| Continuously excessive nitrogen input in greenhouse is one of the major problems in Shouguang, the famous vegetable production base in China, and this led to deteriorated product quality, high risk of nitrate leaching, soil secondary salinity, and low fertilization profit, etc. The objectives of the study were to develop a feedback control system of nitrogen topdressing recommendation and related target values of nitrogen supply at different growth stages for greenhouse tomato in Shouguang. Two-year field trials in different cultivating seasons from 2002 to 2003 were conducted using real-time nitrogen control index, i.e. nitrate in petiole sap, fruit expanding rate, marketable yield and quality, and recommending techniques of PSNT (pre-sidedress nitrate test) and nitrogen balance method for nitrogen topdressing, in comparison with farmer's conventional practice, on the basis of enough basal fertilizer application. The main results were as follows:Optimized nitrogen supply levels (nitrate content in effective root zone plus applied N rate at each topdressing) were 237, 173 and 153 kg-hm-2 N at the first, second and fourth cluster fruit expanding stages of autumn-winter tomato, respectively. During the intervals between the three times of topdressing and harvest, apparent nitrogen mineralization amounts from soil organic matter were 53, 13 and 21 kg-hm-2 N, respectively, and 41, 8 and -17 kg-hm-2 N were released from applied organic manure, respectively. Meanwhile irrigated water also supplied 11,5 and 5 kg-hm-2 N during the investigated periods. Including nitrogen input from organic nitrogen and irrigated water, target values of nitrogen supply for the three topdressing stages were 342, 199 and 162 kg-hm-2 N, respectively, for autumn-winter greenhouse tomato, and nitrogen target value for the whole growth stage was 481 kg-hm-2 N in order to achieve the target yield of 73 t hm-2 in the region.In the trial of winter-spring greenhouse tomato, N topdressing was conducted at the first, second, forth and fifth cluster fruit expanding stages. The results indicated that the optimum nitrogen supply levels at each time of topdressing were 180 kg-hm-2 N at the first, second, forth and fifth cluster fruit expanding stages, and 143 kg-hm-2 N at mid harvest stage. Apparent nitrogen mineralization amounts from soil organic matter were -13, 36, 52, 23, and -9 kg-hm-2 N, and nitrogen supply amounts from irrigated water were 7, 6, 7, 13, and 6 kg-hm-2 N at the intervals between the five times of topdressing and harvest, respectively. So nitrogen target values were 174, 222, 239, 216, and 140 kg-hm-2 N at the five topdressing stages, and the total nitrogen target value was 415 kg-hm-2 N at the target yield of 87 t-hm-2 in winter-spring season.The analysis of apparent nitrogen balance in the year-round greenhouse tomato cultivating system showed that apparent nitrogen loss increased with nitrogen supply level. Positive correlation between apparent nitrogen loss and total nitrogen supply level was investigated while soil nitrogen supply level was from 200 to 800 kg-hm-2 N. Apparent nitrogen loss was higher in autumn-winter season than in winter-spring season due to relatively higher nitrogen input. Compared with farmer's conventional treatment, significant effects on reduction of soil Nmin accumulation, soil nitrate leaching and saltsaccumulation in surface soil layer were investigated in the recommended treatment. So it could be concluded that soil quality of cultivating was improved in the recommended treatment, and this could prolong the use of greenhouses.Very low nitrogen use efficiencies were investigated in farmer's conventional treatment in the greenhouse tomato trials. Nitrogen use efficiency was less than 20% in all the treatments in the trials with different seasons. There was no obvious difference in profit return of fertilization between the two treatments in autumn-winter season. However in winter-spring season, profit return of fertilization in farmer's conventional treatment was much lower than the reco...
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