Variations in soil phosphorus biogeochemistry across six vegetation types along an altitudinal gradient in SW China
Introduction
The understanding of soil phosphorus (P) processes is essential for maintaining sustainable development of mountain ecosystems (Wu et al., 2013a). While most studies on P regimes along altitudinal gradients consider the impact of P on vegetation, the opposite direction (e.g., influences of vegetation on soil P) is poorly understood (Lucas-Borja et al., 2012, Ushio et al., 2010). Soil P bioavailability in natural ecosystems is often simultaneously impacted by lithology (Mage and Porder, 2013), climate (Vincent et al., 2014), soil age (Zhou et al., 2013a), soil properties (Turner and Blackwell, 2013), and biological activity (Richardson et al., 2011). However, most previous studies have focused mainly on one single factor impacting P bioavailability. For example, chronosequences and climosequences have often been used to discuss the influences of soil age (Walker and Syers, 1976) and rainfall (Miller et al., 2001) on biogeochemical P cycling, respectively. Recently, some researchers have noted the need to investigate the interactions (Porder and Chadwick, 2009) and determine the relative importance (Mage and Porder, 2013) of these factors in order to understand P cycling in terrestrial ecosystems. Furthermore, to predict the P status of ecosystems regionally or even globally and to adequately represent P bioavailability in global biogeochemistry-climate models, the relative importance of the impact factors and their synergistic effects on P biogeochemistry should be explored comprehensively (Yang et al., 2013). The vertical vegetation and soil zones on high mountains provide an opportunity for determining the relative importance of the influences of lithology, climate, soil properties, vegetation, and their interactions on the biogeochemical P cycles in natural ecosystems.
There are two important theories that are suitable for explaining variations in soil P bioavailability along altitudinal gradients with similar bedrocks. The first theory is that the soil P bioavailability will decrease with increasing elevation because the decomposition rates of organic P will be reduced due to the decrease in temperature. For example, Vitousek et al. (1988) reported a decreasing trend in available P concentration with increasing elevation in a Hawaiian montane rainforest (760–1585 m asl). In a tropical montane forest in Ecuador, Soethe et al. (2008) also observed that the P availability in the organic horizon decreased with increasing elevation (1900–3000 m asl). Moreover, Vincent et al. (2014) found that the available P concentration decreased upslope in a subarctic tundra landscape (500–1000 m asl).
The second theory is the conceptual model of P evolution with pedogenesis proposed by Walker and Syers (1976). According to this model, the soil P bioavailability will decrease with increasing weathering intensity (and thus soil development) because large amounts of available P can be transformed into occluded P and organic P in more developed soils. Therefore, the available P in soils is speculated to decrease downslope as a result of increasing weathering intensity due to the increasing temperature and more developed vegetation.
Mt. Gongga in southwestern China provides an ideal place for examining P biogeochemical cycles across different climates and vegetation types. On the eastern slope of Mt. Gongga, there is an altitudinal difference of 6600 m and, thus, a large temperature and precipitation gradient (Zhong et al., 1999). Seven vegetation zones are distributed between the alpine frigid meadow to the evergreen broad-leaved forest on the slope (Liu and Qiu, 1986). The soil P bioavailability on Mt. Gongga will decrease with altitude according to the first theory but increase with altitude according to the second one. If the effects of vegetation types are considered, the spatial pattern of the P bioavailability along this slope is likely more difficult to predict. For example, a recent study suggested that changes in temperature significantly influenced PO43 −–P concentrations in a subarctic region, but the effects of temperature depended strongly on the dominant vegetation type (Sundqvist et al., 2014).
Here, in combination with the relevant geochemical and biological factors, the forms and stocks of soil P were investigated using a modified Hedley fractionation technique (Tiessen and Moir, 1993) on Mt. Gongga. In particular, the aims of the study are the following:
- (1)
Revealing the spatial distribution of the soil P bioavailability and forms along the altitudinal zones on Mt. Gongga.
- (2)
Because the variations in soil P biogeochemistry along the altitudinal slope are likely influenced by temperature, precipitation, soil properties, vegetation type and microbial activities, the second goal is to identify the main driving factors among these potential factors.
Section snippets
Study area
Mt. Gongga is located in southwestern China at the southeastern edge of the Tibetan Plateau and has an elevation of 7556 m (Fig. S1). The area within 29 km of Mt. Gongga has a total topographic relief of 6600 m because it is located in the transition zone between the Tibetan Plateau and the Sichuan Basin. Due to this huge altitudinal difference and its distance from cities, there are seven pristine altitudinal vegetation zones on the eastern slope of Mt. Gongga (Fig. 1). The climate of the eastern
Physical and chemical soil properties
The mineral compositions of the parent materials were similar and were dominated by granites at all the sites along the slope. The average contributions of feldspar, quartz, hornblende, chlorite, and muscovite were 43.7%, 32.7%, 17.9%, 4.9% and 0.8%, respectively (Fig. S3).
The degree of soil development on the eastern slope of Mt. Gongga increased with decreasing elevation based on the characteristics of the soil profiles (Table 1). The thickness of the A horizons increased downslope. The
Altitudinal pattern of TP
The spatial distribution of TP concentrations shows an altitudinal pattern: the concentrations and stocks of TP in the alpine sites were higher than those in the sub-alpine and the low altitude sites (Fig. 2a, b). This altitudinal pattern of TP is likely a result of the interaction of climate, vegetation type and P contents in the parent materials. First, in the alpine zones, the P in the primary minerals is less depleted due to weaker weathering resulting from the lower temperatures, higher pH
Conclusions
The variations and driving factors of soil P biogeochemistry across six different vegetation types along an altitudinal gradient on Mt. Gongga were investigated. The concentrations and stocks of soil total P in the alpine zones were higher than those in the sub-alpine and low altitude zones. This pattern is likely related to climate, P content in the parent materials, vegetation type and soil erosion. The concentrations and stocks of available P showed a parabolic pattern along the slope, which
Acknowledgments
The authors thank three anonymous reviewers for their constructive comments and suggestions that are helpful in improving the quality of this manuscript. This study was supported by the National Natural Science Foundation of China (Grant No. 41401253 and No. 41272200) and the Chinese Academy of Sciences (Grant No. KZZD-EW-TZ-06).
References (51)
- et al.
Impact of forest tree species on feldspar weathering rates
Geoderma
(2000) - et al.
Measurement of microbial biomass phosphorus in soil
Soil Biol. Biochem.
(1982) - et al.
Forms and profile distribution of soil phosphorus in alpine Inceptisols and Spodosols (Pyrenees, France)
Geoderma
(2000) - et al.
Phosphorus fractionation in lowland tropical rainforest soils in Central Panama
Catena
(2010) - et al.
Phosphorus requirements of microbial biomass in a regosol and an andosol
Soil Biol. Biochem.
(1998) - et al.
Redox control of phosphorus pools in Hawaiian montane forest soils
Geoderma
(2001) - et al.
A modified single solution method for the determination of phosphate in natural waters
Anal. Chim. Acta
(1962) - et al.
Synchrotron-based P K-edge XANES spectroscopy reveals rapid changes of phosphorus speciation in the topsoil of two glacier foreland chronosequences
Geochim. Cosmochim. Acta
(2013) - et al.
A theory of gradient analysis
Adv. Ecol. Res.
(1988) - et al.
Tree species effects on soil enzyme activities through effects on soil physicochemical and microbial properties in a tropical montane forest on Mt. Kinabalu, Borneo
Pedobiologia
(2010)