Effects of long-term application of inorganic fertilizer and organic amendments on the amounts of fractionated soil organic carbon and their determining factors in paddy fields

To investigate how the long-term application of inorganic fertilizer and organic amendments affects the accumulation of organic matter in physico-chemically fractionated components in paddy soils and how the accumulation is determined by soil properties and anthropogenic carbon input, we studied the effect of variable management of inorganic fertilizer and organic amendments for > 50 years on the amounts of accumulated soil organic carbon (SOC) in fractionated components in three paddy fields. SOC was fractionated into four components based on their physical and chemical properties: (1) light fraction (LF) derived from plant residues, (2) heavy fraction (HF) containing stable aggregates, (3) oxidizable fraction (OxF) and (4) non-oxidizable fraction (NOxF) forming organo-mineral complexes with fine-textured minerals. On average, the amount and percentage of accumulated C in the four fractions were as follows: OxF (6.63 gC kg⁻¹_soil, 46%) > NOxF (5.32, 35) > LF (1.68, 12) > HF (1.12, 7.2), suggesting about 80% of SOC was in fine, stable fractions. The amount of accumulated C in all fractions increased slightly with the application of inorganic fertilizer, while it increased considerably with the use of organic amendments. In addition, the application of fertilizers and amendments raised the proportion of C content in LF and HF, which are labile and related to soil fertility. Stepwise multiple regression analysis using two principal component analysis scores of soil properties and anthropogenic C input further revealed that the labile fractions (LF and HF) were more strongly determined by the management of fertilizers and amendments, while the stable fractions (OxF and NOxF) were more strongly determined by soil-specific properties, mainly the amorphous nature of the soil. These results indicate that the use of certain amounts of organic amendments would be recommended to carry out rational management to improve soil fertility and C sequestration in paddy fields, with the application of amorphous minerals as an additional option.

为探究长期施用无机肥料与有机物料如何影响稻田土壤物理化学分级组分中的有机质积累，以及该积累过程如何由土壤性质与人为碳输入所决定，我们针对3个稻田开展了为期超50年的无机肥料与有机物料差异化管理试验，分析其分级组分中土壤有机碳（SOC, soil organic carbon）的积累量。本研究依据物理化学性质将土壤有机碳划分为4个组分：(1) 源自植物残体的轻组组分（LF, light fraction）；(2) 包含稳定团聚体的重组组分（HF, heavy fraction）；(3) 易氧化组分（OxF, oxidizable fraction）；(4) 与细粒矿物形成有机-矿物复合体的难氧化组分（NOxF, non-oxidizable fraction）。整体而言，4个组分的碳积累量及占比依次为：易氧化组分（6.63 gC kg⁻¹土壤，46%）> 难氧化组分（5.32 gC kg⁻¹土壤，35%）> 轻组组分（1.68 gC kg⁻¹土壤，12%）> 重组组分（1.12 gC kg⁻¹土壤，7.2%），表明约80%的土壤有机碳赋存于精细稳定的组分中。单施无机肥料可使各组分的碳积累量小幅提升，而施用有机物料则能显著促进碳积累。此外，肥料与有机物料的施用提升了轻组与重组组分的碳占比，这两类组分属于活性碳库，与土壤肥力密切相关。进一步采用基于土壤性质与人为碳输入的两个主成分分析得分开展逐步多元回归分析，结果显示：活性碳组分（轻组与重组）更多受肥料与有机物料管理措施的调控，而稳定碳组分（易氧化与难氧化组分）则主要由土壤固有性质决定，尤以土壤无定形矿物特征的影响最为显著。本研究结果表明，合理施用一定量的有机物料可作为稻田土壤肥力提升与碳固存的优化管理方案，配施无定形矿物可作为额外辅助手段。