Characteristics of soluble organic nitrogen and soil enzyme activities in subtropical forest along a restoration gradient

The transformation of soil nitrogen (N) along a forest succession is a key process of the ecosystem, and the study of its relationship is receiving more and more attention. Exploring the characteristics of the SON and enzyme activity is of great significance for maintaining the N cycling in forest ecosystem, vegetation restoration, and forest management. In order to study the characteristics of soil SON and enzyme activity at different succession stages of subtropical forests and their relationship with soil physical and chemical properties, one hectare long-term observation plot of three forests at different succession stages (coniferous forest, deciduous broad-leaved forest and evergreen broad-leaved forest) in Dashanchong Forest Park in Changsha County, Hunan Province was selected to collect surface soil samples to determine SON and glucosamine glycosidase (NAG), glucosidase (BG) and xylosidase (BX) enzyme activities. Results showed that: soil SON concentrations in three forests were in the order of evergreen broad-leaved forest, deciduous broad-leaved forest, and coniferous forest. In addition, the ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), microbial biomass nitrogen (MBN) and total nitrogen (TN) in coniferous forest were lower than evergreen broad-leaved forest and deciduous broad-leaved forest (P<0.05). Soil NAG, BG, and BX in broad-leaved forests were significantly higher than those in coniferous forest, evergreen broad-leaved forest soil had the strongest enzyme affinity, while deciduous broad-leaved forest soil had the highest enzyme activity. Redundancy analysis showed that SON have a significant positive correlation with the activities of the three enzymes, and NAG activity and affinity were significantly positive correlation with soil TN, NH4+-N, and MBN. Studies have shown that there were significant differences in SON and enzyme activities in forest soils at different succession stages in the subtropical area. SON concentration increased during forest succession, and the enzyme activity related to SON transformation also increased. The positive succession of subtropical forest is accompanied by a rapid cycling process of soil N nutrients.