Microbial biomass and activity in salt affected soils under arid conditions
Introduction
Salinization is a process of soil degradation that is increasing in importance throughout the world (Keren, 2000, Liang et al., 2005). Primary salinization is a natural phenomenon involving accumulation of salts through natural processes due to high salt contents in parent materials or groundwater. Secondary salinization occurs frequently mainly as a consequence of overirrigation caused by improper management of irrigation facilities, poor soil internal drainage condition and unsuitable quality of irrigation water.
Excessive amounts of salts have a range of adverse effects on the physical and chemical properties of soil, microbiological processes and plant growth. While a plethora of information is available on the effects of salinity on the physical and chemical properties of soil and plant growth (Sumner, 1995, Keren, 2000, Levy, 2000, Sardinha et al., 2003, Liang et al., 2003), soil microbiological aspects of saline environments have been studied less intensively. Some investigations have been undertaken in naturally saline soils (Zahran et al., 1992, Sarig and Steinberger, 1994, Sarig et al., 1996, Batra and Manna, 1997, Zahran, 1997, Rietz and Haynes, 2003, Sardinha et al., 2003), and the depressive influence that salinity inflicts on soil microbial communities and their activities has been reported in most studies. However, both increases and decreases in mineralization of C and N with increasing salinity have been observed (Singh et al., 1969, Laura, 1974, McClung and Frankenberger, 1987, Nelson et al., 1996, Pathak and Rao, 1998). Since soil organic matter input, and consequently microbial biomass and activity, are typically concentrated in the top few centimeters of the soil (Lavahun et al., 1996, Murphy et al., 1998), salinization near the surface of soil may greatly affect a series of microbially mediated processes. This is a great concern in that microbial processes in soils control ecological function and soil fertility.
Shuangta region is in the western section of Hexi Corridor in Gansu Province of the arid northwestern interior of China, where both primary and secondary salinization occur. Saline drylands cover a large area of the region, where the water table is between 2 and 3 m from the surface of the soil. The extremely low rainfall makes irrigation essential for crop production, and secondary salinity in combination with very low levels of soil organic matter is a major threat to agriculture. Although soil physicochemical properties in the area have been studied thoroughly, microbiological aspects have not received the same attention.
The aim of our research was to investigate how a gradient in salinity under arid conditions affected biomass, activity and community structure of soil microorganisms in 11 salt affected soils in Shuangta region of west central Anxi County, Gansu Province, China.
Section snippets
Study site
The study was conducted in Shuangta region in west central Anxi County, Gansu Province, China. The region, located between 40°25′–40°36′N latitude and 94°49′–96°19′E longitude, is the floodplain area in the middle reaches of the Shule River basin, one of the three largest rivers in Hexi Corridor. It covers an area of about 7.56 × 104 ha, of which saline soils account for about 78%. The climate of the area is medium temperate arid, with an annual mean air temperature of 8.8 °C, annual mean
Soil properties
The soils differed in particle size distribution (Table 1), indicating their alluvial origin. Soil S11 (soil at site 11) showed the highest sand content and lowest silt and clay contents. Soil pH varied between 7.9 and 8.7 (Table 1), and it was highest in soil S7 in combination with highest EC. All soils contained soluble salts, and the EC varied significantly from 0.32 to 23.05 mS cm−1 (Table 1). Soil organic C, total N, total P and total K ranged between 3.07 and 13.82, 0.28 and 1.20, 0.34 and
Soil properties
Soils are generally classified as saline when they have an EC of 4 mS cm−1 or more (Sumner, 1995). Under this guideline, soils S5, S6, S7, S8 and S10 were saline and the others were not. All soils contained soluble salts, which resulted in a group of soil samples showing a wide range of EC values. In arid climates, salinity is usually combined with high soil pH because of CaCO3 enrichment in the uppermost soil layers. Although the effects of pH on the activities of soil microorganisms have been
Acknowledgements
We thank two anonymous reviewers for their valuable comments. We are grateful to Dr. Tian-Peng Gao for technical assistance, Dr. Xiao-Zhuo Han for useful discussion. This work was supported by the National Social Science Foundation of China (No. 04AJL 007) and National Natural Science Foundation of China (No. 03470298).
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