Effects of the natural restoration time of abandoned farmland in a semiarid region on the soil denitrification rates and abundance and community structure of denitrifying bacteria

Diversity and composition of soil bacteria between abandoned and selective-farming farmlands in an antimony mining area

Background and aims

Land abandonment and selective farming are two common management methods to restore the soil conditions of low-pollution farmland in mining areas. The soil bacterial community plays an important role in farmland soil restoration; however, few studies have compared the composition and diversity of soil bacteria between the abandoned farmlands (AFS) and selective-farming farmlands (FFS). Here, the effects of AFS and FFS on soil properties and bacterial diversity were evaluated in an antimony (Sb) mining area in southern China. This study aimed to identify effective land management methods in terms of positive or negative changes in soil environment and bacterial diversity.

Methods

16S rRNA high-throughput sequencing was used to compare the diversity and composition of soil bacteria between AFS and FFS in the Xikuangshan (the largest Sb mine in the world).

Results

Compared to AFS, FFS had higher Sb concentration and nutritional properties (e.g., available N, P, and K) and lower Zn concentration (p < 0.05). The bacterial alpha diversity including Chao1 index, Simpson index, Shannon index and Pielou₋e index in FFS was higher than AFS (p < 0.05). At the phylum level, FFS had higher relative abundances of Chloroflexi, Acidobacteria, Gemmatimonadetes, and Rokubacteria, and lower relative abundances of Firmicutes, Actinobacteria, and Bacteroidetes. At the genus level, FFS had higher relative abundances of Acidothermus, Haliangium, and Rokubacteriales, and lower relative abundances of Bacillus, Rhodococcus, Sphingomonas, and 67-14. Redundancy analysis indicated that soil heavy metal content and soil fertility were closely correlated with the soil bacterial community. Altogether, selective farming of low-pollution farmland in the mining area can improve soil properties and soil bacterial diversity.

K fertilizer alleviates N2O emissions by regulating the abundance of nitrifying and denitrifying microbial communities in the soil-plant system

Potassium (K) fertilizer additions can result in high crop yields of good quality and low nitrogen (N) loss; however, the interaction between K and N fertilizer and its effect on N₂O emissions and associated microbes remain unclear. We investigated this in a pot experiment with six fertilizer treatments involving K and two sources of N, using agricultural soil from the suburbs of Wuhan, central China. The aim was to determine the effects of the interaction between K and different forms of N on the N₂O flux and the abundance of nitrifying and denitrifying microbial communities, using static chamber-gas chromatography and high-throughput sequencing methods. Compared with no fertilizer control (CK), the addition of nitrate fertilizer (NN) or ammonia fertilizer (AN) or K fertilizer significantly increased N₂O emissions. However, the combined application (NNK) of K and NN significantly reduced the average N₂O emissions by 28.3%, while the combined application (ANK) of K and AN increased N₂O emissions by 22.7%. The abundance of nitrifying genes amoA in ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) changed in response to N and/or K fertilization, but the denitrifying genes narG, nirK and norl were strongly correlated with N₂O emissions. This suggests that N or K fertilizer and their interaction affect N₂O emissions mainly by altering the abundance of functional genes of denitrifying microbes in the soil-plant system. The genera Paracoccus, Rubrivivax and Geobacter as well as Streptomyces and Hyphomicrobium play an important role in N₂O emissions during denitrification with the combined application of N and K.

Soil rehabilitation shaped different patterns of bacterial and archaeal community in AMD-irrigated paddy soil

Microorganisms are essential for soil rehabilitation and long-term sustainability of established plants. However, the recovery process of microorganisms in AMD-irrigated paddy soil is poorly understood at present. To verify this, we sampled AMD-irrigated paddy soils before at different rehabilitation stages by characterizing bacteria and archaea community from a chronosequence of AMD-irrigated rehabilitation to pre-disturbance levels from references sites. Next-generation sequencing is used to describe shifts in diversity and taxonomic composition of bacterial and archaeal. Co-occurrence networks are constructed to reveal potential microbial interaction patterns. The result showed bacterial community followed an observable taxonomic transition overtimes, with community structure becoming more similar to that of unmined reference sites. But the archaeal community only showed a seasonal change, which may hint that the archaeal community needs more time in rehabilitation. Both bacterial and archaeal community composition changes were apparent at high taxonomic levels, bacterial communities become dominated by Proteobacteria phylum, and archaeal community was dominated by Crenarchaeota, we proposed the possible reason is bacterial community were mainly derived by soil pH while the archaeal community was impacted by heavy metal. The bacterial co-occurrence networks increased in complexity during succession, improving the community's resistance to environmental disturbance, while the archaeal did not change monotonically with time. This study highlights the distinct recovery pattern of the bacterial and archaeal community during AMD-irrigated paddy soil rehabilitation, which provides a deep understanding of their role in paddy soil, and subsequent harnessing of their potential to pave the way in future rehabilitation strategies for mined sites.

Effects of dissolved oxygen concentrations on a bioaugmented sequencing batch rector treating aniline-laden wastewater: Reactor performance, microbial dynamics and functional genes

This work compared the efficiencies and internal reasons for aniline removal in a bioaugmented sequencing batch reactor at elevated dissolved oxygen (DO) concentrations. Nearly complete aniline removal was achieved while medium DO of 2.65 mg/L was optimal for subsequent nitrogen removal via heterotrophic nitrification-aerobic denitrification. Apart from the highest bacterial diversity richness, favorable DO condition largely enriched putatively aniline degrader, nitrifiers and denitrifiers. Further evidence from qPCR confirmed that moderate DO significantly stimulated the functional genes including aniline degrading gene tad, nitrifying genes amoA, hao, and denitrifying genes nirS and napA, respectively. This study indicates that the significant enrichment of key microorganisms and effective functional genes under optimal DO is the inner mechanism for reliable aniline degradation and subsequent nitrogen removal in the activated sludge reactor.