Effects of organic material application on the structures of humic acids in low yield paddy soil

【Objectives】 The effects of organic material application on the composition of humic substances and the structure of humic acids (HA)in low yield white soil is important to reveal the mechanisms of that how different components of humus improve soil fertility. 【Methods】The “Field experimental study site for enhancing white soil fertility”experiment has been conducted for three years in Nanduzhen, Liyang, Jiangsu Province. Three treatments: straw(ST), swine manure(OM) and no-organic materials (CK,control) were applied.The adjacent soil from the soil monitoring site (2007 - 2013) in Jiangsu Province with annual fertilizer (LAF) and no fertilizer application (NF) was used for comparisons. Soil physicochemical properties and compositions of soil humus were measured. Humic acids (HA) were extracted and the characteristics of HA were measured using infrared spectroscopy and elemental analysis. 【Results】 Soil organic C (SOC), total N in the straw (ST) and swine manure (OM) treatments were significantly higher than those in the no-organic materials treatment (CK). The E4/E6 ratio in HA is in the order of strawswine manurecontrol. According the infrared spectroscopy, the HA from different treatments in both study sites have the characteristic absorptions at 1650 cm-1 (amide I band) and 1550 cm-1 (1500-1800 cm-1 amide II band stretching) and the ratio of 2920/1720 and 2920/1650 of HA from the swine mature and straw treatments were significantly higher than those of the control. C, H and N contents of the HA in the swine mature and the straw treatments were also significantly higher than those in the control treatment and in the monitoring site with annual fertilizer and no fertilizer, however, oxygen contents of HA in the swine manure and the straw treatments were lower than those in the control. H/C and O/C ratios of the HA in the swine manure and the straw treatments were also significantly lower than those in the control treatment and the ratios in the experimental study site were lower than those in the monitoring site with annual fertilizer and no fertilizer treatments. 【Conclusions】 Organic material applications in soils increase soil organic carbon content, improve soil physical and chemical properties, and increase the yield and quality of crops.The formation of new humic substances with the organic material application in the soils can lower humification degree. The infrared spectroscopy of HA in different treatments showed that the HA in paddy soils had obvious characteristic of amides compounds. Organic material application in the paddy soils couldenhance the aliphatic group, reduce the carboxyl and aromatic groups, and decrease H/C and O/C ratios in the organic material treatment and straw treatment, while the nitrogen content in the HA increased significantly. The results indicate that dehydrating processes have occurred in the humus and new humus has formed in the paddy soils after the addition of organic materials.