Soil Physics
Soil Management and Grass Species Effects on the Hydraulic Properties of Shrinking Soils A. S. Gregory C. P. Webster C. W. Watts W. R. Whalley* Cross Institute Programme for Sustainable Soil Function Dep. of Soil Science Rothamsted Research Harpenden, Hertfordshire AL5 2JQ, UK
C. J. A. Macleod A. Joynes Cross Institute Programme for Sustainable Soil Function North Wyke Research Okehampton, Devon EX20 2SB, UK
A. Papadopoulos P. M. Haygarth A. Binley Lancaster Environment Centre Lancaster Univ. Lancaster LA1 4YQ, UK
M. W. Humphreys L. B. Turner L. Skot Institute of Biological, Environmental and Rural Sciences Aberystwyth Univ. Gogerddan, Aberystwyth Ceredigion SY23 3EB, UK
G. P. Matthews Environmental and Fluid Modelling Group Univ. of Plymouth Plymouth, Devon PL4 8AA, UK
In this study, we explored the effect of the roots of different forage grasses on soil hydraulic properties at the plot scale. To achieve this, we set up a field experiment in which six different grass cultivars were grown on replicated field plots at North Wyke, UK. We used tension infiltration measurements to assess soil hydraulic properties and structure. These measurements were made over two consecutive seasons. Measurements of shrinkage, water repellence, and the water release characteristic on soil samples taken from the North Wyke site were also made. We also wanted to compare the effects of different grasses on soil structure with the effects of differences in soil management; we therefore made tension infiltration measurements on fallow soil, permanent grassland, and arable land on a longterm experiment at Rothamsted, Harpenden, UK. Our data showed that the saturated hydraulic conductivity of the capillary matrix of the soil sown with grass depended on the grass species. Grass species affected the characteristic pore size estimated from tension infiltration data. At the Rothamsted site, we were able to infer that the development of macropore structure can be ranked grassland > arable > fallow (from the greatest to the least amount of macropores). In the North Wyke site, all the grass plots showed evidence of a macropore structure, consistent with the grassland site at Rothamsted, but there did not appear to be any variation between grass species. We concluded that changes to soil structure were probably due to physical rearrangement of soil particles by shrinkage.
T
he effects of roots on soil structure and hydraulic properties have been summarized in recent reviews by Gregory (2006) and Hinsinger et al. (2009). Detailed studies have shown that the soil adjacent to the root, called the rhizosphere soil, can have different soil hydraulic properties than the bulk soil, which is unaffected by root growth. In comparison with bulk soil, root growth can alter the water release characteristic of the rhizosphere soil (Whalley et al., 2005) as well as its hydraulic conductivity (Hallett et al., 2003; Whalley et al., 2004). For a given matric potential, the rhizosphere soil is likely to be drier than the bulk soil and have a smaller hydraulic conductivity. This is not a general result, but it is the most frequent observation (Gregory, 2006). The explanation for these changes to soil hydraulic function in the rhizosphere has been related to changes in soil structure (Whalley et al., 2005) and differences in the contact angle between water and the soil surface (Read et al., 2003; Hallett et al., 2003). The effects of the growth of different plants on hydraulic properties in the field have been reported by Bodner et al. (2008) and Hu et al. (2009). Both these accounts show that different plant species can affect soil hydraulic properties differently. There is growing interest in how selecting for specific traits in plants can be exploited to alter soil hydraulic properties (Macleod et al., 2007). The purpose for this study was to explore at the plot scale how six related grass species and hybrids developed from them, which are known to have different root characteristics, affect soil structure and hydraulic properties. We used representative cultivars of the principal forage grasses used in U.K. agriculture, chosen on the basis of their good agronomic performance and their varying capabilities to withstand climatic extremes. All the species used were closely related and, as obligate outbreeders, Soil Sci. Soc. Am. J. 74:753–761 Published online 29 Mar. 2010 doi:10.2136/sssaj2009.0284 Received 3 Aug. 2009. *Corresponding author (
[email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
SSSAJ: Volume 74: Number 3 • May–June 2010
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hybridize naturally (e.g., Humphreys et al., 2003; Humphreys and Harper, 2008). The field experiment on the effects of grass on soil hydraulic properties was established at North Wyke Research, Okehampton, UK. Soil hydraulic properties were determined in the field using tension infiltration measurements (Reynolds and Elrick, 1991). With this technique, the flux of water into the soil through a circular area is measured at a number of different negative pressures. The purpose of the negative pressure is to restrict infiltration to pores that are small enough to be saturated by virtue of capillary suction. If infiltration measurements are made at a range of negative pressures, inferences on pores sizes and soil structure can be made. In this work, we used tension infiltration to determine if the roots of different grass species and hybrids have different effects on the macroscopic behavior of the soil’s capillary pore network and its macropore network. To help the interpretation of the tension infiltration data made on the grassland plots at North Wyke, we also made measurements on long-term fallow, grassland, and arable plots on the Highfield long-term experiments at Rothamsted Research, Harpenden, UK. This site provided larger plots and differences in soil structure due to their management regime rather than their grass composition, as on the plots at North Wyke. The larger plots allowed us to use two different measurements approaches. The first was the common practice of leaving the tension infiltration device in one position and then increasing the infiltration pressure incrementally following a steady-state flux (e.g., Messing and Jarvis, 1993). In the second approach, both the location of the tension infiltration device and the infiltration pressure were completely randomized. We used these data to explore the use of the piecewise solution to tension infiltration proposed by Reynolds and Elrick (1991).
MATERIALS AND METHODS The Field Experiment at North Wyke Six different grass species were used. They were Lolium perenne L. cv. AberStar (2x), Lolium multiflorum Lam. cv. AberEpic (2x), Festuca pratensis Huds. cv. Bf993 (2x), Festuca arundinacea Schreb. cv. Dovey (6x), and two hybrid cultivars L. perenne × F. pratensis cv. Prior (4x), and L. multiflorum × Festuca glaucescens Hegetschw. & Heer cv. Lusilium (4x). AberStar is a perennial ryegrass, AberEpic is an Italian ryegrass, Bf993 is a meadow fescue, and Dovey is a tall fescue. Prior is a perennial ryegrass × meadow fescue hybrid, produced initially in the 1970s but has been improved. Lusilium is an Italian ryegrass × F. glaucescens hybrid and a new French festulolium. Ryegrasses are shallow rooting and less well adapted to drought than many other grasses. In general, fescues have deeper roots and better soil water extraction characteristics and this is believed to contribute to their greater drought resistance. Seed was sown at recommended sowing rates used as standard protocols in the Institute of Biological, Environmental and Rural Sciences (Aberystwyth University, Gogerddan, Aberystwyth, Ceredigion, UK) field trials on four replicate 10- by 3-m plots. The surface horizons of all plots were hydrologically isolated from their neighbors with a plastic barrier 35 cm deep. The grass swards were allowed to establish fully for 12 mo to achieve a good overall ground cover before making measurements. The experiment was set out in four fully randomized blocks on the Rowden site at North Wyke. The site has a shallow slope and the blocks were arranged along this slope. The soils (see Table 1) at Rowden are poorly draining and have hydrological features typical of many U.K. grasslands (Boorman et al., 1995), which made it an ideal platform to investigate the potential for comparing the ability of alternative grass species and their hybrids to manipulate soil hydraulic function.
The Field Site at Rothamsted Highfield is a long-term experiment on the effects of different soil management practices on a silty clay soil (see Table 1). It was established
Table 1. Properties of the soils at North Wyke and at the long-term field experiment site at Rothamsted. Property Location Field GB National Grid reference Coordinates Soil type SSEW group† SSEW series‡ USDA† FAO† Sand (2000–63 μm) kg kg−1 dry soil Silt (63–2 μm) kg kg−1 dry soil Clay (0.35) between treatments (grass, fallow, or arable) and measurement methods (random or sequential) in our data. This means that conclusions drawn are independent of the method used to collect the data (i.e., random or sequential) when straight lines are fitted, as in Fig. 3. In some cases, the hydraulic parameters did depend on the method. For example, log10(Kfs) was significantly different (P = 0.013) between the sequential and randomized approaches (−4.05 and −3.95, respectively, for each method), but the there was no treatment effect on log10(Kfs) (see Table 3). The measured value of lnQ at ψ = 0 was significantly affected by soil management (P = 0.033), as was the difference between this value and the intercept of Eq. [1], M (P < 0.004) (see Table 3). These differences could be ranked grassland > arable > fallow, from largest to smallest. None of the other hydraulic properties were significantly affected by the treatment at Rothamsted. For both lnQ at ψ = 0 and M, the measurement method had no significant effect (P = 0.474 and 0.064, respectively).
The North Wyke Field Experiment Tension Infiltration Data The effects of grass species and time on the measured and derived soil hydraulic properties are summarized in Tables 4 and 5. The main effect of grass cultivar on Kfs was significant at P = 0.038. There was also a significant reduction in Kfs with time (P = 0.001; Table 5). The effect of the interaction between time and
Fig. 3. Steady-state water flux plotted against infiltration pressure for the arable, fallow, and grass plots at the Rothamsted site. The standard error of the mean is shown. The curves are fitted to infiltration data obtained at pressures of −0.20, −0.15, −0.10, and −0.05 m (closed symbols). The open symbols are the measured data at ψ = 0 (lnQ at ψ = 0 for the arable and grassland treatment in (A) are coincident). The dotted line in (A) corresponds to the proposed piecewise method of Reynolds and Elrick (1991).
grass on Kfs was not significant (P = 0.11). The values of Kfs determined in the field experiment at North Wyke are consistent with those data measured directly on consolidated samples of the same soil (Matthews et al., 2010) subjected to a hydraulic gradient. The extrapolated intercepts to Eq. [1] were significantly (P = 0.046) affected by grass cultivar (Table 4) and time (P = 0.001), but there was no significant interaction (P = 0.376). The measured value of lnQ at ψ = 0 was only affected by time (P
P
SED
0.138 0.138 0.326 0.033 P
SED
−17.338 0.429
−17.190 0.835
−17.842 −17.221 −17.580 0.492 0.607 0.633
−17.278 0.046 0.648 0.041
0.208 0.113
−6.827 13 −15.767
−6.442 31 −15.256
−6.973 9 −15.81
−6.636 −6.758 9 15 −15.501 −15.61
−6.758 0.038 15 −15.636 0.544
0.1483
1.511
2.105
1.864
1.613
1.539
1.855
0.702
† α, constant that depends on soil type; Kfs, field saturated hydraulic conductivity; Q, flux; ψ, infiltration pressure.
0.001). We found no significant differences in the difference between the measured lnQ at ψ = 0 and the intercept of Eq. [1] (M in Fig. 2) due to the effects of different grasses, but time had a significant effect (P = 0.021).
Water Release Characteristic, Water Repellence Measurements, and Shrinkage Characteristics There was no effect of grass cultivar on either the drying or the wetting paths of the water release characteristic, so the means for all treatments are given (Fig. 4). These data did confirm that the soil would have been unsaturated during the tension infiltration, as was implicit from the linear relationship between lnQ and ψ (see Reynolds and Elrick, 1991; Whalley et al., 2004). We found no significant effects of grass cultivar on water repellence (P = 0.117). The mean contact angle between the soil minerals and water was 58.7° (data not shown). The natural aggregates shrank as they dried (Fig. 5).
Grass and Environment in the North Wyke Experiment The extreme performers in terms of water extraction were AberStar and Bf993 (data not shown). The cumulative rainfall and evapotranspiration for one calendar month before each tension infiltration measurement is given in Fig. 6. With the exception of the first measurement, rainfall exceeded evapotranspiration. Rainfall was approximately uniform in time for 1 mo before each measurement. Both 2007 and 2008 were wetter than average for the United Kingdom. The mean annual rainfall (1961– 2000) is 1056 mm but in 2007 and 2008 the annual rainfall was 1151 and 1122 mm, respectively. The summer (April–October) rainfall was 552 and 574 mm for 2007 and 2008, respectively, compared with a long-term mean (1961–2000) of 392 mm.
DISCUSSION The Nature of the Relationship between Flow and Infiltration Pressure Inspection of the data in Fig. 3 shows that when the position and infiltration pressure are completely random, there is weak evidence of a piecewise linear relationship between lnQ and ψ for these data. This can be seen most clearly in the data from the grass plots. Randomizing infiltration pressure at each measurement location (as opposed to applying a series of increasing infiltration pressures at a fixed location) allows Eq. [1] to be treated as a continuous function of ψ for all ψ < 0. In the Rothamsted data, using the straight lines fitted in Fig. 3, we have shown that the method of measurement did not affect the conclusions to be drawn because we did not detect any statistically significant interaction between the data collection method and the treatment. It should also be noted that the Gardner function (Eq. [1]) is a continuous function of matric potential (Ghezzehei et al., 2007) across a much wider range of matric potentials than is normally considered in tension infiltration. The continuous nature of Eq. [1] is implicit in Fig. 4. Thus, in a general context, the piecewise application of Eq. [1] is the exception rather than the norm. A single curve was fitted to the data from the North Wyke experiment, which has a similar soil to that in the Rothamsted experiment.
Development of Macropore Structure In Fig. 3, the difference between the extrapolated intercept of the line fitted to lnQ and ψ (for ψ < 0) and the measured value of lnQ at ψ = 0 can be seen clearly. This occurs because lnQ at ψ < 0 is related only to the capillary matrix, while infiltration at ψ = 0 also includes the macropore network (noncapillary). It follows that if the extrapolated estimate of lnQ at ψ = 0 (intercept of Eq. [1]) is equal to the measured value, then there is no macropore network present and the size of any discrepancy
Table 5. Inferred hydraulic data for four dates from the North Wyke field experiment. These data are the means for the main effect of time obtained from analysis of variance and standard errors of differences (SED). Parameter†
May 2007
Oct. 2007
May 2008
July 2008
log10 α, m−1 0.615 0.948 0.352 0.514 −6.489 −6.268 −7.015 −7.055 log10 Kfs, m s−1 Back-transformed Kfs, mm d−1 28 46 8 8 Intercept of Eq. [1] −16.973 −16.933 −17.66 −18.066 Measured lnQ, m3 s−1 (ψ = 0) −16.177 −14.975 −15.313 −16.077 Difference between extrapolated intercept and measured lnQ at ψ = 0 1.339 2.069 1.664 2.011 † α, constant that depends on soil type; Kfs, field saturated hydraulic conductivity; Q, flux; ψ, infiltration pressure. 758
F>P
SED
