Reza Soleimani · Ahmad Hosseini

Abstract Root activity has an important impact on soil development but we have little knowledge of the interaction of the root zone and soil genetic horizons. The aim of this investigation was to study the interactions between soil chemical characteristics and root zone processes in a declining Persian oak forest ( Quercus brantii Lindl.). A randomized complete block design was used to investigate the long-term effect of Persian oak on soil horizons, and the chemical and biological properties in two soil zones(under and outside the canopy). Results indicate that the rhizosphere zone had signif icantly higher total organic carbon (TOC) than outside the canopy soil in the upper soil horizons. In subsurface horizons, water-extractable organic carbon values were signif icantly higher in rhizosphere than in non-rhizospheric zone. Microbial biomass carbon (MBC)values in the rhizospheric zone decreased from the f irst to the second horizon. The MBC/TOC ratio indicated signif icant differences between the rhizosphere and soils outside of the canopy, with the exception of the subsurface horizon. In the subsurface horizon of the rhizosphere, there was greater respiration of organic carbon (∑CO 2/ TOC) than in outside of canopy soil. In addition, root processes inf luenced pH,nitrogen fractions, and availability of phosphorus, potassium, iron, zinc and manganese. Differences in soil characteristics between rhizospheric and non-rhizospheric zones were signif icant in surface horizons due to higher root density than in deeper soil layers. The f indings indicate that Persian oak ( Quercus brantii Lindl.) roots inf luenced the main soil chemical properties, even in calcareous soils.

Keywords Declining f orest · Persian o ak · Rhizosphere ·Soil hor izons

Introduction

Throughout their development, soils undergo continuous changes driven by pedogenesis factors that affect chemical and biological properties (Shanmugam et al. 2017). Among these factors, tree roots and associated organisms promote soil development (Huot et al. 2015). There are reports that root activities change soil properties and organic matter decomposition. Exudation of photosynthates into the soil immediately around roots (i.e., the rhizosphere; Fresno et al.2017) can result in greater availability of nutrients (Meena et al. 2017). Because of rhizodeposition, the rhizosphere shows chemical and biological variations compared to the rest of the soil (Fan et al. 2017). Chemical substances secreted by roots are exudates that change the chemical and biological characteristics of the rhizosphere (Yan Dam and Bouwmeester 2016). Microorganism populations in the rhizosphere are in greater frequency than in the bulk soil(Subke et al. 2018). A wide variety of microorganisms are able to produce auxin and auxin precursors form benef icial relationships with plants and alter host root development. Given that regional changes alter the f lux of carbon resources belowground, these changes are necessary for developing rhizosphere processes (Finzi et al. 2015). In addition, soil development, as affected by the rhizosphere,is dependent on soil type. For example, compared to noncalcareous soils, rhizosphere processes take longer to impact calcareous soils. Persian oak (Quercus brantiiLindl.) forests in western Iran have been showing symptoms of an unusual decline since late 2011. The forest ecosystem is fragile because of drought, thin calcareous soils and high pH.The effect of these factors resulted in extensive occurrences of leaf curling (Caglayan et al. 2016), a major indicator of drought stress. In 2014, when oak decline was most prevalent, leaf curling occurred on 70% of the stands throughout the northern Ilam province in Iran. Variations in the response of soil characteristics to vegetation cover have been reported for different forests (Schlesinger et al. 2016), but only a few studies have considered the effects of the rhizosphere on soil horizons under a forest in decline. Chemical and biological processes, as inf luenced by the rhizosphere, are involved in the thermodynamic development of soils (Lehmann and Kleber 2015). The aim of the present study was to investigate rhizosphere functions, soil horizons and the signif icance of their interactions in a declining oak forest (Quercus brantii).The approach was on the basis of chemical and biological differences between the rhizosphere and non-rhizosphere areas of soils.

Materials a nd methods

Study s ite

This research was conducted in the Ilam forest located on a typical calcareous soil in western Iran (46°262′21″;E33°28′45″N). The elevation is 1365 m a.s.l. The forest is dominated by oak (Quercus brantii). The region has a semi-arid climate with a mean annual precipitation of 571 mm (2006—2017) occurring in late autumn, winter and early spring (Fig. 1). The study area received 362 mm and 341 mm of rain in 2010 and 2014, respectively. Mean annual temperature is 15 °C (2006—2017), with a monthly average temperature ranging from 3.3 °C in January to 19.3 °C in June. The main soil formation is limestone, and the soils are classif ied as inceptisols in the US soil taxonomy system(Burt 2014).

Experimental de sign

Fig. 1 Monthly rainfall of study area

A randomized complete block design was established in the summer of 2012 and consisted of two factors (position and soil depth). Forty-eight measurements (2 positions × 4 soil depths × 6 replicates) were selected and samples collected from under and outside of oak canopies (Otieno et al.2015). Soil prof iles are described in Table 1. Approximately 3 kg of each horizon was sampled. The oak rhizosphere soil was separated from the samples by picking up the roots and adhering soil particles isolated from roots by gentle shaking.Thin soil layers adhering to the roots were the rhizosphere zone. Soil particles from outside the canopy were collected as non-rhizosphere zone soil (Otieno et al. 2015).

Plot d escription

Plots were selected beneath and outside of the canopies under medium to high oak declining conditions. The plots were in six replicates of 3 m × 3 m quadrates (Otieno et al.2015).

Soil anal yses

Determination of soil pH was based on the potentiometric method in water (solid: aqueous ratio of 1:2.5). Total organic carbon (TOC) was determined by potassium dichromate digestion (Skjemstad and Baldock 2007). For measuring water-extractable organic carbon (WOC), 1.0 gm of soil was submerged in distilled water and shaken overnight(120 rpm). The mixture was centrifuged at 1200gfor 8 min and the f iltrate analyzed with a TOC analyzer (TOC-500A,Shimatzu, Tokyo, Japan) (Tao and Lin 2 000). Basal respiration of soils was determined by NaOH (1 M) absorption of CO2generated during the period of incubation by titration of the residual OH with HCl (1 M) (Ekblad and Hogberg 2000). The values of microbial biomass carbon (MBC) were determined by the fumigation extraction method (Witt et al.2000). The amounts of total N (TN) were determined by the Kjeldahl method (McGill and Figueiredo 1993). For determining NO3--N and NH4+-N (inorganic nitrogen), samples were treated with 2 M KCl solution, shaken (1 h) with an orbital shaker (120 rpm), and the mixtures f iltered. Organicnitrogen levels were calculated by subtracting TN and inorganic nitrogen (Rutherford et al. 2008). For the determination of exchangeable calcium (Ca 2+ ), magnesium (Mg2+),potassium (K + ) and sodium (Na + ), the samples were submerged with BaCl 2 (0.2 M) solution and shaken for 15 min.The suspensions were filtered and analyzed by atomic absorption (Shimadzu 4500, Kyoto, Japan) (Wang et al.2004). Available phosphorus and potassium were determined by the Olsen method and extraction by ammonium acetate, respectively (Jones 2001). Micronutrients (zinc,iron, manganese and copper) in the extracts (by DTPA) were determined by atomic absorption (Shimadzu 4500, Kyoto,Japan) (Jones 2001). Electrical conductivity (EC), soluble anions (HCO 3-, SO 4-2 , Cl - ), and cations (Ca 2+ , Mg 2+ , K + )were determined using methods by Rayment and Lyons( 2011).

Table 1 Morphological description of a typical soil prof ile under Persian oak ( Quercus brantii Lindl.), Ilam forest, Zagros Mount (Iran)

Statistical anal ysis

These were performed on all soil horizons of the six prof iles in each location. The amounts from the six prof iles were averaged (n= 6). In all soil characteristics, standard errors were calculated forn= 6. Two-way analysis of variance was performed and means comparison was done by Tukey’s test.Signif icant differences were evaluated atp＜ 0.05. All data analyses were performed using SAS v 9.2 (SAS Institute 2017).

Results

Organic a nd microbial c arbon

TOC (total organic carbon), WOC (water extractable organic carbon) and MBC (microbial biomass carbon) decreased with increasing depth under the oak canopy and non-rhizosphere zone (Table 2). The results indicate that TOC values were signif icantly higher in the rhizosphere than in the non-rhizosphere zone in the upper soil levels (A and AB horizons), whereas there were no signif icant differences observed between rhizospheric and non-rhizospheric in subsoil horizons (Bk1 and Bk2). The WOC values were signif icantly higher in the rhizosphere than in the non-rhizosphere in the AB and Bk1 horizons (Table 2). The values of MBC in the rhizosphere decreased from the A to the Bk1 horizon and in the non-rhizosphere zone, no signif icant differenceswere observed in the AB horizon compared to the A horizon. The MBC/TOC ratio indicated signif icant differences between the rhizosphere and non-rhizosphere zone with the exception of the Bk2 horizon, although the Bk2 of the rhizosphere respired greater OC (∑CO 2 /TOC) (Table 2). This result was supported by the comparison of ∑CO 2 /WOC of the rhizosphere in the Bk2 horizon with the A horizon where the ratio was the highest (Table 2). The values of respiration in the oak rhizosphere were greater than those in the non-rhizosphere zone with the exception of the Bk2 horizon where ∑CO 2 -C was similar in the two zones.

Table 2 Values of total organic carbon (TOC), water-extractable organic carbon (WOC), microbial biomass carbon (MBC), amount of evolved CO 2 during two weeks of incubation (∑CO 2 -C), MBC /TOC and ∑CO 2 -C/TOC for the two soil zones (non-rhizosphere and rhizosphere) of Persian oak ( Quercus brantii Lindl.)

Total o rganic a nd inorganic n itrogen

The values of total nitrogen (organic and inorganic) were signif icantly different between the rhizosphere and nonrhizosphere in the A and AB horizons (p＜ 0.05). For the rhizosphere and non-rhizosphere soils, organic and inorganic nitrogen declined with depth (Table 3). For both the rhizosphere and non-rhizosphere, NH 4+and NO 3-from 0.9 to 1.4% were low components of total nitrogen.

Total n eutralizing v alue(TNV)

TNV values were highest in the rhizosphere (28.2%—31.4%)and showed no signif icant trend with depth (Table 4). The TNV values in the rhizosphere (23.1%—1.3%) decreased from the A to the Bk1 horizon.

Soil ac idity

The pH values in water and in paste increased with depth for both the rhizosphere and non-rhizosphere (Table 4). The pH was signif icantly lower in the rhizosphere than in the outside of canopy only in the A horizon.

Electrical c onductivity

Persian oak signif icantly affected soil EC (0.52 dS m -1 ) in the A horizon (p＜ 0.05). Differences of EC in other horizons were insignif icant (Table 4).

Table 3 Amounts of total nitrogen (TN), organic nitrogen(ON), NO 3 and NH 4 + for the two soil zones (non-rhizosphere and rhizosphere) of Persian oak( Quercus brantii Lindl.)

Table 4 Values of total neutralizing value (TNV),pH, EC and cation exchange capacity (CEC) for the two soil zones (non-rhizosphere and rhizosphere) of Persian oak( Quercus brantii Lindl.)

Cation e xchange c apacity(C EC)

The signif icant differences in CEC between the rhizosphere and non-rhizosphere, with the exception of the Bk2 horizon where the CEC was similar (Table 4). Among exchangeable elements, calcium was the most abundant (approximately 56% of CEC). Exchangeable calcium and magnesium decreased with depth but exchangeable potassium were constant throughout the soil prof ile at 0.27—0.33 cmol (+)kg -1 (Table 5). Exchangeable sodium amounts were lower than 0.09 cmol (+) kg-1in the all horizons.

Available P and K

Available phosphorous P in the rhizosphere was greater than in the non-rhizosphere from the A to the Bk1 horizon (Table 6). Available P was low in the Bk2 horizon of the rhizosphere and non-rhizosphere zones (3.11 and 3.43 mg kg -1 , respectively) (Table 6), whereas available potassium ranged from 306 to 381 mg kg -1 .

Available mic ronutrients

Zinc was present in low amounts (＜ 1 mg kg -1 ) from the A to the Bk1 horizon in both the rhizosphere and non-rhizosphere zones and in higher amounts in the Bk2 horizon (Table 6).The two soil positions showed similar amounts of available Fe (iron). In the rhizosphere, Mn (manganese) decreased from the A to Bk1 horizon, and then increased with depth. In the non-rhizosphere zone, after a decrease from A to the AB,Mn increased with depth. Cu (copper) levels of the rhizosphere were greater than in the non-rhizosphere A- and AB-horizons, and similar in the rhizosphere and non-rhizosphere zone of the Bk1 and Bk2 horizons (Table 6).

Soluble c ations and anions

In both rhizosphere and non-rhizosphere zones, calcium was the most abundant soluble cation fraction (Table 7).The rhizosphere had higher levels of soluble calcium in the A horizon than in the subsurface soil. For soluble cations,the order of abundance was Ca 2+ ＞ Mg 2+ ＞ K + ; for solubleanions, the order wasGenerally, the non-rhizosphere zone contained lower soluble cations and anions than the rhizosphere because of greater leaching and lower plant residues in non-rhizosphere zone. This zone had similar levels of soluble calcium in the A horizon than subsurface soil. In this zone, for soluble cations, the order of abundance was Ca 2+ ＞ Mg 2+ ＞ K + ; for soluble anions, the order was HCO 3-＞ Cl - ＞ SO42-(Table 7).

Table 5 Contents of exchangeable calcium (Exch.Ca), magnesium (Exch.Mg), sodium (Exch. Na) and potassium (Exch. K) in nonrhizospheric and rhizospheric soils under Persian oak( Quercus brantii Lindl.)

Table 6 Amounts of available P, K, Zn, Fe, Mn and Cu for the two soil zones (non-rhizosphere and rhizosphere) of Persian oak ( Quercus brantii Lindl.)

Table 7 The values of soluble anions and cations for the two soil zones (non-rhizosphere and rhizosphere) of Persian oak ( Quercus brantii Lindl.)

Discussion

The continuous effect of the rhizosphere over decades and the effects of calcium carbonate on soil acidity are responsible for the slight differences in pH between the rhizosphere and nonrhizosphere zones (Weiss et al. 2004). However, the higher non-rhizosphere pH observed from the A to the Bk1 horizons suggests that the alkalizing process induced by carbonate formation gradually increased with depth. The lower pH of the rhizosphere in A and AB horizons compared to other horizons suggests that the activity of f ine roots (i.e., an acidifying process) was higher in these horizons than in deeper horizons.[The uptake of cations by oak is one of the major factors that remediate the effects of calcium carbonate on soil pH of rhizosphere.] Uptake of cations leads to the release of hydrogen ions increases acidity responsible for minerals weathering with associated nutrient release (Ballard 2000). TOC (total organic carbon) amounts were higher in the rhizosphere than in the non-rhizosphere zone only in the A and AB horizons. The lack of contrast between the TOC in deeper horizons was attributed to low plant residues (Herbst et al. 2018). Increases in TOC in the oak rhizosphere were observed in comparison with the non-rhizosphere zone. The movement of labile C compounds from A horizon to deeper levels is ref lected by higher WOC(water-extractable organic carbon) contents in the rhizosphere of intermediate AB and Bk1 horizons. Nitrogen levels in both organic and inorganic forms differed between rhizosphere and non-rhizosphere zones. This is similar to studies reporting that the rhizosphere is rich in both N and C fractions (e.g.,Cocco et al. 2013). Depletion of N in deeper horizons may be explained by a high uptake of N compounds by oak roots and excretion of N-poor compounds. High NO 3--N/NH 4+-N ratios in the rhizosphere compared to the non-rhizosphere zone indicate higher nitrif ication (Li and Wang 2013). Higher nitrif ication in the rhizosphere may be attributed to higher organic carbon (Chowdhury et al. 2017). Increased OC contributes to available forms of nutrients. Therefore, available-P concentration in the rhizosphere was greater than in the non-rhizosphere zone in upper horizons but lower in both rhizosphere and non-rhizosphere of the Bk1 horizon. This was due to the presence of high calcium carbonate which converts available-P to weakly active compounds (e.g. calcium phosphates). In the rhizosphere ofCarexspecies, available-P amounts decreased with distance from roots to where no rhizosphere effect was detected (Gusewell and Schroth 2017). The higher available-P concentration in the rhizosphere than in the non-rhizosphere zone may be due to a greater microbial mobilization of P compounds (Zhu et al. 2017).This could be due to the decomposition of oak leaves, twigs, f lowers and fruits. In a forest, the soil nutrient pool is affected by the nutrient input through leaves,branches and roots. In soils derived from calcareous parent materials, the process of weathering controlled by the rhizosphere has a key role in making nutrients available such as phosphorous and micronutrients, and plays an important factor in development of soils (Cross and Lambers 2017).

Conclusion

Root activities are one of the main processes that differentiated soil horizons. Persian oak (Quercus brantii) roots inf luenced the main properties of calcareous soil horizons.The trends of some soil properties with decreasing depth and differences between rhizosphere and non-rhizosphere zones show that Persian oak trees over a long time have altered many soil properties, especially those directly associated with plant residues and root exudates. While activities of roots affected all soil horizons, although to a lesser extent with depth, the effect of plant residues on soil characteristics was mostly conf ined in the upper horizons and ref lects aboveground accumulation of organic matter. On these calcareous soils, continues changes due to the rhizosphere and plant residues were more expressed in the core of the subsoil and topsoil, respectively. However, the signif icant differences of the rhizosphere from the non-rhizosphere zone may develop even in soils derived from calcareous parent materials when mechanisms of soil formation have lasted long enough (＞ 65 years old). Our results suggest that the presence of Persian oak may help in environmental sustainability.

AcknowledgementsThe managing director of Ilam Agricultural and Natural Resources Research and Education Center is highly appreciated for their cooperation in conducting this investigation.