Nutrient balance in farmlands and the resulting environmental risk in Hainan Province

Objectives Cash crop production accounts for a large proportion of agriculture in Hainan Province, China. Here, we compared the three typical crop production systems in Hainan, focusing on nutrient input rate and balance and the potential for environmental risks.

Methods According to the Hainan Statistical Yearbook, the crops in Hainan Province are categorized into grains, vegetables, fruits, and other cash crops. Farmers were randomly surveyed, and 1199 effective samples were obtained. We investigated farmlands’ nutrient source, input rate, and annual nutrient surplus. Further, we quantitatively evaluated environmental risks using reactive nitrogen loss, greenhouse gas emission, and eutrophication effect.

Results The average chemical N, P₂O₅, K₂O application rates in Hainan were 261, 206, and 225 kg/hm², which were 10%, 101%, and 148% higher than the national average levels, and the excessive P and K application was particularly prominent in vegetables, reaching 164% and 138% higher than the national levels. Across the whole region of Hainan, coastal cities/counties had higher nutrient intensity than the central part, e.g. the nutrient input densities in Changjiang and Haikou were 1.0–2.2 times higher than those in Baisha and Qiongzhong. The most applied chemical fertilizers were urea, superphosphate, potassium chloride, and compound fertilizer in the N-P₂O₅-K₂O of 15-15-15. Organic fertilizers were used in low proportion, accounting for 4%, 20%, and 12% of the total N input in grains, vegetables, and fruits, respectively, which are lower than the national average organic proportion. The annual N surplus in rice-pepper rotation, banana, and pineapple plantations were 520, 675, and 667 kg/hm², respectively. Similarly, the annual P surplus were 217, 277, and 228 kg/hm², respectively. The current N surplus in banana and pineapple production was over 8 times above the environmental safety threshold. Pineapple production exhibited the highest reactive nitrogen loss, global warming, and eutrophication potential. NO₃⁻ leaching contributed the highest N loss during the field application stage (85%). This was followed by NH₃ volatilization, nitrous emission, and reactive nitrogen loss during fertilizer production and transportation processes. The quantity of greenhouse gas emissions during the fertilizer production and field application period was similar. Fertilizer application period had higher eutrophia potential than fertilizer production period. N and P application contributed 62.5% and 35.7% of the surface water eutrophia.

Conclusions Hainan's agricultural production is characterized by a high proportion of cash crops with a high nutrient input rate, especially the excessive application of P and K, with a high nutrient surplus and low proportion of organic fertilizer. Coastal cities and counties have more cash crop production than inland areas, leading to higher P and K load in offshore waters. The environmental risks related to fertilizer application are higher than nutrient production. Nitrate leaching is the main source of N loss, and N and P applications primarily contribute to water eutrophia. Pineapple production creates the highest environmental risk in Hainan, requiring urgent optimization of nutrient management.