Drought decreases symbiotic nitrogen fixation and nitrogen transfer in a Populus tomentosa-Hippophae rhamnoides mixed plantation on the semiarid Loess Plateau, China

Nitrogen-fixing tree species (NFTs) have been widely planted in mixed plantations to increase ecosystem available N. However, it is not clear whether symbiotic nitrogen fixation by NFTs, interspecific N transfer in mixed plantations, and the factors that affect them vary under different soil water conditions in semiarid regions. Therefore, symbiotic N fixation by Hippophae rhamnoides and N transfer from H. rhamnoides to Populus tomentosa in a mixed plantation were detected by the N-15 dilution method in 2019 (drier year) and 2020 (normal year) on the Loess Plateau, China. This study also measured the leaf net photosynthetic rate and stomatal conductance of these plantation species. The daily net photosynthetic rate (A(ndaily)), the percentage N derived from atmospheric (%Ndfa) and the N fixed amount (NFA) for H. rhamnoides was significantly lower in 2019 than in 2020 (P < 0.05). Drought also significantly decreased the percentage N transferred (P-transfer) and the N transferred amount (NTA) from H. rhamnoides to P. tomentosa, although there was no significant difference in A(ndaily) for P. tomentosa in the mixed plantation between 2019 and 2020. In addition, %Ndfa and NFA were significantly and positively correlated with A(ndaily) for H. rhamnoides in the mixed plantation in both 2019 and 2020. However, P-transfer and NTA from H. rhamnoides to P. tomentosa were significantly and positively correlated with %Ndfa for H. rhamnoides and with A(ndaily) for P. tomentosa in the mixed plantation in 2020 but not in 2019. The results suggested that drought decreased both symbiotic N fixation ability and N transfer and that N transfer was also associated with the symbiotic N fixation ability of NFTs.