Previous and Current Crop Effects on Early-Season Root Growth and Growing Season’s Soil Moisture Under Dryland Agriculture in Temperate Climate

Understanding the root growth and changes in soil moisture content during the growing season for dryland agriculture crops can improve crop production. It was hypothesized that early-season root growth might be influenced by previous crop and current crops, and soil moisture content and depletion pattern during the growing season and residual soil moisture may be affected by the crop type. A study was conducted on the early-season root growth of canola (Brassica napus L.), wheat (Triticum aestivum L.), and flax (Linum usitatissimum L.) in 2015; and changes in soil water content during the 2013, 2014, and 2015 growing seasons under canola, flax, wheat, barley (Hordeum vulgare L.), and pea (Pisum sativum L.). Early-season root growth of the canola and flax crops was better on wheat than canola stubble, while for wheat it was similar on the stubbles of both wheat and canola. Soil moisture depletion started relatively earlier under the barley and wheat and later under the flax compared to the canola and pea crops. Flax continued to deplete soil moisture for a longer period than the other crops. With some exceptions, all crops could deplete soil moisture to a similar level (down to about 15% or somewhat lower) by the end of their growing seasons. Generally, almost equal amounts of residual soil moisture remained after the different crops.


Introduction
Dryland agriculture is practiced in the study area. The water used by crops comes from rain during the growing season, plus the stored soil moisture from rain/snow before seeding and depletion from the root zone (soil moisture in the soil profile at seeding-harvesting of crops). Under such conditions, the temporal and spatial changes in the soil moisture during the growing season of crops depend on root characteristics and their distribution in the soil profile.
The ability of plants to obtain water and mineral nutrients from the soil is related to their capacity to develop their root systems. Gan et al. (2011) suggested that a crop's root system can compensate by increasing or relocating maximal root growth to higher soil moisture regions, helping maintain plant growth under dry soil conditions (Rendig & Taylor, 1989). Jalota et al. (1980) observed more root growth of wheat with higher moisture content in the soil profile at seeding. The root length density of crops during the dry season tends to decrease mid-season at shallow soil depths, whereas it continues to increase throughout the growing season at deeper soil depths (Moroke et al., 2005). These changes parallel the trend in maximum soil moisture depletion from successively deeper layers as the season progresses.
Understanding soil moisture depletion by crops during the growing season and its residual level in the soil profile at crop harvest can help choose appropriate subsequent crops. To enhance the sustainability of dryland cropping systems, the sequence of crops is arranged to grow low water-using crops followed by high water-using crops so that excess soil water unused by the previous crop is available to the next crop (Unger et al., 2006;Lenssen et al., 2014). Stratified soil moisture depletion in crop rotations may improve the overall water use efficiency (Bunting & Kassam, 1988;Gregory, 1989;Roder et al., 1989). Lenssen et al. (2014Lenssen et al. ( , 2018 and Schlegel et al. (2017) found greater soil water storage and water use efficiency with diversified crop rotations than monocropping. Schlegel et al. (2019) stated that crop rotations could efficiently use soil water and enhance dryland crop yields compared to monocropping.
There has been considerable research on the root growth and soil moisture changes for various crops under several climatic, agronomic and soil conditions. However, limited research is available on early-season root growth based on the influence of previous and current crops, and on pattern of soil moisture depletion during the growing season and its residual level for different crops under similar rainfed agriculture conditions that prevail in the study area. We hypothesized that a crop grown in the rotation might have better early-season root growth compared to the same crop grown under monoculture, because some negative factors associated with monoculture are alleviated. We also hypothesized that soil moisture content and depletion pattern during the growing season and residual soil moisture might be affected by crop type. Thus, the early-season root characteristics of crops and the depletion of soil moisture by the crops during the growing season were monitored in a field study.

Site, Soil, and Weather
The field experiment was located south of Donnelly in a field with the legal land location of NW7-77-20W5 (GPS: 55°39′38.43′′ N, 117°6′10.64′′ W), southeast Peace Region, Alberta, Canada. The soil at the site was a clay loam Luvisol (Soil Classification Working Group 1998). The 0-15 cm soil depth had 48 g kg -1 organic matter, 6.1 pH (water), and 16.4 cmol (+) kg -1 CEC. Spring soil moisture (SSM) and monthly precipitation during the growing season data were obtained from the weather station at Ballater in Alberta, located 5 km from the site (Table 1).

Experimental Details
A study on crop rotation was conducted from 2009 to 2015. A randomized complete block design was used to compare ten crop rotations that included canola, wheat, pea, barley, and flax, plus the monocultures of canola and wheat. The details on treatments, agronomic practices, and crop yield results have been presented by Gill (2018). The current research measured periodic soil moisture content during the 2013, 2014, and 2015 crop growing seasons and root growth in the early-season of 2015. The relevant procedures for these measurements are described below.
The crops were seeded on May 14, 21, and 11 in 2013May 14, 21, and 11 in , 2014May 14, 21, and 11 in , and 2015  For root measurements, selected treatments to compare the effects of 2014 canola and wheat (termed as previous crops) on the early-season root growth of canola (Brassica napus L.), wheat (Triticum aestivum L.), and flax (Linum usitatissimum L.) in 2015 (subsequent crops). Ten intact plants (shoots and roots) plus surrounding soil from representative areas of plots under wheat, canola, and flax crops were collected after 37 days of seeding, on June 17, 2015 (168 Julian day). The soil and roots were soaked in water for several hours. This was followed by gently shaking under water and repeated rinsing to remove the soil. The remaining debris was carefully picked off using tweezers. Then the shoots of five representative plants were cut so that all roots were still attached to the base. These representative plant's roots were spread in trays and scanned using a WinRHIZO Regular V.2013, XL Rhizo plus STD4800 scanner imaging machine (www.regentinstruments.com). WinRHIZO assigns root lengths to predefined diameter classes and counts the tips. Then the scanner provides estimates of the length, surface area and volume measurements for different diameter sized root classes, i.e., for the 0 < 0.5, 0.5 < 1.0, 1.0 < 1.5, 1.5 < 2.0, 2.0 < 2.5, 2.5 < 3.0, 3.0 < 3.5, 3.5 < 4.0, 4.0 < 4.5, > 4.5 mm diameter roots. The roots and shoots of the five plants were dried to determine their dry masses. For presentation, data for roots of different sizes were grouped into thin (< 0.5 mm), medium (> 0.5 < 1.5 mm), and thick (> 1.5 mm) categories (Tables 2  and 3). The data were subjected to the Paired T-test to determine the effect of previous wheat and canola crops on root and shoot measurements of canola, wheat, and flax crops. Differences were considered significant at p < 0.5.
Soil moisture measurements were done for all the crops being grown in different treatments of experiment during the given growing season. Thus soil moisture could be monitored only under four crops (barley, canola, pea, and wheat) in 2013, two crops (canola and wheat) in 2014, and four crops (canola, flax, pea, and wheat) in 2015. Soil moisture was measured at depths of 0-10, 10-20, 20-30, and 30-40 cm under the wheat, canola, flax, barley (Hordeum vulgare L.), and pea (Pisum sativum L.). A profile probe (PR2-UM-3.0) and a moisture meter (HH2 version 4.0) of the Delta T Devices Ltd, 2008, 130 Low Road, Burwell, Cambridge, CB25 0EJ (www.delta-t.co.uk) were used to measure soil moisture at different depths. The profile probe has a sealed polycarbonate rod (~2.5 cm diameter) with paired stainless steel electronic sensors at fixed intervals along its length. When power is applied, each pair of sensors generates a simple analog DC voltage (100 MHz) that transmits an electromagnetic field extending about 10 cm in the soil. The moisture meter was used to apply power to the profile probe sensors, measure the output signal voltage returned, and convert it to soil moisture units (volumetric) using a linearization table and soil-specific parameters. The signal's strength is related to the permittivity of soil, predominantly dependent on water (≈ 81 permittivity compared to ≈ 4 for soil and ≈ 1 for air). Specified fiberglass access tubes (2.5 cm diameter) were installed at the start of each growing season to insert the profile probe for readings at different soil depths. The soil moisture values and standard deviations for the 0-20 cm (average for the 0-10 and 10-20 cm) and 20-40 cm (average for the 20-30 and 30-40 cm) depths are presented in Figures 1, 2, and 3.

Previous Crop Effects
The total length, surface area, volume, and number of tips for canola roots showed significantly greater values on wheat than on canola stubble (Table 2). Similarly, the values for the thin, medium, and thick canola roots were also significantly greater on wheat than on canola stubble; except for the length, surface area, and volume of thick roots and the number of tips for the medium roots. Flax roots length, surface area, volume, and the number of tips also tended to be greater on wheat than on canola stubble, both for the different sizes and total values, but the differences were not statistically significant. Unlike the canola and flax roots, the wheat roots length, surface area, volume, and the number of tips for the thin, medium, and thick roots and their total values were not consistently influenced by the previous crops of canola and wheat.
The root and shoot masses of canola and flax tended to be greater on the wheat than on the canola stubble, with significant differences for the shoot mass of canola (Table 3). The stubble type did not influence the root and shoot masses of wheat.  Note. a The NS and * refers to the effects of previous crop being not significant and significant at the p < 0.5, respectively. Note. a The NS and * refers to the effect of previous crop being not significant and significant at the p < 0.5, respectively.

Root Size Distribution of Crops
Thin roots comprised 78 to 87% of the total root length of the canola, wheat, and flax, with much lower values for the medium (9.6 to 18.0%) and thick (0.8 to 6.7%) roots (Table 2). Almost all the tips (97 to 99%) for the canola, wheat, and flax roots were on the thin roots. The thin roots also had the highest surface area for the wheat (42-45%) and flax (55-60%) crops, followed by medium and thick roots in decreasing order. In contrast, the canola root's surface area was highest for the thick (34.1-38.6%) and lowest for the medium (26.6-28.7%). Unlike the length, surface area, and number of tips, the thick canola and wheat roots had the highest volume, followed by the medium and thin roots in decreasing order. The flax roots volume values were evenly distributed in the thin, medium, and thick classes.

Soil Moisture Content and Depletion Under Different Crops
Under dryland agriculture practiced in the study area, crops are sown in May and harvested during the fall, followed by a cold winter with no crop until sowing during the following May. Stored soil moisture plus rain and depletion of soil moisture during the growing season provide water for crops. Rain and snow during winter jas.ccsenet.org Journal of Agricultural Science Vol. 13, No. 5; increase the moisture content of the soil. With rare exceptions, soil moisture depletion occurs during the growing period of crops. Crop yield suffers in years with low rainfall, when soil moisture content drops to a low level and the crops cannot extract water.

2013 Season
The season started with 75% of the average spring soil moisture (SSM) and received 84% of the average rainfall during the growing season (Table 1). For the crops seeded on 134 Julian day, the first measurement on 157 Julian day showed that the barley plots had slightly less soil moisture in the 0-20 cm soil than the plots under other crops (Figure 1a). The earlier growth of barley roots compared to other crops may have depleted the soil moisture. Between the 157 and 172 Julian days, soil moisture was depleted under barley but not under other crops, further increasing soil moisture differences under barley and other crops. Greater than average rain (Table  1)  For the 20-40 cm depth, the soil moisture content between the 157 and 179 Julian day increased (Figure 1b), likely a result of little water use by the crops from this soil layer and above average rain during this period (  Vol. 13, No. 5; 4. Discussion

Early-Season Root Growth
The roots of a crop play vital role of linking the plant to soil and thereby soil to the atmosphere. Thus roots influence soil moisture and nutrient use efficiency by crops. Root growth is modified by soil factors such as temperature, fertility, aeration, and structure; and plant factors like vigor and ability of shoot to supply carbohydrates and other nutrients to developing roots. The root growth pattern may depend on the soil moisture content and depletion during the crop growing season (Ju et al., 2015).
Compared to canola stubble, the beneficial effects of wheat stubble on the early-season root and shoot growth of canola and flax (Tables 2 and 3) is supported by earlier research. Monreal et al. (2011) reported early-season arbuscular mycorrhizal fungi (AMF) colonization, lateral root area, and root biomass of flax to be greater when wheat rather than canola was the preceding crop. The establishment, early-season biomass, phosphorus accumulation, and seed yield of flax were greater on wheat than canola stubble (Grant et al., 2009). Grant et al. (2005) stated that spring wheat readily forms AMF association while canola does not, and the stubble and roots left by the wheat crop can create a favorable environment in the rhizosphere by increasing pore volume and nutrients for root proliferation of the subsequent crops.
The canola and flax root growth being better under the stubble of wheat than canola are supported by beneficial effects on canola yield from a 1 or 2 years break from canola in crop rotations (Gill, 2018). Similarly, Johnston et al. (2005) found that seeding canola two years in a row resulted in the lowest yield compared with canola in rotations, and O'Donovan et al. (2014) reported higher canola yield after a legume than after canola. Kutcher et al. (2013) speculated that lower canola yield on canola stubble might have been a reflection of less available soil N due to the increased demand from canola compared with other crops.
In other studies, the root vigor of crops was improved by rotations (Nickel et al., 1995), as were the root-related nutrient and water uptake functions (Copeland & Crookston, 1992;Copeland et al., 1993). Cresswell and Kirkegaard (1995) observed that a canola crop did not improve a subsequent wheat crop's rooting depth. In contrast, Williams and Weil (2004) observed soybean (Glycine max L. Merr.) roots growing through compacted soil using channels made by roots of a previous canola crop. This suggests that a prior crop specie's benefit may depend on the following crop species and environmental conditions. Similar early-season growth of wheat roots on the wheat and canola stubble in the present study indicates that some other crop rotation-related factors were preventing wheat stubble benefits. Gill (2018) reported the wheat yield on canola and wheat stubble was not consistently different. Evidence of more wheat root growth following maize (Zea mays L.) crop than rice (Oryza sativa L.) exists in literature (Sur et al., 1981). Other crop sequences research in western Canada indicates that more diverse rotations tend to have fewer pest problems and lower production risk than rotations based heavily on either cereal or broadleaf crops (Bailey et al., 2000;Johnston et al., 2005;Kutcher et al., 2011).

Soil Moisture Content and Depletion
Soil moisture data in 2013, 2014, and 2015 indicated an earlier start of soil moisture depletion by the cereals than other crops (Figures 1, 2, and 3). Compared to other crops, soil moisture depletion under flax started later and then continued slowly and steadily for a more extended period. Similar to our results, wheat used water faster than pulse and oilseed crops (Gan et al., 2008). It is not only the total root system that affects the soil water depletion, but the depth-wise distribution of roots also influences soil water depletion (Fan et al., 2016). Similar to our results, the rate of root growth and water depletion was significantly faster under sunflower (Helianthus annuus) than sorghum (Sorghum bicolor), but sorghum reached maturity later in the growing season and water depletion was approximately the same for sunflower and sorghum (Moroke et al., 2005). They also observed that soil water depletion was significantly greater under sorghum than cowpea (Vigna unguiculata). Thus, the residual water content at the end of the growing season was greater under cowpea than under sorghum. Cowpea may exert a lower suction, hence lesser water uptake than sorghum and sunflower (Bunting & Kassam, 1988). Comparisons of water depletion by Merrill et al. (2003) demonstrated more soil water content after dry pea than sunflower. However, the soil water gains following a pulse crop seemed to be partially offset by lower soil moisture gain during winter fallow.
Less soil moisture depletion from the 20-40 cm soil compared to the 0-20 cm soil during the initial period of crop growth and increase in the later season indicated the delayed establishment of roots in the deeper soil. Root systems of crops have the ability to increase or relocate maximal root growth to regions with greater water content in the soil profile, thereby maintaining plant growth under dry conditions (Rendig & Taylor, 1989). Moroke et al. (2005) observed that the root length densities of sorghum and sunflower near the soil surface increased rapidly after planting but then declined, they increased throughout the growing season in deeper soil, and the soil water depletion corresponded to root length density of the crops. The root length density of crops during an extremely dry season tended to decrease during mid-season at shallow soil depths, whereas it continued to increase throughout the growing season at deeper soil depths (Moroke et al., 2005). Fan et al. (2016) stated that it is not only the total root system that affects the soil water depletion, but soil water depletion is according to the depth-wise distribution of roots. These statements support our result of more soil moisture depletion from deeper soil as the season progressed.
Different residual amounts of soil moisture after crops may have positive and negative aspects. Positively, more depletion capability means better water use efficiency and more rain infiltration, with less water runoff and soil erosion in years with adequate rain. Negatively, if the crop root zone soil is not fully charged before the start of the next crop, the subsequent crop can suffer from water stress (likely in dry areas).

Conclusions
Wheat stubble was beneficial compared to canola stubble in making the soil environment congenial to promote early-season root growth of the subsequent canola and flax crops. Similar growth of wheat roots in wheat and canola stubble indicated that some other monoculture-related factors were preventing the benefits of wheat stubble. About 80% of the total length for canola, wheat, and flax roots came from the thin roots. About 98% of the tips were on the thin roots. Root measurements data showed that an earlier crop could affect the early-season root growth of crops, and the effect may vary with the previous as well as the current crop type.
The 2013, 2014, and 2015 soil moisture depletion data indicated a relatively earlier start under barley and wheat and a delayed start under flax compared to canola and pea crops. The flax continued to deplete soil moisture for an extended period. With a few exceptions, all crops could deplete soil moisture to a similar level (down to about 15% or lower) by the end of the growing season. Thus, there was almost equal amount of residual soil moisture after the different crops and only a limited effect of the crops on the residual soil moisture content. The soil moisture availability probably limited crop production during the 2014 and 2015 seasons. Under the area's dryland agriculture conditions, the crop type could alter the soil moisture depletion pattern during the growing season, but had limited influence on the residual soil moisture at crop harvest. These findings indicated only limited influence of previous crops on the soil moisture availability for subsequent crops.