[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.