Dynamic variability of soil diazotrophs in bulk‐rhizosphere and phenological stages under long‐term mulching in an eroded area in the Loess Plateau

Abstract

Free‐living diazotrophs play a significant role in the process of soil biological nitrogen fixation. Long‐term field management has a cumulative effect on soil microbial communities. However, after long‐term mulching measures in erosion areas, the stability difference in diazotrophs in bulk‐rhizosphere soil and the temporal dynamic changes remain unclear. In this study, we analysed the dynamic variability of soil diazotrophic community structures using high‐throughput sequencing. The result of Chao1 diversity index showed that mulching effectively increased the richness of soil diazotrophs. Combining the community composition, it was found that the diversity level of the soil diazotrophic community under mulching decreased more drastically along the development stages compared with no‐tillage, and Bradyrhizobium played a major role in nitrogen fixation. In rhizosphere soil, mulching promoted the relative abundances of Azohydromonas, Bradyrhizobium, and Skermanella. Co‐occurrence network analysis indicated that mulching measures strengthened the connection between the dominant components to improve the aggregation of the network. The partial least squares‐path model showed that the richness of diazotrophs in rhizosphere soil had an indirect effect on mulching through the available carbon and nitrogen nutrients of bulk soil, thereby improving the response rate to environmental variables. Overall, our findings showed that under long‐term mulching, the richness and aggregation of dominant components of the soil diazotrophic community increased with the phenological period and distance from roots, revealing the importance of mulching in eroded areas to improve farmland soil nitrogen fixation capacity. This study provides a theoretical reference for microecology to effectively improve soil fertility in similar areas.

Bacterial communities in proso millet root‐associated compartments are regulated by growth period and mulching regime

“…Keystone taxa have an essential role in governing the microbial community structure and function (Hartman et al, 2018). In our work, we noted that the keystone taxa changed from Proteobacteria and Actinobacteriota at jointing period to Proteobacteria at flowering period, revealing that with the progress of growth, the microbial network structure changed dynamically with changes in soil nutrients (Hao, Zhang, et al, 2021). Interestingly, we also confirmed that FM significantly enhanced the abundance of keystone taxa in the three compartments than non‐mulching at both periods of proso millet.…”

“…Furthermore, we identified that the alpha diversities of the bulk soil were always the highest, followed by that of the rhizosphere soil and root compartments regardless of mulching regime; this phenomenon is very common and has been widely demonstrated in farmland ecosystems (Fan et al, 2018; Hao, Zhang, et al, 2021; Peiffer et al, 2013; Wu et al, 2021). This may be attributed to the reason that when the rhizosphere is close to the root surface, root exudates activated specific microbial species from the bulk soil microbial seed bank (Liu et al, 2020; Schlemper et al, 2017).…”

“…Recent studies have also confirmed this phenomenon, which may be attributed to the improvement in soil temperature and reduction in periodic physical disturbance (Dong et al, 2017; Huang et al, 2020; Liu et al, 2009). However, it is also reported that mulching regimes can reduce soil nutrient contents, which we believe may be related to the difference between soil fertility levels and sampling stages (Hao, Zhang, et al, 2021, Li et al 2021, Zhang et al, 2019). Compared with NM, the FM and RF treatments considerably decreased the nutrient contents of bulk and rhizosphere soils when our sampling at flowering period.…”

“…Compared to traditional flat planting, mulching regimes can greatly improve the hydrothermal environment in the root zone of crops by inhibiting soil evaporation, collecting rainwater from the ridge to the furrow, and reducing water loss (Dong et al, 2017; Hao, Zhang, et al, 2021; Huang, Liu, et al, 2019a; Zhu et al, 2018). Similarly, we also confirmed that RF treatment enhanced the soil moisture, while FM treatment considerably improved the soil temperature at the vegetative growth period (jointing period) of proso millet; at the reproductive growth period (flowering period), FM treatment significantly enhanced the soil moisture and temperature; this was followed by the RF treatment ( p < 0.05, Table 1).…”

“…FM treatment significantly reduced the alpha diversities in the rhizosphere compartment ( p < 0.05), and the RF treatment significantly reduced the alpha diversities in the bulk soil and root compartments at jointing period ( p < 0.05, Figure 3a). FM improved the soil temperature, and RF altered the micro‐morphology of the soil, resulting in complex and diverse microbial communities, which restricted bacterial growth (Fu et al, 2019; Hao, Zhang, et al, 2021). Conversely, the responses of alpha diversities and specific species in the rhizosphere soil and root compartments to mulching regimes were positive when proso millet entered the reproductive growth period (Figure 3b, c).…”