Effect Of Nitrogen Addition On Interspecific Relationship And Nitrogen Balance Of Alfalfa And Smooth Bromegrass

Planting more legumes in agricultural production systems, or including them in crop rotation, reduces the dependence of plants on mineral N fertilizer, which is beneficial to agricultural production and sustainable environmental development, legumes through fixation of atmospheric N2by their own rhizobia, provides N not only for their own growth needs, but also for grasses co-cultivated in mixtures.This also helps to avoid the loss of mineral N fertilizer because of the lack of time coordination between the soil-available N supply and the plants’needs. So, in legume-grass mixture, the amount of nitrogen fixation of forage legumes and nitrogen transferred from legumes to grasses, the timing of forage legumes start to fixing N, which is key for efficient use of nitrogen. It is of great significance that understanding mechanism of biological nitrogen fixation of legume in response to effective nitrogen in soil to maintain stability and improve nitrogen use efficiency of mixed grassland.A greenhouse pot experiment was conducted to study the temporal dynamics of N2fixation of alfalfa (Medicago sativa L.) in monoculture and in mixture with smooth bromegrass (Bromus inermis Leyss.) in response to the addition of N fertilizer, including3levels of15N-labeled urea, namely NO (0.001g/pot), N75(1.07g/pot) and N150(2.14g/pot), using the15N isotope dilution technology and repeatedly sampling method. The purpose of this experiment was to studied:1) Response mechanism of smooth bromegrass and alfalfa to nitrogen fertilizer in monoculture and mixture;2) Dynamic competitive process between legume (alfalfa) and grass (smooth bromegrass) in mixture;3) Response mechanism of biological nitrogen fixation of alfalfa to nitrogen fertilizer in monoculture and mixture;The main results of this study are as follows:(1). With increasing of N application (from0to150kg/hm2), the above-and below-biomass and tiller number of smooth brome significantly increased (P<0.05), the above-and below-biomass and branch number of alfalfa were no significantly difference in monoculture(P>0.05). The above-and below-biomass and tiller number of smooth brome increased, which in a certain extent inhibited biomass and branch number of alfalfa in mixture (P<0.05).(2). Smooth bromegrass dry weight(shoot and root) and tiller number were significantly higher when grown with alfalfa than when grown alone, regardless of N application level (P<0.05). Oppositely, individual alfalfa plant dry weight (shoots+roots) and branch number were significantly lower in the mixture than when grown alone at all N levels (P<0.05). this result proved than the ability of smooth brome expand above-ground space by means of increase its tiller number was stronger than that of alfalfa in mixture. In addation, competitive ratio of smooth brome in mixture was always greater than1.0, the competitive rate of alfalfa was always less than1.0, which means that competitiveness of smooth brome was stronger than that of alfalfa, and in the whole growth period, competitiveness of smooth brome would gradually weak, and the competitiveness of alfalfa increased.(3). The amount of nitrogen fixation by alfalfa in a monoculture negatively correlated with the amount of N fertilizer (R2=-0.9376, P=0.0057); however, nitrogen fixation by alfalfa in mixture in response to the addition of N fertilizer were different. Due to a large amount of nitrogen was removed from the soil by plant at each sampling, resulting in a significant reduction in soil nitrogen content, competitive advantage of smooth bromegrass disappeared in mixture and alfalfa gradually dominant, this also increased the amount of nitrogen fixation by alfalfa. On the other hand, the ability of smooth bromegrass competing soil available N was greate than that of alfalfa, smooth brome growing consumpted soil available N in mixed culture, resulting in a significant reduction for nitrogen in alfalfa rhizosphere, further promoting N2-fixing by alfalfa. These two aspects seem contradictory, but it was the response mechanism of alfalfa to soil available N in mixed pasture. So, the amount of N2-fixing by alfalfa was affected by competition result between legumes and grasses in the mixed culture.(4). The dominant uptake of N by smooth bromegrass resulted in the reduced availability of N for alfalfa, which led to a higher nitrogen fixation efficiency of alfalfa in a mixture compared to a monoculture. The high efficiency was that:1) the timing of nitrogen fixation of alfalfa in the same treatment began earlier in a mixture than in a monoculture.2) the amount of nitrogen fixation of alfalfa was higher in a mixture than in a monoculture (above-ground:|t|=3.39, P=0.0096; below-ground:|t|=3.57, P=0.0073). The conclusion was fully proved: Compared to alfalfa in monoculture, alfalfa In mixture was greater reliance on biological nitrogen fixation. In addation, with the increase of nitrogen fertilizer, the amount of N transfer form alfalfa to smooth bromegrass significantly reduced in mixture. So, the amount of N transfer from alfalfa to smooth bromegrass depends on the content of soil available nitrogen.