Soil physical and chemical properties in response to long-term cattle grazing on sloped rough fescue grassland in the foothills of the Rocky Mountains, Alberta

Geoderma 346 (2019) 75–83

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Soil physical and chemical properties in response to long-term cattle grazing on sloped rough fescue grassland in the foothills of the Rocky Mountains, Alberta Bin Zhanga,b, Ryan Beckb, Qingmin Panc, Mengli Zhaoa, Xiying Haob,

T

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a

College of Grassland, Resources and Environment, Inner Mongolia Agricultural University, Hohhot 010011, China Agriculture and Agri-Food Canada, Lethbridge Research and Development Centre, 5403-1st Ave. S., Lethbridge, Alberta T1J 4B1, Canada c Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b

A R T I C LE I N FO

A B S T R A C T

Handling Editor: Morgan Cristine L.S.

Soil physical and chemical properties are critical indicators for soil health assessment. It is important to understand how these qualities respond to different grazing intensities at different landscape slope positions. This study evaluated how soil physical and chemical properties respond to cattle grazing intensity and slope position on a rough fescue grassland. Cattle have been grazing at four stocking rates, 0, 1.2, 2.4 and 4.8 animal-unitmonths (AUM) ha−1 since 1949 to simulate Control (CK), Light (L), Heavy (H), and Very Heavy (VH) grazing intensities. Surface soil penetration resistance was measured using a portable penetrometer and soil samples were taken from the top and bottom slope positions for each grazing paddock (but only from the top position for CK) in September 2016. Soil texture, aggregate stability, pH, available phosphorus (AP), water-soluble ions concentration and soil cation exchange capacity (CEC) were determined. Bulk density, porosity and hydraulic properties were measured using a HYPROP system. The S-index, the slope at the inflection point on the soil moisture retention curve, was calculated. Soil clay and silt contents were lower in VH than L treatments; total porosity, air capacity, and saturated water content followed the same pattern. However, the opposite was true for sand content, penetration resistance, and bulk density, with differences between the two grazing treatments greater at the top than the bottom slope position. Grazing changed the shape of soil moisture retention curve in both slope positions with the curve steeper in L than H and VH treatments. The S-index and saturated hydraulic conductivity were reduced by grazing but their responses to slope position and its interaction with grazing intensity were not significant. Soil pH, AP, and water-soluble Na+, K+, Mg2+, Ca2+ and SO42− concentration increased with the animal stocking rate while Cl− and CEC were not affected. Similarly, greater increases in soil pH with increased animal stocking rates were observed in the top than bottom position, while AP, Na+, K+, and SO42− increased with animal stocking rate only in the bottom position. Our results indicate that soil physical properties may be more sensitive to grazing at the top than bottom of a slope. Thus, slope should be considered when developing rangeland soil health assessment indices and grazing management strategies.

Keywords: Slope position Soil hydraulic properties HYPROP system Soil moisture retention curve S-index

1. Introduction Demand for livestock production has increased worldwide and “the total area occupied by grazing is equivalent to 26 percent of the ice-free terrestrial surface of the planet” (Steinfeld et al., 2006). Over-grazinginduced soil degradation has become a global issue and is getting increasing attention by scientists and ranch owners around the world. Assessment of soil health, which is defined as the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans (NRCS, 2018), is an essential first step for the

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restoration of degraded grassland, followed by development of proper grazing strategies. Soil physical, chemical and biological attributes are three type of indicators, related to different functional soil processes, which are typically used to evaluate soil health (Karlen et al., 1997). Soil physical and chemical qualities are essential to maintain ecosystem function and protect land from degradation. Sustainable management practices also seek a balance between increasing livestock production and sustaining soil physical and chemical properties (Burke et al., 1995). Previous studies have shown that the activities of large herbivory,

Corresponding author. E-mail address: [email protected] (X. Hao).

https://doi.org/10.1016/j.geoderma.2019.03.029 Received 11 July 2018; Received in revised form 27 February 2019; Accepted 18 March 2019 0016-7061/ © 2019 Published by Elsevier B.V.

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2. Materials and methods

such as hoof action, defoliation and defecation, can affect soil physical and chemical properties directly or indirectly (Bilotta et al., 2007; Drewry et al., 2008; Greenwood and McKenzie, 2001). However, the response of these properties to grazing can also be affected by an array of biotic and abiotic factors, including vegetation composition and productivity, soil water and nutrient availability, grazing strategy (livestock type, grazing time and intensity), and topography (Cournane et al., 2011; Eldridge et al., 2017). Therefore, the response of soil properties to grazing is highly context-dependent leading to varying results across different experiments. The role of grazing strategy in regulating soil physical and chemical properties has been extensively studied in recent years. For example, previous research has demonstrated that due to the animal body weight, cattle grazing can have greater detrimental effects on soil physical properties than smaller animals such as sheep and deer (Cournane et al., 2011; Eldridge et al., 2017). In another study, Teague et al. (2011) reported that multi-paddock grazing may facilitate maintaining soil physical and chemical qualities compared to continuous grazing by ensuring light to moderate use in the growing season and providing adequate recovery time. Several studies have also shown that soil physical and chemical properties generally respond positively to light or moderate grazing but negatively to heavy grazing (Greenwood and McKenzie, 2001; Steffens et al., 2008). The importance of topography, especially slope, in regulating soil response to grazing has received increasing attention in recent years (Sigua and Coleman, 2010; Zhang et al., 2018a). In contrast to relatively flat soil, steeply-sloped land may be more sensitive to grazing (Mwendera and Saleem, 1997; Nguyen et al., 1998). For example, Mwendera and Saleem (1997) found that under heavy grazing activity, more soil loss occurred from plots on steep than gentle slopes, and that gentle slopes could withstand more grazing pressure. Studies have shown that topography can induce spatial heterogeneity of resources and vegetation, which affects grazing distribution (Willms, 1988; Kölbl et al., 2011). Along a toposequence of catena, the bottom landscape position may contain more water and nutrients that flow down from higher positions and hence have higher forage productivity and quality. On the one hand, highly productive soil is expected to be more stable and resistant to grazing impacts due to greater plant cover and biodiversity (Loreau, 2010). However, more livestock are attracted to highly productive areas and their activities may be more intensive and exert greater stress on the soil. Together, these effects complicate the response of soil to grazing. Despite many studies on how topography regulates soil response to grazing, little is known about how different grazing intensities affect soil physical and chemical properties along a landscape slope. The native grasslands of the Foothills Fescue prairie in southwestern Alberta occupy about 289,000 ha and are managed primarily for cattle grazing (Bailey et al., 2010). In a recent publication, Zhang et al. (2018a) evaluated how some soil biological indicators respond to grazing intensity under different landscape positions, including soil organic carbon, total nitrogen, active carbon, microbial respiration‑carbon, etc. To get a fuller understanding of grassland soil response to grazing intensity and topography and to provide data for a comprehensive soil health assessment for this rough fescue grassland, we present how key soil physical and chemical indicators respond to grazing intensity and topography. Therefore, the aims of this study were to (1) evaluate the response of soil physical and chemical properties to cattle grazing and (2) estimate how slope position regulates the response of those physical and chemical properties. Since grazing has detrimental effects on most biological indicators, especially in the bottom slope positions (Zhang et al., 2018a), we hypothesized that (1) heavy and very heavy grazing intensity will have negative effects on soil physical and chemical properties and (2) greater negative changes will occur in the bottom position since cattle prefer to graze at the bottom than the top of a slope.

2.1. Study site and grazing treatments This study was conducted at the Agriculture and Agri-Food Canada (AAFC) Range Research Substation (50°12′N, 113°54′W), located in the rough fescue Grasslands in the foothills region of southwestern Alberta near Stavely, about 100 km SW of Calgary. The climate is sub-humid without deficiency of precipitation in any season. Mean annual precipitation (1997–2013) is 494 mm; mean annual temperature is 5.3 °C. Elevation ranges from 1280 to 1420 m with an average of 1350 m above sea level. The topography is gently rolling to hilly. Soil is Orthic Black Chernozem with a clay to clay loam texture. In the undisturbed and lightly grazed areas, vegetation is dominated by rough fescue (Festuca campestris Rydb.). The grazing area is co-dominated by Parry's oat grass (Danthonia parryi Scribn.) and Kentucky bluegrass (Poa pratensis L.). The grazing treatments started in 1949 when three paddocks were constructed with areas of 65, 32 and 16 ha. Each paddock has been stocked with 13 cows and their calves for 6 months (from mid-May to mid-November each year since then) to produce three stocking rates, 1.2, 2.4 and 4.8 animal unit months (AUM) ha−1, respectively. The recommended stocking rate for rangeland in this area is 1.6 AUM ha−1 (Wroe et al., 1988). Therefore, we labeled these three stocking rates as Light (L), Heavy (H) and Very Heavy (VH) grazing treatments. Additionally, one 0.75 ha non-grazed exclosure (CK) was included in this study. 2.2. Soil penetration resistance measurement, sample collection and analysis Within each grazing paddock, a 50-m transect was laid perpendicular to the slope at the top and bottom positions. Only one transect was selected in the un-grazed control since the slope was not obvious in this area. Further analysis showed that this transect had similar microclimate and species composition with the top position of the grazing paddocks, so for the present study we deemed this transect as the top position. Ten sampling locations were evenly spaced and treated as ten replications within each transect. Surface soil penetration resistance (0–40 cm) was measured for each sampling location in September 2016 using a portable penetrometer (Eijkelkamp, Netherlands). Samples for texture, aggregate stability and chemical properties analyses were taken in September 2016 and samples for hydrological properties analysis were taken in September 2017. On 19 September 2016, within each sampling location, three soil cores (diameter = 6.5 cm, ≈1 m apart) with 15-cm depth were taken to make a composite sample. Soil samples were air-dried in the laboratory, and then coarsely ground through a 2-mm sieve after stones, plant crowns and visible root fragments had been removed. Subsamples of this sieved soil were further finely ground to pass through a 0.15-mm sieve. Soil texture was determined using the hydrometer method (Day, 1965). Organic matter was removed from sediment using 30% hydrogen peroxide. Soil aggregate stability was measured using the method described by Angers et al. (2007). Soil pH was determined in a 1:2 soil to deionized water ratio. Soil available phosphorus (AP) was extracted with a solution of 0.5 M NaHCO3, using a solid-to-liquid ratio of 1:10 (Tiessen and Moir, 2007). The P concentration in the extract was determined by automated colorimetry on an Astoria analyzer (Astoria-Pacific, Clackamas, Oregon, USA). The water-soluble ion (Sodium: Na+; Potassium: K+; Calcium: Ca2+; Magnesium: Mg2+; Chloride: Cl−; and Sulphate: SO42−) concentrations (solid-to-liquid ratio at 1:2, shaken for 30 min) were determined using a Dionex ion chromatograph (Dionex, Model DX-600 and ICS-1000, Sunnyvale, CA, USA). Soil cation exchange capacity (CEC) was obtained from the ammonium acetate extractable K+, Ca2+, Mg2+ and Na+ concentrations. On 20 September 2017, three soil cores were collected within each transect where soil samples had been taken in 2016. Soil was collected 76

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(Liebig et al., 2013). We assumed that the subsampling error was representative of the experimental error. A repeated measures ANOVA was conducted using PROC MIXED in SAS (SAS Institute Inc., 2008), considering stocking rate and landscape position as fixed factors. Various types of covariance structures (e.g., UN, AR (1), and CS) were fitted and the one with the lowest AIC value was selected for the final analysis. Normality and homoscedasticity were tested first before the ANOVA was conducted. The protected LSD test was used to separate means when fixed effects were significant at the 0.05 probability level. Since there is only one top position for CK, data were analyzed in two steps. In the first step, a two-way ANOVA was performed among the three grazing treatments without CK to assess the effect of grazing intensity, slope position, and their interactions. For the second step, a one-way ANOVA was conducted among the four top positions (CK-top, L-top, H-top and VH-top) to estimate their differences.

at the 5–10 cm depth increment using stainless steel cylindrical cores (inner diameter = 80 mm). In pasture and grassland ecosystems, the upper 5-cm soil layer consists primarily of roots, litter and undecomposed organic matter rather than mineral soil; therefore, land use effects will be less apparent at this depth (Hebb et al., 2017). Thus, in the present study, the upper 5 cm of soil was carefully removed before core insertion. Soil cores were sealed with plastic covers to prevent sample loss and stored at 4 °C in the lab prior to analysis. Soil hydraulic properties were determined using the extended evaporation method (Schindler et al., 2010). A HYPROP system (Hydraulic Property Analyzer, METER Group, Inc., Pullman, WA, USA) was used and the experiments were conducted following the standard procedure described by the HYPROP-FIT Software User's Manual (UMS, 2015). Briefly, two tensiometer probes with different lengths (3.75 cm and 1.25 cm) connected to a sensor unit were inserted into the pre-saturated soil cores to quantify the tension at two depths within the soil core. Sensor units were connected to a computer to record changing tensions under typical drying conditions in the lab. Samples were weighed twice daily until the top tension curve reached the cavitation phase. Then, all soil was cleaned off the stainless-steel cylindrical cores, oven dried for 72 h (105 °C) and the weight was recorded again. The moisture retention curve was computed using volumetric water content and the soil water potential data. Measured data were fitted to the van Genuchten (1980) model using HYPROP Fit® ver. 3.5.4 (UMS, Germany) as:

3. Results 3.1. Soil physical properties Soil resistance to penetration (soil hardness) was affected by grazing intensity and their interaction with slope position, with the main effects occurring in the top 20 cm (Fig. 1; Table 1 and S1). In the 0–10 cm soil layer, H and VH treatments increased soil hardness in both slope positions; in the 10–20 cm soil layer, VH treatment increased soil hardness

θ = θr + (θs − θr)[1 + (αh) n]−m where θ is the volumetric water content (cm3 cm−3), θr is the residual water content (cm3 cm−3), and θs is the saturated water content (cm3 cm−3); α (hPa−1), n and m (m = 1–1/n) are empirical shape parameters, where α is a negative inverse of the air entry potential, h is the tension potential (-hPa) and n is a shape parameter related to the curve smoothness. Soil saturated water content, field capacity (water held by soil against gravity following an irrigation or rainfall event) and wilting point (when soil suction holds water so strongly that plants cannot use any) were obtained from the moisture retention curve. The pF (log cm H2O pressure) ranged from 0 to 7; the corresponding pF values for the soil saturated water content, field capacity, and permanent wilting point were 0, 2.5, and 4.2, respectively (Tan, 2005). Air capacity (AC), defined by the value of air-filled porosity for a soil at field capacity (White, 2006), is calculated as:

AC = θs − θFC where θFC (cm3 cm−3) is the field capacity water content. Plant-available water capacity was calculated as the difference in volumetric water content between field capacity and permanent wilting point. Relative field capacity (RFC) which indicates the soil's ability to store water and air relative to soil's total pore volume (Reynolds et al., 2008) was calculated as:

RFC = θFC /θs S-index, the slope of the moisture retention curve at its inflection point, was calculated based on the Dexter (2004) procedure as follows: 1

2n − 1 −( n − 2) ⎞ S‐index = −n (θs − θr) ⎛ ⎝ n−1⎠ 2.3. Statistical analysis Due to the lack of replication of grazing treatments, sampling sites within each paddock served as pseudo-replicates. This is not ideal and restricts our ability to draw strong conclusions that can be considered representative of grassland landscapes. However, it does provide a unique opportunity to assess the effects of long-term grazing (almost 70 years), so use of this approach may be justified given the value of the grazing treatments which are rare among long-term ecosystem studies

Fig. 1. Response of soil penetration resistance to grazing intensity at two slope positions on a rough fescue grassland (CK: Control; L: Light grazing at 1.2 AUM ha−1; H: Heavy grazing at 2.4 AUM ha−1 and VH: Very Heavy grazing at 4.8 AUM ha−1). 77

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Table 1 The effects of cattle stocking rate and its interaction with slope position on soil hardness (0–10 cm and 10–20 cm), soil texture (clay, sand and silt content), soil aggregate stability and bulk density (BD) on a rough fescue grassland. Parameter

Position

0–10 cm hardness (psi) 10–20 cm hardness (psi) Clay (%) Sand (%) Silt (%) Aggregate (%)

BD (g cm−3)

Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Average Top Bottom Average

Grazing intensity (AUM ha−1) 0

1.2

2.4

4.8

147 D

222 C a 192 B b 324 B a 242 A b 45.31 A a 35.37 A b 29.86 C a 32.86 C a 24.83 B b 31.77 A a 93.79 B b 95.90 A a 94.84 A 0.53 C a 0.50 C a 0.52 C

253 B a 245 A a 317 B a 270 A b 34.08 C a 29.71 B b 37.48 B a 38.74 B a 28.44 B a 31.55 A a 94.93 AB b 96.74 A a 95.84 A 0.91 B a 0.75 B b 0.83 B

318 A a 258 A b 407 A a 262 A b 34.59 C a 32.96 B a 47.83 A a 39.00 A b 17.58 C b 28.05 A a 94.99 AB a 95.66 A a 95.33 A 0.97 A a 0.87 A b 0.92 A

245 C 40.69 B 28.35 C 30.96 A 95.74 A

0.55C

Average

94.86 b 96.10 a 0.74 a 0.71 b

Numbers in a row followed by different uppercase letters differ at P < .05; numbers in a column followed by different lowercase letters differ at P < .05. Table 2 The effects of cattle stocking rate and its interaction with slope position on soil porosity, field capacity, air capacity, relative field capacity and plant available water on a rough fescue grassland. Parameter

Position

Porosity

Field capacity (%) 3

Air capacity (cm cm

−3

Relative field capacity

Plant available water (cm3 cm−3)

)

Top Bottom Average Top Bottom Top Bottom Average Top Bottom Average Top Bottom

Grazing intensity (AUM ha−1)

Average

0

1.2

2.4

4.8

0.83 A

0.73 B a 0.73 A a 0.78 A 26.81 B b 33.22 A a 0.36 A a 0.35 A a 0.35 A 0.42 B a 0.49 A a 0.46 B 0.12 B a 0.09 B a

0.64C a 0.64 B a 0.66 B 33.74 A a 32.44 A a 0.30 B a 0.27 B a 0.29 B 0.53 A a 0.54 A a 0.53 A 0.22 A a 0.17 A b

0.59C b 0.65 B a 0.64 B 27.42 B b 31.29 A a 0.26 B a 0.29 B a 0.28 B 0.51 A a 0.52 A a 0.51 A 0.14 B a 0.18 A a

32.75 A 0.36 A

0.48 A

0.09 B

0.70 a 0.67 b

0.32 a 0.30 a 0.49 a 0.52 a

Numbers in a row followed by different uppercase letters differ at P < .05; numbers in a column followed by different lowercase letters differ at P < .05.

occurring in the top position (Table 1).

only in the top slope position and no differences were found among the three grazing treatments in the bottom slope position. Soil texture was affected by grazing intensity, slope position and their interactions (Table 1 and S1). At both slope positions, sand content was higher and clay content was lower in H and VH treatments than L treatments. Soil clay content was higher in the bottom than top position with the L and H treatments, but no difference was found between the two slope positions with VH. Soil sand content was the same between the two slope positions for the L and H treatments; however, it was higher at the top than the bottom with VH. Grazing decreased soil silt content only in the top position and no differences were found among the three treatments in the bottom position. Soil aggregate stability was not affected by grazing intensity, but differed significantly between the two slope positions. For the L and H treatments, higher soil aggregate stability was found in the bottom position, and there was no difference between the two slope positions for VH. Grazing and slope position had a significant effect on soil bulk density (Table 1 and S1). In the top position, soil bulk density increased from 0.55 g cm−3 to 0.97 g cm−3 as grazing intensity increased from CK to VH, while in the bottom position soil bulk density increased from 0.50 g cm−3 to 0.87 g cm−3 when grazing intensity increased from L to VH. Overall, our results show that cattle grazing increased soil hardness, bulk density and sand content significantly (while decreasing clay and silt content) with a greater increase

3.2. Soil hydrological properties Most soil hydraulic properties respond significantly to grazing intensity and their interaction with slope position (Table 2 and S2). Soil total porosity decreased with cattle grazing at both slope positions; when grazing intensity increased from L to VH, total porosity decreased from 0.73 to 0.59 and from 0.73 to 0.65 for top and bottom positions, respectively. Grazing does not affect soil field capacity in the bottom position, whereas in the top position higher field capacity occurred for both CK and H but not LH and VH treatments. Soil air capacity was lower in H and VH than CK and L treatments in both slope positions, but no differences were found between the two slope positions for all grazing treatments. Relative field capacity was not affected by grazing intensity in the bottom position. In the top position, L had lower relative field capacity than the other treatments. Plant-available water was higher for H and VH than CK and L treatments in the bottom position, but a higher value was only observed for the H treatment in the top position. Grazing also changed parameters in the Van Genuchten model, as well as the shape of soil moisture retention curves (Fig. 2; Table 3 and S3). In general, cattle grazing decreased saturated volumetric water 78

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3.3. Soil chemical properties Soil pH increased with grazing intensity, from 6.15 in CK to 6.92 in the VH treatment top position and from 6.19 in L to 6.51 in the VH treatment bottom position. Available P, water-extractable Na+, K+ and SO42−concentrations were higher in VH than other grazing treatments, but only in the bottom position; no differences were found among the three grazing treatments in top position and all of them were lower than CK (Tables 4 and S4). The water-extractable Ca2+ and Mg−2 concentrations increased with grazing intensity, with a greater increase in Ca2+ concentration in the bottom position. Water-extractable Cl− concentration was higher in VH than other treatments in the top position, but no differences were found among the three grazing treatments in the bottom position. 4. Discussion 4.1. Response of soil physical and chemical properties to cattle grazing Soil physical and chemical properties have significant effects on the dynamics of soil resources such as water, air and nutrients, as well as plant growth and community development and, ultimately, they will affect grassland service functions (Proffitt et al., 1993). Thus, it is important to understand the response of soil physical and chemical properties to anthropogenic disturbance. In our study, we quantified the response of soil physical and chemical properties to cattle grazing intensity and slope position on a rough fescue grassland in the Foothills of the Rocky Mountains, Alberta, Canada. In partial agreement with our first hypothesis, we found a significant increase in soil penetration resistance, sand content and soil bulk density, and decreased clay and silt contents, porosity, air capacity and most hydraulic properties, for the H and VH grazing treatments. Consistent with other research (Drewry et al., 2008; Pulido et al., 2018), our results provide evidence that overgrazing can have detrimental effects on soil physical properties. Together with our previous results that grazing has negative effects on total and labile organic carbon and nitrogen contents (Zhang et al., 2018a), our results suggest that over-grazing may undermine overall soil health. The impact of livestock grazing on soil physical properties are attributed primarily to animal trampling (Greenwood and McKenzie, 2001; Bilotta et al., 2007; Drewry et al., 2008). Mwendera and Saleem (1997) estimated that the static pressure of an adult cow is about 144 kPa and this pressure may increase by two to four times when the animal is moving (Abdel-Magid et al., 1987). Those mechanical stresses may induce soil structure deformation, increase soil hardness and bulk density and accordingly change soil hydraulic properties. Under light grazing intensity, soil may recover to its original condition through mechanisms of structural resilience such as the growth and decomposition of plants and roots, and the activities of soil microorganisms and smaller animals (Kay, 1990). However, under H or VH grazing intensity, frequent disturbance may override the soil's natural recovery ability and result in soil structural deterioration and consolidation which will negatively affect soil porosity and vital hydraulic properties (Naeth et al., 1990; Mwendera and Saleem, 1997). Consistent with this, our results show that as the grazing intensity increased from L to VH, on average soil bulk density increased from 0.52 g cm−3 to 0.92 g cm−3 but soil porosity decreased from 0.78 to 0.64. In addition to direct trampling, grazing can affect soil physical properties indirectly through their effects on vegetation structure and function. First, vegetation acts as a protective mulch which can buffer the livestock pressure and hence plays a pronounced role in protecting soil from destruction and erosion (da Silva et al., 2003; Bilotta et al., 2007). Second, plant residue is the main resource of organic matter input into soil. Soil physical properties strongly depend on the content of organic matter level and the constituents of that organic matter (Quiroga et al., 1998). Using the same study sites, our previous research

Fig. 2. Influence of cattle grazing intensity and slope position on soil moisture retention characteristics on a rough fescue grassland (CK: Control; L: Light grazing at 1.2 AUM ha−1; H: Heavy grazing at 2.4 AUM ha−1 and VH: Very Heavy grazing at 4.8 AUM ha−1).

content in both slope positions (Table 3). No differences were found for the α parameter among different grazing intensities and between the two slope positions although overall porosity was changed by grazing. Saturated hydraulic conductivity (Log K) decreased with increased grazing intensity in both slope positions, varying from 1.020 cm day−1 for CK to 0.813 cm day−1 for VH treatment in the top and from 1.157 cm day−1 in L to 0.863 cm day−1 for VH in the bottom slope position. The shape of the soil moisture retention curve was different among the four grazing treatments. Specifically, in the top position, the CK and L treatments had a steeper curve than the H and VH treatments and, in the bottom position, L also had a steeper curve than H and VH (Fig. 2). At the same time, our results show that grazing intensity decreased the S-index significantly. Further analysis shows that the Sindex was positively related to soil porosity but negatively to soil bulk density (Fig. 3), suggesting that the S-index is a good proxy of soil physical quality in this fescue grassland.

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Table 3 The van Genuchten model parameters, S-index and saturated hydraulic conductivity response to cattle grazing and slope position. θs is the saturated volumetric water content, α is a negative inverse of the air entry potential, n is a shape parameter related to the curve smoothness, θi is the volumetric water content at the inflection point of the curve, S-index is slope at θi on soil moisture retention curve, and K is saturated hydraulic conductivity. Parameter

θs (cm cm 3

Position

−3

)

α (hPa)

n

θi (cm 3 cm−3)

S-index

Log K (cm day−1)

Top Bottom Average Top Bottom Average Top Bottom Average Top Bottom Average Top Bottom Average Top Bottom Average

Grazing intensity (AUM ha−1)

Average

0

1.2

0.687 A

0.632 0.678 0.658 0.024 0.022 0.023 1.594 1.617 1.605 0.433 0.507 0.470 0.133 0.126 0.130 1.020 1.157 1.088

0.017 A

1.913 A

0.467 A

0.172 A

1.285 A

2.4 Aa Aa A Aa Aa A Aa Aa A Aa Aa A Ba Aa A Ba Aa A

0.641 0.601 0.582 0.026 0.023 0.025 1.371 1.416 1.393 0.393 0.410 0.402 0.118 0.105 0.112 0.890 0.921 0.906

4.8 Aa Ba B Aa Aa A Aa Aa B Aa Ba B Ca Ba AB Ca Ba B

0.541 B a 0.605 B a 0.573 B 0.024 A a 0.028 A a 0.026A 1.478 A a 1.390 A a 1.434 AB 0.387 A a 0.420 B a 0.403 B 0.098C a 0.103 B a 0.101 B 0.813 C a 0.863 B a 0.838 B

0.625 a 0.628 a 0.023 a 0.024 a 1.589 a 1.384 a 0.420 a 0.446 a 0.130 a 0.111 a 0.995 a 0.980 a

Numbers in a row followed by different uppercase letters differ at P < .05; numbers in a column followed by different lowercase letters differ at P < .05.

regulating soil water dynamics and plant water-use efficiency. This is especially important in water limited grassland. Research has pointed out that grazing-induced hydrological changes typically have negative effects on soil ecological functions (Greenwood and McKenzie, 2001), which may accelerate soil loss and degradation. Our results show that cattle grazing changed the soil water retention curve and reduced soil total porosity, saturated water content and hydraulic conductivity. This is consistent with many other studies that reported cattle grazing has suppressive effects on soil hydrological processes (Vandandorj et al., 2017; Mwendera and Saleem, 1997). The L treatment had a steeper moisture retention curve which indicates that soil in this plot may be able to deliver water more effectively across a fixed gradient of pressure. Soil pores provide the habitat for microbes and micro-fauna, which in turn are responsible for biotic mechanisms of soil structural resilience (Taboada et al., 2011). Decreased soil porosity associated with heavy grazing may primarily arise from soil compaction-induced collapse of macropores and larger mesopores, as is documented in other studies (Villamil et al., 2001; Wheeler et al., 2002). Root development can produce more macropores and facilitate the formation of soil porosity. However, root biomass at our study site remained stable under different grazing intensities (Zhang et al., 2018b), so it is likely that grazing activity rather than root development played greater role in regulating soil porosity. The lower saturated water conductivity and changed soil water retention curve may also reflect the decrease in pore volume and the change in pore size distribution. The S-index, a measure of soil microstructure, represents many key soil physical properties (such as bulk density and porosity) and hence was used as a comprehensive index for assessment of soil physical quality (Dexter, 2004). Field research and models all indicate that the Sindex can represent soil quality well (Xu et al., 2017), enabling direct comparison of soil physical quality among different management treatments. In the present study, we found that as grazing intensity increased from L to VH, the S-index decreased from 0.130 to 0.101. Our results are consistent with many other studies which found that anthropogenic disturbance has negative effects on the S-index as well as on soil physical quality (Hebb et al., 2017; Naderi-Boldaji and Thomas, 2016). In south-central Alberta, Hebb et al. (2017) reported that the Sindex is in the order of annual crop land < introduced pasture < native grassland. In their study, the S-index of native grassland was only 0.048 and lower than any of our treatments; this difference may be attributable to differences in soil texture, pH and organic carbon

(Zhang et al., 2018b; Li et al., 2009) indicated that increased grazing intensity significantly decreased vegetation productivity. This may reduce the capacity of vegetation to buffer trampling pressure and thus increase the amount of soil structure alteration. In addition, decreased organic matter input resulting from lower vegetation production may jeopardize both the resistance and resilience of soil physical quality to grazing, and together those effects may make the surface soil more susceptible to loss (Nguyen et al., 1998; Aubault et al., 2015). Third, vegetation may affect soil physical properties indirectly through their effects on soil microbial composition and activities. Vegetation is not only the major source of soil organic matter; it also plays an important role in maintaining soil microclimate by buffering the soil against fluctuations in soil moisture and temperature (Sayer, 2006). Grazinginduced decreases in litter and vegetation may increase the surface soil temperature (Yates et al., 2000) and hence greatly affect soil microbial composition and activities (Classen et al., 2007). A previous study found that soil microbial composition (such as fungal/bacterial ratio) could indicate soil water-holding capacity and nutrient availability and retention (Teague et al., 2011). The higher sand content and lower silt and clay contents found in the heavily grazed treatment in our study may imply the occurrence of soil loss in those paddocks. Decreased vegetation cover coupled with increased bulk density and destruction of soil structure as a consequence of overgrazing may lead to a preferential loss of fine soil particles and a relative accumulation of coarse soil particles (Neff et al., 2005). For example, in southeastern Utah, Neff et al. (2005) found that grazing significantly decreased fine soil particles and afterwards, even without grazing for almost 30 years, the historically grazed areas still had 38% to 43% less silt relative to never-grazed areas. In addition, the increase of pH with heavy grazing may be another indicator of soil loss, since grazing can decrease the depth of the soil profile and result in carbonates being closer to the surface. The reduction in the depth of Ah horizon under grazing has been reported in many studies (Basher and Lynn, 1996; Dormaar and Willms, 1998; Villamil et al., 2001). At the same study site, Dormaar and Willms (1998) found that the depth of the Ah horizon changed from 22 cm in CK to 7.5 in H treatment after 43 years of grazing. Dormaar and Willms (1998) attributed this change to a grazing-induced increase in bulk density and water and wind erosion-induced surface soil loss. Soil hydrological properties, such as total porosity, saturated water content and hydraulic conductivity, exert a pronounced role in 80

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4.2. Slope position mediates response of soil hydrological, physical and chemical properties to cattle grazing Most parameters evaluated in our study responded significantly to the interaction of grazing intensity and slope position, suggesting that slope position plays an important role in regulating soil physical and chemical properties in response to cattle grazing. However, in contrast to our second hypothesis, we found that an increase in grazing intensity led to greater soil property changes in the top slope position. For example, when comparing L with the VH treatment, soil clay and silt content, porosity, and saturated water content decreased more in the top position than the bottom; meanwhile, sand content, hardness and bulk density increased more in the top position. In contrast, in the same study site, our previous research found cattle grazing caused greater changes in total and labile organic carbon, as well as carbon and nitrogen stocks, in the bottom position (Zhang et al., 2018a; Zhang et al., 2018b). Since forage utilization was higher in the bottom than top position (Willms, 1988), we speculate that even under the same treatment grazing disturbance is more intensive in the bottom than top position. Our results suggest that along a given slope, soil hydrological and physical properties may respond to grazing differently than biological properties. This complicates the overall soil health response to grazing. Greater changes occurring in the top position suggest soil hydrological and physical properties may be more sensitive to grazing in this position than the bottom position. More intense disturbance in the bottom than top position does not induce a corresponding response, suggesting that other mechanisms may be involved in those processes. First, in contrast to the bottom position, the top position may suffer greater water and wind erosion and leaching (Mwendera and Saleem, 1997), which will make the top position more sensitive to grazing disturbance. Second, the bottom position may possess a strong structural resilience capability due to the vigorous plant growth and microbial activities which result from downward movement of water and other resources (Knapp et al., 1993). This means that the bottom position may have greater resistance and resilience capacity to grazing disturbance than the top position. Water runoff is one of the most important ways that induces soil loss in this fescue grassland, which can translocate water and mineral nutrients from the top to bottom position. In the same study site, Naeth and Chanasyk (1996) revealed that most annual runoff occurs during spring snowmelt and a few summer storms; a further study found that large snowmelt-induced and summer storm-induced runoffs were present in the H and VH treatment (Chanasyk et al., 2003). Other previous studies have found that the runoff sediment concentrations increase with grazing pressure (Wilcox and Wood, 1988; Mapfumo et al., 2002). Grazing-induced decline of vegetation cover coupled with a decreased infiltration rate will increase the potential for water runoff, further magnifying the resource heterogeneity between top and bottom positions resulting in confounded grazing effects. Further research is needed to study the effect of grazing on water runoff and positional response to grazing. This is important especially given that the frequency of heavy precipitation events with longer inter-rainfall intervals is predicted to increase in the future (IPCC, 2013).

Fig. 3. The relationships of soil structure stability index (S-index) to soil bulk density (A) and soil total porosity (B) on a rough fescue grassland.

content between the two study sites. When used in agricultural soil, Dexter (2004) postulated an S-index of 0.035 as the threshold between good and poor soil structure. However, our results show that although the S-index changed with other soil physical properties (such as bulk density and porosity), it was always higher than 0.035. This suggests that the S-index threshold for rangeland soil may be higher than agriculture soil. Grazing increased most soil chemical constituents such as AP, K+, + Na , Ca2+, Mg2+ and SO42−. Livestock defecation is an important mechanism that returns most chemical elements to the soil, which can accelerate rangeland ecosystem nutrients cycling. Meanwhile, livestock trampling can reduce litter particle size and create better litter-soil contact. This may facilitate litter decomposition and favor nutrient incorporation into soil (Naeth et al., 1991). For most water-extractable chemical constituents (such as AP, Na+, K+, and SO42−), our results show that they only increased in the bottom position. This may be attributed to the more intense cattle activities in the bottom position (as discussed below) which can speed up nutrient turnover. Another reason may arise from the downward movement of water and minerals from the top to bottom position which may deposit more nutrients in the bottom position.

5. Conclusion Cattle grazing significantly affected most soil physical properties with the magnitude of response influenced by slope position. With increasing grazing intensity, soil clay and silt contents, porosity, and air capacity decreased while soil hardness, sand content and bulk density increased more in the top position than the bottom. Grazing changed the soil moisture retention curve in both slope positions with L treatments steeper than H and VH treatments. The S-index and saturated hydraulic conductivity were reduced by grazing in both slope positions. At the same time, we found grazing increased soil pH, AP and most water-soluble ions concentrations with pH increasing more in the top 81

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Table 4 The effects of cattle stocking rate and its interaction with slope position on soil pH value, available P (AP), water-soluble ions (Na+, K+, Ca2+, Mg2+, Cl−, SO42−) concentrations and soil cation exchange capacity (CEC) on a rough fescue grassland. Parameter

Position

pH AP (mg kg +

Na

K

+

Ca

−1

(mg kg

)

−1

)

(mg kg−1)

2+

(mg kg

−1

)

Mg2+ (mg kg−1)

Cl− (mg kg−1)

SO42− (mg kg−1) CEC (cmolkg−1)

Top Bottom Top Bottom Top Bottom Average Top Bottom Top Bottom Average Top Bottom Average Top Bottom Average Top Bottom Top Bottom

Grazing intensity (AUM ha−1)

Average

0

1.2

2.4

4.8

6.15 B

5.94 B a 6.19 B a 56.44 A a 44.08 B b 3.24 A a 1.77 B a 2.51 B 47.97 AB a 38.81 B a 44.46 A a 37.25 B a 40.86 B 7.12 BC a 6.71 B a 6.92 B 6.41 A a 10.31 A a 8.36 B 6.93 B a 7.09 B a 37.54 AB a 36.38 B a

6.18 B a 6.24 B a 70.10 A a 56.36 B b 3.34 A a 2.84 AB a 3.09 AB 38.30 B a 43.59 B a 34.41 B a 49.67 AB a 42.04 B 6.06C a 7.93 AB a 7.00 B 8.36 A a 14.02 A a 11.18 A 4.14 B a 4.46 B a 31.28 B b 43.60 A a

6.92 A a 6.51 A b 65.54 A b 98.13 A a 3.70 A a 4.32 A a 4.01 A 43.53 B b 77.73 A a 48.53 A a 57.41 A a 52.97 A 8.96 A a 8.66 A a 8.81 A 12.55 B a 9.30 A a 10.93 A 6.16 B b 13.52 A a 29.23 B b 40.87 AB a

74.22 A 3.37 A

50.61 A 48.64 A

8.39 AB

9.67 AB

13.96 A 56.92 A

3.41 a 2.98 a

44.01a 48.11 a 7.63 a 7.77 a 9.25 a 11.21 a

Numbers in a row followed by different uppercase letters differ at P < .05; numbers in a column followed by different lowercase letters differ at P < .05.

position but AP and most water-soluble ions concentrations only increasing in the bottom position. Our results suggest that soil physical quality may be more sensitive to grazing in the top position than the bottom position. This is important and should be considered when developing rangeland soil health assessment indices and grazing management strategies.

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