Continuous application of conservation tillage affects in situ N2O emissions and nitrogen cycling gene abundances following nitrogen fertilization
Fertilized agricultural soils can be a major source of soil nitrous oxide (N2O) emissions to the atmosphere. Conservative soil management may have the ability to reduce N2O emissions through affecting a number of Ncycling-related soil biophysical properties. Using in situ N2O measurements combined with the techniques of quantitative polymerase chain reaction (qPCR), amplicon sequencing, and metagenomic sequencing, we aimed to understand the effects of long-term (>10 y) conservation tillage (i.e., zero- and chisel-till vs. conventional plow-till) on soil N2O production and associated microbial guilds following inorganic N fertilizer application in maize. Between 2017 and 2019, continuous in situ measurements of N2O fluxes indicated that both zero- and chisel-till significantly lowered cumulative emissions within the growing season, compared to plow-till, mainly through shortening the duration and reducing the magnitude of post-fertilization emission events. Conservative soil management, in particular zero-till, consistently increased the Shannon diversity index of bacterial community over the growing season, compared with plow-till. High-frequency qPCR analyses further revealed a clear tillage-induced niche differentiation between nosZI- and nosZII-N2O reducers, as evidenced by the dominant gene abundance of nosZII compared to nosZI in the conservation tillage soil, which eventually probably contributed to the transformation of N2O to N-2. Moreover, compared to plow-till, zero-till significantly decreased gene abundances involved in N2O production including the nirS, nirK, and narG genes, but increased abundances of N2O reduction genes such as nosZ during peak N2O emissions. Critically, the abundances of detected species involved in denitrification, such as Deltaproteobacteria_bacterium spp. and Alphaproteobacteria_bacterium spp. were clearly inhibited by zero-till. Overall, the reduced soil N2O emissions under reduced tillage positively and strongly depended on the nosZI-to-nosZII ratio, while increased emissions due to conventional tillage were positively associated with intensified denitrification. Such improvements in understanding of the responses of N-cycling gene abundances to tillage intensity can certainly help in the development of updated soil management practices and adaptative N application strategies to reduce reactive N emissions in agricultural ecosystems.