Bamboo forest occupies more than 10 million hectares in the world, and 80% of the forests are located in tropical and subtropical Asia [1
]. Bamboo is an important woody grass that provides various ecosystem services such as carbon (C) sequestration [2
], biofuel production [3
], furniture materials [4
] and food for animals [5
]. Bamboo grows quickly and can increase its height up to a rate of 17 cm·day−1
]. Because bamboo provides fast and easily decomposed C sources to soil, it provides good plantation material for ameliorating badland soils that originally contained poor organic matter and poor physicochemical properties [8
Moso bamboo (Phyllostachys edulis
) is one of the commonly growing bamboo species in Asia. However, because of its high growth rate, moso bamboo can expand rapidly and replace surrounding forests via its aggressive rhizomes [9
]. Moreover, the fast-growing shoots of moso bamboo can reach their full height and occupy the canopy within 2 to 3 months [11
]. Additionally, the allelopathic compounds produced by moso bamboo may suppress the growth of understory or eliminate neighboring plants [12
]. Previous research has reported the impact of bamboo invasion on ecosystem functioning, such as changing plant diversity [13
], altering soil water content [9
], decreasing soil trace elements [15
], and changing microbial diversity [7
]. In addition, composition of soil total organic C (TOC) may degrade with bamboo invasion [18
]. Wang et al. [18
] also found that frequent human disturbance in bamboo plantations further induces decomposition of soil organic matter and results in bamboo soil with high humification.
Because moso bamboo litter contains more easily decomposable substances than other coniferous forests [18
], the soil quality and fertility may degrade after the invasion of moso bamboo. However, the direct evidence of changing soil labile C and N content due to moso bamboo invasion has been seldom addressed. Information on these soil labile C and N stocks can be beneficial to better understand the change of ecosystem functioning before and after bamboo invasion.
Potassium chloride (KCl) and hot-water extraction methods are successful analytical methods to determine soil conditions with vegetation changes [19
]. The KCl extraction method can evaluate labile nitrogen (N) content in soil because much of the N is adsorbed by soil particles (i.e., NH4+
and soluble organic N (Sb
ON)) and can be extracted by ion exchange of K+
]. However, soluble organic C (Sb
OC) is more soluble in hot than cold water [21
] and its content was found highly correlated with microbial biomass C (Cmic
) and soil TOC content [20
To provide a better understanding of how moso bamboo migrates and invades forest ecosystems and how the soil quality is altered with bamboo invasion, we measured soil labile C and N content with different extraction methods from the soil where moso bamboo has invaded Japanese cedar forests. We hypothesized that with bamboo invasion and harvesting disturbance, soil C and N contents would significantly decrease because bamboo provides mostly labile C and N.
With both KCl and hot-water extracts, the Sb
OC content was significantly higher in the cedar than the transition and bamboo plantation soils (Table 1
and Table 2
). The Sb
content was nearly 2.5 times higher in the cedar than bamboo soils and the Sb
content was more than 3.5 times higher in the cedar than bamboo soils. In addition, the Sb
content was more than four times higher than the Sb
content in all locations.
Similarly, both the soil NH4+ and TDN content was higher in cedar than transition and bamboo plantation soils with both KCl and hot-water extracts. In addition, NH4+HW content was higher than NH4+KCl content at all locations. Soil SbONKCl content was higher than SbONHW content in the three vegetation types.
The SbOCHW/TOC ratio was higher in the cedar than the transition and bamboo plantation soil, with no spatial variation found between the cedar and bamboo plantation soil in the SbOCKCl/TOC ratio. In addition, the SbONKCl/TN ratio was higher in the bamboo than cedar plantation soil, with no spatial variation of SbONHW/TN ratio found between the three vegetation types. In addition, the total mineralizable N content was similar among the three vegetation types.
Results from PCA showed that three principal components had eigenvalues >1 and explained 91.1% of the variance in the total data. The C-related parameters, TOC, Sb
, AHPI-C, AHPII-C, and RP-C, and N-related parameters, TN and NH4+HW
, appeared to be the most important soil parameters affected by the bamboo invasion in PC-1 and explained 64.6% of the variance. The Nmic
were the most important parameters in PC-2 and explained 16.5% of the variance. The total mineralizable N, NO3−HW
were the most important parameters in PC-3 and explained 10.0% of the variance (Table 3
The calculated SQI based on PCA results showed the cedar plantation with higher soil quality than the transition zone and bamboo plantation (0.75 ± 0.03 vs. 0.38 ± 0.04 and 0.41 ± 0.02) (Figure 1
Bamboo tissue contains low lignin and has a relatively fast soil C cycle, whereas cedar tissue is strongly resistant to degradation and has a slow soil C cycle [28
]. Previous research showed that bamboo decreased soil C and N content when it invaded a broad-leaved forest [29
], perhaps because the bamboo litter contained more O
-alkyl-C, which can be more easily decomposed than that from coniferous forests [18
]. Because the slow decomposing speed of cedar tissue helps accumulate TOC, high TOC further helps provide high Sb
OC content in the cedar plantation soil. In addition, frequent human activities, such as bamboo shoot harvests, may speed up the removal of organic C from bamboo plantation soils [18
]. These reasons may explain the low contents of TOC and Sb
OC in the bamboo plantation soil.
OC has been considered the most important indicator of soil quality [30
] because increasing Sb
OC content provides energy sources that can be easily utilized by the soil microbial community [31
]. With the results of Cmic
as well as total phospholipid-derived fatty acids from Chang and Chiu [16
] in a parallel study, Cmic
content was found the highest in cedar plantation soils. This finding could be evidence that increasing Sb
OC content benefits microbial growth [32
]. Moreover, Cmic
content was positively correlated with Sb
OC content and TOC in this study, so Sb
OC content may be derived from the decomposition of soil TOC as a result of microbial activity [32
The overall Sb
content that we found were positively correlated with TOC and Cmic
content (Figure 2
). However, Sb
content had steeper slopes when fitting to both TOC and Cmic
content than Sb
content, so Sb
content better reflected the labile portion of soil C that can be readily utilized by soil microbes than did Sb
Although Wang et al. [18
] found that bamboo litter contained more easily decomposable substances and the ratio of hydrolysable C to TOC was higher in bamboo than cedar soil, a substantial amount of labile C still existed in cedar soil. The higher Sb
/TOC ratio in the cedar plantation than bamboo plantation confirmed that the Sb
content was attributed to the increase in TOC pools through the cedar litter.
In addition, hot-water-extracted soil organic C is typically considered readily metabolisable [33
], whereas acid-hydrolysable C is considered bioreactive C that is not readily used by microbes [34
]. The lower Sb
but higher acid-hydrolysable C content [18
] in bamboo than cedar soil may imply that the labile C pool in our bamboo soils was more likely to be bioreactive C rather than readily metabolisable C.
/TOC ratio was higher in moso bamboo soils than other moso bamboo forest soils in the elevation gradient in Central Taiwan, particularly higher than those in low elevation plantation soils [20
]. Because the bamboo plantation in the present study was established later (1970s) than those in low elevation plantation soils (1950s–1960s) [18
], the higher Sb
OC content in the bamboo soils in this study than in other bamboo plantation sites at lower elevation may result from shorter cultivation history that removed less C from the ecosystem.
and TDN contents were distinctly higher in cedar than bamboo plantation soil with both the hot-water and KCl extracts. However, only Sb
but not Sb
content was highest in the cedar plantation soil. This finding could be due to KCl helping to extract clay particle-absorbed Sb
]. In addition, the higher SbONKCl
/TN ratio in the bamboo plantation soils could be due to the lower TN content at the sites.
contents were significantly correlated with TN but not Nmic
content (Figure 3
), so the Sb
ON in soil may not directly affect microbial growth. In addition, the total mineralizable N has been previously considered an active fraction of soil organic N. Thus, the high total mineralizable N content at the bamboo plantation in our study implies that the bamboo plantation contained high levels of active soil organic N.
As compared with our published parallel study in a badland soil (i.e., high clay and calcium carbonate content soil) systems, the changes in soil TOC composition with the bamboo plantation resulted in different outcomes as compared with the present study [8
]. A possible reason for the different outcomes is the difference in net C balance between the two ecosystems. Because soil TOC from a bamboo plantation is mostly in labile forms, it can improve badland soil quality with easily decomposable organic C. However, in the present study, the bamboo plantation was not able to reimburse recalcitrant organic C loss from the soil that was previously grown with hardwood forests, which resulted in an overall organic C loss and degradation from the bamboo soils.
The results from PCA and SQI also provided supportive evidence for the soil property changes that bamboo invasion may introduce. PC-1 was composed of C-related parameters, which clearly indicates that soil TOC was the most sensitive soil property altered by the bamboo invasion. The reduction of TOC among the three vegetation types may further affect soil microbial growth, which resulted in Nmic as the most important parameter in PC-2. NO3− is one important nutrient in soil that supports plant and microbial growth, so NO3− was the most important parameter in PC-3.
Lu et al. [26
] found SQI values of <0.3, 0.3–0.5 and >0.5 for low-, intermediate- and high-quality soil. The calculated SQI among the three vegetation types clearly showed that the soil quality was degraded with the invasion of moso bamboo. The labile C and N provided by the bamboo plantation may still help the soil remain at intermediate quality.