LETTER
doi:10.1038/nature11917
Enhanced nitrogen deposition over China
Xuejun Liu 1*,Ying Zhang 1*,Wenxuan Han 1,Aohan Tang 1,Jianlin Shen 1,Zhenling Cui 1,Peter Vitousek 2,Jan Willem Erisman 3,4,Keith Goulding 5,Peter Christie 1,6,Andreas Fangmeier 7&Fusuo Zhang 1
China is experiencing intense air pollution caused in large part by anthropogenic emissions of reactive nitrogen 1,2.These emissions result in the deposition of atmospheric nitrogen (N)in terrestrial and aquatic ecosystems,with implications for human and ecosys-tem health,greenhouse gas balances and biological diversity 1,3–5.However,information on the magnitude and environmental impact of N deposition in China is limited.Here we use nationwide data sets on bulk N deposition,plant foliar N and crop N uptake (from long-term unfertilized soils)to evaluate N deposition dynamics and their effect on ecosystems across China between 1980and 2010.We find that the average annual bulk deposition of N increased by approximately 8kilograms of nitrogen per hec-tare (P ,0.001)between the 1980s (13.2kilograms of nitrogen per hectare)and the 2000s (21.1kilograms of
nitrogen per hectare).Nitrogen deposition rates in the industrialized and agriculturally intensified regions of China are as high as the peak levels of deposi-tion in northwestern Europe in the 1980s 6,before the introduction of mitigation measures 7,8.Nitrogen from ammonium (NH 41)is the dominant form of N in bulk deposition,but the rate of increase is largest for deposition of N from nitrate (NO 32),in agreement with decreased ratios of NH 3to NO x emissions since 1980.We also find that the impact of N deposition on Chinese ecosystems includes significantly increased plant foliar N concentrations in natural and semi-natural (that is,non-agricultural)ecosystems and increased crop N uptake from long-term-unfertilized crop-lands.China and other economies are facing a continuing chal-lenge to reduce emissions of reactive nitrogen,N deposition and their negative effects on human health and the environment.Atmospheric N deposition results from emissions of reactive nitro-gen (N r )species and their atmospheric transport;it expands the foot-print of local alterations to the N cycle 3.Although both natural and anthropogenic sources contribute to atmospheric N deposition,anthropogenic N r emissions (largely from the agricultural,industrial and transport sectors)have increased substantially since the industrial revolution began 9;they now make the dominant contribution to N deposition in many regions 3,10.Increased concentrations of N r in the atmosphere and,through deposition,in terrestrial or aquatic ecosys-tems,or both,degrade human health 1(notably through driving the formation of particulate matter and tropospheric ozone),alter soil and water chemistry 3,influence greenhouse gas balance 4and reduce biological diversity 5.
The human and environmental costs associated with anthropogenic N r are well recognized,and active measures in Western Europe and North America have stabilized or reduced N r deposition in those regions 6,11,12.Even so,very large costs of excess N r have been reported in the European Union 8(J 70–320billion per year)and the United States 13.In contrast,over the past 30years China’s emissions have increased to the point that it has become by far the largest creator and emitter of N r globally 2.However,the rates and trends of N deposi-tion in China since the 1980s are not clear.We would also like to know
the consequences of N deposition,for the people and ecosystems of China,its region and the world.
Following rapid economic growth since the early 1980s,China’s gross domestic product was estimated at US$5.9trillion in 2010,mak-ing China the world’s second largest economy after the United States (moneyn/news/economy/world_economies_gdp).In around the year 2000,China surpassed the United States and the European Union (combined)in its production and use of N fertilizers.Moreover,less than half of the fertilizer N applied in China is taken up by crops 14;the rest is largely lost to the environment in gaseous (NH 3,NO,N 2O and N 2)or dissolved (NH 41and NO 32)forms 15,16.These fluxes—along with N r emitted during fossil fuel combustion—have resulted in some of the most pronounced air pollution on Earth 1.Increased N r emissions must have
influenced atmospheric N depo-sition in and near China,but information on the magnitude,scope and consequences of any change has been lacking.Here we summarize available data nationwide on the bulk deposition of N r in terrestrial ecosystems.Also,we show that the N cycle has been altered in Chinese ecosystems,both within and outside croplands.
Nitrogen deposition includes wet and dry deposition of both inor-ganic and organic N forms 2,17,but in most cases only the bulk depo-sition of inorganic N (NH 4-N and NO 3-N)has been measured systematically 6,18,19.Bulk N deposition denotes N input from precipi-tation as measured by an open sampler (Supplementary Methods);it is a relatively simple measure that includes wet deposition and a fraction of the dry deposition,and it is suitable for regional comparisons.We constructed a national data set incorporating all the available bulk N deposition results from monitoring sites throughout China between 1980and 2010(Supplementary Fig.1).This data set was used to test the magnitude and trend of atmospheric N deposition in relation to anthropogenic emissions of reduced and oxidized forms of N.
In spite of site-to-site variability in the data,bulk N deposition increased significantly with time (P ,0.001),with an average annual increase of 0.41kilograms of nitrogen per hectare (kgN ha 21)between 1980and 2010(Fig.1a and Supplementary Table 1).The increase in bulk N deposition wa
s driven mainly by increased volume-weighted N concentrations in rain water (0.063mgN l 21yr 21on average;Fig.1b)because annual precipitation in the study area has not changed significantly in the past 30years (Supplementary Fig.2and Sup-plementary Table 1).NH 4-N was the dominant form in bulk deposi-tion,but the ratio of NH 4-N to NO 3-N in bulk precipitation decreased significantly with time (Fig.1c,Supplementary Fig.3and Supplemen-tary Table 1).Overall,annual bulk N deposition averaged 13.2and 21.1kgN ha 21in the 1980s and 2000s,respectively,showing an increase of approximately 8kgN ha 21,or 60%(P ,0.001).
The increase in overall bulk N deposition and the change in the ratio of NH 4-N to NO 3-N in precipitation and deposition (Fig.1)are similar to the increasing trends of anthropogenic gaseous N r (NH 3and NO x )emissions and changes in their ratio since 1980(Fig.2a and Supplementary Fig.4a).The ratio of NH 4-N to NO 3-N in measured
*These authors contributed equally to this work.
1
College of Resources &Environmental Sciences,China Agricultural University,Beijing 100193,China.2Department of Biology,Stanford University,Stanford,California 94305,USA.3VU Unive
rsity Amsterdam,1081HV Amsterdam,The Netherlands.4Louis Bolk Institute,Hoofdstraat 24,3972LA Driebergen,The Netherlands.5The Sustainable Soils and Grassland Systems Department,Rothamsted Research,Harpenden AL52JQ,UK.6Agri-Environment Branch,Agri-Food and Biosciences Institute,Belfast BT95PX,UK.7Institute of Landscape and Plant Ecology,University of Hohenheim,70593Stuttgart,Germany.
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bulk deposition decreased from about 5to 2,and the ratio of NH 3-N to NO x -N in calculated emissions decreased from about 4to 2.5;these changes are highly correlated (P ,0.01).Emissions of NH 3doubled (Fig.2a),reflecting increased agricultural production in that both the use of N fertilizer and the number of domestic animals (expressed as standard livestock units)have also doubled since the 1980s (Fig.2b and Supplementary Fig.4b).Fossil fuel power plants,industrial production and motor vehicles are the major sources of NO x in China and Asia 20.Coal consumption and the number of motor vehicles increased 3.2-and 20.8-fold,respectively,between the 1980s and the 2000s (Fig.2c and Supplementary Fig.4c),driving a more rapid percentage increase in NO x emission than in NH 3emission (Fig.2a),although the net increase in emission was still larger for NH 3than for NO x (about 6TgN versus 4TgN between the 1980s and the 2000s).The ratio of NH 4-N to NO 3-N in bulk
precipitation (Fig.1c)changed in the same direction and by approximately the same magnitude as the ratio of NH 3-N to NO x -N emission over the same period (Fig.2a),despite uncertainties in ammonia emission inventories 2,21.
We analysed the dynamics of bulk N deposition regionally by divid-ing deposition data into six areas:northern,southeast,southwest,north-east and northwest China and the Tibetan plateau.Human influences
on the N cycle differ substantially among these regions.In general,bulk N deposition showed increasing trends and ratios of NH 4-N to NO 3-N showed decreasing trends for all six regions between the 1980s and the 2000s (Supplementary Table 1);highly significant (P ,0.001)increases in bulk N deposition were found in northern,southeast and southwest China,significant (P ,0.05)increases were found in the Tibetan plat-eau and northeast China,and no significant (P 50.199)increase was found in northwest China (Supplementary Figs 5and 6).By comparison with the national average,we found both higher overall rates of depos-ition and higher annual rates of increase in deposition in the industria-lized and agriculturally intensified northern,southeast and southwest China.Annual bulk deposition rates were 22.6,24.2and 22.2kgN ha 21in northern,southeast and southwest China in the 2000s,respectively,with average rates of increase of
0.42,0.56and 0.53kgN ha 21yr 21.A more detailed study 22of all major deposition pathways shows that total annual N wet and dry deposition on the northern China plain (the central area of northern China)was about 80kgN ha 21.These levels are much higher than those observed in any region in the United
B u l k N d e p o s i t i o n (k g N h a –1)
B u l k N c o n c e n t r a t i o n (m g N l –1)
1980
1985
1990
deposition19952000
2005
2010
N H 4-N /N O 3-N i n p r e c i p i t a t i o n
a
b c
Year
Figure 1|Trends in N deposition and its components in China between 1980and 2010.a ,Bulk N deposition;b ,bulk N concentration;c ,ratios of NH 4-N to NO 3-N in bulk precipitation.In spite of large site-to-site variability,both bulk N deposition and N concentration have increased significantly since 1980,and the ratio of NH 4-N to NO 3-N in bulk precipitation has decreased
significantly according to linear mixed models (all P ,0.001;Supplementary Table 1).Data sources are included in Supplementary Information.
N f e r t i l i z e r u s e (T g N y r –1)
Livestock unit (106 heads)
N H 3 o r N O x e m i s s i o n (T g N y r –1)
NH 3-N/NO x -N
Year
N o . o f v e h i c l e s (106)
20
40
60
80100
Coal consumption (109 t)
c a
b
Figure 2|Trends in NH 3and NO x emissions and their main contributors between 1980and 2010.a ,NH 3and NO x emissions and ratios of NH 3-N to NO x -N emission;b ,number of domestic animals (expressed as livestock units)and N fertilizer consumption;c ,number of motor vehicles and coal consumption.Data sources are cited in Supplementary Information.
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States 12,and are comparable to the maximum values observed in the United Kingdom 6and the Netherlands 7when N deposition was at its peak in the 1980s 8.
Extensive long-term environmental inventories and experiments in China allow us to evaluate some of the consequences of this substantial and continuing increase in N deposition.We have summarized results of an ongoing survey of foliar N concentrations from non-agricultural ecosystems throughout China and from detailed studies of crop N uptake from croplands in long-term trials without N fertilizer (des-cribed as zero-N plots hereafter),which are used as reference plots in fertilization experiments.The foliar N data set provides information on how changes in N deposition have influenced plant tissue chemistry in unfertilized,non-agricultural ecosystems.Foliar N increased signifi-cantly (all P ,0.001)between the 1980s and the 2000s for woody,herbaceous and all plant species (Fig.3a,Supplementary Fig.7a and Supplementary Table 1).Foliar N increase for all species averaged 32.8%(24.069.2mg g 21(2000s)versus 18.167.2mg g 21(1980s)),with a higher increase in herbaceous plants than in woody plants (Fig.3a).In contrast,foliar phosphorus (P)did not change signifi-cantly (P 50.085)over the same period (Supplementary Fig.7b and Supplementary Table 1).Foliar N is largely determined by plant spe-cies and plant N nutritional status;foliar N of specific plant species should be stable in natural and semi-natural ecosystems unless some process changes the availability of N relative to other plant resources 23.
Plant species in this study were sampled widely (Supplementary Fig.1)and analysed by standard procedures (Supplementary Information)—and the evaluation of foliar P should correct for any bias towards sampling high-nutrient plant species late in the record (suggesting no apparent changes in the soil environment)—so the increase in foliar N in unfertilized ecosystems most probably represents a widespread increase in plant N nutritional status caused by the cumulative effects of enhanced N deposition.
In agricultural ecosystems,crop N uptake from zero-N plots in long-term experiments is controlled primarily by N deposition,because soil N pools are relatively stable after 5to 10years without N fertiliza-tion 22,24.We summarized the available data on N uptake from zero-N plots in long-term experiments between 1980and 2010(Supplemen-tary Information);N uptake by rice,wheat and maize in zero-N plots was significantly higher in the 2000s than in the 1980s (Fig.3b;all P ,0.05).The increase in N uptake averaged 11.3kgN ha 21across the three major cereals (Fig.3b and Supplementary Fig.8).
Overall,the temporal patterns in bulk N deposition,foliar N and N uptake from zero-N plots are consistent with rapidly increased anthro-pogenic NH 3and NO x emissions over the past three decades.The lower ratio of reduced N to oxidized N in measured deposition agrees well with the decr
ease in the ratio of calculated emissions of NH 3to NO x ,reflecting a more rapid proportional increase in N r emissions from industrial and traffic sources than from agricultural sources.All these changes can be linked to a common driving factor,strong economic growth,which has led to continuous increases in agricul-tural and non-agricultural N r emissions and,consequently,increased N deposition.
Although we did not measure the impact of atmospheric N r emis-sions and deposition from China on the global environment,recent studies indicate that N r deposited by China may be moving to sur-rounding marine ecosystems 25and perhaps to tropical and subtropical forests 26.Another study 27reported a strong abnormal spring increase in free tropospheric ozone concentrations in western
North America between 1995and 2008,and suggested that NO x -induced ozone trans-port from Asia (mainly from China and India)to North America could be a major source.
Clearly,N deposition has increased significantly in China and has affected both non-agricultural and agricultural ecosystems.So far,China’s economic growth model has relied mainly on the consump-tion of raw materials,and it has caused large anthropogenic N r emis-sions in addition to other environmental perturbations 28.For example,the emitted NH 3and NO x gases form secondary aerosols such as NH 4NO 3in PM 2.5(particulate matter with aerodynamic diameter #2.5m m)under f
avourable conditions,decrease visibility and damage human health.The Chinese government has recognized the impor-tance of protecting the environment while developing the economy;recently,it approved the first national environmental standard for limiting the amount of PM 2.5(ref.29).
Our results demonstrate that deposition of reduced forms of N r continues to be of greatest importance in China (which is responsible for approximately 2/3of total deposition)but emission and deposition of oxidized N r are increasing more rapidly.Current environmental policy needs to focus more strongly on reducing present NH 3emis-sions from agricultural sources,whereas control of NO x emissions from industrial and traffic sources will become more important in the near future.It is time for China and other economies to take action to improve N-use efficiency and food production in agriculture and reduce N r emissions from both agricultural and non-agricultural sectors.These actions are crucial to reducing N deposition and its negative impact locally and globally.
METHODS SUMMARY
Data sets on bulk N deposition,plant foliar N concentration and crop N uptake from non-N-fertilized soils were summarized from published data and measure-ments across China.Using 315references and our own deposition monitoring
F o l i a r N (m g g –1)
a
Figure 3|Comparisons of foliar N concentrations and crop N uptake between the 1980s and 2000s.a ,Foliar N in woody,herbaceous and all plant species in non-agricultural ecosystems;b ,N uptake by rice,wheat and maize from zero-N plots in long-term experiments.Both foliar N (all P ,0.001)and N uptake (all P ,0.05)are significantly higher in the 2000s than in the 1980s.The black and red lines,lower and upper edges,bars and dots in or outside the boxes represent median and mean values,25th and 75th,5th and 95th,and ,5th and .95th percentiles of all data,respectively.
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network(CAUDN),we constructed a nationwide data set of the amount of annual precipitation,volume-weighed N concentrations in precipitation and bulk N deposition,as well as the ratio of NH4-N to NO3-N deposition.A total of866 data points of annual volume-weighted N concentrations in precipitation and671 data points of annual bulk N deposition rates at270monitoring sites were sum-marized for the period1980–2010.To clarify regional variations,bulk N deposi-tion data from six separate regions were also summarized.Additionally,a total of 981observations of plant
foliar N concentration and859observations of foliar P were collected from666natural and semi-natural terrestrial plant species or varieties at245sites distributed across the whole of China(based mainly on ref.30;Supplementary Fig.1),and a total of278data points of crop N uptake by rice,wheat and maize were summarized from non-N-fertilized soils in long-term experiments.Emissions of national anthropogenic NH3and NO x were sum-marized from published data2and updated to2010(Supplementary information). Data on N deposition,foliar N and crop N uptake,and other related parameters, were fitted(with year)using linear mixed models or nonlinear regression models for the interval1980–2010(SPSS13.0,SPSS Inc.).Differences in these data between the1980s(1980–1989)and the2000s(2000–2010)were compared statistically using an unpaired two-tail Student’s t-test.A significant difference is assumed when the P value is,0.05or as otherwise stated.Further details on the data sets and statistical methods are given in Supplementary Methods.
Full Methods and any associated references are available in the online version of the paper.
Received25June2012;accepted15January2013.
Published online20February2013.
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Supplementary Information is available in the online version of the paper. Acknowledgements We thank X.Chen,A.Bleeker,X.Ju,J.Shen and R.Jiang for their comments on an earlier version of the manuscript or assistance during the manuscript revision,and we thank H.Liu,J.Lu¨,F.Chen,L.Wu and
S.Qiu for providing data from long-term experiments.The authors also acknowledge all those who provided local assistance or technical help to the CAU-organized Deposition Network.This work was financially supported by the Chinese National Basic Research Program
(2009CB118606),an Innovative Group Grant from the NSFC(31121062,41071151, 40973054)and the Sino-German Research Training Group(GK1070).
Author Contributions X.L.and F.Z.designed the research.X.L.,Y.Z.,W.H.,A.T.,J.S.and ducted the research(collected the data sets and analysed the data).X.L.,Y.Z. and P.V.wrote the manuscript.J.W.E.,K.G.,P.C.,A.F.and F.Zmented on the manuscript.
Author Information Reprints and permissions information is available at
www.nature/reprints.The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper.Correspondence and requests for materials should be addressed to F.Z.(zhangfs@cau.edu).
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METHODS
Data sources for bulk N deposition.Bulk N deposition data are from two sources:monitoring results from a regional atmospheric deposition monitoring network(that is,the China Agricultural University-organized Deposition Network(CAUDN));and results published during the period1980–2010.Only bulk deposition data for inorganic N(NH4-N and NO3-N)were summarized in this study because no dry deposition data for N were reported in China in the 1980s and1990s2.Our data sets include year of monitoring at every site;location of every monitoring site;annual amount of precipitation;concentration and deposi-tion of NH4-N,NO3-N and total inorganic N(TIN);and ratios of NH4-N to NO3-N concentration and deposition in precipitation.Some sites that contained only portions of deposition data(that is,only concentration or deposition of inorganic N)were also included in our data sets.Briefly,bulk N deposition samples from the CAUDN were collected using always-open rain gauges(different from wet-only samplers)on a daily basis and measured by colorimetry(that is,continuous flow analysis)or ion chromatography.For literature deposition data,these are the two most common methods for measuring inorganic N(NH4-N and NO3-N)con-centrations in precipitation(for details,see references in Supplementary Table2). Bulk deposition rates of NH4-N,NO3-N and TIN were then calculated by mul-tiplying N concentration in precipitation by the amount of precipitation19.
In this paper,we summarize866data points of N concentrations in rainwater and671data points of bulk N deposition rates from between1980and2010.All of the data originated from publications and our own results from the CAUDN.In all,we collected315references on annual N concentration and deposition results (including276journal articles,19dissertations and20monographs)(Supplemen-tary Table2)in this data set,covering270monitoring sites widely distributed in China(Supplementary Fig.1).The current bulk N deposition data sets are the most complete deposition data sets in China in spite of some minor weaknesses (that is,relatively fewer monitoring sites and data points in northeast China, northwest China and the Tibetan plateau).Therefore,our meta-analysis based on the data set should be reliable.The same results published in different sources (that is,journals,dissertations or monographs)were cited only once and only one reference source was listed in the following priority:English-language journals, Chinese-language journals,dissertations and monographs.
To clarify regional variations,bulk N deposition data were also summarized on a regional basis(Supplementary Fig.1):northern China,comprising Beijing, Tianjin,Hebei,Henan,Shandong,Shanxi and Shaanxi provinces;southeast China,comprising Shanghai,Jiangsu,Zhejiang,Anhui,Hubei,Hunan,Jiangxi, Fujian,Guangdong,Hong Kong,Macau,Taiwan and Hainan provinces;south-west China,comprising Sichuan,Chongqing,Guizho
u,Yunnan and Guangxi provinces;the Tibetan plateau,comprising Tibet and Qinghai provinces;northeast China,comprising Liaoning,Jilin and Heilongjiang provinces;and northwest China,comprising Xinjiang,Inner Mongolia,Ningxia and Gansu provinces. Data sources for plant foliar N from non-agricultural vegetation types.A total of981observations of plant foliar N content and859observations of foliar P were collected from666natural and semi-natural terrestrial plant species(including woody and herbaceous species,non-N-fixing and N-fixing species,and evergreen and deciduous species,according to various classification methods)between1980 and2010at245sites distributed across the whole of China(Supplementary Fig.1), on the basis of our field measurements and the literature(for details,see references in Supplementary Table3).Leaves were sampled mainly during the growing season (July to September).Leaf samples were oven-dried,ground and then measured for N concentrations using the Kjeldahl method.To avoid systematic deviation caused by chemical determination,N samples determined with C and N elemental analysers30(after the year2000)were not included in our analysis.For the few leaf samples lacking detailed time records,the sampling year was assumed to be two years before the associated paper was first submitted(for example,the sampling year was assumed to be2004if the paper was submitted in2006).Mean foliar N was calculated for each species at the same sites within the same sampling year. Data sources for crop N uptake from zero-N croplands.A total of278data points of crop N uptak
e were collected from non-N-fertilized(zero-N fertilizer input for at least five years)croplands(described as‘zero-N plots’hereafter)during the1980s and2000s across China,on the basis of our field experiments and published data22,31–37.Nitrogen uptake by rice,wheat and maize includes N accu-mulation in grain plus straw at harvest(normally from May to October)of the three main cereal crops on zero-N plots.Grain and straw samples were oven-dried, ground and measured for N concentrations using the Kjeldahl method when the harvest process was completed in the field.Nitrogen accumulation in grain or straw was calculated as N concentration multiplied by grain or straw dry matter; crop N uptake was then the sum of grain and straw N accumulation.For a few publications that did not provide crop N uptake data,we used conversion coefficients38of grain yield for estimating N uptake by rice,wheat and maize, respectively.
Data sources for anthropogenic NH3and NO x emissions and their main con-tributors.NH3and NO x(sum of NO and NO2)emission inventories in China during1980and2010were obtained from all published data available2and updated to2010on the basis of data from the National Bureau of Statistics of China(v/english/statisticaldata/yearlydata/)and the reported NH3and NO x emission factors39;if several emission values were available in one specific year only,an averaged emission value was used.Briefly,China’s national emission inventories for NH3and
NO x were based on different emission sources and emission factors of specific N r species.Compared with the NO x emission inventory(mainly point sources),the NH3emission inventory(mainly non-point-source emission)has a relatively large uncertainty2,21.The ratios of NH3-N to NO x-N emission were then calculated on the basis of averaged annual emission data over the period1980–2010.
Data on N fertilizer use and domestic animal numbers(expressed as livestock units)were from Chinese Agriculture Statistics(1982–2010).The transforma-tion of domestic animal numbers to livestock units was based on some widely used conversion factors in Europe(uropa.eu/statistics_ explained/index.php/Glossary:LSU).Data on coal consumption(as standard coal) and motor vehicle numbers were from the National Bureau of Statistics of China (v/english/statisticaldata/yearlydata/).
Statistical analysis.Annual precipitation;N concentration in precipitation;bulk N deposition;ratios of NH4-N to NO3-N in bulk precipitation;foliar N,foliar P and crop N uptake from zero-N plots;NH3and NO x emissions and ratios of NH3-N to NO x-N emission;N fertilizer use;and numbers of domestic animals,numbers of motor vehicles and coal consumption in China were fitted(with year)by linear mixed models or nonlinear regression models for the interval1980–2010.We used mixed models40,4
1instead of simple linear regressions because of the large site-to-site variability.The selection of linear versus nonlinear regression depended on the distribution of the‘scatter diagram’(initially judging the temporal variation fol-lowed by a linear or nonlinear trend)and on the correlation coefficients(r)and P values in the linear or nonlinear regression equations41.Correlation coefficients were tested by a statistical model(SPSS13.0,SPSS Inc.).Differences between all of the above-mentioned parameters as measured in the1980s(1980–1989)and the 2000s(2000–2010)were compared statistically using an unpaired two-tail Student’s t-test.Significant difference is assumed when the P value is,0.05or as otherwise stated.
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