Assessing nitrogen flow and nitrogen footprint in the food system of a subtropical island with a scenario to mitigate nitrogen load impacted by trade-dependent agriculture

Figure 1 illustrates the nitrogen flow in the food system of Ishigaki Island. Nitrogen flow in the year 2018 was calculated using official statistical data (Ishigaki 2018). Major crops and livestock listed in the statistics were used as agricultural products in the calculation. For unavailable data on the city scale, statistical data on food/feed production on the prefectural and national scales, as well as data from previous studies, were applied. See supplementary tables 1–4 for details.

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For sugarcane, we considered BNF based on Ando et al (2001), who reported that 1/3 of nitrogen in sugarcane bodies is supplied by BNF. Based on the interview, we assumed that the bagasse ash left over after the combustion process in the sugar factory is returned to the sugarcane fields as fertilizer. We took pumpkin as a representative of vegetables because it is the most widely cultivated vegetable on Ishigaki Island (Okinawa 2012). With the exception of sugarcane, we assumed food processing waste and food loss on the island were incinerated, i.e. their Nr was released into the environment. All livestock excreta were assumed to be converted into manure. The nitrogen volatilization from livestock barns and during the conversion was calculated in accordance with NIES (2022) (supplementary table 5). For simplification, all manure Nr used on the island was assumed to be cattle manure Nr because the amount of other livestock manure Nr was relatively small (see section 3.1.1.). Based on the interviews, we assumed that no manure was applied to the sugarcane or pineapple fields or grasslands. In beef cattle production, we considered grazing and assumed that excreta was returned to the grassland during the grazing period. In this study, we did not consider denitrification and ammonia volatilization from agricultural land. We assumed that no feed was applied in seafood production.

All concentrate feed and chemical fertilizer were assumed to be imported. Import of food was calculated to meet the demand of food supply of the island (see section 2.2.5.). Exports were considered for the major export-oriented products of beef calves (live animals), sugarcane (coarse sugar), pineapples, vegetables, and seafood (seaweed and natural tuna). We also regarded the livestock shipped out for slaughter (dairy cattle, pigs, broilers, and egg-laying hens) as export. We could not find data regarding milk and egg exports. Thus, for this study, we assumed no export of milk or eggs occurs on Ishigaki Island. The removal of human excreta Nr by denitrification during sewage treatment was calculated as previously described by Shibata et al (2014).

For each of the major food items produced on the island, Nr losses from different pathways (crop-unused in the field, crop processing waste, livestock manure, slaughter waste, food processing waste, food loss, and human excreta; figure 1) were calculated to determine large pathways of nitrogen release on the island. We also estimated the Nr loss in overseas regions for concentrate feed production to evaluate the influence of livestock production on environmental load. Assuming that concentrate feed was primarily formulated from corn, we calculated the amount of Nr loss and Nr input during grain corn production in overseas regions using parameters for the U.S. reported by Leach et al (2012) because Japan imports corn mainly from the U.S. (Ministry of Agriculture, Forestry and Fisheries (MAFF 2021)).

The nitrogen footprint is defined as the sum of Nr losses to the environment due to various human activities. It is generally determined in relation to consumer behavior (Leach et al 2012). In this study, we focused on products that are produced and consumed within the island, because these products affect the nitrogen load on the island directly through the consumers' diets. We estimated the nitrogen footprint excluding Nr loss to the environment for exported production and imported food to identify the island products for the consumers' diets that affect the nitrogen load the most. Per-capita nitrogen footprint for food for island consumers was calculated based on Nr loss to the environment (both on Ishigaki Island and in overseas regions) and the population (=59 392; number of tourists included).

We estimated the amount of imported coarse food Nr by compensating for the deficit of coarse food on the island, that is, by subtracting coarse food produced for consumption on the island from the average values of coarse food demand for Japan (Eguchi and Hirano 2019, Hirano et al 2023). The amounts of net food Nr and consumed Nr of imported coarse food were calculated using data from previous studies (Eguchi and Hirano 2019, Hirano et al 2023).

To estimate Nr loss in overseas regions for imported coarse food production, we applied the following equation:

$$\begin{equation*}{\text{Loss}} = {\text{ Consumed}} \times \left( {{\text{VNF}} + {\text{1}}} \right) - {\text{Coarse}},\end{equation*}$$

where Loss indicates Nr loss in overseas regions (kgN); Consumed indicates consumed Nr of imported coarse food on Ishigaki Island (kgN); VNF indicates virtual nitrogen factor and is defined as total Nr loss to the environment excluding consumed Nr divided by consumed Nr; Coarse indicates the imported coarse food Nr (kgN). The value of VNF for each production was adopted from Eguchi and Hirano (2019). The values used are listed in supplementary table 6.

The nitrogen flows for all food items were summed to comprehensively assess the present nitrogen flow in the island's food system. We estimated the manure utilization ratio (used cattle manure/total produced livestock manure) to understand the present manure use situation and to judge whether it is possible to replace 30% of chemical fertilizer. A scenario to mitigate Nr loss on the island was created following the agri-environmental policy of the Japanese government strategy MeaDRI (MAFF 2021). In the analysis, we assumed that manure was applied to all crops and that crop yields were kept constant even after chemical fertilizer Nr was replaced with manure Nr based on the previous studies (e.g. Anai et al 2010, de Ponti et al 2012, Liu et al 2021). We chose the use of cattle manure in the analysis because beef cattle production is a key and indispensable industry on the island, and increasing the manure utilization ratio can be controlled by society. In contrast, other measures, such as increasing fertilizer use efficiency, require more scientific knowledge.

For agricultural activity, Nr loss on the island was 1 523 967 kgN, whereas that in overseas regions was 184 133 kgN for concentrate feed production (figure 2). The major Nr loss pathways in Ishigaki Island were 'crop-unused' and 'manure' occupying 37% and 43% of Nr loss on the island, respectively. Sugarcane and beef had the highest Nr losses among the crops/animals because their cultivated areas/numbers of animal heads were also the largest. According to Nakanishi (2008) and Miyamoto et al (2018), chemical fertilizer and livestock manure are the main sources of nitrogen loads on Japanese subtropical islands including Ishigaki Island. This is consistent with our results (figure 2). Kaji and Nagatomo (2014) reported that approximately 60% of the nitrogen applied to sugarcane is unused (i.e. leached, volatilized, or remained in the soil). The calculation results showed that 45% of Nr applied by the chemical fertilizer Nr and BNF was lost as crop-unused Nr (supplementary figure (a)) indicating higher nitrogen use efficiency than in the report. Note that the nitrogen flow of sugarcane production had a large amount of food processing waste, occupying 41% of total Nr loss, which is mainly induced by bagasse combustion, although emitted NO_x concentration is lower than the regulation level (based on interview). For pineapple and vegetables, 90% of Nr loss on the island was estimated to be crop-unused (figure 2). This indicated that the nitrogen use efficiency was very low and that the improvement of fertilizer application could significantly reduce the Nr loss. Unused Nr of paddy rice production showed the lowest ratio (71%) because of lower fertilizer application rate (supplementary table 4).

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In animal production, Nr loss also occurs overseas because of imported concentrate feeds. To cattle, goats, and horses, coarse feed produced on the island was applied; therefore, crop-unused Nr was involved. The largest manure producer was cattle (beef and dairy cattle), which accounted for 88% of the total manure Nr (figure 2).

The calculation revealed that, in total, 28% of Nr loss to the environment is related to food production for island consumers (table 1). Per-capita nitrogen footprint was large for vegetables and beef cattle. We considered that this high footprint is due to low nitrogen use efficiency and import rate. Regarding nitrogen use efficiency, the prefectural standard for fertilizer application for pumpkin is higher than the Japanese average (supplementary table 4(d)). On Ishigaki Island, the total fertilizer application was 700 kgN ha^–1 (=chemical fertilizer 280 kgN ha^–1 + manure 420 kgN ha^–1), whereas that of the Japanese average is 540 kgN ha^–1 (=chemical fertilizer 400 kgN ha^–1 + manure 140 kgN ha^–1) (Mishima 2002, Mishima et al 2012). Our result indicated that unused Nr accounted for 90% of Nr loss (figure 2). For beef cattle, 86% of feed Nr (=coarse feed Nr + concentrate feed Nr) was released as manure Nr. In contrast, the 'other animals' released about 70% of feed Nr as manure Nr (except goat and hoarse = 92% and 91%, respectively) (supplementary figures (e)–(k)). Our estimation of food import (section 3.1.3.) revealed that the import rate of vegetables and beef were 51.5% and 0%, respectively, while those of most of the other products were higher than 65% (except egg and other meat = 56% and 50%, respectively) (table 2). This low import rate with low nitrogen use efficiency resulted in the large per-capita nitrogen footprint of vegetables and beef. Note that for egg, the per-capita nitrogen footprint was smaller than that of vegetables because egg had higher consumed Nr than vegetables (7 490 and 4 524 kgN, respectively) (supplementary figures (d) and (i)) and Nr loss to the environment for island consumers' diets per consumed Nr of egg was 37% of that of vegetables (4.2 and 11.3, respectively). Other meat (goat and horse) has low per-capita nitrogen footprint because their coarse food production was small (supplementary table 7).

Our results showed that Nr loss to the environment for food production for the consumers was small, and most of the Nr loss was related to export production indicating that Ishigaki Island shouldered a burden of Nr loss for food consumption in overseas regions. Whereas food production on the island was not enough to satisfy consumer demand. We, therefore, estimated imported food Nr on the island.

Except for beef, coarse food production on Ishigaki Island was not sufficient to meet the demand of the consumers (table 2). The produced coarse food was 55 977 kgN (supplementary table 7), whereas imported food was estimated to be 5.1 times larger than produced food on the island, reflecting the island's characteristic of heavy dependence on food imports (Maesato et al 2016). The amount of Nr loss in overseas regions for imported food (=470 281 kgN; table 2) was higher than that on Ishigaki Island (=408 665 kgN; table 1). This indicated that Nr loss on the island is made smaller by placing part of the burden of Nr loss on overseas regions to support the island's needs.

On the island, half of the imported Nr originated from chemical fertilizers for agriculture, followed by concentrate feeds for animal production (figure 3(a)). In overseas regions, the amount of Nr input was relatively similar to that on the island (concentrate feed, BNF, chemical fertilizer, and manure = 1 747 665 kgN). Moreover, Nr loss in overseas regions was close to the amount of Nr in the imported products (food and concentrate feeds = 773 962 kgN). A breakdown of chemical fertilizer and Nr flow in overseas regions is shown in supplementary table 8. As previously described in section 3.1.2., most of the Nr loss on the island was a result of export production. These results show that Ishigaki's food system is characterized as trade-dependent, that is, the island shoulders a burden of Nr loss for the food consumed in overseas regions; it simultaneously places a burden of Nr loss for imported food and feed on overseas regions.

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Agriculture on Ishigaki Island uses mainly chemical fertilizer; however, less than 13% of cattle manure was reused for agricultural production. This indicated the necessity of resource recycling, and the amount of remaining manure was enough to replace 30% of chemical fertilizer.