A high-resolution map of reactive nitrogen inputs to China

Database structure

The Coupled Human And Natural Systems Nitrogen Cycling Model Spatial Distribution (CHANS-SD) 1.0 database consists of three files (Fig. 1). The ‘data file’ provides N inputs of 6 land use types, including cropland, forest, grassland, water, built-up area and unused land. The ‘readme file’ explains the abbreviations used in the ‘data file’ and ‘source file’, and provides the units of all variables (Fig. 1). The ‘source file’ includes the full references and input data used in the database (Fig. 1).

Data compilation

We applied the CHANS model to calculate N inputs by land use type. The CHANS model calculates all nitrogen (N) fluxes that can be identified, together with the linkages among subsystems, within a country, state (province), city or watershed, with a mass balance principle. The system is divided into 14 subsystems: cropland, grassland, forest, livestock, aquaculture, industry, human, pet, urban green land, wastewater treatment, garbage treatment, atmosphere, surface water, and groundwater. N cycling starts from the entry of reactive N (Nr) that activated from N₂ into the system or from Nr direct input to the system from outside, and terminates when Nr is transformed to N₂ or lost to outside the system.

N input to cropland subsystem is the largest component of N input to China, including N fertilizer, atmospheric N deposition, N from irrigation, N from livestock manure, N from human excretion, cropland biological N fixation (BNF) and straw recycle. N input to forest subsystem, including forest BNF and N deposition, is the second largest component of N input to China given the large area of forest. N input into surface water subsystem, including Nr runoff from cropland, livestock and forest, human wastewater discharged, industrial waste water, WTP (Water Treatment Plant) effluent and N deposition, is the third largest component of N input in China (Fig. 2). The details of the CHANS model including all the code and parameters of N cycling, and the protocols for the calculation of all related N fluxes can be found from https://person.zju.edu.cn/en/bjgu#930811.

The National Bureau of Statistics of China¹ provides data of cropland N fertilizer use, planting area, livestock and population in the statistical yearbook of each province. Taiwan, Hong Kong, and Macao were not included owing to data limitations. Gridded datasets of land use and GDP is derived from the Resource and Environment Data Cloud Platform¹², and data on China’s hydro-basin distribution is collected from the FAO website GeoNetwork¹³.

In the cropland subsystem, livestock manure input to cropland (MANURE_an, kg/yr) was calculated based on animal population (POP_an, head), excretion factor (EXCRE_an, kg N/head/yr) and rate of livestock manure applied to cropland (RE_an, %) according to Eq. (1) (Fig. 3a),

Amounts of chemical N fertilizer consumption at county scale were obtained from the statistical yearbook of counties (Fig. 3b). Human manure to cropland (MANURE_hu, kg /yr) was calculated by urban population (POP_ur, person), rural population (POP_ru, person), excretion factor (EXCRE_hu, kg N/person/yr), rate of urban excretion return to cropland (RE_ur, %) and rate of rural excretion return to cropland (RE_ru, %) according to Eq. (2) (Fig. 3c).

Cropland BNF (CBNF, kg/yr) was calculated by planting area (area, ha) and N fixation rate (r_fix, kg N/ha/yr) according to Eq. (3) (Fig. 3d),

Straw recycling and N input from irrigation are calculated by using nationally uniform values (Table 1).

N input to the grassland subsystem include N fertilizer, manure recycle, N deposition, grassland BNF and irrigation for artificial grassland. N input in forest subsystem include N fertilizer to artificial forests, N deposition and forest BNF.

In the surface water system, N inputs to each watershed include runoff from cropland, livestock, and forest areas, human wastewater discharge, industrial wastewater, WTP effluent and atmospheric N deposition. Cropland runoff was calculated by spatial distribution of cropland input (Fig. 4a). Livestock runoff using the spatial distribution of livestock manure (Fig. 4b). Finally, forest runoff was calculated by applying the spatial distribution of forest areas. Human wastewater discharge was calculated based on the spatial distribution of the human population (including urban and rural populations) (Fig. 4c). Industrial wastewater was calculated by using the spatial distribution of GDP since industrial output is highly correlated with GDP on regional scale (Fig. 4d). Similarly, the distribution of WTP and its effluent Nr are highly correlated with urban population on regional scale, therefore, WTP effluent was calculated by spatial distribution of urban population.

For N deposition, the satellite observations on NO₂ columns were derived from GOME-2. GOME-2 overpass times provided global coverage of NO₂ with a variable ground spatial resolution of 80 km × 40 km (every day). We used the monthly TEMIS NO₂ product at a spatial resolution of 0.25° latitude × 0.25° longitude downloaded from the website of Tropospheric Emission Monitoring Internet Service¹⁴. The satellite observations on NH₃ columns were derived from IASI onboard the meteorological platforms MetOp-A and MetOp-B¹⁵ with an elliptical footprint of 12 × 12 km up to 20 × 39 km depending on the satellite-viewing angle. The daily NH₃ columns were downloaded from IASI Portal¹⁶. We processed the daily data into the monthly NH₃ columns averaged by daily observations at a horizontal resolution of 0.25° latitude × 0.25° longitude using the arithmetic mean method^17,18. N deposition of every subsystem was obtained from the national N deposition spatial distribution map by ‘spatial join’ tool of ArcGIS 10.6.

Source: Ecology - nature.com