Nitrate denitrification rate response to temperature gradient change during river bank infiltration.

Nitrate attenuation during river bank infiltration is the key process for reducing nitrogen pollution. Temperature is considered to be an important factor affecting nitrate attenuation. However, the magnitude and mechanism of its impact have not been clear for a long time. In this study, the effects of temperature and temperature gradient on the nitrate denitrification rate were investigated via static batch and dynamic soil column simulation experiments. The results showed that temperature had a significant effect on the denitrification rate. Temperature effects were first observed in denitrifying bacteria. At low temperatures, the microorganism diversity was low, resulting in a lower denitrification rate constant. The static experimental results showed that the denitrification rate at 19 °C was approximately 2.4 times that at 10 °C. The dynamic soil column experiment established an exponential positive correlation between the nitrate denitrification decay kinetic constant and temperature. The affinity of denitrifying enzymes for nitrate in the reaction substrate was ordered as follows: decreasing temperature gradient (30 °C → 10 °C) > zero temperature gradient (10 °C) > increasing temperature gradient condition (0 °C → 10 °C). This study provides a theoretical basis for the biogeochemical processes underlying river bank infiltration, which will help aid in the development and utilization of groundwater resources.

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.

Denitrification accounts for the production of mobile forms of nitrogen (N) for plant uptake, N leaching, and gaseous losses. However, few studies have investigated the potential effects of the natural restoration age on denitrification rates and denitrifying microorganisms, especially in fragile ecosystems in semiarid regions. The potential N gas (N2O and N2) emissions and denitrification rates significantly decreased after abandonment (< 9 years) compared to those of active farmland and then steadily increased as the restoration proceeded, leading to an enhanced soil N loss. The total bacterial and napA gene abundances significantly decreased after abandonment (< 9 years) compared to that of farmland and then significantly increased as the restoration proceeded. The abundances of the narG, nirK, nirS, qnorB, and nosZ genes steadily increased with the restoration age of abandoned farmland. The community compositions of denitrifying bacteria exhibited different fluctuating patterns, suggesting different response patterns of community traits of N gas emission-related functional guilds to the restoration age of abandoned farmland. Changes in N gas emissions and in the abundance and diversity of denitrifying microorganisms exhibited similar patterns, suggesting an increased population and diversity of denitrifying bacteria are responsible for the enhanced N gas emissions. We observed clear patterns of plant coverage and denitrifying microorganisms that were associated with increases in the organic C, NH4+-N, and NO3--N contents and decreases in the soil bulk density as well as increases in the abundance and diversity of denitrifiers with the restoration age of abandoned farmland that were linked to an increase in N gas emissions. It is therefore recommended that effective measures (i.e., modest levels of grazing) may be able to be undertaken to assist with decreasing greenhouse gas nitrous oxide (N2O) and N loss after 32 years of farmland abandonment.

[Effects of Organic Fertilizer Combined with Biochar on Denitrifying Microorganisms and Enzyme Activities in Orchard Soil].

To study the effects of organic fertilizer combined with biochar on soil denitrification and denitrifying microbial community structure, this study took lemon orchard soil as the research object and adopted a pot experiment, setting up five fertilization treatments:no fertilization(CK), conventional fertilization(F), organic fertilizer(P), fertilizer+biochar(FP), and organic fertilizer+biochar(PP). The abundance and community structure of denitrifying microorganisms were studied using real-time quantitative PCR and T-RFLP. Redundancy analysis(RDA) was used to explore the environmental factors affecting the denitrifying microbial community structure, and PLS-PM analysis was used to explore the environmental factors affecting the denitrification potential of lemon orchard soil. The results showed as follows:① compared with that under the single fertilizer treatment(F), the organic fertilizer and biochar(P, FP, and PP) treatments significantly increased the denitrification potential of the soil, ranging from 147.8% to 1445.3%. The denitrification rate of soil treated with organic fertilizer combined with biochar was 23.8% lower than that treated with organic fertilizer alone. ② Compared with that in the CK treatment, fertilization treatment significantly increased the abundance of nirS and nirK denitrification microorganisms. Fertilizer treatments(F and FP) significantly reduced the abundance of nosZ denitrifying microorganisms. Biochar treatment significantly changed the diversity and uniformity of denitrifying microorganisms, but the specific law and mechanism quality remained unclear. ③ The results of RDA analysis showed that fertilization could affect the community structure of nirS, nirK, and nosZ denitrifying microorganisms by changing C/N, WC, NO3--N, SOC, AK, and AP. ④ PLS-PM analysis showed that soil denitrification was positively correlated with pH and the abundance of nirK denitrification microorganisms, and NO3--N indirectly affected soil denitrification by affecting the abundance of nirK denitrification microorganisms. In addition, the nirK microbial community was the dominant microbial community in soil denitrification in lemon orchards. In conclusion, organic fertilizer directly affected soil denitrification by regulating soil pH, whereas regulating NO3--N content affected nirK denitrification microbial abundance, indirectly affecting soil denitrification. The application of organic fertilizer combined with biochar could slow down the improvement of soil denitrification caused by single application of organic fertilizer, which is more suitable for promotion in orchards in this region.

Response of gene abundance of ammonia-oxidizing microorganisms and denitrifying microorganisms to nitrogen and phosphorus addition in subtropical forest.

We conducted a nitrogen (N) and phosphorus (P) addition experiment in Qianjiangyuan National Park in 2015, to investigate the response of ammonia-oxidizing microorganisms and denitrifying microorganisms. There were four treatments, including N addition (N), P addition (P), NP, and control (CK). Soil samples were collected in April (wet season) and November (dry season) of 2021. The abundance of amoA gene of ammonia-oxidizing microorganisms (i.e., ammonia-oxidizing archaea, AOA; ammonia-oxidizing bacteria, AOB; comammox) and denitrifying microbial genes (i.e., nirS, nirK, and nosZ) were determined using quantitative PCR approach. The results showed that soil pH was significantly decreased by long-term N addition, while soil ammonium and nitrate contents were significantly increased. Soil available P and total P contents were significantly increased with the long-term P addition. The addition of N (N and NP treatments) significantly increased the abundance of AOB-amoA gene in both seasons, and reached the highest in the N treatment around 8.30×107 copies·g-1 dry soil. The abundance of AOA-amoA gene was significantly higher in the NP treatment than that in CK, with the highest value around 1.17×109 copies·g-1 dry soil. There was no significant difference in N-related gene abundances between two seasons except for the abundance of comammox-amoA. Nitrogen addition exerted significant effect on the abundance of AOB-amoA, nirK and nosZ genes, especially in wet season. Phosphorus addition exerted significant effect on the abundance of AOA-amoA and AOB-amoA genes in both seasons, but did not affect denitrifying gene abundances. Soil pH, ammonium, nitrate, available P, and soil water contents were the main factors affecting the abundance of soil N-related functional genes. In summary, the response of soil ammonia-oxidizing microorganisms and denitrifying microorganisms was more sensitive to N addition than to P addition. These findings shed new light for evaluating soil nutrient availability as well as their response mechanism to global change in subtropical forests.

Effects of water level on nitrous oxide emissions from vegetated ditches.

Nitrous oxide (N2O) is considered a powerful greenhouse gas. Vegetated ditches are an important source of N2O emissions in the agricultural systems. However, few studies have examined on the relationship between N2O emissions and the water level in vegetated ditches. To investigate the effect of water level on the N2O emissions, three pilot-scale ditches vegetated with Myriophyllum aquaticum were constructed with low (LW), medium (MW), and high (HW) water levels. The examined results indicated that the M. aquaticum ditches decreased N2O emissions by 38.4% and 67.9% in MW and HW, respectively, as compared to the LW ditch. In addition, the N2O emission factor decreased with increasing water level in the order of: LW (0.18%) > MW (0.11%) > HW (0.06%). The MW and HW ditches reduced the N2O emissions by controlling the sediment nitrogen contents, in which the ammonia nitrogen increased with increasing the level of water, while nitrate nitrogen decreased with increasing the level of water. The increase in the level of water significantly reduced the gene abundance of ammonia-oxidizing archaea (AOA) (p < 0.05), thereby reducing the N2O emissions in the MW and HW conditions due to the positive correlation between N2O emissions and AOA gene abundances. The unclassified_k_norank_d_Bacteria was the dominant denitrifying bacterial genus observed in the M. aquaticum ditches, and its highly relative abundance yielded low N2O emissions in the HW ditch. These findings indicate that reducing N2O emissions may be achieved by controlling the water level in vegetated ditches.