Effects of Water Control and Nitrogen Addition on Functional Traits and Rhizosphere Microbial Community Diversity of Haloxylon ammodendron Seedlings

Abstract

Water and nitrogen sources have always been the primary limiting factors for vegetation growth in arid and semi-arid regions and play an important role in the physiological ecology of vegetation. In this work, we studied the effects of water deficit and nitrogen addition on the physiological traits and rhizosphere bacterial microbial community of Haloxylon ammodendron seedlings in sterilized and non-sterilized soil habitats. A pot experiment was conducted to control the water and nitrogen sources of H. ammodendron seedlings. The water deficit treatment was divided into two groups based on gradient: a normal water group (CK, 70% field water holding capacity) and water deficit group (D, 30% field water holding capacity). The nitrogen addition treatment was divided into a no addition group (CK, 2.8 mg·kg−1) and addition group (N, 22.4 mg·kg−1). At the end of the growing season, the biochemical indexes of H. ammodendron seedlings were measured, and the rhizosphere soil was subjected to 16S rDNA-high-throughput sequencing to determine the rhizosphere bacterial community composition of H. ammodendron seedlings under different treatments. The results showed that the root-to-crown ratio of H. ammodendron seedlings increased significantly (p < 0.05) under the water deficit treatment compared to the control and nitrogen addition treatments, indicating that H. ammodendron seedlings preferred to allocate biological carbon to the lower part of the ground. In contrast, plant height and root length were significantly lower (p < 0.05) under water deficit treatment compared to the control, and no significant change was observed under water deficit and nitrogen addition compared to the control, indicating that water deficit inhibited the growth of H. ammodendron seedlings and nitrogen addition mitigated the effect of water deficit on the growth of H. ammodendron seedlings. Under sterilized soil conditions, both water deficit and nitrogen addition significantly increased the abundance and diversity of bacterial communities in H. ammodendron seedlings (p < 0.05). Conversely, under non-sterilized conditions, both inhibited the diversity of microbial bacterial communities, and the microbial characteristic species under different controls were different. Therefore, in the short-term experiment, H. ammodendron seedlings were affected by water deficit and allocated greater quantities of biomass to the underground part, especially in the non-sterile microbial environment; different initial soil conditions resulted in divergent responses of rhizosphere bacterial communities to water deficit and nitrogen addition. Under different initial soil conditions, the same water deficit and nitrogen addition treatment will lead to the development of distinct differences in rhizosphere bacterial community composition.