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• Foliar C:N:P:K ratios were driven by changes in nutrient concentrations and seasons. • Warming + drought increased foliar N, N:P, and δ15N across seasons. • Drought and warming had significant interactive effects on foliar δ15N but not δ13C. • Foliar δ15N may be a useful indicator of systematic changes in N cycling in forests. Carbon (C), nitrogen (N), phosphorus (P), and, more recently, potassium (K) have been examined in ecological stoichiometry because they are the most abundant elements in organic matter. An increasing number of studies are investigating the potential responses of plants to future global climate change scenarios, and although warming and drought may greatly influence ecosystem function and services, their combined effects on C:N:P:K stoichiometry remain unknown. In the present study, we assessed the stoichiometry of a major subtropical tree species Cunninghamia lanceolata (Lamb.) Hook, in southern China in terms of its responses to warming and drought stresses. To measure C, N, P, and K concentrations, C:N:P:K stoichiometric ratios, and stable isotope abundance, we sampled C. lanceolata needles from seedlings subject to four treatments: control (CT), warming (W, with 5 °C increase in temperature), drought (D, with 50% decrease in precipitation), and the abovementioned warming and drought conditions combined (WD). The warming and drought conditions applied simulated the climate changes predicted for the next decades in southern China. We found that variations in foliar C:N:P:K stoichiometric ratios were driven by changes in nutrient concentration, season, and treatment. Unexpectedly, differences in foliar intrinsic water use efficiency across sampling seasons were not significant. The WD conditions increased foliar N concentration (57%) and the δ15N value (111%) across the different seasons, and significant interactive effects between warming and drought on foliar N concentration and δ15N were clearly demonstrated. The N cycle is likely to accelerate in subtropical forests, with more of the "heavier" N being mobilized for plant use, and thus the combination of warming plus drought may alleviate plant N restrictions in the future. Thus, foliar δ15N might be used as an indicator of the changes in N cycling in C. lanceolata forests. [ABSTRACT FROM AUTHOR]

[Display omitted] • Low N addition is beneficial to copiotrophs and reduces bacterial Shannon diversity. • N addition strengthens the association between BCC-DOM. • The key role of rare bacterial taxa on DOM components was emphasized. • Total nitrogen directly affects key bacterial taxa and association between BCC-DOM. • The importance of rare bacterial taxa to forest ecosystem is highlighted. The interaction between soil bacterial communities and dissolved organic matter (DOM) is the central link of soil carbon and other nutrient cycle. However, how nitrogen (N) addition changes the relationship between forest soil bacterial community composition and DOM components (BCC-DOM) and which key bacterial taxa affect DOM components is not clear. After two years of N addition, we used parallel factor analysis and high-throughput sequencing to explore the changes of DOM fluorescent components, microbial community composition, and diversity at three N addition levels in a subtropical Pinus taiwanensis forest and presented the relationship between BCC-DOM through co-occurrence networks. The results show that low N addition, but not high N addition, transitions the surface bacterial community into a eutrophic state and reduces the Shannon index in the subsoil. Short-term N addition decreases the dissolved organic carbon (DOC) content but increases DOM humification degree and humus-like components (C1 and C2) in the topsoil. Network analysis revealed that the relationships between BCC-DOM are more complex after N addition and the negative associations were dominant. Random forest analysis showed that Chloroflexi (TK10, Ktedonobacteria, and JG37.AG.4) and Nitrospirae (Nitrospira) are key factors for predicting humus-like DOM. Protein-like DOM was mainly related to Firmicutes (Clostridia). Furthermore, structural equation models revealed that soil pH and DOC content do not determine the key bacterial taxa and DOM components, but total N content significantly affects these parameters. Short-term N addition reduces soluble substances in forest soil but improves the obstinacy of carbon substrates. Further, N addition helps to regulate forest soil bacterial communities and strengthen the interactions with DOM. [ABSTRACT FROM AUTHOR]

The priming effect (PE) induced by fresh carbon inputs generally stimulates soil organic matter (SOM) mineralization. However, the magnitude of the PE will vary according to the chemical composition of plant litter and nitrogen (N) deposition, due to the effect of these factors on soil microbial community composition and activity. This needs to be considered explicitly in ecosystem models, for more accurate prediction of soil carbon dynamics and stocks. Thus, we conducted a 100-day incubation experiment with additions of three 13C-labeled litters and inorganic N to evaluate the effects of plant litter in combination with N additions on SOM mineralization, PE, microbial community composition and activity in a subtropical Cunninghamia lanceolata forest soil. We found no difference in SOM mineralization and PE among three litter types, but the addition of litter together with N increased the PE by approximately 11–42%, apparently through a microbial stoichiometry decomposition process. When N was added, the PE response, biomass of microbial groups and enzyme activities were related to the litter chemical composition. The direction of PE varied with time, and different microbial mechanisms operated at different decomposition stages. Moreover, microbial activity seemed to be energy-limited by the late decomposition stage, based on the incorporation of litter-derived carbon into soil microbial phospholipid fatty acids with high-lignin litter than other litter types. In conclusion, our findings emphasize that different chemical composition litters in combination with N inputs into soil can have a strong effect on soil carbon stocks by stimulating the priming of native SOM mineralization in subtropical forests. Our results also highlight that changes in litter chemical composition, nutrient availability and microbial processes need to be considered from short to long term, which is of great importance to accurately evaluate the variability of the PE under future climate change scenarios. • We studied the effects of litter and inorganic N addition on SOC mineralization. • We included three litters with different chemical composition and a subtropical forest soil. • N addition stimulated the magnitude of litter-induced PE across all litter types. • Only under N addition, the litter-induced PE was related to litter chemical composition. • The direction of PE varied over time, associated with different mechanisms. [ABSTRACT FROM AUTHOR]

Microorganisms play an integral role in driving phosphorus (P) transformation in forest soils; however, studies on soil P cycling and the molecular mechanisms of microbes activated in response to elevated nitrogen (N) deposition are limited. In this study, we conducted a multilevel field N enrichment experiment in a subtropical P-deficient Moso bamboo (Phyllostachys heterocycla) system to evaluate the microbial ecological traits of P transformation (e.g., organic P mineralization and inorganic P solubilization) over three consecutive years. N addition significantly decreased available and organic P levels in the soil and increased the microbial biomass C:P and N:P ratios, indicative of severe microbial P limitation. Consequently, N addition increased the absolute abundance of P starvation response regulation genes (phoU and phoR), which further induced an increase in organic P mineralization (phoN , phoD, appA), but not that of inorganic P solubilization genes (ppx and gcd). This suggests that microbes enhance P availability by organic P mineralization rather than inorganic P solubilization to ameliorate reduced P availability. Furthermore, a bacterial functional module (B_Mod#0) consisting of Proteobacteria, Actinobacteria , and Firmicutes accounted for more than 60% of the changes in the abundance of genes responsible for organic P-mineralization in the soil, suggesting that B_Mod#0 acts as a keystone phylotype in enhancing functional P-cycling potential. This study provides novel insights into microorganism-driven P cycling in P-deficient forest soils with N addition. • N addition reduced soil P availability and intensified microbial P limitation. • N addition activated microbial P starvation response and enhanced expression of P mineralization genes. • B_Mod#0 acts as a keystone phylotype in enhancing functional potential for organic P mineralization. • Organic P mineralization is more important than inorganic P desorption for microbial amelioration of P limitation. [ABSTRACT FROM AUTHOR]