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Context: Carbon (C) and nitrogen (N) transformation processes in soils play an important role in the fluctuation of soil pH. Incorporation of chitosan and crop straws, byproducts from fishery and agriculture, into acidic soils can increase soil pH through decarboxylation, decomposition, N immobilisation and ammonification. Aims: The study was designed to evaluate the transformation of organic N and C from chitosan and/or crop straws and their effects on soil physicochemical properties. Methods: Chitosan, rice straw and maize straw were incubated with two acidic Ultisols from Langxi (Soil 1) and Yingtan (Soil 2) differing in initial pH. Six treatments were prepared in triplicate: control (no amendment), 4% chitosan, 4% rice straw, 4% maize straw, 2% chitosan + 2% rice straw, and 2% chitosan + 2% maize straw. Soil pH, N transformation and CO2 evolution were estimated at different time intervals. Key results: After 40 days of incubation, control soil pH decreased by 0.35 and 0.32 units for Soils 1 and 2, respectively. Rice straw, maize straw, chitosan, rice straw–chitosan and maize straw–chitosan significantly increased soil pH by 0.51, 0.17, 2.27, 1.78 and 2.02 units for Soil 1, and 0.71, 0.16, 0.67, 0.49 and 0.68 units for Soil 2 (P < 0.01). The respective treatments decreased exchangeable acidity by 62%, 51%, 95%, 95% and 95% for Soil 1 and 75%, 69%, 88%, 88% and 87% for Soil 2. In treatments containing chitosan, the pH increase resulted from ammonification of organic N and mineralisation of organic C, with the effect higher in Soil 1 than Soil 2. Conclusions: Amending acidic soils with chitosan and crop residues can effectively increase soil pH and slow soil acidification rate. Implications: This study provides useful information for amelioration of acidic soils. Chitosan and crop straws, byproducts from fishery and agriculture, can be used to increase soil pH in acidic soils through decarboxylation, decomposition, N immobilisation and ammonification. Combined application of chitosan and crop straws showed greater ameliorating effect on soil acidity than single applications of straw. This practice provides an effective strategy for improving soil acidity, inhibiting soil acidification, regulating N transformation, and reducing N loss in farmlands. [ABSTRACT FROM AUTHOR]

To evaluate the effects of biochar on pH buffering capacity (pHBC) of paddy soils, rice straw biochar (RC) was modified by hydrogen peroxide (H2O2) and nitric acid/sulphuric acid (HNO3/H2SO4), respectively, and then added to the paddy soils developed from sandy Ultisol, clay Ultisol and granite Ultisol at the same proportion. After submerging and draining incubation, soil pHBC, soil pH and the available cadmium (Cd) extracted with calcium chloride were determined. The results indicated that the two modified biochars effectively promoted pHBC of three paddy soils and HNO3/H2SO4 modified biochar (HNO3/H2SO4-RC) led to greater increase in soil pHBC. Compared with control, HNO3/H2SO4-RC apparently increased pHBC of the paddy soils from sandy Ultisol, clay Ultisol and granite Ultisol by 32.7%, 37.4% and 25.3%, respectively. Correspondingly, soil pH increased by 1.59, 1.15 and 0.88 units, and the available Cd decreased by 74.1%, 67.5% and 75.0% in the paddy soils due to the incorporation of HNO3/H2SO4-RC. HNO3/H2SO4 modification significantly increased the carboxyl groups on the surface of RC. The dissociation of these functional groups produced a large number of organic anions, which could consume exogenous protons, and thus retarding the decline of soil pH and decreased soil available Cd. [ABSTRACT FROM AUTHOR]

The dual role of biochar for inhibiting soil acidification induced by nitrification was determined through two-step incubation experiments in this study. Ca(OH) 2 or biochar was added respectively to adjust soil pH to the same values (pH 5.15 and 5.85), and then the amended soils were incubated in the presence of urea for 70 days. The results showed that compared with Ca(OH) 2 treatment, both rice straw biochar and peanut straw biochar inhibited the decrease in soil pH and the increase in exchangeable acidity during the incubation. The application of biochars suppressed soil nitrification during the incubation, and thus reduced 7.5 mmol kg−1 and 1.4 mmol kg−1 protons released from nitrification compared to Ca(OH) 2 treatments. Compared with Ca(OH) 2 treatment, the ammonia-oxidizing bacteria population size was decreased by 8% and 12% in rice straw biochar and peanut straw biochar treatments respectively, which was the main responsibility for the inhibited nitrification after biochar application. In addition, the application of rice straw biochar and peanut straw biochar increased soil pH buffering capacity (pHBC) respectively by 22% and 32%. The increased pHBC played the main role (75%) in inhibiting the acidification of the soil amended with peanut straw biochar, while the rice straw biochar inhibited soil acidification mainly through suppressing nitrification during the incubation. Overall, compared with lime application, biochars can inhibit soil acidification caused by urea application through suppressing the nitrification process and improving the resistance of soils to acidification. The crop residue biochars presented a longer-lasting effect on ameliorating acidic soils than mineral lime. Image 1 • Biochar played a dual role in inhibiting soil acidification from nitrification. • Biochar decreased proton release through suppressing soil nitrification. • Biochar increased soil pH buffering capacity and the resistance to acidification. • Biochar would be a better option to ameliorate acidic soils than liming. [ABSTRACT FROM AUTHOR]