Original articleChanges of soil physico-chemical properties and enzyme activities in relation to grassland salinization
Highlights
► We found that grassland salinization had different effects on soil nutrients. ► The tested soil enzyme activities were highly correlated with one another. ► The tested soil enzyme activities were highly correlated with some soil properties. ► Not all enzymes studied were sensitive to salinity.
Introduction
Soil enzymes play an important role in organic matter (OM) decomposition and nutrient cycling. For example, the β-glucosidase is an important enzyme in terrestrial carbon (C) cycle in producing glucose, which constitutes important energy source for microbial biomass [1]. Thus, determination of β-glucosidase activity has been suggested as a good indicator of soil quality among other hydrolytic enzyme activities [2]. Phosphatase plays an important role in transforming organic phosphorus (P) into inorganic forms for plants, particularly when P availability limits plant productivity [3]. Urease is an important enzyme involved in soil nitrogen (N) cycling. Urease catalyzes the hydrolysis of organic N to inorganic forms, the former using urea-type substrates and the latter ammonia or ammonium ion substrates [4], [5]. Soil enzyme activities have been proposed as indicators for measuring the degree of soil degradation for their sensitivities to small changes in soils [6], [7]. However, the reliability of soil enzyme as indicators of soil quality has been questioned recently [8], [9], [10], [11].
Salinization, one of the most serious types of land degradation, has become a major concern throughout the world [12], [13]. Salt-affected soils occur mainly in arid or semiarid region, where evaporation greatly exceeds precipitation and salts dissolved in the ground water reach and accumulate at the soil surface through capillary movement [13]. Worldwide, approximately 932 million ha soils are estimated to be salt-affected [14]. Salt-affected soils cover about 10% of the total dry land surface [14], and this situation is getting worse in many parts of the world [15]. According to the database of China's second national soil survey, soil salinization affects an estimated 35 million ha in China, of which 29.3 million ha is in grassland.
Salinity not only results in soil degradation and severe decreases in land potential productivity [16], and it also has adverse effects on microbiological processes [17]. However, soil microbiological aspects of natural saline environments, such as soil enzyme activities, have been studied less intensively [17], [18]. Several studies that have been conducted to evaluate the effects of controlled or irrigation-induced saline conditions on soil enzyme activities, and in most of these studies the depressive effects have been reported [17], [19], [20]. For example, Frankenberger and Bingham (1982) [21] found that salinity depressed enzyme activities under laboratory conditions. Irrigation-induced salinity also detrimentally influenced soil enzyme activities [17], [22]. However, controlled conditions or anthropogenic alterations hardly reflect the natural process of this studied grassland soil. Inconclusive and/or at times contradictory results may be produced with the variation of experimental location, the extent of salinity/sodicity (levels of EC, SAR/ESP and pH) and their distribution pattern, soil temperature, and the duration of the experiment ranging from days to years [23].
The experiment was conducted in the naturally salt-affected grassland in the middle reaches of the Hexi Corridor region, Gansu Province, China. In this area, salt-affected grassland is estimated to 1.4 million ha, accounting for 79% of the total salt-affected soils [24], but less information concerning soil physico-chemical properties and microbial activities in relation to soil salinization in the Hexi Corridor region is available. With a view to determining which of various enzyme activities, if any, would be useful as indicators of the degree of salinization in the grasslands, we studied: (1) changes in soil physico-chemical properties in the salinization process; (2) how salinization had affected soil enzyme activities.
Section snippets
Study area
This experiment was conducted at the Linze Grassland Ecological Test Station (100°02′E, 39°15′N) in the middle reaches of the Hexi Corridor region at an average altitude of 1400 m above sea level in Gansu Province, PR China. The station is located at the southern edge of the alluvial fans of the Heihe River. The Heihe River runs down from the Qilian Mountains, and melt water from glaciers and snow cover is the principal source of surface runoff. The underground water table is at a depth of
Soil physico-chemical properties
As shown in Fig. 1B, the averaged soil pH ranged from 8.47 to 8.73, with no significant difference across the landscape (P > 0.05). The averaged EC of the soils increased significantly, while SWC decreased significantly with salinization development (P < 0.05) (Fig. 1A, C). Compared to the lightly salinized grassland, the averaged EC of the soils increased by 348.9%, from 368.3 mS m−1 to 1653.3 mS m−1, and SWC decreased by 55.0%, from 43.8% to 19.7% in the severely salinized grassland.
The
Soil physico-chemical properties
With salinization development, EC increased significantly, but no significant change was observed in soil pH. The effects of pH on the activities of soil microorganisms have been reported [6], [19]. However, the pH value variations in the current study do not seem sufficiently great to establish differences in biological activity (Table 3).
Changes in soil attributes (e.g. SOC, TN, TP, AN and AP concentrations) are important indicators of changes in soil fertility and long-term ecosystem
Conclusion
Salinization is one of the most serious types of land degradation in arid and semiarid regions. It is evident that grassland salinization has extremely adverse affects not only on some soil physical and chemical properties but also on some enzyme activities. Soil water content, organic C, total N, available N concentrations, as well as β-glucosidase, alkaline phosphatase and urease activities, all decreased significantly with an increase in salinity, while available P concentration increased
Acknowledgements
We thank all technicians for their help with field investigation. This research was funded by one of the Chinese National Key Projects for Basic Scientific Research (2009CB421303) and one of Chinese National Fund Projects (30972422) and one of the Chinese National Support Projects of Science and Technology (2011BAC07B02-06).
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