Spatial variations of soil phosphorus in bars of a mountainous river
Graphical abstract
Introduction
Bars are an important part of the riparian zone, as the interface between aquatic and terrestrial ecosystems, they protect the habitats of wildlife, decrease the shear stress during high flows, increase channel stability and decrease erosion (Noe et al., 2019; Noe and Hupp, 2009). Moreover, Coleman and Kupfer (1996) found that streams with bars in an agricultural watershed had buffer widths far larger than the proposed widths compared to streams without bars. Therefore, bars can help in controlling agricultural non-point source pollution.
Phosphorus is one of the restrictive elements of non-point source pollution in agriculturally dominated watersheds. Large amounts of P fertilizers are applied to support intensive agricultural production (Carpenter, 2008; Khaledian et al., 2018; Smith et al., 2015). However, only a small amount of the applied P-rich fertilizer can be absorbed by the crop, and most of the P fertilizer is lost to the surrounding environment through erosion, leaching, runoff, and/or drainage, which later contributes to eutrophication of waterbodies (Houria et al., 2020; Liu et al., 2014; Nkansah et al., 2019). In mountainous areas, farmland is located along the banks of rivers, and revetments are constructed to prevent hydraulic scouring and control rainfall, tides, and other adverse processes (Yan et al., 2019). Thus, the P in mountainous farmland soils migrates to rivers mostly through underground leaching due to the revetments, and bars adjacent to the revetments can be regarded as the last barrier for the interception of runoff polluted with P (Mckergow et al., 2003; Olley et al., 2015). Therefore, bars reduce underground P pollution via leaching into rivers, mainly through the filtering of interception (Yan et al., 2019). However, natural bars are increasingly being destroyed due to the acceleration of urbanization worldwide, which may seriously affect their ecological functions (Lewis and Maslin, 2015; White and Greer, 2006).
Moreover, it is necessary to understand the spatial variations in soil P to control agricultural non-point source pollution (Ye et al., 2019). Recent studies have reported on spatial variations in soil P in riparian zone such as bar, floodplain, wetland, bank and water-level fluctuation zone. In the longitudinal direction, soil TP showed an increasing trend from upstream to downstream in floodplains (Moustakidis et al., 2019). However, a decreasing trend of soil TP in the water-level fluctuation zone in a mountainous river has also been reported (Wang et al., 2020). In the vertical direction, soil P has been commonly reported to have less vertical variability at shallow depths of floodplain (Moustakidis et al., 2019). However, Watson et al. (2019) found that soil TP contents have a great vertical difference at shallow depths of floodplain. Therefore, no consensus regarding the spatial pattern of soil P in riparian buffers has been reached.
To improve the understanding of the spatial variation in P, many studies have explored the relationship between the spatial distribution of soil P and environmental factors in riparian zone such as bar, floodplain, wetland, bank and water-level fluctuation zone in mountainous river. Factors such as land use, hydrological regime and vegetation type have been found to affect the spatial variation in soil P (Liu et al., 2020; Wamelink et al., 2018; Ye et al., 2019). In the longitudinal direction, soil fine particle contents have been found to affect the longitudinal distribution of soil P (Moustakidis et al., 2019). In the cross-sectional direction, the variation of streamwise velocity has been found to affect the cross-sectional distribution of soil P in floodplain (Bai and Zeng, 2019). In the vertical direction, the anti-season hydrological regime and rainfall erosion had major impact on the spatial variation of soil P in the water-level fluctuation zone in mountainous river (Wang et al., 2020). Previous studies have explored the dominant factors that influence the distribution of soil P in riparian zone such as floodplain and water-level fluctuation zone in one or two dimensions. However, the dominant factors that influence the distribution of soil P in three dimensions (longitudinal, cross-sectional and vertical) have not been fully understood in mountainous river bars.
P in soil is scarcely found in southern China, which is affected by unique climatic conditions, and the annual application rate of phosphate fertilizers is high to support the intensive agricultural production (Yan et al., 2018), and the agricultural land is mainly distributed along the banks of the mountainous river. Therefore, studying the spatial variations in P and the dominant factors affecting the distribution of P in bars of mountainous rivers is crucial to employ appropriate management strategies to protect these rivers. At present, no consensus regarding the distribution of P in bars of mountainous rivers has been reached, and dominant factors of distribution of soil P in different directions have not been fully understood in mountainous river bars.
In this study, soil samples in three directions (longitudinal, cross-sectional and vertical) were collected in a mountainous river bar. The soil TP, soil available P, soil clay content, soil silt content, soil sand content, soil pH, and vegetation biomass were measured. The objectives of this study were as follows: 1) to investigate the spatial variation of P in the bars of a mountain river; 2) to analyze the dominant factors that influence the variation in P; 3) to predict the risk of P release from the mountainous river bars. The paper is organized as follows. Description of study area, field sampling methods, laboratory analysis methods and data analysis methods are derived in Section 2. In Section 3, our results analyze the spatial variation of P in different directions (longitudinal, cross-sectional and vertical) in mountainous river bars, and discuss against previously published analytical of the variation of P in riparian zone. Our studies also analyze the dominant factors that influence the variation in P in different directions (longitudinal, cross-sectional and vertical) in mountainous river bars. Conclusion is presented in Section 4.
Section snippets
Study area
The study area is located in Longyou country, Quzhou City, Zhejiang Province, south China (28°47′10″N-29°03′15″N; 119°8′41″E-119°12′52″E, Fig. 1), and Lingshan River is an important tributary of the Qujiang River, which is the upstream section of the Qiantang River. The total length and the watershed area of the Lingshan River are 43.79 km, 367.6 km2, respectively. The environment of the study area is characterized by a subtropical monsoon climate with four distinct seasons. The mean annual
Descriptive statistics
Soil P is a limiting factor for plant growth and biomass production. Soil TP and available P are two main indicators for the measurement of P levels (Yu et al., 2014). Table 3 shows the summary statistics for the soil TP and soil available P levels at the different sampling sites. The soil TP and available P concentrations ranged from 373.25 mg kg −1 to 1256.48 mg kg −1. TP contents in soils generally range between 0.1 and 3 g kg−1 (Lemanowicz, 2018), and the nutrient status of the soil is
Conclusions
Results of this study indicated that the soils in the study area had low TP (lower than the threshold of 1 g kg−1) and available P (lower than the threshold of 25 mg kg−1) levels, and there was a low risk of P leaching in the study area. The results further showed the spatial variation of soil P and its dominant factors in mountainous river bars. Longitudinally, soil TP exhibited a decreasing trend, mainly influenced by the erosion effect of flowing water, whereas soil available P exhibited a
CRediT authorship contribution statement
Chuanbin Dou:Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Visualization, Writing - original draft, Writing - review & editing.Jihong Xia:Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.Yingjun Wang:Investigation.Wangwei Cai:Methodology, Resources.Zhuo Zeng:Methodology, Resources.Xingxue Zhu:Investigation.Yuezhou Cheng:Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the National Key Research and Development Program of China (2018YFD0900805), the Postgraduate Research & Practice Innovation Program of Jiangsu Province [Grant No. KYCX20_0493], the Key Program of Water Conservancy Science and Technology of Zhejiang Province [Grant No. RB1915], the National Natural Science Foundation of China [Grant No. 41471069], the Fundamental Research Funds for the Central Universities [Grant Nos. SJKY19_0522 and 2019B67714].
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