Effects of reclamation age on soil microbial communities and enzymatic activities in the sloping citrus orchards of southwestern China
Introduction
Soil microbes, which affect the development and function of soil and benefit plants by providing nitrogen, phosphorus and other mineral elements and a variety of organic nutrients, participate in soil nutrient cycling in the ecosystem (Li et al., 2015). Soil enzymes, which mainly come from soil animals, plant roots, various microbial secretions and decomposable remnant substances (Sinsabaugh et al., 2009; Joniec and Frąc, 2017; Joniec, 2018), play an essential role in all biochemical processes in the soil, including the release and storage of various nutrient elements, the formation and development of humus and changes in soil physical conditions. The geometric mean of enzymatic activities (GMeas) can be used as an index to indicate the metabolic activity of a microbial community in the soil (Raiesi and Kabiri, 2016; Luo et al., 2018; Raiesi and Salek-Gilani, 2018). Since microorganisms and soil extracellular enzyme activities are general indicators in various ecosystems, it is essential to study their contributions to soil matrix fertility and nutrient information during the development and utilization of slope orchards.
An increasing number of reports support that there is a close linkage between soil attributes and the soil microbial community (Zuber and Villamil, 2016; Qu et al., 2017). When the soil microenvironment changes because of fertilization conditions (Geisseler and Scow, 2014; Hartmann et al., 2015; Francioli et al., 2016), land use history (Hazarika et al., 2014; Kang et al., 2018) and farming practices (Mbuthia et al., 2015; Kabiri et al., 2016), the soil microbial community and activity can respond quickly and substantially (Li et al., 2013; Hou et al., 2018). It has been suggested that under reclamation conditions, mainly for agricultural purposes, microbial components respond differently to drastic environmental changes (e.g., soil properties) (Hartmann et al., 2015; Francioli et al., 2016). All of the soil acidification (Fierer et al., 2009), nutrient addition (Yao et al., 2011) and degradation of complex organic compounds (Mbuthia et al., 2015) can result in compositional shifts in microbial functional groups. For example, bacterial groups were more affected than fungal groups by soil pH (Fierer et al., 2009). In addition, fungi and prokaryotes show different adaptability to extreme environmental stress due to the distinct abilities of each group to resist degradation and use certain types of nutrients (Mbuthia et al., 2015). In summary, the general response of different functional groups is a more comprehensive and accurate indication of soil fertility.
With the increases in stand age, soil carbon composition, soil microbial metabolism and microbial community composition will change, including in forest or orchard ecosystems (Zhang et al., 2017; Kyaschenko et al., 2017; Antisari et al., 2018; Wu et al., 2020). Wu et al. (2020) found that under the background of various stand ages in citrus orchards, soil moisture and the C:N ratios are important factors influencing the soil microbial community composition. Similarly, soil C:N ratios and microbial biomass carbon content are important indicators of soil carbon metabolic diversity and microbial community activity (Chen et al., 2013). However, in addition to the changes in soil physical and chemical properties, litterfall production (Zhou et al., 2015), root biomass (Fujimaki et al., 2007) and even mycorrhizal infection of fine roots (Hishi et al., 2016) also change with stand age. Therefore, it is very interesting to explore whether there is a correlation between citrus tree indicators and below-ground ecosystems with increasing stand age.
Recently, both the above-ground and below-ground ecosystems have become part of important reclamation evaluations (Dangi et al., 2012). It is well known that ecosystem productivity and diversity are dependent on various soil physical, chemical and biological variables (Huang, 2008). Moreover, various soil indices can be influenced by the growth and development of above-ground vegetation due to an integral connection between above-ground and below-ground processes (Whitehead et al., 1982; Meijer et al., 2011). Therefore, we wanted to explore how much these corresponding above- and below-ground indicators contribute to the changes in the soil microbial community following the orchard planting year.
Orchards are a critical way for farmers to earn a living and gain economic benefits. Large-scale dry valleys are distributed in many basins in the area of the Hengduan Mountains. This region is characterized by relatively high temperatures, long and intense sunshine exposure, little precipitation and large evaporation rates. Because of these particular and advantageous natural resources, this region breeds unique fruit as well as vegetable products; therefore, sloping citrus orchards have become a typical land use type. Nevertheless, the combination of limited land resources and large population concentrations has led to the intensive development of the land, including the development and utilization of steep slopes, which usually leads to water erosion due to extensive soil disturbance (Zhang et al., 2017; Chen et al., 2019; Wang et al., 2019b; Liu et al., 2020). Whether and how slope orchard reclamation affects microbial community composition and activity are rarely studied, but this relationship is crucial for the sustainable management of slope orchards. It is noteworthy that in slope orchards, soil can be greatly affected by artificial management, such as ploughing, landfilling, pruning and fertilizing. These disturbances may induce changes in major physical, chemical or biological factors, which can further result in shifts in the structure and activity of soil microorganisms.
Chronosequences are an effective way to assess the changing trends and rates of soil properties using space to replace time under the strict control of environmental factors (Chaudhuri et al., 2013; Qu et al., 2017). Therefore, we chose citrus orchards of different stand ages to measure changes in the microbial community structure and activity and to identify the factors that led to these variations. The goal of this study was to investigate whether the stand ages (3, 7, and 11 years) of citrus slope orchards could change soil fertility and whether there exists a linkage between above-ground variations and below-ground microorganisms. We hypothesized that (1) microbial biomass and enzymatic activity will increase with increasing stand age in citrus slope orchards; and (2) the shift in the soil microbial community and activity is related not only to soil carbon and nitrogen but also to fine roots and plant above-ground biomass.
Section snippets
Study site and experimental design
This study was conducted in the slope orchards of the citrus cultivar Huangguogan in Hanyuan, Ya'an, Sichuan Province, China (29°16′20″-29°18′49″ N, 102°31′24″-102°35′36″ E). The study site is located in the dry valleys of the Hengduan Mountains region, which spans multiple river basins and has a semi-arid or arid climate. The annual mean temperature is 17.9 °C, and the annual mean precipitation is 741.8 mm. The soil type is sandy loam soil, which is characterized by good permeability and high
Soil physicochemical properties
The soil pH ranged from 5.5 to 8.0; among the sampling sites, the pH value in the 7-year-old reclaimed plots was significantly higher than that in the 3-year-old plots (Table 1). Furthermore, the contents of soil moisture, NH4+-N and DON in the uncultivated plots were significantly higher than those in the other plots. However, the contents of NO3−-N and DOC were significantly different between the cultivated and uncultivated sampling sites. Moreover, the soil total organic carbon and nitrogen
Shifts in total microbial biomass and soil organic carbon
In the current study, the total soil microbial biomass in orchards with a longer time since reclamation (i.e., 7 and 11 years since reclamation) was significantly higher than that in the uncultivated land and the 3-year-old orchards (Fig. 1). At the same time, the content of TC and TN displayed an increasing trend during in the 11 years following the initial citrus orchard reclamation, which is consistent with the findings of Qu et al. (2017). These results confirm the positive relationships
Conclusion
Changes in soil fertility are fundamental to and inevitable in the process of agricultural management. Soil microbial biomass and activity are sensitive indicators of soil quality and are crucial to the ecological function and sustainability of slope orchards. In this study, the soil microbial biomass and activity increased in citrus slope orchards that had been reclaimed many years prior to sampling, and the important roles of soil organic carbon, nitrogen and plant biomass indices in
Acknowledgements
This research was funded by the National Key Research and Development Program of China (No. 2017YFC0505104), the National Natural Science Foundation of China (No. 31770658), the Key Research Program of the Chinese Academy of Sciences (KFZD-SW-427) and the Sichuan Science and Technology Program (No. 2018SZDZX0030). We also thank American Journal Experts for their language editing services.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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2022, ChemosphereCitation Excerpt :Moreover, the SOC contents increased significantly with increasing cultivation age, and much higher values were observed in most soil layers from the citrus orchards with 10 years and 20 years of cultivation age. The results were in line with those of Liao et al. (2018) and Qiang et al. (2020), who observed an increasing trend in SOC content following long-term citrus orchard cultivation. The most dominant NEO in soil profiles was IMI, accounting for more than 36% of ∑5NEOs detected (Fig. 4), ranging from 0 to 16.03 ng/g dw (Fig. 3).