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Aims: Our study aimed to determine whether, and to what extent, stand characteristics and topography affected spatial variations in soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) concentrations in subtropical forests. Methods: Soil samples were taken from a Choerospondias axillaris deciduous broadleaved forest and a Lithocarpus glaber-Cyclobalanopsis glauca evergreen broadleaved forest. Spatial variations in SOC, TN and TP concentrations and the factors affecting them were investigated using geostatistical analysis and stepwise linear regression, respectively. Results: The L. glaber-C. glauca forest exhibited higher coefficients of variation (CVs) of SOC (35 %) and TN (34 %) concentrations than the C. axillaris forest (27 % for SOC and 21 % for TN), but the CV of TP concentration in the L. glaber-C. glauca forest (17 %) was lower than that in the C. axillaris forest (24 %). Stand characteristics contributed the most to spatial variations in SOC and TP, while soil texture made the greatest contribution to variations in TN. Topography contributed the least to variations in SOC, TN and TP. Conclusions: Stand characteristics, together with topography and soil texture, contributed to spatial variations in SOC, TN and TP concentrations. The contributions of stand characteristics differed in SOC, TN and TP due to their different cycling characteristics. [ABSTRACT FROM AUTHOR]

• Change of soil N contents as stands age was pronounced in the topsoil. • Ratios of NO 3 −: NH 4 + increased while MBN: (NH 4 + + NO 3 −) decreased as stands aged. • Soil N availability increases and N cycling becomes more open in older stands. • Soil total N is co-regulated by trees' N demand and litter biomass return. • Gross N mineralization and nitrification control mineral N availability. Soil nitrogen (N) is frequently limiting forest productivity, especially in plantations. To investigate the soil N dynamics and the consequences of N accumulation as well as the losses in the ecosystem, we studied a chronosequence of Chinese-fir (Cunninghamia lanceolata [Lamb.] Hook) plantations with stands aged 3, 16, 25, 32, and >80 years. Total N (TN), dissolved organic N (DON), microbial biomass N (MBN) and mineral N (i.e., ammonium (NH 4 +) and nitrate (NO 3 −)) in the top 0–5 cm and 5–15 cm soils were compared with the annual N accumulation by trees, annual litter N return and microbial N transformation rates. Stronger changes with stand age were observed in all N forms in the top 5 cm compared to the 5–15 cm layer. Topsoil TN and DON increased steadily in stands from ages 3- to >80-years-old by 1.9-times and 2.1-times, respectively. MBN increased from 206 mg kg−1 to 327 mg kg−1 in the first 25 years and stabilized thereafter. NH 4 + content increased sharply from 3 to 16 years and then stabilized, while NO 3 − increased linearly with stand age. Both N demand of Chinese-fir and litter N input were clearly age dependent and defined the soil N status: rapid tree biomass accumulation in younger stands greatly depleted soil N, while the older stands with larger litter return and slower growth enabled N re-accumulation in the topsoil. The close positive correlation between total mineral N and gross mineralization rate, as well as between NO 3 − and gross nitrification rate indicated that the mineralization of soil organic matter and the nitrification of released NH 4 + were the two primary microbial processes controlling the available N supply. The topsoil NO 3 −: NH 4 + ratio in older stands (32- and >80 years) was greater than 1.0, and the MBN: (NH 4 + + NO 3 −) ratio was greatly reduced. This indicates that N availability increase and N cycling is accelerated with stands age. [ABSTRACT FROM AUTHOR]

Stand development affects soil properties, nitrogen (N) dynamics, and soil microbial community composition, but the question remains whether differences in N mineralization rates are mirrored by the abundance of relevant functional genes. In this study, we used the 15N pool-dilution method to estimate N mineralization (i.e. , ammonification and nitrification) rates across a Chinese fir (Cunninghamia lanceolata) chronosequence, with stands aged 7, 16, 29, 36, and >80 years. Gene copy numbers of bacteria (16S rRNA), fungi (ITS), ammonia-oxidizing archaea (AOA) and bacteria (AOB) (amoA), denitrifiers (nirS , nirK), N 2 fixers (nifH) and organic N decomposers (chiA) were quantified by qPCR. Gross ammonification and nitrification rates increased linearly with stand age in the topsoil (0–5 cm depth) and were strongly positively correlated with the abundance of the bacterial 16S rRNA gene and AOA amo A, respectively. Higher net nitrification but lower NO 3 − immobilization rates in older stands (32 and > 80 years) drove higher N availability for vegetation than in young stands (7 years). Older stands also had higher rates of NH 4 + consumption than younger stands due to the increased fungal ITS abundance and higher microbial biomass N (MBN), and AOA amoA was more abundant and active than AOB amoA due to the more acid conditions characteristic of mature forests. Redundancy analysis showed that functional gene abundance was strongly affected by soil properties such as pH, NH 4 +-N content, and MBN. We also found that microbial N storage potential was lower, and the NO 3 −-N leaching and gaseous N loss potential were higher in older stands than in younger stands. Collectively, stand developmental stage gave rise to the observed spatial gradient of gross ammonification and nitrification rates by altering the abundance of microbial functional genes, which affected plantation productivity via its modulation of the supply of bioavailable N. [Display omitted] [ABSTRACT FROM AUTHOR]

• Contents of soil total nitrogen (N) and N forms increased with vegetation restoration. • The availability of soil N decreased as vegetation restoration proceeded. • Soil N cycle was still closed when forests restored to climax communities. • Soil N availability was co-regulated by plant diversity index and soil available N pools. Nitrogen (N) is an essential element for the functioning of terrestrial ecosystems and is often the primary limiting resource for the productivity of many terrestrial ecosystems. Existing knowledge on the effect of vegetation restoration on soil N dynamics is inconsistent. The aim of this study was to investigate the dynamics of soil N contents and availability with vegetation restoration and other influencing factors. Soil samples were collected to a depth of 40 cm from four distinct plant communities along a restoration gradient from scrub-grassland (4–5 years), shrub (10–12 years), coniferous and broadleaved mixed forest (45–46 years) to the evergreen broadleaved forest (>90 years). Total N (TN), soluble organic N (SON), microbial biomass N (MBN), inorganic N (IN, including ammonium (NH 4 +), nitrate (NO 3 –)), IN/SON, NO 3 –/NH 4 +, and MBN/IN were compared across vegetation factors, soil physicochemical properties and microbial activities. TN, SON, MBN, and NH 4 + contents increased remarkably with vegetation restoration, while NO 3 – decreased from scrub-grassland to shrub and then increased. The dominant influencing factors of soil N forms were fine root biomass, soil organic carbon, C/N ratio, pH, microbial biomass carbon, and ureases, which indicated that vegetation factors, soil physicochemical properties, and microbial activities collectively control soil N accumulation. IN/SON and NO 3 –/NH 4 + ratios were < 1 and decreased with vegetation restoration, while the MBN/IN ratio increased. The results implied that soil N availability reduced with vegetation restoration and N cycling remained closed, although it has been restored to the broadleaved forest stage. The close correlation between plant diversity index and soil N availability indicated that soil N availability reduces largely due to increasing plant N demands induced by the improvement in plant species diversity. Therefore, vegetation restoration facilitates long-term soil N accumulation and retention in this region. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tree mycorrhizal associations drive soil N cycling pattern in subtropical forests. • Soil nitrate N decreased with increasing ectomycorrhizal (ECM) tree dominance. • ECM tree and associated fungi reduce soil N mineralization and nitrification rates. • High NAGase activity and C:N ratio but low pH was found in ECM-dominated soils. Trees and their associated arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi drive carbon (C) and nitrogen (N) cycling patterns. However, the mechanisms underpinning this relationship and the degree to which mycorrhiza mediate the effects of forest tree composition on soil N cycling outside of the temperate zone remain unclear. Here, we conducted field surveys to establish a natural gradient with increasing ECM tree dominance in subtropical forests, and explored the effects of tree mycorrhizal associations on soil N cycling. Across this mycorrhizal gradient, we quantified soil available N, microbial biomass N, net N transformation rates, as well as N -acquiring enzyme activities, litterfall biomass and quality, and soil pH during the one-year growing season. We found that with increasing ECM tree dominance, the dissolved organic N and nitrate N decreased, while the microbial biomass N increased linearly. Soils dominated by AM trees had 1.8–2.3 times larger mineral N contents, net N mineralization rate, and net nitrification rate than ECM-dominated stands, suggesting a rapid N cycling in AM-dominated stands. ECM trees and their associated fungi reduce the net N mineralization rate in topsoil mainly by producing recalcitrant litterfall (i.e., higher C:N) and by secreting β- N -acetylglucosaminidase to deplete N directly from soil organic matter, which together increased N limitation for free-living decomposers. The low pH and high C:N ratio in ECM-dominated soils inhibit the proliferation of ammonia-oxidizers and thus decrease the net nitrification rate. Our results demonstrate that the increasing ECM tree dominance increase soil N -acquiring enzyme activity and C:N ratio but decrease pH value, all of them critically mediating soil N availability. Consequently, by altering the relative abundances of tree mycorrhizal associations shifts in forest composition under global changes and plantation establishment can be expected to result in altered soil N cycling. [ABSTRACT FROM AUTHOR]