A survey on current fertilizer practices and their effects on soil fertility and soil salinity was conducted from 1996 to 2000 in Beijing Province, a major vegetable production area in the North China Plain. Inputs of the major nutrients ͑NPK͒ and fertilizer application methods and sources for different vegetable species and field conditions were evaluated. Excessive N and P fertilizer application, often up to about 5 times the crop requirement in the case of N, was very common, especially for high-value crops. Potassium supply may have been inadequate for some crops such as leafy vegetables. Urea, diammonium orthophosphate ͑͑NH 4 ͒ 2 HPO 4 ͒ and chicken manure were the major nutrient sources for vegetable production in the region. Over 50% of N, 60% of P and nearly 90% of K applied originated from organic manure. Total N application rate for open-field Chinese cabbage from organic manure and inorganic fertilizers ranged from 300 to 900 kg N ha -1 on 78% of the farms surveyed. More than 35% of the surveyed greenhouse-grown tomato crops received Ͼ 1000 kg N ha -1 from organic and inorganic sources. A negative K balance ͑applied K minus K removed by the crop͒ was found in two-thirds of the surveyed fields of open-field Chinese cabbage and half of the surveyed fields of greenhousegrown tomato. Plant-available N, P and K increased with increasing length of the period the greenhouse soils had been used for vegetable production. Similarly, soil salinity increased more in greenhouse soils than in open-field soils. The results indicate that balanced NPK fertilizer use and maintenance of soil quality are important for the development of sustainable vegetable production systems in this region.

Numerous models have been developed for calculating optimum decision rules for nitrogen fertilization based on remote sensing techniques. New technologies related to digital image analysis may provide an alternative method to estimate nutrient status faster and more efficiently than current techniques. A series of field studies was conducted to determine the applicability of digital image analysis for nitrogen demand estimates in broccoli plants. Digital images were taken under constant light conditions in various wavelength ranges (380–1300 nm) using a digital imager. Images were processed for the parameters and in the color system. The image analysis showed a close correlation between the nitrogen status of broccoli plants and the parameter of the color system especially in the wavelength ranges and  nm. The relationship between nutrient concentration in leaf dry matter and the parameters was used to determine the N fertilizer demand within the cultivation period. Estimated N amounts were applied as top dressing four weeks after setting and were 100 kg lower than the standard fertilizer rate. Calculated N balances indicated a total uptake of applied N amounts in the image-based N treatments without considerable yield loss. Thus, digital image analysis proved to be an effective means of determining nitrogen status and adjusting fertilizer applications to preserve or enhance crop quality and yield considering sustainability.

A series of field experiments were conducted over a three-year period to test a modified N-Expert system (the decision support system for nitrogen recommendations) with different irrigation regimes for a rotation of amaranth (Amaranthus tricolor L.), spinach (Spinacia oleracea L.), and cauliflower (Brassica oleracea L.). Local commercial fertilizer practice based on farmer surveys was selected as the control, and irrigation treatments followed conventional practice with a balanced irrigation schedule. Some parameters, notably target yields and net mineralization rates, were modified according to local conditions, and most were referenced from the database of the N-Expert system in the first cultivation year of the rotation. Parameters were tested in earlier field experiments and were fine-tuned in later experiments. No significant yield reduction occurred below conventional nitrogen (N) practice using the recommended N treatment (N2) with different water treatments, except for irrigation treatments between 60% and 90% of PESW (plant extractable soil water: the difference between field capacity and wilting point) in 2000 and conventional irrigation practice in 2001 for cauliflower. There was a significant decrease in residual N min at harvest in treatments using the modified N-Expert recommendation system compared with conventional N practice. This indicates that there is considerable potential for use of the N recommendation system for sustainable vegetable production in the North China Plain.

Advanced nitrogen (N) advisory systems require target values of N supply for the crop and the results of soil testing for inorganic nitrogen (N min ) before cultivation and estimates of N release from mineralization. Two field experiments with different N supply levels were conducted in the Beijing region to determine the target values of N supply and N mineralization rates for optimization of N fertilization of Chinese cabbage (Brassica campestris L. ssp. Pekinensis) and carrot (Daucus carota L.). Crop yield, N uptake, and ORDER REPRINTS soil inorganic N were investigated during the crop growth periods. Marketable yields of the two crops increased significantly with N application rate, and were described as linear-plateau functions (Chinese cabbage: Y ¼ 0.16N þ 80.5, when 0 < N 319; Y ¼ 112.7, when N ! 319; and carrot: Y ¼ 0.12N þ 50.5, when 0 < N 247; when Y ¼ 67.1, N ! 247; where: Y was marketable yield, t FW ha À1 ; N was N supply level, the sum of soil inorganic N at preplanting and applied fertilizer N, kg N ha À1 ). Similar trends were also observed in crop N uptake. The agronomically effective N supply levels for Chinese cabbage and carrot growth in Beijing region were 349 and 277 kg N ha À1 , respectively, for target marketable yields of 120 t FW ha À1 for Chinese cabbage and 65 t FW ha -1 for carrot considering extra 30 kg N ha À1 of fertilizer to avoid the risk of crop yield reduction. During the crop growth period net N mineralization in the root zone was 38 kg N ha À1 for Chinese cabbage and 46 kg N ha À1 for carrot. Weekly N mineralization rate was about 3.2 kg N ha À1 . Nitrogen loss coefficient, defined as the ratio of average weekly apparent N loss to N supply level, ranged from À0.011 to 0.053 for the different N supply levels in the two experiments. The N net mineralization rates determined, N loss rates and target values were found to be useful for prediction of optimum N supply based on N balance calculations for sustainable vegetable production.

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