Soil has the nature of acidity and alkalinity, mostly indicated by soil pH that could greatly affect soil ecological processes and functions. With exogenous inputs of acidic materials (such as acid rain), soils may more or less resist to maintain their pH levels within specific thresholds by various buffering processes. It has been well established that soil properties such as cation exchange capacity (CEC), soil organic matter (SOM), and clay content play important roles in mitigating the effects of acid inputs, but the factors varied across soils. This microcosm experiment was conducted to investigate changes in the soil pH and quantitatively estimate the critical pH threshold of simulated acid rain for three highly weathered soils (red soil, lateritic red soil, and latosol) that are typical soil types widely distributed across the world’s subtropical and tropical climatic zones, as well as important influential factors, after continuously adding different levels of simulated acid rain on the surface of soil cores. The results showed that the change in the soil pH was not significantly different among the three soils, although it was exponentially related to soil CEC and clay content. Resultantly, the latosol that had high soil CEC and clay content was more resistant to simulated acid rain, especially when relatively weak simulated acid rain treatments were applied. The lateritic red soil that contained the lowest soil CEC and clay content showed the greatest decline in the soil pH under the strongest simulated acid rain treatment of pH being 2.5. Furthermore, we estimated the critical pH threshold of simulated acid rain for the three soils and observed that it was considerably different among the soils. Surprisingly, the pH threshold of simulated acid rain was also positively related to the soil CEC and clay content, therefore making the highest pH threshold in the latosol. Our results imply that soil CEC and clay content may play critical roles in the soil acid-buffering processes from two aspects; it could not only contribute to the soil acid-buffering capacity, but also affect the threshold of acidity of acid rain below which abrupt soil acidification may occur.

With the continuation of industrialization and urbanization, acid rain (AR) has aroused extensive concern because of its potential negative effects on ecosystems. However, analysis of the current status and development trends in AR research area has seldom been systematically studied. Therefore, we motivated to conduct a bibliometric analysis of AR publications (1900–2018) using HistCite and CiteSpace software programs. Compared to traditional reviews by experts, this study offers an alternative method to quantitatively analyze and visualize the development of AR field at a large time scale. The results indicated that the overall concern of AR research studies had increased from 1900 to 2018. The most productive country was the United States, while the institution with the most publications was Chinese Academy of Sciences. “Environmental Sciences” was the most popular subject category, Water, Air, and Soil Pollution was the dominant journal, and C.T. Driscoll was the most prominent author in AR field. There were three hotspots in the field of AR, including analyzing AR status and its control policies in Europe, the United States, and China in the past few decades, investigating the ecological consequences of AR on plant histological, physiological, and biochemical traits, as well as surface water and soil properties, and the model application for quantitatively assessing AR and its effects on terrestrial and aquatic ecosystems at regional scale. Further, “behavior”, “phosphorus”, “fractionation”, “soil acidification”, “corrosion”, “performance”, “recovery”, “rainwater”, “trace element”, and “surface water” have been emerging active topics in recent years. This study can help new researchers to find out the most relevant subject categories, countries, institutions, journals, authors, and articles, and identify research trends and frontiers in the field of AR.

Soil acidification is a global environmental issue that decreases soil functions, and it has been significantly accelerated by anthropogenic activities in recent decades. Soils can resist acidification upon receiving acid inputs due to the resistance or/and resilience capacity of soils, which is termed the acid buffering capacity of soils, and it is often indicated by the soil pH buffering capacity (pHBC). An increasing number of studies have been conducted to quantify soil pHBC at various sites, but to date, integration of global data is lacking; therefore, the variations in large-scale soil pHBC and the factors that influence these variations are still unclear. In this study, we collected previously published data on soil pHBC to analyze its variations on a large scale, as well as investigate the underlying factors influencing these variations. The results showed that soil pHBC varied substantially from site to site, with a mean of 51.07 ± 50.11 mmol kg⁻¹ pH⁻¹. Soil texture and pH, separately or collectively, explained a considerable proportion of the total variation of global soil pHBC. It is well-established that a series of processes contribute to the soil acid buffering capacity in different pH ranges, and the global data analyses showed that pH 5.5 could be a key threshold value; different buffering systems may be active at pH > 5.5 and pH < 5.5. Moreover, tropical soils were more acid-sensitive than temperate and subtropical soils, and forest soils had significantly lower soil pHBCs than grassland and cropland soils. This could be attributed in part to the different soil properties, such as soil texture or pH, among the different climatic zones and ecosystems.

Soil acidification is a severe environmental problem around the world. Soil pH buffering capacity (pHBC) is the intrinsic factor affecting the soil acidification rate and is intensively impacted by anthropogenic and natural conditions. However, composite assessments of the effects of land use and soil type on soil pHBC are still limited. Therefore, we collected samples of five soil types (red soil, lateritic red soil, latosol, paddy soil and acid sulphate soil) from two land use patterns of agricultural and adjacent forest fields at different depths (0–10 cm, 10–20 cm and 20–30 cm) in South China, aiming to investigate the effects of land use and soil type on soil pHBC in this region. The results show that land use, soil type and their interactions greatly influence soil pHBC and physico-chemical properties. Forest soils have a significantly higher pHBC (11.40–49.50 mmol·kg⁻¹ soil·unit⁻¹ pH), cation exchange capacity (CEC), exchangeable Al³⁺ (EAl³⁺) and clay content than agricultural soils. Acid sulphate soil has the highest pHBC (49.27–117.83 mmol·kg⁻¹ soil·unit⁻¹ pH) values and exchangeable acid (EA) content among all investigated soil types, whereas lateritic red soil has the lowest pHBC (10.56–31.71 mmol·kg⁻¹ soil·unit⁻¹ pH). In agricultural fields, soil pHBC is positively related to CEC, soil organic carbon (SOC) and EA, indicating that agricultural soils may be in a cation exchange buffering stage. The soil pHBC of forest fields is positively correlated with SOC and EAl³⁺, implying that forest soils may be in the Al buffering stage. In conclusion, soil pHBC would vary with different land use forms and soil types, in which a series of key complex physico-chemical processes and interactions would occur to regulate soil pH buffering capacity.