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The effects of long-term reforestation on soil microbial communities and biomass are poorly understood. This study was conducted on two coniferous plantations: Cunninghamia konishii Hayata, planted 40 years ago (CONIF-40), and Calocedrus formosana (Florin) Florin, planted 80 years ago (CONIF-80). An adjacent natural broadleaf forest (BROAD-Nat) was used as a control. We determined microbial biomass C and N contents, enzyme activities, and community composition (via phospholipid fatty acid [PLFA] assessment). Both microbial biomass and PLFA content were higher in the summer than in the winter and differed among the forests in summer only. Total PLFA, total bacterial, gram-positive bacterial, gram-negative bacterial, and vesicular arbuscular mycorrhizal fungal contents followed the same pattern. Total fungal content and the ratios of fungi to bacteria and of gram-positive to gram-negative bacteria were highest in CONIF-40, with no difference between the other forests. Principal component analysis of PLFA contents revealed that CONIF-40 communities were distinct from those of CONIF-80 and BROAD-Nat. Our results suggest that vegetation replacement during reforestation exerts a prolonged impact on the soil microbial community. The understory broadleaf shrubs and trees established after coniferous plantation reforestation may balance out the effects of coniferous litter, contributing to bacterial recovery. [ABSTRACT FROM AUTHOR]

Soil microbial communities play vital roles in nutrient cycling and ecosystem functioning but these communities could be affected by land use change. To understand the impacts of land use change on soil microbial communities, we assessed the relative abundance of soil microbial communities and soil physicochemical properties following stand conversion from native broadleaf forests to mixed and bamboo forests in Feng yang Mountain Nature Reserve, China. We used phospholipid fatty acid (PLFA) profiling analysis to determine the composition of microbial communities, quantified soil bulk density, pH, organic carbon, nitrogen, and phosphorus concentrations to determine soil physicochemical properties, and assessed species richness and evenness to determine vegetation structure. We found that the abundance of anaerobic bacteria was significantly higher in the bamboo forests than in broad-leaved or mixed forests, while the abundance of 16:1 ω5c was significantly lower in the mixed forests than other forests. The relative abundance of 16:1 ω5c was positively correlated with soil pH, while the abundance of anaerobic bacteria was negatively correlated with soil phosphorus concentration. Among the three different land use types, bamboo forest was characterized by significantly higher soil pH, while the broad-leaved forest had significantly higher nitrogen concentration, and mixed forest had significantly higher soil bulk density. Overall, the composition of microbial communities in native broad-leaved forest was distinct from converted forests in the relative abundance of anaerobic bacteria and 16:1 ω5c, underscoring the fact that land use change can have a profound impact on soil microbial composition. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha year) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (NO), carbon dioxide (CO) and methane (CH), as well as natural abundance isotopic signatures of NO, nitrate (NO) and ammonium (NH). In the control plots during dry season, we deduced limited NO production by denitrification in the topsoil (0.05-0.40 m) as indicated by: ambient NO concentrations and ambient N-NO signatures, low water-filled pore space (35-60%), and similar N signatures of NO and NO. In the subsoil (0.40-2.00 m), we detected evidence of NO reduction to N during upward diffusion, indicating denitrification activity. During wet season, we found that NO at 0.05-2.00 m was mainly produced by denitrification with substantial further reduction to N, as indicated by: lighter N-NO than N-NO throughout the profile, and increasing NO concentrations with simultaneously decreasing N-NO enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between O-NO and N-NO site preferences. Long-term N addition did not affect dry-season soil NO-N contents, doubled wet-season soil NO-N contents, did not affect N signatures of NO, and reduced wet-season N signatures of NO compared to the control plots. These suggest that the increased NO concentrations have stimulated NO production and decreased NO-to-N reduction. Soil CO-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO-C contents. The N-addition plots showed reduced dry-season soil CH-C contents and threshold CH concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH uptake rates remained similar between treatments possibly because diffusive CH supply from the atmosphere largely limited CH oxidation. [ABSTRACT FROM AUTHOR]