (e.g. Jenkinson et al., 1988). 2.4. Chemical analysis Total C in soil, in macroorganic matter fractions and in the particle size fraction 20–150 mm was defined as dichromate-oxidizable C according to Kurmies (Mebius, 1960). Total C in the particle size fraction ,20 mm was determined with a CHN autoanalyzer (Carlo Erba NA 1500).
2.5. Statistical analysis The relationships between the percentage of soil particles ,20 mm and total soil C and C associated with the particle size fractions ,20 mm, 20–150 mm and .150 mm were analyzed with correlation and regression techniques (Genstat, 1987).
3. Results The identification of soil organic matter fractions that are sensitive to changes in input of organic materials (sandy and clay soil of objective 1) At all sampling times, more than 90% of all soil carbon was present in the fractions 20–150 mm and ,20 mm in the treatments where no organic material was applied to the soil (Table 2tablehere>). The amount of C in the heavy macroorganic matter fraction and the 20–150 mm fractions did not change significantly with time, while C in the other fractions decreased with time. The relative changes in the amounts of C in size and density fractions due to the application of chaff, lucerne and FYM are expressed as the amounts of C in the size and density fractions in the chaff, lucerne and FYM treatments, divided by the corresponding amounts in the treatments where no organic inputs were applied 0.5, 3, 8, 15 and 25 years after the start of the treatments. There were no
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The relative changes were larger for the FYM treatment than for the lucerne and chaff treatments. The application of FYM increased the amount of C in the heavy macroorganic matter fraction relatively more than the application of lucerne or chaff (Figs. 1 and 2). 3.1. Relationship between soil texture and C in organic matter fractions in grassland soils (soils of objective 2) Fig. 1. The amounts of C in the light, intermediate and heavy macroorganic matter fractions and the 20–150 mm and ,20 mm fractions in the chaff treatment divided by the corresponding amounts of C in the fractions in the no input treatment 0.5, 3, 8, 15 and 25 years after the start of the experiment. Average of the sandy and clay soil.
differences in the relative changes of C in the fractions between the sand and clay soils and between the lucerne and chaff treatments. The amounts of C in each of the fractions derived from the chaff and FYM treatments divided by the corresponding amounts in the treatments receiving no input were averaged for the sand and clay soils (Figs. 1 and 2). Generally, when organic materials were added to the clay and sand soil pronounced changes took place quickly in the amounts of C in the light and intermediate fractions of the macroorganic matter pool. The heavy fraction of the macroorganic matter pool responded more slowly, while changes in the fractions 20–150 mm and the ,20 mm were relatively slow and small (Fig. 1 and 2). Within a few years of application of organic materials, changes in the light and intermediate fractions of the macroorganic matter fraction could be determined easily. The relative increases in the light and intermediate fractions of the macroorganic matter pool reached a maximum within 8–15 years after the start of input of organic material. At this maximum, the amounts of C in the light and intermediate macroorganic matter fractions were 6–23 times greater in the treatments where organic material was applied than in the treatment where there was no input of organic material. The amount of C in the heavy macroorganic matter fraction and the fractions 20–150 mm and the ,20 mm fraction continued to increase relative to the nil input treatments. After 25 years, the relative increases ranged from 4 to 19 for the heavy macroorganic matter fraction and from 1.3 to 4.4 for the 20–150 mm and the ,20 mm fractions.
We investigated the relationships between soil texture and organic C in different organic matter fractions. For the fraction .150 mm, we present the sum only of the light, intermediate and heavy fractions, because the density fractions did not give any additional information with respect to the capacity of soils to protect C physically. There was a highly significant correlation (r = 0.91) between the clay and silt content of grassland soils and the amount of C associated with this fraction (Table 1; Fig. 3; C associated with the fraction ,20 mm = 6.9 + 0.29× % particles ,20 mm). The amounts of C in the fractions 20–150 mm and .150 mm did not correlate significantly with the clay and silt fraction and varied considerably between soils with similar clay and silt content (Table 1). C in size fractions in soils with similar textures but differing in organic matter input (Tynaarlo and Cranendonck soil of objective 2 and the sandy and clay soil of objective 1) In spite of the fact that the arable field from Tynaarlo, the maize field from Cranendonck and the
Fig. 2. The amounts of C in the light, intermediate and heavy macroorganic matter fractions and the 20–150 mm and ,20 mm fractions in the FYM treatment divided by the corresponding amounts of C in the fractions in the no input treatment 0.5, 3, 8, 15 and 25 years after the start of the experiment. Average of the sandy and clay soil.
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J. Hassink et al. / European Journal of Agronomy 7 (1997) 189–199 Table 3 Amounts of C in different size fractions and total soil C in the top 10 cm of the grassland and arable soil in Tynaarlo and the top 20 cm and the soil layers at 30–40 and 60–80 cm depth in the grassland and maize field in Cranendonck (g/kg) Location
Tynaarlo Grassland Arable Cranendonck 0–20 cm grass 30–40 cm grass 60–80 cm grass 0–20 cm maize 30–40 cm maize 60–80 cm maize
C in fraction (treatment)
Total soil C
,20 mm
20–150 mm
.150 mm
11.9 11.8
9.6 5.4
11.6 2.8
36.3 23.8
3.6 3.4 3.4 3.6 3.5 3.1
4.7 4.0 3.6 3.1 2.1 1.7
6.7 3.5 2.8 2.6 1.2 0.9
15.9 12.1 9.7 8.8 7.1 7.2
fractions was generally close to the total amount of soil C (Table 3). The application of chaff, lucerne and FYM to the sand and clay soil for 25 years (long term experiment of objective 1) resulted in a considerable increase in total soil C in comparison with the treatment where no organic residues were applied (Table 4). In the sandy soil, the increase was concentrated in the 20–150 mm fraction, while in the clay soil most of the increase took place in the ,20 mm fraction (Table 4). In the clay soil, the sum of C in the fractions was close to the total amount of soil C; in the sandy soil, the sum of C in the fractions was 40% higher than total soil C for the no input treatment, while in the other treatments the sum of C in the fractions was again close to total soil C (Table 4). No explanation can be offered for the high recovery in the sand soil without input. 3.2. Modelling (sandy and clay soil of objective 1 and Czech soil at Ruzyne)
deeper layers of the grassland and maize field from Cranendonck had much lower total amounts of soil C than the corresponding top layers of the grassland fields, the amounts of C associated with the clay and silt fraction were not less (Table 3). The amounts of C in the fractions 20–150 mm and .150 mm were less in the arable field from Tynaarlo and the maize field from Cranendonck than in the corresponding grassland sites, and decreased with increasing depth at Cranendonck (Table 3). The sum of C in the different
The simulations of the increase in total soil C in soil with addition of lucerne or chaff, or the decrease in C in soil in the treatments without addition were excellent (Figs. 4 and 5) in both the sand and clay soils. The simulated build-up of organic matter in the soils receiving FYM also agreed well with the measurements during the first 13 years. During the second half of the experiment, however, a sharp increase in soil C was measured, which could not be simulated
Table 4 Amounts of C in different size fractions after 25 years of no input or annual applications of chaff, lucerne and FYM (g/kg) and the percentage of increase in soil C (in comparison with the no input treatment) that is associated with the fraction , 20 mm for the sand and clay soil in Haren
Sandy soil No input Chaff Lucerne FYM Clay soil No input Chaff Lucerne FYM
,20 mm
20–150 mm
.150 mm
5.1 7.5 8.5 12.4
3.9 9.4 10.1 21.5
0.2 1.6 1.1 5.5
25.5 32.0 24.1
6.6 16.8 16.5 42.5
10.2 15.4 16.5 26.0
6.4 9.7 10.0 19.2
0.4 1.6 0.9 4.3
53.2 60.8 48.6
14.7 25.9 25.0 52.7
Increase in fraction , 20 mm (% of total increase)
Total soil C
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(Figs. 4 and 5). In all treatments receiving additions, an odd decline in soil C during the last 3 years of the experiment was measured. The build-up and decline of organic C at Ruzyne was simulated well (Fig. 6) and it is particularly interesting to see that simulating the change in bulk density allowed us to reproduce the extra increase in concentration of organic C (in g per C/kg soil) in the plot receiving 160 t/ha 20/cm each year despite the fact that the addition on this plot was nominally the same as that on the plot receiving 80 t/ ha 10/cm.
4. Discussion The first objective was to test how fast different size and density fractions respond to input of organic material. We found that the light and intermediate fractions of the macroorganic matter fraction were much more sensitive to input of organic residues than the other fractions. This is in line with the results of studies in other agricultural systems (Janzen et al., 1992; Barrios et al., 1996). In a previous study Hassink (1995b) found that the C:N ratio as well as the amounts of the light and intermediate macroorganic matter fractions were affected more by the long term differences in residue input than other fractions. The light and intermediate fractions of the macroorganic matter fraction can be used as sensitive indicators of changes in SOM. This enables us to detect effects of
Fig. 3. Relationship between C in the particle size fraction ,20 mm (clay and silt in g/kg soil) and the percentage of soil particles ,20 mm in the top 10 cm of grassland soils, the top 10 cm of an arable field in Tynaarlo, the top 20 cm of a maize field in Cranendonck and the top 25 cm of the FYM treatments of the long term experiment in the sandy and clay soil.
Fig. 4. The dynamics of organic carbon in the sandy soil with no input (O), and annual inputs of chaff (W), lucerne (B) and FYM (X) during a 25 year period. Lines are simulations, points are measured values.
agricultural practices on SOM dynamics within a much shorter period. The second objective was to test whether the amount of C that can become associated with clay and silt particles is limited and to quantify the relationship between soil texture and the maximum capacity of a soil for C to be associated with clay and silt particles. We observed a close positive correlation between the percentage of soil particles ,20 mm and the amount of C associated with these particles. We found that although the top layers of the sandy grassland soils in Tynaarlo and Cranendonck contained more C than the corresponding arable sites and the deeper soil layers, the amounts of C associated with the clay and silt fraction were not different. The increases in soil C were only observed in the larger size fractions. This suggests that the amount of C that can become associated with the clay and silt fraction had reached a maximum in these sandy soils. The hypothesis that soils have a limited capacity to protect C by their association with clay and silt particles is also confirmed by the observation that the relationship between soil texture and the amount of clay and silt associated C in the sandy and clay soil that had received FYM for 25 years was the same as for the grassland soils (Fig. 3). The observation that total soil C did not increase any further with FYM application (both in the sandy and clay soil; Figs. 4 and 5) also suggests that the protective capacity of the soils was saturated after 25 years of FYM application. We assume that the observed relationship between the percentage of soil particles ,20 mm and the amount
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Fig. 5. The dynamics of organic carbon in the clay soil with no input (O), and annual inputs of chaff (W), lucerne (B) and FYM (X) during a 25 year period. Lines are simulations, points are measured values.
of C associated with this fraction can be used as a first estimation for the capacity of a soil to preserve C. Not only soil texture but also clay type may affect the capacity of soils to protect organic C physically. Specific surfaces of clays vary from 2–4 m2/g for quartz (Wilding et al., 1977), 6–39 m2/g for kaolinite (Dixon, 1977), 50–100 m2/g for illite to 800 m2/g for smectite and vermiculite (Robert and Chenu, 1992). Soil dominated by clays with a high specific surface area are expected to adsorp more humic substances than soils dominated by clays with a low specific surface area (Tate and Theng, 1980). Although the dominant clay minerals in Dutch sandy soils are quartz and kaolinite (clay minerals with a low specific surface) and the dominant clay minerals in fine-textured Dutch soils are illite and smectite (clay minerals with a higher specific surface) (Favejee, 1949; Marel, 1949; Breeuwsma, 1990), the clay and silt particles in the sandy soils had higher C contents than the clay and silt particles in fine-textured soils (Hassink et al., 1997). In most cases a decrease in pH favours the bonding of organics to clay particles (Varadachari et al., 1994). This might explain why the amount of clay and silt associated C was greater for the Tynaarlo soil (pH 4.4) than for the Cranendonck soil (pH 5.4; Table 3). The results suggest that exposure of clay particles to free organic matter and aggregation plays an important role in determining the actual protective capacity (X) in soil. Moreover, most clays with a low specific surface area have a relatively high ratio of external to internal surface area, whereas for clays with a high specific surface area the opposite is true (Robert and
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Chenu, 1992). Possibly, many organic compounds do not penetrate the interlayer space and adsorb only on the external surfaces (Tate and Theng, 1980). The third objective was to test whether a simple model that explicitly describes physical protection as a function of the capacity of clay and silt particles to hold organic matter could simulate the dynamics of soil C in different soils with different inputs assuming that the protective capacity (X) was directly related to soil texture as described by Hassink and Whitmore (1997). Except for the second half of the FYM experiment in the sand and clay soil, the results of the model were in line with the experimental values. The unexplained increase in soil C in the FYM experiment in the sand and clay soil might be due to the presence of chemically resistant C in FYM that accumulated during the course of the experiment, or due to the fact that with the application of FYM, soil particles are applied to the soil, possibly leading to a gradual increase in the protective capacity of the soil. In the coarse sandy soils of Tynaarlo and Cranendonck, increases in soil C under grassland were concentrated in the sand size fraction; in the heavier textured sandy soil of the long term experiment, the accumulation of C in the chaff, lucerne and FYM treatments mostly took place in the 20–150 mm fraction in contrast to the treatment with no application, while in the clay soil of the long term experiment the accumulation of C was concentrated in the ,20 mm fraction. The percentage of C accumulating in the ,20 mm fraction was less in the FYM treatment than in the chaff and lucerne treatments (Table 4).
Fig. 6. The dynamics of organic carbon in the clay-loam soil in the Czech Republic receiving 0 (X), 80 (B) or 160 (O) t/ha FYM annually for 31 years and no addition during a subsequent 5 years. Lines are simulations, points are measured values.
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This may be due to the fact that the ,20 mm fraction in the FYM treatments has reached its capacity. The results strengthen the assumption that the degree of saturation of the clay and silt fraction determines the physical protection of applied C and that with this description of physical protection we are able to simulate the long term dynamics of soil C under different conditions. We conclude that the light and intermediate fractions of the macroorganic matter are most sensitive to changes in C input and that the amount of macroorganic matter depends on soil management and is independent of soil texture, while the amount of clay and silt associated C is strongly affected by soil texture and only affected by management as long as the protective capacity of this fraction is not yet saturated. Our results are in agreement with results of Quiroga et al. (1996) who found for Argentinian soils that the amount of C associated with particles ,50 mm was only affected by soil texture and not by soil management, while coarse organic matter was strongly affected by soil management, but not by soil texture. We can conclude that the light and intermediate fractions of the macroorganic matter pool can be used as early indicators of effects of changes in soil management on the amount and quality of SOM. The capacity of soils to preserve SOM can be estimated from the clay and silt content of the soil. The observation that the capacity of soils to preserve SOM is limited has important consequences for the estimation of the long term behaviour of SOM and the estimation of the amounts of SOM that can be stored in soils in a sustainable way. References Amato, M. and Ladd, J.N., 1992. Decomposition of 14C-labelled glucose and legume materials in soil: properties influencing the accumulation of organic residue C and microbial biomass C. Soil Biol. Biochem., 24: 455–464. Barrios, E., Buresh, R.J. and Sprent, J.I., 1996. Organic matter in soil particle size and density fractions from maize and legume cropping systems. Soil Biol. Biochem., 28: 185–193. Bonde, T.A., Christensen, B.T. and Cerri, C.C., 1992. Dynamics of soil organic matter as reflected by natural 13C abundance in particle size fractions of forested and cultivated oxisols. Soil Biol. Biochem., 24: 275–277. Breeuwsma, A., 1990. Mineralogical composition of Dutch soils. In: W.P. Locher and H. de Bakker (Editors), Soil Science of the
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