Effects of nitrogen addition on the productivity of the Medicago sativa ‘Gannong No.9’-LL2 Rhizobium symbiotic system

doi:10.11686/cyxb2025425

Abstract

Abstract:

The “Gannong No. 9 alfalfa（Medicago sativa）-Rhizobium strain LL2” symbiotic system is recognized as an efficient nitrogen-fixing association. To clarify the effects of nitrogen application rates on the productivity of this system， four nitrogen （N） addition treatments were established： 0 kg·ha^-1 （R）， 35.09 kg·ha^-1 （NR₁）， 46.79 kg·ha?¹ （NR₂）， and 58.48 kg·ha^-1 （NR₃）. Corresponding non-inoculated controls at each nitrogen level were included： 0 kg·ha^-1 （CK₁）， 35.09 kg·ha^-1 （CK₂）， 46.79 kg·ha^-1 （CK₃）， and 58.48 kg·ha^-1 （CK₄）. Measurements were conducted at the branching， budding， and early flowering stages. These included the number of effective nodules per plant， effective nodule weight per plant， nitrogenase activity， plant height， root length， aboveground and belowground dry weight， crude protein （CP）， ether extract （EE）， neutral detergent fiber （NDF）， acid detergent fiber （ADF）， and forage grading index （FGI）. This study evaluated the effects of nitrogen addition on nodulation and nitrogen fixation， plant growth， and forage quality in the Gannong No.9-LL2 symbiotic system. The results showed that in the NR₂ treatment， the number of effective nodules per plant in the Gannong 9-LL2 symbiotic system was 22， 32， and 35 at the branching， budding， and initial flowering stages， respectively. Effective nodule weight per plant was 0.0353， 0.0745， and 0.1013 g·plant^-1， and nitrogenase activity was 4.79， 26.51， and 28.13 μmol·g^-1·h^-1 at the same development stages， displaying an increasing trend as growth progressed. At the early flowering stage， the plant height was 51.92 cm and CP content was 20.42%. Root length and both aboveground and belowground dry weight were significantly higher than those of other nitrogen treatments and the controls （P<0.05）. Meanwhile， NDF and ADF contents were significantly lower than those under other nitrogen addition treatments and the control （P<0.05）. The strong nodulation and nitrogen-fixing responses and the growth-promoting effect of NR₂ occurred mainly at the branching stage， where the number of effective nodules per plant was 84.44% higher than that of the R treatment， and nodule weight per plant and nitrogenase activity were 2.5-fold and 3.1-fold higher than those of R treatment， respectively. Increases in root length， aboveground dry weight， and belowground dry weight were also greater at the branching stage than at the budding and early flowering stages. At the budding stage， fiber fractions were reduced in NR₂， with NDF and ADF contents reduced by 3.82% and 6.03%， respectively， compared with R treatment， and this effect was strongest than at the branching and early flowering stages. At the early flowering stage， crude protein was markedly enhanced in NR₂ where CP content was increased by 9.04% relative to R treatment. Similar， but smaller effects were observed at earlier growth stages. During grading at the early flowering stage， the FGI value of NR₂ was 54.91 MJ·d^-1， corresponding to a premium forage grade. To summarize， the NR₂ nitrogen application rate of 46.79 kg·ha^-1 was identified as the optimal N level for maximizing the productivity of the Gannong No.9-LL2 symbiotic system. These findings provide research data useful for development of cultivation practices aimed at improving the productivity of the Gannong No.9 alfalfa-LL2 symbiotic system.

Key words: alfalfa, rhizobium, nitrogen application level, nitrogen fixation, productivity

Bing-sen ZHAO, Wen-juan KANG, Shang-li SHI, Jian GUAN, Bao-fu LU, Yi-lin HAN, Jing-jing WANG, Feng-jiao CHEN. Effects of nitrogen addition on the productivity of the Medicago sativa ‘Gannong No.9’-LL2 Rhizobium symbiotic system[J]. Acta Prataculturae Sinica, 2026, 35(10): 48-60.

Figures/Tables 6

Table 1 Grade standard of roughage quality（MJ·d-1）

分级 Grade	饲草分级指数 Forage grading index
特级Special grade	>53.68
1级Grade 1	33.50~53.68
2级Grade 2	19.20~29.29
3级Grade 3	11.10~16.44
4级Grade 4	6.28~10.67
5级Grade 5	<6.28

Fig.1 Nodulation difference of the Gannong No.9-LL2 symbiotic system under different nitrogen levels

Fig.2 Differences in the nitrogen-fixing capacity of the Gannong No.9-LL2 symbiotic system under different nitrogen levels

Fig.3 Phenotypic changes of the Gannong No.9-LL2 symbiotic system under different nitrogen levels

Fig.4 Nutritional quality changes of the Gannong No.9-LL2 symbiotic system under different nitrogen levels

Table 2 Forage grading index grades of 8 treatments

处理 Treatments	饲草分级指数 Forage grading index （MJ·d^-1）	饲草分级指数级别 Forage grading index grade
CK₁	42.67	1级Grade 1
R	45.80	1级Grade 1
CK₂	43.30	1级Grade 1
CK₃	42.21	1级Grade 1
CK₄	42.25	1级Grade 1
NR₁	51.12	1级Grade 1
NR₂	54.91	特级Special grade
NR₃	47.46	1级Grade 1

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