叶类蔬菜的硝态氮累积及成因研究

The cause of nitrate accumulation in leafy vegetables

在菜园土壤上进行的田间试验 ,用禾谷类作物冬小麦作比较 ,研究了菠菜、小白菜、大青菜和油菜等叶类蔬菜累积硝态氮的特点。结果表明 :硝态氮累积是一般旱作作物的共性 ,苗期更为明显 ,无论蔬菜还是冬小麦均有较高的硝态氮含量 ( 367.8～ 1 4 1 3.4 μg/ g) ;但随生育期后延 ,蔬菜的硝态氮含量波动升高 ,冬小麦波动降低。盆栽试验表明 ,施入土壤的氮肥是蔬菜硝态氮累积的主要来源 ;过量施用氮肥所导致的蔬菜硝态氮吸收与还原转化不平衡是产生累积的根本原因 ;

Field and pot experiments were carried out to study nitrate accumulation in leafy vegetables.N application rates at 0,90,180,270kg N/hm 2 were used in the field experiments with Chinese cabbage(\%Brassica Chinensis var. Oleifera Makino et Nenoto),green cabbage(\%Brassica Chinensis \%L.),spinach(\%Spinacia oleracea\% L.)and repe(\%Brassica campestris\% L.)as vegetables,and winter wheat(\%Triticusp aestvum\% L.)as a comparison crop.and they were planted on a vegetable field.The N rates of 0,0.20,0.40,0.60 and 0.80g N/kg soil were adopted for the pot experiments,using Chinese cabbage,spinach and rape as indicators,and the pots were placed in agreenhouse.\;The field experiment results showed that the nitrate accumulation was a very common phenomenon for all crops in the experiments,especially at their seedling stages.The nitrate concentrations of the 4 leafy vegetable species ranged from 367.8 to 1413.4 μg/g,while those in winter wheat ranged from 730.1 to 807.4μg/g at the early stages.For comparison of species differences in nitrate accumulation,a mean value of different N rates were calculated for each crop.The data showed that the concentration in winter wheat was 16.4% lower than that in Chinese cabbage,14.6% lower than that in green cabbage and 39.9% lower than that in rape,while 59.2% higher than that in spinach.It is difficult to distinguish the crop more prone to accumulate nitrates between leafy vegetables and winter wheat at their early stage.However,with plant growth,the nitrate concentrations in the leafy vegetables increased,whereas those in winter wheat decreased in a fluctuating way.The average concentrations of the entire growing stage were significantly higher for the vegetables than those for wheat.It was 1069.3μg/g in Chinese cabbage,452.6μ/g in spinach,while only 338.7μg/g in winter wheat.Obviously,as a whole,the leafy vegetables had higher abilities to accumulate nitrate than winter wheat did.\;Pot experiments showed that application of N fertilizer increased the vegetable growth at lower N rates,but inhabited it at higher N rates.Chinese cabbage reached its highest yield at 0.60g N/kg soil,and rape and spinach at 0.40g N/kg soil.When the N rates were increased to 0.80g N/kg soil,the yields of all vegetables were significantly decreased.Different from the yields,the nitrate\|N concentrations in the leafy vegetables were in continuous increase as the N rates increased.Compared with that without N fertilization,nitrate\|N concentrations were increased 5.4 to 79.7 times for Chinese cabbage,3.4 to 104.4 times for rape,and 12.1 to 126.4 times for spinach.The nitrate\|N concentrations were positively correlated with the N rates,the coefficients ranging from 0.933 to 0.957,significant at 0.01 level.Obviously,the N fertilizers added to soil were the major source for vegetables having high nitrate accumulation,being able to account for 87.0% to 91.6% of the nitrate accumulated in the 3 leafy vegetables.\;With the increase of nitrate\|N concentration,nitrate reductase activities were obviously increased.The activities of the nitrate reductase reached 1.19μg NaNO\-2/g·h.for Chinese cabbage and rape,and 1 97μg NaNO\-1/g·h.for spinach at 0.60g N/kg soil,while the corresponding values were only 0.16,0.21,and 0 10 μg NaNO\-2/g·h.at 0.20g N/kg soil.The nitrate reductase is a key enzyme for plants to reduce nitrate,and high activities of the enzyme usually mean high nitrate reduction abilities.However,in this experiment the high activities did not lead to reduction of the nitrate in the leafy vegetables to a low concentration.The reason for this phenomenon may be related with the physiological characteristics of the nitrate reductase.The nitrate reductase is a substrate\|inducible enzyme and its activities closely depend on supply of the substrates:the more nitrate in the medium,the higher activity of the enzyme will be.With the increase of N rates,the vegetables took up more nitrate that stimulated the activities of the nitrate reductase in plant tissues.However,the increase of nitrate reduction abil