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

Abstract 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 N 2 -fixation by a legume shrub ( Caragana microphylla ) and its N transfer to coexisting non-legumes with the 15 N dilution technique. A paired experimental design (legume vs. non-legume quadrats) was used. A small amount (0.03 g N m −2 ) of 15 N-labeled KNO 3 (99.5% 15 N enrichment) was evenly distributed in these paired labeled quadrats, with only water additions as the control. One month after the 15 N 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 N 2 -fixation and transfer to non-legumes were estimated based on the aboveground biomass and its 15 N 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 N 2 -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.