Does Increasing the Diversity of Small Grain Cropping Systems Improve Aggregate Stability and Soil Hydraulic Properties?

Abstract

Wheat (Triticum aestivium L.) and barley (Hordeum vulgare L.) are two commonly grown cereal crops in the northern Great Plains. Adding other crops such as field pea (Pisum Sativum L.), canola (Brassica napus L.), or camelina (Camelina sativa L.) to wheat or barley cropping systems may improve soil quality. However, little is known about the effects of including oilseeds in small grain cropping systems on soil physical properties. We sampled an 8-year dryland study with 10 different cropping systems including continuous spring wheat, continuous winter wheat, continuous barley, pea–spring wheat, pea–barley, pea–winter wheat, pea–barley–camelina–spring wheat, pea–barley–canola–spring wheat, pea–winter wheat–camelina–spring wheat and pea–winter wheat–canola–spring wheat. We measured dry aggregate stability, wet aggregate stability, water retention, hydraulic conductivity, bulk density and total carbon. Continuous barley and winter wheat had a higher fraction of large dry soil aggregates, whereas the pea–barley–canola–spring wheat and pea–spring wheat cropping systems had a higher fraction of small aggregates in the 0–15 cm depth. However, wet aggregate stability, water retention, bulk density, hydraulic conductivity and soil carbon concentration were not affected by the cropping system in the 0–15 cm depth. Diversifying small grain cropping systems by adding canola or camelina oil seeds and peas generally did not affect soil physical properties at this location.

1. Introduction

Small grains including winter and spring wheat (Triticum aestivium L.) and barley (Hordeum vulgare L.) are widely grown crops throughout the northern Great Plains (NGP) in the United States and Canada [1]. Wheat and barley have frequently been planted in a continuous cropping system either every year or having an alternative fallow year in semi-arid areas such as the NGP [2]. Growing continuous wheat or barley can lead to weed and disease issues and reduced grain yield over time in the NGP [3,4,5]. Growing wheat or barley in a rotation with crops such as camelina (Camelina sativa L.), canola (Brassica napus L.) or field peas (Pisum sativum L.) may not only improve small grain yields but may also provide additional revenue-generating crops, reduce nitrogen inputs, and enhance soil quality and water use [3,4,5,6,7]. However, little is known about how soil physical properties change when these three alternative crops are added to wheat or barley cropping systems.

Crop rotations are important for maintaining agricultural sustainability. Diversifying cropping systems has many benefits ranging from improving environmental quality to increasing grain yields [8,9]. One important reason for having a diverse crop rotation is to control weeds, pests and diseases [9]. In addition, increasing the diversity of cropping systems can improve nutrient use efficiency and reduce the needed fertilizer inputs to optimize grain yields [8,9,10]. Crops such as field pea and soybean (Glycine max L.) can fix nitrogen from the atmosphere and convert it into organic nitrogen in plant biomass, which can be mineralized into useable nitrogen for the following crop and reduce the inorganic nitrogen fertilizer needs [9]. Reducing the need for inorganic chemicals such as herbicides, pesticides and inorganic fertilizers in cropping systems benefits the environment by reducing the amount of chemicals applied that can become pollutants in ground and surface waters [11,12,13]. Additionally, having more diverse cropping systems can be more resilient to changing climatic conditions such as droughts [11,13]. In addition, more diverse cropping systems can have a higher water use efficiency [9,11]. Alsomore divers cropping systems can have higher crop grain yields [10,11,12,13]. Additionally, having more diverse cropping systems provide multiple sources of income for agricultural producers. It is also known that increasing crop rotational diversity may improve soil health [13]

Soil aggregate stability, water retention, and hydraulic conductivity are important indicators of soil health. Soil wet and dry aggregate stability, hydraulic conductivity and water retention are essential soil quality properties in small grain-cropping systems. Rainfall in the NGP is limited, making the ability of the soil to store water important [14]. Moreover, fields in the NGP region experience high wind during a portion of the year, making the stability of dry soil aggregates near the surface important [15,16]. Additionally, the stability of wet soil aggregates, water retention and hydraulic conductivity are meaningful because they allow the limited water from rainfall to infiltrate into the soil surface to be stored throughout the year [17]. These soil properties also are linked to environmental quality due to their indication of soil water storage and movement rate though the soil.

Previous studies have been conducted to analyze the effects of cropping system diversity in wheat and barley cropping systems on soil water retention, aggregate stability and bulk density [18,19,20,21,22,23,24,25]. In some cases, increasing the diversity of cropping systems containing wheat may increase soil carbon concentration, wet and dry aggregate stability, and water retention [20,22,23]. However, a majority of these studies reported no differences in soil aggregate stability, water retention, hydraulic conductivity and bulk density when wheat or barley cropping system diversity was increased [18,19,20,21,24,25]. In some of the studies where there were differences, there was greater precipitation and longer growing seasons than in the NGP [22,23]. Differences in the soil organic carbon concentration caused by cropping systems and or the quantity of residues returned annually may be what leads to differences in soil physical properties in different small grain cropping systems [22,23]. Additionally, differences in total carbon and the carbon to nitrogen ratio (C/N) of crop residues that are produced in the cropping system may affect the soil physical properties [20]. Canola and camelina oilseeds and field peas generally produce less residue than wheat or barley and have lower C/N, which may lead to differences in residue cover on surface and soil carbon concentration [26,27]. Few of these previous studies on the effects of small grain cropping systems on soil physical properties contained field peas or oilseed crops such as camelina and canola in the cropping system.

Oilseed crops are crops that can be rotated with wheat, which may increase the net annual income return by producers and improve water use and soil quality [28,29,30]. Oilseed crops such as canola and camelina can be used to create renewable biofuels, which may have an increasing demand in the future [31]. However, little is known about the effects of adding oilseed crops to wheat and barley cropping systems on soil physical properties. The objective of this study is to determine whether diversifying wheat or barley small grain cropping systems by adding field pea, camelina and canola improves soil dry and wet aggregate stability, bulk density, water retention, hydraulic conductivity and soil carbon concentration compared to continuous wheat or barley cropping systems on a loam textured soil. We hypothesized that increasing the diversity of wheat and barley cropping systems would improve soil physical and hydraulic properties.

2. Materials and Methods

2.1. Experimental Layout

This experiment was located in Sidney, MT, USA (47°46′ N, 104°16′ W) and established on a field containing Williams loam soil series (fine loamy mixed superactive frigid Typic Argiustoll). This study was established on a field that was previously under continuous spring wheat and spring wheat–safflower (Carthamus tinctorius L.) cropping systems with no tillage [32]. The average annual temperature is 8 °C and average annual precipitation is 340 mm at this location [33]. The average sand, silt, and clay contents were 35, 32.5, and 32.5 percent, respectively; soil pH was 6.1 in the 0–20 cm depth; soil organic carbon was 12.5 g kg⁻¹ and bulk density was 1.25 g cm⁻³ in the 0–15 cm depth prior to study initiation [32,33]. This study was initiated during fall of 2013 with four replications in a randomized block design. Each phase of every cropping system was present in each study year. Ten cropping system treatments were established including continuous spring wheat, continuous winter wheat, continuous barley, pea–spring wheat, pea–winter wheat, pea–barley, pea–barley–camelina–spring wheat, pea–barley–canola–spring wheat, pea–winter wheat–camelina–spring wheat and pea–winter wheat–canola–spring wheat. Winter wheat was planted in September and other crops were planted during April or May and harvested during late July and August throughout the study (

Table 1. Annual planting and harvest dates for spring wheat, winter wheat, barley, field pea, canola and camelina.

Crop	Spring Wheat	Winter Wheat	Barley	Field Pea	Canola	Camelina
Year	Planting Date
2014	4/24	9/24/2013	4/25	4/22	4/25	4/25
2015	4/16	9/17/2014	4/16	4/14	4/15	4/15
2016	4/9	9/18/2015	4/9	4/11	4/9	4/9
2017	4/17	9/14/2016	4/17	4/15	4/18	4/18
2018	5/5	9/22/2017	5/5	5/5	5/7	5/7
2019	4/16	9/12/2018	4/16	4/15	4/16	4/16
2020	4/28	9/27/2019	4/28	4/28	4/28	2/28
2021	4/29	9/15/2020	4/29	4/29	4/30	4/30
	Harvest Date
2014	8/14	8/14	8/14	8/29	7/28	7/22
2015	8/18	7/24	8/19	NH	8/13	8/4
2016	8/15	7/21	8/2	7/19	8/1	7/19
2017	8/1	7/14	7/31	7/16	7/31	7/25
2018	8/15	7/25	8/9	7/26	8/7	8/3
2019	8/15	7/30	8/16	7/31	8/16	7/31
2020	8/11	8/10	8/6	7/29	8/11	8/12
2021	8/12	8/11	8/11	7/27	7/26	7/21

NH means not harvested.

). Only the grain portion of the crops were harvested. All crops were planted using a no-till drill and weeds were controlled using appropriate herbicides for each crop and rotation. Soil sampling was conducted in fall of 2021 after two rotation cycles of the four-year crop rotations. Samples used to measure dry and wet aggregate stability and carbon concentration were collected from the winter wheat phase of the pea–winter wheat and continuous winter wheat cropping system, the barley phase of the continuous barley and pea–barley cropping system and the spring wheat phase of the other six cropping systems. The soil water retention curve, hydraulic conductivity and bulk density were only measured on the continuous spring wheat, pea–spring wheat, pea–barley–camelina–spring wheat, pea–barley–canola–spring wheat, pea–winter wheat–camelina–spring wheat and pea–winter wheat–canola–spring wheat cropping systems.

2.2. Dry Aggregate Stability (DAS)

Bulk soil samples were collected from three locations per plot on 21 September 2021 using a spade shovel and then mixed into one composite sample per plot. Samples were collected from the 0–7.5 and 7.5–15 cm depths with minimal disturbance to dry aggregates by gently placing them into a plastic bag, which was followed by setting the bag into a storage tote. Aggregates were then air-dried in an aluminum baking pan for 7 days. Approximately 500 g of air-dry aggregates were weighed and placed on top of a nested set of sieves of 19-, 8-, 4.75-, 2-, 1-, 0.83- and 0.25-millimeter diameters with a catch pan on the bottom. The set of nested sieves were then placed on a vibratory shaker and fastened into the shaking device [34]. The vibratory shaker was run for 5 min at an amplitude of 9. After five minutes, the vibratory shaker was turned off; then, the contents remaining on top of each sieve were weighed. Samples then were placed into plastic bags in aggregate size groups of >8 mm, 4.75–8 mm, 2–4.75 mm and 0.25–2 mm. For each plot, two samples were run on the set of nested sieves. The fraction of aggregates retained on each sieve, mean weight diameter (MWD) and geometric mean diameter (GMD) of dry soil aggregates was then calculated. Equation (1) shows how MWD was calculated where n is the number of aggregate size classes, x_i is the mean diameter of an aggregate size class i, m_a is the mass of aggregates of a given size class, and m_s is mass of the total sample prior to sieving [35].

$$\mathrm{MWD}={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}\left({\mathrm{x}}_{\mathrm{i}}\times \frac{{\mathrm{m}}_{\mathrm{a}}}{{\mathrm{m}}_{\mathrm{s}}}\right)$$

(1)

Equation (2) was used to calculate GMD where n, m_a, m_s and x_i are the same as was used to calculate MWD [35]. Wind erodible fraction was considered to be the fraction of soil aggregates less than 0.83 mm [36].

$$\mathrm{GMD}={\mathrm{e}}^{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}(\frac{{ \mathrm{m}}_{\mathrm{a}}}{{\mathrm{m}}_{\mathrm{s}}}\times \ln {\mathrm{x}}_{\mathrm{i}})}$$

(2)

2.3. Wet Aggregate Stability (WAS)

Soil wet aggregate stability (WAS) was measured using nested sieves on a reciprocating sieving device [35]. First, approximately 50 g of soil aggregates of a certain size group was weighed out. Aggregates in the size classes > 8 mm and 4.75–8 mm were placed on filter paper resting on a set of nested sieves that had 4.75-, 2-, 1-, 0.5- and 0.25-millimeter mesh openings and were immersed in a container full of water. This separated the aggregates into size classes of 4.75–8 mm, 2–4.75 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm and <0.25 mm. Water was poured into the container to the level of the filter paper on top of the 4.75 mm sieve. Then, the aggregates were wet by capillary forces for 10 min. Following wetting, the aggregates were immersed on top of the 4.75 mm sieve, and the filter paper was gently removed. The wet sieving device was run for 10 min at 35 revolutions per minute after which the sample remaining on top of each sieve was washed into pre-weighed cans and oven dried at 105 °C for 48 h. Percent sand of the fraction <0.25 mm also was determined by pouring container contents less than <0.25 mm through a 53 µm sieve after sieves were removed. Contents retained on the 53 µm sieve were then oven dried at 105 °C for two days and weighed. Following weighing, aggregates and the fraction < 0.25 mm were corrected for sand content by dispersing with 5 g L⁻¹ sodium hexametaphosphate solution and washing through the 53 µm sieve. Then, contents remaining on top of the sieve were washed into pre-weighed cans, oven-dried for 48 h at 105 °C and weighed. The fractions of water-stable aggregates, MWD and GMD of these soil aggregates were then calculated. Furthermore, 50 g of aggregates from the 0.25–2 mm and 2–4.75 mm aggregates size classes was weighed out. The water level in the container was raised to the level of the filter paper on top of a 0.25 mm sieve. Samples were placed on top of filter paper and wetted by capillarity for 10 min; then, they were immersed in water and wet sieved for 10 min at 35 revolutions per minute. Contents remaining on top of the 0.25 mm sieve were then washed into pre-weighed cans. Contents of the container were poured through a 53 µm sieve, and contents > 53 µm were washed into a pre-weighed metal can and oven-dried at 105 °C for 48 h. Aggregates were then corrected for sand content using the same procedure as used for the larger aggregates. The fractions of water-stable aggregates of these two dry aggregate sizes were then determined.

2.4. Water Retention Curve, Hydraulic Conductivity and Bulk Density

Soil cores were collected on 1 October 2021 from the 0–7.5 and 7.5–15 cm depths using a Hyprop sampling device (Meter Group, Pullman, WA, USA). Cores were covered with plastic end caps, placed in a plastic bag, and then refrigerated until they could be processed. Prior to running on the Hyprop device, samples were trimmed flush with the metal ring; then, a porous cloth was placed on the bottom of the sample prior to saturating in tap water for 48 h. Water retention curves and hydraulic conductivity were then determined by the extended evaporation method using the Hyprop system [37]. After the top Hyprop tensiometer cavitated, the gravimetric water content, oven dry soil mass and bulk density of the sample were determined. The oven-dry soil mass was entered into the Hyprop Evaluation Software to determine the soil water retention curve and saturated hydraulic conductivity [38]. The soil water retention curve and conductivity data were fit with the van Genuchten constrained model with the Mualem conductivity function

$$\mathsf{\theta }\left(\mathrm{h}\right)=\frac{{\mathsf{\theta }}_{\mathrm{s}}-{\mathsf{\theta }}_{\mathrm{r}}}{{[1+{(\mathsf{\alpha }\mathrm{h})}^{\mathrm{n}}]}^{\mathrm{m}}}+{\mathsf{\theta }}_{\mathrm{r}}$$

(3)

$$\mathrm{m}=1-\frac{1}{\mathrm{n}}$$

(4)

$$\mathrm{K}\left(\mathrm{h}\right)={\mathrm{K}}_{\mathrm{s}}{\left(\frac{\mathsf{\theta }-{\mathsf{\theta }}_{\mathrm{r}}}{{\mathsf{\theta }}_{\mathrm{s}}-{\mathsf{\theta }}_{\mathrm{r}}}\right)}^{\mathsf{\tau }}{\left[1-{\left\{1-{\left(\frac{\mathsf{\theta }-{\mathsf{\theta }}_{\mathrm{r}}}{\mathsf{\theta }-{\mathsf{\theta }}_{\mathrm{r}}}\right)}^{\frac{1}{\mathrm{m}}}\right\}}^{\mathrm{m}}\right]}^2$$

(5)

in the Hyprop Evaluation Software [39,40]. Here, θ is volumetric soil water content, θ_s is saturated water content, θ_r is residual water content, α is the van Genuchten air-entry value parameter, n is the pore size distribution parameter, K is soil hydraulic conductivity, K_s is soil-saturated hydraulic conductivity, and τ is the tortuosity factor. The soil water retention and hydraulic conductivity are presented as pF vs. water content and pF vs. hydraulic conductivity where pF is the base 10 logarithm of the water potential in cm (hPa) of water. We analyzed the effect of the cropping system on the van Genucthen α, n, θ_s, and θ_r parameters and Mualem Ks and τ parameters.

2.5. Soil Particle Size Analysis and Carbon Content Measurements

Approximately 100 g of the representative aggregates used to determine aggregate stability was crushed and passed through a 2 mm sieve. Particle size analysis was determined using the hydrometer method [41]. About 25 g of the 100 g soil subsample was crushed with a mortar and pestle to pass through a 0.425 mm sieve (No. 40). Samples were then run on a LECO (LECO, St Joseph Michigan) total C/N analyzer using the dry combustion method at 1300 °C to determine the soil carbon concentration [42]. Previous sampling at this location in 2015 and 2021 found no carbonates in the 0–15 cm depth, so the total carbon concentration is primarily soil organic carbon [Chloe Turner-Meservy, Personal Communication 2021 and [32]. Inorganic carbon was determined on the previously collected samples using the calcimeter method [43]. Organic carbon can be estimated by subtracting the inorganic carbon concentration from the total carbon concentration. The results presented as total carbon concentration in Section 3.4 for the 0–7.5 and 7.5–15 cm depths can be considered the same as soil organic carbon concentration due to samples previously collected from these plots having no inorganic carbon in the 0–15 cm depth.

2.6. Statistical Analysis

Statistical analysis on soil properties was conducted in SAS 9.4 using a mixed model ANOVA (SAS Institute 2017). Fixed variables were crop rotation and depth, and replication was the random variable. Effects were considered statistically significant at p ≤ 0.05. Soil properties of MWD of dry soil aggregates, GMD of wet soil aggregates, average fraction water-stable aggregates, MWD of 4.75–8 mm and >8 mm aggregates, bulk density, water content at −330 hPa (pF 2.5) and −15,000 hPa (pF 4.2), θ_s, θ_r, α, n, Ks, τ soil carbon and clay percentage were correlated to each other using the correlation procedure in SAS. Other soil properties were not included in the correlation analysis. The least squared differences were calculated for water retention and hydraulic conductivity data for the 0–7.5 and 7.5–15 cm depths.

3. Results

3.1. Dry Aggregate Stability

The fractions of aggregates > 8 mm (p = 0.0005), 2–4.75 mm (p = 0.0152), and <0.25 mm (p = 0.0058), MWD of dry soil aggregates (p = 0.0006) and wind erodible fraction (fraction < 0.83 mm) (p = 0.0079) were affected by the cropping system (Figure 1). The fractions of aggregates > 8 mm (p = 0.0023), 4.75–8 mm (p = 0.0030), 2–4.75 mm (p = 0.0001), 0.25–2 mm (p < 0.0001) and <0.25 mm (p < 0.0001), MWD (p < 0.0001) and GMD of aggregates (p = 0.0001) and wind erodible fraction (p < 0.0001) were affected by depth (

Table 2. Effect of cropping system on fraction size of dry soil aggregates greater than 8 mm (>8 mm), 4.75–8 mm, 2–4.75 mm, 0.25–2 mm and less than 0.25 mm (<0.25 mm), mean weight diameter of dry soil aggregates (MWD) and geometric mean diameter of dry soil aggregates (GMD) at the 0–7.5-cm and 7.5–15-cm soil depths.

Cropping System	>8 mm	4.75–8 mm	2–4.75 mm	0.25–2 mm	<0.25 mm	MWD	GMD
Units	Fraction Size					mm
0–7.5 cm Depth
Continuous Winter Wheat	0.182 A	0.041	0.255 BC	0.407	0.115 c	4.375 A	3.241
Continuous Spring Wheat	0.092 B	0.039	0.223 AB	0.467	0.177 ab	2.776 B	3.031
Continuous Barley	0.239 A	0.044	0.164 BC	0.414	0.137 bc	4.726 A	3.720
Pea–Winter Wheat	0.148 B	0.042	0.252 AB	0.432	0.124 c	3.813 B	3.374
Pea–Spring Wheat	0.138 B	0.039	0.167 BC	0.452	0.17 a	3.288 B	2.704
Pea–Barley	0.074 B	0.043	0.234 AB	0.460	0.202 a	2.573 B	2.857
Pea–Winter Wheat–Canola–Spring Wheat	0.095 AB	0.035	0.204 BC	0.477	0.188 ab	3.007 B	2.725
Pea–Winter Wheat–Camelina–Spring Wheat	0.095 B	0.047	0.251 A	0.467	0.162 ab	2.403 B	2.907
Pea–Barley–Canola–Spring Wheat	0.068 B	0.037	0.222 AB	0.485	0.187 a	2.802 B	2.713
Pea–Barley–Camelina–Spring Wheat	0.117 B	0.073	0.187 BC	0.445	0.176 ab	4.342 B	3.018
7.5–15 cm Depth
Continuous Winter Wheat	0.374 A	0.048	0.257 BC	0.256	0.065	7.322 A	3.827
Continuous Spring Wheat	0.182 B	0.077	0.360 AB	0.312	0.069	5.618 B	4.239
Continuous Barley	0.285 A	0.065	0.278 BC	0.305	0.067	5.321 A	3.764
Pea–Winter Wheat	0.132 B	0.063	0.306 AB	0.396	0.102	3.677 B	3.634
Pea–Spring Wheat	0.158 B	0.074	0.290 BC	0.373	0.104	3.736 B	3.708
Pea–Barley	0.093 B	0.045	0.365 AB	0.425	0.072	3.559 B	3.494
Pea–Winter Wheat–Canola–Spring Wheat	0.248 AB	0.070	0.293 BC	0.321	0.068	5.772 B	4.059
Pea–Winter Wheat–Camelina–Spring Wheat	0.119 B	0.065	0.399 A	0.348	0.068	3.361 B	4.026
Pea–Barley–Canola–Spring Wheat	0.123 B	0.050	0.338 AB	0.389	0.098	3.776 B	3.591
Pea–Barley–Camelina–Spring Wheat	0.180 B	0.063	0.326 BC	0.357	0.075	5.278 B	3.756
Statistical Significance
Cropping System	0.0005	NS	0.0152	0.0624	0.0058	0.0006	NS
Depth	0.0023	0.0030	0.0001	0.0001	0.0001	0.0001	0.0001
Cropping System × Depth	NS	NS	NS	NS	0.0500	NS	NS
Replication	0.0354	0.0005	NS	0.0014	0.0001	0.0186	0.0345

Different uppercase letters following numbers indicate statistical significance at p = 0.05 for cropping system effect and lowercase letters following numbers indicate statistical significance at p = 0.05 for cropping system × depth effect. NS means not significant at p = 0.1.

). Replication affected the fractions of >8 mm, 4.75–8 mm, 0.25–2 mm and <0.25 mm aggregates, MWD and GMD of dry soil aggregates and wind erodible fraction (Table 2). Cropping system × depth interaction affected the fraction of aggregates < 0.25 mm (p = 0.05) but did not affect other soil properties related to dry aggregate stability (Table 2). All interactions not mentioned were not significant. In the 0–7.5 cm depth, the pea–barley cropping system (0.188) had the highest fraction of <0.25 mm aggregates, whereas continuous winter wheat (0.114) had the lowest fraction of aggregates < 0.25 mm (Table 2). The fraction of aggregates < 0.25 mm was not affected by the cropping system in the 7.5–15 cm depth (Table 2). Across the two depths, the fractions of aggregates > 8 mm were higher in continuous barley and continuous winter wheat than in the other cropping systems (Table 2). The fraction of 2–4.75 mm aggregates was higher in the pea–winter wheat–camelina–spring wheat (0.325 averaged across 0–7.5 and 7.5–15 cm depths) cropping system than the continuous barley (0.221 averaged across 0–7.5 and 7.5–15 cm depths) and continuous winter wheat cropping systems (0.256 averaged across 0-7.5 and 7.5-15 cm depths) (Table 2). In addition, the mean weight diameter of dry soil aggregates was higher in the continuous winter wheat and barley cropping systems than the other eight cropping systems (Table 2). Additionally, the wind erodible fraction was lower in the continuous winter wheat (0.232 averaged across 0–7.5 and 7.5–15 cm depths), continuous barley (0.263 averaged across 0–7.5 and 7.5–15 cm depths), winter wheat–pea (0.286 averaged across 0–7.5 and 7.5–15 cm depths) and pea–winter wheat–camelina–spring wheat (0.291 averaged across 0–7.5 and 7.5–15 cm depths) cropping systems than the spring wheat–pea (0.363 averaged across 0–7.5 and 7.5–15 cm depths) and pea–barley–canola–spring wheat (0.348 averaged across 0–7.5 and 7.5–15 cm depths) cropping systems.

3.2. Wet Aggregate Stability

The fractions of water-stable aggregates, MWD and GMD of wet soil aggregates were not affected by the cropping system and cropping system × depth affects, but they were affected by the depth effect (Appendix A,

Table A1. Effects of cropping system averaged across depths and depth averaged across cropping systems on fraction of water-stable aggregates for size ranges of 0.25–2 mm (0.25 mm), 2–4.75 mm (2 mm), 4.75–8 mm and >8 mm (8 mm), average fraction of water-stable aggregates (Average), mean weight diameter of 4.75–8 mm aggregates (4.75 mm MWD), geometric mean diameter of 4.75–8 mm aggregates (4.75 mm GMD), mean weight diameter of 8 mm and larger aggregates (8 mm MWD) and geometric mean diameter of 8 mm and larger aggregates (8 mm GMD).

Treatment	0.25 mm	2 mm	4.75 mm	8 mm	Average	4.75 mm MWD	4.75 mm GMD	8 mm MWD	8 mm GMD
Units	Fraction Water Stable Aggregates					mm
Cropping System Effect
Continuous Winter Wheat	0.644	0.454	0.777	0.805	0.670	3.146	1.865	4.850	2.522
Continuous Spring Wheat	0.661	0.579	0.769	0.786	0.698	3.287	2.137	5.683	3.233
Continuous Barley	0.593	0.534	0.793	0.813	0.683	3.415	2.103	7.178	3.151
Pea-Winter Wheat	0.557	0.569	0.737	0.725	0.647	2.431	1.426	4.037	2.733
Pea-Spring Wheat	0.534	0.489	0.770	0.721	0.629	2.389	1.356	3.415	1.431
Pea-Barley	0.571	0.479	0.804	0.779	0.659	2.751	1.523	5.059	2.210
Pea-Winter Wheat-Canola-Spring Wheat	0.474	0.499	0.687	0.835	0.624	2.729	1.590	6.196	3.648
Pea-Winter Wheat-Camelina-Spring Wheat	0.646	0.513	0.752	0.799	0.678	2.910	1.912	4.972	2.464
Pea-Barley-Canola-Spring Wheat	0.653	0.509	0.756	0.772	0.671	2.612	1.331	5.666	3.131
Pea-Barley-Camelina-Spring Wheat	0.529	0.451	0.743	0.799	0.631	2.696	1.507	6.351	3.331
Depth Effect
0–7.5	0.631 a	0.560 a	0.809 a	0.834 a	0.708 a	3.610 a	2.229 a	6.916 a	4.073 a
7.5–15	0.541 b	0.454 b	0.708 b	0.733 b	0.609 b	2.065 b	1.120 b	3.587 b	1.497 b
Statistical Significance
Cropping System	NS	NS	NS	NS	NS	NS	NS	NS	NS
Depth	0.0363	0.0007	0.0012	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
Cropping System × Depth	NS	NS	NS	NS	NS	NS	NS	NS	NS
Replication	0.0728	NS	NS	NS	NS	NS	NS	NS	0.0903

Different letters after numbers indicate statistical significance at p ≤ 0.05. NS means not significant at p = 0.1.

and Figure 2). Replication did not affect the fractions of water-stable aggregates of any aggregate size or GMD and MWD of wet soil aggregates between 4.75–8 mm and > 8 mm (Appendix A, Table A1). The fractions of water-stable aggregates were 0.71 and 0.61 for 0–7.5 and 7.5–15 cm depths, respectively when averaged across aggregate sizes and cropping systems. Moreover, the MWD and GMD values of wet soil aggregates were higher for the 0–7.5 cm depth than for the 7.5–15 cm depth. Across the 4.75–8 mm and >8 mm aggregate sizes, the MWD of wet soil aggregates was 5.26 mm in the 0–7.5 depth and 2.82 mm in the 7.5–15 cm depth, whereas the GMD of wet soil aggregates was 3.15 mm in the 0–7.5 cm depth and 1.31 mm in the 7.5–15 cm depth averaged across cropping systems.

3.3. Water Retention Curve, Hydraulic Conductivity, van Genuchten Fitting Parameters and Bulk Density

The van Genuchten α and θ_s parameters, water content at pf 4.2 (15,300 hPa) and bulk density were affected by depth. Cropping system or cropping system × depth did not affect any of the soil hydraulic properties measured in this study (

Table 3. Effect of cropping system averaged across depth and depth averaged across cropping systems on van Genuchten α (air entry value), pore size distribution parameter (n), residual water content (θ_r), saturated water content (θ_s), saturated hydraulic conductivity (Ks), τ parameter, water content at pF 1.8 (63 hPa), water content at field capacity (pF 2.5, 333 hPa), water content at wilting point (pF 4.2, 15,300 hPa) and bulk density.

Treatment	α	n	θr	θs	Ks	τ	1.8	2.5	4.2	Bulk Density
Units	cm−1		cm3 cm−3	cm3 cm−3	cm Day−1		% Water Content			g cm−3
Cropping System Effect
Spring Wheat	0.038	1.292	0.069	0.550	670.7	−0.819	42.5	31.0	15.4	1.38
Pea-Barley-Camelina-Spring Wheat	0.032	1.311	0.043	0.513	124.3	−1.86	40.4	29.0	13.3	1.44
Pea-Barley-Canola-Spring Wheat	0.037	1.372	0.106	0.546	505.1	−2.00	41.9	30.7	17.4	1.42
Pea-Winter Wheat-Camelina-Spring Wheat	0.037	1.399	0.136	0.539	1279.2	−1.72	42.4	31.2	18.6	1.43
Pea-Winter Wheat-Canola-Spring Wheat	0.033	1.270	0.043	0.530	1428.3	0.628	43.2	32.3	15.2	1.47
Pea-Spring Wheat	0.0367	1.327	0.085	0.555	276.2	−1.77	44.1	32.5	17.7	1.42
Depth Effect
0–7.5	0.042 a	1.317	0.059	0.568 a	1256.3	−1.23	42.9	30.6	14.4 b	1.32 b
7.5–15	0.029 b	1.340	0.102	0.510 b	9.00	−1.29	41.9	31.7	18.1 a	1.54 a
Statistical Significance
Cropping System	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
Depth	0.0130	NS	0.0894	0.0001	0.0794	NS	NS	NS	0.0108	0.0001
Cropping System × Depth	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
Replication	0.028	NS	NS	NS	0.0297	0.0137	NS	NS	NS	0.085

Different letters after numbers indicate statistical significance at p ≤ 0.05. NS means not significant at p = 0.1.

and Figure 3A,B and Figure 4A,B). However, at pF ranges of 2.1–2.3 (125–200 hPa), the spring wheat–pea rotation tended to have a higher hydraulic conductivity than the other five rotations containing spring wheat in the 7.5–15 cm depth (Figure 4B). Additionally, in the 7.5–15 cm depth, the pea–barley–camelina–spring wheat rotation retained less water than the other spring wheat cropping systems, but the variation in the data was high (Figure 3B). The α parameter was 0.0425 cm⁻¹ for the 0–7.5 cm depth and 0.0295 cm⁻¹ for the 7.5–15 cm depth across cropping systems (Table 3). The θ_s was higher in the 0–7.5 cm depth (0.568 cm³ cm⁻³) than in the 7.5–15 cm depth (0.511 cm³ cm⁻³) across rotations (Table 3). Additionally, the bulk density was lower for the 0–7.5 cm depth (1.32 g cm⁻³) than for the 7.5–15 cm (1.54 g cm⁻³) depth across treatments (Table 3). The water content at pf 4.2 (15,300 hPa) was 14.4% in the 0–7.5 cm depth and 18.1% inthe 7.5–15 cm depth (Table 3).

3.4. Soil Carbon Concentration and Particle Size Analysis

Soil carbon concentration was affected by depth (p < 0.0001) and replication (p = 0.0074) but was not affected by cropping system (p = 0.1894) or cropping system × depth (p = 0.9696) effect (Figure 5). Carbon concentration was 1.58% for the 0–7.5 cm depth and 1.13% for the 7.5–15 cm depth when averaged across cropping systems. Percent silt and clay were affected by depth, but the other interactions did not affect soil sand, silt or clay percentage. Sand content was 34% in the 0–7.5 cm depth and 33% in the 7.5–15 cm depth, silt content was 42% in the 0–7.5 cm depth and 39% in the 7.5–15 cm depth, and clay content was 24% in the 0–7.5 cm depth and 28% in the 7.5–15 cm depth when averaged across cropping systems.

3.5. Correlation between Soil Properties

Organic carbon concentration may be responsible for variations in soil physical properties when cropping systems are changed. Only the soil properties of wet aggregate stability (fraction water-stable aggregates, MWD of 4.75–8 mm and > 8 mm wet soil aggregates) were positively correlated to soil carbon content, but the dry aggregate stability (MWD and GMD diameter of dry soil aggregates) was negatively correlated to soil carbon concentration. The r values for fraction water-stable aggregates, MWD of 4.75–8 mm and >8 mm wet soil aggregates, and MWD and GMD of dry soil aggregates, were 0.51 (p < 0.0001), 0.42 (p < 0.0001), and 0.51 (p < 0.0001), −0.25 (p = 0.0235), and −0.36 (p = 0.0005), respectively. Additionally, the soil hydraulic properties of Ks (r = −0.35 and p = 0.0124) and van Genuchten α (r = −0.53 and p < 0.0001) and θ_s (r = −0.78 and p < 0.0001) were negatively correlated to soil bulk density.

4. Discussion

Generally, the cropping system did not have any significant effects on soil physical properties except for dry aggregate stability. These findings suggest that adding additional crops to wheat or barley cropping systems in the NGP does not have any negative effects on wet aggregate stability, water retention and hydraulic conductivity. This suggests that increasing the diversity of cropping systems in the NGP does not affect the ability of water to infiltrate and be stored in the soil. This is important for the ability of the soil to store the limited and inconsistent precipitation that occurs in the NGP region. These findings are beneficial to agricultural producers in this region by showing that adding additional crops to wheat or barley cropping systems will not have a negative effect on important soil physical properties.

Our finding that there were limited effects of small grain cropping system on soil physical properties contradicts what we expected in our hypothesis. This may be due to there being no differences in the soil carbon content among cropping systems. Moreover, block and depth tended to have a greater effect on many of the soil properties than cropping system treatments. This may mask differences between cropping systems due to differences occurring in some of the soil properties across the study site. Only wet and dry aggregate stability were correlated to soil carbon content. Wet aggregate stability was positively correlated to carbon content, but dry aggregate stability was negatively correlated to carbon content. This likely was due to carbon content being higher in the 0–7.5 cm depth than in the 7.5–15 cm depth. Dry aggregate stability was higher in the 7.5–15 cm depth, whereas wet aggregate stability was higher in the 0–7.5 cm depth.

However, dry aggregate stability was affected by cropping system. Dry aggregate size distribution may have been higher in the continuous winter wheat and barley rotations due to there being more residue on the surface compared to cropping systems containing peas and oil seeds [27]. Higher dry aggregate stability indicates that the soil is generally less susceptible to wind erosion, which is desirable in areas of high winds such as the NGP [15,16]. Wind erodible fraction is considered as the fraction of aggregates < 0.83 mm which was higher in the pea–spring wheat and pea–barley–canola–spring wheat than in the continuous winter wheat and continuous barley cropping systems [36]. However, the increased susceptibility to wind erosion in the pea–spring wheat and pea–barley–canola–spring wheat cropping systems did not lead to changes in soil carbon content, wet aggregate stability, water retention and hydraulic conductivity after 8 years.

Our study agrees with other studies that found no differences in wet aggregate stability, bulk density, hydraulic conductivity and soil water retention when comparing different small grain cropping systems [18,19,25]. Chang and Lindwall [18] compared a continuous winter wheat to a winter wheat–fallow and a winter wheat–barley–fallow cropping system in Alberta after 8 years in a loam textured soil at depths between 0 and 120 cm. They found no differences in water retention, saturated hydraulic conductivity and bulk density, which agrees with our study. Arshad et al. [19] measured soil organic carbon, wet aggregate stability, water infiltration and bulk density after 11 years comparing a continuous spring wheat to a spring wheat–spring wheat–canola, spring wheat–spring wheat–pea and spring wheat–spring wheat–fallow rotation in a silt loam in Alberta Canada. They also found that increasing the diversity of wheat cropping systems had no effect on these four soil properties. Hammel [25] measured bulk density after 10 years when comparing a winter wheat–spring pea to a winter wheat–spring barley–spring pea cropping system on a silt loam textured soil in Idaho, USA in the 0–60 cm depth and generally found no differences in bulk density between the two cropping systems. Pikul et al. [20] measured bulk density, dry aggregate stability and infiltration rate at eight locations across the Great Plains region of USA and Canada. This study found infiltration was higher under winter wheat–fallow rotation than a winter wheat–corn (Zea mays L.)–millet (Cenchrus americanus L.) cropping system on a silt loam in Colorado, USA, and it was higher in a spring wheat–winter wheat–sunflower (Helianthus annuus L.) rotation than a spring wheat–fallow rotation in North Dakota, USA on a silt loam, but there were no differences at other locations [20]. Additionally, this study found no differences in bulk density between cropping systems at three of the locations but found a lower bulk density under a continuous winter wheat cropping system than a winter wheat–sorghum (Sorghum bicolor L.)–fallow rotation in Texas, USA on a silty clay textured soil, a lower bulk density in a spring wheat–winter wheat–sunflower rotation than a spring wheat–fallow rotation in a silt loam in North Dakota, USA, a lower bulk density in corn–soybean–sorghum–oat (Avena sativa L.)+ clover (Trifolium repens L.) than continuous corn in Nebraska, USA on a silty clay loam and a lower bulk density in a spring wheat–fallow than a continuous spring wheat rotation in a loam textured soil in Montana, USA [20]. This study found that the MWD of dry soil aggregates was not affected by cropping system at most of the locations but found a higher MWD in a continuous spring wheat than a winter wheat–sorghum–fallow rotation in Texas, USA in a silty clay loam [20]. Although this study found some effects of cropping system diversity on the soil properties that they measured, some of the studies found that increasing cropping system diversity had improved soil physical properties, while others found the opposite effects. Additionally, a study conducted in Ontario, Canada found that wet aggregate stability was higher in a soybean–winter wheat rotation than a continuous corn, soybean–corn and continuous soybean rotation, but there was no differences water retention and soil organic matter in the five rotations that were compared in this study after 14 years in a clay loam soil [22]. This study was conducted in a climate with greater precipitation and had different crops than in our study. When comparing our study with other previously published studies, in most of the cases, diversifying wheat and barley cropping in the Great Plains region of the United States and Canada does not improve soil organic carbon, aggregate stability, hydraulic conductivity and water retention in a variety of soil textures and mean annual temperatures. In the minority of cases where there were differences, the cropping systems were different than our study [20]. However, our study compared how 10 different cropping systems affected soil physical properties at a location compared to five or less analyzed at a given location in the previous studies [18,19,20,21,22,23,24,25].

In many of the studies where there were no differences in aggregate stability and soil water retention, there were also no differences in soil carbon concentration between cropping systems [18,20,21,25,44]. Crops planted in our study may have only had small differences in residue produced or root biomass in the different cropping systems. Oilseed crops and field peas generally produce less annual residue than wheat or barley, which may not lead to an increase in soil carbon concentration and improvements in physical properties near the soil surface [26,27]. Moreover, in dryland cropping systems in water-limited areas such as eastern Montana, crop yields are highly variable depending upon the precipitation received each year, leading to inconsistent biomass production [45]. In cropping systems with years of low residue production, it may take long periods of time for soil physical properties to change from management.

Increasing cropping system diversity with small grain rotations did not improve soil aggregate stability and soil water retention in comparison to continuous wheat or barley cropping systems in this study. The tillage practice can have a larger effect on the soil physical properties than the cropping system in annual small grain cropping systems [18,22,46]. No tillage can lead to better soil quality in these studies than conventional tillage [18,22,46]. Van Eard et al. [22] found that wet aggregate stability and soil organic matter was higher in no tillage than conventional tillage in their cropping system study. Chang and Lindwall [18] found that water retention was higher under no tillage than conventional tillage at −75, −500 and −1500 kPa water potentials, and bulk density was less under no tillage near the soil surface, but saturated hydraulic conductivity was not affected in the 0–30 cm depth in their cropping system study. Mahli et al. [46] found that no tillage reduced bulk density but increased the MWD of dry soil aggregates and saturated hydraulic conductivity in their cropping system study near the soil surface. However, our study only had one tillage practice, which was no tillage. To improve the soil physical properties in the NGP, dryland agriculture crop rotations which contain biannual or perennial crops such as kernza or hay may be needed. Some studies have found cropping systems with perennial crops had higher soil organic carbon concentration and wet aggregate stability but lower bulk density compared to annual cropping small grain cropping systems [19,47,48].

5. Conclusions

Our study found limited effects of cropping system diversity on soil aggregate stability, bulk density, hydraulic conductivity, water retention and soil organic carbon concentration. Adding canola and camelina oilseed crops into small grain rotations has no negative effects on these properties but also did not improve these soil properties. This finding is important because diversifying wheat or barley cropping systems by adding canola, camelina and field pea does not have negative effects on important indicators of soil physical quality. Our finding shows having more diverse cropping systems that can provide a more sustainable source of income and be better for environmental quality will not cause negative effects to soil physical properties and soil organic carbon concentration.

For benefits to occur by diversifying small grain cropping systems, it may take a longer duration than the eight years of this study for differences between cropping systems to occur. Annual precipitation and crop residue yields are highly variable in the NGP region, which may lead to slow changes in soil properties due to management. In the NGP, crop rotational diversity had limited effects on soil physical properties after 8 years. Longer term studies may be needed to better understand if wheat or barley rotated with oilseed crops and peas have any benefit to the soil physical properties considered in this study.