Science of the Total Environment 502 (2015) 280–286

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Understory vegetation leads to changes in soil acidity and in microbial communities 27 years after reforestation Xiaoli Fu ⁎, Fengting Yang, Jianlei Wang, Yuebao Di, Xiaoqin Dai, Xinyu Zhang, Huimin Wang Qianyanzhou Ecological Station, Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China

H I G H L I G H T S • Improving understory abundance resulted in an elevation of pH. • The understory abundance increment induced an increase in the fungal:bacterial ratio. • Tree diversity positively influenced the fungal:bacterial ratio.

a r t i c l e

i n f o

Article history: Received 6 May 2014 Received in revised form 30 August 2014 Accepted 7 September 2014 Available online xxxx Editor: F.M. Tack Keywords: Aboveground diversity Understory vegetation Soil acidification Microbial community composition

a b s t r a c t Experiments with potted plants and removed understories have indicated that understory vegetation often affects the chemical and microbial properties of soil. In this study, we examined the mechanism and extent of the influence of understory vegetation on the chemical and microbial properties of soil in plantation forests. The relationships between the vegetational structure (diversity for different functional layers, aboveground biomass of understory vegetation, and species number) and soil properties (pH, microbial community structure, and levels of soil organic carbon, total nitrogen, and inorganic nitrogen) were analyzed across six reforestation types (three pure needleleaf forests, a needle-broadleaf mixed forest, a broadleaf forest, and a shrubland). Twenty-seven years after reforestation, soil pH significantly decreased by an average of 0.95 across reforestation types. Soil pH was positively correlated with the aboveground biomass of the understory. The levels of total, bacterial, and fungal phospholipid fatty acids, and the fungal:bacterial ratios were similar in the shrubland and the broadleaf forest. Both the aboveground biomass of the understory and the diversity of the tree layer positively influenced the fungal:bacterial ratio. Improving the aboveground biomass of the understory could alleviate soil acidification. An increase in the aboveground biomass of the understory, rather than in understory diversity, enhanced the functional traits of the soil microbial communities. The replacement of pure plantations with mixed-species stands, as well as the enhancement of understory recruitment, can improve the ecological functions of a plantation, as measured by the alleviation of soil acidification and increased fungal dominance. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Pure plantation forests comprise approximately 80% of all plantation forests in China (Peng et al., 2008). To a certain extent, pure plantations yield a quick profit but at the expense of forest ecological functions. The practice of planting monospecific forests can lead to a decline in litter quality (Chen et al., 2005) and soil fertility (Lian and Zhang, 1998; Liu et al., 1998; Wang et al., 2011; K Yang et al., 2013; W Yang et al., 2013) and to frequent pest outbreaks (Ji et al., 2011) and can alter the soil food web (Scheu et al., 2003).

⁎ Corresponding author. Tel.: +86 010 64889913. E-mail address: [email protected] (X. Fu).

http://dx.doi.org/10.1016/j.scitotenv.2014.09.018 0048-9697/© 2014 Elsevier B.V. All rights reserved.

The dense canopies and low light availabilities of pure plantations lead to low levels of understory species richness and biomass compared with mixed plantations (Chen et al., 2005; Duan et al., 2010; Zhang et al., 2010). The floristic components that represent only a small proportion of the total plant biomass, however, can also greatly affect ecological processes (Wardle and Zackrisson, 2005; Wardle et al., 2008). Removal experiments have provided a clear evidence that understory vegetation can decrease net nitrification (Matsushima and Chang, 2007), thereby minimizing the leaching of nitrates (Baba et al., 2011), increasing the soil organic matter content (Wang et al., 2013), and improving the litter decomposition rate (Xiong et al., 2008). Some short-term experiments (less than five years) have shown that removal of the understory vegetation had no significant effect on soil pH (Xiong et al., 2008; Wang et al., 2013). Invasions of some exotic understory plants in deciduous forests,

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however, have increased the soil pH (Ehrenfeld et al., 2001), indicating the important role of understory vegetation in alleviating soil acidification. These varying effects of the understory vegetation on soil pH indicate that further study is necessary. The composition of the soil microbial community can indicate the status of an ecosystem for assessing restoration targets and the effectiveness of management interventions (Harris, 2009). Recent findings have shown that both over- and understory vegetations affect soil microbial communities. For example, removal of the understory significantly reduced the amount of fungal biomass and the ratio of fungi to bacteria (Wu et al., 2011). The species richness of shrub and herb layers has also been independently linked to the composition of the soil microbial community and to the performance of the soil biota (Carney and Matson, 2005; Nilsson and Wardle, 2005; Eisenhauer et al., 2011). Mitchell et al. (2012) recently observed in a pot experiment that the interaction between trees and a shrub (Calluna vulgaris) altered the soil microbial composition. The effects of the interaction once the trees were older, however, were not determined but may be expected to be reduced. Stand age can also influence soil microbial communities (Chatterjee et al., 2009), and the effects of the understory on soil microbial communities may be highly context-dependent (Wardle and Zackrisson, 2005; Busse et al., 2006). Further studies to test the effect of understory vegetation on soil microbial communities in plantation forests are thus required. In this study, we explored the properties describing ecological functions (e.g. soil carbon (C) and nitrogen (N) concentrations, pH, and microbial community composition) in six types of reforestation in subtropical China. Our goal was to determine the effect of the understory vegetation in plantation forests on these soil properties. We hypothesized that (1) the reforested areas with higher aboveground understory biomasses would have higher pHs and (2) the tree layer would increase the fungal:bacterial ratio by increasing the diversity of tree species, whereas the understory would increase the fungal:bacterial ratio by increasing the aboveground biomass of the understory, because the understory layer generally contributes less to litter inputs than the tree layer (Finzi et al., 2001; Hart et al., 2005), and the biomass/abundance of the understory would thus be more important than its diversity.

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and broadleaf forests were weathered from mudstone. Soil samples to a depth of 10 cm were collected in 1983 for each soil type at 15 random locations. The soil organic carbon (SOC) content, soil total nitrogen (TN) content, and pH from these two soil types were statistically similar in 1983 (Table 1). 2.2. Soil sampling and analyses We collected soil samples in October 2010 to a depth of 10 cm using a hand auger 45 mm in diameter. Three sampling locations separated by more than 100 m were selected for each reforestation type. Nine soil cores were collected at each sampling location, and every three soil cores were combined into one composite soil sample. The composite soil samples were passed through a 2-mm sieve and then divided into three subsamples. One subsample was used to determine the SOC content, TN content, and pH; one subsample was used to determine inorganic nitrogen (IN) content (including ammonium and nitrate N); and one subsample was used to assess the microbial community structure by analyzing the composition of phospholipid fatty acids (PLFAs). The subsamples for measuring the SOC and TN contents and the pH were further air-dried and passed through a 0.25-mm sieve. The subsamples for the IN content and PLFA analyses were stored at 4 and −20 °C, respectively. The soils had no calcareous material, so the SOC and TN contents were measured using a CN-element analyzer (Elementar VarioMax CN, Mt. Laurel, NJ). The soil pH was measured potentiometrically in a 1:2.5 soil:water suspension. The IN was extract− ed in a 2 M KCl solution, and the concentrations of NH+ 4 -N and NO3 -N in the extracts were analyzed by continuous flow on an autoanalyzer (Bran + Luebbe, Norderstedt, Germany). A PLFA profile was determined for each sample by extraction as described by Wu et al. (2011). Fatty acids were expressed in nmol g−1 soil. Bacteria were represented by the PLFAs i15:0, a15:0, 15:0, i16:0, 16:1ω9, 16:1ω7t, i17:0, a17:0, 17:0, cy17:0, 18:1ω7, and cy19:0 (Frostegård and Bååth, 1996); fungi were represented by 18:2ω6, 9 and 18:1ω9 (Joergensen and Wichern, 2008; Frostegård et al., 2011); and arbuscular mycorrhizal fungi (AMF) were represented by 16:1ω5c (Olsson, 1999). 2.3. Analysis of vegetational structure

2. Materials and methods 2.1. Study site This study was conducted at the Qianyanzhou Ecological Station (26°44′39″N, 115°03′33″E, 102 m a.s.l.) in the Jiangxi Province in southeastern China. The station occupies approximately 204 ha, and has a typical subtropical climate. The mean annual temperature is 17.9 °C, with a mean daily minimum temperature of 6.4 °C in January and a maximum of 28.8 °C in July. The mean annual precipitation is 1489 mm, mostly occurring between March and June. The soils are weathered from red sandstone and mudstone and are classified as Typic Dystrudepts by the USDA system (Soil Survey Staff, 1975). The zonal vegetation at the station, a subtropical evergreen broadleaf forest, had almost disappeared prior to the 1980s due to deforestation, and the dominant vegetation had become grassland and scattered shrubland, and severe soil degradation had occurred. Widespread reforestation was implemented in 1983 to revert the soil degradation. Six reforestation types were established, including three pure needleleaf forests (Pinus elliottii, Pinus massoniana, and Cunninghamia lanceolata), a needle-broadleaf mixed forest (P. massoniana mixed with Schima superba), a broadleaf forest (mainly Liquidambar formosana and Magnolia denudata), and a shrubland regenerated by natural revegetation. By 2003, the shrubland was dominated by Loropetalum chinense, Serissa serissoides, Quercus fabric, Rubus corchorifolius, Vaccinium bracteatum, Rhus chinensis, Smilax ferox, and Lespedeza davidii. The soils of the P. elliottii and P. massoniana forests and the shrubland were weathered from sandstone. The soils of the C. lanceolata, needle-broadleaf mixed

The vegetational structure of the reforestation types varied slightly and had stabilized by 20–30 years after reforestation (Li et al., 1999; Wang et al., 2007), and the responses of the soil organisms to disturbance lagged (by more than three years) relative to the aboveground organisms (van der Putten et al., 2001; Hedlund et al., 2003), so we used the data for vegetational structure surveyed in 2003. Three square plots (10 × 10 m), at the soil-sampling locations, were approximately demarcated to survey the tree layer in each of the six reforestation types. In each plot, one 5 × 5 m and two 1 × 1 m subplots were delimited for surveys of the shrub and herb layers, respectively. The species composition, height, and diameter at breast height were surveyed for the tree layer. The species composition was determined for the shrub and herb layers. The surveys were based on species identification, and no species overlapped among the different layers. Shannon's diversity index was then calculated for the tree, shrub, and herb layers and for all the plant species as a whole. Based on allometric models (Ma et al., 2007), the tree aboveground biomass was calculated for the P. elliottii, P. massoniana, C. lanceolata, needle-broadleaf mixed, and broadleaf forests. The aboveground biomass of the understory was determined by the harvesting method. We collected all surface litter material within a 0.5 × 0.5 m square in the centers of the plots. The standing litter was oven-dried at 65 °C for 48 h and weighed. 2.4. Statistical analysis We used a one-way analysis of variance (ANOVA) and a multiple comparison procedure (Tukey's honest significant difference test) to

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Table 1 The basic soil properties (mean ± SE, n = 15) for the different reforestation types in 1983. The values with the same letters within columns are not significantly different. Parent material

Sandstone

Mudstone

Reforestation type

Soil properties

Pinus elliottii Pinus massoniana Shrubland (spontaneous revegetation) Cunninghamia lanceolata Needle-broadleaf mixed forest Broadleaf forest

test for the influence of the reforestation type on the chemical properties of the soil and on the microbial community composition using SPSS Version 10.0 (SPSS Inc., Chicago, Illinois, USA). Significant differences were assessed at the p b 0.05 level. We performed a redundancy analysis (RDA) (CANOCO 4.5, Ithaca, NY, USA) to determine the extent to which the soil properties and the litter mass could explain the variation of the soil microbial communities among the reforestation types. Based on the highest model coefficients, the response of the tree aboveground biomass to the aboveground biomass of the understory was regressed using a polynomial model. The regression was performed using Microsoft Excel 2003 (Microsoft, Redmond, Wash., USA). Stepwise multiple linear analyses examined the relationships of the pH, AMF:fungal ratio, and fungal:bacterial ratio with the aboveground biomass of the understory, species number, and Shannon's diversity index for the tree, shrub, and herb layers and for all the plant species using SPSS Version 10.0 (SPSS Inc., Chicago, Illinois, USA). The assumptions of the ANOVA and linear regression for the data set were verified prior to analysis. One outlier with excessive influence was excluded from the regression analysis.

Soil organic carbon (g kg−1)

Total nitrogen (g kg−1)

pH

24.12 ± 2.17a

1.04 ± 0.10a

5.26 ± 0.06a

32.47 ± 5.54a

0.95 ± 0.11a

5.50 ± 0.19a

3.2. Soil chemistry and microbial community properties The reforestation measures tended to decrease the SOC content between 1983 and 2010 but did not change the TN content, with the exception of the C. lanceolata forest (Fig. 1). All six reforestation measures, however, decreased the C:N ratio (p b 0.01). The pH significantly decreased by an average of 0.95 across the reforestation types, the decline being highest in the C. lanceolata forest. Soil pH was statistically similar in the other five reforestation types. The total PLFAs, bacterial PLFAs, and fungal PLFAs remained more similar over the 27 years in the shrubland compared with those in the P. elliottii, C. lanceolata, needle-broadleaf mixed, and broadleaf forests (Fig. 2). The levels of AMF were higher in the shrubland and the broadleaf forest than in the P. massoniana forest. The C. lanceolata forest had similar AMF levels and AMF:fungal ratios compared with the P. elliottii and P. massoniana forests. The fungal:bacterial ratio in the shrubland was similar to that of the broadleaf forest but was significantly higher than under the C. lanceolata forest. An RDA analysis indicated that the chemical properties of the soil and the standing litter mass together explained approximately 78.4% of the variation of the PLFA composition (Fig. 3).

3. Results 3.3. Vegetational structure and soil properties 3.1. Vegetational structure Shannon's diversity index changed significantly among the reforestation types for the tree and herb layers, but reforestation type did not influence the diversity index for the shrub layer (Table 2). The needlebroadleaf mixed forest had the highest diversity index for the tree layers and for the total of all plant species. The shrubland had the lowest diversity index for the herb layer and for the total of all plant species. The P. elliottii forest had the lowest tree-layer diversity index. The needlebroadleaf mixed forest and the shrubland had the highest and lowest species numbers, respectively. The shrubland had the highest understory aboveground biomass. The standing litters in the shrubland and the P. elliottii forest were the lowest and highest, respectively. The standing litters were statistically similar among the other four reforestation types.

Table 3 shows the results of the multiple stepwise regression analysis for the relationships of the pH, AMF:fungal ratio, and fungal:bacterial ratio with the aboveground biomass of the understory, species number, and Shannon's diversity index for the various plant functional layers (i.e. trees, shrubs, and herbs) and for all plant species considered as a whole. None of the parameters of vegetational structure were correlated with the AMF:fungal ratio. Soil pH increased with aboveground biomass of the understory. pH was not significantly correlated with the diversity indices of the vegetation. The fungal:bacterial ratio was positively correlated with the aboveground biomass of the understory and the Shannon's diversity index for the tree layer. The aboveground biomass of the understory and the diversity index for the tree layer explained 27.2% and 26% of the changes in the fungal:bacterial ratio, respectively, indicating that

Table 2 The effect of reforestation types on the Shannon's diversity index values for different functional layers and for all the plant species considered as a whole, species number, aboveground biomass of understory, as well as standing litter. The values with the same lower-case letters within columns are not significantly different. Reforestation type

Pinus elliottii Pinus massoniana Cunninghamia lanceolata Needle-broadleaf mixed forest Broadleaf forest Shrubland

Shannon's diversity index Tree layer

Shrub layer

Herb layer

Total plant species

0.50c 0.85b 0.87b 1.66a 1.49a 1.03b

2.25a 2.40a 2.18a 2.42a 2.11a 2.39a

1.87a 1.65a 1.56a 1.98a 1.05b 0.64c

2.26b 2.20b 2.23b 2.75a 2.14b 1.69c

Species number

Aboveground biomass of understory (g m−2)

Standing litter (g m−2)

19.00b 21.33b 20.33b 25.33a 19.00b 14.67c

117.50b 131.42b 92.88 b 101.83b 100.50b 340.58a

494.43a 324.97b 286.20b 242.10bc 265.53bc 145.80c

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in the mixed stand observed by Wang et al. (2008). De Schrijver et al. (2012) suggested that the poor quality of the leaf litter from needleleaf trees slows the decomposition of the litter and, consequently, leads to its accumulation on the forest floor and to soil acidification. Our results, however, showed that the P. elliottii needleleaf forest had a higher amount of standing litter and soil pH compared with the needlebroadleaf mixed and broadleaf forests. This inconsistent result indicated that the litter quality of the tree layer alone may not be sufficiently robust to predict soil pH. Earlier findings on a continental scale suggested that the relationship between soil pH and species richness was monotonically positive in forests with more acidic soils (Chytrý et al., 2007). Our results support the premise that vegetational structure contributes differently to changes in soil pH which was positively correlated with the aboveground biomass of the understory but not with the species number. 4.2. PLFA composition changes with the vegetational structure

Fig. 1. The effect of reforestation types on changes in the soil organic carbon, total nitrogen, and pH from 1983 to 2010. PE, PM, and CL are the three needleleaf forests of P. elliottii, P. massoniana, and C. lanceolata, respectively. The values with the same letter are not significantly different at the p b 0.05 level. Error bars represent the standard error of the mean.

the aboveground biomass of the understory and the tree-layer diversity had similar influences on the fungal:bacterial ratio. However, a substantial portion of the increase of the aboveground biomass of the understory was at the expense of the tree aboveground biomass, because the latter marginally decreased when the former exceeded 113 g m− 2 (Fig. 4). 4. Discussion 4.1. Soil chemistry changes with the vegetational structure We expected the SOC and TN contents to increase over the 27 years following reforestation. The SOC content, however, decreased, and the TN content did not change, except in the C. lanceolata forest. These results were generally consistent with global patterns, indicating that C and N levels are either depleted or unchanged in the first 30 and 50 years respectively after afforestation (Li et al., 2012). The decrease of SOC content may have been due to the enhancement of C mineralization due to soil disturbance during reforestation. Nearly all the increase in ecosystem C during the 30 years of forest development in another ecosystem was invested in stands growth, but not soil C (Compton and Boone, 2000). A similar process may also have been responsible for our decrease in SOC levels. Soil acidification showed an average change in pH of 0.95 across reforestation types over the past 27 years, likely due to atmospheric deposition; this finding was consistent to that from unfertilized agricultural sites in the same study region (Dong et al., 2012). The soil pH declined more rapidly in the C. lanceolata forest in agreement with the significantly lower pH in the pure C. lanceolata stand relative to the pH

The total, bacterial, and fungal PLFA levels remained similar in the shrubland and the broadleaf forest over the 27 years of reforestation. The AMF levels were higher in the shrubland and broadleaf forest than in the P. massoniana forest. Zechmeister-Boltenstern et al. (2011) reported similar findings indicating that soil AMF levels were higher in deciduous than in coniferous forests. The similar AMF level and AMF:fungal ratio in the C. lanceolata stand compared with the P. elliottii and P. massoniana stands in our study, however, were opposite to what was expected, because C. lanceolata is colonized by AMF whereas P. elliottii and P. massoniana are colonized by ectomycorrhizal fungi (Chen and Chen, 1983). We also found no correlations between the parameters of vegetational structure and the AMF:fungal ratio. These results may be partially due to the significantly different AMF community compositions of the soil and roots (K Yang et al., 2013; W Yang et al., 2013). The limitation of the PLFA method may also have contributed, because PLFA 16:1w5 can be a good indicator of arbuscular mycorrhizae when bacterial biomass is low but a poor indicator in soil with high bacterial biomass (Frostegård et al., 2011). The chemical properties of the soil and the mass of the standing litter explained most of the variation in the PLFA composition, but approximately 22% of the variation among the reforestation types could not be explained. Soil properties and aboveground plant communities can both drive belowground microbial diversity (Nielsen et al., 2010; Thoms et al., 2010). Vegetational structure varied with reforestation types 27 years after reforestation, as determined by the species number, aboveground biomass of the understory, and Shannon's diversity index for the tree, shrub, and herb layers and for all plant species considered as a whole. Our findings demonstrated that the aboveground biomass of the understory and the diversity of the tree layer positively influenced the fungal:bacterial ratio. Such influences of the understory vegetation have generally been overlooked in the past studies, which have instead focused on the effect of the tree-layer diversity on the soil community. The competition for soil nutrients among the understory, trees, and soil microbes may partially account for the effects of the understory vegetation on soil microbial communities, as suggested by Wu et al. (2011). Additionally, the understory may affect the soil microbial communities indirectly by inhibiting the belowground biomass of the trees (Mitchell et al., 2012) or directly by producing special root exudations in their rhizospheres to support beneficial symbioses (Bertin et al., 2003). The positive influence of the aboveground biomass of the understory on the fungal:bacterial ratio, combined with the lack of an effects of the shrub-layer diversity on the fungal:bacterial ratio, suggests that the enrichment of the understory aboveground biomass, rather than increased diversity, influenced the soil microbial communities. Nielsen et al. (2010) similarly found that fungal dissimilarity between a birch woodland and a heather moorland was most strongly correlated with the difference in the shrub cover. These results provide evidence supporting the suggestion by Smith et al. (2003) that changes in the

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Fig. 2. Differences in the total PLFA, bacterial PLFA, fungal PLFA, AMF PLFA, AMF:fungal ratio, and fungal:bacterial ratio among the reforestation types. PE, PM, and CL are the three needleleaf forests of P. elliottii, P. massoniana, and C. lanceolata. The values with the same letter are not significantly different at the p b 0.05 level. Error bars represent the standard error of the mean.

structure of soil microbial communities are related to the traits of the individual aboveground components (not necessarily to the dominant component) rather than to changes in diversity per se. Examples of

the understory aboveground biomass enrichment include replacing pure plantations with mixed-species production stands, removing the needle litter layer (Navarro-Cano et al., 2010), carefully spacing the trees to create canopy gaps, or thinning the stands (McGuire et al., 2001; Duan et al., 2010). Management strategies maximizing understory abundance, however, may produce significantly negative effects on the tree aboveground biomass (Fig. 4). Managing different ecosystem services and quantifying the trade-offs between ecosystem functions and commodity values in plantations are thus challenging. The aboveground biomass of the understory and the tree-layer diversity had similar influences on the fungal:bacterial ratio, but earlier findings have shown a stronger influence of the understory biomass on the microbial community. For example, Šnaidr et al. (2013) reported that only a minor part of the effect of the tree layer on the microbial community composition could be explained by differences in litter or in soil chemistry and suggested that the understory is likely an important mediator of the effects of the vegetation. Mitchell et al. (2010) also inferred that the soil microbial community was influenced more by grass-dominated ground vegetation than by birch trees in a Table 3 Results of multiple stepwise regression analysis (n = 18) for the relations of the pH, AMF: fungal ratio, and fungal:bacterial ratio with the aboveground biomass of understory, species number and Shannon's diversity index for different plant functional layers (e.g. tree, shrub, and herb) and for all the plant species. Soil property

Variables

Significance level

Explained variability (%)

pH

Aboveground biomass of understory Aboveground biomass of understory Shannon's diversity index for tree layer

0.035

25.0

0.005

27.2

0.011

26.0

Fungal:bacterial ratio Fig. 3. Redundancy analysis of PLFA data. Environmental factors included the pH, standing litter (L), total nitrogen (TN), soil organic matter (SOC), carbon:nitrogen ratio (C/N), + nitrate nitrogen (NO− 3 ), ammonium nitrogen (NH4 ), and inorganic nitrogen (IN).

No correlations were found between the vegetation structure parameters and the AMF: fungal ratio.

Tree aboveground biomass (kg m-2)

X. Fu et al. / Science of the Total Environment 502 (2015) 280–286

12 10 8 6 4 2

2 y = -0.0049x + 1.1067x - 52.945 2

R = 0.3971, p =0.0975 0 80

100

120

140

Understory aboveground biomass (g m-2) Fig. 4. Relation between the understory aboveground biomass and the tree aboveground biomass. The regression line is shown together with the model equation and with the coefficient.

degenerating birch woodland., Ferns and herb plants had a greater effect than the tree layer on soil microbes in a native mixed-oak forest (Rodríguez-Loinaz et al., 2008). Our results and these earlier findings collectively suggest that greater attention should be given to the importance of the understory in the structure and function of soil microbial communities in forests with low levels of tree diversity. The effects of the understory vegetation on an ecosystem, however, may be highly context-dependent (Wardle and Zackrisson, 2005; Busse et al., 2006). We therefore caution that the effect of the understory in a more mature ecosystem is unclear. 5. Conclusions Our results suggest that increases in the understory may alleviate soil acidification and that increasing the understory biomass and tree diversity may be beneficial for improving fungal dominance within soil microbial communities in the studied plantations. Too large an increase in the aboveground biomass of the understory, however, may decrease the tree aboveground biomass. The advisable aboveground biomass of the understory in our study sites should not exceed 113 g m−2. In drawing these conclusions, we note that the study was performed in reforestation stands in subtropical China. In summary, our findings are encouraging, because they provide us with more explicit management options for improving ecosystem functions in plantations. Acknowledgments We are grateful to S. Fu and L. Zhou for their help with the PLFA analysis. This research was funded by the National Program on Key Basic Research Projects of China (2012CB416903), the NSFC Projects of International Cooperation and Exchanges (31210103920), the National Natural Science Foundations of China (41101203, 31070559), and the Gan-Po Distinguished Researcher Program. References Baba M, Abe S, Kasai M, Sugiura T, Kobayashi H. Contribution of understory vegetation to minimizing nitrate leaching in a Japanese cedar plantation. J For Res 2011;16:446–55. Bertin C, Yang XH, Weston LA. The role of root exudates and allelochemicals in the rhizosphere. Plant Soil 2003;256:67–83. Busse MD, Beattie SE, Powers RF, Sanchez FG, Tiarks AE. Microbial community responses in forest mineral soil to compaction, organic matter removal, and vegetation control. Can J For Res 2006;36:577–88.

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