Abstract:
Objectives The conversion of paddy field to upland is accompanied by acidification. We studied the main driving factors and suitable amendments to provide the soil from acidification.
Methods Incubation method was used for the study, with two test paddy soils that are developed from granite and sand shale, respectively. The treatments composed of flooding condition (water content 35%, w/w) without and with N application (W, WN), and aerobic condition to simulate paddy to dry land conversion (15%, w/w) without and with N application (D, DN); Under DN, amendments were added to make treatments of lime (DNL), nitrification inhibitor DCD (DNF), lime + straw (DNS), lime + DCD (DNT), lime + DCD + straw (DNP), and biochar (DNC). 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 Nitrogen application will intensify the soil acidification during converting from paddy fields to uplands. The oxidation of iron and sulfur is the main factors driving the acidification by contributing predominant protons. The together application of lime and nitrification inhibitor is an effective measures to alleviate soil acidification.
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