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.

In order to provide a theoretical basis for understanding the underground physiological and ecological processes mediated by fine roots in different root orders, the variations of fine root functional traits of three dominant trees in Zijin mountain were investigated.

Three 20 m×20 m plots were set up by using a random block design in Zijin mountain. Inside the plots, the fine roots of three dominant tree species, including Aphananthe aspera, Celtis sinensis and Liquidamabar formosana, were collected. The root diameter, specific root length, specific surface area, root tissue density, C content, N content, and P content of these fine roots in orders were quantified and compared.

1) Species identity significantly affected all parameters mentioned above, while fine root order only had significant effects on fine root parameters, i.e. root diameter, specific root length, specific surface area, N content, P content, and C/N ratio. 2) With the increase of fine root order, root diameter and C/N increased, specific root length, specific surface area, N content, and P content decreased, while root tissue density and C content had no response. 3) Among the three species, C. sinensis had the smallest diameter, the largest specific root length and specific surface area, and the highest C content, A. aspera had the highest root tissue density, and L. formosana had the smallest C/N in 1^st-5^th root orders; A. aspera had the highest N content, and the of C. sinensis had the highest P content in the 1^st root order. In addition, the fine root biomass of A. aspera and L. formosana increased with the increase of root order, and the 1^st order root biomass in C. sinensis was higher than that of the 2^nd and 3^rd grade roots. 4) The principal component analysis showed that the fine root functional traits of the three tree species could be divided into two variation dimensions. A. aspera preferred resource acquisition type, C. sinensis preferred autonomous foraging resource acquisition type, and L. formosana preferred the trade-off of different strategies.

The functional characteristics of fine roots and multi-dimensional root strategy are different among tree species. This indicate that the nutrient absorption efficiency of fine roots of A. aspera and C. sinensis are higher, while that of L. formosana is lower. However, the fine root decomposition rate of L. formosana may be faster than that of A. aspera and C. sinensis.

Soil organic carbon (SOC) is important for soil quality and fertility in forest ecosystems. Labile SOC fractions are sensitive to environmental changes, which reflect the impact of short-term internal and external management measures on the soil carbon pool. Organic mulching (OM) alters the soil environment and promotes plant growth. However, little is known about the responses of SOC fractions in rhizosphere or bulk soil to OM in urban forests and its correlation with carbon composition in plants.

A one-year field experiment with four treatments (OM at 0, 5, 10, and 20 cm thicknesses) was conducted in a 15-year-old Ligustrum lucidum plantation. Changes in the SOC fractions in the rhizosphere and bulk soil; the carbon content in the plant fine roots, leaves, and organic mulch; and several soil physicochemical properties were measured. The relationships between SOC fractions and the measured variables were analysed.

The OM treatments had no significant effect on the SOC fractions, except for the dissolved organic carbon (DOC). OM promoted the movement of SOC to deeper soil because of the increased carbon content in fine roots of subsoil. There were significant correlations between DOC and microbial biomass carbon and SOC and easily oxidised organic carbon. The OM had a greater effect on organic carbon fractions in the bulk soil than in the rhizosphere. The thinnest (5 cm) mulching layers showed the most rapid carbon decomposition over time. The time after OM had the greatest effect on the SOC fractions, followed by soil layer.

The frequent addition of small amounts of organic mulch increased SOC accumulation in the present study. OM is a potential management model to enhance soil organic matter storage for maintaining urban forest productivity.