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
|Crop||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)§||Ratio of biomass burning in situ (%)†||Ratio of biomass burning as household fuel (%)†||Cf¶|
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||Population†||Nitrogen excreta rate (kg N year−1 head−1 of human or animal)‡||FraclossMS (%)§|
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.