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Although the effects of adding nitrogen (N) to grasslands have been studied worldwide, little is known about how those effects are, in turn, influenced by climate, soil, and vegetation. Data from such experiments conducted in two main types of grassland in China, alpine and semi-arid, which differed widely in terms of climate, soil, and vegetation, were compiled and analysed to examine the response of above-ground biomass to N. Adding N increased the above-ground biomass by 51% on average. Although the response of biomass showed no obvious trend with the dose of N in the alpine grasslands, the response in the semi-arid grassland peaked at 15–20 g m −2 of N applied annually. Nitrogen addition efficiency in the alpine grasslands was higher than that in the semi-arid grasslands. The difference was mainly due to the differences in total soil N and its interactions with the above-ground biomass, which were particularly influenced by the soil's capacity to provide available N. These results suggest that soil N and features of local vegetation should be taken into account in arriving at the appropriate dose of N for a given site. [ABSTRACT FROM AUTHOR]

Biochar has emerged as an efficient tool to affect bioavailability of heavy metals in contaminated soils. Although partially understood, a carefully designed incubation experiment was performed to examine the effect of biochar on mobility and redistribution of Cd, Cu, Pb and Zn in a sandy loam soil collected from the surroundings of a copper smelter. Bamboo and rice straw biochars with different mesh sizes (<0.25 mm and <1 mm), were applied at three rates (0, 1, and 5% w/w). Heavy metal concentrations in pore water were determined after extraction with 0.01 M CaCl 2 . Phytoavailable metals were extracted using DTPA/TEA (pH 7.3). The European Union Bureau of Reference (EUBCR) sequential extraction procedure was adopted to determine metal partitioning and redistribution of heavy metals. Results showed that CaCl 2 -and DTPA-extractable Cd, Cu, Pb and Zn concentrations were significantly ( p < 0.05) lower in the bamboo and rice straw biochar treated soils, especially at 5% application rate, than those in the unamended soil. Soil pH values were significantly correlated with CaCl 2 -extractable metal concentrations ( p < 0.01). The EUBCR sequential extraction procedure revealed that the acid extractable fractions of Cd, Cu, Pb and Zn decreased significantly ( p < 0.05) with biochar addition. Rice straw biochar was more effective than bamboo biochar in decreasing the acid extractable metal fractions, and the effect was more pronounced with increasing biochar application rate. The effect of biochar particle size on extractable metal concentrations was not consistent. The 5% rice straw biochar treatment reduced the DTPA-extractable metal concentrations in the order of Cd < Cu < Pb < Zn, and reduced the acid extractable pool of Cd, Cu, Pb and Zn by 11, 17, 34 and 6%, respectively, compared to the control. In the same 5% rice straw biochar treatments, the organic bound fraction increased by 37, 58, 68 and 18% for Cd, Cu, Pb and Zn, respectively, compared to the control, indicating that the immobilized metals were mainly bound in the soil organic matter fraction. The results demonstrated that the rice straw biochar can effectively immobilize heavy metals, thereby reducing their mobility and bioavailability in contaminated soils. [ABSTRACT FROM AUTHOR]