Comparison of food supply system in China and Japan based on food nitrogen footprints estimated by a top-down method

In the N-Input method, synthetic nitrogen fertilizer input, BNF for crop production and nitrogen in captured and non-fed aquaculture seafood were regarded as new nitrogen input. Atmospheric deposition was not taken into account because the origin of ammonium deposition is primarily fertilizer and animal manure and was considered as recycled nitrogen. Although nitrogen oxides deposition due to fossil fuel combustion is new nitrogen input, its contribution to the total input is small in Asian agricultural areas according to our previous research (Shindo et al 2012).

New nitrogen input for domestically produced food was represented as direct input and those for imported food as indirect input (figure 1). For the estimations, food balance sheet (FBS) and other statistics on food production and trade of FAOSTAT (FAO 2019), database of International Fertilizer Industry Association (IFA; Heffer et al 2017) on fertilizer application and OECD database (OECD 2016, 2019) on forage consumption and BNF were primarily used. Ninety-four food commodities from 19 food types listed in the FBS including secondary products such as sugar, vegetable oil and alcoholic beverages were considered. The data that support the findings of this study are available upon reasonable request from the authors.

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The method to estimate new nitrogen input had been developed by our previous paper (Shindo and Yanagawa 2017) and was modified mainly for applying the method to China where some necessary data were not available. The detailed calculation method is shown in the supplementary material (available online at stacks.iop.org/ERL/16/045003/mmedia).

For Japan, the data in 2011 used in Shindo and Yanagawa (2017) were used in this paper too. For China, data in 2013, the latest data, were chosen. It is reasonable to compare these data from different years because food production and consumption in Japan are almost same in 2011 and 2013 as shown below.

The FBS 2000 allocates food supply into six streams: food, processing, feed, seed, losses, and other-uses. According to the handbook of FBS (FAO 2001), other-uses includes the portion consumed by tourists as well as the quantity used for manufacture of non-food products (e.g. oil for soap). The other-uses stream was considered unsuitable for inclusion in the food N-footprint; therefore, both direct and indirect nitrogen input for each commodity was adjusted as follows before calculating the N-footprint:

$$\begin{equation}{adjNN} = {NN}\,\left( {{A_{{\text{Supply}}}} - {A_{{\text{Other}}}}} \right){ \cdot {A_{{\text{Supply}}}}}^{ - 1}\end{equation} \tag{ 2 }$$

where ${adjNN}$ represents adjusted new nitrogen input, $NN$ is new nitrogen input for total supply of a commodity (Mg N yr⁻¹). ${A_{{\text{Supply}}}}$ and ${A_{{\text{Other}}}}$ are quantity of that commodity supplied and of the portion for other-uses (Mg yr⁻¹), respectively, in the country of interest.

For oilcrops and sugar crops, the major portion of which were used as feedstocks to produce vegetable oil and sugar, respectively, the calculation of new nitrogen input was adjusted by reducing the other-uses of the commodity and also that of its secondary products. The adjusted new nitrogen input was calculated for soybean as an example:

$$\begin{align} & {adjNN}_{{\text{Soybean}}} = {NN}_{{\text{Soybean}}}\left( {{{A}}_{{\text{Supply}},\,{\text{Soybean}}}} \right. \nonumber\\ & \quad \left. - {A_{{\text{Other}},\,{\text{Soybean}}}} - {A_{{\text{Manu,Soybean}}\,}} \cdot {A_{{\text{Other,Soybean}}\,{\text{oil}}\,}} \cdot \right. \nonumber\\ & \quad \left. {A_{{\text{Supply}},\,{\text{Soybean}}\,oil}}^{ - 1} \right) \cdot {A_{{\text{Supply}},\,{\text{Soybean}}}}^{ - 1}\end{align} \tag{ 3 }$$

where ${A_{{\text{Manu,Soybean}}}}$ represents quantity of soybeans used for soybean oil production, and other variables are same as in equation (2), but for soybean or soybean oil.

The N-footprint was calculated as the total ${adjNN}$ of all food commodities.

As an indicator of nitrogen inefficiency, input–output ratio of individual food commodities supplied in the country of interest was estimated based on the adjusted new nitrogen input (total of direct and indirect) and nitrogen output to each food commodity supplied (produced + imported−exported), excluding other-uses and losses (supplementary material 3):

$$\begin{align}{R_{\text{I/O}}} & = \left( {{adjNN}_{{\text{Direct}}} + {adjNN}_{{\text{Indirect}}}} \right) \cdot \nonumber\\ & \quad {({{N}_{{\text{Supply}}}} - {{N}_{{\text{Other}}}} - {{N}_{{\text{Losses}}}})^{ - 1}}\end{align} \tag{ 4 }$$

$$\begin{align}{R_{{\text{I/O}},\,{\text{Secondary}}}} & = \left( {{adjNN}_{{\text{Direct}}} + {{adjNN}_{{\text{Indirect}}}}} \right) \cdot \nonumber\\ & \quad {({A_{{\text{Supply}}}} - {A_{{\text{Other}}}} - {A_{{\text{Losses}}}})^{ - 1}}\end{align} \tag{ 5 }$$

where ${R_{\text{I/O}}}$ (Mg N Mg N⁻¹) represents the ratio for crop, livestock products and fed-aquaculture seafood and ${R_{\text{I/O,Secondary}}}$ (Mg N Mg⁻¹) represents the ratio for secondary products; ${adjNN}_{{\text{Direct}}}$ and ${adjNN}_{{\text{Indirect}}}$ indicate the adjusted direct and indirect new nitrogen input, respectively; ${{N}}_{{\text{Supply}}}$, ${{N}}_{{\text{Other}}}$, ${{N}}_{{\text{Losses}}}$ represent the nitrogen contained in food supplied, other-uses and losses, respectively; ${{{A}}_{{\text{Losses}}}}$ represent the quantity of losses of that food (FAO 2019). As the secondary products contain no or very less nitrogen, we used the product quantity instead of nitrogen content in equation (5).

VNF and ${R_{\text{I/O}}}$ are convertible each other. However, in FAOSTAT terminology, food implies crude food before food processing (FAO 2001), whereas food consumption in the N-calculator method represents food consumed by humans, not including the quantities lost or discarded in processing or consumption (Leach et al 2012). Therefore, VNF is derived from ${R_{\text{I/O}}}$ as follows:

$$\begin{equation}{{VNF}}_{{\text{Dr}}} = {R_{\text{I/O}}} \cdot {({Tr}_{{\text{Processing}}} \cdot {{Tr}_{{\text{Consumption}}}})^{ - 1}} - 1\end{equation} \tag{ 6 }$$

$$\begin{align}{{VNF}}_{{\text{Dr}},\,{\text{Secondary}}} & = {R_{{\text{I/O}},\,{\text{Secondary}}}} \cdot \nonumber\\ & \quad {({{Tr}}_{{{Processing}}} \cdot {{Tr}}_{{\text{Consumption}}})^{ - 1}}\end{align} \tag{ 7 }$$

where ${VNF}_{{\text{Dr}}}$ is derived VNF, ${{Tr}}_{{\text{Processing}}}$ and ${{Tr}}_{{\text{Consumption}}}$ represent nitrogen transfer rates to the next product in food processing and consumption, respectively. ${{VNF}}_{{\text{Dr,Secondary}}}$ is derived VNF of secondary products, defined as the ratio of total nitrogen losses during production of secondary product to the amount of that product (Hayashi et al 2020).

We used the values of ${{Tr}}_{{\text{Processing}}}$ and ${{T}}_{{\text{Consumption}}}$ proposed by Guo et al (2017). The same values were used for Japan according to Oita et al (2020). ${{VNF}}_{{\text{Dr}}}$ reflects the VNFs of each food commodity both in the country of interest and in major source countries, which can be used to estimate the N-footprint with the N-calculator method by using equation (1). NUE of individual food commodity in different countries can be compared by ${R_{\text{I/O}}}$ as well as by ${{VNF}}_{{\text{Dr}}}$. In order to investigate the food supply system of Japan and China in terms of nitrogen efficiency, we estimated the N-footprint for China by the N-calculator procedure with Japanese ${{VNF}}_{{\text{Dr}}}$ and compared it with the footprint estimated with Chinese ${{VNF}}_{{\text{Dr}}}$. A similar exercise was carried out for the N-footprint for Japan.

New nitrogen input for supplied food eventually becomes the nitrogen load to the environment, which is either in the country of interest or in foreign countries connected with food trade. The conceptual scheme was shown in figure 2. The new nitrogen input and the nitrogen loads for the food handled by a country of interest were calculated as follows:

$$\begin{equation}{{NN}} = {{NN}_{{\text{DP}}}} + {{NN}_{{\text{IM}}}} - {{NN}_{{\text{EX}}}}\end{equation} \tag{ 8 }$$

$$\begin{equation}{{NL}_{{\text{WC}}}} = {{NN}_{{\text{DP}}}} + {{{N}}_{{\text{Food}},\,{\text{IM}}}} - {{{N}}_{{\text{Food}},\,{\text{EX}}}}\end{equation} \tag{ 9 }$$

$$\begin{equation}{{NL}_{{\text{FC}}}} = {{NN}_{{\text{IM}}}} - {{N}}_{{\text{Food}},\,{\text{IM}}} + {{N}}_{{\text{Food}},\,{\text{EX}}}\end{equation} \tag{ 10 }$$

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where ${NN}$ represents the total new nitrogen input for supplied food in the country of interest, which is N-footprint of the country; ${{NN}_{{\text{DP}}}}$, ${{NN}_{{\text{IM}}}}$ and ${{NN}_{{\text{EX}}}}$ represent those for domestic production in the country, for imported and exported food, respectively; ${{NL}_{{\text{WC}}}}$ and ${{NL}_{{\text{FC}}}}$ represent the nitrogen load to the environment within the country of interest and in foreign countries, respectively; ${{{N}}_{{\text{Food}},\,{\text{IM}}}}$ and ${{{N}}_{{\text{Food}},\,{\text{EX}}}}$ represent the nitrogen in imported and exported food, respectively. The values in equations (8)–(10) are only for the food, excluding other-use.

The food N-footprints estimated by the N-Input method for China in 2013 and Japan in 2011 were shown in figure 3 and table A1in supplementary material. The N-footprint in China was estimated to be 21.96 kg N capita^–1yr^–1. Approximately 93% of China's new nitrogen input was direct, mainly attributable to the application of synthetic nitrogen fertilizer used within the country (figure 3). The indirect input for the imported food was relatively small; imported crops, especially soybeans, were the main contributors. This estimated N-footprint was a little larger than the published estimates using the N-calculator method: 21 kg N capita^–1yr^–1 in the 2000s (Guo et al 2017) and 19.8 kg N capita^–1yr^–1 in 2013 (Oita et al 2020). The N-footprint in Japan, 18.44 kg N capita^–1yr^–1, was mainly attributable to indirect nitrogen input (figure 3), which is within the range of previous estimates with the N-calculator method: 28.1 kg N capita^–1yr^–1 in 2009 (Shibata et al 2014), 15.2 kg N capita^–1yr^–1 in 2011 (Oita et al 2018), 18.3 kg N capita^–1yr^–1 for 2011–2015 (Hayashi et al 2018), and 21.8 kg N capita^–1yr^–1 in 2013 (Oita et al 2020). The large variation of the previous estimates by N-footprint was probably caused by the different VNFs for some food commodities and different setting of the food processing and consumption (in household) losses.

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We found possible fluctuation of N-footprint estimate by the N-Input method. For example, the consumption of nitrogen fertilizer in China is 25.1 Tg Nyr^–1 in the IFA database (Heffer et al 2017), whereas FAOSTAT reports the quantity as 30.8 Tg Nyr^–1, while data on Japan were not different each other in these two databases. The result of N-footprint, 21.96 kg N capita^–1yr^–1 for China was based on the IFA data, but the N-footprint was 24.68 kg N capita^–1 yr^–1 when the FAOSTAT data was used instead. Fertilizer consumption data reported in China Statistical Yearbook (National Bureau of Statistics in China 2014) is 23.9 Tg N yr^–1, closer to the IFA data, that is a reason we selected the IFA value. However, as both databases are open to the public and widely used, the estimate of N-footprint by N-input method can have uncertainty attributable to uncertainties in the data used.

In China, 17% of maize, 24% of cassava, and 69% of vegetable oil were allocated to other-uses (i.e. non-food uses) according to FBS data of FAOSTAT (FAO 2019). The nitrogen content of the entire food supply (including all uses) in China was 21.45 Tg N, 23% of which was allocated to other-use. Similarly, the total new nitrogen input for the entire food supply in China in 2013 was 35.24 Tg N, 14% of which was due to other-uses. This ratio (14%) was smaller than the ratio of other-use for the nitrogen content in food products (23%), because those commodities which were largely allocated to other uses tended to have relatively small ${R_{\text{I/O}}}$ values, e.g. less than 1.0 for oilcrops. In contrast, in Japan, only 2.9% of the nitrogen in the entire food supply and 2.7% of new nitrogen input were allocated to other uses.

Table 1 shows the ${R_{\text{I/O}}}$ and ${{VN}}{{\text{F}}_{{\text{Dr}}}}$ of some food commodities derived with equations (6) and (7), and VNFs used in the N-calculator method by the prior researches. Nitrogen transfer rates at food processing and consumption used to derive ${{VNF}_{{\text{Dr}}}}$ were also shown in the table. ${R_{\text{I/O}}}$ and ${{VNF}_{{\text{Dr}}}}$ were small for crops and large for meat in both countries, with some notable exceptions (table 1). Among crops, fruits had extremely large ${R_{\text{I/O}}}$ and ${{VNF}_{{\text{Dr}}}}$ in both China and Japan. Vegetables also had large values compared with other crop commodities. Among the livestock products, beef had the highest ${R_{\text{I/O}}}$ and ${{VNF}_{{\text{Dr}}}}$. These were generally true for VNFs proposed in the prior researches by other authors (table 1), although previous VNFs shows the large fluctuation by the different authors even for the VNFs of the same country. A remarkable exception is those by Guo et al (2017), who found the VNF of beef to be the lowest among the livestock products and of starchy roots to be very high (table 1). They might consider the particular agricultural and livestock farming system in China, whereas our results are based on the open-access data and some data (forage consumption etc) were estimated according to the general trend (supplementary material). ${{VNF}_{{\text{Dr}},\,{\text{Secondary}}}}$ for palm oil was almost same with the literature value, where palm oil used in Japan and China were mostly imported from Malaysia and Indonesia.

Table 1. Ratio of new nitrogen input to nitrogen content in food commodity (${R_{\text{I/O}}}$), virtual nitrogen factor (VNF) values for China and Japan calculated with the N-input method, published VNF values, and published nitrogen transfer rates

	N input/output ratio ${R_{\text{I/O}}}$ for crop, livestock and seafood products, ${R_{\text{I/O, Secondary}}}$ for secondary products		${{VNF}_{{\text{Dr}}}}$, ${{VNF}_{{\text{Dr}},{\text{Secondary}}}}$ derived from ${R_{\text{I/O}}}$		VNF from literatures						Nitrogen transfer rates for food processing and consumption from literatures b
	China	Japan	China	Japan	China a	China b	Japan c	Japan d	USA e	Global ave f	${\text{T}}{{\text{r}}_{{\text{Processing}}}}$	${\text{T}}{{\text{r}}_{{\text{Consumption}}}}$
Cereals total	1.73	1.39	1.37	0.90	2.10		1.50	1.17	1.40
Wheat	1.32	1.28	1.09	1.03		1.10		1.40			0.70	0.90
Maize	1.37	2.42	0.70	2.00		0.80		1.00			0.95	0.85
Rice	3.11	1.07	3.16	0.43		1.10		0.93			0.85	0.88
Vegetables	3.41	3.50	5.69	5.87	4.00	7.70	5.50	3.05	9.60		0.60	0.85
Fruits	18.54	10.60	21.96	12.12	8.50	19.00		13.10			0.95	0.85
Starchy roots	2.60	2.48	2.73	2.57	3.00	15.90	4.90	2.19	1.50		0.80	0.87
Soybean	0.85	0.70	3.34	2.56		1.30	1.30		0.50		0.23	0.85
Poultry meat	1.88	2.92	1.88	3.47		5.70	6.00	3.85	3.20		0.71	0.92
Pork	3.78	6.95	5.32	10.63		7.90	6.70	6.99	4.40		0.65	0.92
Beef	7.32	12.81	12.73	23.01		5.20	12.40	11.52	7.90		0.58	0.92
Eggs	1.31	2.54	0.40	1.70		7.20		5.48			1.00	0.94
Milk and dairy products	1.41	2.17	0.48	1.29		7.00	2.70	7.41	4.30		1.00	0.95
Fish and seafood total	2.67	1.64	2.22	0.98		4.10	2.90	0.60	4.10		0.90 d	0.92 d
Fed aquaculture	4.49	4.35	4.43	4.26			4.3, 3.4 g
Non-fed and captured	1.42	1.11	0.71	0.35			0.2 g
Secondary products h
Sugar and sweeteners	0.037	0.011	0.037	0.011							1.00	1.00
Soybean oil	0.439	0.270	0.439	0.270							1.00	1.00
Palm oil	0.027	0.021	0.027	0.021						0.024 ± 0.0032	1.00	1.00
Wine	0.018	0.013	0.018	0.013							1.00	1.00
Beer	0.005	0.002	0.005	0.002							1.00	1.00
Other alcoholic beverages	0.035	0.025	0.035	0.025							1.00	1.00

Unit: Mg N Mg N^–1 for ${R_{\text{I/O}}}$, VNF, ${{VNF}_{{\text{Dr}}}}$ and nitrogen transfer rate; Mg N Mg^–1 for ${R_{\text{I/O,Secondary}}}$. ^a Zhang et al (2018). ^b Guo et al (2017), with-trade case assuming the import only from USA, data in 2000s. ^c Shibata et al (2014) for crop and livestock products with-trade case assuming the import only from USA. ^d Oita et al (2020) for average values from 2011 to 2013. ^e Leach et al (2012). ^f Hayashi et al (2020). ^g Oita et al (2018): Freshwater fish: 4.3, marine fish:3.4. ^h For the secondary products, we assumed ${{Tr}_{{\text{Processing}}}}$ and ${{Tr}_{{\text{Consmption}}}}$ are 1.0 and N content in secondary products (${{{N}}_{{\text{Scondary}}}}$) is zero.

In the present study, both ${R_{\text{I/O}}}$ and ${{VNF}_{{\text{Dr}}}}$ in China were slightly larger than those in Japan for many types of crops, whereas for livestock products, the values were smaller in China than in Japan. These differences were considered partly caused by the larger application rate of chemical fertilizer in crop farming and usage of unaccounted feed such as food waste for livestock farming in China. ${{VNF}_{{\text{Dr}}}}$ reflects the nitrogen use efficiencies in major countries exporting the food to the country of interest and was used for N-footprint estimation by N-calculator method in the next section.

Magnitude of food N-footprint is determined by the amount and composition of food commodities consumed and their nitrogen efficiencies. Taking less food or taking more efficient food with lower VNF realizes the smaller N-footprint. Food N-footprint in China in 2013 was 19.1% larger than that in Japan in 2011 (figure 3). Based on FAOSTAT data (FAO 2019), the total quantity of nitrogen supplied as food in Japan was 5.09 kg N capita^–1 yr^–1 in 1961; this gradually increased to 6.59 kg N capita^–1 yr^–1 in 1989 and then decreased gradually; this was almost stable after 2009 and the value in 2011 was 5.86 kg N capita^–1 yr^–1 (figure 4(a)). In China, the total nitrogen in supplied food was 2.85 kg N capita^–1 yr^–1 in 1961; this increased gradually with occasional stagnations through the 1960s and the 1970s, and then increased steadily to reach 6.95 kg N capita^–1 yr^–1 in 2013 (figure 4(b)). This quantity exceeded the quantity in Japan in 2011 by 18.7%, showing that the larger N-footprint in China was mainly attributable to the larger protein consumption in China. According to the changes in food supply in China in figure 4(b), per capita food N-footprint has increased rapidly and it will continue to increase for a while.

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In order to evaluate the effect of diet composition and nitrogen efficiency, we compared the N-footprint of Japan (and of China) calculated with equation (1) using ${{VNF}_{{\text{Dr}}}}$ for Japan and for China. Additionally, we proportionally reduced the consumption of each commodity of China by 19.1% so that the total nitrogen in food consumption equaled to that of Japan. Table 2 showed that N-footprint for China was larger than that for Japan as with the results by the N-Input method, and N-footprint calculated with Japanese ${{VNF}_{{\text{Dr}}}}$ was larger than that with Chinese ${{VNF}_{{\text{Dr}}}}$ for all three cases (food consumption in Japan and China and reduced consumption in China); this suggests that average efficiency of food supply is higher in China than in Japan in terms of VNFs. Comparison of the second and forth columns of the table shows that Chinse footprint was still slightly larger than Japanese footprint even when the total nitrogen in consumed food are same each other: this was caused by the different diet composition. The largest difference in diet composition is higher Chinese consumption of vegetables, which have a relatively high VNF. China's per capita vegetable protein consumption is the highest in the world, and is 3.5 times that of Japan (FAO 2019). The fraction of animal protein in the total protein of the human food is 36% in China, less than that in Japan (i.e. 47%). However, 56% of the animal protein in China is from meat, which has a high VNF. In contrast, in Japan it is 41% and milk and fish, which have smaller VNF, are contributing much of the remainder.

Figure 5 shows the nitrogen flow associated with production and trade of food (${{{N}}_{{\text{Food}}}}$) and new nitrogen input for production of these food (NN) for Japan and China based on figure 2 and equations (8)–(10). In China, supplied food was mostly covered by domestic production. Contribution of food trade was relatively small: 2.36 Tg N (14% of the supply) and 0.28 Tg N (1.7% of the supply) was imported and exported, respectively. As a result, almost all new nitrogen input for the supplied food became the environmental load within China. Japan imports 1.14 Tg N of food (78% of the supply) and new nitrogen input for imported food (1.55 + 0.18 = 1.72 Tg N) occupied 73% of the total new nitrogen input for the supplied food. Nevertheless, nitrogen load in foreign countries (${{NL}_{{\text{FC}}}}$) was 0.60 Tg N that was only 18% of the total load and remaining became the load within Japan, because large portion of new nitrogen input is embedded in food and carried from exporting countries to Japan.

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Food import from China to Japan was 0.11 Tg N: China was the fifth countries exporting food nitrogen to Japan after USA, Australia, Canada and Brazil. This food import increased the nitrogen load to China by 0.07 Tg N.

Although the N-Input method intended to produce a food N-footprint that is comparable to that from the N-calculator method, the results are unlikely to be identical for various methodological reasons. Among these is the inclusion of secondary products in N-footprint estimation with the N-input method. This method estimates the new nitrogen input to produce the secondary products such as. sugar, vegetable oil and alcoholic beverages, while these food commodities are not included in the N-calculator method because the VNF of nitrogen-free food cannot be defined. The estimated new nitrogen input for imported secondary food were 0.15 kg N capita^–1 yr^–1 and 0.42 kg N capita^–1 yr^–1 for China and Japan, respectively (figure 3; supplementary table A1 contains additional details). The new nitrogen input for the domestic production of secondary products estimated by the same method as for imported products were 1.81 kg N capita^–1 yr^–1 and 1.64 kg N capita^–1 yr^–1 for China and Japan, respectively. Thus, secondary products significantly contributed, 9.0% and 11.2%, to the total N-footprints of China and Japan, respectively.

Another reason for the differences between results of the two estimation methods is associated with the nitrogen contained in by-products that are utilized in food systems, e.g. protein meal. In the N-Input method, oil was regarded as the main product and protein meal as the by-product, and hence new nitrogen input to produce oilseed crops used for oil production was included in the N-footprint of oil but not in the N-footprint of protein meal. In the N-calculator method, on the other hand, the new nitrogen input for protein meal production may be included in the estimation of VNFs of livestock products, although this is not clearly described in the relevant published researches. If this is indeed the case, then the uncounted nitrogen for secondary products (vegetable oil etc) in the N-calculator method may be partly compensated by the nitrogen input for protein meal production fed to livestock.

Some of the oil was not used for food and therefore the new nitrogen input for that portion was not included in the food N-footprint in the N-Input method. Beside the oil, some food products are being allocated to other-uses: 14% of total new nitrogen inputs in China were sent off to other-uses as shown in 3.2. For example, 1.02 million t of bio-ethanol was produced in China in 2005, with maize and cassava as the main feedstocks (Wang et al 2009). Food products are widely used as feedstocks of biofuels and industrial goods in many other countries too; e.g. 28.9% of supplied maize was used for bio-ethanol and 9.8% for other industrial application in US in 2016–2017 (Mohanty and Swain 2019). The new nitrogen input for the food products allocated to other-uses should be counted in energy or goods and services sectors, but it was not included in N-footprint for these sectors as far as the authors know.