X. YAN, Tokyo University of Agriculture and Technology Graduate School of Agriculture, Department of International Environmental and Agricultural Science, Fuchu 183-8509, Japan. Email: email@example.com
To evaluate the atmospheric load of reactive gaseous nitrogen in the fast-developing Eastern China region, we compiled inventories of nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions from a typical rural catchment in Jiangsu province, China, situated at the lower reach of the Yangtze River. We considered emissions from synthetic N fertilizer, human and livestock excreta, decomposition of crop residue returned to cropland and residue burning, soil background and household energy consumption. The results showed that, for the 45.5 km2 catchment, the annual reactive gaseous emission was 279 ton N, of which 7% was N2O, 16% was NOx and 77% was NH3. Synthetic N fertilizer application was the dominant source of N2O and NH3 emissions and crop residue burning was the dominant source of NOx emission. Sixty-seven percent of the total reactive gaseous N was emitted from croplands, but on a per unit area basis, NOx and NH3 emissions in residential areas were higher than in croplands, probably as a result of household crop residue burning and extensive human and livestock excreta management systems. Emission per capita was estimated to be 18.2 kg N year−1 in the rural catchment, and emission per unit area was 56.9 kg N ha−1year−1 for NH3 + NOx, which supports the observed high atmospheric N deposition in the catchment. Apparently, efficient use of N fertilizer and biological utilization of crop straw are important measures to reduce reactive gases emissions in this rural catchment.
Modern agricultural activities have a great impact on the global atmospheric environment. It has been estimated that approximately 81% of nitrous oxide (N2O), 35% of nitrogen oxides (NOx, NO + NO2) and 85% of ammonia (NH3) originate from agricultural activities (Krafenbauer and Wriessning 1995). N2O is a potent greenhouse gas and also participates in the destruction of stratospheric ozone (Crutzen 1970). NOx contributes to the build-up of tropospheric ozone and is a precursor of nitric acid for acid rain (Crutzen 1979). NH3 is the only natural alkaline gas in the air and it influences the pH of aerosols, cloudwater and rainfall. Moreover, NH3 is an important contributor to ammoniated aerosols (Brasseur et al. 1999). Excessive reactive N may result in eutrophication of water bodies, soil acidification and changes in biodiversity (Galloway and Cowling 2002).
The lower reach of the Yangtze River, located in Eastern China, has long been one of the most densely populated agricultural regions and is legendary in China for its many centuries of productive rice-based agriculture (Wu et al. 2009). It has also been a fast-developing economic region over the past three decades. With the increased standard of living in rural areas, farmers have had increasing access to chemical fertilizer and high-quality fuel (e.g. LPG, coal). Crop residue that was traditionally used as organic fertilizer is now largely burned in the fields. Anthropogenic reactive N has far exceeded the bio-fixed N in natural terrestrial ecosystems in this region (Xing and Zhu 2002). In addition, this area is considered to be a hot spot area of N deposition (Galloway et al. 2004). Therefore, environmental problems such as air and water pollution are becoming a major concern.
Emission inventories provide crucial information for designing effective emission control strategies. Over the past two decades, concerns about N2O, NOx and NH3 emissions have been raised in China. Xing and Yan (1999) and Lu et al. (2006) used different methods to estimate N2O emission from agricultural fields. Li and Lin (2000) compared the emissions of N2O, NOx and NH3 from fuel combustion, industrial processes and agricultural sectors. Yan et al. (2003) estimated the emissions of N2O, NOx and NH3 from croplands in East, South-East and South Asia. Most of these earlier studies, however, have focused on the emission of one or more gases at regional or national levels. In such large-scale estimations, activity data are usually obtained from official statistics, which have often been questioned because of administrative interference with the generation of the statistical data. Inventories compiled using first-hand activity data for a typical area may provide more insight into the ongoing situation. In addition, emission estimates made on a per capita or per unit area basis may be more suitable for comparison among different regions or systems.
The purpose of the present study was to: (1) understand the contribution of anthropogenic activity to the reactive N emissions in a typical rural catchment in Eastern China, (2) identify major contributors to the reactive N emissions, (3) quantify the per capita and per unit reactive gaseous emissions. In contrast to previous inventory studies, the present study was supported by household investigations and experimental monitoring.
Materials and methods
The study catchment is located in Jurong, 40 km south-east of Nanjing city, Jiangsu province, China (32°01′N, 119°13′E), with an annual mean temperature of 15°C and annual mean precipitation of 1,050 mm (monitored results for the year 2007–2008 by the automatic weather station [Vantage Pro Plus; Davis Instrument Corporation, San Francisco, CA, USA] installed in the catchment). The total area of the catchment is 45.5 km2, of which 31.2% is paddy fields, 22.8% is uplands, 9.3% is tea garden and forest land, 27.5% is construction and roads, and the remainder is covered by water bodies. The total residential population is 18,092 (data from local statistical office). It is a typical rice-based agricultural catchment. The major soil type is paddy soil developed from Quaternary losses. Rice, cotton, maize and soybean are planted in the summer season, and wheat and oil rape are planted in the winter season. Rice–wheat and cotton–oil rape are the major annual cropping rotations in the paddy fields and upland fields, respectively. In conventional farming practice, synthetic compound fertilizer and urea are the major N nutrient sources for the croplands, and are usually applied at a rate of 500–600 kg N ha−1 year−1 to summer and winter crops.
There is no industry or intensive livestock farming in the catchment. A small number of livestock are raised for self-consumption. Agricultural activity is the dominant source of N contamination.
Source of the agricultural activity data
Nitrogen-related agricultural activity data were obtained through household investigation. A questionnaire table was designed to include items such as cultivation and type of crops, fertilization rate, crop yield, fate of crop residue, livestock population, and management of human and animal excreta. Approximately 600 households in the catchment were investigated in 2007 and 2008.
Household fossil fuel consumption was not surveyed in the catchment. Instead, we adopted the per capita consumption of fossil fuel energy obtained through a survey in a neighboring county that has a similar economic development level to the catchment (Wang et al. 2002).
Methodology for estimating N 2 O, NO x and NH 3 emissions
Emission of N2O was estimated principally by following the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Emission Inventories (hereafter referred to as the 2006 IPCC Guidelines). The emission sources considered include soil background emissions, synthetic N fertilizers (FSN, kg N), managed human and livestock excreta (FMON, kg N), organic N applied as fertilizer (FON, kg N), N from crop residue returned to croplands (above-ground and below-ground) (FCR, kg N), crop residue burning in situ (FRB, kg N), household residue consumption (FHRB, kg N) for cooking and animal feeding, as well as household fossil fuel energy consumption (FHE, TJ) for cooking and animal feeding. Biological N fixation was removed as a direct source of N2O emission in the 2006 IPCC Guidelines because of the lack of evidence of significant emissions arising from the fixation process itself (Rochette and Janzen 2005). There is no organic soil (FOS) or significant land-use changes (FSOM) in the study catchment and thus FOS and FSOM were not considered in the present study. The number of grazing animals in the catchment is negligible, thus urine and dung N deposited on pasture, range and paddock by grazing animals were not counted. As a result, the total N2O emission from the catchment in a year was estimated as:
where EFFIELD (kg N2O-N kg−1 N) is the N2O emission factor for N applied to soils (synthetic nitrogen fertilizer, organic nitrogen fertilizer and crop residue N returned to soil), EFMON, EFRB and EFHRB (kg N2O-N kg−1 N) are the N2O emission factors for N in managed human and livestock excreta, N in crop residue burned in situ and N in crop residue burned as household fuel, respectively, EFHE is the N2O emission factor for household fossil fuel energy consumption (kg N2O-N TJ−1) and EFBK is the background emission factor (kg N2O-N ha−1) on an area basis for croplands. Area is the total area of croplands in the catchment (ha).
The total consumption of synthetic nitrogen fertilizer (FSN) was calculated from the N application rates and areas of different crops, both were obtained through household surveys. The values are shown in Table 1.
Table 1. Parameters for estimating nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions from synthetic N fertilizer and N in crop residue
Total area (ha)†
Synthetic N fertilizer application rate (kg N ha−1)†
Yield (kg ha−1)†
Above-ground residue/seed ratio
Below-ground residue/above-ground residue ratio‡
Nitrogen content in above-ground residue (kg N kg biomass−1)
N content in below-ground residue (kg N kg biomass−1)§
The amounts of managed human and livestock excreta (FMON) and organic N applied as fertilizer (FON) were calculated as follows:
where N(T) is the number of head of livestock species T in the catchment; Nex(T) is the annual average N excretion (kg N animal−1year−1); FraclossMS is the fraction of managed manure N loss in the manure management system (%). In the 2006 IPCC Guidelines, human excreta were not included in the calculation of FMON and FON. However, human excreta have long been used as organic fertilizer in China. It was estimated that 33% of human excreta was used as organic fertilizer (Xing and Yan 1999). Therefore, we included human excreta as a source of organic fertilizer. The parameters required for calculating FMON are shown in Table 2.
Table 2. Parameters for estimating nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions from human and livestock excreta
Human or animal
Nitrogen excreta rate (kg N year−1 head−1 of human or animal)‡
†Investigated results from the present study. ‡Values from Xing and Yan (1999). §FraclossMS is the amount of managed manure nitrogen loss in the different manure management systems; values from the Intergovernmental Panel on Climate Change (2006). ¶This number is the corrected adult population from the permanent population using a factor of 0.85 reported by Zhu (1997).
FCR, FRB and FHRB were estimated from the total amount of above-ground crop residue N and its fate (the proportion of crop residue that is returned to the soil, burned in situ and burned as fuel). The below-ground crop residue was also included in the FCR as recommended by the 2006 IPCC Guidelines. The total amount of above-ground crop residue N was estimated from the crop yield, the above-ground residue/yield ratio and the N content in the above-ground crop residue. The below-ground crop residue N was estimated from the crop yield, the above-ground residue/yield ratio, the below-ground residue/above-ground residue ratio and the N content in the below-ground residue (IPCC 2006). For FRB, the total amount of crop residue N burned in situ should be multiplied by the corresponding combustion factor of each crop owing to incomplete burning in the field. Residue/yield ratios and N contents in crop residues were measured for the major crops (rice, maize, wheat and oil rape), with 10 samples for each crop in the catchment. The parameters for the other crops were adopted from the literature. The data used in the calculation are shown in Table 1.
The FHE was calculated according to the permanent population and the annual consumption of fossil fuel energy per capita (852 MJ coal and 505 MJ LPG per capita) adopted from the report of Wang et al. (2002).
Because NOx and NH3 emissions share the same sources as N2O, the estimation method was the same as that for N2O.
The N2O emission factor from rice paddy was adopted from Zou et al. (2010). The N2O emission factor from upland was derived from equations in Lu et al. (2006) to take into account the effect of fertilization rate and precipitation. The NOx emission factors from all soils were adopted from Yan et al. (2003). Because NH3 emission often accounts for a significant proportion of applied synthetic fertilizer N, we measured NH3 emissions from synthetic N fertilizer applied to the four major crops in the catchment (rice, maize, wheat and oil rape) using the method reported by Dong et al. (2006). For each of the four crops, emission factors were measured on three farmlands for two crop seasons (2007–2009), replicated three times in every farmland. The applied N fertilizers were urea and compound fertilizer; at 210–310 kg N ha−1 for rice and 75–300 kg N ha−1 for upland crops. Average results are shown in (Fig. 1). For NH3 emissions from synthetic fertilizer applied to the other summer crops (soybean and cotton), we used the average emission factor measured from maize land over 2 years. Annual average NH3 emission factor from maize and oil rape lands was used for synthetic fertilizer applied to the tea land. The NOx and NH3 emission factors from biomass burning were reported to be related to the residue N content (Delmas et al. 1997). We corrected the emission factors of NOx and NH3 from Andreae and Merlet (2001) using the measured N contents in the crop residue. Other emission factors for various sources are shown in Table 3.
Table 3. Emission factors of nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) from different sources used in the estimation
N2O (kg N2O-N kg−1 N)
NOxa (kg NOx-N kg−1 N)
NH3 (kg NH3-N kg−1 N)
aThe amount of NOx emission was calculated as NO-N. bValue from Zou et al. (2010). cValues from Yan et al. (2003). dThis NH3 emission factor was obtained from the combined emission factors of NOx and NH3 reported in the Intergovernmental Panel on Climate Change (2006) Guidelines minus the NO emission factor from the corresponding sources reported by Yan et al. (2003). eValues from Lu et al. (2006). fExperimental results from the present study. gValues from Intergovernmental Panel on Climate Change (2006) Guidelines. hThis factor was the corrected factor according to the N content in the crop residue from the reported emission factor of Andreae and Merlet (2001). iValues from Geadah (1985).
Synthetic N fertilizer applied to rice paddies
0.17 in maize landsf
Synthetic N fertilizer applied to uplands
0.07 in rape landsf
0.03 in wheat landsf
Crop residue applied to rice paddies
Crop residue applied to uplands
Human, swine, poultry and non-dairy cattle excreta applied to rice paddies
Human, swine, poultry and non-dairy cattle excreta applied to uplands
Sheep excreta applied to rice paddies
Sheep excreta applied to uplands
Managed excreta of human and swine
Managed excreta of poultry
Managed excreta of sheep
Managed excreta of non-dairy cattle
Residue burning in situ and household residue burning
Household coal consumption (kg N T J−1)
Household liquefied petroleum gas consumption (kg N T J−1)
Soil background (kg N ha-1 year−1)
N 2 O, NO x and NH 3 emissions from different sources
Annual N2O, NOx and NH3 emissions resulting from synthetic N fertilizer application were estimated to be 12.6, 4.13 and 103 ton N, respectively (Fig. 2). NH3 was obviously the dominant gas emitted by N fertilizers, and NH3 emissions from rice paddies accounted for more than half of the total NH3 emissions from synthetic N fertilizer, owing to the high application rate. On average, NH3 emissions accounted for 10% of the total synthetic N applied to croplands in the catchment.
Annual N2O, NOx and NH3 emissions resulting from human and livestock excreta during management and after application as organic fertilizer to the fields were 1.99, 1.10 and 73.6 ton N, respectively (Table 4). The amount of NH3 emission was much greater than the emissions of N2O and NOx. Much more NH3 was emitted during the management process than after being applied to the soils. Owing to the small population of livestock, human excreta were the dominant source of NH3 emission.
Table 4. Annual nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions from human and livestock excreta
N2O (ton N)
NOx (ton N)
NH3 (ton N)
Managed human excreta
Managed livestock excreta
Fertilized human excreta
Fertilized livestock excreta
The N2O, NOx and NH3 emissions from crop residues were estimated to be 1.79, 36.7 and 33.5 ton N year−1, respectively (Table 5). These emissions occurred in three processes, that is, field burning, burning for household energy and decomposition of above-ground and below-ground crop residue in the field. Irrespective of whether it was N2O, NOx or NH3, more than half of the emissions occurred during the field burning of crop residue, followed by residue burning for household energy.
Table 5. Annual nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions from crop residue burning in situ (FRB), household residue burning (FHRB) and crop residue returned to cropland (FCR)
N2O (ton N)
NOx (ton N)
NH3 (ton N)
Reactive gas emissions from household fossil fuel energy consumption (including coal and LPG) were small, with annual N2O and NOx emissions of 0.01 and 1.70 ton N, respectively. NOx emission mainly occurred during household coal consumption.
Background emissions of N2O, NOx and NH3 were also considerable at 3.69, 1.35 and 3.55 ton N per year, respectively.
Total emissions of N 2 O, NO x and NH 3
Total N2O, NOx and NH3 emissions from the different sources are summarized in Fig. 3. The total annual N2O emission was estimated to be 20.1 ton N. Synthetic N fertilizer application was the largest contributor to N2O emissions, accounting for 63% of the total emissions. Soil background emission contributed 18% of the total emissions. N2O emission from the other sources (FMON, FON, FCR, FRB, FHRB and FHE) only accounted for 19% of the total emissions (Fig. 3).
For the total 45.0 ton of NOx-N emission, crop residue burning in situ (FRB) and in households (FHRB) was by far the dominant source. NOx emissions from crop residue burning in the fields and in households as biofuels accounted for 52 and 29%, respectively, of the total emissions. Synthetic N fertilizer application was responsible for 9% of the total NOx emission (Fig. 3).
The total annual NH3 emission was estimated to be 214 ton NH3-N. Similar to N2O emission, synthetic N fertilizer application was the largest source and accounted for 48% of the total emissions. Another important NH3 emission source was managed human and livestock excreta, which accounted for 29% of the total emissions. Emissions from residue burning in the field and residue burning for household energy comprised 10 and 6%, respectively, of the total emissions (Fig. 3).
The total reactive gaseous emissions in the catchment amounted to 279 ton N year−1 (Table 6). The emitted N2O, NOx and NH3 comprised 7, 16 and 77%, respectively. The averaged emissions per capita were 1.31 kg N2O-N, 2.93 kg NOx-N and 13.9 kg NH3-N, and the per unit area emissions were 4.42 kg N2O-N ha−1, 9.89 kg NOx-N ha−1 and 47.0 kg NH3-N ha−1, respectively, totaling 61.3 kg reactive gaseous N ha−1 year−1.
Table 6. Comparison of nitrous oxide (N2O), nitrogen oxide (NOx) and ammonia (NH3) emissions in the field and residential areas in the study rural catchment
†Values in parentheses indicate the percentage of total emission. ‡The corrected adult population from the permanent population using a factor of 0.85 reported by Zhu (1997) was used.
Subtotal emissions (ton N)
Emission per area (kg N ha−1)
Subtotal emissions (ton N)
Emission per area (kg N ha−1)
Total emission (ton N)
Emission per area (kg N ha−1)
Emission per person (kg N person−1)‡
Emissions over croplands and residential areas
Emissions of N2O, NOx and NH3 from residential areas and croplands are separated in Table 6. The emission sources from croplands include FSN, FON, FRB, FCR and soil background emission, and the FMON, FHRB and FHE emissions were attributed to residential areas.
The results showed that more reactive gaseous emissions occurred in croplands, accounting for 67% of the total emissions. The proportion of N2O emissions in the cropland areas even reached 92%. However, the per unit area emissions of NOx and NH3 in the residential areas were more than those in the croplands, with 11.5 kg NOx-N and 54.8 kg NH3-N ha−1 in the residential areas, and 9.21 kg NOx-N and 43.7 kg NH3-N ha−1 in the croplands. The per unit area emission for N2O from croplands (5.77 kg N ha−1) was higher than that from residential areas (1.26 kg N ha−1).
Emissions of N 2 O, NO x and NH 3
On a global scale, livestock waste is the largest contributor to N2O emissions, and is responsible for threefold the amount emitted by synthetic N fertilizer (Mosier and Kroeze 2000). In Australia, estimated N2O emissions originating from N fertilizers and animal waste are almost equal, accounting for 32 and 30%, respectively, of the total (Dalal et al. 2003). In this rural catchment area of China, although there is a large area of rice paddy fields that have a lower fertilizer-induced N2O emission factor than the uplands, synthetic N fertilizer was by far the dominant source of N2O, contributing 63% of the total emission. This is in agreement with the finding of Yan et al. (2003) that, in East, South-East and South Asia, China was the only country where fertilizer was the dominant source of N2O emission, apparently because of the intensive application of N fertilizer for crop production. According to our surveys conducted in 2007 and 2008, synthetic N application rates to croplands in the catchment reach up to 500–600 kg N ha−1 year−1 for summer and winter crops.
Although NOx emissions from crop residue burning in situ and from houses as biofuels only accounted for <4% of the total NOx emissions in China (Streets and Waldhoff 2000), it appears to be a serious environmental problem in the study rural area. The intensive use of synthetic fertilizer has also decreased the necessity of crop residue return as a nutrient source. As a result, most crop residue in the catchment was burnt in houses as biofuel or in the field, leading to large NOx emissions from crop residue burning and contributing 81% of the total NOx emissions. Our investigations showed that 64% of crop straw was burnt in situ after harvest in the study catchment. This is comparable with Cao et al. (2008), that is, approximately 15.2–60% of crop straw was burnt in the field directly in China, and the highest NOx emission from field burning occurred in eastern China.
Emission of NH3 was the biggest contributor to reactive gases emission in the study catchment and accounted for 77% of the total N2O, NOx and NH3 emissions. Heavy synthetic N application to croplands and extensive human waste management in the rural catchment resulted in high NH3 emission. The NH3 emission rate of synthetic N fertilizer was generally higher for summer crops (rice and maize) than for winter crops (wheat and oil rape), probably because of the temperature difference. The average NH3 emission rate of 10% observed in the present study was comparable to the values reported by Zhu and Chen (2002).
Globally, domestic animals are the largest source of NH3 emissions (40%), which is approximately threefold that from synthetic fertilizer and agricultural crops combined (Schlesinger and Hartley 1992). Roe et al. (1998) found a similar trend in the USA; NH3 emissions from livestock waste accounted for 55% of the total NH3 emissions and synthetic N fertilizer application contributed 7%. Owing to the small livestock population and intensive use of synthetic N fertilizer (particularly in the rice fields) in the current rural catchment, synthetic N fertilizer contributed 48% of the total NH3 emissions, which was more than the contributions of total managed and applied excreta (34%).
Although the total emission of NH3 from the croplands was more than that from the residential areas in the catchment, NH3 emission per unit area in the residential areas was higher than that in the croplands. This might be the reason for the higher annual average air concentration of NH3 in the residential areas (5.4 μg m−3) than in the croplands (4.1 μg m−3; R Yang, unpublished data, monitored from October 2007 to September 2008) measured 1.5 m above the surface.
High wet N deposition has been observed in the lower reach of the Yangtze River (26.8 kg N ha−1 year−1; Zhao et al. 2009), as well as in this catchment (25.2 kg N ha−1 year−1; R Yang, unpublished data, monitored from October 2007 to September 2008), whereas the average wet deposition in China was estimated to be 9.9 kg N ha−1 year−1 (Lü and Tian 2007). The total emissions of NOx and NH3 in this catchment averaged 56.9 kg N ha−1 year−1, which was more than double the observed wet N deposition. The high observed N deposition was, therefore, plausible. The other half of the emitted NOx and NH3 probably contributed to the dry N deposition or participated in the photochemical reaction of gases.
Uncertainty in the estimated emissions
We have done our best to reduce the uncertainties in the estimated emissions. First, the N-related activity data were obtained through first-hand investigation. Second, the largest reactive gaseous N source in the catchment, that is, NH3 emission from synthetic fertilizer, was directly measured for four major crops and for two consecutive years. Third, we used local fertilization rate and precipitation to correct the N2O emission factors for upland and local crop residue N data to correct NH3 and NOx emission factors for biomass burning. Nevertheless, there still exist large uncertainties in the estimated emissions owing to the large number of emission factors and parameters that were not measured specifically for the study catchment. The largest source of uncertainty is probably the NH3 emission factor for human and animal excreta, which is difficult to measure and data are rare, except for the default IPCC value. Biomass burning emission factors also have large uncertainty owing to a lack of sufficient measurements.
In a typical rice-based agricultural catchment of 45.5 km2, the annual N2O, NOx and NH3 emissions were estimated to be 20.1 ton N2O-N, 45.0 ton NOx-N and 214 ton NH3-N, totaling 279 ton N. The contributions of the various sources to the reactive gaseous emissions were quite different for the different gases. Synthetic N fertilizer and soil background emission contributed 63 and 18%, respectively, to the total emission of N2O. Crop straw burning in fields and in houses as biofuel was the dominant source of NOx emissions in the study catchment, contributing 52 and 29%, respectively. The total emission of NH3 was far greater than the total emissions of N2O and NOx, and was emitted from synthetic N fertilizer (48%) and managed human and livestock excreta (29%), particularly from synthetic N fertilization of rice fields and human excreta. More N2O, NOx and NH3 emission occurred in croplands than in residential areas. However, the NOx and NH3 emissions per unit area in the residential areas were higher than those in the field area. The average per unit area emission of NOx and NH3 reached 56.9 kg N ha−1 year−1, supporting the high inorganic N deposition rate observed in the catchment. Clearly, more efficient use of N fertilizer and biological utilization of crop straw would be effective in reducing the emission of reactive gases in this rural catchment.
This research was financially supported by the National Natural Science Foundation of China (No. 40721140018). We are grateful to Xiaosan Jiang and Jianchao Liu of the Nanjing Agricultural University and Aijun Ma of the Jiangsu Polytechnic College of Agriculture and Forestry for their assistance in conducting household surveys on agricultural activities.