Parameters Related to Nodulating Ability of Some Legumes

Parameters related to nodulating ability of some legumes were studied in a pot trial in the Institute of Forage Crops, Pleven, Bulgaria during the 2013 and 2014 years. Birdsfoot trefoil (Lotus corniculatus L.), sainfoin (Onobrychis viciifolia Scop.) and subterranean clover (Trifolium subterraneum L.) were pure cultivated and in mixture with perennial ryegrass (Lolium perenne L.) in the next ratios: birdsfoot trefoil + perennial ryegrass (50:50%); sainfoin + perennial ryegrass (50:50%); subterranean clover + perennial ryegrass (50:50%); birdsfoot trefoil + subterranean clover + perennial ryegrass (33:33:33%); sainfoin + subterranean clover + perennial ryegrass (33:33:33%). From the pure cultivated crops, birdsfoot trefoil showed both, the highest nodulating ability and root biomass to nodule number ratio. Birdsfoot trefoil and subterranean clover in mixtures with perennial ryegrass formed more nodules as compared to pure crops by 16.0% and by 24.4%, respectively. Sainfoin showed weak nodulating ability, both, pure and in mixtures. In terms of root to aboveground biomass ratio, the legumes were arranged as follows: subterranean clover, birdsfoot trefoil and sainfoin. Research Article Parameters Related to Nodulating Ability of Some Legumes Viliana Vasileva* Institute of Forage Crops, 89 “General Vladimir Vazov” Str., 5800 Pleven, Bulgaria Dates: Received: 09 February, 2017; Accepted: 08 March, 2017; Published: 10 March, 2017 *Corresponding author: Viliana Vasileva, Institute of Forage Crops, 89 “General Vladimir Vazov” Str., 5800 Pleven, Bulgaria, Tel: +359 64 805 882; Fax: +359 64 805 881; E-mail:


Introduction
Forage legumes have signifi cant role in agriculture and are often integrated in mixed farming systems to enhance crop yields through their nodulating and nitrogen fi xing ability [1], Every year on the global level about 50 million tons nitrogen is fi xing in the agro ecosystems [2,3], consider that legume species fi x between 15 and 25 kg nitrogen per tone of dry matter.
Legumes often are grown in mixtures where they are stimulated to fi x more nitrogen from the air moreover, they are less competitive with grasses for soil nitrogen [4], There is a dynamic relationship between the legume and the grass component, where soil nitrogen absorption by the grass lowers its inhibitory effect on nitrogen fi xation. Climate changes force forage crops and mixtures to adapt to the altered conditions [5]. Legumes species that can provide self-sowing and persist continuously in the sward as subterranean clover become of practical importance [6,7], Choosing the right components is important because of their competitiveness depends on the success of the mixture [8,9], In the present work some parameters related to nodulating ability of valuable forage crops as birdsfoot trefoil, sainfoin and subterranean clover alone and in mixtures with perennial ryegrass were studied.
Two cuts for forage were harvested, the root biomass was washed with tap water and the next characteristics were recorded: nodulating ability (nodule number per plant); roots to number of root nodules ratio (dry root biomass, dried at 60 o C was devided to number of root nodules); root biomass to aboveground biomass ratio, weight (dried at 60 o C). These characteristics were compared to the characteristics of pure legumes (birdsfoot trefoil, sainfoin and subterranean clover).
Data were averaged for two years and statistically processed standard error fi nding using softweer [10].

Results and Discussion
Symbiotic fi xations of nitrogen take place in the root nodules [11], in our study birdsfoot trefoil and subterranean clover formed the most number of nodules (Table 1).
According to [4,12], legumes effectively regulate the processes of nodulation and nitrogen fi xation in mixtures depending on the accessibility of mineral nitrogen. Taking into account the nodulation in mixtures of birdsfoot trefoil, we can see that there are no signifi cant differences for its mixtures with ryegrass and these with ryegrass and subterranean clover. In both mixtures nodulation was higher as compared to pure birdsfoot trefoil by 16.0% and 17.6%, respectively.
The major factor infl uenced the nodulation in mixture is the competitiveness of legume with subsequent grass component [13,14] consider that the uptake of mineral nitrogen in mixture is more as compared to pure growing of legumes and this refl ect on the nodule number. However, the competitiveness for soil nitrogen may have benefi cial effect on the stimulating of nitrogen fi xing processes because mineral nitrogen inhibits N-fi xation process. The positive interaction in mixture of alfalfa and timothy (1:1 and 1:2) was found [15]. Timothy involving as a component increased the proportion of fi xed nitrogen from alfalfa.
Sainfoin as compared to other legumes tested formed less number of nodules, pure, as well in mixtures. In mixtures with ryegrass and in these with ryegrass and subterranean clover, nodule number not differs signifi cantly from the pure sainfoin.
Relatively less amount of fi xed nitrogen in comparison with other legumes was found in sainfoin from many authors [16][17][18][19], As a possible reason they and others [8,20], showed that this crop needs twice as much CO 2 (approximately 20 mol) for 1 mol N 2 . Moreover, in sainfoin less amount of assimilates usually were used for the leaf formation as well the leaves have a smaller surface.
Subterranean clover in mixture with ryegrass formed by 24.3% more number of nodules as compared to pure subterranean clover. Apparently, there was positive synergy observed in regard to nodulating ability.
As a whole birdsfoot trefoil, sainfoin and subterranean clover formed higher number of nodules in mixtures and on average there were any signifi cant differences between the mixtures.
Specifi c nodulating ability follows the tendency of nodulation. This characteristic varied from 0.1269 to 0.1438 in birdsfoot trefoil, from 0.0237 to 0.0269 in sainfoin and showed closed values in subterranean clover (0.0362 -0.0375) ( Figure  1).
Having in a mind the characteristics included in the calculation of root biomass to nodule number ratio, lower values of this ratio indicated better provision of root biomass with root nodules. The roots to number of root nodules ratio was the best found in birdsfoot trefoil grown alone and in subterranean clover grown in mixtures with ryegrass ( Table 2) We assume that it was due to the higher nitrogen use effi ciency of subterranean clover under intercropped conditions because of the successful competition between the two species for available nitrogen.
Components included in mixtures have different nitrogen metabolism. In ryegrass total accumulated nitrogen was result only of the uptake of nitrate nitrogen through the roots by leaf nitrate reductase and effi ciency of their use was lower. In subterranean clover nitrogen comes from nitrogen fi xation. Root biomass to aboveground biomass ratio is determining by the plant development. Formation of biomass of the plants and its various organs is the result of the assimilation activity of photosynthetic tissues, as well as of the root system functioning [21].
It has been known long ago that grass/legume intercropping systems are highly effi cient in relation to productivity than separately grown crops [22][23][24], Root biomass to aboveground biomass ratio varied from 0.5211 in subterranean clover in mixtures with ryegrass to 1.1091 in sainfoin in the three component mixtures (Figure 2). A large amount of root biomass in mixtures increased the possibility for taking of immobile nutrients which contributed the better plant development as considered [25,26],

Conclusion
Nodulating legumes in pastures are the basis for economic and environmental sustainability of agricultural systems.
From the birdsfoot trefoil, sainfoin and subterranean clover studied, birdsfoot trefoil showed the highest nodulating ability and root biomass to nodule number ratio. Birdsfoot trefoil and subterranean clover in mixtures with perennial ryegrass formed by 16.0% and by 24.4%, respectively more nodules as compared to pure grown crops. Sainfoin showed weak nodulating ability, both, pure and in mixtures. In terms of root to aboveground biomass the legumes were arranged as follows: subterranean clover, birdsfoot trefoil and sainfoin. The role of mixtures based on legumes for sustainable agriculture was shown.