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Soil Physics

Tillage Effects on Hydraulic Properties and Macroporosity in Silty and Sandy Soils

^a Univ. of Rostock, Institute for Land Use, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germany
^b Brandenburg Univ. of Technology, Cottbus, Chair of Soil Protection and Recultivation, P.O. Box 101344, D-03013 Cottbus, Germany

^* Corresponding author (uwe.buczko{at}uni-rostock.de)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In agricultural soils, macroporosity and hydraulic properties are influenced by tillage practices. The objective of this study was to characterize macroporosity and surface soil hydraulic properties in two soils of different texture (Lietzen: sandy loam–Humic Dystrudept; Adenstedt: silt loam; Typic Hapludoll) under conventional (CT) and conservational (RT) tillage systems. Soil hydraulic conductivity was assessed in situ by ponded infiltration with single rings (n = 70) and tension infiltration by means of a "closed-top" hood infiltrometer (HIF; n = 48). Macroporosity (pore diameters >1 mm) was estimated from differences in infiltration at saturation and at –3 cm H₂O soil matric potential. Mean saturated hydraulic conductivity (K_s) for Lietzen was 3.1 x 10^–5 m s^–1 and for Adenstedt was 4.3 x 10^–5 m s^–1. These values are by one order of magnitude higher than values estimated from soil texture. This implies that soil structure has a dominant influence on hydraulic conductivity. Mean values of macroporosity were 0.005% for Lietzen and 0.018% for Adenstedt (using the method of Watson and Luxmoore). The respective values were 0.0008 and 0.0013% when the method of Bodhinayake et al. was used. For Adenstedt, RT showed higher macroporosity than CT (not significant at P < 0.05 for n = 12). Such treatment-induced differences were less developed for Lietzen. The K_s values measured with the ponded ring infiltrometer (RIF) at the sandy Lietzen site were higher than the corresponding values measured with the tension infiltrometer. These differences may be caused by subcritical soil water repellency (i.e., contact angles of the soil-water-air interface below 90°), although further factors could also be important (e.g., air entrapment, differences in water saturation, geometry of infiltration devices).

Abbreviations: CT, conventional tillage • HIF, hood infiltrometer • RIF, ring infiltrometer • RT, reduced (conservation) tillage • TIF, tension infiltrometer • WRC, water retention characteristics

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
HYDRAULIC CONDUCTIVITY and the infiltration capacity of soils are governed to a large extent by macropores, that is, the macropore volume fraction, diameter distribution, and the continuity and connectivity of the macropore network (Ehlers, 1975; Logsdon et al., 1990; Shipitalo et al., 2000). In agricultural soils, management and tillage practices have a strong impact on the physical properties of the topsoil and the characteristics of the macropore system (e.g., Frede et al., 1994; Tebrügge and Düring, 1999; Shipitalo et al., 2000). The generic term "conventional" tillage refers to soil management systems that involve plowing and harrowing, whereas "conservation tillage" is a practice with less soil disturbance, reduced penetration depth and without inversion of the topsoil. In no-till systems, the seeds are directly drilled into the soil without any further tillage practice.

For RT and no-till, many studies reported enhanced infiltration capacity and hydraulic conductivity compared with CT (e.g., Logsdon et al., 1993; Frede et al., 1994; Reynolds et al., 1995, 2000; Tebrügge and Düring, 1999; Cameira et al., 2003). In several cases, however, lower hydraulic conductivity and infiltration rates under RT practices were reported (e.g., Culley et al., 1987; Zachmann et al., 1987; Heard et al., 1988; Unger, 1992). In other studies, no significant differences of hydraulic properties under different tillage practices were observed (e.g., Starr, 1990; Ankeny et al., 1990), or the differences of infiltration properties changed with time (Dunn and Phillips, 1991). The greater numbers of macropores and higher connectivity of the macropore system in vertical direction, which is found commonly in RT compared with CT systems is mostly attributed to greater abundances of earthworms and less disturbance of the topsoil (Ehlers, 1975; Zachmann et al., 1987; Meek et al., 1990; Dunn and Phillips, 1991; Reynolds et al., 1995).

Soil hydraulic properties can be characterized by various methods. Widely used are in situ measurements of infiltration rates, either under saturated conditions with ponded infiltration rings and pressure infiltrometers (Bond and Collis-George, 1981; Reynolds and Elrick, 1990), or tension infiltrometers under slightly negative hydraulic potentials (Ankeny et al., 1988; Perroux and White, 1988). A special kind of tension infiltrometer is the "closed-top" (Dixon, 1975; Topp and Zebchuk, 1985) or hood infiltrometer (HIF). Compared with the standard tension infiltrometer, measurements with the HIF require much less effort to prepare the soil surface and consequently less disturbance. Moreover, the effect of the contact material on measured hydraulic parameters (Reynolds and Zebchuk, 1996) is eliminated.

It is generally accepted that, especially in structured macroporous soils, field methods to evaluate hydraulic properties are preferable to laboratory methods, because the volume of soil cores for laboratory measurements is often too small to be representative, and continuous macropores are often dissected by the core walls (Angulo-Jaramillo et al., 2000). The correlation between hydraulic parameters measured with field methods and values determined using soil cores in the laboratory is often poor (Reynolds et al., 2000).

Tension disc permeameters allow estimation of the proportion which different size ranges of macropores contribute to total soil water flow by adjusting the hydraulic potential of the water supply. From the differences in infiltration rates under different hydraulic supply potentials, numbers and volume fractions of hydraulically effective macropores have been derived in agricultural (Dunn and Phillips, 1991; Trojan and Linden, 1998; Bodhinayake and Si, 2004) and forest (Watson and Luxmoore, 1986, 1988; Buttle and McDonald, 2000) soils. Use of contact sand with traditional membrane-based tension infiltrometers can cause clogging of the macropores near the soil surface. Therefore, characterization of hydraulically active macroporosity could be a strong motivation for the use of HIFs, which do not require preparation of the soil surface with contact sand. Other methods to characterize macroporosity in soils include dye tracer experiments (Andreini and Steenhuis, 1990; Droogers et al., 1998), inventory of visible macropores in the field (e.g., Logsdon et al., 1990; Ela et al., 1992), image analysis (e.g., Edwards et al., 1988), resin impregnation techniques (Singh et al., 1991), X-ray tomography (Anderson et al., 1990), calculation from laboratory-measured water retention characteristics (Carter, 1988) or from soil water contents directly beneath tension infiltrometer measurement sites (Bodhinayake and Si, 2004). While there is no universally accepted threshold diameter for macropores, many studies use a minimum value of 1 mm. The different methods to characterize macroporosity seem to yield systematically different results for macroporosity: Whereas macroporosities derived from tension infiltrometry are typically within the range of 0.001 to 0.05%, the macroporosities obtained by other methods are generally higher. Direct comparisons (Bodhinayake and Si, 2004) revealed macroporosities derived from water retention characteristics, which are by a factor of about 1000 higher than macroporosities derived from tension infiltrometry. Infiltrometry-derived macroporosity can be interpreted as hydraulically active porosity, whereas macroporosity calculated with other methods represents the total, "static" macroporosity.

The objectives of this study are twofold:

1. To characterize and compare the surface saturated soil hydraulic conductivity, infiltration properties and macroporosity of two differently textured agricultural soils which are subject to long-term CT and RT sytems;
   
2. To compare hydraulic properties determined with ponded ring infiltration measurements with data derived by a new type of "closed-top," membrane-free tension infiltrometer device, the "hood infiltrometer."
   

The novelty of this study compared with similar previous work by other authors is threefold: (i) application of a relatively new measurement device (HIF); (ii) comparison of tension infiltration with falling head ponded infiltration measurements; (iii) utilization and comparison of two different methods to calculate hydraulically active macroporosity based on tension infiltration data (namely, the approaches of Watson and Luxmoore, 1986, and Bodhinayake and Si, 2004).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Experimental Sites
The investigations were performed on two agricultural sites in Northern Germany: Lietzen in Brandenburg and Adenstedt in Lower Saxony. The most pertinent characteristics of those sites are compiled in Table 1. On both sites, different tillage practices have been applied in parallel for several years. Whereas Lietzen is a typical site for the eastern German dry sandy soils, Adenstedt is an example for the fertile soils of the central German loess belt.

Infiltration Measurements
Surface soil hydraulic properties were measured in situ between April 2000 and July 2002. Details for measurements at the Lietzen site are compiled in Table 2; at Adenstedt, all measurements were done in April/May 2000. Infiltration at near-saturated conditions was measured using a HIF, whereas ponded infiltration was measured with a single RIF. For each plot, measurements were done within an area of approximately 100 m² and at locations unaffected by wheel traffic.

The exact pressure heads applied during the measurements varied slightly, because in many instances, it was not possible to adjust the supply pressure head precisely to the predetermined value (although actually applied values could be measured with a precision of 0.1 cm). In such cases, the supply pressure head was recorded, and corresponding infiltration rates for 0 and –3 cm H₂O were obtained by interpolation and extrapolation, assuming logarithmic scales of both pressure heads and infiltration rates.

Pore radii r (L) are calculated from the hydraulic head (L) using the equation of capillarity:

[1]

Here, is the surface tension of water (M T^–2) (= 71.3 mN m^–1 at 15°C), the contact angle between the water-air interface and the solid surface, the density of water (M L^–3) (= 1000 kg m^–3), and g the acceleration due to gravity (L T^–2) (= 9.81 m s^–2). Assuming the pore space consists of capillary tubes, and = 0, a hydraulic head of –3 cm H₂O corresponds to a pore diameter of 1 mm, which is often adopted as a threshold diameter for macropores (Watson and Luxmoore, 1986; Trojan and Linden, 1998; Buczko et al., 2003).

However, measurements with the capillary rise method (Wöllecke, 2006) yielded contact angles distinctly greater than 0° (Table 1). For contact angles of 60°, the pore diameter corresponding to –3 cm H₂O would be only 0.5 mm. However, the exact contact angles in the soil are strongly dependent on water content and probably approach 0° after prolonged infiltration. Therefore, the macropore threshold of –3 cm H₂O is adopted here, and consequently, infiltration at –3 cm H₂O excludes macropores, and the difference between infiltration rates at 0 cm H₂O and –3 cm H₂O, q_m (L T^–1), can be used to estimate the hydraulically active macroporosity of a soil. An upper bound for the number of macropores per unit area, N_m, results from the Hagen–Poiseuille equation for laminar flow through a capillary tube (Watson and Luxmoore, 1986; Dunn and Phillips, 1991):

[2]

Here, denotes the dynamic viscosity of water (M L^–1 T^–1) (taken here as 0.00115 Pa s for a temperature of 15°C), and r_m (L) is the minimum radius for macropores (here: 0.5 mm). Implicitly assumed in Eq. [2] is a unit hydraulic gradient, that is, steady-state conditions during infiltration flow. The effective macroporosity, _m, is estimated as:

[3]

Since r_m is the minimum radius of macropores, _m as calculated with Eq. [2] and [3] is an estimate of the maximum value for hydraulically active macroporosity, because r_m occurs in the denominator of Eq. [2].

Recently, Bodhinayake et al. (2004) proposed a modified method for calculating the hydraulically effective macroporosity, which takes into account the hydraulic conductivity, K(), in the pressure head interval corresponding to the two pore radii a and b. The resulting contribution of hydraulically active macroporosity for this interval of pore radii is calculated as:

[4]

For known hydraulic property functions, an exact analytical solution for Eq. [4] may be obtained by direct integration. Here, Gardner's exponential model was used, and the resulting analytical solution given in Bodhinayake et al. (2004; Eq. [21]). In the present study, macroporosities calculated with both methods, Watson and Luxmoore (1986) and Bodhinayake et al. (2004), are given.

Hydraulic conductivity, K(), was calculated from HIF data with the double tension approach according to Ankeny et al. (1991): Wooding's (1968) analytical solution for steady-state flow from a circular disc with radius r_d may be written for two different hydraulic heads ₁ and ₂ and corresponding water volume flux rates Q₁ and Q₂ (L³T^–1) as:

[5]

[6]

An exponential relationship is assumed for the hydraulic conductivity function K() (LT^–1), with K_s (LT^–1) denoting the saturated hydraulic conductivity. The parameter of this relation is calculated by dividing Eq. [5] by Eq. [6]:

[7]

The hydraulic conductivity functions are then calculated with:

[8]

and

[9]

Ponded infiltration with variable ponding depths was measured using single rings (RIF) of sheet iron of 20 cm diameter, which were pressed 8 cm into the soil. Measurements were initiated with a ponded depth of about 20 cm and infiltration rates as a function of ponding depth recorded with a time resolution of 30 s, until all the water in the ring had infiltrated. Immediately thereafter, the filling and measurement procedure was repeated.

The calculation of K_s from the ponded infiltration data follows the procedure outlined in Reynolds and Elrick (1990) and Elrick et al. (1995). According to Reynolds and Elrick (1990), the water flux Q [L³ T^–1] from a RIF with a constant ponding depth H (L) is given by

[10]

Here, _m denotes the matric flux potential (L² T^–1), and G is a dimensionless geometry factor, which for H > 5 cm can be calculated as (Reynolds and Elrick, 1990)

[11]

with d = depth of ring insertion; for d = 8 cm, and r = 10 cm, a value of G = 0.4368 is calculated with Eq. [11]. Based on two flux rates Q₁ and Q₂ (with Q₂ > Q₁) measured at the corresponding ponding depths H₁ and H₂ (H₂ > H₁), K_s is calculated as:

[12]

and the corresponding matric flux potential

[13]

This calculation procedure was developed for stationary infiltration at constant ponding depths. Further assumptions are, that the soil is homogeneous, the soil matrix is rigid, and hydraulic properties are isotropic.

A modification of this procedure for falling head conditions was developed by Elrick et al. (1995). Equation [10] may be written as

[14]

Here, A is the base area of the ring (L²) and H(t) the time-variable ponding depth. Separation of variables and integration results in a forward formulation for H(t):

[15]

The unknown parameters K_s and _m in Eq. [15] were estimated by an inverse calculation. An objective function was minimized which incorporates measured (H*) and predicted (H) values of ponding depths and prior estimates of K_s and _m (similarly as in Zeleke and Si, 2005):

[16]

Here, ² denotes the variance of the measurement error of H, _K² that of K_s, and ² that of _m; n is the number of H(t) data pairs for each measurement sequence. The superscripts "ss" and "fh" denote "steady state" and "falling head,", respectively. Prior estimates of K_s^ss and _m^ss were obtained with Eq. [12] and [13], assuming a quasi steady state infiltration flux. The calculation was based on regression lines of ponding depths vs. infiltration rates, from which infiltration rates for ponding depths of 20, 15, 10, and 5 cm were estimated. These were subsequently used to calculate K_s and _m with Eq. [12] and [13]. The unknown variances of measurement errors were simply assumed to be 10% for H, K_s, and _m (Zeleke and Si, 2005).

Statistical Analysis
One-way analysis of variance (ANOVA) was used to test for statistical significance of differences among means of K_s and macroporosity, using the program SPSS (version 11.0.0). Comparisons between individual means were made using Fisher's Least Significant Difference when the F-value in the ANOVA was statistically significant. Customarily, the null hypothesis (equal means) is rejected at a probability P < 0.05. A test for normal distribution revealed, that macroporosity was better approximated by a log-normal distribution. For K_s values, some datasets proved to be log-normally distributed, whereas others were better approximated by a normal distribution. In ANOVA, log-transformed values were used for macroporosity, whereas both untransformed and log-transformed data were used for K_s values.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Hydraulic Conductivity
For the Lietzen site, K_s values (Fig. 1 , Table 3) are slightly higher when determined with the RIF in comparison with the HIF values and higher for conventional tillage (CT) than for conservation (reduced) tillage (RT). None of these differences is, however, significant at P < 0.05 (Table 3). The overall geometric mean of K_s values for all measurements at the Lietzen site is significantly lower than the overall geometric mean of K_s values at the Adenstedt site (Table 3).

View larger version (12K): [in this window] [in a new window]   Fig. 1. Boxplots of Ks values for the conventional and conservation tillage plots of the Lietzen site, determined with the hood (HIF) and the ring (RIF) infiltrometer. The numbers of measurements are given in Table 3. Boxes denote the 25th and 75th percentiles positions, the continuous bar inside the box shows the median and the dash-dotted line the mean value; the whiskers denote the 10th and 90th percentiles; outliers are indicated as triangles. This notation applies also to subsequent box plots.

In contrast to the Lietzen data, at the Adenstedt site, K_s values derived from RIF measurements are not consistently higher than K_s values derived from HIF measurements (mean values are lower for RIF-K_s than for HIF-K_s values; Table 3; Fig. 2 ). For the Lietzen site, the geometric mean of RIF-K_s values is 1.6 times higher than the HIF-K_s data. The different relations between K_s values derived from HIF and RIF measurements at the two different sites may be caused by slight differences in soil water repellency, and concomitantly, different degrees of wetting during HIF measurements. For both sites, soil water repellency measured with the water drop penetration time (WDPT) and the ethanol percentage (EP) test was very low (WDPT values were consistently below 5 s, and EP values 0%). However, measurements with the capillary rise method revealed "subcritical" repellency levels (i.e., contact angles of the solid-liquid-gas interface below 90°) with contact angles of about 60° (Wöllecke, 2006; Table 1). For the RT plot at the Adenstedt site, the measured contact angle is clearly lower (about 30°). However, the contact angle data given in Table 1 are based on only few soil samples, and a statistical treatment was not possible. A further indication of a slightly higher repellency level at the Lietzen site is given by the coarser texture compared with the Adenstedt site and the dryer climatic conditions (Table 1). Both factors, dryer climate and coarser texture, favor the formation of soil water repellency (Doerr et al., 2000). It is well known that in severely water repellent soils (i.e., contact angles > 90°) infiltration with positive hydraulic heads is necessary to overcome the initial resistance to wetting and that the hydraulic head corresponds to the water entry pressure which is directly related to the repellency level (Wang et al., 2000).

View larger version (13K): [in this window] [in a new window]   Fig. 2. Boxplots of Ks values for the conventional and conservation tillage plots of the Adenstedt site, determined with the hood (HIF) and the ring (RIF) infiltrometer. The numbers of measurements are given in Table 3.

An exact correlation between RIF- and HIF- K_s values at the Lietzen and Adenstedt sites is problematical, because corresponding RIF and HIF measurements were done at neighboring (about 50 cm apart), but not absolutely identical positions. Between these neighboring datasets of RIF– and HIF– K_s values there is no correlation (not shown here). Similar poor correlations between K_s values determined with ponded and tension infiltration measurements are reported by Reynolds et al. (2000).

Concerning K_s values of different tillage treatments, only for the Adenstedt site values for the RT plot are significantly higher (by a factor of about 2) than values at the CT plot. However, these differences are not significant when only RIF-K_s values are considered (Table 3, Fig. 2). Similarly, higher K_s values under RT compared with CT was described for many sites worldwide: Cameira et al. (2003) reported for a silt loam soil under corn higher K_s values (measured with TIF) under RT than under CT and Reynolds et al. (2000) reported for soils with loamy and clay-loamy texture higher K_s values at no-till sites compared with CT. According to Logsdon et al. (1993), ponded infiltration rates for RT under corn in loam soil are significantly higher than infiltration rates in CT plots. Under trafficked treatment, Ankeny et al. (1990) described for a silty clay loam soil higher TIF infiltration rates for no-till compared with CT.

The lack of significant differences in hydraulic properties between CT and RT at the Lietzen site (Table 3) can possibly be attributed to the short duration of the tillage experiment (since 1996; Table 1) at the time of the studies, and in general to the lower earthworm activity in this sandy soil compared with the Adenstedt site (Hangen et al., 2002).

A factor not accounted for in the present analysis is temporal variability of hydraulic conductivity: for many agricultural soils, seasonal variations of K_s values and infiltration rates have been described (Angulo-Jaramillo et al., 1997; Heddadj and Gascuel-Odoux, 1999; Cameira et al., 2003; Bagarello and Sgroi, 2004). Temporal variations of K_s were also observed at the Lietzen site (see Table 2), but the observed temporal trends were inconclusive and too few measurements were available for each date for a proper statistical treatment of the data. Moreover, the amplitude of temporal variation seemed low enough to justify a bulk treatment of all the data measured throughout the course of the year.

Also, spatial variability of hydraulic properties is often observed at agricultural sites (Logsdon and Jaynes, 1996; Mohanty et al., 1996a). Since the exact measurement locations at the Lietzen and Adenstedt sites were slightly different at the different dates, spatial variability probably plays a role, but could not be quantified and was superimposed by temporal variability.

Macroporosity
Values of hydraulically active macroporosities (Table 4, Fig. 3 ) range for both sites largely between 0.001 and 0.2% when using the calculation procedure of Watson and Luxmoore (1986). These values are similar to those determined in many other studies (see Table 5). When the procedure of Bodhinayake et al. (2004) is used, macroporosities are by a factor of 5 to 10 lower (Table 4). Similarly large differences for macroporosities estimated with the two methods are reported by Bodhinayake et al. (2004). For better comparability with results obtained by other authors in previous studies (Table 5), we focus here on the values obtained with the method of Watson and Luxmoore (1986), bearing in mind the limitations of this method (see Bodhinayake et al., 2004). For Lietzen, the geometric mean macroporosity is 0.005%, whereas it is significantly higher at Adenstedt (0.018%). This is in line with a more abundant soil fauna (mainly earthworms) observed and the higher macropore continuity determined with dye-tracer experiments at Adenstedt (Hangen et al., 2002). For both sites, hydraulically active macroporosities are higher for the RT treatment (although not significant at P < 0.05, Table 4). For Adenstedt, macroporosities estimated from HIF infiltration data are similar to values obtained from classical tension infiltrometer measurements (Buczko et al., 2003; Table 6).

View larger version (16K): [in this window] [in a new window]   Fig. 3. Boxplots of macroporosity values for the conventional and conservation tillage plots of the Adenstedt and Lietzen site, determined with the hood infiltrometer (HIF). The numbers of measurements are given in Table 4. For each study plot, the left side box plot denotes macroporosity values calculated with the approach of Watson and Luxmoore (1986), and the right side box plot denotes the values calculated with the method of Bodhinayake et al. (2004).

View larger version (13K): [in this window] [in a new window]   Fig. 4. Measured water retention curves (0- to 10-cm soil depth) for the conventional and conservation tillage plots of the Adenstedt and Lietzen sites. The number of replications for each suction head and treatment/plot is n = 3. For better clearness of this graphic, error bars are not given.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	ACKNOWLEDGMENTS

 
This investigation was financially supported by the German Ministry of Education and Research (BMBF) as subproject A 4.3 of the German Research Network ‘Natural Disasters’ (DFNK) under grant number 01SF9971/4. Thanks to B. Wöllecke (Cottbus) for assistance with soil sampling and water retention data, and to Dr. N.A. Wahl (Univ. Aalborg, Denmark) for assistance with infiltration measurements.

Received for publication February 1, 2006.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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