Soil pH regulates soil biogeochemical processes and has cascading effects on terrestrial ecosystem structure and functions. Afforestation has been widely adopted to increase terrestrial carbon sequestration and enhance water and soil preservation. However, the effect of afforestation on soil pH is still poorly understood and inconclusive. Here we investigate the afforestation-caused soil pH changes with pairwise samplings from 549 afforested and 148 control plots in northern China. We find significant soil pH neutralization by afforestation—afforestation lowers pH in relatively alkaline soil but raises pH in relatively acid soil. The soil pH thresholds (T_pH), the point when afforestation changes from increasing to decreasing soil pH, are species-specific, ranging from 5.5 (Pinus koraiensis) to 7.3 (Populus spp.) with a mean of 6.3. These findings indicate that afforestation can modify soil pH if tree species and initial pH are properly matched, which may potentially improve soil fertility and promote ecosystem productivity.

Forest stand age is a meaningful metric, which reflects the past disturbance legacy, provides guidelines for forest management practices, and is an important factor in qualifying forest carbon cycles and carbon sequestration potential. Reliable large‐scale forest stand age information with high spatial resolutions, however, is difficult to obtain. In this study, we developed a top‐down method to downscale the provincial statistics of national forest inventory data into 1 km stand age map using climate data and light detection and ranging‐derived forest height. We find that the distribution of forest stand age in China is highly heterogeneous across the country, with a mean value of ~42.6 years old. The relatively young stand age for Chinese forests is mostly due to the large proportion of newly planted forests (0–40 years old), which are more prevailing in south China. Older forests (stand age > 60 years old) are more frequently found in east Qinghai‐Tibetan Plateau and the central mountain areas of west and northeast China, where human activities are less intensive. Among the 15 forest types, forests dominated by species of Taxodiaceae, with the exception of Cunninghamia lanceolata stands, have the oldest mean stand age (136 years), whereas Pinus massoniana forests are the youngest (18 years). We further identified uncertainties associated with our forest age map, which are high in west and northeast China. Our work documents the distribution of forest stand age in China at a high resolution which is useful for carbon cycle modeling and the sustainable use of China's forest resources.

Since the mid-20th century, nitrogen (N) deposition has shown an increasing trend in the Tibetan Plateau (TP), where alpine ecosystems are sensitive to elevated N deposition. However, the quantitative characterization of N deposition in the TP remains unclear, due in most part to the lack of in situ measurement. Using the Tibetan Observation and Research Platform network, we conducted short-term in situ measurements of major ions (NO₃⁻, Cl⁻, SO₄²⁻, NH₄⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺) wet deposition at five remote sites in the TP during 2011–2013. At Southeast Tibet Station, Nam Co Station, Qomolangma Station, Ngari Station, and Muztagh Ata Station, the NH₄⁺–N wet deposition was 0.63, 0.68, 0.92, 0.36, and 1.25 kg N ha⁻¹ yr⁻¹, respectively; the NO₃⁻–N wet deposition was 0.28, 0.24, 0.03, 0.08, and 0.30 kg N ha⁻¹ yr⁻¹, respectively; and the inorganic N wet deposition was 0.91, 0.92, 0.94, 0.44, and 1.55 kg N ha⁻¹ yr⁻¹, respectively. The inorganic N wet deposition mainly occurred in the form of NH₄⁺–N during summer at all sites. Results of enrichment factor analysis and principal component analysis demonstrated that both NH₄⁺–N and NO₃⁻–N wet deposition in the TP were mainly influenced by anthropogenic activities. Backward trajectory analysis showed that the inorganic N deposition at Muztagh Ata Station was mainly transported from central Asia and the Middle East through westerlies. At Southeast Tibet Station, Nam Co Station, Qomolangma Station, and Ngari Station, the inorganic N deposition was mainly contributed by anthropogenic sources in south Asia, and was mainly transported by the Indian monsoon. Combining site-scale in situ measurements of inorganic N wet deposition in this and previous studies, the average wet deposition of atmospheric NH₄⁺–N, NO₃⁻–N, and inorganic N in the TP was estimated to be 1.06, 0.51, and 1.58 kg N ha⁻¹ yr⁻¹, respectively. The average NH₄⁺–N : NO₃⁻–N ratio in precipitation in the TP was approximately 2 : 1. Results from the present study suggest that earlier estimations based on chemical transport model simulations and/or limited field measurements likely overestimated substantially the regional inorganic N wet deposition in the TP. To clarify the total N deposition in the TP more clearly, it is essential to conduct long-term monitoring of both wet and dry deposition of atmospheric N in various climate zones in the TP in the future.