Search

An ex-situ 15 N–18O tracing experiment with soils collected from the valley and slope, respectively, of a subtropical secondary karst forest with three N addition levels, i.e., 0, 50, and 100 kg N ha−1 year−1 for each topographic position to investigate N2O production pathways. Autotrophic nitrification pathways (ammonia oxidation, nitrifier denitrification, and nitrification-coupled denitrification) accounted for > 70% of total N2O production, but denitrification pathways (heterotrophic denitrification and co-denitrification) were the minor source of N2O at both topographic positions. In the valley, chronic N addition stimulated ammonia oxidation-derived N2O, which was paralleled by increased ammonia-oxidizing archaea (AOA) amoA gene transcript abundance, but inhibited nitrifier denitrification- and nitrification-coupled denitrification–derived N2O along with suppressed ammonia-oxidizing bacteria (AOB) amoA gene transcript abundance and stimulated nosZII gene transcript abundance, respectively. On the slope, chronic N addition stimulated ammonia oxidation-derived N2O along with increased AOB amoA gene transcript abundance, and enhanced nitrifier denitrification-derived N2O congruent with increased AOB amoA and decreased nirK gene transcript abundances. In addition, chronic N addition reduced the relative contribution of heterotrophic denitrification to N2O production but had no significant influence on heterotrophic denitrification-derived N2O on the slope. Overall, our results provide a comprehensive view in terms of how topography-driven soil properties regulate N2O production and its pathways in a subtropical forest. [ABSTRACT FROM AUTHOR]

Hydrophilic and hydrophobic organic compounds extracted from 13C-labelled maize residues were incubated with soils to evaluate their mineralization and priming effect (PE) caused by their utilization by microbial groups. Two soils with contrasting soil properties were collected from well-drained upland and water-logged paddy. Mineralization of the 13C-labelled fractions and their PE were quantified by monitoring the CO2 efflux and 13C enrichment during a 40-day incubation. The composition of main microbial groups (bacteria and fungi) involved in the utilization of 13C-labelled fractions was determined based on phospholipid fatty acids (PLFAs) analysis. At the initial stage (6–24 h), hydrophilic fraction had faster mineralization rate (3.6–70 times) and induced 1.5–10 times stronger PE (positive in upland soil and negative in paddy soil) than those of hydrophobic fraction. The 13C-PLFAs data showed that the incorporation of hydrophilic fraction into bacteria was 11.4–16.4 times greater than that into fungi, whereas the hydrophobic fraction incorporated into fungi was 1.0–1.5 times larger than that into bacteria at day 2. This indicated greater contributions of r-strategists (fast-growing bacteria) for the uptake of hydrophilic fraction versus K-strategists (slow-growing fungi) for hydrophobic fraction. Compared with K-strategists, the r-strategists possessed a much faster metabolism and thus triggered stronger apparent PE by accelerating microbial biomass turnover, resulting in higher mineralization and stronger PE for the hydrophilic than hydrophobic fraction. The slower and less mineralization of both fractions in paddy than in upland soils is due to the suppression of microbial activity and substrate utilization under flooding. At the end of 40-day incubation, the cumulative mineralization of hydrophilic and hydrophobic fractions was similar. Consequnently, microbial mechanisms underlying the utilization of organic compounds with contrasting solubility (hydrophilic or hydrophobic) are crucial for evaluating the stabilization and destabilization (e.g., priming) processes of soil organic matter. [ABSTRACT FROM AUTHOR]

Determining the effects of nitrogen (N) and phosphorus (P) application on arbuscular mycorrhizal fungi (AMF) communities is important for predicting AMF responses to nutrient deposition. The AMF parameters and soil properties were monitored in karst grassland after 2 years of N and P addition. Then, AMF abundance, diversity, and community composition significantly differed between seasons. AMF abundance was higher in July (summer) than in December (winter), whereas richness and Chao1 estimator values showed the opposite results. The numbers of the genera Funneliformis and Sclerocystis were significantly more abundant in December, but the proportions of Scutellospora, Redeckera, and Diversispora were significantly higher in July. N and NP treatments significantly increased AMF abundance; richness and Chao1 values in the P treatment were significantly higher than those of the control in July. AMF community composition changed substantially between December and July but did not respond to fertilization. AMF abundance was significantly correlated with total N (TN), while AMF richness was also significantly correlated with available P (AP) and pH. pH and nitrate N (NO3−-N) strongly affected AMF community composition. These results suggested that P became more limiting with N fertilization, AMF investment increased access to more P, and richness was lower when certain AMF taxa (Diversisporales) increased in abundance during the growing season and under more P-limiting conditions. These results also suggested that N and P addition have specifically different effects on AMF abundance and diversity, and consequently potential effect on long-term vegetation composition and productivity. [ABSTRACT FROM AUTHOR]