Unravelling the Functional Diversity of the Soil Microbial Community of Chinese Fir Plantations of Different Densities

Chaoqun Wang,Yuhong Dong,Yihui Wei,Lin Xue,Ruzhen Jiao

doi:10.3390/f9090532

Abstract

The structure and function of forest ecosystems are directly or indirectly affected by their stand density. However, what effect the density of Chinese fir plantations has on the functional diversity of the soil microbial community remains unclear. The microbial metabolic functional diversity of soils sampled at the topsoil (0–20 cm) of 35-year-old Chinese fir plantations of five initial densities (D1: 1667 stems∙hm−2, D2: 3333 stems∙hm−2, D3: 5000 stems∙hm−2, D4: 6667 stems∙hm−2, and D5: 10,000 stems∙hm−2) was studied by using Biolog ECO technology. The results showed that the soil pH, oxidizable organic carbon (SOOC), available N (AN), available P (AP), and available K (AK) contents all showed a gradual increase from D1 to D4 and a decrease from D4 to D5, while the number of culturable bacteria and total microorganisms, the average well color development (AWCD) values for the single carbon substrate and six types of carbon sources used by the microbial community, as well as the Shannon-Wiener diversity index (H’), Pielou evenness index (J), and McIntosh Diversity Index (U), were the opposite, suggesting that low-densities favored C and N mineralization and the nutrient cycle. The density of Chinese fir plantations had a significant effect on the use of carbohydrates, amino acids, carboxylic acids, and phenolic acids by the soil microbial community, but it had no significant effect on the use of polymers (p < 0.05). Principal component analysis (PCA) revealed that carbohydrates, polymers, and phenolic acids were sensitive carbon sources that caused differences in the metabolic functions of soil microbial communities in Chinese fir plantations. Redundancy analysis (RDA) showed that physicochemical factors have a significant influence on the metabolic function of soil microbial communities (RDA1 and RDA2 explained >85% variance). The changes in density affected the soil physicochemical properties, the composition, and the metabolic functional diversity of microbial communities in Chinese fir plantations, which is certainly useful for the stand density regulation of Chinese fir plantations.

Highlights

As one of the important compositions in the forest soil ecosystem, soil microorganisms play an important role in the degradation of organic matter, nutrient conversion, and energy flow [1,2].Forests 2018, 9, 532 ; doi:10.3390/f9090532 www.mdpi.com/journal/forestsIn addition, soil microorganisms can serve as indicators of soil health [3], on-site tree planting conditions, and soil toxicology [4], as soil microbes are most sensitive to changes in the soil microenvironment [5,6]
This study revealed that the density of Chinese fir plantations can directly or indirectly drive changes in soil physical and chemical properties, the quantity of cultivatable micro-organisms, and the community functional diversity in topsoil (0–20 cm), which provides more theoretical support for the fact that the forest density is a factor that causes plant-soil feedback
The results showed that the soil pH, SOOC, available N (AN), available P (AP), and available K (AK) contents positively responded to changes in density from D1 to D4 and negatively responded to density from D4 to D5 when the Chinese fir plantation was at least 35 years old, while the number of culturable bacteria and total micro-organisms; the intensity of utilization of carbon sources by culturable microbial communities; and the H’, J, and U diversity index were the opposite

Summary

Introduction

As one of the important compositions in the forest soil ecosystem, soil microorganisms play an important role in the degradation of organic matter, nutrient conversion, and energy flow [1,2].Forests 2018, 9, 532 ; doi:10.3390/f9090532 www.mdpi.com/journal/forestsIn addition, soil microorganisms can serve as indicators of soil health [3], on-site tree planting conditions, and soil toxicology [4], as soil microbes are most sensitive to changes in the soil microenvironment [5,6]. Biolog ECO technology has been widely used to determine the functional diversity of different types of soil microbial communities [8]. This technique is based on the principle that culturable microbial communities [9] use a carbon substrate in a microplate to develop a color reaction with a redox dye to obtain the physiological characteristics of the microbial community [10] and functional diversity [11]. Stand density directly affects plant growth [12], and indirectly affects the composition of undergrowth vegetation and the function of the soil ecosystem by changing the canopy density and root distribution [13,14].

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