Abstract:
Objectives The conversion of paddy field to upland is accompanied by acidification. We studied the main driving factors and suitable amendments to protect the soil from acidification.
Methods Incubation method was used for the study, with two test paddy soils developed from granite and sand shale, respectively. A total of 10 treatments were set up: 1) flooding (W); 2) flooding + nitrogen fertilizer (WN); 3) paddy-to-upland Conversion (D); 4) paddy-to-upland conversion + nitrogen fertilizer (DN); 5) paddy-to-upland conversion + nitrogen fertilizer + lime (DNL); 6) paddy-to-upland conversion + nitrogen fertilizer + DCD (dicyandiamide) (DNF); 7) paddy-to-upland conversion + nitrogen fertilizer + lime + straw (DNS); 8) paddy-to-upland conversion + nitrogen fertilizer + lime + DCD (DNT); 9) paddy-to-upland conversion + nitrogen fertilizer + lime + DCD + straw (DNP); 10) paddy-to-upland conversion + nitrogen fertilizer + biochar (DNC). The moisture content for the flooding treatment was set at 35%, while that for the paddy-to-upland conversion treatment was set at 15%. The pH, NO3−, available S, Mn2+, Fe2+, exchangeable acid and Al, and exchangeable base cations were measured during the incubation period.
Results Compared with W treatment, D treatment decreased soil pH by 2.49 and 1.99 units, increased exchangeable Al by 0.26 and 0.10 cmol(+)/kg, and NO3− by 53.57 and 117.70 mg/kg, respectively. Compared with WN treatment, DN treatment decreased soil pH by 2.57 and 2.49 units, increased exchangeable Al by 0.42 and 0.13 cmol(+)/kg and NO3− by 113.24 and 213.47 mg/kg, respectively. Compared with the DN treatment, DNT treatment was the most effective in enhancing pH by 2.88 and 3.25 units, followed by the DNP, DNF, DNL, DNS, and DNC treatments with the increased pH by 1.67 and 2.93, 0.86 and 1.56, 0.48 and 0.87, 0.15 and 0.69, 0.16 and 0.08 units for the soils derived from granite and sand shale, respectively; the DNL, DNF, DNS, DNT, DNP, and DNC treatments reduced soil exchangeable Al by 0.77 and 0.40, 0.29 and 0.30, 0.78 and 0.40, 0.76 and 0.40, 0.71 and 0.20, 0.60 and 0.22 cmol(+)/kg, respectively; DNF, DNT and DNP treatments decreased soil NO3− by 61.17 and 143.10 mg/kg, 101.04 and 129.00 mg/kg, 80.75 and 183.52 mg/kg, respectively, while DNL, DNS and DNC treatments increased soil NO3− significantly. Compared with DN, DNL, DNS, DNT and DNP treatments increased the exchangeable Ca of two soils by 2.99 and 3.29, 1.92 and 2.15, 2.98 and 3.25, 1.78 and 1.86 cmol(+)/kg, respectively. During the conversion, the proton production of the two soils in DN treatment was increased by 7.34 and 7.16 cmol(+)/kg, relative to W treatment, and Fe2+ oxidation contributed 50.44% of the proton production in granite derived soil, and sulfur oxidation contributed 62.16% of the proton production in sand shale derived soil. Compared with DN, DNF, DNT, and DNP treatments reduced the proton production in granite and sand shale derived soils by 3.64 and 3.26, 2.66, and 5.44, 3.41 and 3.08 cmol(+)/kg, respectively, while DNL, DNS, and DNC treatments did not exhibit significant reduction effect. Correlation analysis showed that soil pH was positively correlated with NH4+ and Fe2+ content, and negatively correlated with NO3− and S content. The random forest model showed that the protons produced by iron and sulfur oxidation contribute the most to the proton production during change from paddy to upland.
Conclusions After the conversion of paddy soils developed from granite and sandy shale to upland conditions, soil acidification is exacerbated. The production of protons through iron and sulfur oxidation is the primary cause of soil acidification, followed by proton generation from nitrogen nitrification. The application of lime combined with a nitrification inhibitor is one of the effective measures to alleviate soil acidification.
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