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A massive rise in atmospheric nitrogen deposition (ND) has threatened ecosystem health through accelerating soil nitrogen (N) cycling rates. While soil microbes serve a crucial function in soil N transformation, it remains poorly understood on how excess ND affects microbial functional populations regulating soil N transformation in tropical forests. To address this gap, we conducted 13-year N (as NH

Plant hydraulic traits have been shown to be sensitive to changes in nitrogen (N) availability in short-term studies largely using seedlings or saplings. The extent and the magnitude of N-sensitivity of the field grown mature trees in long-term experiments, however, are relatively unknown. Here, we investigated responses of leaf water relations and morphological and anatomical traits of two dominant tree species (Castanopsis chinensis and Schima superba) to a six-year canopy N addition in a subtropical forest. We found that N addition increased leaf hydraulic conductivity in both species along with higher transpiration rate and less negative water potential at 50% loss of leaf hydraulic conductivity and at leaf turgor loss point. Examination of leaf morphological and anatomical traits revealed that increased leaf hydraulic efficiency was at least in part due to increased vessel diameter which also compromised the hydraulic safety under increased water stress. Moreover, reduced vessel reinforcement and increased thickness shrinkage index further interpreted the increases in leaf hydraulic vulnerability under N addition. Our results demonstrated that N deposition may lead to increases of plant water loss to the atmosphere as well as tree vulnerability to drought.

Developing an understanding of the response of soil organic carbon (SOC) to N addition is critical to quantify and predict the terrestrial carbon uptake under increasing N deposition in the future. However, results from field studies on the response of SOC content and composition to N addition are highly variable across different ecosystems. The interpretation of SOC responses to N addition are often complicated by the differences in climate, soil substrate and other factors. To address this question, we measured SOC and its components in adjacent broadleaved and coniferous subtropical forests after 14 years of N addition. SOC in the top 50 cm increased by 2.1 kg m−2, 1.8 kg m−2 and 1.2 kg m−2 for low, medium and high rates of N addition in the broadleaved forest, but did not change significantly in the coniferous forest. Increased SOC in the broadleaved forest was contributed by the significant increases in particulate organic carbon (POC), humus organic carbon (HOC) in the 0–10 cm and 30–50 cm soil layers and resistant organic carbon (ROC) in the 0–10 cm soil layer. 13C nuclear magnetic resonance (NMR) spectra of coarse SOC revealed a decrease in easily decomposed carbon (C) and a shift in recalcitrant C. The increased SOC accumulation in the broadleaved forest was largely driven by altered rates of organic matter decomposition, rather than C inputs to soil. Land-history and low nutrient availability may have contributed to the lack of significant impact of N addition on SOC in the coniferous forest. Our results suggested the different controls of SOC accumulation and less sensitivity of SOC chemical composition at the molecular level to N addition in the two subtropical forest soils.

Urbanization, the migration of populations from rural to urban areas, has been causing great stress on natural environments, leading to air pollution and nitrogen (N) deposition, negatively affecting forest health. Although there is evidence that urbanization has changed forest N cycling, little is known about whether urbanization also changes the availability of phosphorus (P), which is important for plant growth and forest productivity. To address this question, we carried out a survey in the Pearl River Delta region, the world's largest urban area in southern China, using two types of representative forests, the evergreen broadleaf forests (BFs) and pine plantations (PPs). The leaf N:P ratios in the two forest types were high (20-50) with a significant increasing pattern along the rural-to-urban gradient. The ratios of leaf P:K and P:Na declined along the rural-to-urban gradient, whereas leaf P content did not change in BF but decreased in PP along the rural-to-urban gradient, suggesting that leaf P became limiting along urbanization. The abundance of actinomycetes and gram-negative bacteria decreased along the rural-to-urban gradient, indicating the negative effects of urbanization on soil microorganisms. Principal component analysis indicated that divergent key factors respond to the urbanization and affect plant P limitation in the two forest types. In BF, broadleaf trees showed a greater response to N deposition from urbanization indicating direct leaf N uptake from N deposition is a key factor for plant P limitation. Alternatively, in PP, our findings suggest soil acidification is an important factor accelerating plant P limitation. Our study revealed that urbanization intensifies plant P limitation in subtropical forests, and the effects vary depending on forest types. Our findings provide empirical information to support the management of forest ecosystems and evaluation of urbanization effects on forest health.