CSIRO PUBLISHING
www.publish.csiro.au/journals/ajsr
Australian Journal of Soil Research, 2003, 41, 949–961
Influence of soil treading on sediment and phosphorus losses in overland flow R. W. McDowellA,D, J. J. DrewryA, R. J. PatonA, P. L. CareyB, R. M. MonaghanA, and L. M. CondronC A
AgResearch Ltd, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand. AgResearch Ltd, Soil and Physical Sciences Group, PO Box 84 Lincoln University, Canterbury, New Zealand. C Centre for Soil and Environmental Quality, PO Box 84, Lincoln University, Canterbury, New Zealand. D Corresponding author; email:
[email protected] B
Abstract This study investigated the effect of simulated cattle treading on soil infiltration rate (saturated hydraulic conductivity: Ksat) and macroporosity, and the consequent loss of sediment and phosphorus (P) via overland flow from a grassland and cultivated soil used for dairy farming in southern New Zealand. Treading decreased soil macroporosity and Ksat, and hence time to ponding, which increased the volume of overland flow. Mean suspended sediment concentration was greater in the cultivated treatments (0.076 g/L) compared with the grassland treatments (0.014 g/L). In the grassland soil, sediment and particulate P fractions in overland flow increased with treading due to increased soil disturbance and decreased protection from erosion by grass cover. In contrast, for the cultivated soil, sediment and P concentration and load decreased with increasing treading, due to greater ponding which decreased the erosive power of raindrop impact. Dissolved and particulate P fractions followed similar trends, although mean total P (mostly particulate P) was greater in cultivated treatments (1.07 mg/L) than the grassland treatments (0.64 mg/L). Relationships were generated between macroporosity and the loss of sediment and P, showing the wider application of macroporosity for environmental assessment than solely an agronomic measurement. SR021 8 ReP.thaWosl.pMhcorDuoswaendlsedmi entlos inoveralndfolwfolowingtreadnig
Additional keywords: infiltration, particulate, hydrology, erosion, macropores, compaction.
Introduction The loss of phosphorus (P) and sediment from land to surface waters via overland flow is a significant cause of impaired water quality. In much of New Zealand where surface water drains hard rock catchments (as opposed to volcanic catchments), P is the limiting nutrient for accelerated eutrophication (ANZECC-ARMCANZ 2000). The reactive and sorptive nature of P means that the loss of P from land to water is often associated with sediment transport. Factors that are likely to accelerate the movement of P and/or sediment are therefore of principal concern. Soil treading damage caused by grazing animals has been linked to altered soil physical properties such as decreased infiltration rate and porosity, increased bulk density, and decreased pasture growth (Singleton and Addison 1999; Drewry et al. 2000). By changing these parameters with treading it is reasonable to assume that the likelihood of overland flow is increased. Since overland flow represents a quicker mechanism and often carries with it a greater sediment and P load than subsurface flow (McDowell et al. 2001), our main objective was to examine the loss of sediment and P via overland flow in relation to the amount of treading damage and its effect on soil hydrological parameters (e.g. infiltration rate and porosity). In New Zealand, while the wintering of animals on pastures is common in areas with mild climates and good pasture growth year round, in cooler areas such as southern New © CSIRO 2003
10.1071/SR02118
0004-9573/03/050949
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Zealand, the lack of pasture growth requires an alternative strategy. In these cooler environments, it is common for animals to be placed in a field where a forage crop has been grown for winter feed (e.g. Brassica rutabaga L.). This often has the effect of damaging the soil via treading at a time when the soil is likely to be saturated. Treading damage above the cultivation depth on cropped soil can be easily repaired by re-cultivation; damage beneath this, and treading damage in permanent pasture, is less easily rectified. While the damage to soil physical properties in grassland has been well documented (e.g. Drewry et al. 2000; Drewry and Paton 2000), it is not known if this damage accelerates the loss of sediment or P. Thus, our secondary objective is to assess the influence of treading damage on sediment and P loss in these cultivated soils compared with grassland soils. Materials and methods Soil and preparation The soils used were taken from 2 locations (one under grass and one in an adjacent cultivated field) in the Muddy Brook catchment (NZ Map Grid East 2257950, North 5450100) near Balclutha, South Otago, New Zealand. The soil is a Waitahuna silt loam (NZ Classification: Mottled Fragic Pallic soil) that until 2 years earlier had been used for sheep and beef cattle grazing. Conversion to dairy farming has resulted in increased fertiliser inputs together with increased grazing frequency and occasional winter cropping with Brassica rutabaga L. (swede) and B. oleracea L (kale). Samples were taken manually using a metal cutting blade of intact grassland soil (top 10 cm, 95% ground cover) and soil from a grassland site that had been cultivated six months prior to sampling and sown with B. rutabaga and B. oleracea (c. 10–20% ground cover). Soils were placed in boxes of 0.1 m2 and 12.5 cm depth with 3 holes drilled into the base for drainage. Soils were then randomly trodden upon with a mechanical cow hoof approximately 8 cm at its longest length (Di et al. 2001) at frequencies of 0, 10, 20, 30, 40, 60, 90, or 120 imprints/m2. Duplicates were made of the 20, 30, 60, and 90 treading frequencies giving a total of 12 grassland soils and 12 cultivated soils. Soils were then transported to an indoor rainfall simulation facility. During treading and until soils were rained upon (within 3 days), soil moisture was kept as near to their antecedent condition as possible (c. 55% v/v). After treading, considerable damage was visually noted on both the pasture and crop, although no specific measure of cover was made. Overland flow Overland flow was generated by applying artificial rainfall (tap water, P less than detection limit of 0.005 mg P/L) at 15 mm/h to each boxed and trodden soil, inclined at 5% slope. The rainfall simulator uses one TeeJet 1/4HH-SS30WSQ nozzle (Spraying Systems Co., Wheaton, IL), approximately 250 cm above the soil surface to gain terminal velocity (Sharpley et al. 1999). The nozzle, plumbing, in-line filter and pressure gauge were fitted onto a 305 by 305 by 305 cm aluminium frame with tarpaulins on each side to provide a windscreen. The simulated rainfall had drop-size, velocity, and impact energies approximating natural rainfall (Shelton et al. 1985). The 15 mm/h rainfall-intensity has a return frequency of approximately thrice per year for a 15 min event. Samples of overland flow were taken for 1 h after flow had initiated. The rainfall simulation and soil boxes used in this study were designed to study interactions between soil and overland flow and mechanisms controlling the release of soil P to overland flow (Sharpley 1985, 1995). Although these boxes accurately represent field processes, and as such mimic processes involved in saturation-excess overland flow, they are not designed to quantify P losses on the field scale per se (Sharpley et al. 1982). Water and soil analyses Soils were analysed in triplicate for bicarbonate-extractable P (Olsen P; Olsen et al. 1954) and total P following digestion of 250 mg (ground 30 µm) should give a good estimate of flow potential. Furthermore, by decreasing the number of macropores within the soil, the space (or void volume) available to hold or conduct water also decreases. Our experiment examined mainly saturation-excess overland flow due to restricted drainage out of the base of the box and the interruption of pores by treading. Consequently, the water holding capacity of the soil, and thus macroporosity, should be related to the potential for overland flow. As such, a significant negative linear relationship was evident for both land uses between macroporosity and the volume of overland flow produced (Fig. 1). Significant positive relationships occurred between macroporosity and the time to ponding (i.e. the initiation of overland flow) for both land uses and would suggest that macroporosity is a good indicator of the potential for saturation-excess overland flow. However, no significant relationship could be found between Ksat and macroporosity.
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Table 1. Parameter
Mean
Summary statistics for parameters s.e.m.A
Median
Min.
Max.
25th 75th percentile percentile
8.0 0.57 0.001 0.09 0.11 0.19 0.12 0.46 0.02 0.06 0.08 0.14 0.09 0.27 4.0 0.84 7
27.0 0.90 0.040 0.24 0.20 0.40 0.52 0.91 0.32 0.19 0.17 0.35 0.47 0.82 16.6 1.04 1616
10.3 0.78 0.004 0.19 0.16 0.36 0.17 0.49 0.03 0.14 0.12 0.28 0.13 0.39 9.8 0.87 162
14.8 0.86 0.029 0.21 0.19 0.40 0.39 0.79 0.25 0.18 0.16 0.34 0.33 0.66 15.2 0.95 770
4.0 0.35 0.040 0.02 0.03 0.05 0.39 0.48 0.34 0.02 0.03 0.05 0.36 0.44 6.7 0.93 2
40.0 0.97 0.110 0.12 0.33 0.45 1.28 1.61 0.81 0.08 0.11 0.17 1.09 1.16 25.6 1.24 418
6.0 0.70 0.063 0.03 0.05 0.08 0.72 0.78 0.43 0.02 0.04 0.07 0.47 0.60 7.8 1.02 3
19.5 0.94 0.098 0.08 0.12 0.20 1.16 1.40 0.72 0.06 0.08 0.14 0.88 0.97 14.1 1.19 81
Grassland Ponding (min) B Volume of flow (L)C SS (g/100 mL) DRP (mg/L) OP (mg/L) TDP (mg/L) PP (mg/L) TP (mg/L) SS (g) DRP (mg) OP (mg) TDP (mg) PP (mg) TP (mg) Macroporosity (% v/v) Bulk density (Mg/m3) Ksat (mm/h)
13.8 0.80 0.014 0.19 0.17 0.36 0.28 0.64 0.12 0.16 0.14 0.29 0.23 0.52 12.2 0.91 588
1.4 0.02 0.004 0.01 0.01 0.02 0.04 0.04 0.03 0.01 0.01 0.02 0.04 0.05 1.1 0.02 137
Ponding (min) B Volume of flow (L) SS (g/100 mL) DRP (mg/L) OP (mg/L) TDP (mg/L) PP (mg/L) TP (mg/L) SS (g) DRP (mg) OP (mg) TDP (mg) PP (mg) TP (mg) Macroporosity (% v/v) Bulk density (Mg/m3) Ksat (mm/h)
12.8 0.82 0.076 0.06 0.09 0.15 0.92 1.07 0.59 0.04 0.06 0.10 0.72 0.82 10.6 1.10 78
3.1 0.05 0.007 0.01 0.02 0.03 0.09 0.11 0.05 0.01 0.01 0.01 0.07 0.07 1.6 0.03 43
13.5 0.80 0.007 0.20 0.18 0.39 0.24 0.58 0.06 0.17 0.14 0.32 0.18 0.47 12.8 0.90 546 Cultivated 8.5 0.89 0.076 0.04 0.07 0.09 0.97 1.04 0.62 0.03 0.06 0.09 0.77 0.89 8.9 1.10 6
A
Standard error of the mean (all grassland or cultivated soils combined). Time to the initiation of overland flow. C Measured 1 h after flow initiation. B
Perhaps unexpected was the lack of any significant relationship between overland flow variables (time to ponding or volume) and Ksat. However, this can be explained by the occurrence of different mechanisms and pathways of overland flow such as saturationexcess and infiltration-excess (Hortonian). In our situation, the restricted drainage out of the bottom of the boxes would have resulted in the gradual wetting-up and saturation of soils, causing saturation-excess overland flow. As mentioned above this would be a function of the soils ability to hold water and thus, macroporosity. However, due to treading, the
5
10
15
20
25
0
40
60
80
100
No. of treading imprints/m2
20
120
Grassland Cultivated Grassland fit Cultivated fit
5
15
20
25
30
Time to ponding (min)
10
35
40
0.0
0.4
0.6
0.8
Overland flow volume (L)
0.2
1.0
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Fig. 1. Variation of macroporosity and Ksat with the number of treading imprints, time to ponding, and volume of overland flow over 1 h (after initiation). Note the log axis for Ksat values. All curve fits are linear relationships except the exponential decrease for sediment.
1
10
100
0 1000
Ksat (mm/h)
Macroporosity (% v/v)
30
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rainfall rate of 15 mm/h would have exceeded the Ksat values of some soils, causing ponding to either occur or not, and eliminating the likelihood of a linear relationship between the 2 variables. This is indicated in Fig. 1 as points below the dashed line (at 15 mm/h), and shows that infiltration was exceeded (and infiltration-excess overland flow likely) in a much greater proportion of the cultivated soils than grassland soils. However, this is also subject to the time taken for the infiltration rate in unsaturated soil to decrease to the infiltration rate under saturated conditions, and as such the influence of any tortuous preferential flow pathways (Dunne et al. 1991). Unfortunately, we cannot ascertain the exact proportion of flow as infiltration- or saturation-excess mechanisms without knowledge of the variation of hydraulic conductivity with time and moisture conditions. Additional variation of Ksat values would have occurred depending upon soil microtopography, whereby the preferential flow paths, which influence Ksat are more prevalent in higher sections usually associated with vegetation, compared with the microdepressions (Dunne et al. 1991). The influence of treading on sediment loss The loss of sediment from the soil surface by water erosion in our experiment involves 3 main processes: (1) the impact of raindrops and soil ‘wetting-up’ causing slaking and dispersion; (2) the detachment of soil particles into flow; (3) the transport of detached material by gravity or overland flow. As evident from data presented in Figs 2 and 3, treading obviously has a significant impact on the concentration and load of sediment lost in overland flow. Factors pertinent to sediment loss affected by treading and subsequent compaction include, increasing soil bulk density, soil strength (resistance to compaction) and cohesion, and decreasing compressibility and infiltration (Climo and Richardson 1984; Bradford and Peterson 2000). Coupled to these factors are the increased physical disturbance and movement of soil by hoof impact. All factors should be common to both cultivated and grassland soils. However, it is clear from both the large differences in sediment concentration and load, and the trends of these with treading, that the relative effect of each factor differs and/or other processes are also involved. For untrodden cultivated soils, sediment losses were an order of magnitude greater than losses from untrodden grassland soils. However, the negative relationship between treading and sediment losses for cultivated soils and the positive relationship for grassland soils resulted in equivalent sediment losses between cultivated and grassland soils trodden 120 times/m2 (Fig. 2). If physical disturbance of the soil were the primary factor influencing sediment loss, then one would expect a positive relationship between treading and sediment loss. Clearly this is not the case for cultivated soils. However, it is likely that compression of the soil due to treading will have altered the performance of the soil surface in response to the erosive power of rainfall and subsequent overland flow. Two of the main reasons for decreasing sediment loss with treading in cultivated soils are likely to be: • The increase in soil strength and cohesion and, thus, resistance to detachment by raindrop impact and during flow; • The increasing ponding with treading (due to the decrease in Ksat and macroporosity and an increase in surface roughness from imprints), causing a decrease in erosion by raindrop impact (Torri and Borselli 2000). Compared with cultivated soils, grassland soils exhibited a greater mean macroporosity and Ksat, implying a greater resistance to treading damage (Table 1). This would also imply less surface roughness, and hence, the likelihood for ponding to influence sediment loss would be greatly decreased, while the relative influence of other factors, such as physical disturbance and the smearing of soil, would be enhanced compared with cultivated soils.
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0.30 Grassland Cultivated Grassland fit Cultivated fit
0.10
0.25 0.20
0.08
0.15 0.06 0.10 0.04
DRP (mg/L)
Suspended sediment (g/100 mL)
0.12
955
0.05 0.02
0.00
0.00 0.5 0.3
0.2
0.3 0.2
0.1
TDP (mg/L)
DOP (mg/L)
0.4
0.1 0.0 0.0
1.6
1.2
1.4 1.2 0.8
1.0
0.6
0.8
TP (mg/L)
PP (mg/L)
1.0
0.6
0.4
0.4 0.2
0.2
0.0 0
20
40
60
80
100
120 0
20
40
60
80
100
120
0.0
No. of treading imprints/m2
Fig. 2. Concentration of sediment and P fractions as a function of the number of treading imprints. The arrow in the suspended sediment graph represents a threshold at 30 imprints/m2 above which SS is accelerated. This used a fit of a split-line model to the grassland data (70% variance accounted for and standard error of 0.014). All curve fits (except split-line) represent either an exponential rise to maximum or exponential decrease, except the linear relationships for sediment, PP, and TP.
Nguyen et al. (1998) noted that the loss of sediment due to treading in grassland soils was much greater above a visual assessment of soil damage of 40%. Our results showed that although a linear relationship was found between sediment loss and the number of treading imprints (Fig. 2), that more sediment loss occurred when more than 30 imprints/m2 were present (as evident in the significant fit of a split-line relationship, which identifies 2 linear relationships either side of a threshold value: McDowell and Sharpley 2001a).
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Suspended sediment (g)
0.08
0.18 0.16
Grassland Cultivated
0.14
0.06
0.12 0.10
0.04
Grassland fit Cultivated fit
0.08
DRP (mg/L)
956
0.06 0.02
0.04 0.02
0.00
0.00
0.4
0.3 0.2 0.2
TDP (mg/L)
DOP (mg/L)
0.3
0.1 0.1
0.0
0.0 1.2
1.0
PP (mg/L)
0.8 0.6 0.6 0.4
TP (mg/L)
1.0
0.8
0.4 0.2
0.2
0.0
0.0 0
20
40
60
80
100
120 0
20
40
60
80
100
120
No. of treading imprints/m2
Fig. 3. Load of sediment and P fractions as a function of the number of treading imprints. All curve fits represent either an exponential rise to maximum or exponential decrease, except the linear relationships for sediment, PP, and TP.
Grass cover greatly decreases the kinetic energy of raindrop impact by intercepting rainfall and dissipating the energy of impact (Torri and Borselli 2000). Surface soil erosion is further decreased by the much greater trapping efficiency of grass compared with bare soil. This is clearly evident by a comparison of suspended sediment from the untrodden grassland and cultivated soils (Figs 2 and 3). However, as treading increases, more grass is crushed, killed, and buried negating the beneficial effect of cover and allowing rainfall to impact upon the soil surface. Indeed, Greene et al. (1994) showed that removing perennial grasses and treading the uncovered soil can have a significant effect on the flow and
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sediment generated. Reported differences in soil physical responses to treading, due to moisture status or soil mineral composition (e.g. Climo and Richardson 1984; Nguyen et al. 1998; Sheath and Carlson 1998) were unlikely to be an influence here (mean soil moisture (v/v) of cultivated and grassland soils before simulated rainfall was applied: 58 ± 5% and 60 ± 4%, respectively). A significant positive relationship between the load of suspended sediment and treading is apparent in grassland (Fig. 3). However, no relationship occurred for cultivated soils. Since we have identified ponding to be a significant influence on the soil erosion process, this would also imply that the selective erosion of fines (soil particles
