N-fixing trees in wetland restoration plantings: effects on nitrogensupply and soil microbial communities

Impact of diversified cropping systems and fertilization strategies on soil microbial abundance and functional potentials for nitrogen cycling

Diversified cropping systems and fertilization strategies were proposed to enhance the abundance and diversity of the soil microbiome, thereby stabilizing their beneficial services for maintaining soil fertility and supporting plant growth. Here, we assessed across three different long-term field experiments in Europe (Netherlands, Belgium, Northern Germany) whether diversified cropping systems and fertilization strategies also affect their functional gene abundance. Soil DNA was analyzed by quantitative PCR for quantifying bacteria, archaea and fungi as well as functional genes related to nitrogen (N) transformations; including bacterial and archaeal nitrification (amoA-bac,arch), three steps of the denitrification process (nirK, nirS and nosZ-cladeI,II) and N2 assimilation (nifH), respectively. Crop diversification and fertilization strategies generally enhanced soil total carbon (C), N and microbial abundance, but with variation between sites. Overall effects of diversified cropping systems and fertilization strategies on functional genes were much stronger than on the abundance of bacteria, archaea and fungi. The legume-based cropping systems showed great potential not only in stimulating the growth of N-fixing microorganisms but also in boosting downstream functional potentials for N cycling. The sorghum-based intercropping system suppressed soil ammonia oxidizing prokaryotes. N fertilization reduced the abundance of nitrifiers and denitrifiers except for ammonia-oxidizing bacteria, while the application of the synthetic nitrification inhibitor DMPP combined with mineral N reduced growth of both ammonia-oxidizing bacteria and archaea. In conclusion, this study demonstrates a strong impact of diversified agricultural practices on the soil microbiome and their functional potentials mediating N transformations.

Soil microbial nitrogen-cycling gene abundances in response to crop diversification: A meta-analysis

Single planting structure has a significant impact on the maintenance of nitrogen in managed ecosystems. Although the effect of crop diversity on soil nitrogen-cycling microbes is mainly related to the influence of environmental factors, there is a lack of quantitative research. This study aims to determine the effect of diversified cropping mode on the abundance of functional genes in the soil nitrogen cycle based on the quantitative integration of a meta-analysis database containing 189 observation data pairs. The results show that the soil nifH (nitrogenase coding gene), nirS and nirK (nitrite reductase coding gene), and narG (nitrate reductase coding gene) abundances are positively affected by the diversity of plant species, whereas the amoA (ammonia monooxygenase coding gene) and nosZ (nitrous oxide reductase coding gene) show no response. Diversification duration and ecosystem type are important factors that regulate soil nitrogen fixation and nitrification gene abundances. Denitrification genes are mainly affected by categorical variables such as the planting pattern, soil layer, application species, duration, and soil texture. Among them, the long-term continuous diversification is mainly manifested in the reduction of soil nifH and increase of nirK abundances. Soil organic carbon and nitrogen linearly affect the responses of nifH, amoA, nirS, and nirK. Therefore, to maintain the soil ecological function, diversity of planting patterns needs to be applied flexibly by regulating the abundance of nitrogen-cycling genes. Our study draws conclusions in order to provide theoretical references for the sustainability of nitrogen and improvement of management measures in the process of terrestrial managed ecosystem diversification.

Di(2-ethylhexyl) phthalate and dibutyl phthalate have a negative competitive effect on the nitrification of black soil

Di (2-ethylhexyl) phthalate (DEHP) and dibutyl phthalate (DBP) are the most widely used plasticizers for agricultural mulching films and one of the most common organic pollutants in black soil. However, little is known about the effect of these two contaminants on nitrification in black soil. This study investigated the changes of 20 mg/kg DEHP and DBP on the diversity of nitrification microbial communities, the abundance of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) related genes, and the activities of key enzymes involved in nitrification. During ammonia oxidation, DEHP and DBP had uncompetitive inhibition of urease, reducing the copy number of amoA gene, and microorganisms (Azoarcus, Streptomyces and Caulobacter) would use inorganic nitrogen as a nitrogen source for physiological growth. During nitrite oxidation, the copy number of nxrA gene also reduced, and the relative abundance of chemoautotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) decreased. Moreover, the path analysis results showed that DEHP and DBP mainly directly or indirectly affect AOB and NOB through three ways. These results help better understand the ecotoxicological effects of DEHP and DBP on AOB and NOB in black soil.

The contrasting effects of biochar and straw on N2O emissions in the maize season in intensively farmed soil

This study evaluated the combined effects of biochar and straw on N2O flux and the community compositions of nitrifiers and denitrifiers in the maize season in an intensively farmed area in northern China. The experiment consisted of four treatments: (1) CK (only chemical fertilizer application); (2) C (biochar application); (3) SR (straw application to the field); and (4) C+SR (the application of both biochar and straw). The results indicated that during the maize growing season, N2O flux decreased by 30.3% in the C treatment and increased by 13.2% and 37.0% in the SR and C+SR treatments compared with CK, respectively. NO3--N, NH4+-N, and microbial biomass carbon (MBC) were the main soil factors affecting N2O flux, and they were positively correlated with NO3--N and negatively correlated with MBC in the C treatment and positively correlated with NH4+-N in the SR and C+SR treatments. Both biochar addition and straw return shifted the community compositions of nitrifiers and denitrifiers. N2O production was mainly reduced by promoting the ammonia-oxidizing bacteria (AOB) gene abundance and inhibiting the nirK gene abundance in the C treatment but promoted by inhibiting the AOB and nosZ gene abundances in the SR and C+SR treatments. Nitrosospira (AOB) and Rhizobium (nirK) were the main contributors among the treatments. NO3--N, NH4+-N, and MBC were the main soil factors affecting the denitrifier communities. The predominant species associated with the nirK, nirS, and nosZ genes were positively correlated with NO3--N and MBC and negatively correlated with NH4+-N. These results provide valuable information on the mechanism of N2O production and reduction in biochar- and straw-amended soil under field conditions.

Diversity of soil nitrogen-fixing bacteria in the rhizosphere and non-rhizophere soils of Ebinur Lake Wetland

Ebinur Lake Wetland is an understudied desert wetland ecosystem, particularly regarding nitrogen cycling. This study aimed to ascertain the diversity and richness of nitrogen-fixing bacterial communities in the Ebinur Lake Wetland. The diversity of the nitrogen-fixing bacteria community of nifH genes from the rhizosphere and non-rhizosphere soils of four plants in different seasons were examined using Illumina HiSeq PE250 high-throughput sequencing technology. The correlation between soil environmental factors and diversity and richness of nitrogen-fixing bacteria was studied using the redundancy analysis (RDA). The results showed that the diversity of nitrogen-fixing bacteria in the rhizosphere soil of the constructive plants was higher than that in the non-rhizosphere soil; also, the diversity in July was higher than that in October and April. Geobacter, Pseudomonas and Bradyrhizobium were the dominant common bacteria in different samples of Ebinur Lake Wetland. The RDA showed that the total nitrogen, available potassium and available phosphoruswere significantly correlated with the diversity and richness of nitrogen-fixing bacteria. The diversity and community structure of nitrogen-fixing bacteria in soil samples also changed over time. The community structures of nitrogen-fixing bacteria in the rhizosphere and non-rhizosphere soils of the four plants were not the same during the same period. The correlation between soil environmental factors and the community structure and abundance of nitrogen-fixing bacteria can provide data basis and theoretical support for the degradation and restoration of Ebinur Lake Wetland.

Assessing passive rehabilitation for carbon gains in rain-filled agricultural wetlands

Wetland ecosystems have a disproportionally large influence on the global carbon cycle. They can act as carbon sinks or sources depending upon their location, type, and condition. Rehabilitation of wetlands is gaining popularity as a nature-based approach to helping mitigate climate change; however, few studies have empirically tested the carbon benefits of wetland restoration, especially in freshwater environments. Here we investigated the effects of passive rehabilitation (i.e. fencing and agricultural release) of 16 semi-arid rain-filled freshwater wetlands in southeastern Australia. Eight control sites were compared with older (>10 year) or newer (2-5 year) rehabilitated sites, dominated by graminoids or eucalypts. Carbon stocks (soils and plant biomass), and emissions (carbon dioxide - CO₂; and methane - CH₄) were sampled across three seasons, representing natural filling and drawdown, and soil microbial communities were sampled in spring. We found no significant difference in soil carbon or greenhouse gas emissions between rehabilitated and control sites, however, plant biomass was significantly higher in older rehabilitated sites. Wetland carbon stocks were 19.21 t C_org ha^-1 and 2.84 t C_org ha^-1 for soils (top 20 cm; n = 137) and plant biomass (n = 288), respectively. Hydrology was a strong driver of wetland greenhouse gas emissions. Diffusive fluxes (n = 356) averaged 117.63 mmol CO₂ m² d^-1 and 2.98 mmol CH₄ m² d^-1 when wet, and 124.01 mmol CO₂ m² d^-1 and -0.41 mmol CH₄ m² d^-1 when dry. Soil microbial community richness was nearly 2-fold higher during the wet phase than the dry phase, including relative increases in Nitrososphaerales, Myxococcales and Koribacteraceae and methanogens Methanobacteriales. Vegetation type significantly influenced soil carbon, aboveground carbon, and greenhouse gas emissions. Overall, our results suggest that passive rehabilitation of rain-filled wetlands, while valuable for biodiversity and habitat provisioning, is ineffective for increasing carbon gains within 20 years. Carbon offsetting opportunities may be better in systems with faster sediment accretion. Active rehabilitation methods, particularly that reinstate the natural hydrology of drained wetlands, should also be considered.