[Effects of Drip Irrigation Patterns and Biochar Addition on Soil Mineral Nitrogen and Microbial Regulation of Greenhouse].

Drip irrigation and biochar amendment could affect the nitrogen form and transformation. Creating a deep understanding of the interacting effects of drip irrigation patterns and biochar on soil mineral nitrogen, as well as the key functional genes and microbial community involved in nitrogen transformation is helpful for improving facility agricultural management, increasing water and nitrogen use efficiency, and reducing the nitrate accumulation and groundwater pollution caused by nitrogen leaching. Four treatments [surface drip irrigation (D), insert drip irrigation (ID, insert depth 15 cm), surface drip irrigation +10 t·hm-2 of biochar (DB), and insert drip irrigation +10 t·hm-2 of biochar (IDB)] were conducted in a solar greenhouse, and non-rhizospheric and rhizospheric soils of pepper plants were studied. There was no effect of drip irrigation patterns and biochar on ammonium-nitrogen in the non-rhizospheric and rhizospheric soils. Compared with surface drip irrigation, insert drip irrigation decreased the nitrate-nitrogen concentration in the non-rhizosphere soil (P<0.05), but biochar addition weakened the difference. Biochar addition decreased the nitrate-nitrogen concentration in the rhizosphere soil under the same drip irrigation patterns. In the D treatment, biochar significantly decreased the number of copies of AOA, AOB, and nirK genes in the non-rhizospheric soil, and AOA gene copies in the rhizospheric soil (P<0.05); however, there was an increase in the number of copies of AOB and nirK genes in the rhizospheric soil of the D and ID treatments (P<0.05). Based on the structural equation model (SEM), in the non-rhizospheric and rhizospheric soils, pH and electrical conductivity were the environmental factors with the greatest influence on the ammonium-nitrogen and nitrate concentrations, respectively, and the gene copy number of AOB was the biotic factor with the greatest influence on the nitrate-nitrogen concentration. Based on PICRUSt, the γ-Proteobacteria contributed mostly to ammonia monooxygenase gene (K10945) expression, whereas the α-Proteobacteria, especially the rhizobia members, contributed mostly to nitrite reductase gene (K00368) expression. Biochar addition regulated the bacterial community structure that participated in K10945 gene expression in the non-rhizospheric soil and K00368 gene expression in the rhizospheric soil (P<0.05). Overall, biochar addition contributed more to nitrate-nitrogen and microbial mineral nitrogen-transformation processes in the agricultural soil than did the drip irrigation patterns.

[Influence of Biochar Amendment on Soil Denitrifying Microorganisms in Double Rice Cropping System].

At present, it is not explicit how biochar regulates the microbial process of denitrification in paddy fields. Therefore, a field experiment was carried out in a double rice cropping system with three wheat straw biochar treatments:no biochar treatment (CK), added 24 t·hm-2 biochar (LC), and added 48 t·hm-2 biochar (HC). Real time PCR (qPCR) and terminal-restricted fragment length polymorphism (T-RFLP) technology were adopted to analyze the abundances and microbial community structures of denitrification functional genes (narG, nirK and nosZ) in the fallow season and rice season. Due to its alkalinity, biochar amendment increased soil pH by 0.2-0.8. Biochar amendment also increased soil NH4+-N and NO3--N contents by 21.1%-32.5% and 63.0%-176.0% in the fallow season due to the presence of soluble N. Nevertheless, it reduced NH4+-N content by 48.8%-60.1% in the rice season due to the adsorption of biochar. The amendment increased soil MBN content in the fallow season, which may be a result of the large surface of biochar supplying nutrients and a suitable survival environment for the microorganisms. In the fallow season, compared to CK treatment, the increased soil NH4+-N and NO3--N with biochar amendment promoted the conversion of NH4+-N to NO3--N, and thus increased the abundances of narG and nosZ (P<0.05). The higher soil pH with biochar addition increased the abundances of nosZ and altered the community structures of narG and nosZ in the fallow season. Biochar amendment altered the denitrification process, but it did not change N2O emissions in the fallow season, which might reduce NO3--N leaching losses. In the rice season, biochar amendment increased nosZ abundance (P<0.05). HC increased the nirK gene abundance, which contributed to increased N2O emission in the rice season (P<0.05). Biochar converted the community structures of nirK and nosZ by decreasing the NH4+-N content in the rice season. The changes of the narG community structure with HC treatment contributed to the increased N2O emission. In conclusion, biochar amendment can influence the microbes involved in soil denitrification by changing the soil properties, and thus impact the N2O emissions and NO3--N leaching.

[Effects of Biochar Amendment on Soil Microbial Biomass Carbon, Nitrogen and Dissolved Organic Carbon, Nitrogen in Paddy Soils].

Biochar can influence soil microbial biomass. It is not clear how biochar amendment affects soil microbial biomass carbon and nitrogen (MBC and MBN) and dissolved organic carbon and nitrogen (DOC and DON) in double-cropping rice soils. To address this problem, two subtropical double-cropping rice soils (S1 and S2) were selected for an incubation experiment. S1 is developed from granite-weathered red soil and S2 is developed from Quaternary red clay. The following three wheat straw-derived biochar application rates were used, without N fertilizer, in each paddy soil:0%, 1%, and 2% of soil weight, represented by CK, LB, and HB, respectively. After a 70 d incubation, soil mean MBC was 877.03 mg·kg-1, 832.11 mg·kg-1, and 849.30 mg·kg-1 in S1 for the three application rates, and 902.94 mg·kg-1, 874.19 mg·kg-1, and 883.22 mg·kg-1, respectively, in S2. S1+LB, S1+HB, and S2+LB treatments reduced soil mean MBC compared to the CK treatment (P<0.05). This may be attributed to biochar inhibiting microbial growth by adsorbing soil organic carbon and other low-molecular-weight organic matter. Low biochar application rates decreased mean soil MBN by 9.45% compared to the CK treatment in S1 (P<0.05). No significant differences in mean MBC/MBN were observed among the S1 treatments, but LB reduced MBC/MBN in S2 (P<0.05). Due to the soluble organic carbon content and strong alkalinity of biochar, biochar amendment increased mean soil DOC by 4.42%-22.20% and 10.57%-35.47% in S1 and S2, respectively (P<0.05). However, biochar amendment (except for the S2+HB treatment) decreased mean soil DON in both paddy soils. This may have resulted from the adsorption of soil organic nitrogen by biochar and N consumption during the decomposition of the organic carbon within biochar. Biochar amendment increased mean soil DOC/DON in both paddy soils (P<0.05) and mean DOC/DON increased with an increase in the biochar application rate. Based on these results, biochar amendment increased soil dissolved organic carbon, decreased soil microbial biomass, and enhanced the nitrogen deficit in double-cropping paddy soils. Therefore, biochar should be combined with the application with fertilizer in double-cropping rice systems in subtropical central China.

[Effect of long-term fertilization on lignin accumulation in typical subtropical upland soil].

To investigate the effect of long-term fertilization on lignin accumulation and clarify its influencing factors in subtropical agricultural upland soils, alkaline CuO oxidation and gas chromatography was performed to quantify the amount of lignin and its monomers components (V, S and C). The soil samples were collected from the fertilization treatments of NPK and NPKS (NPK combined with straw) in Huanjiang County, Guangxi Province (limestone soil) and Taoyuan County, Hunan Province (red soil). The results showed that NPK had no significant effect on the lignin content (Sumvsc) of limestone soil, whereas the content in red soil significantly increased by (55 ± 1)%. For the NPKS treatment, the lignin content in limestone and red soil increased by (328 ± 4)% and (456 ± 9)%, respectively. After the same fertilization treatment, the proportion of cinnamyl (C)-type significantly increased in red soil, while a significant increase of vanillyl (V)-type monomers occurred in limestone soil, indicating that lignin degradation in agricultural soils was monomer specific. Furthermore, the acid-to-aldehyde ratios of syringyl-type [(Ac/Al)] or vanillyl-type [(Ac/Al)v] monomers tended to decrease after long-term fertilization with the higher value for limestone soil, suggesting the degree of lignin degradation in limestone was higher than that in red soil. Soil organic matter and total nitrogen were not correlated with lignin content, but were significantly correlated with the composition of VSC monomers. Meanwhile, the available nutrient content in the soil (available nitrogen, phosphorus, and potassium) was closely related to the contents and components of V, S, and C-type monomers (P<0.05). It indicated that the availability of soil nutrition should be considered as a key factor for the accumulation of lignin.