Effect of Flooding and Air-drying on Nutrition Content of Soil in Embankment WLFZ of Chaohu Lake

Wu KUANG,Tiantian WU,Yanhui ZHANG,Yunzhi YAN,Ming RUI

1.Provincial Key Lab of Research on Wastewater Treatment Technology,Anhui Research Academy for Environmental Science,Hefei 230022,China;

2.College of Life Sciences,Anhui Normal University,Wuhu 241000,China

Responsible editor:Na LI Responsible proofreader:Xiaoyan WU

As the last ecological protective screen for lake water body,the water-level fluctuating zone(WLFZ)is of great significance for lake water quality and safety.The previous study of Svensson et al.[1-2]has shown that the phosphorus in the wetland soil can be released into the overlying water,and the loss of soil nitrogen depends on the soil erosion resistance.Due to the importance of soil in the ecological system,it has been paid more and more attention in recent years.In China,the WLFZ of the Three Gorges Reservoir area is the major study focus.CHENG et al.[4]investigated the physical and chemical properties of soil in WLFZ of the Three Gorges Reservoir,and the results showed that seasonal water flooding destroyed the soil structure,making the soil quality gradually poorer.ZHAN et al.[5]explored the effects of flooding and air-drying on the isothermal adsorption and desorption of soil nitrogen and phosphorus in WLFZ of the Three Gorges,and the results showed that the WLFZ that had absorbed certain amount of ammonia nitrogen and phosphorus would release them into the overlying water again after flooding again,the release strength of which depended n the absorbing amount of exogenous nitrogen and phosphorous.As the bonding zone of land and water,WLFZ was closely related with water body.During the rainfall process,the rich vegetation in WLFZ could reduce the water surface erosion and the erosion of topsoil,thereby reducing water pollution[6].After experiencing water level rising and falling,the soil in WLFZ would go through great changes in physical properties,and the soil nutrients in WLFZ presented overall downward trend.Moreover,the variety of plants may decreased dramatically[7].In order to protect the water body,it is necessary to improve the environment in WLFZ.FAN et al.[8]proposed using biological buffer zone,ecological composite,sloping agricultural land,watershed ecology,artificial wetland and ecological riverbank technologies to carry out the ecological environment protection and control measures.As one of the three lakes that has been most seriously polluted,Chaohu Lake has got a lot attention to its control and management from all over the world.In order to comprehensively and effectively,effectively carry out the governance of Chaohu Lake,it is of great importance to conduct the vegetation restoration and ecological system reconstruction,make use of the vegetation in WLFZ to control and prevent soil erosion and land degradation[9],improve the fertility of the soil and reduce water pollution[10].It has proved that there exists significant corresponding relation between the land use patterns and surface runoff in the Chaohu River Basin[11].Different land use patterns show great significance in the phosphorous transfer fluxes (referring to the phosphorous losses with surface runoff per unit area)[12].Through the analysis on the organic matters as well as nitrogen and phosphorous of soil in northwest WLFZ of Chaohu Lake under natural and different artificial conditions,we studied the variation patterns of organic matter as well as nitrogen and phosphorous of soil in WLFZ under different conditions after flooding with the aim to provide reliable data support and scientific support for the WLFZ reconstruction and ecological function recovery.

Materials and Methods

General situation of the study area

The study area was located in northwest WLFZ of Chaohu Lake,where yellow cinnamon soil and paddy soil were the major soil types.And soil parent material was mainly composed of loess parent material and sedimentary parent material.The area belonged to the warm temperate transition of subtropical monsoon climate zone,where the weather was mild and humid with annual average temperature of 15-16 ℃;the area showed significant interannual variability in precipitation with the multi-year average precipitation of 1 100 mm,in which the precipitation in summer(June-August)accounted for 39% of the annual precipitation;the predominant wind direction of this area was southeaster in summer and northeaster in winter;annual average humidity was 77% with the maximum humidity of 81%.

Sampling point setting and sample collection

The sampling point setting took full account of the current water level fluctuations in Chaohu Lake.In 2008,the lowest water level was 8.30 m,and the highest was 9.53 m.According to meteorological data,the rainfall in Hefei city was mainly concentrated in May-August; the water level in May-December was relatively low,when most of the WLFZ was exposed; in August,due to the large amount of rainwater,the water level rose,which flooded the WLFZ; in Decemeber,the WLFZ had been dried.Therefore,the investigation on the soil of the northwest WLFZ of Chaohu Lake was carried out in May,August and December,and samples were collected.

Sampling point settingThe sampling position was located in the northwest of Chaohu.The distribution of sample points and basic information was shown in Fig.1 and Table1.Soil sampling points were arranged according to the natural condition and artificial conditions.

Natural conditionsThe natural bank caving zone (BA) was a part of the bank caving formed naturally in the northwest bank of Chaohu Lake,which had not conducted any human-induced restoration before.Farmland emergent vegetation zone (NT),which was mainly covered with reed in the lakeside zone and farmland in the landside zone,was mainly planted with various economic crops of rapeseed,peanut,cotton and sweet potato.

Artificial conditionsAccording to different rehabilitation programs,the artificial restoration area mainly included brushwood emergent vegetation zone(GT),self-organizing recover zone (ZZZ),riprap protection forest zone (PF),and riprap shrub and herbaceous plant zone (PG).The upper zone of WLFZ which had gone above the water level of Chaohu Lake was set as the control to take samples as references.

Sample collection and analysisThe samples were collected in accordance with the relevant provisions of the Technical Specification for Soil Environment Monitoring.In the land part,sappers shovel was used to dig the 0-10 cm soil layer; in the flooded WLFZ,Peter Johnson dredge was used to collect soil.

Soil pretreatmentAfter the soil samples were air dried,stones,roots,plant residues and other obvious debris were removed,and then the samples were porphyrized in the ceramic mortar,and then meshed by the 100-mesh.After fully mixed,the samples were sealed in the polyethylene ziplock bag for later use.

Table1 Sampling point information

Analysis methodThe dried sample was braised at 550 ℃for 3 h,and then the weight loss was calculated,obtaining the organic matter content[13-14]; total phosphorus content was measured by alkali melting molybdenum antimony anti colorimetric method[15]; total nitrogen by using semi micro Kjeldahl method[15]; improved Goltman method was used for the phosphorus fractionation[16].Data analysis was completed by SPSS 2003 and Excel 21,Origin 8.

Results and Analysis

Spatial distribution characteristics of soil organic matters,nitrogen and phosphorus in WLFZ

As was shown in Table2,the average organic matter contents in the soil of the WLFZ and control group were(41.54±18.87)and(36.86±18.44)g/kg,respectively,which were mainly distributed in the range of 10.34-93.83 and 6.64-89.72 g/kg.The normality test results showed that the organic matter contents in soil samples from both WLFZ and the control group were fitted well with the normal distribution,and the confidence interval of the soil organic matter with the average below 0.95 was (32.52,41.19) in WLFZ,while that was (37.60,45.72) in the control group.The variable coefficient of the soil organic content from the samples in WLFZ (50.0%)was higher than that in the upper part of WLFZ,indicating that the soil organic matter content in WLFZ showed high dispersion degree,and the process of flooding and air-drying affected soil organic matter content in a complex way.It was related with the natural environment of each sampling point,the pway of pollution and so on.On the whole,the average organic matter content of the soils from WLFZ was higher than that from the un-flooded zone,which indicated that the dry-wet alternation(flooding and air-drying)made the organic matter content in the soil increased.

The average nitrogen contents of the soil samples from WLFZ and the control group in the study area were respectively (420.05 ±323.54) and(417.99 ±232.42) mg/kg,mainly distributed in the range of 84.76-2 130.04 and 78.31-1 360.04 mg/kg.The k-s test results showed that the nitrogen content in the soil from the control group was well-fitted in the normal distribution,and the confidence interval for those with the average with the confidence coefficient of lower than 0.95 was (368.46,467.53),while the total nitrogen content in the soil samples from WLFZ was not fitted in the normal distribution.Therefore,compared to the distribution of organicmatter,the distribution of total nitrogen in the study area was more uneven,which was most obvious in the soil of WLFZ.It indicated that after going through the long-term repeated process of air-drying-flooding-air-drying,WLFZ showed significant difference in total nitrogen content in different sampling points,which increased the spatial heterogeneity.

Table2 Statistical analysis of organic matter,nitrogen and phosphorus content in the study area

The average organic phosphorous contents of the soil samples from WLFZ and the control area were(114.62±137.35) and (99.67±99.56)mg/kg,respectively,which were mainly distributed in the range of 16.28-757.09 and 17.35-544.06 mg/kg.The average total phosphorous contents in the soil samples from WLFZ and the control area were (330.38 ±223.69)and (288.42±201.90) mg/kg,with the main distribution ranges of (83.13-1 144.11)and(49.85-1 068.97)mg/kg,respectively.The k-s test results showed that the total phosphorous content of the soil in WLFZ was fitted in the normal distribution,while the soil organic phosphorous in WLFZ and organic phosphorous content and total phosphorous content in the soil from the control area were not fitted in normal distribution.The confidence interval of the soil total phosphorous in WLFZ with the average of lower than 0.95 was (277.82,382.95).The variable coefficient of soil organic phosphorous content in WLFZ was 119.8%,showing strong degree of variability; and the variable coefficient of soil organic phosphorous content the control area was 99.9%,which belonged to the moderate degree of variation but was very close to strong degree of variability.It indicated that the distribution of organic phosphorous in the study area was not even,which was closely related with the factors of the rise and fall of lake water,vegetation cover and human disturbance,especially the distribution of organic phosphorous in the soil from WLFZ.The variable coefficient of total phosphorous content in the soil from WLFZ(67.7%)was smaller than that from the control area (70.0%),indicating that the alternation of wetting and drying caused by the rise and fall of the lake in WLFZ could reduce the differences in soil total phosphorous content among different sampling points.

Changes of soil organic content in WLFZ before and after waterflooding

As was shown in Fig.2,after water level rising in summer,the riprap embankment in the RF was easy to impact by the lake water,making the water movement in this area become fierce.Therefore,organic matters were brought into the lake with the surface soil under the washing of overlying water,which made the organic matter content during the waterflooding period in August decreased than that in May.Rich organic matters could promote the formation of soil structure and improve soil physical properties,so compared with the riprap embankment,the WLFZ where the emergent plants were used as the"barrier" between land and lake was more conductive to soil and water conservation and organic matter accumulation,so as to improve the bottom material of WLFZ.Under flooding conditions,the increase of soil organic matter was the most obvious in BA,where there were residents living and construction sites.Therefore,it suggested that the significant increase of organic matter content in summer in this sampling point was related with the production and living pollution in the bordering area.Thus,human factors played an important role in the increase of soil organic matter content.

The content of soil organic matter was increased after flooding from that before flooding.During the study,the soil organic matters of all sampling points were in the accumulation state,which indicated that the process of flooding and air-drying was conductive to the increase of soil organic matters in WLFZ with the increase range of 27.7%-236.0%.

Changes of total nitrogen content in WLFZ before and after waterflooding

As was shown in Fig.3,except NT,the soil total nitrogen contents in flooding conditions were higher than that before flooding.Analysis showed that the eutrophication of the water body was serious in summer in Chaohu Lake,and under flooding status,the nitrogen in overlying water would inevitably influence the submerged soil nitrogen content; on the other hand,this research process was also inevitably influenced by human factors,so the total nitrogen content was subject to the influence of both human factors and natural factors.Due to frequent storm in summer,a great amount of nitrate nitrogen,nitrite nitrogen,ammonia as well as ammonium ions would migrate into the overlying water of WLFZ along with the storm runoff.In NT,the content of total nitrogen in flooding conditions decreased significantly,and the reason was that there were large area of farmland near this sampling points,making it seriously affected by the agricultural cultivation.The releasing amount of total nitrogen would higher if the absorbed amount of ammonia nitrogen by the soil was greater[5].Under the waterflooding condition,the total nitrogen content in the soil from this sampling point was significantly decreased,and the soil released a great amount of nitrogen into the overlying water,increasing the risk of eutrophication in Chaohu Lake.

The total nitrogen content in the soil from WLFZ was higher after flooding and air-drying than before flooding.After flooding-air-drying,the adsorption ability of ammonia nitrogen enhanced[5]; on the other hand,the biomass residues of the dead or withered terrestrial plant in soil would be decomposed by microorganisms and released nitrogen,which could increase the total nitrogen content in soil.

Changes of soil organic phosphorus content in WLFZ before and after waterflooding

As was shown in Fig.4,the soil organic phosphorous contents in flooding conditions were higher than that before flooding.It had proved that the organic phosphorous during the entire waterflooding period would decrease first and then increase,and the organic phosphorous was mainly dissolved in the early stage of flooding and accumulated in the later stage of flooding.In the early stage of flooding,the accumulated phosphorous was released,and the increased organic phosphorous in the later stage of flooding might be caused by the transformation from some iron-phosphorus[17].

Except BA,the soil organic phosphorous content after air-drying was lower than that in flooding condition.Except BA,all sampling points in the study area of WLFZ were covered with a large number of natural grown or artificial planted reeds and other emergent plants.Related study showed that the decrease in organic phosphorous content was caused by the increased mineralization activities from the microbial decomposition because there were a large number of microorganisms in the rhizosphere of reeds,and the good aeration conditions after airdrying could promote the activity of microorganisms[18].However,there was no emergent plants growth in BA,and the mineralization activity of organic phosphorous by microorganisms was relatively small,and therefore,with the accumulation of soil organic matter,the organic phosphorous content also increased.

Changes of soil total phosphorus content in WLFZ before and after waterflooding

As was shown in Fig.5,the soil total phosphorous contents of all sampling points in WLFZ increased during the flooding from that in air-drying status.Due to the frequent rains in summer,the phosphorous element in surface soil was washed down by the storm into WLFZ.Due to the adsorption of soil to phosphorus in WLFZ,the total phosphorous content in soil increased.Under the conditions of airdrying after flooding,in addition to BA,the soil samples from all other sampling points showed decrease in total phosphorous content.The reason was that the phosphorous demand for the vegetation growth in WLFZ decreased the total phosphorous content in soil,but there was no artificially planted vegetation in BA,where the soil surface was in naked status.Therefore,the extraneous phosphorous could only be accumulated in soil,and when the accumulation amount of phosphorus in soil reached a certain level,the soil would release the phosphorous into overlying water in the next flooding.

Conclusions

(1) In the study area,the average soil organic matter content in the WLFZ was (41.54±18.87)g/kg,which was in the range of 10.34-93.83 g/kg;the average total nitrogen content was(420.05 ±323.54) mg/kg,ranging in 84.76-2 130.04 mg/kg; the average organic phosphorus content was(114.62 ±137.35) mg/kg,ranging in 16.28-757.09 mg/kg;the average total phosphorus content was (330.38 ±223.69)mg/kg,and the content was in the range of 83.13-1 144.11 mg/kg.

(2) In general,human activity and the process of flooding and air-drying were beneficial to the increase of soil organic matter content in the soil.The process of flooding and air-drying was beneficial to the adsorption of nitrogen in soil,which could increase the total nitrogen content in soil.

(3) The process of flooding and air-drying could enhance the spatial heterogeneity of soil organic matter,total nitrogen and organic phosphorous.The variable coefficient of organic content in the soil of WLFZ was 119.8%,showing strong degree of variability.The process of flooding and air-drying could reduce the differences in soil total phosphorous content among different sampling points in WLFZ.

(4) During the flooding period,the lake water showed great fluctuations due to the wind waves,which washed against the soil in WLFZ,and carried organic matters into soil.Emergent plants could protect soil,thereby reducing the loss of organic matter.In addition,different types of vegetation in different artificial restoration area objectively enhance the spatial heterogeneity in WLFZ,which was conducive to the protection of biological diversity in WLFZ.In addition,it could become beautiful lakeside landscape.

(5) During the agricultural process,the application of nitrogen fertilizer showed significant effect on the amount of nitrogen in the soil of WLFZ.Under flooding condition,the soil in WLFZ would release nitrogen into the overlying water.

(6)In the condition of air-drying after flooding,the adsorption of phosphorous by plants reduced the total phosphorous content,and in the land with no vegetation coverage,the phosphorous would be accumulated and may be released into the overlying water during the next flooding period.

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