CHEN Zhong-du,Ll Feng-bo,XU Chun-chun,Jl Long,FENG Jin-fei,FANG Fu-ping

China National Rice Research Institute,Hangzhou 310006,P.R.China

Abstract The ecological systems services or multi-functionality of paddy rice cultivation are critical to the functioning of the Earth’s life-support system.We estimated the ecosystem services value (ESV) of paddy rice during 1980-2014 across China.The results indicated that the ESV of the paddy field in China showed an upward trend during this period.The share of ESV on CO2 sequestration was the highest,followed by ESV on temperature cooling and greenhouse gas (GHG)emission.The yield-scaled ESVs of Zones II (southern rice-upland crops rotation regions) and III (southern double rice production regions) were similar and significantly higher than the ESVs of Zones I (northeastern single rice production regions) and IV (Southwest rice-upland crops rotation regions).Between 1980 and 2014,the ESV of each region increased to varying degrees,except for the ESVs of Guangxi,Zhejiang,Fujian,and Guangdong.Such effects suggest the existence of a significant spatial-temporal variation in the total amount,structure,and density of ESV of paddy fields in China,which can further guide the development of future options for the adaptation of healthy rice production in China.

Keywords: ecosystem services value,economic evaluation,rice production,rice ecosystem,spatial-temporal variation,historical change

1.lntroduction

Rice paddies,providing nearly 26.5% of the global area for cereal grains production,play an important role in world food security,especially in Asian countries (FAOSTAT 2016).The paddy field ecosystem,which belongs to the farmland ecosystem,is an artificial-natural complex ecosystem composed of three parts: rice field biological system,environmental system,and artificial control system.In addition to having the full characteristics of the farmland ecosystem,the paddy field ecosystem also has some functions of the wetland ecosystem that provides multiple positive or negative ecosystem services,such as water quality and quantity modulation,organic waste disposal,soil formation,biological nitrogen fixation,biological diversity maintenance,biotic regulation,and contribution to global climatic regulation(Matsunoet al.2006;Yoon 2009).The contribution of these ecosystem services to human welfare may be comparable to that of food and fiber production (Yoon 2009) but is seldom recognized by the public because most of these ecosystem services are indirect (Pimentelet al.1997).The economic valuation of ecosystem service is becoming an effective way to understand the multiple benefits provided by the ecosystem.At present,three recent trends -urbanization,modernization of rice production,and abandonment of rice cultivation -have changed the functions of paddy soil.Therefore,there is a need to develop an effective way of assessing the ecosystem services value (ESV) provided by croplands that is comprehensible to the public (Lillemor and Derqvist 2002).

Recently,several studies in rice-producing countries have been conducted to identify,estimate,and value the multiple outputs from paddy rice cultivation (Lillemor and Derqvist 2002;Natuhara 2013;Westphalet al.2015).In Japan,the estimated total economic values of eight ecosystem services were 7.3×1010USD by paddy fields(Natuhara 2013).Meanwhile,the estimated floodwater storage capacity of all paddy fields in Japan was 8.1 billion m3,which by far exceeds 2.4 billion m3,the total flood detention capacity of flood control dams in Japan(Shimura 1982).A study that evaluated the ecological service functions such as purifying the atmosphere and temperature cooling in paddy fields in Korea has estimated the total ESV in paddy fields to be 1.3×1011-7.4×1011CNY (Kimet al.2006).

Existing studies have focused predominantly on the analysis of a part of ecological service functions such as gas regulation,flood control,and environmental pollution in the typical rice system or have described the trends of functions only (Xiaoet al.2005).A few studies have explored the regional differences in the ecological functions exhibited by the rice ecosystem.Studies have shown great spatial variation in the ESV of rice production because of the differences in climate,soil,and agronomy factors in different rice-growing regions (Xiaoet al.2005;Burkhardet al.2013).For example,field experiment results showed that the greenhouse gases emitted from paddy fields were significantly higher in south double rice cropping areas than in north single rice cropping areas,largely due to the high temperature during rice-growing season and double rice planting system in south areas(Yanet al.2003;Saddamet al.2015).

Rice ranks the first among the cereal crops in China,the largest rice producer in the world.China accounts for 16 and 28% of the global rice area and global rice production,respectively (FAOSTAT 2016).However,China still finds it difficult to further increase its rice production due to many constraints,such as population growth,sharp decreases in cultivated land,lack of water resources,pollution,and frequent natural disasters.Additionally,as a global rice producer,China must ensure that its rice production processes adhere to global food safety standards.

Over the last half-century,however,China’s rice production system has displayed fluctuating planted areas,increasing total production,and significant geographic shifts.Between 1949 and 2010,the annual average planted area of rice in China increased from 25.7 to 29.6 million ha and peaked at 36.2 million ha in 1976.Annual average yields increased as well,for instance,from 1 892 kg ha-1in 1949 to 6 553 kg ha-1in 2010.Due to the increased yields in various areas planted with rice,annual Chinese rice production has been stable at 170-220 million tons since 1984 (NBSC 2021).

Meanwhile,since the 1960s,China has used increasing amounts of chemical fertilizer in agriculture as its most important way to enhance soil fertility and grain yield.However,chemical fertilization has also resulted in severe environmental problems,such as eutrophication and nitrite pollution of groundwater (Juet al.2009).According to Yuet al.(2005),the integrated value per unit area of ecosystem services increased with nitrogen fertilizer dose,peaked at the fertilizer dose of 278 kg N ha-1with the average of 17 967 USD ha-1yr-1,and then decreased with higher fertilizer doses.Moreover,during the past decades,rice cultivation in China has migrated northward due to natural,social,and economic factors (Anwaret al.2013;Abrahamet al.2014).Some studies have reported that the north migration of rice cultivation worsened the shortage of agricultural water in northern regions,reduced the yield potential,and increased the transport cost of the rice grain,all of which generated adverse effects on food security (Youet al.2011;Xuet al.2013).However,most of the aforementioned studies focused on the analysis and value estimation of regional rice field ecological service function and lacked in-depth analysis of the quantitative characteristics of ESV.Accordingly,the objectives of this study were to: (1) evaluate the temporal change in the total amount of ESV for rice production;(2)assess the composition of ESV for rice production;and(3) understand the regional distribution of ESV for rice produced in the different rice regions of China.

2.Materials and methods

2.1.Study regions

Rice is mainly cultivated in four agro-eco zones in China,as shown in Fig.1,which mainly refers to the regional distribution map of rice in China (Meiet al.1988).The four regions are comprised of 18 provinces (autonomous region,municipality) based on their agro-ecological conditions (Xiao etal.2005) and are sub-divided into Zones I,II,III,and IV.Zone I includes the northeastern single rice production regions: Heilongjiang,Jilin,and Liaoning.Zone II includes the southern rice-upland crop rotation regions: Henan,Hubei,Anhui,and Jiangsu.Zone III covers the southern double rice production regions:Fujian,Guangdong,Guangxi,Hainan,Jiangxi,Zhejiang,and Hunan.Zone IV includes the Southwest rice-upland crop rotation regions: Yunnan,Sichuan,Chongqing,and Guizhou.Zones I,II,III,and IV comprise 15.9,27.0,39.5,and 14.9% of the total rice production in China,respectively,which cover the country’s main areas of rice production.This study did not include Hong Kong of China,Taiwan of China,and Macao of China due to data acquisition difficulties.The primary rice cropping system is single middle rice and double rice (early and late rice)in Zones I and III,respectively.Zones II and IV employ middle rice-upland crop rotation as their dominant rice cropping system.

Fig.1 The spatial distribution of four primary rice production zones in China.

2.2.lnventory data collection

The relative inventory database used in this study was compiled from the annual rice production data from 1980 to 2014 that were collated from national statistical reports and related literature.The agricultural inputs data of 18 provinces (autonomous region,municipality) were directly or indirectly collected from theNational Cost-Benefit Survey for Agricultural Product(1980-2014) from 1980 to 2014 in China (http://#cnki.net/kns55/Navi/HomePage.aspx?id=N2013100048&name=YZQGN&floor=1).Synthetic fertilizers consisted of N,P2O5,K2O,and compound fertilizers.Data for double rice planting areas and rice yields of different regions were obtained from the Chinese National Statistical Database (CNSD,1980-2014) (http://www.stats.gov.cn/tjsj/) during 1980-2014.Meteorological observation data,including the daily maximum temperatures,minimum temperatures,precipitation,wind speed,barometric pressure,relative humidity,net radiation,and total radiation,were also collected from the China Meteorological Administration for the same period (http://www.cma.gov.cn/2011qxfw/2011qsjgx/).Exactly 710 of the agrometeorological stations were located at the same sites as the meteorological stations,and the remaining 68 agro-meteorological stations were located near the meteorological stations.Using the interpolated daily temperature data,the daily mean temperature was estimated as the average of the daily minimum and maximum temperatures for each agrometeorological station.

2.3.Calculation of ecosystem service values

In each case,this study calculated the ESV of six functions present in rice paddies,namely CO2sequestration,O2production,temperature cooling,flood alleviation,greenhouse gas (GHG) emission,and chemical pollution during the rice-growing season.These functions were used to evaluate the positive and negative effects of the rice paddy field on the ambient environment.Estimation of total ESV of rice ecosystem was calculated using the eq.(1) below:

whereVC,Vf,VGHG,andVCPstand for the ESV on CO2sequestration,O2production,temperature cooling,flood alleviation,GHG emission,and chemical pollution,respectively.

Ecosystem service values on CO2 sequestrationThe ESV on CO2sequestration in this study was calculated by the Sweden C tax (Zhanget al.2007) by using the following eq.(2):

where,VCis the ecosystem service value on CO2sequestration by rice plants (CNY ha-1);Qis the annual net biomass of rice (Wanget al.2011);1.63 is the CO2absorbing coefficient by rice plants through photosynthesis;12/44 is the coefficient for converting CO2into C;Pcis the international CO2trade price (0.924 CNY kg-1) (Zhanget al.2007).

Ecosystem service values on O2 productionRice can release O2into the atmosphere through photosynthesis to improve the air quality during the growing season.In this study,the O2cost of the paddy field ecosystem was estimated by the cost of oxygen generation by industrial means using the following eq.(3):

Ecosystem service values on temperature coolingThe ESVs on temperature cooling from paddy fields were calculated by the replacement cost method (Liuet al.2015) shown below:

whereVETCis the ESV of temperature cooling (CNY ha-1);indicates the total amount of evapotranspiration during the high temperature in the rice-growing season.

In this study,the Penman-Monteith model recommended by FAO was used to calculate the evapotranspiration of paddy ecosystems (Sunet al.2007).is the cost of water evaporation (10.39 CNY mm-1ha-1).

Ecosystem service values on flood alleviationPaddy fields can divert and store large amounts of rainwater,play flood control,and reduce the incidence of flooding.If there are no paddy fields,the flood has to be controlled by flood control dams.This study assumed that the average bund height is 20 cm and the ponding water depth is 5 cm.The remaining 15 cm height of the bund can be used for rainfall storage.Therefore,the ESVs on flood alleviation were calculated using the shadow engineering method (Liuet al.2015) shown below:

where,Vfis the ecosystem service value of flood alleviation (CNY ha-1);indicates the total amount of water retained by the paddy field (mm ha-1);Pfis the cost of the reservoir engineering (1.52 CNY m-3).

Ecosystem service values on GHG emissionPaddy fields are found to be a major source of GHG emissions among many agricultural systems.The GHG emissions of rice include agricultural inputs and non-CO2GHGs emissions (i.e.,CH4and N2O) from paddy fields.In this study,agricultural inputs include irrigation,tillage,and harvest practices that consume diesel fuel,fertilizers,pesticides,seed,and other special materials used in the paddy soil (e.g.,film,tray).The GHG emissions from paddy soil were computed using the Intergovernmental Panel on Climate Change (IPCC) method (2006).The economic effect of the paddy field for GHG emission was computed using the Sweden C tax method (Zhanget al.2007) shown below:

where,VGHGis the ESV of GHG emission (CNY ha-1);TGHGis the global warming potential of all GHG emissions (kg CO2-eq ha-1);Pcis the international CO2trade price (CNY kg-1);ECH4is the CH4emission per unit area of paddy field(kg ha-1);EN2Orepresents the N2O emission from the unit nitrogen applied to the paddy field (kg ha-1);InandCnrefer to agricultural input and its GHG emissions coefficient,respectively.The conversion coefficients of CO2equivalent for most of the inputs were from the Chinese Life Cycle Database (CLCD v0.7,IKE Environmental Technology CO.,Ltd.,China).

Ecosystem service values on chemical pollutionThe extensive use of chemical fertilizers and pesticides during the rice-growing season pollutes the water bodies and the soil environment.The ESVs on chemical pollution are estimated using the environmental cost method.The economic valuation equation is defined as follows:

whereVcpis the ecosystem service value of chemical pollution (CNY kg-1);Vpris the value of the chemical pollution caused by the production of pesticide (CNY kg-1);Veurepresents the fishery loss (CNY kg-1) caused by eutrophication;Vniis the value of the nitrite pollution of drinking water (CNY kg-1);Vfameans the health loss of farmers (CNY kg-1) in the application of pesticides;Vbirepresents the loss of biodiversity resulting from the use of pesticides (CNY kg-1).

3.Results

3.1.Change of the total amount of ecosystem service value of paddy field

The total ESV in the paddy fields of China increased from 1.74×1012CNY in 1980 to 2.37×1012CNY in 2014 (Fig.2).The ESV in Zone III accounted for 48% of the total ESV,and it was significantly higher than the ESVs in other regions.The ESV in Zone III showed an increasing trend from 2002 after a slight decrease during 1980-2002.From 1980 to the present,the ESV in Zone II showed an upward trend and reached the highest value in 2013(9.57×1011CNY).

Notably,the total ESVs in Zones IV and I were relatively lower than the ESVs in other rice production regions.Since 1980,the total ESV in Zone I showed an upward trend,increasing by 637.6% as compared to that in 1980.The total ESV in Zone IV was stable and low,at an average of about 2.80×1011CNY during 1980-2014.From the different types of rice maturity,ESV of early and middle rice were similar in the 1980s,accounting for 36.0 and 39.6%,respectively,which were higher than the ESV of late rice (24.3%) (Table 1).However,since the 1980s,the ESV of middle rice has risen sharply,while that of early rice has dropped significantly (Fig.2).

Table 1 Change of the percentages of six ecosystem service values (ESV) for four rice cultivation regions and three rice types compared between 1980-1984 and 2010-2014 (%)

Fig.2 Change in the total amount of ecosystem service value (ESV) of paddy field in four main rice production regions (A) and three rice types (B) during 1980-2014 in China.

3.2.The structure of ecological service value in paddy field

The ESV on CO2sequestration was the most significant fraction of the total ESV,then followed by the ESVs on temperature cooling (36.61%) and GHG emission(30.98%) (Fig.3).The ESV on O2production,flood alleviation,and chemical pollution was relatively lower,only accounting for 6.52,10.02,and 1.08%,respectively.Since 1980,the share of ESV on CO2sequestration had declined from 80.5% (1980-1984) to 77.1% (2010-2014),while that on temperature cooling had increased from 35.9% (1980-1984) to 37.1% (2010-2014).The percentage of ESV on flood alleviation and GHG emission showed a downward trend,from 12.9% and-34.9% (1980-1984) to 8.6% and -28.1% (2010-2014)(Fig.3).The proportion of ESV on CO2sequestration in Zones IV and I showed a decreasing trend,while Zones II and III exhibited an increasing trend.The ESV on chemical pollution showed the opposite regularity compared to ESV on CO2sequestration in each rice production region.From the different rice maturity types,the proportion of ESV on CO2sequestration,O2production,and flood alleviation in early,middle,and late rice showed a decreasing trend,while the opposite trend appeared in the other three ESVs.

Fig.3 Change in the percentages of six ecosystem service values (ESV) in China during 1980-2014 in China.GHG,greenhouse gas.

3.3.Ecological service intensity in paddy field

The average area-scaled ESV in China was 9.57 CNY m-2,showing an increasing trend over three decades,reaching the highest value in 2013,which was 40.5%higher than that in 1980 (Fig.4).The area-scaled ESV in Zone III was higher than that in other regions,with an average of 12.87 CNY m-2.The area-scaled ESVs in Zones IV and I were similar and low.Overall,during 1980-2014,area-scaled ESV in all the rice production regions increased,with the maximum increase rate in Zone II,where the average in 2010-2014 was higher by 39.8% compared to that in 1980-1984 (Fig.5).The increasing ranges of the other three regions were similar in value,20.3% (Zone I),19.5% (Zone III),and 19.5%(Zone IV).The yield-scaled ESV in China was stable,maintained at 12.22-13.67 CNY kg-1during 1980-2014.The yield-scaled ESV in Zone III was significantly higher than that in Zone II in the 1980s,but there was no significant difference since the 1990s.The yield-scaled ESVs in Zones IV and I were similar,significantly lower than the values in Zones III and II.The area-scaled ESV of early and middle rice (6.53-9.24 CNY m-2) showed no significant difference,yet both were higher than that of late rice (4.49-7.26 CNY m-2).The yield-scaled ESV of early rice (13.99-16.30 CNY m-2) was significantly higher than those of middle (11.81-12.21 CNY m-2) and late rice (10.76-12.54 CNY m-2) (Fig.5).From the 2000s onwards,no significant difference was shown between middle rice and late rice (Fig.5).

Fig.4 Change in area-scaled ecosystem services value (ESV) (A) and yield-scaled ESV (B) during 1980-2014 in China.

Fig.5 Change in area-scaled and yield-scaled ecosystem services values (ESVs) for four rice cultivation regions and three rice types during 1980-2014 in China.A and B,the area-scaled ESVs for four rice cultivation regions and three rice types,respectively.C and D,the yield-scaled ESVs for for four rice cultivation regions and three rice types,respectively.Bars are SE.

3.4.Spatial distribution change of ecological service value of paddy field

In Zone I,the area-scaled ESV was the highest for Jilin compared with the other regions,valued at 7.19 CNY m-2(Fig.6).The area-scaled ESVs of Heilongjiang and Liaoning were similar and low,valued at 6.60 and 6.93 CNY m-2,respectively.In Zone II,the area-scaled ESV in Hubei reached 14.52 CNY m-2and was higher than that in Anhui,Jiangsu,and Henan.In Zone III,the area-scaled ESV in different regions followed the order:Guangdong＞Guangxi＞Jiangxi＞Hainan＞Zhejiang＞Fujian.In addition to Fujian,the area-scaled ESV in other regions showed an increasing trend each year.In Zone IV,the area-scaled and yield-scaled ESVs peaked in Chongqing,which were much greater than those in other regions.Taking the rice yields into account,on average,the yield-scaled ESVs in Jilin,Heilongjiang,and Liaoning in Zone I were of similar values.The yieldscaled ESV was higher in Hubei,Anhui,and Henan thanin Jiangsu in Zone II and showed similar data among other regions.In Zone III,the yield-scaled ESV in different regions followed the order: Hainan＞Guangxi＞Jiangxi＞Zhejiang＞Guangdong＞Fujian＞Hunan.In Zone IV,the yield-scaled ESV in Chongqing was the highest.

Fig.6 Spatial distribution of total ecosystem services value (ESV) (A),area-scaled ESV (B),and yield-scaled ESV (C) during 2010-2014 at four primary rice production zones across China.

4.Discussion

4.1.Ecological service value in rice-cropping systems

The role of ecological service functions of paddy field systems is now receiving increasing attention in regional sustainable development (Pearce 1995;Kimet al.2006).In the last decade,studies in China that quantitatively evaluate the regional or national ESV of the rice ecosystem have been increasing (Xiaoet al.2011;Chenet al.2015;Fanget al.2017).In our study,the total annual amount of ESV (2.3×1012CNY) during 1980-2014 in the rice cropping system of China was slightly lower than the estimate of an earlier study (Xiaoet al.2005).Meanwhile,another study’s estimate was comparable to our study,where the total monetary value of ecological functions provided by the Chinese paddy ecosystem was 1.9×1012CNY in 2003 (Sunet al.2007).Discrepancies in ESVs may be explained by different procedures and methods used in these studies.Most scholars have adopted the ecosystem service value classification framework proposed by Pearceet al.(1995)that divides the ecosystem services into the use-value(direct use value,indirect use value,and option value)and non-use value (existence value and bequest value);few scholars have considered the negative effects of rice ecosystem’s functions on the ecosystem service value classification system,such as cost value.What is more,the different farmland ecological services are generally associated with crop growth curves due to the changes in crop biomass and the environmental factors over time.

Our results indicated that CO2sequestration from paddy soil was the biggest contributor to total ESV,followed by temperature cooling and GHG emission.Xiaoet al.(2005) also pointed out that the variation range of CO2sequestration in paddy soil was 6 100-11 790 kg C ha-1,which was an important part of the ESV of paddy soil,with an economic value of 1.7×1011CNY.It has been suggested that when the rice field is flooded,the anaerobic condition discourages soil organic carbon (SOC) decomposition and favors SOC accumulation.The long flooding period in the rice system may result in a high sequestration rate (Chenet al.2021).Pan (2008) also indicated that paddy soil not only has high SOC,but also has great CO2sequestration potential.Many scholars have estimated the SOC density and quantity of paddy soil in China based on the second National Soil Census.Pan (2008) showed that the density of SOC in paddy soils in China ranged from 12-226 t C ha-1,and the storage of SOC in the surface layer was 1.3 Pg.Xieet al.(2007) estimated that the SOC density of surface paddy soil in China was 97.6 t C ha-1on average from the 1980s to 2000s,which was higher than that of dryland soil by 13%.If all paddy fields were converted to dry land,carbon would be saved by 3.4×1011kg.The temperature cooling effect of water surfaces is well known (Oke 1987).The solar energy absorbed near the ground evaporates water from the vegetation and soil.The paddy field is filled with water for a long time,which is always involved in the circulation of atmospheric water,among which evapotranspiration can drive the change of temperature(Dhageet al.2017).Especially in the hot summer,due to the water evaporation and foliar transpiration on the surface of rice fields,water will turn into water vapor into the air and increase the humidity of the air.

4.2.Differences in area-and yield-scaled ESVs

Area-and yield-scaled ESVs are two important references for the ecological compensation of the rice ecosystem (Fanget al.2017).The result presented in this study showed that the area-scaled ESV of Zone III was significantly higher than that in the other three rice regions,with Zone II in the second and no significant difference between Zones I and IV.The high ESV was mainly due to the relatively higher rice yield in Zones III and II.Additionally,the values of carbon sequestration,O2production,and temperature control in these two rice regions were significantly higher than those in the other rice production regions.Moreover,the north migration of China’s rice production is one of the key reasons for the spatial and temporal variability in the ESV of the rice cropping system (Anwaret al.2013;Abrahamet al.2014).“North migration” reduced not only the value of positive functions but also the negative functions.Fanget al.(2017) indicated that “north migration” reduced the nationally area-scaled ESV by 0.9-14.2% compared with the “no migration” scenario.Based on our study results,the values of CO2reduction and O2production were mainly determined by rice production.The yield of middle rice was higher in southern regions than in northern regions.Moreover,the double rice cropping system in southern China gained more harvest index and higher yield than the single middle rice cropping system in northern China.Therefore,this study found that the ESV of CO2reduction and O2production in south regions was higher than those in northern regions.From different maturity types of rice,early and middle rice have higher area-scaled ESV than late rice,which might be due to the differences in the GHG emissions from paddy fields among the different rice production regions.A large amount of straw residues in the rice fields after the harvest of early rice can decompose faster under the relatively high temperature (but below 32°C)during the late rice-growing season,generating many organic compounds and leading to an increase in CH4emissions (Shenget al.2008).The spatial and temporal variability in ESV of paddy fields ecosystems may be greater due to meteorological factors,soil properties,and cultivation measures (Xiaoet al.2005;Fanget al.2017).For example,the reduction in the values of flood mitigation and temperature cooling was mainly because of higher precipitation and temperature in the southern regions than in the northern regions during the ricegrowing season.Previous studies have reported that the emission coefficient of GHG (CH4and N2O) was in this order: Zone I＜Zone II＜Zone III＜Zone IV (Yanet al.2003).Thus,the ESV on GHG emissions in the north region was significantly lower than that in the south region.

4.3.Potential limitations of this study

The ecological service function of the rice paddy system plays an important role in safeguarding regional ecological balance.How to quantitatively evaluate the value of ecological service function in the rice field is a complex issue in the research on rice cropping systems.Though more than 10 positive and negative benefits of rice paddy have been identified in previous studies,the quantitative evaluation of ESVs in paddy fields has not been well established in the research on the ecological economy of paddy field ecosystem.Several studies have used empirical formulas and statistical data to assess the value of various ecological service functions (Liet al.2011;Chenet al.2015;Fanget al.2017),while a few studies have calculated the ESV of paddy fields ecosystem based on field survey and experiment data (Xiaoet al.2005;Yinet al.2008).In this study,the value of GHG emission was calculated using the integrated results of field-measured CH4and N2O emissions during 1980-2014 in China,and the value of temperature cooling and flood mitigation was estimated by using the daily meteorological data during the rice-growing season.At present,many scholars have diverse views on whether to consider the energy consumption caused by labor work when calculating the GHG of the farmland ecosystem.West and Marland(2002) believe that normal respiration occurs whether the laborer works or not,so the energy consumption of the laborer is not considered in the calculation.Energy from human labor was not considered in the study despite the presence of considerable human labor involved in the current agricultural management conditions of paddy rice production.As agricultural mechanization accelerates in China,more energy will be derived from diesel oil rather than human labor.However,the value of CO2production,O2production,and chemical pollution was only calculated by using annual statistical data.The field experiment conducted to measure these benefits remains limited.As such,more work is needed to investigate the methodologies to precisely estimate the non-production benefits of the rice ecosystem in future studies.

5.Conclusion

This study discovered a significant spatial-temporal variation in the total amount,structure,and intensity of ESV of paddy fields in China during 1980-2014.The study results showed that the total ESVs in paddy fields presented an increasing trend in the peroid.The total amount of ESV in 2014 was 2.37×1012CNY,which was 36.5% higher than that in 1980.Furthermore,the percentage of CO2sequestration was the highest among the six ecosystem service functions in the rice ecosystem,followed by temperature cooling and GHG emission.The area-scaled ESV increased significantly during 1980-2014,while the yield-scaled ESV did not present an obvious increasing trend.The total ESV and area-scaled ESV were the highest in Zone IV among all regions.The yield-scaled ESVs of Zones IV and II were similar,and both were significantly higher than those in Zones I and III.The percentage of CO2sequestration in Zones I and III increased during 1980-2014,while those in Zones II and IV decreased.Therefore,these results enhanced our understanding of the effects of temporal and spatial changes on paddy rice ecosystem services in China.The findings offer evidence for exploring appropriate agricultural strategies and policies that can be adapted to different regional environments in the paddy soil and in other countries with a similar ecology.

Acknowledgements

This study was supported by the Natural Science Foundation of Zhejiang,China (Q21C130007) and the National Key Research and Development Program of China (2016YFD0300210).We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Declaration of competing interest

The authors declare that they have no conflict of interest.