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Published in Vadose Zone Journal 3:714-721 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

ORIGINAL RESEARCH

Hydraulic Properties of Soil–Straw Mixtures

P. Garnier*,a, N. Ezzinea, S. De Gryzeb and G. Richarda

a INRA, rue Fernand Christ, 02000 Laon, France
b Laboratory for Soil and Water Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium
* Corresponding author (garnier{at}laon.inra.fr).

Received 18 June 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Crop residue management at the soil surface controls C sequestration and soil erosion. The size and distribution of crop residues incorporated into the topsoil affect soil physical properties. The objective of this work was to analyze the immediate effect of straw incorporation on the hydraulic properties of soil. Laboratory experiments were performed to determine the retention and the hydraulic conductivity curves of soil and wheat (Triticum aestivum L.) straw mixtures. The samples consisted of 1-cm straw pieces mixed with loamy soil aggregates of 2 to 3 mm in diameter. Volumetric straw contents were 0, 0.1, 0.2, and 0.3 cm3 cm–3. Suction table and pressure extractor methods were used to measure the water retention curve. Hydraulic conductivity curves were obtained using Wind's evaporation method. At a constant water pressure head, gravimetric water content increased with increasing straw content. The relationship between water pressure and volumetric water content did not differ significantly between the treatments. An additive model based on the sum of the water retention of straw and soil measured separately gave a good estimation of the water retention measured in the mixture. Hydraulic conductivity decreased with increasing straw content for the same water pressure head. Tomographic images showed that mixing straw into the soil created new pores in the samples. Results from simulations of straw decomposition indicated that (i) the uptake of water by straw has to be taken into account when calculating how much water should be added gravimetrically in an incubation experiment and (ii) changes in hydraulic conductivity due to the presence of straw can have an impact on water and C dynamics.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE MANAGEMENT OF organic residues has received much attention in recent years, especially with respect to the control of soil erosion and C sequestration. Changes in practices aimed at sustainable agriculture have resulted in very diverse methods of organic residue management with larger areas under minimum or no tillage. The fragmentation and distribution of residues are influenced by the type of residue management implemented. These will affect soil physical properties and contact between soil and residues. The conditions of initial residue placement have consequences on water infiltration, thermal regime, and residue decomposition.

Many studies described the effect of crop residue left at the soil surface on the thermal and hydraulic regime (Bristow et al., 1986; Bussière and Cellier, 1994). From these studies, it was generally concluded that mulch protects soil from evaporation. In addition, mulching increases soil roughness and decreases runoff. Soil crusting is reduced and infiltrability is increased (Baumhardt and Lascano, 1996). To simulate these situations, it is necessary to obtain the hydraulic properties of the mulch (e.g., retention curve) (Gonzalez-Sosa et al., 1999; Findeling, 2001).

A number of studies also evaluated the effect of exogenous organic matter on soil bulk density, aggregation, water retention, and water infiltration. First, organic amendments decrease soil bulk density due to the "dilution effect" of the added organic matter with the denser mineral fraction (Gupta et al., 1977). Second, added organic matter increases soil aggregation due to organic C produced by decomposition (Tisdall and Oades, 1982). Furthermore, additions of organic matter lead to more abundant macropores. The amount of water retained by the soil increases with an increasing amount of organic amendments incorporated in the soil (Gupta et al., 1977). Martens and Frankenberger (1992) showed that adding organic matter increases macroporosity and water infiltration rates, but additions can decrease hydraulic conductivity at lower pressure heads (Gupta et al., 1977). However, the effect of organic matter incorporation on hydraulic properties is often neglected when simulating water flow in agricultural fields.

Microbial decomposition of crop residues depends on the physical environment in the soil, including water content and water potential (Andrén et al., 1992). Myrold et al. (1981) and Quemada and Cabrera (2001) compared water retention curves of crop residues and soil. They found a similar shape but a higher water-holding capacity for the residues than for the soil. According to Myrold et al. (1981), the retention curve of residues should be used to calculate the gravimetric water content of the soil–residue mixture. In most laboratory incubation experiments, water content is mainly calculated on the basis of gravimetric water content of soil without the addition of residue. Only few studies performed incubation experiments by applying the same water pressure heads for all treatments (soil with and without residues) (e.g., Corbeels et al., 2003). Taking into account the impact of residues on the retention curve of soil is necessary to calculate C and N mineralization more accurately under standard conditions. Subsequently, it would be useful to extrapolate the decomposition with varying water content changes under field conditions.

The goal of this work was to determine and analyze the effect of undecomposed straw on the water retention and hydraulic conductivity curves of soil. We tested whether classic methods to measure water retention curves of soils (e.g., suction table and pressure extractor) could also be used for straw residues. We analyzed the contribution of straw to the water retention curve of the straw and soil mixture. Finally, we performed numerical tests on the impact of these properties on the residue decomposition rate.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Straw and Soil
The straw (Scipion cultivar), already described by Annoussamy et al. (2000), came from an experimental field at Boigneville (35 km south of Paris). Annoussamy et al. (2000) analyzed the geometry of the straw. From 100 wheat internodes of 20-cm length, they found an external mean diameter of 3.22 mm, a mean thickness of 0.53 mm, and a mean density of 0.116 g cm–3. They used a micrometer (Mitutoyo, Kanagawa, Japan) for diameter and thickness measurements. In our experiment, the straw was immersed in NaClO solution (0.05 M) for 3 h at 3°C to avoid decomposition. It was then cut into 1-cm pieces (wet straw was easier to cut than dry straw). The straw pieces were subsequently dried for 24 h at 80°C.

The soil came from the experimental field of Mons-en Chaussée in Northern France (49°80'N, 3°60'E). The soil was a deep loam with the following textural composition for the 0- to 30-cm layer (g g–1): 0.15 clay, 0.76 silt, and 0.09 sand. Organic matter content was 0.021 g g–1. It was sieved at a gravimetric water content of 0.2 g g–1 to obtain 2- to 3-mm aggregate diameters and sterilized with {gamma} rays at 60 kGy to kill microorganisms. The soil was then stored in a chamber at 3°C.

Soil aggregates (0.2 g g–1 of gravimetric water content) and dry straw were first mixed in a large and flat container. We could sight check that the mixture was homogeneous. The mixture was then poured into cylinders. Volumetric straw contents in these mixtures were: 0, 0.1, 0.2, and 0.3 cm3 cm–3 (volume of straw/volume of soil–straw mixture). We used small cylinders for tomographic analyses (43-mm diam., 40 mm high), intermediate cylinders for water potential measurements (70-mm diam., 24 mm high) and larger cylinders for Wind measurements (150-mm diam., 70 mm high). The sample composition of the intermediate and large cylinders is given in Table 1. We calculated the volume of soil (compacted at a bulk density of 1.2 g cm–3) corresponding to the volume of straw added. To constitute the samples, we removed this quantity of soil from the initial quantity of soil corresponding to the sample without straw. We assumed that the straw tubes were not crushed between soil aggregates during the compaction of the sample (at low pressure). The samples were compacted until the soil reached a density of 1.2 g cm–3. Thus, the dry bulk density of the samples decreases as the straw content increases (Table 1), but the density of the soil around the straw is assumed to be equal to 1.2 g cm–3. Samples were stored in a refrigerator at 4°C to reduce straw decomposition. In the field, the amount of straw residues after harvest is around 8 t ha–1. The volumetric straw contents of 0.1, 0.2, and 0.3 cm3 cm–3 in the samples correspond to superficial incorporation to the 7-, 3.5-, and 2.3-cm depths, respectively.


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  		Table 1. Composition of soil–straw mixtures of the samples used for water retention measurements (small samples, 7-cm diameter and 2.4-cm height) and hydraulic conductivity measurements (large samples, 15-cm diameter and 7-cm height).

 
Tomography Images
The four samples with different amounts of straw added (0, 0.1, 0.2, and 0.3 cm3 cm–3 of volumetric straw content) were air dried before further analysis. The samples were scanned using a microfocus computer tomography system at the Department of Metallurgy and Materials Engineering of the University of Leuven, Belgium. This system comprised a Philips HOMX 161 X-ray source (Philips, Eindhoven, The Netherlands) and an AEA Tomohawk TH9428 HX 12 bit 1024 x 1024 detector , turntable, and software for scanning and reconstruction (AEA, Abingdon, UK). The energy used for the scans was 132 kV. To avoid beam hardening, a copper filter of 0.81 µm was placed in front of the source. Step size was 0.5°. The size of 1 pixel was 51.4 µm.

Retention Curves with Suction Table and Pressure Extractor Methods
The retention curve expresses the relationship between water content and water pressure head. We used the suction table method to obtain the retention curve at high water pressure heads (greater than –100 cm) and the pressure extractor method (Klute and Dirksen, 1986) to obtain the retention curve at low water pressure heads (from –100 to –15000 cm of water). Measurements were made for straw, the soil samples without straw, and the soil–straw mixture samples. For the retention curves of pure straw, the pieces of straw ({approx}10 cm long) were directly put on the suction table or in the pressure extractor. We used three replicates for each mixture. Neither method measures osmotic pressure, which can be high when studying plant material. We assumed that osmotic pressure was negligible in our experiment. Indeed, some of its solutes were lost due to the immersion of the straw in NaClO solution during sample preparation. Myrold et al. (1981) measured a very low osmotic pressure under similar straw preparation conditions. Furthermore, under natural conditions, crop residues are subjected to leaching, which considerably reduces osmotic pressure. The samples were saturated on the suction table by applying zero water suction at the bottom for 24 h and progressively applying zero water potential at half the height of the samples for 8 d. Straw pieces were saturated in water at 4°C for 24 h. Preliminary experiments were performed to determine the minimum number of days to reach equilibrium for soil and straw, using both methods. We found that a minimum number of 6 d was necessary. The fact that straw had also reached equilibrium indicated that water loss by evaporation was negligible, although the straw had a large contact surface with the air.

In the suction table method, the saturated samples were placed on a sand layer whose bottom was in contact with a water column. Then, the samples were equilibrated with suctions imposed by a water column height of –1, –3, –5, –10, –30, –50, –70, and –95 cm. The samples remained 6 d at each equilibrium stage and weighed.

Using the pressure extractor method, the saturated samples were put on a saturated porous plate inside a pressure chamber. Air pressure was used to extract water from the soil sample by the porous plate. The porous plate was initially covered with kaolinite to ensure hydraulic continuity between the porous media and porous plate. Samples were extracted using the following pressures: –500, –750, –1000, –10000, and –15000 cm of water. The samples were weighed after 6 d of equilibration time.

Retention and Hydraulic Conductivity Curves with Wind's Evaporation Method
We used Wind's (1968) evaporation method to estimate the hydraulic properties of the soil–straw mixture. Wind's method, already described by Tamari et al. (1993) and Wendroth et al. (1993), is based on an evaporation experiment. The sample is placed on a balance to determine evaporative water loss through time. Tensiometers are inserted horizontally at different depths to measure the water pressure head profile. Water content is estimated at each depth of water pressure head by using an analytical function to describe the water content–pressure head relation. The mean water content measured by weighing the sample can be calculated from the estimated water content of each water pressure measurement compartment. The retention curve was optimized by an iterative procedure until agreement between the measured and calculated mean water content was satisfactory. Hydraulic conductivity is calculated using Darcy's Law from the flux measured at the soil surface and the water pressure gradients measured in the sample. It cannot be estimated well above –50 cm of water because the water pressure head gradients in the samples are too small (Wendroth et al., 1993).

In our experiment, six tensiometers were inserted horizontally in the cylinder at 0.5, 1, 2, 3, 4.5, and 6 cm from the soil surface. We had two replicates for each straw proportion. The samples were saturated according to Richard et al. (2001) with distilled and degassed water by applying zero water potential to the bottom for 24 h and progressively increasing the level of water until it reached the top of the sample (1 cm h–1). Zero water potential had been applied to the top for 8 d. The water retention curve (relation between volumetric water content {theta} and water pressure head h) and the hydraulic conductivity curve (relation between hydraulic conductivity K and water pressure head h) were described using the van Genuchten (1980) model, which gives the following relations:

[1]

[2]
where Se(h) is the effective saturation, {theta}r (cm3 cm–3) and {theta}s (cm3 cm–3) are the residual and saturated volumetric water contents, Ks (cm h–1) is the saturated hydraulic conductivity, and {alpha} (cm–1), n, and m are empirical parameters. The Mualem condition (m = 1 – 1/n) is used. {theta}s, {alpha}, and n were fitted for the retention curve. {alpha}, n, and Ks were fitted for the hydraulic conductivity. The residual volumetric water content {theta}r was fixed at 0. In our fitting, we did not consider water pressure head measurements above –50 cm of water.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Tomography Images
In the tomographic sections, the aggregates are white, whereas the pores are black (Fig. 1) . The interior of the small cylindrical straw pieces ({approx}1 cm long, {approx}3-mm diam.) is black, while the wall of the straw tube has an intermediate, gray color ({approx}0.5 mm). The addition of straw greatly modified the appearance of the samples. The porosity was difficult to measure because the software could not distinguish between black pores and gray straw tubes. Total black and gray area increased more than the increase in straw content, which indicated that additional porosity formed by mixing straw pieces within the soil. More work has to be done to distinguish the straw and thus quantify this additional porosity. Moreover, the straw pieces were not located homogeneously within the sample. In the sections to which straw had been added, it could be seen that the straw pieces were drawn to each other or to the (dry) wall of the PVC core. This latter observation was observed only in the small cylinders used for tomography and not in the larger cylinders used for retention curve or hydraulic conductivity curve.



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  		Fig. 1. Four tomographic sections of cylindrical samples with 0, 0.1, 0.2, and 0.3 cm3 cm–3 of straw content. This was visualized using microfocus computer X-ray tomography. The aggregates are white, the pores are black, and the straw is black with a gray wall.

 
Hydraulic Properties
Retention Curves
Straw and Soil. The retention curves of soil and straw are presented in Fig. 2 . Water pressure head was expressed as a function of gravimetric (Fig. 2a) or volumetric (Fig. 2b) water content. The scatter in the retention curve of the straw may be explained by the difference in the physical characteristics between wheat internodes because thickness and diameter can vary from 0.4 to 0.7 mm and from 2.6 to 3.9 mm, respectively (Annoussamy et al., 2000). Water storage capacity was higher for straw than for soil in both units. For gravimetric water content, the difference between the retention curves of the two media was very large. For example, at –10 cm of water, the gravimetric water content was 0.31 g g–1 for soil and 3.64 g g–1 for straw. At this pressure, a mass of dry straw stored about 11 times more water than the same mass of dry soil. For volumetric water content, both retention curves were quite similar even though storage capacity was slightly higher for straw than for soil. At –10 cm of water, the volumetric water content was 0.34 cm3 cm–3 for soil and 0.42 cm3 cm–3 for straw. At this pressure, a volume of straw stored 20% more water than the same volume of soil.



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  		Fig. 2. Retention curves of soil and straw expressed as a function of gravimetric water content (a) and volumetric water content (b).

 
The gravimetric retention curves of straw obtained by Myrold et al. (1981) using a psychrometer and a hydraulic press were close to these obtained in this work with the pressure extractor method (Fig. 2a). This result indicates that conventional methods used to measure the retention curves of soils, such as the pressure extractor method, can also be used at these water pressures for residue samples like straw.

Figure 2b shows that from saturation to –3 cm of water pressure head, the volumetric water content of straw decreased by 0.44 cm3 cm–3. Using the data of Plisson Annoussamy (1999), we calculated that the pore volume within the straw internal cylinder was equal to 0.45 cm3 cm–3. Therefore, we assumed that this decrease of volumetric water content of 0.44 cm3 cm–3 was due to the drainage of water inside the cylinder near saturation.

Plisson Annoussamy (1999) measured the diameters of the different cells of this straw variety on a transversal section obtained with a microscope. The cell diameters were from 20 to 100 µm. We assumed that they can be assimilated with pores that drain water for an equivalent water pressure head of –10 to –1000 cm. Chesson et al. (1997) showed that the diameters of pores in the cell wall are from 0.1 to 10 nm, which are too small to contribute to pore drainage at these water pressure heads.

The differences between gravimetric and volumetric water content were explained partly by the difference between soil and straw density. Gravimetric and volumetric water contents (w and {theta}, respectively) are related via the dry bulk density {rho}d: {theta} = w{rho}d. The dry bulk density of soil, imposed in the experiment at 1.2 g cm–3, was 10 times higher than the density of straw (i.e., 0.116 g cm–3 calculated by Plisson Annoussamy, 1999). Because the volumetric water contents of soil were quite similar to those of straw, the differences in gravimetric water content between soil and straw (the gravimetric water content of straw was 11 times higher than the gravimetric water content of soil) were explained by differences in the bulk density of the two media.

Soil–Straw Mixtures. The retention curves of the soil–straw mixtures measured with both methods (suction table and pressure extractor methods and Wind method) are presented in Fig. 3 . Each recorded value was the average of three replicates, and the error bars were the standard error distributed around the average. The standard errors were generally very low. For a similar water pressure head, water content increased as straw content increased. Retention curves were closer together when water content was expressed in terms of volumetric water content than in terms of gravimetric water content, as shown previously. The van Genuchten parameters are given in Table 2. The coefficients of determination between measured and estimated water pressure heads were very large, being equal to 0.99. The volumetric retention curves were quite similar; thus, the parameters of Table 2 were also close to each other, except for the samples with 0.3 cm3 cm–3 straw content. This latter difference may be explained by the high correlation between parameters. We found slight differences between both methods. The Wind method systematically overestimated the water content obtained by the other methods at a given water pressure head. Similar results were also found by Richard et al. (2001).



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  		Fig. 3. Retention curves of the soil–straw mixtures (straw content expressed in cm3 cm–3) expressed as a function of (a) gravimetric water content and (b) volumetric water content. Retention curves obtained with the suction table and pressure extractor methods are drawn with open symbols. Retention curves obtained with Wind's method are drawn with black symbols.

 

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  		Table 2. Parameters of the retention curve of van Genuchten (1980) model obtained with Wind's methods for straw content of 0, 0.1, 0.2, and 0.3 cm3 cm–3.

 
Additive Model for Retention Curves. Several studies (Sharma et al., 1993; Fiès et al., 2002) have shown that water retention in mixtures of different constituents may be calculated as the weighted sum of the water content of these two constituents measured separately at the same water pressure head. The results of this additive model were compared with the retention curves of our "real" soil–straw mixtures. The additive model states that, for each water pressure head, the water content of the soil–straw mixture is calculated as the weighed sum of the water content of soil and straw at the same pressure according to their proportion. The following equation was used:

[3]
where w(h), wsoil(h), and wstraw(h) are the gravimetric water content of the mixture, of the soil, and of the straw, respectively, at water pressure head h, and p is the proportion of straw in the sample (p = mass of straw/total mass of the soil–straw mixture). We took the experimental data of the soil water retention curve and a linear model to describe the water retention curve of straw due to dispersion of the experimental data [w = 6.56 – 1.402Xln(h)]. The error bars of the model presented in Fig. 4 show the data dispersion of the straw retention curve.



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  		Fig. 4. Comparison between measured and calculated retention curves for the different soil–straw mixtures (straw content expressed in cm3 cm–3). The model values are shown with the error bars.

 
Calculated retention curves were close to experimental data (Fig. 4). The model gives a good description of the increase of water content with increasing straw content. However, we noticed some discrepancies near saturation. The calculated water content was higher than the measured water content at saturation. The model underestimated measured water content from –10 to –1 cm of water pressure head. At saturation, the straw cylinders could be filled only partially with water due to air entrapment inside the cylinder during the saturation process. Indeed, we observed that some straw tubes had aggregates at their extremities, which could prevent air to escape when the aggregates are wet. In this case, the model would overestimate the measured water contents. The soil–straw mixtures may contain new pores at the contact between straw cylinders or between straw and aggregates. These larger pores compared with those of the soil without straw could explain why, between a potential of –10 and –1 cm, the theoretical water content may be smaller than the measured one. This extra porosity was observed in the tomography images of Fig. 1. We observed in the images that the soil with straw contained more large pores (>1 mm) between the straw and soil aggregates than the soil without straw. This result was also obtained by Fiès et al. (2002) comparing soil mixed with different proportions of glass. In their study, the additive model was not appropriate to model the retention curve of their samples near saturation because of this specific porosity created by the mixture.

Hydraulic Conductivity Curves
The hydraulic conductivity function of water potential was measured on the samples using Wind's method. The parameters fitted with the van Genuchten model are given in Table 3. The coefficients of determination were 0.6 to 0.91. They were lower than the coefficients of determination obtained from the retention curves, as was observed by Richard et al. (2001). The curves are presented in Fig. 5 . Each recorded value was the average of two replicates. The error bars were the standard error distributed around the average. For clarity, they were shown for only a few data. The standard errors were generally low except for straw content, with a standard error of 0.3 cm3 cm–3. Near saturation, we assumed that the straw was still wet and contributing to water transfer. From –50 to –300 cm of water, the conductivity curves were close and there was no clear relation between hydraulic conductivity and straw content for a given water pressure head. Below –300 cm of water, hydraulic conductivity decreased as straw content increased for a given pressure head. At these water pressure heads, we assumed that water transport occurred mainly between soil aggregates and the water pathway was more sinuous because of the presence of straw. Straw increased soil tortuosity. We assumed that hydraulic discontinuity existed between the straw and soil with empty pores. Thus, the decrease of hydraulic conductivity was linked to a decrease of cross-sectional area of soil that contributes to transfer of water (as it is shown on the pictures of Fig. 1). Gupta et al. (1977) also reported that hydraulic conductivity decreased as the amount of incorporated sludge in soil increased. They attributed this to repellency to wetting in the soil receiving high rates of sludge.


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  		Table 3. Parameters of the hydraulic conductivity curve of van Genuchten (1980) model obtained with Wind's methods for samples with straw content of 0, 0.1, 0.2 and 0.3 cm3 cm–3 and the coefficient of determination r2 of the fitting.

 


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  		Fig. 5. Hydraulic conductivity curves for the soil–straw mixtures (straw content expressed in cm3 cm–3).

 
Impact of Hydraulic Properties of Soil–Straw Mixtures on Carbon Mineralization
Impact of the Retention Curve
Soil water content and soil water potential have an impact on microbial activity and thus on organic matter decomposition. Several models have been proposed to simulate the impact of moisture on microbial activity such as decomposition (Paul et al., 2003). In the model described by Garnier et al. (2001), for example, organic matter decomposition depends on a moisture function fw:

[4]
where C is the C content of fresh organic matter and k is the decomposition rate constant of the pool in question (d–1). Other factors like temperature and N limitation also have an impact on decomposition (Garnier et al., 2001) but are not considered here. The moisture function fw has been defined relative to "reference" conditions (Rodrigo et al., 1997), which are href = –100 cm of water. The formula is derived from Andrén et al. (1992):

[5]
where h' is the water pressure at which microbial activity ceases, which is equal to –75800 cm of water. For h < h', fw = 0 and for h > href, fw = 1.

Incubation experiments are usually conducted under controlled conditions for water content and water pressure head. A gravimetric water content and corresponding water pressure head are chosen on the water retention curve obtained on soil samples without crop residue. The gravimetric water content is then imposed on the sample of soil and crop residue.

However, Fig. 6a shows that the water pressure heads of the mixtures (obtained from our retention curves Fig. 2a and 3a) decreased as straw content increased for a given gravimetric water content of 0.24 g g–1. Therefore, the water pressure head of an incubation experiment could be overestimated if the straw is not considered in the retention curve. The function fw calculated from these pressure heads also decreased as straw content increased (Fig. 6a). Function fw could also be overestimated if straw is not considered in the retention curve. This could lead to an overestimation of C mineralization. Figure 6b shows that the calculated organic C (Eq. [4]) was lower, at a given time, when function fw was higher than what we could expect if fw were equal to 0.7 (for a pressure head of –700 cm). After 14 d of decomposition, the difference in C mineralization was 12.5% at a gravimetric water content of 0.24 g g–1. This difference is large enough to consider the effect of organic matter on water retention curves when calculating how much water is needed in incubation experiments.



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  		Fig. 6. (a) Pressure heads and moisture function fw at the gravimetric water content of 0.24 g g–1. (b) Change through time of organic C decomposition for the different moisture functions fw.

 
Impact of Hydraulic Conductivity
We simulated organic matter decomposition in a plough layer using the N and C transport and transformation model of Garnier et al. (2001). A soil layer of 30 cm, initially at a water pressure of –100 cm, was subjected to evaporation at the top, at a constant water pressure of –1000 cm (with no flux at the bottom). The range of water pressure head (–1000 to –100 cm) was chosen because hydraulic conductivity values were not measured outside this range (Fig. 5). In our simulation, straw was mixed into the soil only in the first 7 cm of the topsoil. The biological parameters and the initial conditions were those described in Garnier et al. (2003). The initial amount of NO–3–N in the soil was 1 g L–1. The amount of C in the fresh organic matter of the straw layer was 19.8 g of C kg–1 soil. In Simulation 1, the hydraulic conductivity curve of the first layer with straw represents, the hydraulic properties of the soil with 0.3 cm3 cm–3 of straw content (Sample 1 with straw content of 0.3 cm3 cm–3 in Table 3), and in Simulation 2, the hydraulic properties of the soil without straw (Sample 1 with straw content of 0 cm3 cm–3 in Table 3). In Simulation 2 we neglected the effect of straw on hydraulic conductivity. The soil below the first layer had the hydraulic conductivity of the soil without straw (Sample 1 with straw content of 0 cm3 cm–3 in Table 3). We analyzed the difference between both simulations. There was a significantly different water pressure head in the middle of the straw layer (Fig. 7a) . In Simulation 2 the soil dried faster than in Simulation 1 because the hydraulic conductivity of the soil without straw was higher compared with the soil with straw content of 0.3 cm3 cm–3. Figure 7b shows that CO2 fluxes were faster in Simulation 1 than in Simulation 2 during the first 5 d because the soil was wetter. After 5 d, N limitation was the most important factor controlling C mineralization. If the effect of straw on hydraulic conductivity was neglected, there could be flaws in the simulation of the hydraulic processes and organic matter decomposition.



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  		Fig. 7. Change through time of (a) the pressure head at 3.5 cm from the top and (b) CO2 emission with hydraulic conductivity curve of soil with 0.3 cm3 cm–3 of straw content (Simulation 1) and with hydraulic conductivity curve of soil without straw (Simulation 2).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The presence of straw strongly influenced the gravimetric water retention curve. Simulations showed that this should be considered when gravimetric water content has to be controlled in laboratory incubation experiments. Straw has little effect on volumetric water content due to the porosity of these media. The presence of straw in soil decreased hydraulic conductivity. This happened presumably because of the increase in tortuosity. Simulations showed that these changes influence organic matter decomposition. Tomographic images indicated that the addition of straw in soil created new pores. These could have an impact on hydraulic conductivity near saturation and then influence the movements of contaminants. In further research, these tomographic images could be used to calculate saturated hydraulic conductivity using the software developed by Delerue and Perrier (2002). Furthermore, the changes in hydraulic properties during straw decomposition could be analyzed.


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




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G. A. Johnson, J. G. Davis, Y. L. Qian, and K. C. Doesken
Topdressing Turf with Composted Manure Improves Soil Quality and Protects Water Quality
Soil Sci. Soc. Am. J., October 27, 2006; 70(6): 2114 - 2121.
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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
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Environmental Quality		 The Plant Genome