Changes of anaerobic to aerobic conditions but not of crop type induced bulk soil microbial community variation in the initial conversion of paddy soils to drained soils
Introduction
Soil is an important component of terrestrial ecosystems and plays a crucial role in plant productivity and ecosystem services. Soil microorganisms are involved in all of the biochemical processes that occur in soils, and play an important role in organic matter decomposition, nutrient transformation and cycling, and trace gas emission (Creamer et al., 2015). Land use conversion, particularly within cropland, is a common occurrence driven by market economy (Houghton et al., 1999). The alteration of land use changes plant species, the associated management strategies and disturbance intensity, and thus strongly influences soil physicochemical properties, soil microbial biomass and community composition (Bossio et al., 2005, Lauber et al., 2008, Nishimura et al., 2008, Wang et al., 2014). However, few studies have been conducted to identify the effects of the changes in plant species and in soil environment.
Previous studies have shown that shifts in the soil microbial community structure can be associated with changes in a number of soil properties, including the soil water content (Yang and Zhang, 2014, Yuan et al., 2015), the soil total phosphorus, available phosphorus (Sheng et al., 2013), NO3-N, and soil organic carbon contents, pH (Sheng et al., 2013, Yang and Zhang, 2014, Yuan et al., 2015), and so on. The different relationships between microbial communities and soil properties are mainly affected by land use type and land use history (Lisboa et al., 2014) and has been shown to be site-specific (Williams et al., 2013). The development of different plant species associated with land use conversion would have different influences on soil microbial communities in the rhizosphere, which depends on the differences in the quantity and quality of carbon resources different plant species produce (Sanon et al., 2009). It is crucial to differentiate the effects of plant species from soil environment for carbon and nitrogen cycle modeling and agriculture production management. However, there is conflicting evidence of plant influences on bulk soil across individual fields (Jangid et al., 2011).
Land use conversion results in dynamic changes in soil processes (Arevalo et al., 2011). For example, Alberti et al. (2010) showed that there was a net loss of carbon in terms of net biome production 2 years after conversion from corn to alfalfa, but this difference decreased over the next few years (Robertson et al., 2000). Sun et al. (2011) suggest an initial increase in soil nutrients (SOC and total N) within 10 years after rice conversion to vegetables, followed by stability in the nutritional status during an extended period of vegetable cultivation between 10 and 100 years. Land use conversion can have significant and long-lasting effects on soil properties and microbial communities (Murty et al., 2002). Consequently, the understanding of the conversion effects at different timescales is important and urgently required. However, little research has been conducted to understand the effect on soil properties and microbial communities during the initial years after land conversion.
Rice is the second most important crop in the world after wheat and is the most important crop in China. The rice area in China accounts for approximately 19% of the global rice area and plays an important role in China's food security and agricultural production (Zhang et al., 2005). However, in recent years, a considerable quantity of rice fields has been converted to maize, soybean and vegetable fields due to water shortages and economic benefits (Nishimura et al., 2008, Sun et al., 2011, Yuan et al., 2016). For example, in the Huai river basin in northern China, the main crops approximately 20 years ago were winter wheat and lowland rice (in summer); however, farmers started replacing lowland rice with crops such as maize, soybean, and cotton because of increasing water shortages (Tong et al., 2003). However, few studies have been carried out to compare the soil properties and microbial community composition between double rice paddies and maize or soybean fields, especially for newly established drained fields. After land use conversion from a paddy to a drained field, the soil environment changes from anaerobic to aerobic; this significantly influences the soil water content and soil pH, which have profound effects on the soil microbial community (Wang et al., 2014). Moreover, converted drained fields have been planted to many different types of crops (Nishimura et al., 2008), which would also influence the soil microbial community. However, few studies have focused on converted crop types to determine how they influence the soil microbial community. It is important to distinguish between the effects of the changes of anaerobic to aerobic conditions and of crop type conversion on the soil microbial community to develop sustainable agriculture.
Accordingly, in this study, we compared the soil properties and microbial communities of double rice (RR) paddies and newly converted maize-maize (MM) or soybean-peanut (SP) fields. The objectives were 1) to identify the variation in the soil properties and soil microbial community structure 2 years after land conversion from RR to MM or SP; and 2) to compare the difference in soil properties and microbial community structure between MM and SP. We hypothesized that 1) recent conversion from paddy fields to drained fields would change the soil properties and microbial communities; 2) conversion to MM and SP would have similar effects on the soil microbial communities in the initial years after land conversion; and 3) pH is the main driver of the variation in the soil microbial communities.
Section snippets
Site description
An experimental site was set up at the Qianyanzhou Ecological Station (26°44′ N, 115°03′ E) of the Chinese Academy of Sciences (CAS), located in Jiangxi Province, southern China. The site is a typical red soil, hilly region with a subtropical monsoon climate. The average annual temperature varies between 17.4 and 18.9 °C. Annual precipitation varies between 945 and 2144 mm, with 24%, 41%, 23% and 12% of the total precipitation occurring in the four respective quarters of the year. The soils
Soil physical and chemical properties
The soil properties varied significantly after the conversion from paddy fields to drained fields (Table 1, Table 2), and land conversion explained 67.2% of the variation. However, no significant difference in the soil properties was found between MM and SP (Table 1, Table 2). The soil water content significantly decreased by 26.3%, and the pH decreased by 0.50 and 0.52 for MM and SP, respectively, compared with RR. The pH in MM was not significantly different from that in SP. Soil inorganic N
Discussion
The perMANOVA results showed that land use conversion from paddy fields to drained fields has significant effects on soil properties (Table 1, Table 2). In accordance with our first hypothesis, the recent conversion from paddy fields to drained fields significantly changed the soil properties. These influencing patterns and extents on soil properties were similar for conversed MM and SP, which confirms the role of land use conversion (from paddy fields to drained fields) in altering soil
Conclusions
The results showed that the changes of anaerobic to aerobic conditions from paddy soils to drained soils but not of crop type significantly influenced the soil properties and microbial communities during the initial conversion. Soil microbial communities shifted from G + and G − to fungi after conversion, in which soil pH was the key driver for the variations in the soil microbial communities. It is important for soil models to predict the microbial responses to land use conversion from anaerobic
Acknowledgements
This study was supported by the National Basic Research Program of China (973 Program, 2012CB417103), the National Natural Science Foundation of China (41001179) and the Strategic Science Plan of the Institute of Geographic Sciences and Natural Resources Research (2012ZD004). The authors also thank the academic editor and anonymous reviewers for their constructive comments, which helped to improve the manuscript.
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