soil aggregation and organic carbon as affected by topography and land use change in western iran
TRANSCRIPT
Soil & Tillage Research 121 (2012) 18–26
Soil aggregation and organic carbon as affected by topography and land usechange in western Iran
Shamsollah Ayoubi a,*, Parisa Mokhtari Karchegani b, Mohammad Reza Mosaddeghi a,1, Naser Honarjoo b
a Department of Soil Science, College of Agriculture, Isfahan University of Technology, 84156-83111 Isfahan, Iranb Department of Soil Science, College of Agriculture, Islamic Azad University, Khorasgan Branch, Isfahan, Iran
A R T I C L E I N F O
Article history:
Received 10 October 2011
Received in revised form 3 December 2011
Accepted 19 January 2012
Keywords:
Soil organic carbon
Total nitrogen
Physical fractionation
Slope gradient
Aggregate stability
Micromorphology
A B S T R A C T
The study was conducted to investigate the effects of slope gradient and land use change on soil
structural stability, and soil organic carbon (SOC) and total nitrogen (TN) pools in aggregate-size
fractions in western Iran. Three land uses in the selected site were natural forest (NF), disturbed forest
(DF) and cultivated land (CL); and three classes of slope gradient (0–10%, S1; 10–30%, S2; and 30–50%, S3)
were used as a basis for soil sampling. The results showed that DF and CL treatments significantly
decreased soil structural stability indices in the three slope classes. The highest percentages of macro-
aggregates (i.e. 2.00–4.75 mm) and meso-aggregates (0.25–2.00 mm) were found in the lowest slope
class (S1) which was related to high SOC stock in this position. The highest percentage of macro-
aggregates was observed in the NF soil; but the highest percentages of micro-aggregates (0.053–
0.25 mm) were observed in the CL treatment. Micromorphological observations confirmed that topsoil
under natural forest mainly consisted of highly-porous crumb microstructure, excremental pedo-
features or passage features, which are indicators of enhanced SOC and biological activity. The lowest
values of SOC and TN were observed at the steep slope class (S3) presumably coincided with accelerated
soil erosion. Overall, enhanced aggregation and aggregate-associated organic carbon pools were
observed in the forest soils on the steep slopes indicating the importance of land management on C
sequestration in natural environments.
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1. Introduction
The importance of soil structure for soil tillage, water retention,root penetration and erosion potential has attracted a lot ofattention for agricultural productivity and environmental quality(Jastrow et al., 1998). Knowledge on the formation and stabiliza-tion of soil aggregates in natural and disturbed ecosystems isnecessary to address a variety of environmental concerns, rangingfrom the fate and transport of hazardous pollutants to the use ofsoils as potential C sink. Also, the conceptual links betweenaggregate stability and associated SOC are essential in the soilorganic matter dynamics (Jastrow, 1996; Jastrow et al., 1998).
Physical fractionation techniques consisting of size fraction-ation (primary and secondary particles) and density fractionationemphasize the role of physical fractions (i.e. soil minerals andaggregates) in SOC stabilization and turnover. These methods are
* Corresponding author. Tel.: +98 311 3913475; fax: +98 311 3913471.
E-mail addresses: [email protected] (S. Ayoubi), [email protected]
(P. Mokhtari Karchegani), [email protected] (M.R. Mosaddeghi),
[email protected] (N. Honarjoo).1 Tel.: +98 311 3913470; fax: +98 311 3913471.
0167-1987/$ – see front matter � 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2012.01.011
considered less destructive than chemical fractionation proce-dures, and the results are expected to be better related to the soilstructure and function of SOC (Christensen, 1992). The dynamics ofaggregate formation seems to be closely linked with SOC storage insoils (Golchin et al., 1997).
It is frequently observed that native soils usually have lowerbulk density (BD), and higher SOC content, aggregate stability andsaturated hydraulic conductivity when compared with thecultivated counterparts. Among several studies, are those ofCelik (2005) on Mediterranean soils in Turkey, and Khormali et al.(2009) on loessial soils in semi-arid region of northern Iran, whoreported diminishing soil quality indices upon cultivation ofnative soils. The land use is an important factor affecting SOCaccumulation and storage in soils, which controls the magnitudeof SOC stock and also greatly influences the composition andquality of organic matter in soils (Six et al., 2002; John et al., 2005;Helfrich et al., 2006). Land use and soil cultivation not only affectthe total amount of SOC, but also influence the organic carbondistribution in physical fractions and the SOC protectingprocesses. The knowledge of SOC in aggregate-size fractions(i.e. secondary particles) can help to assess and predict the effectsof land use on SOC storage and/or pools and soil aggregate soilstability.
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–26 19
The effects of land use conversion and different managementpractices on the distribution of SOC in different aggregate-sized aswell as in primary particle-associated fractions have beenfrequently reported in different soils and diverse climates, e.g.,by Helfrich et al. (2006) for silty soils in sub-humid region ofBavaria, Germany, Bronick and Lal (2005) for two soils in northernOhio, US, by Hoyos and Comerford (2005) for Andisols from theColombian Andes, and by Doaei (2008) for calcareous soils in semi-arid regions, southwestern Iran.
When a soil is subjected to cultivation (i.e. native land use ischanged), the unstable macro-aggregates are disrupted rapidly,making the occluded (i.e. physically protected) SOC morevulnerable to decomposition and mineralization (Six et al.,1998; Cambardella and Elliott, 1993). The SOC and aggregatesmutually protect each other since SOC is physically protected by itsassociation with soil primary particles in aggregates; at the sametime aggregate stability is enhanced by this association (Six et al.,1999, 2000, 2002). Thus, changes in the land use, such as cropcultivation in natural ecosystem (e.g. forest), often influence boththe quantity and quality of organic matter, which is the majorbinding agent for the stabilization of soil aggregates (especially thebigger ones). Caravaca et al. (2004), for example, found the lowestSOC (3.3 g kg�1) and TN (0.35 g kg�1) contents in the areas that hadbeen cultivated for more than 50 years in a semi-arid region ofSpain. Similar results were found by Vagen et al. (2006) whoinvestigated Oxisols in Madagascar. Six et al. (2000) reported thatcultivation reduced SOC content and changed the distribution andstability of soil aggregates. Celik (2005) observed that cultivationcaused 61 and 64% decreases in MWD of forest and pasture soils forthe 0–10 cm layer and 52 and 62% for the 10–20 cm layer,respectively. Similarly, cultivation resulted in, on average, a 34%decrease in the percent of water-stable aggregates (%WSA).
Erosional processes are enhanced after land use change in hillyregions and affect the soil properties considerably (e.g. Afshar et al.,2010). Wang et al. (2001) reported a considerable decrease in SOCconcentration caused by erosion on soils of the Loess Plateau inChina. When the physically-protected organic matter pools areexposed due to aggregate disruption upon land use change, thestructural stability will be diminished ultimately accelerating soilerosion especially on the sloping lands.
Soil micromorphological observations can help to interpret theprocesses occurring in soils exposed to deforestation. Soil faunaand flora affect many soil processes, in particular soil aggregation
Fig. 1. Location of the study area and distribution of soi
and the pore space architecture. Excremental pedo-featuresconstitute most of the micro-aggregates in organic soil horizonsand they can accumulate to such extents that dominate a horizon(Davidson et al., 2002). Soil micromorphology provides a methodof studying the interactions between animals and soil, withemphasis on features indicative of faunal activity, such asexcremental pedo-features and void space, which persist evenafter the organisms were no longer present. Micro-structure,porosity (shape, size and distribution), and biological activity areconsidered as soil quality indicators (Khormali et al., 2009).
The natural Oak forest in hilly and mountainous region ofwestern Iran has been disturbed for the past fifty years ago; and theforested area has been comprehensively cleared in some parts andthe lands have been cultivated for over 50 years (Afshar et al.,2010). There is little documentation about the impact of land usechanges on soil structure and physical fractions of organic matterin the hilly region of western Iran. The objectives of present studywere: (i) to evaluate the effect of land use change and slopegradient on soil structure using aggregate stability indices andmicromorphological characteristics, and (ii) to assess their impactson SOC and TN pools in water-stable aggregate size-fractions inhilly region of western Iran.
2. Materials and methods
2.1. Site description
This study was conducted in hilly regions of uplands Lordeganwatershed located in western Iran (Fig. 1). The study area is locatedwithin 508120 to 508370E longitude and 318580 to 328030N latitude.The mean elevation of the area is approximately 1860 m a.s.l. Themean annual temperature and precipitation at the site are 15 8Cand 600 mm, respectively. The hill slopes of the study area havebeen developed by extensive dissection of sedimentary Quaternarydeposits.
The study area, naturally forested by Oak, has been experienc-ing anthropogenic pressure and land disturbance, and has been putto a variety of land uses: (i) natural forest land (NF), (ii) disturbedforest land (DF), and (iii) cultivated land (CL) after clear forestcutting about 50 years ago, which has been cultivated with rainfedwheat and barley. The forest was sparsely cut by local farmersabout 50 years ago in the DF land use. It was used for agroforestryfor 10 years and then abandoned up to the time of this study. These
l sampling points in the study region, western Iran.
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–2620
three main land uses were chosen as a basis for soil sampling topredict the effects of land use on physical fractions of soil organicmatter. From each land use, 33 soil samples were collected underthree classes of slope gradients: 0–10% in toeslope position (S1),and 10–30% (S2) and 30–50% (S3) both located in backslopeposition.
A total of 99 soil samples were collected from the region withsampling pattern as shown in Fig. 1. Soil samples were collected inlate August 2010 from the 0–10 cm layer using an auger; three sub-samples per 1 m2 area in each position were mixed to form onecomposite sample reducing the effect of natural micro-variabilityon the soil samples. At the time of soil sampling, the CL treatmentwas cultivated by winter wheat and was under fallow during thesummer.
2.2. Aggregation analysis and soil organic matter fractionation
A 100 g soil sample, without disturbing the aggregates, waspassed through a 4.75-mm sieve, capillary-wetted to a matricsuction of 33 kPa (i.e. field capacity). The soil water content at33 kPa was determined on a separate batch of aggregates using apressure plate apparatus (Klute, 1986). Care was taken to avoidthe disruption of aggregates, and to minimize the effect ofentrapped air and uneven swelling upon rapid water adsorption inthe wet-sieving method. The wetted samples were used for thewet-sieving analysis with sieves set of 2, 1, 0.5, 0.25, 0.1 and0.053 mm. The aggregates were separated into seven size ranges(4.75–2, 2–1, 1–0.5, 0.5–0.25, 0.25–0.1, 0.1–0.053 and<0.053 mm). However, for the sake of simplicity and easierinterpretation of the results three combined aggregates sizes of4.75–2, 2.00–0.25 and 0.25–0.053 mm were used for the organicmatter fractionation as suggested by Cambardella and Elliot(1993). The aggregates were wet-sieved in a water bucket for30 min with a vertical stroke of 1.3 cm and a speed of30 strokes min�1.
By the end of wet-sieving, the sieves were gently pulled outfrom the water bucket, and aggregate fractions were recovered,oven-dried (at 50 8C) for 3 days and weighed. The percents of sandand gravel were determined by passing the disaggregated fractionsthrough their corresponding sieves. Percent of water-stableaggregates (%WSA) was calculated (Kemper and Rosenau, 1986)as follows:
%WSA ¼Xn
i¼1
WiðaþsÞ � WiðsÞ
Wt �Pn
i¼1 WiðsÞ
!� 100 (1)
where Wi(a+s) is the dry weight of the particles on sieve i, Wi(s) is thedry weight of sand or gravel on sieve i, Wt is the total dry weight ofthe soil (i.e. 100 g), and n is the number of aggregate fractions (i.e.6). Mean weight diameter (MWD, mm) of aggregates wascalculated by (Kemper and Rosenau, 1986):
MWD ¼Xn
i¼1
wi � X̄i (2)
Table 1Some soil physical and chemical properties in the surface soil layer (0–10 cm) in selec
Land use Soil properties
Clay (%) Silt (%) Sand (%) Soil texture
NF 41.00 35.80 23.20 Clay
DF 38.20 35.30 26.50 Clay loam
CL 37.90 35.10 27.00 Clay loam
NF: natural forest; DF: disturbed forest; CL: cultivated land, CCE: calcium carbonate eq
where X̄i is the arithmetic mean of aggregates size on sieve i, and wi
is the fraction of stable aggregates on sieve i, calculated using:
wi ¼WiðaþsÞ � WiðsÞPn
i¼1 WiðaþsÞ �Pn
i¼1 WiðsÞ(3)
Geometric mean diameter (GMD, mm) of aggregates representinglog-normal size distribution was also calculated (Kemper andRosenau, 1986):
GMD ¼ expXn
i¼1
wi log X̄i
!(4)
The SOC content of the aggregate-size fractions was determined bythe wet-oxidation method (Nelson and Sommers, 1982). Totalnitrogen (TN) was also measured by the Kjeldahl method (Krik,1950) in the mentioned three aggregate fractions.
2.3. Micro-morphological studies
Thin sections of about 80 and 40 cm2 were prepared from air-dried, undisturbed and oriented clods using standard techniquesdescribed by Murphy (1986). Soil samples were impregnated bypolyester (crystic 17449) which mixed with accelerator. Thecatalyst for crystic resin was methyl ethyl ketone peroxide (MEKP)and the accelerator was cobalt octoate (cobalt 2-ethylhexanoicacid). Micromorphological description was made as described byStoops (2003). A Zeiss polarizing microscope was used to study thethin sections prepared under both plain and cross polarized lights.
2.4. Statistical analysis
Factorial arrangement in a completely random design witheleven replicates was used for the statistical analysis. The overallstatistical arrangement of the treatments was: three land usetreatments (NF, DF and CL) and three slope gradients (S1, S2 and S3).The data were analyzed using analysis of variance (ANOVA) in theSAS statistical program (SAS Institute, 1990). Mean comparisonwas done using the Duncan’s multiple range method. Statisticalsignificant was evaluated at the p < 0.05 probability level.
3. Results and discussion
Some chemical and physical properties of the studied soils areshown in Table 1. Soil properties such as particle size distribution,texture, calcium carbonate equivalent (CCE), and SOC weresignificantly different at selected land uses. Land use changeconsiderably affected the particle size distribution, soil texture,SOC and CCE contents. The pH and EC did not vary much at the siteand land use change had no effect on these properties.
3.1. Soil structural stability and aggregate fractions
Continuous cultivation for fifty years after deforestationsignificantly decreased the %WSA, MWD and GMD in all of theselected slope gradients (Table 2). The highest values of %WSA,
ted land uses in the studied site.
CCE (%) SOC (%) pH EC (dS/m) BD (g cm�3)_
44.06 2.61 7.5 0.45 1.38
51.34 1.96 7.7 0.76 1.54
55.26 1.78 7.8 0.69 1.75
uivalent; SOC: soil organic carbon; EC: electrical conductivity; BD: bulk density.
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Fig. 2. Linear relationship between soil organic matter (SOM) and mean weight
diameter (MWD) for whole soil samples in the studied site.
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–26 21
MWD and GMD were found in the NF-S1 combination (i.e. naturalforest with slope gradient less than 10%). However, there were nosignificant differences in these indices between the disturbed forestand cultivated lands (see Table 2). Investigating the effects of landuse change and landscape position on aggregate stability in arangeland in southwest of Iran, Doaei (2008) reported that thehighest aggregate stability was found under pasture in the footslopeposition. Khormali et al. (2009) studying the role of deforestation onsoil quality attributes in northern Iran, showed that MWD decreasedfrom 1.49 mm in forest to 0.88 mm in the adjacent cultivated land asa result of considerable loss of SOC and breakdown of aggregates.The lower structural stability of cultivated soil may be attributed tothe disturbance by cultivation operations, which disrupt aggregatesand expose the occluded and physically-protected organic matterpools to rapid decomposition (Six et al., 2000).
The loss of SOC by cultivation might be related to thedestruction of macro-aggregates. The SOC plays a key role in soilaggregate stability (Lu et al., 1998). Correlation analysis showedthat there was a strong significant correlation between MWD andSOC (r = 0.83, **p < 0.001) (Fig. 2). Thus, the observed differences inthe overall aggregate stability (as quantified by MWD) due to landuse mainly resulted from the differences in the quantity of organicmatter under different land uses. Aggregate stability reflects theinteraction between primary particles and organic constituents toform stable aggregates, which are influenced by various factorsrelated to soil environmental conditions and managementpractices (Elustondo et al., 1990; Lu et al., 1998). Caravaca et al.(2004) indicated that WSA of cultivated soils was significantlylower (on average 40%) than that of forested counterparts (onaverage 82%). Findings of Celik (2005) also indicated thatcultivation has caused 61 and 52% decreases in MWD of 0–10 cm and 10–20 cm layers, respectively. The higher aggregation inforested soils may have protected the easily decomposable organicmatter pools from microbial degradation (Celik, 2005; Evrendileket al., 2004).
The natural forest soils had higher aggregate stability probablydue to extensive and stronger rootings, higher SOC content,permanent plant cover, conservation of the soil and lack of soildisturbance. The restoration of the soil macro-structure in forestecosystem was mainly driven by the direct and indirect effects ofroots and external hyphae as binding agents (Jastrow et al., 1998).Accordingly, Balabane and Plante (2004) observed the greaterMWD and stable macro-aggregates in the pasture soils comparedto the cultivated counterparts. Lal et al. (1994) also observed thattwo Ohio soils had higher soil aggregate stability under long-termreduced and no-tillage systems compared to conventional (inten-sive) tillage practices.
Fig. 3 shows the distributions of the aggregate-size classes asaffected by land use changes. Distribution of soil aggregates
Fig. 3. Distribution of aggregate-size fractions as affected by the land use changes at different slope gradients; NF: natural forest, DF: disturbed forest, CL: cultivated land, S1:
slope 0–10%, S2: slope 10–30%, and S3: slope 30–50%. Means with at least one dissimilar letter indicate significant differences (p < 0.05) among land use-slope treatments
based on Duncan’s mean test. For easy comparison and consistency, letters with similar style were used for each land use-slope combination.
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–2622
differed significantly among the studied land uses. The highestpercents of two groups of large aggregates (i.e. 2.00–4.75 and 0.25–2.00 mm) were found in the lower slope (S1). The highestpercentage of macro-aggregates (i.e. 2.00–4.75 mm) was observedin the natural forest soil and that of micro-aggregates (i.e. 0.053–0.25 mm) was noted in the cultivated and disturbed forest soils.Our results are in line with the observations of Besnard et al. (1996)who found that macro-aggregates are the most frequent aggregatefraction (52.4%) in a forest soil and that after 7 years of conversionto maize cultivation, 28.6% of the total soil were located in the>200 mm, 18.0% in the 50–200 mm and 49.5% in the <50 mmfractions.
Cultivation probably accelerated the rate of soil erosion,resulting in destruction of weak macro-aggregates especially atthe steep slopes. The cultivated soils had significantly (p < 0.01)higher amount of small aggregates (<0.05 and 0.05–0.10 mm) thanthe other two land uses (Fig. 3). Since small aggregates were foundto be a useful indicator of soil degradation, tillage in the cultivatedsoils disintegrated the larger aggregates into smaller ones. Thefindings of Celik (2005) also showed significant differencesbetween forested and cultivated soils in terms of aggregate sizedistribution.
Reduction of large aggregates in the cultivated soil (Fig. 3) maybe attributed to the physical disturbance of soil and the lowstability of macro-aggregates. Binding agents such as fungalhyphae and plant roots stabilize large aggregates but aretemporary and unstable. Wright and Hons (2005) showed thatin the no-tillage system, the percents of aggregates >2 mm and0.25–2.00 mm in the 0–5 and 5–15 cm layers were greater thanthose in the conventional tillage system, respectively. Grandy andRobertson (2006) assessed the impact of 10 years tillagemanagement on the soil aggregation and reported that tillagesignificantly reduced the large aggregates (i.e. 2–8 mm) andincreased the small aggregates (i.e. <0.25 mm) in the 0–7 cm soillayer.
3.2. Soil organic carbon, total nitrogen and C/N ratio in aggregate-size
fractions
The effects of slope gradient and land use on SOC, TN and C/Nratio in different aggregate-size fractions are presented in Table 3.The highest SOC associated with macro-aggregate fraction (i.e.2.00–4.75 mm) was found in the NF-S1 combination (22.00 g kg�1),
which probably receives the large amounts of inputs and freshlylitter by surface erosional processes. Within the selected treat-ments, SOC was the highest in all of the fractions for NF land use(Table 3) with higher inputs and lower decomposition rates. TheSOC contents were reduced considerably in the disturbed andcultivated soils as compared to the natural forest counterparts.These differences were much greater for the larger aggregates. Ithas been reported that long-term cropping systems reducedaggregation and macro-aggregate stability (e.g. Jastrow, 1996;Mikha and Rice, 2004). The physical location of SOC in the soilarchitecture appears to be the key factor governing SOC stock anddynamics (Oades, 1988). Tillage breaks down soil aggregates andthereby exposes SOC, which has been physically protected withinthe aggregates.
Further, an increase in SOC concentration was observed withincrease in aggregate size (see Table 3). These results are inagreement with the concept of aggregate hierarchy according towhich micro-aggregates are bound together into macro-aggre-gates by transient binding agents such as microbial- and plant-derived polysaccharides and temporary binding agents such asroot and fungal hyphae (Tisdall and Oades, 1982; Six et al., 2000).Elliott (1986) studied the aggregation and carbon, nitrogen, andphosphorus in native and cultivated soils and confirmed that theconsequence of aggregate hierarchy was an increase in SOCconcentration with increasing aggregate size because SOC is themajor binding agent of large aggregates.
The water-stable macro-aggregates in NF were richer in SOCthan micro-aggregates. This SOC enrichment might be attributedexclusively to the new (young) SOC pools, whereas the old SOCpools of the aggregates remained effectively unchanged. Cambar-della and Elliot (1993) reported that stable macro-aggregates werericher in SOC than meso-aggregates and micro-aggregates, and thisSOC was younger with greater mineralization rate. Our results alsodemonstrated that the SOC stored in micro-aggregates was slightlyaffected by cultivation than SOC in macro-aggregates in the DF andCL treatments. These results are in line with finding of John et al.(2005) who compared the SOC pools in soil aggregate-sizefractions under different land uses. Bronick and Lal (2005) reportedthat virgin soils had greater SOC inside the aggregates compared tothe cultivated counterparts. They found that SOC content of largeaggregate fraction (i.e. 4.75–8.00 mm) was higher than that ofmedium-size fraction (i.e. 0.50–2.00 mm). Six et al. (2000) alsoreported an increase in SOC content in the large aggregate-size
Ta
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S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–26 23
fraction under no-tillage system and an SOC decrease in the small-size fraction under conventional tillage. Our results of SOC and TNenrichment in macro-aggregates are in agreements with resultsreported by Puget et al. (1995), Jastrow (1996).
There were significant differences in organic carbon contentamong the three land uses for macro-aggregates, but no significantdifference was found for the SOC in the meso- and micro-aggregates (see Table 3). Similar trend was found for the TNconcentration (Table 3). The highest SOC and TN were found in NF-S1 combination and the lowest values were observed in steepslopes and cultivated lands (i.e. CL-S3 combination). Although,significant differences were found for SOC concentration inaggregate-size fractions under NF, but the differences betweenthe SOC values in the fractions for disturbed and cultivated soilswere not significant. Also, significant differences were found forthe SOC and TN concentrations in aggregates among the slopegradients; the lowest SOC and TN concentrations were observed inthe steep slopes (S3) most likely due to highest soil erosion rate.These results implied that erosion and soil manipulation presum-ably reduced the SOC and inhibited the formation of stable macro-aggregate fractions, rather than micro-aggregates.
The overall trend of TN distribution among the aggregatefractions was similar to that of SOC distribution (see Table 3). Theresults showed that C/N ratio was in the order: macro-aggrega-tes > meso-aggregates > micro-aggregates for all of the treat-ments (Table 3). The increase in C/N ratio with increasingaggregate size suggests that SOC in macro-aggregates is youngerand more labile than SOC in micro-aggregates (e.g. John et al.,2005). The C/N ratio in aggregate fractions was in the range 15–23for NF soils and in the range 8–11 for DF and CL soils, respectively.These results are in agreement with findings of Shi et al. (2010).This observation presumably shows that decomposition and/orturnover processes of C and N were not similar among differentland uses. Cambardella and Elliott (1993) also reported that 18% ofSOC and 25% of TN in no-tilled soil was associated with fine silt-sized particles in macro-aggregates. The TN decreased as theintensity of tillage increased, but SOC contents remainedunchanged, suggesting that the processes controlling the SOCand N cycles are not coupled. They assumed that SOC turnover isprimarily controlled by physical protection mechanisms, whileorganic N turnover is mainly controlled by chemical protection.This may be applicable for the observed differences of organicmatter quality (e.g. C/N ratio) in our studied land uses but needsfurther investigations.
Fig. 5 shows the contributions of SOC and TN in differentaggregate fractions to the total SOC and N stocks in 100 g of bulksoil under different land uses. The NF land use contained, onaverage, 25.78, 58.16 and 16.06% of total SOC in macro-aggregates,meso-aggregates and micro-aggregates, respectively. The corre-sponding values were 5.29, 56.35 and 38.36%; and 2.12, 62.35, and35.53% for the NF and CL land uses, respectively. Obviously aconsiderable portion of SOC in disturbed ecosystems was stored inaggregate fractions <2 mm compared to that in the natural forest.Conversely, the aggregates >2 mm were the most importantfraction for SOC storage in the natural forest. Little variation wasobserved in the SOC stock of meso-aggregates due to land usechange. Similar results were reported by John et al. (2005) whoobserved that a major portion of SOC was stored in meso-aggregates (250–1000 mm) under wheat and maize cultivations.
Soil macro-aggregates are important for soil physical andchemical fertility maintenance and they are very sensitive to thechange in the land use and management (Oades, 1988). Forest soilshad higher SOC and N associated with macro-aggregates than thetwo other land uses (Table 3). These results suggest that naturalforest can maintain the macro-aggregates structure rather thandisturbed and cultivated soils, and can enhance the ability of soil to
Fig. 4. (a) Crumb micro-structure in the topsoil under natural forest treatment (plain polarized, PPL); (b) excremental pedo-features (e.g. passage features, biologic activity) in the
topsoil under natural forest treatment (PPL); (c and d) massive micro-structure with low porosity in the topsoil under cultivated treatment (c: PPL and d: cross polarized, XPL).
Fig. 5. Distribution of soil organic carbon (a) and total nitrogen (b) contents in
water-stable aggregates under different land uses; NF: natural forest, DF: disturbed
forest, CL: cultivated land. Different letters in a size fraction represent statistical
difference among land use treatments at p < 0.05. Bars on the columns stand for
standard deviations.
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–2624
sequester SOC and N as compared to that in the two disturbedecosystems. These findings are consistent with the finding ofCambardella and Elliot (1993) who found higher SOC and TN inmacro-aggregates of no-till treatments as compared to those ofconventional tillage systems. Similar results were reported byDoaei (2008) who compared the effects of land use and landscapeposition on SOC and TN pools of aggregates in southwestern Iran.
3.3. Micro-morphological observations and soil order changes
The thin sections of the soil samples from forest and cultivatedsoils are illustrated in Fig. 4. Inspection of the thin sections showedcoarse/fine (C/F) distribution of double spaced coarse enualic innative forest soils (Fig. 4a). Porosity included dominantlycompound packing voids which were induced by highly-separatedcrumb microstructure. This high porosity might be as a result of thepresence of high SOC and biological activity in the soil (Fig. 4a andb). The high biological activity could be deduced from the presenceof high amount of excremental pedo-features or passage featuresas discussed by Adesodun et al. (2005) (Fig. 4b).
In contrast, topsoil from the cultivated treatment lackssufficient organic matter and consequently biological activity forthe improvement of soil micro-structure and porosity. The topsoilmicrostructure of cultivated treatment is mainly vesicular orweakly-developed blocky with low planar porosity (Fig. 4c and d).The C/F related distribution in the topsoil of cultivated soilindicated close porphyric distribution (see Stoops, 2003). Cultiva-tion practices have led to soil erosion and compaction, anddiminishing soil quality attributes such as biological activity,porosity, and microstructure. Our finding are consistent with thoseof Khormali et al. (2009) and Khormali and Shamsi (2009) whoused the micro-morphological pedo-features to determine the
S. Ayoubi et al. / Soil & Tillage Research 121 (2012) 18–26 25
effects of land use change on soil microstructure and porosity ofloessial soils of hillslopes in Golestan province, Iran.
The NF soils were mainly classified as Fine, mixed, thermic, TypicHaploxerolls according to Soil Survey Staff (2006). The dominantepipedon and subsurface horizon in this land use were mollic andcalcic horizons, respectively. Almost all the CL soils had sufficientSOC and bore other pertained properties to be classified as Mollisols.In contrast, none of the adjacent cultivated soils had the require-ments of a mollic epipedon but ochric predominates and classified asFine loamy, mixed, thermic, Typic Calcixerepts (Soil Survey Staff,2006). Therefore, the soil orders of NF lands were transformed fromMollisols to Inceptisols due to cultivation. This finding reveals thattillage and erosion has led to thinning of mollic epipedon, losingmuch of its organic matter, formation of blocky structure (Fig. 4c)and a compacted horizon with high BD (see Table 1). These processesultimately induced formation of an ochric epipedon. Khormali andNabiollahi (2009) also investigated the impact of land use change ondegradation of Mollisols in western Iran. The results revealed thatdifferent from the rangelands, cultivated soils lacked enough SOC tomeet the requirements of Mollisols and had only ochric epipedons.Micro-morphological investigations demonstrated that the range-land soils had strong granular and crumb micro-structure. Khormaliet al. (2009), studying land use changes in loessial soils of northernIran, reported that topsoil of the forested land mainly includedhighly porous crumb micro-structure as opposed to the soils indeforested and cultivated treatments.
4. Conclusions
(1) The distribution of aggregate size classes (especially macro-aggregates) was influenced by land cultivation and soil tillageon steep landscapes in western Iran. For the natural forest, thedominant aggregates were macro-aggregates, whereas meso-aggregates and micro-aggregates were dominant in thedisturbed forest and cultivated soils. Micro-morphologicalobservations also confirmed that the cultivation destroyed themacro-aggregates and led to low-porosity blocky structure.The soil orders of native forest lands were transformed fromMollisols to Inceptisols due to cultivation. Tillage and erosionalprocesses changed mollic epipedon to ochric epipedon in thecultivated soils.
(2) The impact of land use on SOC of the aggregate-size fractionswas highly significant in the steep slopes. The overall trends ofTN and C/N ratio distributions among the aggregate-sizefractions were similar to those of SOC. Variations of SOCconcentration and C/N ratio among the aggregate fractionswere in accordance with the concept of aggregate hierarchyconfirming the dominant binding role of organic matter in thestructure of the studied soils. Undoubtedly, natural forestmanagement incorporated more SOC into the aggregates thanthose in the disturbed forest and cultivation treatments.
(3) High water-stable aggregates and aggregate-associated SOC inthe native forest soil showed the role of land management on Csequestration. A greater SOC sequestration potential andenvironmental sustainability exist in soils under natural forest,especially on the steep slopes. Overall, the results suggest thatundisturbed land use such as natural forest improves soilaggregation, structural stability and SOC storage, and reduces Cemission and soil erosion, especially in the mountain regionswith high rainfall in western Iran.
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