Soil acidification instead of alkalization has become a new environmental issue caused by urbanization. However, it remains unclear the characters and main contributors of this acidification. We investigated the effects of an urbanization gradient on soil acidity of Pinus massoniana forests in Pearl River Delta, South China. The soil pH of pine forests at 20-cm depth had significantly positive linear correlations with the distance from the urban core of Guangzhou. Soil pH reduced by 0.44 unit at the 0–10 cm layer in urbanized areas compared to that in non-urbanized areas. Nitrogen deposition, mean annual temperature and mean annual precipitation were key factors influencing soil acidification based on a principal component analysis. Nitrogen deposition showed significant linear relationships with soil pH at the 0–10 cm (for ammonium N ("Equation missing"<!-- image only, no MathML or LaTex -->-N), P < 0.05; for nitrate N ("Equation missing"<!-- image only, no MathML or LaTex -->-N), P < 0.01) and 10–20 cm (for "Equation missing"<!-- image only, no MathML or LaTex -->-N, P < 0.05) layers. However, there was no significant loss of exchangeable non-acidic cations along the urbanization gradient, instead their levels were higher in urban than in urban/suburban area at the 0–10 cm layer. Our results suggested N deposition particularly under the climate of high temperature and rainfall, greatly contributed to a significant soil acidification occurred in the urbanized environment.

Restoration plantations in subtropical regions, often established with fast-growing tree species such as Acacia auriculiformis A. Cunn. ex Benth and Eucalyptus urophylla S. T. Blake, are frequently developed on highly weathered soils characterized by phosphorus deficiency. To investigate strategies for mitigating nutrient imbalances in such ecosystems, a long-term (≥13 years) fertilization experiment was designed. The experiment involved three fertilization regimes: nitrogen fertilizer alone (N), phosphorus fertilizer alone (P), and a combination of nitrogen and phosphorus (NP) fertilizers. The objective of this study was to investigate the effects of long-term fertilization practices on soil quality in subtropical plantations using a soil quality index (SQI). Consequently, all conventional soil physical, chemical, and biological indicators associated with the SQI responses to long-term fertilization treatments were systematically evaluated, and a principal component analysis (PCA) was conducted, along with a literature review, to develop a minimum dataset (MDS) for calculating the SQI. Three physical indicators (silt, clay, and soil water content), three chemical indicators (soil organic carbon, inorganic nitrogen, and total phosphorus), and two biological indicators (microbial biomass carbon and phosphodiesterase enzyme activity) were finally chosen for the MDS from a total dataset (TDS) of eighteen soil indicators. This study shows that the MDS provided a strong representation of the TDS data (R² = 0.81), and the SQI was positively correlated with litter mass (R² = 0.37). An analysis of individual soil indicators in the MDS revealed that phosphorus addition through fertilization (P and NP treatments) significantly enhanced the soil phosphorus pool (64–101%) in the subtropical plantation ecosystem. Long-term fertilization did not significantly change the soil quality, as measured using the SQI, in either the Acacia auriculiformis (p = 0.25) or Eucalyptus urophylla (p = 0.45) plantation, and no significant differences were observed between the two plantation types. These findings suggest that the MDS can serve as a quantitative and effective tool for long-term soil quality monitoring during the process of forest sustainable management.

Nitrogen (N) deposition alters the soil environment for forest trees, particularly in tropical regions, leading to variations in leaf traits. However, the adaptive responses of plantation tree species to chronic N deposition, via leaf trait modifications, remain poorly understood. We conducted a decade-long experiment involving N additions in two typical plantations dominated by Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) in South China, to investigate species-specific leaf trait plasticity under N deposition. Our results showed that long-term N addition did not affect N and phosphorus (P) concentrations, sugar and starch levels, intrinsic water use efficiency (iWUE), and leaf mass per area (LMA), but lowered leaf total Carbon (C) content in both EU and AA. Moreover, it resulted in divergent traits between them, showing an increase in tannin and phenolics but a decrease in leaf water content (LWC) in AA but no such variations in EU. These differential responses were attributed to their unique leaf traits that EU contains high chemical defensive compounds and AA, as an N-fixing tree species, exhibits higher resource levels. The reduced leaf total C was redirected towards defense, without compromising iWUE through unchanged sugar and starch levels, particularly in AA. Our findings demonstrate that long-term N addition intensifies the coupling between C and water, resulting in a shift in C allocation in trees. Consequently, long-term N addition triggers different defensive strategies: a conservative defense in EU and an active defense in AA. This finding offers new insights into the adaptive mechanisms of forest plants under global change scenarios.