Long-Term Effect Of Compost And Inorganic Fertilizer Application On Soil Organic Matter Structure In A Sandy Loam

To clarify the accumulation mechanism of soil organic matter in a sandy loam, long-term effect of compost and chemical fertilizers application on concentration and structure of soil organic matter were studied. Soil samples were collected from a long-term field experiment which had four treatments, i.e. compost (CM), half N in compost plus half N in fertilizer (HCM), fertilizer (NPK) and control without any fertilizer or compost (CK). All soils were separated into macroaggregate (2000～250μm), microaggregate (250～53μm) and silt+clay fraction (<53μm) using the wet-sieving method. The molecular structure of the organic matter in the bulk soils and the aggregates on the fourier transform infrared photoacoustic spectroscope (FTIR-PAS) and solid-state13C-nuclear magnetic resonance (13C-NMR) spectroscope. And the concentration and distribution of carbohydrate and lignin in aggregates were studied by the acid extraction and alkaline copper oxidation methods, respectively.The results showed that aggregate mass and organic carbon concentration in bulk soils and aggregates were greatly changed by long-term compost and chemical fertilizer application. Compost application significantly increased the macroaggregate mass, whereas fertilization had a much weaker effect. Fertilization, especially compost application, significantly increased organic C contents in bulk soils and aggregates. The concentration of organic carbon was different for different aggregates size classes. The organic carbon concentration in macroaggregates was higher (mean value of10.58gC·kg-1) than those in microaggregates and silt+clay fraction (mean value of6.41gC·kg-1and6.69gC·kg-1, respectively). However, carbohydrate content was larger in macroaggregates and silt+caly fraction (mean value of0.93gC·kg-1and0.89gC·kg-1, respectively) than in microaggregates, which was agreed with the results of the infrared spectrums that the weakest absorption of O-H and C-H were found in microaggregates. The largest content of lignin was in the macroaggregates due to the selective accumulation of vanillyl C and syringyl C during organic matter decomposing. With the decreasing of aggregate size, lignin content reduced. Phenolic C was found significantly associated with lignin content (R2=0.384,p=0.011), showing that phenolic C should be a good indicator for lignin content. Different from the aromatic C, the relative content of alkyl-C increased with the decreasing of aggregate size. Thus, the ratios of alkyl-C/O-alkyl-C increased with the decreasing of aggregate size and they were significantly negatively correlated with the C/N ratio (R20.421, p=0.022). These findings suggest that organic matter were decomposed in the finer the aggregates.Compared with CK, the application of compost increased the ratio of O-alkyl-C or carbonyl-C to total organic C in soil, but the decomposition degree of organic matter decreased, whereas the application of chemical fertilizer increased the ratio of O-alkyl-C and alkyl-C to organic C in the soil. The relative content of aryl-C in bulk soils was reduced in all treatments. The increase in the proportion of O-alkyl-C in the soil was mainly attributed to the increase in relative content of methoxyl-C and N-alkyl-C in macroaggregates under compost treatment. However, the increase in the proportion of O-alkyl-C was mainly attributed to’the increase in methoxyl-C or N-alkyl-C in macroaggregates and O-alkyl-C in microaggregates in NPK treatment. Application of compost led to the accumulation of carbonyl-C in macroaggregates and microaggregates of the soil, whereas that of alkyl-C was mainly observed in macroaggergates under NPK treatment. Application of either organic matter or chemical fertilizer reduced the relative content of aryl-C in the soil, which was mainly attributed to the decrease in the proportion of aryl-C in macroaggregates and microaggregates. All the findings indicated that total soil organic matter structure was affected mainly by soil organic matter structure in macroaggregates and microaggregates in sandy loam. Long-term application of compost helps physically protect a large amount of carbohydrates and organic acids in macroaggregates and microaggregates, then raise the content of organic carbon in soil.