Characterization of soil nitrogen pool dynamics and influencing factors of two typical forest stands in Zijinshan, Nanjing, China

To elucidate the dynamic characteristics of soil nitrogen pools and their key influencing factors across different stand types in Zijin mountain, aiming to provide empirical data for enhancing soil nitrogen availability and maintaining soil fertility in the north subtropical region.

This study was conducted in Zijin mountain, Nanjing, focusing on two representative forest stand types: a deciduous broad-leaved forest dominated by Celtis sinensis and Aphananthe aspera, and a mixed coniferous-broadleaf forest composed primarily of Pinus massoniana and Liquidambar formosana. Soil nitrogen content and its fractions were analyzed at two depths (0-20 cm [upper layer] and 20-40 cm [lower layer]) across different growth stages. Additionally, the chemical composition of litter and fine roots, as well as enzymatic activities, were assessed to explore their roles in soil nitrogen dynamics.

1) The ranges of total nitrogen (TN), nitrate nitrogen (NO₃^--N), ammonium nitrogen (NH₄⁺-N), and microbial biomass nitrogen (MBN) in the soils of the deciduous broad-leaved forest and mixed coniferous-broadleaf forest were 3.33-7.63 g·kg^-1 and 1.57-5.70 g·kg^-1, 2.56-6.82 mg·kg^-1 and 1.42-5.14 mg·kg^-1, 4.16-12.04 mg·kg^-1 and 4.92-15.33 mg·kg^-1, and 13.81-37.24 mg·kg^-1 and 10.82-33.74 mg·kg^-1, respectively. Both forest types exhibited a distinct surface accumulation effect of nitrogen; 2) With the exception of nitrate nitrogen, all other nitrogen fractions in the deciduous broad-leaved forest were significantly higher than those in the mixed coniferous-broadleaf forest (P<0.05). In both forest types, total nitrogen (TN) was significantly higher during the peak growing season than in the early and late growing stages (P<0.05). Ammonium nitrogen (NH₄⁺-N) was significantly higher in the peak and late growing seasons than in the early stage (P<0.05). Microbial biomass nitrogen (MBN) was significantly higher in the early growing season than in the peak and late growing seasons (P<0.05); 3) Redundancy analysis (RDA) indicated that litter biomass and litter carbon-to-nitrogen (C/N) ratio were the primary factors influencing nitrogen content variations in the deciduous broad-leaved forest, explaining 40.9% and 26.3% of the variation, respectively. In the mixed coniferous-broadleaf forest, fine root biomass was the dominant factor affecting nitrogen content variation, explaining 43.8% of the variation; 4) Mantel tests revealed significant correlations between enzymatic activities and soil nitrogen fractions. In the deciduous broad-leaved forest, nitrite reductase and urease significantly affected total nitrogen (TN) and ammonium nitrogen (NH₄⁺-N) (P<0.05), while protease and nitrate reductase had highly significant effects on ammonium nitrogen (NH₄⁺-N) (P<0.01). In the mixed coniferous-broadleaf forest, nitrite reductase significantly influenced total nitrogen (TN), nitrate nitrogen (NO₃^--N), and microbial biomass.

In summary, the deciduous broad-leaved forest exhibited higher soil nitrogen content than the mixed coniferous-broadleaf forest, primarily due to its greater litter and fine root biomass, which facilitated rapid nutrient return, enhanced microbial growth, and stimulated enzymatic activities. These findings suggest that in future vegetation restoration efforts in the northern subtropical region, the establishment of deciduous broad-leaved forests should be prioritized to enhance soil nitrogen availability and ecosystem productivity.