Hainan is the province with a large area of rice-vegetable rotation and banana cultivation in China, but the nitrogen (N) and phosphorus (P) fertilizers input by farmers far exceed the nutrient requirements of crops, which may have a negative impact on Hainan’s ecological environment. The aim of this study was to investigate the N and P surpluses, and soil nitrate accumulation in typical cropping systems in Hainan, thus to provide scientific basis for evaluating nutrient losses and their impacts and achieving sustainable nutrient management.

In 2021-2022, a typical area for rice-vegetable rotation and banana cultivation in Chengmai, Hainan, was selected as the research area and 20 rice-vegetable rotation plots and 15 banana orchards were identified. The information of chemical and organic fertilizer application, straw returning method and amount of above fields were obtained by real-time record of farmers’ agricultural activities, crop biomass and the nutrient content were determined at crop harvest, and other nutrient input include nutrient deposition and biological N fixation were obtained by literature survey. Five banana orchards were selected and soil was collected by soil auger method and nitrate N accumulation was measured in the 0-400 cm soil profile.

The N and P fertilizer inputs to the rice-vegetable rotation were 1 308 kg N·hm^-2 (975 kg N·hm^-2 of chemical and 333 kg N·hm^-2 of organic fertilizer) and 515 kg P·hm^-2 (385 kg P·hm^-2 of chemical and 130 kg P·hm^-2 of organic fertilizer); the aboveground N and P uptake of the crop were 248 kg N·hm^-2 and 48 kg P·hm^-2; the surplus of N and P in rice and vegetable rotation was 1 196 kg N·hm^-2 and 484 kg P·hm^-2. The N and P fertilizer inputs to banana orchards were 1 340 kg N·hm^-2 (1 293 kg N·hm^-2 of chemical and 47 kg N·hm^-2 of organic fertilizer) and 447 kg P·hm^-2 (442 kg P·hm^-2 of chemical and 5 kg P·hm^-2 of organic fertilizer); the aboveground N and P uptake were 242 kg N·hm^-2 and 23 kg P·hm^-2; the banana N and P surpluses were 1 271 kg N·hm^-2 and 435 kg P·hm^-2. The nitrate-N accumulation in the 0-400 cm soil profile of banana orchards was 1 131 kg N·hm^-2.

Excessive application of N and P fertilizers has led to the large nutrient surplus in typical soil-crop systems in Hainan, and large amount of nitrate-N has accumulated in banana orchard in the deep soil layer. Hainan produces typical high-value fruit and vegetables at the cost of large nutrient losses and negative environmental impacts, optimized nutrient management should be implemented to ensure its environmental safety.

We aim to quantify aboveground nitrogen (N) uptake in per hundred kilograms grain (N₁₀₀) of maize in different agro-ecological zones at different yield levels in China, and analyze the effects of climate, soil, variety and N fertilization on N₁₀₀ of maize, thus to provide a scientific basis for determining rational N fertilizer rate.

We divided Chinese cropland into six major regions, i.e., northeast, northwest, North China Plain, middle and lower Yangtze River, southwest, and southeast, and collected 349 peer-reviewed papers published during 1980-2022 to analyze the spatial variation of N₁₀₀ and its changes at different yield levels, and compared the differences in calculated theoretical N rate between constant and region-specific N₁₀₀. The effects of climate, soil and fertilization on N₁₀₀ were analyzed using Pearson correlation coefficient, Random forest model and Meta-analysis, to reveal the causes of spatial variation in N₁₀₀.

Under the optimized N management, N₁₀₀ of spring maize was significantly lower than that of summer maize which were 2.21 and 2.46, respectively; and there were significant differences in N₁₀₀ of maize among different agro-ecological zones, which were 2.19 (Northeast spring maize), 2.12 (Northwest spring maize), 2.54 (Northwest summer maize), 2.45 (North China Plain summer maize), 2.77 (Middle and Lower Yangtze River spring maize), 2.38 (Middle and Lower Yangtze River summer maize), and 2.39 (Southwestern maize zone), respectively. The difference between calculated the theoretical N rate based on the national average N₁₀₀ (2.34) and that based on regional-specific N₁₀₀ was -22-31 kg N·hm^-2. Aboveground N uptake, yield, and mean annual temperature were the most important factors affecting N₁₀₀. The N₁₀₀ showed a significant quadratic decrease with increasing yield (P<0.01), and grain yield was a good predictor of N₁₀₀. Varieties significantly affected maize N₁₀₀, the N₁₀₀ of common Chinese maize varieties Zhengdan 958, Xianyu 335, and Denghai 605 are 2.42, 2.12, and 2.39, respectively. New varieties had a significant lower N₁₀₀ than old varieties. The application of N fertilizer significantly increased the N₁₀₀ of maize, and the greatest increase effect of N₁₀₀ caused by N fertilizer application was observed at 200-300 kg N·hm^-2. Once application of slow and controlled release fertilizer, deep placement, reduction of the ratio of basal N fertilization and increasing the frequency of N fertilizer application all significantly increased N₁₀₀.

When calculate the rational N fertilization, we need to considerate the regional differences of N₁₀₀, thus to obtain accurate fertilizer N rate, and the N₁₀₀ of maize is mainly driven by variation in aboveground N uptake, yield and mean annual temperature.