| The ability that plants directly absorb small molecular organic nitrogen from soil has been generally recognized,but the nutritional effects of organic nitrogen on plants and the mechanism of plant uptake organic nitrogen and its influencing factors are still unclear.Some researchers found that the absorption of inorganic nitrogen by plants decreased significantly,but it could continue to effectively absorb organic nitrogen through the reduction of non-structural carbohydrate(NSC)content in plant roots caused by girdling.Therefore,plants may be more preference to absorb organic nitrogen when the supply of NSC is insufficient.However,the above results were obtained by hydroponics culture,the nutritional effects of organic nitrogen on plants and the absorption characteristics of organic nitrogen in plants under natural conditions when the supply of carbohydrates was affected need to be studied.In order to verify whether plants tend to absorb organic nitrogen when their carbohydrate supply is disturbed in natural environment.Seedlings of Betula platyphylla and Larix gmelinii,which are common tree species in northern China,were used as materials in this study,and adopted greenhouse potting methods to cultivate seedlings that carbohydrate supply was changed by pruning the shoot(30%and 60%pruning intensity,respectively)to simulate disturbance caused by machine damage and pest.Different forms of nitrogen(ammonium nitrate as inorganic nitrogen and glycine as organic nitrogen)were used to fertilize seedlings.The growth,carbon and nitrogen accumulation and characteristics of uptake nitrogen of Betula platyphylla and Larix gmelinii seedlings with different forms of nitrogen fertilization responses to pruning were studied by using non-pruning and non-nitrogen nutrient solution as control(CK).The main results are as follows:(1)The biomass of Betula platyphylla and Larix gmelinii seedlings increased with the application of different forms of nitrogen,but seedlings has different responses to pruning and different nitrogen forms.Application of inorganic nitrogen was more beneficial to the biomass accumulation of Betula platyphylla seedlings than application of organic nitrogen under no pruning,and application of organic nitrogen was more beneficial to the biomass accumulation of Betula platyphylla seedlings under pruning than application of inorganic nitrogen.Regardless of whether pruning or not,application of organic nitrogen was more beneficial to biomass accumulation of larch seedlings than application of inorganic nitrogen.Under the condition of no pruning,light pruning and heavy pruning,the biomass of Betula platyphylla seedlings was increased by 36%,9%and 14%with inorganic nitrogen,21%,10%and 17%with organic nitrogen;the biomass of Larix gmelinii seedlings was increased by 7%,13%and 12%with inorganic nitrogen,12%,17%and 23%with inorganic nitrogen,respectively.(2)The application of different forms of nitrogen promoted the nitrogen content and nitrogen accumulation in the seedlings of Betula platyphylla and Larix gmelinii.Under the condition of no pruning and mild pruning,the application of inorganic nitrogen was more beneficial to the increase of nitrogen accumulation in the seedlings of Betula platyphylla and Larix gmelinii than that of organic nitrogen.Under the condition of severe pruning,the application of organic nitrogen was more beneficial to the increase of nitrogen accumulation in the seedlings than that of inorganic nitrogen.Under the condition of no pruning,light pruning and heavy pruning,inorganic nitrogen application increased the nitrogen accumulation of Betulaplaryhylla seedlings by 41%,25%and 21%,organic nitrogen application increased by 27%,19%and 34%,and inorganic nitrogen application increased the nitrogen accumulation of larch seedlings by 25%,37%and 38%,and organic nitrogen application increased by 21%,32%and 58%,respectively,compared with the control.(3)Nitrogen forms had a significant effect on soluble sugar accumulation in the aerial part of Betula platyphylla seedlings,but had no significant effect on NSC accumulation in the underground part.Among the two forms of nitrogen fertilization,the application of organic nitrogen was more conducive to the accumulation of soluble sugar in the aerial part.Compared with the control,the aerial soluble sugar accumulation of Betula platyphylla seedlings decreased with the application of inorganic nitrogen and organic nitrogen,but the soluble sugar accumulation with the application of organic nitrogen was higher than that with the application of inorganic nitrogen.Under the condition of heavy pruning,the aerial part soluble sugar accumulation of Betula platyphylla seedlings with the application of organic nitrogen increased by 8%compared with the control,the application of inorganic nitrogen in seedlings decreased by 42%compared with the control.The interaction of pruning and nitrogen forms had a significant effect on the accumulation of NSC in the aerial part of larch seedlings.The application of inorganic nitrogen was more beneficial to the accumulation of NSC in the underground part than that of organic nitrogen under the condition of no pruning and heavy pruning.Compared with the control,the application of inorganic nitrogen and organic nitrogen reduced the accumulation of NSC in the underground part by 23%and 30%,respectively.Under the condition of heavy pruning,the application of inorganic nitrogen increased by 3%and the application of organic nitrogen decreased by 24%.Under mild pruning,the application of organic nitrogen was more conducive to the accumulation of NSC in underground part.Under mild pruning,the application of inorganic nitrogen and organic nitrogen reduced the accumulation of NSC in underground part by 72%and 43%,respectively.(4)There are differences in the absorption capacity of organic nitrogen between Betula platyphylla and Larix gmelinii seedlings.The absorption capacity of Larix gmelinii seedlings to organic nitrogen is higher than that of Betula platyphylla seedlings.The proportion of molecular organic nitrogen uptake by birch seedlings increased after pruning.Without pruning,mild pruning and severe pruning,the proportion of molecular organic nitrogen uptake by birch seedlings without fertilization was 54%,58%and 76%,inorganic nitrogen application was 97%,100%and 100%,and organic nitrogen application was 19%,91%and 100%,respectively.Under the condition of no fertilization and application of inorganic nitrogen,the absorption ratio of molecular organic nitrogen of larch seedlings decreased with the increase of pruning intensity.The absorption proportion of molecular organic nitrogen of larch seedlings without fertilization was 88%,72%and 12%,respectively,and that of inorganic nitrogen was 94%,73%and 31%,respectively.Under the condition of application of organic nitrogen,the absorption proportion of molecular organic nitrogen of larch seedlings would be increased.After heavy pruning treatment,the absorption proportion of molecular organic nitrogen were 99%,100%and 51%,respectively.In summary,the biomass and nitrogen accumulation of both seedlings responded in the same way to different nitrogen forms after pruning.After pruning,the application of organic nitrogen was more beneficial to both seedlings biomass than the application of inorganic nitrogen.Only in the case of severe pruning,the application of organic nitrogen is more conducive to the increase of nitrogen accumulation in both seedlings than the application of inorganic nitrogen.The absorption proportion of intact molecular organic nitrogen in birch and larch seedlings was different after pruning.The absorption proportion of birch seedlings increased,and except for the application of organic nitrogen fertilizer,the absorption proportion of the other fertilization treatments decreased after pruning. |
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