Effects of long-term inorganic and organic fertilizations on the soil micro and macro structures of rice paddies
Introduction
Soil structure is a fundamental property of soil health because it impacts the storage and movement of water, gas and nutrients, root growth, and microbe activity (Bronick and Lal, 2005). Soil structure can be assessed over several orders of scales from mineral–organic complexes, aggregates, typically referred as soil microstructure, to peds and clods in the soil profile, usually considered soil macrostructure (Carter, 2004). And the size of the corresponding pores ranges from μm to mm or even larger. Tisdall and Oades (1982) proposed that the factors and processes controlling the formation of soil structure are different at contrasting scales in an aggregate hierarchy concept model. Management practices, e.g. tillage and fertilization, have been proven to impact each level of soil structure either directly or indirectly (Bronick and Lal, 2005). Kravchenko et al. (2011) showed large intra-aggregate pores in no tillage and native succession vegetation treatments are more heterogeneous than those in conventional tillage treatment. Macropores (> 0.75 mm) were more abundant in pastureland than under arable crops and provided pathways for preferential flow at the core scale (Luo et al., 2008). Despite the numerous evaluations of land use and management effects on soil structure, most studies have been limited to a specific scale and knowledge of the responses of soil structure at different scales is lacking.
Information about a soil's inner structure has usually been inferred from soil properties (e.g., hydraulic properties and gas permeability) (Hill et al., 1985; Marshall, 1958; Moldrup et al., 2001). These calculations were based on assumptions of ideal pore shapes and typically could not provide information regarding the architecture of soil pore system. Therefore, a direct study of soil structure is necessary. Direct observation and quantification of the structure of soil were typically conducted on soil thin sections (Pagliai et al., 2004; Mooney et al., 2007). However, soil thin section can only provide two-dimensional (2D) information of soil structure. And the preparation of thin sections is time consuming (Murphy, 1986). Computed tomography (CT) offers a rapid and non-destructive way to study soil structure over a range of scales (Taina et al., 2008; Wildenschild et al., 2002; Helliwell et al., 2014). High-resolution CT can show the detailed organization of soil aggregates and has been used to study aggregation processes (Atkinson et al., 2009; Zhou et al., 2013), soil microstructure (Peth et al., 2008), and soil biophysical interactions (Martin et al., 2012; Vos et al., 2013) at the aggregate scale. CT with a low resolution, on the other hand, can scan large samples and is frequently used to study macropores and their relationship with soil hydraulic properties (Luo et al., 2008) at the soil core scale. The study of the micro and macro scale soil structure is possible by using a combination of CT systems with different resolution capabilities. Schlüter et al. (2011) studied soil structure development at two different scales and proposed a method to combine soil pore size distribution (PSD) acquired at the different scales. Dal Ferro et al. (2013) found that both the micro- and macro-scale soil structures were affected by fertilization from the scanning of soil aggregates and soil cores using micro-CT.
Rice is the most important staple food in China and the cultivation area of rice is 25 million ha, accounting for 25% of the national arable land area (Li, 1992). Long-term traditional cultivation of rice, specifically flooding during most of the growing season, drastically changed soil physical, chemical, and biological properties and resulted in a special anthropogenic paddy soil (Gong, 1986). The structure of paddy soil is more dynamic at the aggregate to soil core scales compared with those of upland soils. The plow layer of the paddy soil is homogenized before each growing season to prepare the seedbed, which destroys surface soil structure considerably (Eickhorst and Tippkötter, 2009; Kirchhof et al., 2000; Sharma and Datta, 1986). Moreover, paddy soil experiences frequent swell-shrink cycles caused by periodic flooding and drying management (Zhang et al., 2013). These processes are accompanied by the creation and closing of cracks which has critical importance for the evolution of the structure of paddy soil (Liu et al., 2003; Sander and Gerke, 2007). At the micro-scale, aggregation of paddy soil is greatly influenced by the oxidation-reduction conditions caused by flooding and drainage cycles (Kögel-Knabner et al., 2010). For example, Fe oxides are important binding agents of soil aggregates, but their effects vary among different Fe species (Duiker et al., 2003). Poorly crystalline Fe has a larger and more reactive surface area and therefore is more effective in soil aggregation than crystalline Fe (Duiker et al., 2003; Yan et al., 2013). Repeated flooding and drainage cycles have been shown to increase Feo oxides while reducing Fed oxides; therefore, these processes are beneficial to soil aggregation (Zhang et al., 2003).
The application of organic or inorganic fertilizers can directly or indirectly introduce different ions and organic matter to the soil, which may cause soil disaggregation or aggregation (Haynes and Naidu, 1998). In the past few years, research regarding the effects of fertilization on paddy soil has mostly focused on SOC sequestration (Anders et al., 2012; Brar et al., 2013; Das et al., 2014), greenhouse gas emissions (Yagi and Minami, 1990), and microbial and geochemical processes (Zhong and Cai, 2007) due to environmental and ecological concerns. These processes are closely linked with soil structure, which determines the transport of water, gas and solutes and provides a habitat for soil microorganisms (Young and Crawford, 2004). Although the change in aggregate stability under fertilization in paddy soils has been evaluated (Li and Zhang, 2007; Huang et al., 2010; Yan et al., 2013), the effect of fertilization on the formation and dynamics of the structure of paddy soil is still unclear.
To better understand the sustainability of paddy soil to continuously received inorganic and organic fertilizers, this study aimed to evaluate the soil micro and macro structure of a long-term fertilization experiment. The specific objectives were: (1) to evaluate the effects of fertilization on aggregate- and core- scale structure using synchrotron based micro-CT and industrial micro-CT and (2) to investigate the mechanisms of the structure evolution of paddy soil.
Section snippets
Experimental site
Soil was taken from long-term experiment established in 1982 at the Jiangxi Institute of Red Soil, Jinxian County, Jiangxi Province, China (116°10′ E, 28°21′ N). The experiment site lies in a flat area of the hilly region of Southern China. The experiment site has a subtropical climate and a mean annual temperature and precipitation of 17.7 °C and 1706 mm, respectively. The paddy soil (Typic Stagnic Anthrosols, Chinese Soil Taxonomic Classification, 2002) is clay loam (20% sand, 48% silt, and 32%
Soil properties
Table 2 shows some selected properties of the paddy soil and yields of different fertilization treatments. No significant difference was found for the clay, silt, and sand content among the treatments. SOC content of NPKOM treatment was 28% higher than those of CK and NPK treatments while the latter two treatments did not differ. Similar to SOC, total porosity (calculated from bulk density and soil density, 2.65 g cm− 3), saturated hydraulic conductivity (Ks), and PAWC significantly increased
Effects of inorganic and organic fertilizations on soil properties
Long-term application of NPKOM increased SOC due to the addition of organic manure and increased input of stubble and roots as a result of increased yields (Yan et al., 2013). As SOC increased, bulk density decreased and total porosity increased. Similar positive effects of using NPKOM on SOC, bulk density, and AWC have been reported in previous studies (Edmeades, 2003; Haynes and Naidu, 1998; Rasool et al., 2007; Naveed et al., 2014). The long-term application of inorganic fertilizer, however,
Conclusions
The intra-aggregate and inter-aggregate structures of paddy soil was assessed by scanning multi-scale soil samples at different resolutions. The porosity of the paddy soil increased with the increasing samples size due to the incorporation of more cracks in the larger samples. However, the trends in soil porosity between the different treatments were similar at the aggregate, small core, and large core scales, respectively.
Long-term fertilization affected soil structure at all scales. Soil in
Acknowledgments
We greatly appreciate the assistance from Hans-Jörg Vogel and Steffen Schlüter on image analysis. We thank the Shanghai Synchrotron Radiation Facility (SSRF) for providing the beam time. This work was financially supported by the National Natural Science Foundation of China (41471183 and 41101200), the Chinese National Basic Research Program (2015CB150400), and the Innovation Program of Institute of Soil Science, CAS (ISSASIP1111). SJM is supported by the ERC FUTUREROOTS project.
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