Response of some soil attributes to different land use types in calcareous soils with Mediterranean type climate in north-west of Iran

There was marked differences on the morphological behavior of soils among the three land use types in both Cambisols and Vertisols (Table 3). In this context, weakening in soils structure, hardening of consistency, and lightning of color occurred for cropland under comparable conditions with grassland and forest. In general, strong to medium granular structure of grassland and forest was replaced by fine angular blocky structure in cropland soils as result of traffic activities or mechanical stresses. Soil color was modified from reddish brown (10YR4/3) and dark brown (10YR3/3) in grassland and forest to yellowish brown (10YR5/4 and 10YR5/6) in cropland. Indeed, cultivation led to the lightning of soil color through increase in soil value (as an indicator of lightness or darkness) mainly due to destruction of the organic residues. The change in land use also exerted a major influence on soil consistency. Consistency degree of friable to very friable in grassland and forest was replaced by firm to very firm in the tilled soils (Table 3).

Particle-size distribution among land use types indicated some changes in both the studied soil types (Table 3). However, there was no statistically significant difference in percent distribution of sand, silt, and clay among the three land use types. The cultivated soils were noticeably lower in silt and clay contents and higher in sand fraction compared to those of the adjoining grassland and forestland. By the mean, silt and clay contents were in the order of forestland > grassland > cropland whereas sand content was in the order of cropland > grassland > forestland (Table 3). Through cultivation, sand content was increased by 4–35 % while silt and clay contents were decreased by 2–22 % and 2–18 %, respectively, compared to those of the adjoining grassland and forest. Presence of greater silt and clay contents in soils from the grassland and forestland could be attributed to: (1) accelerated alteration induced by natural and suitable conditions in the grassland and forest ecosystems; for instance, increasing of weathering processes in the grassland and forest could be highlighted through increasing of the secondary clay silicates (smectite) compared with the cropland (as will be discussed later). Indeed, accelerated weathering of primary minerals like micaceous minerals probably led to increased production of layer silicate clays in the grassland and forest; (2) well-covered vegetation in the grassland and mainly forest protects the soil against erosion (Abbasi et al. 2007). The grassland and forest have a long-growth period and high vegetation and biomass, therefore, land surface was well-covered and directly protected from strong erosion. On the contrary, the cultivated soils had a short-growth period along with low biomass and land surface was left bare after harvest (Zhao et al. 2005), resulted in accelerating soil erosion and losing clay and silt fractions. As a consequence, it can be said that the grassland and mainly forest through the tree canopy, surface litter, biomass, and root may have stabilized and protected these soils from significant surface erosion (Stolt et al. 1993). This pattern can provide favorable conditions for activation of soil forming processes mainly weathering and synthesis of clay fraction.

XRD analyses of clay fraction suggest that the examined soils are predominantly composed of illite, smectite, chlorite, and kaolinite. The semi-quantitative estimate of clay minerals is exhibited in Table 4 and the pertaining X-ray patterns are presented in Fig. 2. Changes in land use types and soil type did not cause noticeable modification in the XRD pattern of chlorite and kaolinite regarding the quantity and the intensity or in the position of peaks of them. This trend is not surprising, especially, in view of the fact that chlorite and kaolinite are very stable against weathering process in calcareous and alkaline soil like the region investigated (Khormali and Abtahi 2003; Rezapour et al. 2009). Variability of illite and smectite was strongly evident following changes in land use and soil type. Variation of illite was in the rank of cultivated soil > grassland > forestland, whereas smectite was in the order of forestland > grassland > cultivated soil. Indeed, two opposite patterns in illite and smectite distribution were highlighted in the forestland. This pattern suggested that these two clay minerals are largely related together through genetic pathways following water–plant activity-soil processes interaction in the forest land ecosystem. In this context, smectitization process as result of expanse of illite layer (peak of 10 Å) to smectite (peak of 17–18 Å) through a disordered stacking sequence (interstratified illite/smectite phase) in a K removal environment can be an acceptable reason for trends of illite and smectite. This assessment is in agreement to other researcher (Barre et al. 2007, 2009). As emphasized, it can be said that the natural forest promoted remarkably the expansion of illite layers (peak of 10 Å) toward smectite (peak of 18 Å) compared to other land use (cultivated soil and grassland) based on X-ray patterns. On the other hand, increasing pattern of smectite in the forestland was in parallel with an decreasing and increasing pattern, respectively, for clay and sand (as discussed before) indicating that smectite may be associated with weathering of primary minerals such as feldspars (Dixon and Schulz 2002). In addition, inherent origin (from bedrock) along with transformation of primary K bearing (such as mica feldspars) was considered as the main pathway for occurrence of illite (Rezapour et al. 2009).

Mean (±SD) values of selected soil chemical and fertilitical attributes are presented in Table 5. The mean soil pH of the three land use types showed that soil from the forestland had the minimum pH followed by the cropland and grassland in both Cambisols and Vertisols. While the variability of pH (mean) appears to be uniform in the cultivated soil and grassland, continuous cultivation led to a rise of 0.30 unit in soil pH for Cambisols and 0.46 unit for Vertisols compared to the adjacent soils under forest. This pattern may be explained by tillage practices such as (1) the mixing of surface soil with subsurface soil containing carbonates (Kettler et al. 2000) and (2) the irrigation practices containing a noticeable content salts and bicarbonate as showed by the used irrigation water (Table 1). Although the determined pH values were in an optimal range (6–7.5) for all the studied soils, it was found that cultivation had a negative effect on soil quality because of any increase in soil pH can be considered as a negative aspect in the alkaline and calcareous soils (Smith and Doran 1996). In contrast, the grassland and forest had no change and positive changes on soil quality, respectively, than the soils under cultivation with respect to soil pH.

Long-term cultivation significantly changed the EC and SAR of the soils compared to the adjacent soils under grass land and forest (Table 5). On an average, the soil from cropland had 1.78 (Cambisols) to 4.38 (Vertisols) and 2.0 (Cambisols) to 5.5 (Vertisols) times higher EC than soils from grassland and forest, respectively. Similarly, SAR contents of the cropland were 10 % (Cambisols) to 45 % (Vertisols) and 12 % (Cambisols) to 51 % (Vertisols) greater than those of the grassland and forest, respectively, indicating that the general distribution SAR was matching with distribution pattern of EC. In view of the fact that the applied irrigation water had a remarkable concentration of salts as displayed by its EC and TDS (Table 1), increasing pattern of EC and SAR was not surprising in the cultivated soils. In spite of soil quality being affected negatively by increasing soil salinity in the cultivated soils, the EC values were in the acceptable range (0–1.5 dS m⁻¹) for “general plant growth and microbial activity” (Smith and Doran 1996).

Calcium carbonate equivalent values under forest declined significantly (P ≤ 0.05) compared to those of the adjoining grassland and cropland in both Cambisols and Vertisols (Table 5). The mean CCE of the soils from grassland and cropland was almost similar (i.e. 96.7 and 96.8 g kg⁻¹ in the Cambisols), whereas the mean CCE value in the forest was 20–23 % (Cambisols) and 51.3–51.5 % (Vertisols) lower than those of grassland and cropland. This pattern can be attributed to high tend of forest land for the dissolution/translocation of calcium carbonates. In the light of this, lithogenic carbonates were dissolved under ambient moisture and level high soil CO₂ partial pressure leading to CCE leaching from soil surface to greater depth with the percolating soil water in the natural forest (Wang and Anderson 1998; Rubio and Escudero 2005). Contrary to forest land, calcareous parent material is tilled in the cultivated soils by farmers to retain a certain depth of cultivated soil leading to increased CaCO₃ in the cropland (Ni and Zhang 2007).

Similarly, general distribution SOC and total N were in the order of forestland > grassland > cropland for both Cambisols and Vertisols. Impact of different land use types on OC indicated that soils from the cropland had 12 % (Vertisols) to 41 % (Cambisols) and 15 % (Vertisols) to 18 % (Cambisols) low OC and total N, respectively, than the adjacent soils under the grassland (Table 5). These differences were so wider as compared to the cropland and forestland. Significantly, OC content and total N (mean) of cropland were 2.4 (Vertisols) to 3.8 (Cambisols) and 1.82 (Vertisols) to 2.5 (Cambisols) times, respectively, lower than that of forest soil (Table 5), showing that degrading effects of cultivation on soil quality are into a crucial category. These findings are comparable to the observation by Hajabbasi et al. (1997) who found that forestland had approximately three times more OC and total N than those of under the cultivation. Overall, the combination of fewer plant residues return and biomass after harvest, continuous cropping and frequent burning, faster decomposition rates of organic matter, and greater erosion can be subjected to declining SOC and total N and destroying soil quality in the studied region following long-term cultivation. Under the cultivation environment, much of the plant material is removed for human and animal food and relatively less remains its way back to the soil, making soil more susceptible to water erosion and runoff. On the other hand, tillage operations aerated the soil and broke up the organic residues, providing them more accessible to decomposition (Brady and Weil 1999). The organic matter content of the Vertisols was generally much higher than those of the Cambisols in the three land use types. This trend may be explained by the higher clay percentage in soils containing Vertisols, making clay–humus complexes that protect the organic matter from oxidation and degradation (Quiroga et al. 1996; Brady and Weil 1999). This assessment was more evident through occurrence of a recorded close correlation between SOC and clay content (Fig. 3a). In fact, the general distribution SOC was matching with distributive pattern of clay fraction. Soil organic carbon positively and significantly correlated with total N (Fig. 3b) indicating that total N is particularly released from SOC mineralization. In summary, loss of soil organic matter and total N under cultivation system are some factors in the examined region involving burning or removal of residues after harvest, clay content, soil types, and erosion.

Soil under continuous cultivation had both negative and positive effects on available phosphorous content as compared to forestland and grassland (Table 5). The mean soil P contents were found in the order of forestland > cropland > grassland for both Cambisols and Vertisols. On average, P content of cropland was 2.4 times higher and 1.65 time lower compared to those of the adjoining grassland and forestland, respectively, in the Cambisols. For the Vertisols, cultivation led to phosphorous pattern as 1.2 times higher and 2.3 times lower than the adjacent soil under grassland and forest, respectively. These results are in line with other authors (Bowman et al. 1990; Hajabbasi et al. 1997; Liu et al. 2002). Significant increase of P content (P ≤ 0.05) in the forestland (Table 5) may be associated with relative high SOC and low erosion in the land use type. High level of OC in the forestland can release soluble phosphorous compounds when organic residues and humus decompose as proposed by other works (Kamprath 1999; Yu et al. 2005). Organic matter can also improve available P by blocking or masking the phosphorus fixation site. By contrast, the combination of tillage activities along with very low SOC input during harvest that leaves the soil bare and unprotected from the ravages of erosion (Brady and Weil 1999) can contribute to the significant differences between cultivated soils and natural ecosystems like forest regarding to available P.

Long-term cultivation specified a significant decline (P ≤ 0.05) in available K compared to those of the adjoining grassland and forestland (Table 5) and the order was forestland > grassland > cropland for both Cambisols and Vertisols. Through the mean, soils under cultivation manifested a drop of 30 % (Cambisols) to 21 % (Vertisols) and 40 % (Cambisols) to 26 % (Vertisols) than the adjacent soil under grassland and forest, respectively. This means that depletion of available K was low in the Vertisols area than the soils under Cambisols area, mainly due to variation in clay content, nature and types of clay minerals, CEC, and organic matter. It seems that the greater clay content, nature and smectite content, and corresponding large surface area and CEC manifested higher available K in the Vertisols (Srinivasarao et al. 2007). Accumulation of significant more available K in the forestland may be explained by a process involving the biocycling of K. Trees in the forest region scavenge K from deeper within the soil, incorporate it into their biomass and return it to the soil surface in litter, making an enrichment aspect of K following litter decomposition (Graham and O’Geen 2010).

Intensive cropping specified a depletion face in exchangeable Ca, Mg and K compared to those of the adjoining grassland and forestland and the order was forestland > grassland > cropland for both Cambisols and Vertisols (Table 5). Accumulation of more exchangeable cations (Ca, Mg and K) in the forest can be associated with a process known as the “nutrient pumps or nutrient uplift”. In the forest land, deep-rooted trees often act as nutrient pumps, taking exchangeable cations form deep subsoil horizons into their root systems, translocating it to their leaves, and re-entering it to the soil surface with litter fall and decomposition, causing much higher exchangeable cations levels in the natural forests.

Soils under long-term cultivation reflected a negligible increase and significant increase in exchangeable Na in the Cambisols and Vertisols, respectively, than the adjacent soils under grassland and forest (Table 5). A similar pattern was observed for ESP, suggesting that the overall distribution of ESP was matching with exchangeable-Na pattern. Increase of sodium on the exchange complex in the cultivation system can be explained by the quality of the irrigation water. The irrigation water samples for the cultivated soil had a remarkable bicarbonate content (Table 1) that can reduce concentration of Ca⁺² and Mg⁺² in the soil solution by precipitating these ions as insoluble carbonates such as CaCO₃ and Ca Mg (CO₃)₂, making an increasing aspect in exchangeable Na through time. Such process, finally, causes an increasing pattern in soil ESP as documented by Corwing et al. (2003) and Pal et al. (2003).

Long-term cropping induced a decrease, although not significant, in CEC compared to those of the grassland and forestland in both Cambisols and Vertisols, probably as a result of destruction in SOC and variation in clay minerals (illite and smectite). In the light of this pattern, cultivation was associated with a drop of 5 % (Cambisols) to 10 % (Vertisols) and 12 % (Cambisols) to 20 % (Vertisols) in mean of CEC than soils from grassland and forest, respectively. Such pattern of CEC was matching with distributive pattern of SOC and clay content, showing that CEC is a function of organic matter and clay fraction (Rezapour and Samadi 2012). This assessment is more evident through appearance of a significant correlation between CEC and SOC (Fig. 3c) and CEC and clay content (Fig. 3d).

Soil scientists have developed the concept of soil quality to help quantify factors that are influencing the ability of the soil to function effectively in a variety of roles (Brady and Weil 1999). In order to better illustrate the impact of different land use types on soil attributes, a soil quality index (SQI) was examined using development programs of USDA to evaluate the influence of environmental and managerial stressors on soil health (Amacher et al. 2007). This index integrating numerous soil physicochemical attributes into a single number can be applied to monitor modifications in soil properties by time. The considered SQI had a significant correlation with soil properties, e.g. with SOC (r = 0.75, P ≤ 0.001), with total N (r = 0.74, P ≤ 0.001), with Ex-K (r = 0.70, P ≤ 0.001), and with SAR (r = −0.58, P ≤ 0.001) as reported by Rezapour and Samadi (2012). Nine major soil physicochemical properties including coarse fragments, soil pH, SOC, total N, exchangeable cations K, Ca and Mg, ESP, and Olsen-P were used for calculating the mentioned SQI. The individual index values for all the mentioned properties are summed to give a total SQI (for details, see Amacher et al. 2007) as:

The total SQI of cropland, grassland, and forest were 10, 11, and 12 for the Cambisols area and 11, 11, and 12 for the Vertisols area, respectively.

The total SQI is then expressed as a percentage of the maximum possible value of the total SQI for the soil properties that are measured as follow:

The results showed that cultivation led to a negative effect on SQI (a drop of 10–17 %) compared to those of the adjoining grassland and forestland mainly in the Cambisols area. On average, the SQI values of the soils from cropland, grassland, and forest in the Cambisols area were 66.7, 73.3, and 80 %, respectively. This pattern indicates that soil from the cropland had 9 and 17 % lower SQI value than the adjacent soils under grassland and forest, respectively, in the Cambisols (Fig. 3a). In the Vertisols area, cropland produced (1) no effect on SQI compared to grassland and (2) 17 % drop in SQI than the adjacent soils under forest (Fig. 3b). Changes in soil organic matter and pH were the major factors affecting the SQI yield. However, we suggest that EC can be also important in SQI values. As mentioned before, a significant rise in soil EC occurred through cultivation in both Cambisols and Vertisols but it was not considered in this SQI. Therefore, it seems that the examined SQI need to be refined mainly in calcareous soils where dynamic soluble salts are significant following agricultural practices (Fig. 4).