Introduction

Top-down effects of species at higher trophic levels on species at lower trophic levels in the food chain can induce cascade effects1. For instance, aboveground consumers (e.g. animals) can affect the belowground consumers such as soil microbes2,3,4,5. However, less attention has been paid to the effects of holoparasite-host interaction on belowground decomposers6. Holoparasites are not but very similar to the primary consumers because they are completely dependent upon their hosts for photosynthates, water and mineral nutrients. Although the interaction between holoparasites and host alters carbon and nutrient cycling in the plant and soil system7,8, very little is known about the ecological consequences such as its effects on microbial communities and their function.

Parasitic plants with over 4,500 known species are among the most ubiquitous generalist parasites in both natural and managed ecosystems worldwide. About 20% of parasitic plants are holoparasites9. Parasitic plants acquire part (hemiparasites) or all (holoparasites) of their demand of water, carbon and nutrients from the hosts and thus influence the hosts' performance10 and further the belowground properties. For example, Bardgett et al. found that the root hemiparasite Rhinanthus minor indirectly regulated the belowground chemical and microbial properties in a grassland ecosystem infected after three years11. Two main mechanisms have been forwarded to explain the ‘top-down’ effects of parasitic plants on belowground microbial communities: (1) an enhanced supply of substrates in the rhizosphere could stimulate soil microbial activity. For instance, Bardgett et al. suggested that an increased host's root growth and root exudation was the primary reason for the enhanced activity of belowground decomposers in a mixed grassland community infected by hemiparasitic R. minor11. Also, Jescke et al. found that the concentration of certain amino acids decreased in the roots of Ricinus communis infected by the holoparasite Cuscuta reflexa12. (2) Beside belowground C inputs, hemiparasitic plants may also affect above-ground litter inputs potentially altering soil C cycling and soil microbial communities. An alternative mechanism could be a positive impact on soil nutrient cycling may occur13, where hemiparasitic plants such as Bartsia alpine and Amyema miquelii are found to accumulate nutrients in their leaves and to produce high-quality litter that decomposes rapidly.

So far, the mechanisms underlying the ‘top-down’ effects of parasitic plants on belowground microbial communities are still poorly understood. Litter, root detritus and root-derived exudates are the main sources of soil organic matter and the carbon sources of soil microbes14. In natural ecosystems, some hemiparasitic plants such as Bartsia alpine and Amyema miquelii accumulate nutrients in their leaves and produce high-quality litter that decomposes rapidly and thus positively influence soil nutrient cycling12. However, Bardgett et al. suggested that a mixed grassland community infected by hemiparasitic R. minor stimulated the activity of belowground decomposers, which was regarded as a result of enhanced supply of substrate rather than increased litter quality because both the host's root growth and root exudation increased11. Jescke et al. also found that the concentration of certain amino acids decreased in the roots of Ricinus communis infected by the holoparasite Cuscuta reflexa12. Thus, we proposed that such changes in the root exudates could affect the belowground properties, although so far, there is no experimental evidence for this view.

The introduction of exotic plant species to a native ecosystem may lead to changes in the interactions between native plant species, herbivores, pathogens, parasites and other biotic compartments at various tropic levels, which shape the structure and functioning of the invaded system15. Biological control of invasive plants uses parasitic plants to infect and control the invasive plants16,17,18. This may affect the ecological interactions between invasive species and other biotic and abiotic factors at different trophic levels in invaded communities, as well as alter soil microbial communities, nutrient cycling and greenhouse gas emission. These changes, in turn, may provide alternative ways to control invasive plants.

Mikania micrantha (Asteraceae) (hereafter Mikania) is native to Central and South America and was introduced into China after 191019. Mikania is now widely distributed in Guangdong Province in South China and threatens native ecosystems19. A native holoparasitic plant, Cuscuta campestris (Convolvulaceae, hereafter Cuscuta), has been suggested to infect the invader Mikania as a biological control measures in southern China8. Several studies found that the parasitism of Cuscuta effectively suppressed photosynthesis and growth of the invasive plant Mikania, leading to a reduced cover and an increasing diversity of native species8,18,19. Yu et al. observed that the levels of soil nutrients in a field Mikania community infected by Cuscuta have already changed after 1–4 years of infection8. In a field experiment with Mikania in southern China, Li et al. found that infection of Cuscuta changed soil chemical properties, enzyme activity and microbial biomass three years after infection7. The effects found in those experiments7,8 may be a combined result of changes in litter quality and quantity, root detritus and root-derived exudates. In our study here, we conducted a short-term (7 weeks) microcosm experiment with Mikania infected by Cuscuta to exclude the effects of parasite litter inputs on the biomass, activity and functional diversity of soil microbial communities. We hypothesized that 1) the short-term infection of Cuscuta decreases the host's biomass and as a result also C inputs to the rhizosphere, which in turn reduces soil microbial biomass, soil respiration, the activity of soil enzymes and the functional diversity of soil microbial communities, 2) the magnitude of such effects increases with increasing levels of infection intensity. In addition, we aimed to better understand the mechanisms of biological control using parasitic plants against invasive plants.

Results

Infection effects on biomass

Parasite infection decreased the aboveground, belowground and total biomass of the host plant, but increased the aboveground and total biomass of the co-occurring grass (Fig. 1). The total pot plant biomass was significantly decreased by the infection, but did not differ among the infection intensities (Fig. 1).

Figure 1
figure 1

Aboveground biomass (a), belowground biomass (b) and the total plant biomass (c) of an invasive plant Mikania growing with a native plant Coix in four treatments (Control = non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta. Mean value + 1 SD are given. Different letters within each category indicate significant difference between treatments (p < 0.05). FMicrantha, FCoix, FBoth indicates F values of treatments (non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta) on biomass of the invasive Mikania, the co-existing native grass Coix and on biomass per pot (Mikania + Coix), tested using one-way ANOVAs. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

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Infection effects on soil microbial communities

Shannon's (F3,16 = 101.092, p < 0.001), Simpson's diversity (F3,16 = 46.078, p < 0.001) and evenness (F3,16 = 340.286, p < 0.001) indices of soil microbial communities decreased significantly with infection intensity (Table 1). The utilization of miscellaneous C sources by soil microbial communities did not differ among treatments (Fig. 2), whereas the low-level infection significantly decreased the utilization of carbohydrates and amines/amides and high-level infection significantly decreased the utilization of polymers, carbohydrates, amines/amides and carboxylic acids by soil microbial communities (Fig. 2). Medium-level infection had no significant effects on the utilization of various carbon sources, but the utilization of polymers, carbohydrates, amines/amides and carboxylic acids were significantly higher in the medium-level infection than in the low- and high-level infection (Fig. 2).

Table 1 Functional diversity indices of soil microbial communities in soils under four treatments (non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta). Different letters within a column indicate significant difference between treatments (p < 0.05)
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Figure 2
figure 2

Carbon utilization ability of soil microbial communities in relation to treatments (Control = non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta).

Data was shown with mean ± standard deviation. Different letters within each category indicate significant difference between treatments (p < 0.05).

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Infection effects on soil microbial biomass, soil enzyme activity and soil respiration

Parasite infection increased concentrations of soil Corg (F3,16 = 4.245, p = 0.045) (Fig. 3a) but decreased soil microbial biomass C (F3,16 = 30.882, p < 0.001) (Fig. 3b). Correspondingly, the ratio of Cmic to Corg was decreased (F3,16 = 24.081, p < 0.001) by parasite infection but not affected by the infection intensity (Fig. 3c). The soil β-D-glucosidase activity tended to decrease with infection intensity, but the decrease was only significant when Mikania was highly parasitized by Cuscuta (Fig. 4a). Soil respiration rates significantly decreased by the low-level infection but increased at the medium- and the high-level infection as compared to controls (F3,16 = 12.161, p < 0.01; Fig. 4b). The microbial metabolic activity did not change with low infection. However, at the two higher levels of infection, the microbial metabolic activity increased by 180% or so (Fig. 4c).

Figure 3
figure 3

Responses of soil Corg (a), Cmic (b) and Cmic/Corg (c) to treatments (Control = non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta). Data was shown with mean ± standard deviation. Different letters indicate significant difference between treatments (p < 0.05).

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Figure 4
figure 4

Soil β-D-glucosidase activity (a), Soil respiration rate (b) and microbial metabolic activity (c) in relation to treatments (Control = non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta). Data was shown with mean ± standard deviation. Different letters within each category indicate significant difference between treatments (p < 0.05).

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PCA analysis of soil properties and the relationships with the biomass of plants

The PCA ordination of soil properties had eigenvalues on the first two axes of 2.607 and 2.100 and 78.4% of the variance was explained by the two axes. Soils under the medium- and high-level infection were clearly separated from soils und the low-level infection and controls according to the PC1 axis, whereas soils under medium-level infection and controls were distinguished from the soils subjected to the low- and high-level infection according to the PC2 axis (Fig. 5). The values of Cmic, Cmic/Corg ratio and β-D-glucosidase activity were strongly positively correlated with the first axis. The carbon utilization ability of soil microbial communities had a strongly positive correlation with the second axis, while soil Corg had a strongly negative correlation with the second axis.

Figure 5
figure 5

Principal components analysis (PCA) ordination of soil carbon-related properties.

Control = non-infected Mikania and low-, medium- and high-level infected Mikania by Cuscuta. Data was shown with mean ± standard deviation.

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Pearson correlation analysis showed that Shannon-Wiener diversity was significantly positively correlated with soil Cmic; The evenness index was significantly positively correlated with soil Cmic, Cmic/Corg ratio and AWCD; Simpson's diversity was significantly positively correlated with soil Cmic, Cmic/Corg ratio, β-D-glucosidase activity and AWCD (Table 2). Both Cmic and Cmic/Corg ratio were significantly positively correlated with aboveground, belowground and total biomass of host Mikania and both Mikania and neighboring Coix, but significantly negatively correlated with aboveground and total biomass of neighboring Coix (Table 2).

Table 2 Pearson correlation coefficients between soil properties and the functional diversity indices of soil microbial communities and between soil properties and the plant biomass. Values in bold are significant at p < 0.05. * p < 0.05, ** p < 0.01, *** p < 0.001
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Discussion

In our study, short-term (7 weeks) parasitism on invasive plants significantly altered the biomass, functional diversity and activity of soil microbial communities, indicating a quick top-down effect of aboveground consumer on the belowground decomposers. In line with our results, previous long-term field studies found that Mikania infected by Cuscuta markedly affected soil physico-chemical properties, enzyme activity, soil microbial biomass7 and soil nutrients8 in Mikania-invaded communities. In a natural grassland ecosystem, Bardgett et al. observed significant changes in belowground properties by an infection with hemiparasitic R. minor2. Similarly to plant parasite infection20, foliar herbivores were also found to have strong top-down effects on soil decomposers in a NERC Soil Biodiversity field site located in Scotland21 and in a well-drained arctic tundra heath system22.

Previous studies have paid less attention to the effects of parasitism on soil microbial biomass but focused on the relationships between root-parasitic nematodes and soil microbial biomass. For example, no effects23 of Heterodera trifolii infection but negative effects24 of Rotylenchulus reniformis infection on soil microbial biomass C were reported. Denton et al. found that low intensity infection of H. trifolii increased but high level infection decreased the microbial biomass in Trifolium repens community25. Our present study found that short-term infection of holoparasites significantly decreased soil microbial biomass C. With increasing level of infection, the microbial biomass C decreased, which may indicate that holoparasite-infection rapidly reduced the C supply to soil microorganisms. The likely reason for this decline was a smaller C input from roots since there were no litter inputs from aboveground foliage in our microcosm with young plants during a short experimental period and hence, C inputs were only derived from the belowground system. Root biomass and root-derived exudates are the primary carbon sources for soil organic matter and soil microbial populations26,27,28. Parasitic plants, especially holoparasitic plants absorb nutrients and water from the host28. In addition, the parasite consumes photosynthetic products from the host's phloem, thereby reducing the amount of carbon supplied to the root29. In this study, parasitism significantly decreased the belowground biomass of Mikania but had no effects on the biomass of Coix (Fig. 1). Pearson correlation analysis showed that the decrease in soil microbial biomass was significantly correlated with a declining belowground biomass of Mikania, indicating that an altered root biomass of parasitized Mikania was primarily responsible for the observed changes in soil microbial biomass. The inhibition effect of parasitism of Cuscuta on Mikania released the neighboring grass Coix from competition and increased the aboveground, belowground and the total biomass of Coix (Fig. 1). Pearson correlation analysis showed that the soil microbial biomass was highly positively correlated with the above- and belowground biomass and the total biomass of Mikania (Table 2), indicating that the invasive plants had strong effects on the structure and function of soil microbial communities. Increases in the aboveground biomass of Coix might have increased the supply of the rhizosphere with assimilates, but this increase in carbon supply was apparently not strong enough to compensate for the negative effects of decreased Mikania biomass on soil microbial biomass. In this study, pots were fertilized to avoid the differences in soil nutrient availability and the nutrient limitation on the growth of host and parasitic plants30. Although the fertilization could boost the timing of infection and magnify the effect of parasitic plants on host, the close correlation of plant biomass with microbial biomass indicates that the nutrient addition has not modified the general responses of on microbial communities and their functions to parasite infection.

In contrast to microbial biomass, soil respiration increased at the two higher levels of infection and hence, the soil microbial metabolic activity, the respiratory activity per unit microbial biomass strongly increased. However, calculating metabolic activity using total soil respiration measured in the field is critical as soil respiration has an autotrophic component, which ideally should be subtracted. In the studied system, the autotrophic contribution to soil respiration is not known, but as root biomass even decreased by plant parasitism, it seems likely that an increase in root respiration was not responsible for the observed increase in microbial metabolic activity. The stimulated C metabolization without a corresponding increase in microbial biomass indicates a higher C flow through the belowground and higher respiratory C losses and hence, a smaller potential of the soil to sequester C under high levels of infection31. This finding also suggests that the infection of Mikania by a holoparasitic plant altered the availability but probably also the quality of substrate for microbial communities. One reason could be a decrease in belowground biomass by the plant parasitism but there might have also been a shift in root exudation. Root exudates are low-molecular-weight compounds that are passively and actively released by living roots32,33. Carbon-rich substrates, such as sugars (50–70% of total exudates), carboxylic acids (20–30% of total exudates) and amino acids (10–20% of total exudates) make up the majority of exudates compounds34 and can provide abundant resources for soil microbial communities26,32. These root exudates are of primary importance for microorganisms, as they are readily assimilable without the need to be synthesized by exo-enzymes35. In our study, parasitism significantly changed the functional diversity indices of soil microbial communities and the utilization of various carbon sources by soil microbial communities, again suggesting that the available carbon substrates in root exudates changed with parasitism. Using 14C-labelled compounds, van Hees et al. demonstrated that 60 to 90% of organic acids but only 10–30% of amino acids are respired in the short-term and hence metabolic activities are higher when organic acids are the dominant root exudates33. Exudation of low-molecular weight organic acids generally increases with environmental stress34. Consequently, our observation of an increased microbial metabolic activity at high levels of infections could be interpreted as a result of a stress induced by parasitism.

Exotic plant species can rapidly alter the structure and function of soil microbial communities36,37,38,39,40 and thus change the ecosystem-level soil properties41 and processes39, which may be an important mechanism for the invader success. In a three-year-long field study, Li et al. observed that the invasion of Mikania increased soil microbial biomass C, N and P, soil microbial quotient and the functional diversity, which could enhance soil nutrient availability and in turn the growth of Mikania42. The present study showed that the short-term infection of Cuscuta suppressed the host's biomass and decreased the soil microbial biomass and altered the functional diversity of soil microbial communities. The positive correlation of microbial biomass with above- and belowground biomass of Mikania and of both Mikania and the neighboring Coix strongly suggests that the biomass of Mikania had an overarching effect on soil microbial communities. While inhibiting the growth of Mikania, infection of Cuscuta released the neighboring Coix from the competition. However, the gain in biomass by Coix did not compensate for the losses by Mikania, resulting in a negative response in the total pot above- and belowground biomass and, as a consequence, also in the microbial biomass. In turn, this decline in microbial biomass and the associated effects on nutrient cycling might be an alternative pathway by which the parasitic Cuscuta can prevent the invasion of Mikania.

In conclusion, short-term (7 weeks) infection of Cuscuta significantly decreased above- and belowground biomass of the invasive host, decreased the soil microbial biomass and altered the function diversity of soil microbial communities, but increased the soil respiration. Our short-term experiment excluded aboveground litter inputs and found a positive correlation of roots with microbial biomass, which suggest that the negative effects of plant parasitism on soil microorganism were caused by decreased root biomass and root exudation of the invasive Mikania. Although holoparasitic plants sucks away all substrate from the host, we believe that in the long-term, also an altered functional diversity, activity and biomass of soil microbial community as observed in our study may affect other ecosystem functions such as nutrient availability which in turn might be a possible pathway by which parasitic plant control the invasive plant and restore the native community.

Methods

Study site

We conducted our microcosm experiment in the Dengshuiling village in the southeastern part of Dongguan City (E 113°31′–114°15′; N 22°39′–23°09′), Guangdong Province, China. The climate is marine subtropical with a mean annual precipitation of 1820 mm, mean annual temperature of 23.1°C and mean annual sunshine time of 1874 hours. Mikania started to invade this area in the early 1990s and has spread extensively in shrublands and abandoned fields.

Experimental design

Our microcosm experiment consisted of seedling of the invasive Mikania and native annual grass Coix both planted together in pots (25 cm in diameter and 20 cm in height). Coix was chosen because it is one of the most common co-occurring species in communities invaded by Mikania. Three weeks after seedling planting, Mikania was infested at three different levels of intensity (low, medium and high) with Cuscuta, a native holoparasitic plant, which is widely distributed in Fujian, Guangdong Province and Xinjiang Uygur Autonomous Region, China43.

Prior the microcosm experiment, seedlings of the invasive Mikania were propagated by cuttings (10 cm long), which were collected from a Mikania population in the field near Dengshuiling village, using sharp pruning shears sterilized with 70% ethanol. Only the upper stem segments of healthy and disease-free plants were used for cutting collection. Half of the leaves were removed from each collected cutting to reduce water losses. The cuttings were vertically inserted 3–4 cm deep in prepared nursery beds on July 16, 2006.

Native Coix seeds purchased from Heze Chinese Medicine Institute of Shandong were immersed in 20% CuSO4 for 10 min to avoid disease infection. Thereafter, the seeds were left in water for 24 hours, placed in 70% ethanol for 1 min, in water for 5 min and in 10% H2O2 for 5 min. Finally, the seeds were rinsed with sterilized water three times. On June 2006, seeds with similar size were sown on prepared nursery beds in the field.

In each pot, one Mikania and one Coix seedling were planted at a distance of 10 cm on July, 2006. At planting, seedlings of both species were ~15 cm in height. The potting soil was a mixture of sand and local soil which was taken from an abandoned field without the invasive species near Dengshuiling village. After removing the vegetation and litter from the soil surface, the red clay soil was sampled to a depth of 15 cm. Then, we removed plant materials (e.g. roots) and stones, homogenized the soil, mixed it with sand (soil-to-sand ratio, 3:1, v/v) and filled 2.5 kg of the soil mixture into the pots. The potting soil had a pH (in distilled water without CO2) of 5.3 and initial contents of total organic carbon, nitrogen and phosphorus of 16 g kg−1, 0.56 g kg−1 and 0.16 g kg−1, respectively.

All pots were fertilized with half-strength Hoagland's nutrient solution weekly44 and irrigated with tap water twice per day. Three weeks after the seedlings had been transplanted, native parasitic Cuscuta collected from a field population near Dengshuiling village was wound around the stems of Mikania for infection. In a pilot study, Li et al. (unpublished data) found that the haustoria number, the number of branches and the proportion of the cover of the parasite to the host plant were positively correlated with the number of the parasite's stem but not the length of the parasite's stem. Therefore, we used one, two and three 15-cm-long Cuscuta stems wound around Mikania stems to represent low-, medium- and high-level infection, respectively. Mikania grown without infection was used as controls. Each treatment was replicated five times. Pots were randomly arranged in the field and moved every week.

Measurements

After seven weeks of infection, soil respiration was monitored in situ using the LCi Portable Soil Respiration system (ADC BioScientific Ltd., Hoddesdon, Herts, England). The elliptical soil collars (14 cm in maximum diameter, 8 cm in minimum diameter and 10 cm in height) were inserted 10 cm into the soil. For each measurement of soil respiration, the soil chamber was placed on the collar and the increase in CO2 was recorded for five minutes. We repeated the measurement cycles 10 times for 30 s-intervals until the CO2 flux was constant.

Seven weeks after infection, Cuscuta was removed from the Mikania host. All plants were harvested, separated into roots and shoots, dried for 48 h at 80°C and weighed to determine biomass. Soil samples were stored at 4°C and transported to the laboratory immediately. The soil samples were sieved through a sterilized 2-mm sieve to remove vegetation, small animals, plant roots and stones. A sub-sample of each soil sample was air-dried and ground for soil chemistry analysis and a second sub-sample was stored at 4°C and used to analyze the carbon utilization pattern within 48 hours after sampling. All the equipments used for processing soil samples were sterilized and cleaned with 70% ethanol.

The total soil organic carbon (Corg) was determined using potassium dichromate oxidation45. Soil microbial biomass carbon (Cmic) was determined using the chloroform-fumigation-extraction method46. Before and after chloroform-fumigation, the water content of the soil was measured gravimetrically and the total dissolved C (TOC) of the soil extracted in 0.5 M K2SO4 was measured using a TOC Analyzer (TOC-Vcph, Shimadzu Scientific Instruments, Inc.). Cmic was calculated from the difference of fumigated and unfumigated soil samples as follows: microbial biomass C = Ec/KEC, where Ec = extractable C of chloroform-fumigated soil (conversion to dry soil mass) – extractable C of unfumigated soil (conversion to dry soil mass) and kEC = 0.4547. Microbial metabolic activity was calculated as the ratio of soil respiration to microbial biomass48. Here, we divided soil respiration rates (mg CO2-C m−2 h−1) by the corresponding microbial biomass C concentrations (mg C kg−1 soil DW).

As a measure of the soil's ability to break down cellulose49, we determined the activity of β-D-glucosidase activity from air-dried soils (<2 mm) as described by Li et al.7. Zornoza et al. found that β-D-glucosidase (EC 3.2.1.21) activity determined in air-dried soils was almost same with those obtained from soils under filed-moist conditions50. The specific substrate p-nitrophenyl β-D-glucoside was used for this determination.

The carbon utilization ability of soil microbial communities was assessed by Average Well-Color Development (AWCD) at 96 h using Biolog 96-well Ecoplates (Biolog, Inc., Hayward, California, USA). Each plate contains 31 different carbon sources each with three replicates, including eight carbohydrates, eight carboxylic acids, four polymers, six amino acids, two amines and three miscellaneous substrates51. Fresh soil (10 g) was prepared and diluted according to the modified method described by Li et al.7. Each well of a Biolog EcoPlate was filled with 150 μl of the final dilution52. Three replicate substrate sets were used to get a mean value for each soil sample. Plates were incubated at 25°C for 96 h and color development was measured as absorbance (A) using a microplate reader (Multiscan MK3, Thermo Lab. Systems) at 590 nm53. The individual absorbance value of the 31 single substrates was calculated by subtracting the value of the blank control (raw difference; RD). Negative RD values were set to zero. To minimize the effects of inoculum densities on the absorbance, data were normalized by dividing the RD values by their respective average well color development (AWCD) values. AWCD values were used to calculate Shannon's, Simpson's and evenness diversity indices, using Biological Tools version 0.20 software.

Data analysis

One-way ANOVA was used to analyze the effects of infection on biomass growth and soil properties, followed by Fisher protected least significant difference (LSD) test at the 0.05 confidence level to examine the difference in means between treatments. A Pearson correlation analysis was used to test the correlation between plant biomass and the carbon-related soil properties. All statistical analyses were performed in SPSS 16.0 for Windows. Soil-carbon-related properties in all treatments were assessed using principal component analysis (PCA) to study the relationships between the properties and their grouping54. The statistical software package PC-ORD was used for PCA analysis55. All figures were created in Sigma Plot 11.0.