Date Published: August 12, 2015
Publisher: Public Library of Science
Author(s): Zeyan Wu, Stacey Elizabeth Haack, Wenxiong Lin, Bailian Li, Linkun Wu, Changxun Fang, Zhixing Zhang, Zhen Jin.
Soil microbes play an essential role in the forest ecosystem as an active component. This study examined the hypothesis that soil microbial community structure and metabolic activity would vary with the increasing stand ages in long-term pure plantations of Pinus elliottii. The phospholipid fatty acids (PLFA) combined with community level physiological profiles (CLPP) method was used to assess these characteristics in the rhizospheric soils of P. elliottii. We found that the soil microbial communities were significantly different among different stand ages of P. elliottii plantations. The PLFA analysis indicated that the bacterial biomass was higher than the actinomycic and fungal biomass in all stand ages. However, the bacterial biomass decreased with the increasing stand ages, while the fungal biomass increased. The four maximum biomarker concentrations in rhizospheric soils of P. elliottii for all stand ages were 18:1ω9c, 16:1ω7c, 18:3ω6c (6,9,12) and cy19:0, representing measures of fungal and gram negative bacterial biomass. In addition, CLPP analysis revealed that the utilization rate of amino acids, polymers, phenolic acids, and carbohydrates of soil microbial community gradually decreased with increasing stand ages, though this pattern was not observed for carboxylic acids and amines. Microbial community diversity, as determined by the Simpson index, Shannon-Wiener index, Richness index and McIntosh index, significantly decreased as stand age increased. Overall, both the PLFA and CLPP illustrated that the long-term pure plantation pattern exacerbated the microecological imbalance previously described in the rhizospheric soils of P. elliottii, and markedly decreased the soil microbial community diversity and metabolic activity. Based on the correlation analysis, we concluded that the soil nutrient and C/N ratio most significantly contributed to the variation of soil microbial community structure and metabolic activity in different stand ages of P. elliottii plantations.
Pinus elliottii is a tree species native to the southeastern United States, and has been widely planted in China since 1980s due to its forage characteristics of high yield and fast growth . Presently, it covers 11 provinces and has become one of the most important economic tree species in China . However, dramatic yield decline and soil degradation observed in long-term pure plantations of P. elliottii have attracted an increasing level of interest from many ecologists and foresters . Previous studies have shown that continuous cultivation of a single species resulted in degradation of the soil . This phenomenon has been observed in many cultivated tree species, such as Cunninghamia lanceolata, Larix gmelinii, Pinus massoniana, Eucalyptus spp., and Populus spp. [5–9]. It has also been demonstrated that soil degradation inevitably results in the imbalance of soil microbial communities . In addition, it is suggested that this imbalance further exacerbates soil degradation in long-term pure plantation cultivation . Due to its long-term importance as a forestry crop in China, a number of studies have addressed above-ground characteristics of P. elliottii continuous pure plantations, including their growth regulation, biomass, and allelopathy [12–13]. However, restricted by soil ecosystem complexity and reliable experimental methods, the effects of P. elliottii long-term pure plantation patterns on the below-ground soil microbial community has not yet been reported.
Our study demonstrated that soil microbial community diversity and metabolic activity decreased with increasing stand ages in P. elliottii plantations. The bacterial biomass decreased with the increasing stand ages, while the fungal biomass increased. The carbon source utilization rate and community diversity of soil microbes gradually decreased with increasing stand ages. Overall, both the PLFA and CLPP illustrated that the long-term pure plantation pattern exacerbated the microecological imbalance previously described in the rhizospheric soils of P. elliottii, and markedly decreased the soil microbial community diversity and metabolic activity. Similar trends were also reported in other tree species . The importance of soil nutrient content in shaping microbial communities has been reported by a number of studies, and has been established as a key determinant for soil microorganism survival, species composition, and metabolism . Therefore, the most important contributor to our observed reduction in microbial diversity and metabolic activity in P. elliottii stands is the decline of soil nutrient content. P. elliottii is a fast-growing tree species, and has a high requirement and utilization rate of soil nutrients to maintain this growth. Although litterfall can return nutrients to the soil and improve fertility to some extent, this rate of natural nutrient cycling is too slow . In this study, the analysis of soil physicochemical properties showed that soil nutrient indicators such as TOC, AN, AP, and TK decreased with the increasing stand ages (Table 1). Correlation analysis also demonstrated close links between soil nutrient indicators and soil microbial diversity (Table 4). The correlation coefficient of community diversity with TOC and AN was greater than 0.945 (p<0.05) and 0.851 (p<0.05), respectively. Consequently, the decline of soil nutrient content inevitably leads to a decrease in soil microbial diversity. Furthermore, the effects of spatial change on soil nutrient content are also very important [29–32]. We will do further studies to compare the variation among different forest ecosystems in subtropical area. Source: http://doi.org/10.1371/journal.pone.0135354