Impact of Soil Pore Size Distribution on Crop Growth in Relation to Plant Available Water (PAW) and Soil Organic Carbon (SOC) in Arid/Semi-Arid Regions

The Experiment of Impact of Soil Pore Size on PAW and Crop Growth in a Dispersive Soil

Measurements: According to Farahani et al. [5], soil composite samples were collected from the soil surface (0– 0.3 m depth) of an agricultural loamy soil (the original soil), which is located at Ferdowsi university of Mashhad campus, north-eastern Iran (Table 1). The experiment was performed as completely randomized design with three replications. The soil has been treated with monovalent cation of K, to project clay dispersion, from concentration of 13.6 to 27.2 and 27.2 to 54.4 cmolc kg-1 in EC=3 and 6 dS m-1, respectively.

OC=organic carbon; ECe= electrical conductivity of saturated extract; pHs= pH of saturated paste; CEC= cation exchange capacity. Table 1: Intrinsic Physical and Chemical Properties of the Original Soil.

The measurements were hereafter done on the treated soil samples. The soil water retention was determined on undisturbed soil cores (diameter: 3.5 cm; height: 4 cm) at matric suctions of 0, 50, 100, 300, 500, 1000 and 5000 hPa, using pressure plate apparatus. Then, the van Genuchten equation (Eq. (1)), see Van Genuchten MT [23], with the Mualem restriction (m = 1–1/n) was fitted to the measured data using the RETC software Genuchten MV, et al. [24]:

1 1 [1 )n n s r h θ θ θ α −  = − +  (1)

( ) (

where θs and θr are the saturated and residual water contents (cm3 cm−3), respectively, α is a scaling factor (hPa−1) and n is a shape parameter of the soil water retention curve.

The PAW was also calculated using soil water retention data and parameters by SAWCal (soil available water calculator) software Asgarzadeh H, et al. [25]. To determine soil pore size distribution the volumetric water content retained in soil at matric suctions up to 50 hPa was defined as macro-porosity, between 50–300 hPa and >300 hPa Marshall TJ [26] were considered as meso-porosity and micro-porosity, respectively.

To determine amount of dispersible clay in water in the soil samples, 20 g of the disturbed treated soil (< 2 mm) was placed in a 250-ml cylinder, and 200 ml of distilled water was added slowly down the sides of the cylinders to avoid soil disturbance Marchuk A, et al. [27]. After 12 h, dispersed particles were gently stirred into suspension and left to stand for 2 h. 10 ml suspension was taken from 10 cm depth, oven dried at 105 °C for 24 h, and dispersible clay in relation to total clay was determined [27].

To assess impact of PAW values on crop growth, maize was planted in the treated soils. NPK and micro fertilizers were also applied to prepare suitable condition for crop growth. Irrigation was done by weight change of the pots in relation to their initial weights using distilled water. At the end of maize vegetative growth, plants were harvested and shoot and root weights were measured to calculate shoot:root ratio per pot, as a well-known parameter to show rate of plant growth.

Data Interpretation: According to [5], adding monovalent cations (K) into the soils created clay dispersion in the treated samples, because monovalent cations may increase diffuse double layer (DDL) thickness of the clay particles and lead to clay dispersion. The differences between CEC of the original soil and the treated samples were not statistically significant, which means K ions were not fixed by clay layers and played a role in dispersion. The dispersible clay percentage increased significantly (p < 0.05) with increasing K concentration compared to the original soil (Figure 1). Also, clay dispersion at EC = 6 was lower than EC = 3 dS m−1, which primarily could be due to the higher electrolyte concentration that reduces dispersion Quirk JP, et al. [28].

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Grey Symbols: EC=3 dS m-1; Black Symbols: EC=6 dS m-1; T1 to T6 Indicate the Treatments (Concentration Of 13.6 To 27.2 and 27.2 To 54.4 cmolc kg-1 at two EC Levels) and Cont. is the Original Soil. Error Bars Indicate ± 1 Standard Error. Figure 1: Percentage of Dispersible Clay as a Function of K Concentration.

The meso- and the micro-porosity increased significantly (p < 0.05) in most of the treated samples in comparison with the original soil at both EC (Figure 2), however the macro- porosity did not differ significantly in the treated samples compared to the original soil. Clay particle rearrangement causes aggregate destruction and reduces pore size, which is referred to as short-range migration of dispersed clay particles into the soil pores Chen Y, et al. [29]. In reference Marchuk A, et al. [30], they worked on changes of pore size distributions in a sandy loam soil contained monovalent cations (Na and K) using X-ray tomography, and found non-connected pores filled with dispersed clay particles. In reference Marchuk S, et al. [31], they studied two soils (sandy loam and clay) as influenced by K treatments and reported that large pores (17.4–69.2 μm) decreased with increase in K concentration, while small pores (0–17.4 μm) increased.

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Grey Columns: EC=3 dS m-1; Black Columns: EC=6 dS m-1; T1 to T6 Indicate the Treatments (Concentration Of 13.6 to 27.2 and 27.2 to 54.4 cmolc kg-1 at two EC Levels) and T0 is the Original Soil. In Each Chart Same Letters are Not Statistically Significant at P<0.05. Error Bars Indicate ± 1 Standard Error.

Results of the fitted parameters of van Genuchten model, see [23], on water retention data demonstrated that PAW(FC- PWP) increased with increasing K concentration, and PAW of the treated soils were larger than in the original soil at both EC levels (Table 2). This observation was because of clay dispersion impacts on soil structure, in particular on soil pore size, which discussed earlier. The maximum PAW were attributed to the highest K amounts at both EC (Table 2).

In each column same letters are not statistically significant at p<0.05. Table 2: Values of PAW in the Treated Samples and the Original Soil.

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Grey symbols: EC=3 dS m-1; black symbols: EC=6 dS m-1; t1 to t6 indicate the treatments (concentration of 13.6 to 27.2 and 27.2 to 54.4 cmolc kg-1 at two EC levels) and Cont. is the original soil.

The PAW and the meso- and micro-porosity positively related at both EC (Figure 3). As discussed earlier, the soil pore size became smaller in most of the treated samples due to clay dispersion, therefore increase in PAW with increasing the meso- and micro-porosity seems logical. Since smaller pores are responsible for water retention at matric suctions of 100 and 15000 hPa (equivalent to FC and PWP, respectively), water contents at FC and PWP were effective on calculation of PAW in this study. The more water retention at FC and PWP, the more PAW amounts. In comparison with the original soil, maximum increments in FC and PWP were observed in the treatments included highest K concentrations at both EC.

In reference Nang ND, et al. [32], they worked on changing PAW in a salt affected Australian soil, and found the highest PAW in the samples with maximum dispersible clay, and justified the shift toward smaller pores was a result of clay dispersion. Furthermore, in Zangiabadi M, et al. [33], they studied the effect of soil pore size distributions on PAW values in 30 different soils from northeastern Iran and demonstrated reduction of soil pores diameter resulted in increase of soil water retention and PAW. However, in Hardy SM [34], they studied sodic soils (with high ESP values), and reported that high concentration of Na led to reduction in PAW, it is in contrast to our findings. High amounts of ESP collapse soil structure and block the soil pores McMullen B [35], which results in PAW reduction.

The shoot:root ratio of maize, as an index of crop growth, enhanced significantly with increasing PAW at EC = 6 dS m-1 (shoot:root = 25.847 PAW - 1.3066, R² = 0.64, p=0.03), whereas no significant relationship was observed at EC = 3 dS m-1. This indicated that crop growth conditions were improved in the treated soils at higher EC level. Generally, negative impacts of clay dispersion on soil structure have been reported in previous literature, however, in this study, a positive impact on plant available water and plant growth was found, because clay dispersion increased water retention. The effects of clay dispersion on soil functions are complicated and dependent on cation concentration Marchuk A, et al. [36], EC [28, 29, 30, 31, 32, 33, 34, 35, 36], soil texture Rengasamy P, et al. [37] and clay mineralogy Farahani E, et al. [38]. In our study, clay dispersion caused by cations and increased water retention and PAW. Because water retention was the physical plant growth limiting factor, plant growth conditions were improved, and plant growth increased by clay dispersion at EC = 6 dS m−1. However at EC = 3 dS m−1, according to root weight data (unpublished data) root development decreased due to the higher clay dispersion and soil hardening and therefore, plant growth did not increase although PAW increased. Consequently, as long as the dispersed clay does not increase resistance against root growth, the increase in water retention, due to change in soil pore size by migration of the dispersed clay particles, may positively influence physical crop growth conditions in arid/semi-arid regions of northeastern Iran.

The Experiment of Relationship between Soil Pore Size, SOC and Crop Growth in Two Tillage Systems

Measurements: According to Farahani et al. [39], plots of tillage systems (conventional and no-till), included wheat- maize rotations and cover crop cultivation, were established in 2017 at the “Agricultural and Natural Resources Research Center”, northeastern Iran (Table 3). The initial SOC content was 0.36% in the studied soil, which is clearly low but looks typical in the arid/semiarid regions of Iran, since 61.6% of agricultural soils contain less than 1% SOC Moshiri F, et al. [40]. No-tillage (NT) included completely avoiding any tillage practice, and seed was sown with a no-till drill and normal row planters for wheat and maize, respectively. In conventional tillage (CT), tillage was done with mouldboard plow (to 25 cm depth), followed by tandem disking twice (to 10 cm depth). Pneumatic precision seeders and row planters were used to grow maize and wheat, respectively.

SOC=soil organic carbon; ECe= electrical conductivity of saturated extract; pHs= pH of saturated paste. Table 3: Physical and Chemical Properties of the Initial Soil.

Details of crop and cover crop (clover) cultivation from the start of the experiment (2017) to sampling (2020) were as follows: wheat was cultivated and harvested in November 2017 and June 2018, respectively. Clover was sown in July 2018 using a wintersteiger drill in NT, and was harvested in May 2019 with a mower. Maize was grown and harvested in June and October 2019, respectively. Wheat in a rotation was planted and harvested in November 2019 and June 2020, respectively. In CT treatments only stubble of the crop was left on the plots, while crop residues were retained and mixed with the soil surface in NT after clover harvest. Soil sampling was done in July 2020 (after wheat harvest) with two sampling depths (0–15 and 15–30 cm) and three replications in a randomized complete block design in both tillage systems.

Percentage of micro- and macro-aggregates (<250 and >250 μm, respectively), as influenced by SOC content, was determined with the 0.25 mm sieve. Air-dried aggregates were put on the sieve and shaken in dry condition and the amount of micro- and macro-aggregates was calculated as mass of aggregates that passed the sieve and the aggregates on the sieve, respectively, divided by total mass of the soil sample Totsche KU, et al. [41]. SOC content in the samples were determined using wet oxidation method Walkley A, et al. [42]. Root growth condition has also been assessed by root residue evidence in the tillage plots.

To investigate soil pore system, 3-dimensional X-ray imaging was performed with a medical scanner (Planmeca Viso G7 - Helsinki, Finland). Three replicates for each soil depth (0–15 and 15–30 cm) of undisturbed core samples (with a diameter and height of 4.5 and 6.25 cm; 100 cm3) were scanned in air-dry condition. The voxel size was 75 μm, maximum X-ray energy of 100 kV and 100 mA was used to scan each soil core.

Data Interpretation: The SOC content after 3 years of establishment (2017–2020) increased significantly compared to the initial soil, however NT plots showed higher SOC content than CT (Figure 4). NT had 92% and 144% increase of SOC at 0–15 and 15–30 cm depth, respectively, in comparison with the initial soil (Tables 3 & Figure 4). Apparently in NT system, the amelioration of physical conditions encouraged root growth in the depth of 15–30 cm, thus root residues content and SOC was increased at this depth after harvesting the plants. An increase of SOC by 28% in CT (0– 15 cm) compared to the initial soil can be attributed to the mixing crop root residues with the soil within the tillage layer of 25 cm. Although, in CT (15–30 cm) 44% decrease in SOC was observed compared to the initial soil, possibly due to the decomposition of organic matter by tillage practices.

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The higher SOC content in NT compared to CT was mostly due to the cover crop (clover) cultivation and crop residues retention in NT system. Cover crops increase SOC content and improve soil properties, leading to more biomass carbon input and extending the advantages of conservation tillage. Seeding cover crops after wheat harvest enhanced SOC in a NT system with wheat–sorghum rotation in Kansas Blanco- Canqui H, et al. [43]. Soils in arid/semiarid areas mostly contain low SOC contents, therefore clay particles can present great impact on soil structure and create organo-mineral complexes, which help with organic matter accumulation Levy GJ, et al. [44]. It was observed that soil structural stability correlated with clay content, and this relationship being stronger in clay contents of ≥35%. High structural stability indices has been reported in two soils contained 38% and 45% clay and 46% and 30% sand, respectively. It means sand content will not influence soil stability if clay content is high enough to ameliorate structural stability [44]. Clay content of the studied soil was 36% (Table 3) and could increase the SOC content in NT, consisted of crop residues retention, compared to the initial soil. Higher clay contents in soils may cause more molecules of organic carbon to adsorb on clay particle surfaces and form more organo- mineral complexes, which consequently can protect SOC from microbial and enzymatic decomposition and increase SOC storage Jagadamma S, et al. [45, 46].

SOC influences porosity and pore size distribution, for example, in Emerson WW, et al. [47], they found enhancement of pore size with increasing SOC percentage in two sandy and silty soils. It was demonstrated that greater SOC content increased soil physical quality indices, especially pore size distribution functions. Aggregate size could also be regarded as a structural stability index in soils, which may be influenced by SOC content Reynolds WD, et al. [48, 49]. Trend of macro-aggregates changes among the treatments in NT and CT systems was similar to the SOC contents in our study, implying that NT (at both depths) with higher SOC had more macro-aggregates. The lowest percentage of macro- aggregates also belonged to CT in depth of 15-30 cm, with the lowest SOC (Figure 4 & Table 4). In reference [43], they found that increasing SOC developed macro-aggregation, hence soil aggregate size can increase by 1.5 mm when SOC increases by 1%, and pore size between aggregates can also increase. A study in a semi-arid region of China compared conventional and conservation tillage and reported that macro-aggregates percentage increased by 41.2% and 56.6% for no-till system without and with crop residue retention, respectively Wu J, et al. [50]. They also mentioned that macro-aggregates are the major pools of SOC and total nitrogen. Therefore, it can be inferred from our results that the highest SOC in NT produced more macro-aggregates at both depths.

CT=conventional tillage; NT=no-tillage; Different letters were statistically different in p<0.05. Table 4: Mean Comparison of Percentages of Micro- and Macro-Aggregates of the Treatment Plots.

According to cross-sectional X-ray images of the soil pores (Figure 5), the core samples at depth of 0–15 cm had a lower soil pore volume than those at 15–30 cm in both tillage systems. Regardless of soil depth, CT had a higher total porosity compared to NT, which is due to ploughing the soil in CT system. Moreover, the highest total porosity was observed in CT (15–30), revealed that it had the lowest compaction (bulk density) among the treatments (values of bulk density in the treatments were not significantly different in this study, so not shown). In general, low soil porosity and high bulk density in a NT system can originate from the absence of soil disturbance Bescansa P, et al. [51, 52, 53, 54]. The loose soil structure of ploughed horizons typically has an isotropic, well-connected pore system, whereas pores in unploughed soils may be more anisotropic and less connected Schlüter S, et al. [55]. Furthermore, in CT (0-15) moderate soil compaction caused by tillage machineries in soil surface, resulted in lower total porosity compared to the subsoil (Figure 5). The greater subsoil porosity in tillage systems could be attributed to soil moisture distribution during growing season in arid/semi-arid region of northeastern Iran. According to measurement of the in situ water content during the experiment, the surface soils had lower moisture than the subsoil in both tillage systems. This probably caused by high evaporation from the soil surface (0–15 cm depth) in spring and summer (corresponds to the most of vegetative and reproductive growth of winter wheat) in arid/semi-arid region of northeastern Iran. In reference Enkova LK, et al. [56], they studied three agricultural soils in western Slovakia (with different textures and SOC) and reported that the more soil moisture the more porosity. Moreover, the soil moisture distribution at different depths could potentially influence root growth. In Hodgkinson L, et al. [57] it was reported that root growth and development was higher under moist condition. Therefore, it can be assumed that there was less root growth and root canals at the depth of 0–15 cm compared to the depth of 15– 30 cm in our study.

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(Dark areas = pores, grey areas = solids, white areas = gravels), CT: conventional tillage; NT: no-tillage. Figure 5: X-Ray Computed Tomography Images of Soil Pores for the Treatments at Two Depths (0–15 and 15–30 cm).

Root density could be a possible reason for the non- significant differences in bulk density values in our study. Based on the visible big root canals in X-ray images in NT (15-30), it apparently had better plant growth and higher root density than CT. The cultivation of cover crop, more SOC, and therefore the stable aggregates increased root growth in NT system. Soil loosening through tillage practices in CT and root density and SOC in NT may therefore explain a non- significant difference in bulk density in two tillage systems. Cover crops created continuous macro-pores and increased air-filled porosity at low matric suctions in a no-till system [25].