Abstract: Large proportion of natural forest has been transformed into plantations in subtropical regions, with consequences on forest ecosystem structure and function. In order to understand the responses of two nitrite reducing genes (nirK and nirS) in N₂O production to forest conversion, we collected soil samples from Castanopsis carlesii natural forest, Cunninghamia lanceolata plantation and Pinus massoniana plantation and examined the abundance of nirK and nirS genes in soils and aggregates. Results showed that forest conversion increased soil pH, while decreased soil ammonium content. Forest conversion did not influence the mass proportion of soil aggregates. The abundance of nirK and nirS genes varied in aggregates with different particle sizes. The abundance of nirK and nirS genes was the highest in small macraoaggregates and the lowest in the silt-clay particles. Moreover, the abundance of nirK was significantly higher than that of nirS in soils of all forest types, indicating that nirK dominated in the acidic forest soils. Conversion of natural forest to plantations significantly increased the abundance of nirK and nirS genes in the bulk soil and aggregates, indicating that forest conversion would be beneficial for the growth of microorganisms bearing nirK and nirS genes, which might be associated with the increases of soil pH. Taken together, conversion of natural forest to C. lanceolata plantation or P. massoniana plantation significantly increased the abundance of nirK and nirS in soils and aggregates, but did not affect the mass proportions of aggregates.

Key words: forest conversion, soil aggregate, nirK, nirS.