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Soil pH is important for controlling many soil properties. The variation in soil pH can be associated with changes in climate, soil buffering system, nitrogen deposition, and plants. However, there still lacks a comprehensive study exploring the effects of all these factors on soil pH simultaneously. Here we aimed to investigate the environmental controls on the spatial variation of soil pH in planted forests across Northern China and reveal its response to different-forms of nitrogen deposition for different species of tree plantations. We sampled 1980 soil profiles from 660 planted forest plots (3 profiles in each plot) in Northern China. We used correlation analyses and structure equation models (SEM) to explore the impacts of multiple environmental factors on soil pH. Climate (water balance, temperature) and soil inorganic carbon accounted for most variations of soil pH. Specifically, the concentration of hydrogen ions ([H+]) varied almost isometrically with soil inorganic carbon, which was also the major buffering system in this region. Nitrogen deposition affected both soil pH values and soil buffering system. Results from structure equation model indicated that nitrate nitrogen directly decreased soil pH, while ammonium nitrogen mostly affected soil pH indirectly through its impacts on soil inorganic carbon. The responses of soil pH to nitrogen deposition were species-specific, and conifer stands tended to have higher soil acidification rate than stands of other tree species. Our study provides important information for understanding mechanisms controlling the spatial pattern of soil pH in planted forests and highlights the need to develop informed policies for soil resource management under increasing threats from anthropogenic nitrogen deposition. • Climate and soil inorganic carbon dominated soil pH variations in planted forests. • Nitrogen deposition indirectly affected soil pH via impacting soil inorganic carbon. • The responses of soil pH to nitrogen deposition were species-specific. [ABSTRACT FROM AUTHOR]

Phosphorus (P) deficiency is one of the main problems limiting crop growth in red soils of southern China. The primary objective of this study was to examine P availability as a function of soil acidity. Soils were sampled from a long-term fertilization experiment and are referred as low-P (No P fertilization) and high-P (120 kg P2O5 ha-1). Both low-P and high-P treatments were incubated adjusting soil pH to seven levels from 3.0 to 6.5 for 10, 20, 30 and 45 days. The pH, DIP, and Olsen-P were determined after each incubation period, and inorganic P fractions were measured at the end of incubation. For both low-P and high-P treatments, DIP decreased with rising pH value and increased with decreasing soil pH. Olsen-P, Ca-P (Ca2-P, Ca8-P, and Ca10-P) and Al-P increased significantly with soil pH in low-P treatment. In high-P, Olsen-P increased with pH between 3.4 and 5.0 and was stable at higher pH. Moreover, Ca-P and Al-P increased significantly but Fe-P decreased with soil pH increase. The redundancy analysis showed that Ca-P, Al-P, and pH had positive effects on Olsen-P, but Occluded-P showed a negative correlation with Olsen-P in both soils. Our results confirmed that soil P availability was influenced by pH and that the changes in DIP and Olsen-P were linked to changes of inorganic fractions from occluded to Ca- and Al-bound forms. Managing soil acidity is a key issue regarding the availability of P in red soils of China and our results suggest that at least a pH of 5.0 should be targeted. [ABSTRACT FROM AUTHOR]

Ultisols are widely distributed in the subtropical regions of China as well as in the world. High acidity of Ultisols limits plant growth and reduces crop yields. Amelioration of an acid Ultisol was investigated by incorporating the residues of canola ( Brassica campestris L.), wheat ( Triticum aestivum L.), rice ( Oryza sativa), corn ( Zea mays), soybean ( Glycine max), peanut ( Arachis hypogaea), faba bean ( Vicia faba L.) and pea ( Pisum sativum) and Chinese milk vetch ( Astragalus sinicus L.) shoots after incubation of the agricultural by-products for a maximum of 75 days, soil pH was increased by each of the plant materials. The degree of amelioration of the soil acidity by the plant materials was found to depend on the ash alkalinity and N content of the materials; the legumes of higher ash alkalinities and lower N contents, such as peanut straw and faba bean straw, led to the largest increases in soil pH, while the legumes of higher N contents showed less amelioration of the acidity to a certain degree, because of the release of protons during nitrification of NH [ABSTRACT FROM AUTHOR]

In this study, four sites (Sites A, B, C and D, which were associated with varying durations of zinc smelting) namely two slag piles (Profile1, Profile 2) at Sites B and C and two water channels at Sites A and D in Hezhang County, Guizhou Province, Southwestern China, were investigated. Significantly elevated mean Pb, Zn and Cd concentrations, 11,192 (±8,991), 20,563 (±16,727) and 137 (±112) mg kg−1 in slag Profile 1( n=10), and 12,535(±4,408), 26,189 (±6,209) and 47.9 (±30.1) mg kg−1 in slag Profile 2 ( n=12) were detected, respectively. Both profiles had high pH values (7.72 ± 0.37 in Profile 1 and 8.33 ± 0.49 in Profile 2). In addition, high mean Pb, Zn and Cd in soils (14,945 (±9,696), 17,059 (±12,534) and 61.9 (±33.6) mg kg−1) and in stream sediments (11,450 (±7,097), 20,768 (±11,404) and 66.8 (±25.5) mg kg−1) were found at Site D. A Composite Pollution Index (CPI) determined for each study Site confirms the anthropogenic origins of the observed Pb, Zn and Cd contamination. Small portions (<3% of the total) of exchangeable Pb and Zn fractions in samples were extracted by using a sequential extraction technique. In contrast, 37.7% (±7.69%), 21.3% (±7.75%) and 22.7% (±13.2%) of total Pb, Zn and Cd was associated with the carbonate bound fraction. This is indicative of the alkaline pH associated with the carbonate based-ores of this region. Further, total metal concentrations were positively and significantly correlated with soil Fe contents. This may in large part be due to the co-precipitation of Pb, Zn and Cd with Fe minerals in the surface weathering environment. However, a potential threat still exists with regards to the environment release of Pb, Zn and Cd as a result of an alteration of the pH environment, such as in the acidic rhizosphere environment of cultivated crops or acid deposition in this region. It is therefore suggested that immediate remediation measures are implemented to minimize potential risks. [ABSTRACT FROM AUTHOR]