Soil soluble organic nitrogen and active microbial characteristics under adjacent coniferous and broadleaf plantation forests

Soil soluble organic nitrogen (SON) is considered as a sensitive indicator of soil nitrogen (N) status and plays an important role in N cycling in forest ecosystems. Most work on forest soil SON to date has been conducted in temperate areas. The information about soil SON pools and dynamics in tropical and subtropical areas is limited. The aim of this study was to investigate the effects of different forest types on soil SON availability and associated microbial properties. Soil samples were collected at two depths (0-20 and 20-40 cm) from adjacent 12-year-old plantation forests of coniferous Chinese fir (Cunninghamia lanceolata (Lamb) Hook; Coniferous Fir), coniferous-broadleaf mixed species of broadleaf Nagai Podocarpus and coniferous Pond cypress (mixed species), and broadleaf Phoebe nanmu (broadleaf Phoebe) in subtropical China. Three extraction methods (water, hot water, and 2 M KCl) were used for measurements of SON. Soil microbial biomass carbon (C) and N and activities of urease, protease, and l-asparaginase were also measured. Hot water and KCl extractable SON pools were greater in the surface soils (0-20 cm) under the broadleaf Phoebe than under the Coniferous Fir, while those under the mixed species were intermediate. The microbial biomass C and N as well as activities of protease and urease were higher in the soils under the broadleaf Phoebe and mixed species than under the Coniferous Fir. Soluble organic C and SON pools were positively related to soil total C and N and microbial biomass C and microbial biomass N. Activities of urease, protease, and asparaginase were highly correlated with SONw, SONhw, and SONKCl. This work has demonstrated that forest type had significant impacts on the pool size of soil SON. This may be attributed to the different quantity and quality of organic inputs from different forest systems. Positive relationships between microbial biomass and enzyme activities with the SON pools clearly demonstrated that microbial communities play a vital role in the dynamics of soil SON in forest ecosystems. It is recommended that soil SON measured by hot water extraction be incorporated into the routine measurement of soil available N in forest ecosystems. Further study on the role of specific microbial functional groups in the SON transformation and the chemical nature and seasonal dynamics of SON pools in forest ecosystems is warranted.