Spatial And Temporal Variation Of Net Primary Productivity Of The Sanjiang Plain And Its Influencing Factors

As one of the most important aspects of the global carbon cycle,the terrestrial carbon cycle is the most complex and involves many problems.As the most likely source of carbon / sink,terrestrial ecosystems have become a hotspot in the research field,and it is also one of the most uncertain ecosystems in the current study.Net primary productivity is an important feature of ecosystem sustainability and ecosystem function,and the study of net primary productivity and its response to global change plays a pivotal role in the study of the whole terrestrial ecosystem carbon cycle.The Sanjiang Plain includes the largest freshwater wetlands in China.Located in the middlehigh latitudes,the Sanjiang Plain is very sensitive to global climate change.At the same time,the Sanjiang Plain was seriously disturbed by human activities.Since the 1950 s,a large number of marsh wetlands have been reclaimed as cropland.Revealing and quantitatively analyzing the impact of different factors on terrestrial ecosystem carbon cycling in the region not only provides a typical case for regional-scale global change research,but also has important guiding value for understanding the carbon cycle process and the formulation of relevant policies.Taking the Sanjiang plain as the study area,this paper developed data sets including the land use / cover data,the climate changes?such as temperature,precipitation?,atmospheric environment changes?CO2 concentration,N deposition?and land management data?N fertilization,irrigation data,etc.?.Additionally,these data were developed as the input data sets of the Dynamic Land Ecosystem Model.Based on the analysis of the changes of environmental factors,the DLEM model was used to simulate the temporal and spatial changes of net primary productivity of terrestrial ecosystem in the Sanjiang Plain under global climate changes and human activities.At the same time,the effects of various factors on the net primary productivity changes in the Sanjiang Plain were analyzed quantitatively by single factor experiments.The main conclusions of this paper are as follows:?1?Under the background of global warming,the average temperature showed an increasing trend from 1954 to 2014 in the Sanjiang Plain.The maximum temperature appears in 2007?4.78 ??,and the lowest temperature appears in 1969?1.03 ??.The average annual minimum temperature growth rate is much higher than the average annual growth rate of the maximum temperature,especially since the 1980 s.The growth of average annual minimum temperature in the Sanjiang Plain in the past 60 years mainly contributed by the growth of the average annual growth.During the period from 1954 to 2014,the average annual precipitation in the Sanjiang Plain was 577.41 mm per year.The maximum annual precipitation can reach to 810.52 mm?1994?and the minimum precipitation was 396.94 mm?1976?.?2?Over the past 60 years,the Sanjiang Plain experienced serious land use changes.Paddy fields,dry farmland and settlement area increased quickly at the expense of forest land,grassland and marsh.During the past 60 years,dry farmland had the largest increase rate,followed by paddy field while marsh and grassland decreased significantly by 25.9% and 9.9%,respectively.The decline of ecosystem service value?ESV?in the study area is severe and the downward trend slowed in the 1980 s.The sharp drop in the size of the marsh is one of the main reasons for the decline in ecosystem service values.The analysis of ESV and its ESV structural changes shows that in addition to protection of forest and marsh,decision makers should also pay attention to the protection and restoration of grassland in the Sanjiang Plain to achieve sustainable development.?3?Based on the site data,this study calibrated the parameters of DLEM and simulated the spatial and temporal changes of the net primary productivity since the 1950 s in the Sanjiang Plain.During the study period,net primary productivity of the Sanjiang Plain increased from 43.8 Tg C?yr^-1 in the 1950 s to 48.0 Tg C?yr^-1 in the 2010 s.In addition,we analyzed the carbon cycle of the marsh ecosystem in the Sanjiang Plain.During the study period,NPP increased from 28.7 Tg C?yr^-1 in the 1950 s to 30.8 Tg C?yr^-1 in the 2010 s.?4?By setting the "single factor" experiments,this paper quantitatively analyzes the impact of environmental factors on the carbon cycle of the Sanjiang Plain in the past 60 years.The results show that the climatic factors made the largest contribution to the growth of NPP in the Sanjiang Plain over the last 60 years,followed by the change of N deposition and CO2 concentration.At the same time,this paper also analyze the contribution rate of each factor in different periods.The results showed that in the 1950 s,1980s,1990 s,2000s and 2010 s,climate change made the greatest contribution to NPP growth,while N deposition made the largest contribution in the 1960 s and 1970 s.During the 1960s-2010 s,the land use / cover change made a negative contribution for the NPP change in the Sanjiang Plain.?5?In the past 60 years,the N deposition of the Sanjiang Plain showed an increasing trend,and the growth rate of NHx_N is much higher than that of Noy_N.The growth of nitrogen deposition in the Sanjiang Plain is mainly due to the growth of NHx_N.At the same time,the amount of nitrogen fertilizer in the Sanjiang Plain showed increased trend over the past 60 years.During the period from 1954 to 2014,the increase of nitrogen deposition and nitrogen fertilization in the Sanjiang Plain promoted the growth of net primary productivity in the study area.The CO2 concentration in the study area also showed increasing trend and has contributed to the growth of NPP in the study area over the past 60 years.