Abstract

Water resources development and utilization (WRDU) is an important way for humans to utilize natural resources, and has a deep effect on ecological environments. Flat topography, groundwater dependence and a high proportion of agricultural water are the main features of WRDU in Sanjiang Plain. Due to large-scale development in the last 60 years, the ecological environment of Sanjiang Plain has changed significantly. In order to identify the eco-environmental problems and make regional ecological environment and water resources sustainable, trend and correlation analyses were performed to analyze the eco-environmental effects of WRDU from the aspects of water resources, land resources, vegetation and climate. The results show that the regional eco-environmental effects caused by WRDU in Sanjiang Plain are significant. The quantity and quality of groundwater resources and the social and ecological functions of land resources are significantly affected by the regional development and utilization of water resources, while the effect of surface water resources is not obvious. The changes of vegetation and climate are also significant, but the response mechanism to WRDU requires further study. With the changes of extent, pattern and degree of surface water utilization in Sanjiang Plain, the effect of surface water resources is becoming more and more prominent.

INTRODUCTION

The large-scale hydrological cycle has a significant impact on many physical and chemical processes of terrestrial ecosystems. Strong interaction and feedback exist between the hydrological cycle and terrestrial ecosystems (Niasse 2002). The development and utilization of water resources has changed hydrological cycle processes (Gleick 2000), and thus it has caused negative responses of the terrestrial ecosystem and serious eco-environmental problems (Evans & Attia 1991; Nilsson & Berggren 2000; Smakhtin 2001; Herrero & Perez-Coveta 2005; Huber-Sannwald et al. 2006; Munoz-Hernandez et al. 2011; Tu et al. 2012).

Domestic and international research on the eco-environmental effects of water resources development and utilization (WRDU) has been carried out for many years. Current research is mainly focused on the eco-environmental effects of hydropower engineering (Poff & Hart 2002; Komatsu & Yasuda 2015), sluice and dam projects (Powell 2002), interbasin water transfer projects (Matete & Hassan 2005), river hydropower development (Kingsford 2000), WRDU of watersheds (Tang et al. 2003) and WRDU of ecological vulnerable areas, e.g. arid regions (Snyder & Tartowski 2006). After 60 years of large-scale land reclamation and crop cultivation, the scale and extent of WRDU has expanded gradually and the target of WRDU has shifted from controlling flood and waterlogging to satisfying industrial, agricultural and domestic water demand. The consequential eco-environmental problems are becoming more and more serious (Guo 1991). Therefore, based on the data of WRDU and the ecological environment of Sanjiang Plain, the eco-environmental effects of WRDU were analyzed using trend and correlation analysis in this study.

STATUS OF WATER RESOURCES IN SANJIANG PLAIN

General situation of water resources

The Sanjiang Plain (Figure 1), located in the east of Heilongjiang Province and northeast of China, is a low alluvial plain formed by the confluence of Heilongjiang River, Songhua River and Wusuli River, a total area of 10.9 × 104 km2. The Sanjiang Plain abounds with rivers. There are 21 major rivers belonging to the above three major river systems. The annual average water resources amount is 287.6 × 108 m3. Uneven spatiotemporal distribution and abundant groundwater are the main characteristics of the water resources in Sanjiang Plain.

Figure 1

River system and terrain of Sanjiang Plain.

Figure 1

River system and terrain of Sanjiang Plain.

Present situation of water supply and demand

In 2012, the total water supply in Sanjiang Plain was 156.5 × 108 m3. The surface water supply was 50.1 × 108 m3 accounting for 32% of the total water supply, and the groundwater supply was 106.4 × 108 m3 accounting for 68% of the total water supply. In the same year, the total water consumption was 156.5 × 108 m3, which was equal to the total water supply, and the agricultural water consumption was the largest part (152.3 × 108 m3) accounting for 97.3% of the total water consumption in Sanjiang Plain.

Sanjiang Plain water projects

In 2012 there were 271 reservoirs in Sanjiang Plain. There were six large-scale, 26 medium-scale and 239 small-scale reservoirs, with the total reservoir capacity of 31.2 × 108 m3. The beneficial reservoir capacity of these reservoirs was 15.8 × 108 m3 accounting for 10.5% of the surface water resources amount (SWRA). In 2012 there were 92,160 pumping wells in Sanjiang Plain, and 72,514 pumping wells were used for farmland irrigation accounting for 78.7% of the total number of pumping wells.

TREND ANALYSIS OF ECO-ENVIRONMENTAL EFFECTS

Years of WRDU have greatly changed the natural resources and ecological environment of Sanjiang Plain and resulted in groundwater drawdown, water area reduction, shrinking wetland, water supply and demand contradiction and water quality deterioration, etc., which have caused extensive concern.

Effect of surface water

Sanjiang Plain has an average annual runoff of 151 × 108 m3, a coefficient of variation of annual runoff of 0.55 and an average annual runoff depth of 158.6 mm. Along with the continuous construction of water projects, the development and utilization of surface water resources in Sanjiang Plain has gradually increased. The effective surface water irrigation area and SWRA in Sanjiang Plain are shown in Figure 2.

Figure 2

Effective surface water irrigation area and SWRA on Sanjiang Plain.

Figure 2

Effective surface water irrigation area and SWRA on Sanjiang Plain.

As shown in Figure 2, from 2000 to 2011, the effective surface water irrigation area in Sanjiang Plain shows a rapid growth trend, with growth of 37.2% in 12 years and an average annual growth of 3.1%. In the same time, the SWRA in Sanjiang Plain shows a slight fluctuation trend, except for 2009 and 2010. This phenomenon indicates that the utilization of surface water is not the key factor affecting the change of SWRA in Sanjiang Plain. The low-level development of surface water in the region may cause this phenomenon. However, the annual SWRA is lower than the annual average SWRA (151 × 108 m3) in Sanjiang Plain except for 2009 and 2010. Along with the continuous construction of reservoir water impoundment, water pumping stations and water systems connecting projects in Sanjiang Plain, the utilization degree of surface water will sharply increase in the future. The impact of WRDU in Sanjiang Plain on SWRA would not be negligible.

On the one hand, the long-term excessive application of chemical fertilizers and pesticides transferring from farmland into natural water bodies with the rainfall-runoff process formed non-point source pollution, which resulted in the deterioration of surface water quality (SWQ) in Sanjiang Plain. On the other hand, along with the improvement of people's living standards and the development of the regional economy, the increasing discharge of domestic and industrial sewage and the inadequate capacity of sewage treatment resulted in further deterioration of SWQ in Sanjiang Plain. The SWQ of the main rivers in Sanjiang Plain is shown in Table 1.

Table 1

SWQ of Sanjiang Plain

AreaYilan CountyJiamusi CityShuangyashan CityJixi CityQitaihe CityHegang CityMuling City
Selected reaches Mudan River Main stream of Songhua River Naoli River Muling River Woken River Wutong River Muling River 
2000 Grade IVa Grade IV Grade V Grade V Grade V Grade V WT Grade V 
2001 Grade IV Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade V 
2002 Grade IV Grade IV Grade IV Grade V Grade IV Grade III Grade V 
2003 Grade IV Grade IV Grade IV WT Grade Vb WT Grade V Grade III Grade IV 
2004 Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2005 Grade V Grade V Grade V WT Grade V WT Grade V Grade III Grade V 
2006 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2007 Grade V Grade V WT Grade V WT Grade V WT Grade V Grade II Grade IV 
2008 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade IV 
2009 Grade V Grade IV Grade V WT Grade V WT Grade V Grade IV Grade V 
2010 Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2011 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade IV 
AreaYilan CountyJiamusi CityShuangyashan CityJixi CityQitaihe CityHegang CityMuling City
Selected reaches Mudan River Main stream of Songhua River Naoli River Muling River Woken River Wutong River Muling River 
2000 Grade IVa Grade IV Grade V Grade V Grade V Grade V WT Grade V 
2001 Grade IV Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade V 
2002 Grade IV Grade IV Grade IV Grade V Grade IV Grade III Grade V 
2003 Grade IV Grade IV Grade IV WT Grade Vb WT Grade V Grade III Grade IV 
2004 Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2005 Grade V Grade V Grade V WT Grade V WT Grade V Grade III Grade V 
2006 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2007 Grade V Grade V WT Grade V WT Grade V WT Grade V Grade II Grade IV 
2008 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade IV 
2009 Grade V Grade IV Grade V WT Grade V WT Grade V Grade IV Grade V 
2010 Grade IV Grade IV Grade IV WT Grade V WT Grade V Grade III Grade V 
2011 Grade V Grade IV Grade IV WT Grade V WT Grade V Grade III Grade IV 

aThe SWQ grade is classified according to ‘Environmental Quality Standards for Surface Water (GB 3838–2002)’ (EPAC 2002).

bWT Grade V means the SWQ is worse than Grade V.

From 2000 to 2011, the water quality of the main rivers in Sanjiang Plain shows a gradual trend of deterioration except for the reaches of Wutong River in Hegang City. Over the 12 years from 2000 to 2011, the SWQ of Sanjiang Plain was always worse than Grade I, and the SWQ of most rivers in Sanjiang Plain was worse than Grade III. This situation indicates that the SWQ in Sanjiang Plain is bad and in some areas is very bad. In 2011, the water quality of the reaches of Muling River in Jixi City and the reaches of Woken River in Qitaihe City are WT Grade V, which is below the minimum standard of SWQ for water environmental function, accounting for 28.6% of the selected reaches. The water quality of the reaches of Mudan River in Yilan County is Grade V, and that of the main stream of Songhua River in Jiamusi City, Naoli River in Shuangyashan City and Muling River in Muling City are Grade IV, together accounting for 57.1% of the selected reaches.

Effect of groundwater

Over the 12 years from 2000 to 2011, the number of pumping wells in Sanjiang Plain increased from 24,739 to 77,370, with a total increase of 3.1-fold and an annual average increase of 17.7%. The rapid and great increase in the development and utilization of groundwater resources caused significant groundwater effects. The efficient groundwater irrigation area and groundwater resources amount (GWRA) in Sanjiang Plain are shown in Figure 3, and the changes of average groundwater level in Sanjiang Plain are shown in Figure 4.

Figure 3

Efficient groundwater irrigation area and GWRA in Sanjiang Plain.

Figure 3

Efficient groundwater irrigation area and GWRA in Sanjiang Plain.

Figure 4

Changes of average groundwater level in Sanjiang Plain.

Figure 4

Changes of average groundwater level in Sanjiang Plain.

From Figure 3, the efficient groundwater irrigation area and GWRA in Sanjiang Plain have a significant negative correlation. Over the 12 years from 2000 to 2011, the efficient groundwater irrigation area had an increase of 238.4% and the GWRA had a decrease of 44.7%. From Figure 4, the minimum groundwater buried depth shows a continuous substantial upward trend and the maximum groundwater buried depth shows a continuous slight downward trend, an increase of 2.64 m and a decrease of 0.64 m over 12 years, respectively. The amplitude of groundwater buried depth increased from 5.95 m in 2000 to 9.23 m in 2011, with an increase amplitude of 55.1%. Because 80% of the paddy field area in Sanjiang Plain is irrigated by groundwater, increasing paddy field area and unreasonable development have affected the groundwater circulation and balance and even caused a large-area groundwater depression in Sanjiang Plain (Zhang et al. 2014; Wang et al. 2015).

According to the ‘Heilongjiang Water Resources Bulletin’ (WRDHP 2000–2011) and ‘Quality Standards for Ground Water (GB/T 14848–93)’ (EPAC 1993), the situation of groundwater quality (GWQ) in Sanjiang Plain from 2000 to 2011 was counted and is shown in Figure 5.

Figure 5

GWQ of Sanjiang Plain.

Figure 5

GWQ of Sanjiang Plain.

As shown in Figure 5, the GWQ of Sanjiang Plain had great variation and showed a general trend of fluctuation over the 12 years from 2000 to 2011. The proportion of Class V reached 42.3% in 2011. The proportion of Class V and IV exceeded 90%. The situation of GWQ in Sanjiang Plain is not optimistic. In consideration of the difficulty improving GWQ, the problems of GWQ protection in Sanjiang Plain need to be solved urgently.

Effect of land

The WRDU (especially irrigation and drainage engineering) has a great impact on topography, which is the dominant factor of soil erosion and soil and water loss (Zhang et al. 2015). Surface topographic change can transform surface flow characteristics, such as the concentration of surface runoff and the acceleration of flow speed, to aggravate soil erosion. The change of soil erosion area in Sanjiang Plain is shown in Figure 6. The waterlogging-prone area and the ratio of waterlogging-prone area to arable area (RWAAA) in Sanjiang Plain are shown in Figure 7.

Figure 6

Soil erosion area and effective surface water irrigation area in Sanjiang Plain.

Figure 6

Soil erosion area and effective surface water irrigation area in Sanjiang Plain.

Figure 7

Waterlogging-prone area and RWAAA in Sanjiang Plain.

Figure 7

Waterlogging-prone area and RWAAA in Sanjiang Plain.

Over the 12 years from 2000 to 2011, the soil erosion area increased from 2.28 × 106 ha to 2.38 × 106 ha, with a total increase of 0.1 × 106 ha and an annual average increase of 4.5%. Along with the continuous increase of effective surface water irrigation area, the soil erosion area in Sanjiang Plain shows an upward trend. The curve of soil erosion area is not a continuously rising curve, but a staircase-like one (Figure 6).

As can be seen in Figure 7, over the 12 years from 2000 to 2011, the waterlogging-prone area increased from 9.35 × 105 ha to 9.54 × 105 ha, with a total increase of 0.19 × 105 ha and an annual average increase of 2.1%. The curve of waterlogging-prone area shows a slightly upward trend. However, compared with the waterlogging-prone area in the late 1980s, nearly 1.5 × 106 ha (Zhu 1998), the area in 2011 has decreased a lot. The decrease owed to the strategy that controls waterlogging by large-scale rice cultivation in Sanjiang Plain (Wang et al. 2015). The RWAAA also fell from 58.8% to 36.6% from 2000 to 2011. However, the waterlogging-prone area will increase along with the expansion of wetland reclamation area in Sanjiang Plain.

Sanjiang Plain is the largest concentrated distribution area of wetlands in China. After decades of WRDU, the area and state of wetland have changed greatly. Because there is no specialized database to query wetland area, we collected the wetland area of Sanjiang Plain in different years from the relevant literature (Zhang et al. 2003; Zhou & Liu 2005; Zhang et al. 2009; Wang et al. 2011; Song et al. 2014; Dong et al. 2015). The curve of wetland area in Sanjiang Plain is drawn and shown in Figure 8.

Figure 8

Wetland area in Sanjiang Plain.

Figure 8

Wetland area in Sanjiang Plain.

From Figure 8, the wetland area has decreased from 5.34 × 106 ha in 1949 to 7.08 × 105 ha in 2010, with a total reduction of 4.63 × 106 ha and a reduction rate of 86.8%. The curve shows two decreasing stages, of which one is sharp and the other is flat. The wetland area in the first stage from 1949 to 1980 decreased by 3.39 × 106 ha, an average annual decrease of 1.06 × 105 ha. In the first stage, the sharp reduction was mainly due to the large-scale transformation of wetland into farmland. The wetland area in the second stage from 1980 to 2010 decreased by 1.24 × 106 ha, an average annual decrease of 3.99 × 104 ha. In the second stage, the flat reduction was mainly due to decreasing reclamation speed and artificial wetland construction.

Effect of vegetation

Surface inundation, land occupation, local climate change and soil degradation triggered by WRDU have direct or indirect impacts on vegetation coverage (Nilsson & Berggren 2000). The forestland and grassland area in Jiamusi City (a city located in the northeast of Sanjiang Plain) are shown in Figure 9.

Figure 9

Forestland and grassland area in Jiamusi City.

Figure 9

Forestland and grassland area in Jiamusi City.

As shown in Figure 9, the forestland and grassland area have changed greatly in the nearly 20 years from 1996 to 2014. The forestland area decreased by 7.45 × 104 ha, a decrease rate of 12.1%. The grassland area decreased by 5.257 × 105 ha, a decrease rate of 84.2%. The two curves both show a staircase-like decrease, which reflects the gradual effect of WRDU on surface vegetation. In recent years, along with the practice of returning farmland to forestland and grassland, the areas have increased slightly. Compared with the forestland area, the degradation degree of grassland area is high, which indicates that grassland is susceptible to WRDU.

Effect of climate

Regional atmospheric physical condition is mainly controlled by atmospheric circulation (Degirmendžić et al. 2004). The WRDU changes regional underlying surface characteristics (such as water area, surface albedo, soil moisture and surface roughness, etc.), resulting in a feedback response of atmospheric circulation (Sud et al. 1988) and further causing fluctuations of local microclimate characteristics (such as rainfall, evaporation, temperature, etc.) (Zhao & Shepherd 2012). According to the ground climate data from Fujin weather station (an international exchange station) in Sanjiang Plain, the curves of annual evaporation capacity, annual precipitation and annual average temperature are shown in Figures 1012.

From Figure 10, the annual evaporation capacity shows an upward trend. The annual evaporation capacity increased from 1,173 mm in 1953 to 1,404.6 mm in 2001, with an annual average increase of nearly 5 mm and a total increase of 231.6 mm, which is equal to the annual precipitation in arid areas (Chen et al. 2011). This increase accounted for 19.7% of the annual evaporation capacity in 1953. The construction of water projects in Sanjiang Plain, such as irrigation canals and ditches, ponds, reservoirs, etc., expanded the water area. The increase of effective irrigation area and paddy rice planting area improved soil moisture. Therefore, the variation trend of annual evaporation capacity in Sanjiang Plain is possible. This result also confirms the regional characteristics of the ‘evaporation paradox’ in China (Cong et al. 2009).

Figure 10

Annual evaporation capacity of Fujin weather station.

Figure 10

Annual evaporation capacity of Fujin weather station.

From Figures 11 and 12, over the 61 years from 1953 to 2013, the annual precipitation shows a slight decrease, but the trend is not significant. The annual average temperature shows a clear upward trend, with a total increase of 1.5 °C, which accounts for 100% of the annual average temperature in 1953. The average increase every 10 years of annual average temperature is 0.25 °C, which exceeds the global average of 0.13 °C (IPCC 2007). The inverse relationship between annual evaporation capacity and annual precipitation conforms to the Budyko hypothesis (Budyko 1963, 1974). The variation trend of annual average temperature is also consistent with the background of global warming.

Figure 11

Annual precipitation of Fujin weather station.

Figure 11

Annual precipitation of Fujin weather station.

Figure 12

Annual average temperature of Fujin weather station.

Figure 12

Annual average temperature of Fujin weather station.

CORRELATION ANALYSIS OF ECO-ENVIRONMENTAL EFFECTS

By the trend analysis of eco-environmental effects of WRDU, the evolution of eco-environmental factors along with the WRDU in Sanjiang Plain was revealed in the time dimension. In order to investigate the response and correlativity between eco-environmental factors and WRDU, three correlation coefficients were applied to carry out the correlation analysis. The correlation coefficients applied in the study were the Pearson, Spearman and Kendall correlation coefficients (PCC, SCC and KCC), respectively.

Set X, Y as -dimensional random variables, and as the i-th components of X and Y. The PCC r, SCC and KCC can be calculated by the following formulas, respectively (Chok 2008):  
formula
(1)
 
formula
(2)
 
formula
(3)
where and are the averages of and ; and are the ranks of and ; and are the averages of and ; P is the number of ordered pairs that is consistent with .

SPSS Statistics 20.0 software was applied to calculate the three correlation coefficients. The results are shown in Table 2.

Table 2

Correlation analysis between WRDU amount and eco-environmental factors

Eco-environmental factorPearson
Spearman
Kendall
SignificanceaSignificanceSignificance
SWRA 0.499 0.098 0.420 0.175 0.364 0.100 
SWQ 0.014 0.966 0.129 0.690 0.082 0.724 
GWRA −0.803** 0.002 −0.748** 0.005 −0.606** 0.006 
Groundwater buried depth 0.585* 0.046 0.617* 0.032 0.500* 0.027 
GWQ −0.638* 0.026 −0.494 0.103 −0.443* 0.046 
Soil erosion area 0.803** 0.002 0.911** 0.000 0.785** 0.001 
Waterlogging-prone area 0.796** 0.002 0.822** 0.001 0.679** 0.003 
Wetland area −0.572 0.108 −0.500 0.170 −0.389 0.144 
Vegetation area −0.893** 0.000 −0.763** 0.002 −0.587** 0.006 
Annual evaporation capacity 0.782** 0.001 0.643** 0.010 0.486* 0.012 
Annual precipitation −0.123 0.551 −0.184 0.368 −0.089 0.523 
Annual average temperature −0.414* 0.036 −0.389* 0.050 −0.301* 0.035 
Eco-environmental factorPearson
Spearman
Kendall
SignificanceaSignificanceSignificance
SWRA 0.499 0.098 0.420 0.175 0.364 0.100 
SWQ 0.014 0.966 0.129 0.690 0.082 0.724 
GWRA −0.803** 0.002 −0.748** 0.005 −0.606** 0.006 
Groundwater buried depth 0.585* 0.046 0.617* 0.032 0.500* 0.027 
GWQ −0.638* 0.026 −0.494 0.103 −0.443* 0.046 
Soil erosion area 0.803** 0.002 0.911** 0.000 0.785** 0.001 
Waterlogging-prone area 0.796** 0.002 0.822** 0.001 0.679** 0.003 
Wetland area −0.572 0.108 −0.500 0.170 −0.389 0.144 
Vegetation area −0.893** 0.000 −0.763** 0.002 −0.587** 0.006 
Annual evaporation capacity 0.782** 0.001 0.643** 0.010 0.486* 0.012 
Annual precipitation −0.123 0.551 −0.184 0.368 −0.089 0.523 
Annual average temperature −0.414* 0.036 −0.389* 0.050 −0.301* 0.035 

aTwo-tailed.

**Indicates very significant at 0.01 levels. *Indicates significant at 0.05 levels.

As shown in Table 2, there is a very significant correlation between WRDU amount and GWRA, or soil erosion area, or waterlogging-prone area, or vegetation area, respectively. The PCC between WRDU amount and vegetation area is highest, which indicates a very significant negative correlation (Frude 1987) and linear correlation. The SCC and KCC between WRDU amount and soil erosion area is highest, which indicates a very significant positive correlation, monotonic relationship and rank correlation. There is a significant correlation between WRDU amount and groundwater buried depth or annual average temperature, respectively. The three correlation coefficients between WRDU amount and groundwater buried depth are higher than that of annual average temperature. However, there is no significant correlation between WRDU amount and SWRA, or SWQ, or wetland area, or annual precipitation, respectively.

Among all the eco-environmental factors, the three correlation coefficients between WRDU amount and GWQ or annual evaporation capacity are inconsistent. The PCC and KCC between WRDU amount and GWQ indicate a significant negative correlation, but the SCC indicates that there is no significant monotonic relationship between them. The PCC and SCC between WRDU amount and annual evaporation capacity indicate a very significant linear and positive correlation and a very significant monotonic and positive correlation, respectively, but KCC only indicates a significant positive rank correlation.

CONCLUSIONS

Trend and correlation analyses were applied to research the eco-environmental effects of the WRDU in Sanjiang Plain. The influences of WRDU on the regional water resources, land resources, vegetation and climate were revealed in the study.

In terms of water resources, the WRDU had significant impacts on the quantity and quality of water resources. The influences were only concentrated on groundwater resources, while surface water resources showed no significant response. Therefore, research on groundwater monitoring and groundwater ecological level in Sanjiang Plain is extremely important.

In terms of land resources, the WRDU has caused significant changes of land resource quantity and function. The soil erosion area and waterlogging-prone area have expanded gradually. Therefore, land consolidation and drainage projects are very necessary for Sanjiang Plain. In recent years, the slowing speed of wetland area reduction has been due to the consciousness for protection and artificial wetland.

In terms of vegetation and climate, although their responses to WRDU were significant, the influence level, way and process are still not very clear. The response mechanism of vegetation and climate to the WRDU in Sanjiang Plain needs further study.

ACKNOWLEDGEMENTS

This study has been financially supported by the National Natural Science Foundation of China (51679040, 51579045), Natural Science Foundation of Heilongjiang Province of China (E2016004, D201403), University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province of China (UNPYSCT-2015006), Academic Backbone Project of Northeast Agricultural University (16XG10), and Young Talents Project of Northeast Agricultural University (14QC47, 14QC45). The authors would like to thank the editor and anonymous reviewers whose comments and suggestions helped to improve the manuscript.

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