Symbiotic nitrogen fixation and interspecific transfer by Caragana microphylla in a temperate grassland with 15N dilution technique

- %Ndfa of C microphylla was estimated to range from 84 to 95%.
- Symbiotic N2-fixation by C. microphylla was 4.3 g m−2 yr−1 in temperate grassland.
- Fixed N can transfer to and transfer amount depends on coexisting plants species.

Quantification of symbiotic nitrogen (N) fixation by legume and its N transfer to the neighboring plants is an important prerequisite to understand the N cycling in the N-limited grasslands. The objective of this study is to explore the symbiotic N2-fixation by a legume shrub (Caragana microphylla) and its N transfer to coexisting non-legumes with the 15N dilution technique. A paired experimental design (legume vs. non-legume quadrats) was used. A small amount (0.03 g N m−2) of 15N-labeled KNO3 (99.5% 15N enrichment) was evenly distributed in these paired labeled quadrats, with only water additions as the control. One month after the 15N addition, aboveground parts of C. microphylla and four non-legumes of reference plants (i.e. Cleistogenes squarrosa, Artemisia frigida, Artemisia capillaris and Leymus chinensis) in both labeled and controlled quadrats were harvested. Symbiotic N2-fixation and transfer to non-legumes were estimated based on the aboveground biomass and its 15N in these plant species. The N derived from the atmosphere (%Ndfa) of C. microphylla was estimated to range from 84 to 95%. According to the aboveground biomass of C. microphylla, symbiotic N2-fixation was estimated to be 4.3 g m−2 yr−1 in the mixed C. microphylla community in the temperate grassland of northern China. The amount of N transfer from C. microphylla to their coexisting plants varied with plant species. About half of above N in C. squarrosa derived from C. microphylla, while L. chinensis showed almost no N derived from C. microphylla. Both A. frigida and A. capillaris showed about 36% and 37% of N derived from C. microphylla, respectively. Totally, about 17% of N fixed in the aboveground part of C. microphylla was transferred to its associated species. Considering the amount of N fixed and transferred by C. microphylla, we conclude that C. microphylla plays an important role in the N cycling of temperate grasslands in northern China.