Effects of Afforestation Restoration on Soil Potential N(2)O Emission and Denitrifying Bacteria After Farmland Abandonment in the Chinese Loess Plateau

Denitrification is a critical component of soil nitrogen (N) cycling, including its role in the production and loss of nitrous oxide (N(2)O) from the soil system. However, restoration effects on the contribution of denitrification to soil N(2)O emissions, the abundance and diversity of denitrifying bacteria, and relationships among N(2)O emissions, soil properties, and denitrifying bacterial community composition remains poorly known. This is particularly true for fragile semiarid ecosystems. In order to address this knowledge gap, we utilized 42-year chronosequence of Robinia pseudoacacia plantations in the Chinese hilly gullied Loess Plateau. Soil potential N(2)O emission rates were measured using anaerobic incubation experiments. Quantitative polymerase chain reaction (Q-PCR) and Illumina MiSeq high-throughput sequencing were used to reveal the abundance and community composition of denitrifying bacteria. In this study, the afforestation practices following farmland abandonment had a strong negative effect on soil potential N(2)O emission rates during the first 33 years. However, potential N(2)O emission rates steadily increased in 42 years of restoration, leading to enhanced potential risk of greenhouse gas emissions. Furthermore, active afforestation increased the abundance of denitrifying functional genes, and enhanced microbial biomass. Actinobacteria and Proteobacteria were the dominant denitrifying bacterial phyla in the 0 to 33-years old sites, while the 42-years sites were dominated by Planctomycetes and Actinobacteria, implying that the restoration performed at these sites promoted soil microbial succession. Finally, correlation analyses revealed that soil organic carbon concentrations had the strongest relationship with potential N(2)O emission rates, followed by the abundance of the nosZ functional gene, bulk density, and the abundance of Bradyrhizobium and Variovorax across restoration stages. Taken together, our data suggest above-ground restoration of plant communities results in microbial community succession, improved soil quality, and significantly altered N(2)O emissions.