Roles of Organic Phosphorus in Modifying Phosphorus Buffering Capacity in Soils along Shoreline of a Chinese Large Shallow Lake (Lake Chaohu)

Keywords: soil; organic phosphorus; degree of phosphorus saturation; phosphorus sorption behavior; land use; large lake

Introduction

Spatial patterns of total nitrogen (TN) and total phosphorus (TP) would change significantly with land use changes currently being implemented to achieve sustainable agriculture development and environmental restoration. Taking land use type into account when considering the spatial variation of TN and TP would increase the accuracy in modeling and prediction of soil nutrient status and nutrient movement at the watershed scale [1]. Phosphorus flux from agricultural landscapes to surface waters may cause eutrophication [2]. Therefore, spatial variations in land use patterns and the relevant soil phosphorus status around the large lakes are of great environmental significant but inadequately studied. The distribution of soil phosphorus among labile and nonlabile forms can be a major determinant of agricultural and natural ecosystem productivity, and in wetland soils, organic phosphorus (OP) was distinguished to be labile [3]. Importantly, the composition of NaOH-extractable OP in the clay fraction was influenced to a greater extent by land use than by fertilizer inputs [4]. Land use and soil management affect soil organic carbon in whole soil and size separates, but knowledge of the accompanying soil OP is limited. In fact, OP compounds in the Bronx River bed sediments were identified, which provided a step toward improving water quality in an urban river system [5]. In the estuarine sediments of the Chaohu Lake Valley in Eastern China, the extensive use of phosphate has affected phosphorus processes throughout the valley and resulted in the TP contents in the substrates and surface sediments increasing significantly. Of all the forms of phosphorus, residual phosphorus was present at the highest levels and accounted for 19.8-74.0% of the TP. Additionally, the degree of phosphorus saturation (DPS) data showed that almost half of the estuarine sediments posed a potential risk of eutrophication [6]. Shortly, the OP status and DPS in the soils surrounding aquatic ecosystem are of great environmental significance. Furthermore, because of water resources eutrophication and the need for water protection strategies, the estimation of diffuse phosphorus leaching losses from agricultural soils has become an important issue. Phosphorus-sorption parameters were found to be more important for phosphorus leaching than the extent of the various phosphorus pools in the soils [7]. The sorption and desorption of phosphorus from eroding soil particles in land runoff are important processes contributing to agriculturally-driven eutrophication [8]. Equilibrium phosphorus concentration (EPC₀) represents the phosphorus concentration maintained in a solution by a solid phase (soil or sediment) when the rates of phosphorus adsorption and desorption are the same [9], it can give a theoretical indication as to whether phosphorus is either adsorbed or desorbed by soil [10]. Hence, it is essential to exam the OP status and its relation to phosphorus sorption behaviors in terms of DPS and EPC₀ in soils along the shoreline of large eutrophic lakes with different land use patterns.

In this study, spatial and seasonal samples were taken from the soils representing the different types of land use (agricultural field, grassland, woods and dilapidated embankment) along the shoreline of a Chinese large shallow lake, and the sediments in littoral zones that directly receive diffusion from these soils were also sampled. The contents of different phosphorus species and alkaline phosphatase activity (APA) in the soils were measured, and phosphorus sorption behaviors were analyzed in the soils. The aim of this study was to test the hypothesis that OP which derived from a variety of land use types can effectively alter phosphorus buffering capacity of the soils surrounding large lakes thereby modifying eutrophication processes.

Fig 1 · Map of Lake Chaohu showing the sampling sites in soils
and sediments along the shoreline (the type of the northwestern
shoreline was cement embankment with no soil covered)

Materials and Methods

Study Area

Lake Chaohu (117°16 ’54”-117°51 ’46”E and 31°25 ’28”-31°43 ’28”N), one of the five largest freshwater lakes in China, locates in the Yangtze-Huaihe region, central of Anhui Province. Its surface area is about 780km², drainage total area is 13,486km², and the shoreline length around the whole lake is 184.66km. In the last decades, the disturbance from human was becoming increasingly serious, and made the shoreline of the lake shrink a lot. Deterioration of water quality, degradation of ecosystem and decline of bio-diversity occurred in Lake Chaohu, due to a great amount of industrial, agricultural and domestic sewage discharged into the lake, made the eutrophication become more and more grievously.

Sample Collection

Field investigations were carried out in different seasons along the shoreline of Lake Chaohu. The number of sampling sites is 16, which contain different types of shoreline such as agricultural field, grassland, woods and dilapidated embankment. Soils under the surface of the vegetation 10-20cm were collected and adjacent surface sediments of littoral zones were obtained using a Peterson grab sampler. The positions and types of sampling sites are shown in Fig 1 and Table 1.

Chemical Analysis

Soil samples were air-dried, sieve to <2mm, and analyzed for following parameters: Olsen-P and algal available phosphorus (AAP) were determined by NaHCO₃ solution [11] and NaOH solution respectively [12].

TP, inorganic phosphorus, and OP were determined as the difference between ignited and nonignited soils extracted with 0.1 M H₂SO₄ [13].

EPC₀ simulated from Freundlich isothermal model; maximum phosphorus sorption capacity (Q_max) and sorption energy (K) simulated from Langmuir isothermal model [14].

Phosphorus sorption index (PSI) that could rapidly determine soil phosphorus sorption capacity was also modified [15].

APA was assayed using the model substrate p-nitro-phenyl phosphate (pNPP). Dry samples were incubated at 37 °C for 1 h with pNPP solution and the absorption of the supernatant was measured at 410 nm [16].

Pearson correlation coefficients and linear regression analysis were performed using SigmaPlot2000 and SPSS13.0 for windows.

Fig 2 · The contents of Olsen-P (a), AAP (b)
and OP (c) in different land use patterns

Fig 3 · The relationships between OP and Olsen-P
as well as AAP, p<0.01 (a); APA and OP (p<0.01)
as well as Olsen-P (p<0.05) (b); EPC₀ values and
Olsen-P (p<0.01) as well as AAP (p<0.05) (c)
in the soil, n=67

Fig 4 · The relationships between EPC₀ and DPS that was normalized by Q_max (a)
and by PSI (b)

Fig 5 · The relationship between OP and Q_max, p<0.01 (a); OP and sorption energy (K),
p<0.05 (b) in the soil, n=67

Fig 6 · The relationship between OP
in the soil and those in the littoral zone
sediment, p<0.01, n=53

Results and Discussion

Phosphorus forms of the surface soils were significantly different among upland land use patterns [17]. Consistently, among the major land use types along the shoreline of Lake Chaohu, the contents of Olsen-P and AAP are less variable, while, the grassland showed the highest contents of OP in the soils (Fig.2), this may be explained with reference to organic matter. Grassland has potential to sequester more carbon than tilled soil because of the stability of soil organic carbon stored in the <20µm fraction [18]. Additionally, in hydrologically isolated wetlands and surrounding pasture uplands, total carbon and phosphorus were tightly coupled in the sandy soils, OP was dominated by phosphomonoesters in both wetland and pasture soils [19]. Clay from permanently vegetated soil had larger proportions of teichoic acid-phosphorus and other diester-phosphorus forms and was richer in resin extractable OP than clay from arable soil [4]. In 21 basaltic grassland soils, OP was consisted of phosphate monoesters (84-100%) [20]. Thus, grassland possesses more organic carbon coupled with OP that may be dominated by phosphatase hydrolysable phosphorus such as phosphomonoesters.

OP was closely linked with bioavailable inorganic phosphorus species, such as Olsen-P and AAP (Fig.3a). The transformation between organic and inorganic phosphorus was mediated by APA, because APA exhibited significantly positive relations with both OP and Olsen-P (Fig.3b). OP could induce APA and consequently liberate inorganic phosphate. Consistently, in the intermittently flooded soils, APA was significantly and positively related to total OP, labile OP, moderately labile OP, and moderately stable OP [21]. There was also a significantly positive relationship between acid soluble OP and APA in the sediment of a Chinese large lake [22]. Furthermore, in a soybean-wheat rotation management system, the bioavailable phosphorus was significantly correlated with the rates of OP mineralization and the activities of root phosphatase and rhizosphere soil alkaline phosphatase, activity of enzymes mediated through microbes are a major factor in controlling organic and inorganic phosphorus [23]. Surface runoff phosphorus was directly derived from soil available phosphorus pools, including H₂O- and NaHCO₃- extractable inorganic phosphorus, water-soluble OP, and NaHCO₃- and NaOH-extractable OP fractions, which are readily mineralized by soil microorganisms and/or enzyme mediated processes, there is a potential relationship between soil phosphorus availability and phosphatase activities, relating to phosphorus loss by surface runoff. Therefore, phosphatase activity may serve as an index of surface runoff phosphorus loss potential [24]. Shortly, by this mechanism of enzymatic hydrolysis, OP could indirectly enhance EPC₀ by linearly increasing Olsen-P (Fig.3b and c). In conjunction, lower soil EPC₀ indicates higher capacity of soil to absorb phosphorus [25]. In soils of a subtropical wetland, both EPC₀ and water-soluble phosphorus were higher at the surface and decreased with depth [26]. Soluble reactive phosphorus concentrations in the stream water were positively correlated to sediment EPC₀ [27].

It was found that, independently of what DPS was expressed, higher DPS led to higher EPC₀, as illustrated by significantly positive relationships between these two variables (Fig.4). Consistently, low EPC₀ paralleled with low DPS [28]. Generally, DPS was defined as ratio of Olsen-P to Q_max or PSI [29, 30]. Noticeably, when the Olsen-P was replaced by OP, the relevant DPS can still be indicative of EPC₀ (Fig.4). Furthermore, as the denominator of DPS, the Q_max was significantly positive related to OP (Fig.5a). In loess-soils, cultivation decreased both OP fraction and phosphorus sorption capacity [31] indicating the coupling between OP and Q_max in principle. More generally, there existed significantly positive relationships between organic matter and Q_max in soils [29, 32, 33, 34]. Shortly, OP either acted as or gave rise to the numerator of DPS. Simultaneously it enhanced the relevant denominator (Q_max), whose effects on phosphorus retention were uncertain.

OP could indirectly affect the other respect of phosphorus sorption. There existed a significantly negative relationship between OP and sorption energy (K) derived from the Langmuir equation (Fig.5b). This can be explained by the interaction between OP and inorganic phosphate in the sorption process. Dissolved organic phosphorus (DOP) was preferentially sorbed over PO₄^3- in sediments and the DOP sorption increased more strongly with the increasing of organic matter content in sediments than the PO₄^3- sorption. The sorption of DOP and dissolved organic carbon (DOC) increased similarly with the increasing of organic matter content in sediments, and DOC was preferentially absorbed. The rank order of sorption strength of sediments was as follows: DOC < PO₄^3- < DOP [35].

Phosphorus status in lake sediments was dependent on the land use in surrounding soils. For example, sediments in Silver Lake are heavily contaminated with soil runoff phosphorus, mostly coming from the surrounding croplands and an active hog lot on the southeastern lakeshore [36]. Terrestrial litter can contribute up to 10% of the total phosphorus supply to lakes with a large surface area relative to that of their drainage basin [37]. Phosphorous was found in sediments and adjacent soil in the organic form, and was used as an indicator of anthropogenic influence on the reservoir banks [38]. OP is abundant in sediments and is an important phosphorus speciation for primary productivity [35]. Our results further give a general linkage between OP in the shoreline soils and littoral zone sediments (Fig.6), strengthening the influence of land use on lake eutrophication.

Conclusions

Land use types in large lakes are generally diverse, yielding quantitatively different OP and less variable inorganic phosphorus in their soils. The OP gave linear rise to maximum phosphorus sorption capacity and in parallel liberated inorganic phosphorus species by the mechanism of enzymatic hydrolysis, thereby modifying DPS together with EPC₀. In the same time, the OP had adverse effects on phosphorus sorption energy. Furthermore, shoreline soils and littoral zone sediments were closely linked in terms of OP. Shortly, derived from the soils with greatly different land use, OP affected the phosphorus buffering capacity in complicated ways, which can reach the lakes involved by its input into the littoral zone. For restoration and management of large eutrophic lakes, land use patterns must be taken into account with soil OP being a suitable indicator. With abundant OP but not higher inorganic phosphorus content, grassland should be reserved and restored around shoreline of large lakes.

Acknowledgement

This work was supported by these grants: the National Key Basic Research and Development Program (2008CB418006), the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (2008ZX07103-004 and 2008ZX07316-003) and Sino-Hungary Bilateral S&T Cooperation (2008-333-4-7). The authors would like to thank Mr. Wei Xing for his careful work on identification of dominant species. The thanks will also go to Mr. Xiaoke Zhang for his help in the sampling collection.

References

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