The effect of freeze-thaw cycles on soil aggregate stability in different salinity and sodicity conditions

Freezing and thawing affect soil aggregate stability. Understanding the effects of freezing and thawing processes on aggregate stability is necessary for the development of strategies for managing saline-sodic soils. This study was performed to determine the effects of freezing and thawing cycles (two, four, and six) on wet aggregate stability in six soils of different salinity and sodicity (54 dS m, 11.7%; 49 dS m, 11.8%; 53 dS m, 31.7%; 85 dS m, 39.7%; 59 dS m, 13.9%; 68 dS m, 36.8%, respectively) and three different aggregate sizes (< 1, 1-2, and 2-4 mm). The experiments were conducted under laboratory conditions using disturbed and non-cropped soil samples. In soils with a high percentage of exchangeable sodium and high electrical conductivity, the effect of freeze-thaw cycles on the wet aggregate stability was not signif icant. However, when the electrical conductivity was high and the percentage exchangeable sodium relatively low, wet aggregate stability was signif icantly reduced by the freeze-thaw cycles. Moreover, for aggregate sizes of 1-2 and 2-4 mm, a significant reduction (P < 0.01) was seen in the wet aggregate stability of samples that underwent six freeze-thaw cycles compared to those that underwent two cycles. Additional key words: saline-sodic soils, soil properties, soil structure.

electrolyte concentrations are unstable and show weak aggregate stability (Abu-Sharar et al., 1987b;Oster et al., 1999).Flocculation is generally enhanced when the soil solution salinity exceeds a value of approximately 1.5 dS m -1 (Hanson et al., 1999).
It is well known that freezing and thawing affect soil structure.Consecutive freeze-thaw cycles may negatively affect soil particle aggregation (Lehrsch et al., 1991).Many studies have shown that freezing and thawing negatively affect soil structure and aggregation.For example, Oztas and Fayetorbay (2003) reported weak aggregate stability in soils with a high PES after freezing-thawing.Hanay et al. (2003) reported that even in the presence of high salinity levels, aggregate stability decreased with increasing numbers of freezethaw cycles in soils with a high PES.
Freezing and thawing processes occur especially at the soil surface.Any disturbance of dispersive soils at the surface is likely to result in a rapid decline of their structure and a subsequent reduction in their permeability (Oster et al., 1999).This enhances the possibility of surface soils becoming exposed to wind and water erosion.
Based on Turkish Advanced Soil Map surveys, salinity and sodicity were detected in 1,518,722 ha of the country's land resources.These survey data indicate that saline soils cover a large part of Turkey's barren lands (74%), which together occupy an area almost equal to 2% of the total surface area, 5.48% of the total cultivable land area, and 17% of the 8.5 million ha of economically irrigable land area (Kendirli et al., 2005).
This study was conducted to evaluate the effects of freezing and thawing processes on the stability of different sized aggregates in soils of the Igdir Plain with different salinity and sodicity levels.
The Igdir Plain in northeastern Turkey is one of the country's most important agricultural production areas.It covers 68,000 ha of irrigated land and has a mean slope of 0.1%.The mean altitude is 850 m.About 36% of its soils (25,000 ha) suffer from some degree of salinity and/or alkalinity (Ardahanlioglu et al., 2003).The mean annual precipitation, temperature, and relative humidity of the area are 253 mm, 11.7ºC, and 71% respectively (Smith, 1993).The soil humidity and temperature regimes are defined as Aridic and Thermic (Soil Survey Staff, 1975).
Six samples from the 0-30 cm top layer of soils with different salinity and sodicity levels (Typic Natrargids) were collected.Their physical and chemical properties were determined using the methods of Klute (1986) and Page et al. (1982) (Table 1).The samples were air dried and sieved into three aggregate size groups (< 1, 1-2, and 2-4 mm).Five hundred grams of each aggregate size group were placed into 1 kg plastic pots with holes.The moisture regime of the aggregates prior to freezing was set to field capacity (-0.33 kPa).The samples of different aggregate size groups were then frozen at -5°C for 24 h in a temperature-controlled freezer and subsequently thawed at a cool room temperature (+5°C) for another 24 h.This freezing and thawing procedure was repeated for two, four or six cycles.This sequence is considered to represent the freezing and thawing conditions of early spring on the Igdir Plain.
The percentage of water-stable aggregates was then determined using the wet sieving procedure (Kemper and Rosenau, 1986).
The treatments followed a factorial design: three replications including six soil types, three aggregate size groups, and three freezing-thawing cycles.Analysis of variance (ANOVA) was performed to evaluate the effects of the different treatments.Duncan's multiple test procedure was used to compare the average values obtained (SAS Institute, 1989).
Table 2 shows the comparison of the mean wet aggregate stabilities of the soil samples for different numbers of freeze-thaw cycles.The effect of these cycles on this variable was not significant in soils I, II, IV and VI.Significant reductions (P < 0.01) were seen, however, for soils III and V after six freeze-thaw cycles.Soil III contains less organic matter and clay than the other soils (Table 1).An increasing number of freeze-thaw cycles may have a negative effect on the aggregate stability of soils with less organic matter (Lehrsch et al., 1991).Further, low clay contents may enhance the negative effect caused by a high PES in such soil.Soil V had the highest amount of silt; Pikul and Allmaras (1985) report that frequent freezing and thawing of silt loam soils may seriously degrade their structure.
The effect of freezing-thawing cycles was not significant for aggregate sizes of less than 1 mm (Table 2).For the 1-2 mm and 2-4 mm aggregate size groups, a significant reduction (P < 0.01) in wet aggregate stability occurred after six freeze-thaw cycles with respect to two freeze-thaw cycles.Benoit (1973) found that the greatest reduction in the percentage of water-stable aggregates following freezing and thawing occurred within the largest aggregate size groups.
At the end of the six freeze-thaw cycles, no significant alterations were seen in the wet aggregate stability of any of the aggregate sizes of soils III, IV and VI (Fig. 1).Soils III, IV and VI had higher PES values (Table 1).These soils initially showed low aggregate stability, in agreement with that reported by Abu-Sharar et al. (1987b) and Oster et al. (1999).Weakly aggregated soils with a high PES may therefore be less affected by freezing-thawing.In conclusion, this study reports the following findings: i) at the end of six freeze-thaw cycles, the wet aggregate stability of soils III and V was significantly reduced; ii) in soils with a high PES and electrical conductivity, the effect of freeze-thaw cycles on the wet aggregate stability is insignificant; however when the electrical conductivity is high and PES relatively low, the wet aggregate stability decreases significantly with increasing numbers of freeze-thaw cycles.

Table 1 .
Some physical and chemical properties of the soils studied

Table 2 .
Wet aggregate stability values for different soils and aggregate sizes after different numbers of freeze-thaw cycles 1 Mean followed by the same letter are not significant (P < 0.01) according to Duncan's multiple range test.