Decomposition, Organic Acids, Thermal And Structure Characteristic In Crop Residues And Soils Among Straw Returning

Crop residues are relatively common solid resources in our country. In recent years, the resources utilization catches the attention of many studies. Crop residues remaining in agricultural fields are not waste products; instead they provide a variety of ecological and agricultural benefits, including nutrient cycling, erosion control, soil organic carbon(SOC) sequestration, and improvement of soil physical properties, which receives more attention by agricultural scientists and environmentalists. The practice of leaving these crop residues in agricultural fields to maintain SOC levels, which is important for sustaining the productivity of agroecosystems, influences components, properties and status of crop residues and soils in agriculture environment. This paper studied the decomposition characteristic change, LMWOAs variation, thermal characterization and structural characteristic of crop residues and soils among three land uses by adding two crop residues with different water conditions and C/N ratios using nylon mesh bags method, high performance liquid chromatography(HPLC), thermal analysis and fourier transforminfrared spectroscopy, which could explain the transformation mechanism in the process of residues decomposition and provide a scientific basis for the rational use of straw resources. The main conclusions are congregated as the following:(1) The decomposition characteristic showed that at the early stage(winter) of the decomposition(0-20 d), the decomposition rates were more than 20% and the decomposition rates of bean straws were faster than corn straws. On 20-120 d of the decomposition, they were relatively stable. Then the decomposition rates increased to 60-70% at the end(300 d). The effects of decompositio n were better in the low C/N ratio and fresh crop residues treatments(P < 0.05). There were no significant differences(P > 0.05) of crop residues decomposition rates among three land uses(farmland, peach orchard, vineyard). The decomposition of crop residues was rather related with soil temperature and precipitations(P < 0.01).(2) LMWOAs have important effects on several processes of many plants and soils. The results of this study demonstrated that the concentrations of LMWOAs were higher at the early and intermediate stages of the decomposition. The LMWOA concentrations were higher in bean than in corn plants, and much higher in fresh plant samples than in dry samples(P < 0.05). High N content in plant caused a rapid change in organic acids of plant residues, but had no significant influences on the organic acids concentrations(P > 0.05). Moreover, fresh samples exhibited more variation than dry samples, and bean straws exhibited more variation than corn straws. Therefore, the organic acids of crop residues were related to type, water condition and C/N ratio of crop residues, and land uses. The addition of plant residues to soils influenced oxalic acid in soils(r = 0.72*) at the early stages but had no significant influence on the organic acids in the soil(un-detected at the end) over the whole 1-year decomposition period. Organic acids in soils were related to land uses and seasons.(3) Thermogravimetry(TG) and differential scanning calorimetry(DSC) have been widely used for the thermal characterization of several biomasses. The results demonstrated that major mass loss ended between 500℃ and 700℃ and followed by a slow and continuo us mass change with a long tail of devolatilization in TG curves. There was a large exothermal peak at 600°C in the DSC curves. The thermal profile varied with straw types and decomposition stages. And bean residues were more stable than corn residues obtained from TG-DTG curves. The decomposition rates and percentages of fixed carbon and volatile matters were significantly different among six crop treatments(P < 0.05). While, there were no obvious differences in three different land uses(P > 0.05). The decomposition of crop residues in soils led to a reduction in the percentage of labile materials( metabolic litter) at low temperatures of TG-DTG curve(300°C), which were negatively correlated to soil temperature and precipitations(P < 0.05), and an increase in structural litter and fixed carbon percentages, which were significantly positive correlated to soil temperature and precipitations(P < 0.05). The percentages of fixed carbon and volatile matters obtained from TG-DTG curves fluctuated with crop residues specie, water condition and C /N ratio of plant residues, soil temperature and precipitations during the decomposition(P < 0.05). The change in heat values of decomposing plant residue treatments varied with differences in soil temperature and land uses(P < 0.05). Thermal analysis properties of crop residues were greatly influenced by the straw unstable components. There was significantly negative correlation between heat values and mass loss of 200-300℃(TG curve). TG and DSC parameters were influenced by the type of crop residues.(4) The FTIR of different crop residues showed that corn and bean residues generated the similar functional groups. But due to the difference of amide compounds and silicon stable matter contents, absorption peaks and intensities were different in 3400, 1640, 1400-1460, 1310 and 1000-1100 cm-1. The absorption peak intensities changed before and after the decomposition. As decomposing, hydroxyl, amide group, methyl, and methylene reduced, and carboxyl increased with the formation of organic acids. In addition, carbohydrates, amide compounds and sugars broke down gradually. The compounds, which were decomposed easily(such as aliphatic structure, amide compounds and sugars), can be directly decomposed, while a part of aromatic substance was firstly decomposed into intermediate such as carboxylic acid esters(1725-1735 cm-1) and aliphatic structure(1450-1460 cm-1). The shoulder peak in 1560-1732 cm-1 was associated with organic acids of crop residues. Urea had a promoting effect on decomposition of corn residues, and related to the variation of the absorption peak in 1310 cm-1. Straw decomposition mainly formed humic acid. The changes of structure were related with the type of crop residues and N contents.(5) The structural changes and characteristics of corn and bean residues decomposition in the vineyard, peach orchard and farmland soils in different seasons were investigated using fouriertransform infrared spectroscopy and thermal analysis. Thermal analysis results showed that different crop soils had different TG-DTG profiles. There were water, alkanes substance, aliphatic substance, polysaccharide, aromatic nucleus and clay mineral in farmland, peach garden and vineyard soils, while more aliphatic substance and polysaccharide in vineyard soil. There were more aliphatic substances and heat values in soils during spring and summer. Thermal analysis parameters of vineyard soils were greatly influenced by their organic matter, and the same as in spring and summer soils. FTIR results indicated that, the structures of different soils were similar, but different in their absorption peak intensities. The absorption peak intensities of 3200-3600, 1610-1630 and 780-800 cm-1 in farmland soils were relatively small, with high weathering degree; The absorption peak intensities at 1425 cm-1 in peach garden soils were weaker, which indicated there were less calcium carbonate contents in peach garden soils; There were more polysaccharide contents(1000-1100 cm-1) in vineyard soils. The FTIR absorption peak intensities varied with the seasons change. The absorption peak intensities in 1425 cm-1 and 1615-1630 cm-1 slightly declined in summer; The absorption peak intensities of 1030 cm-1 in vineyard and peach garden soils in spring and summer increased, whereas there were no obvious changes in farmland soils. In summer, absorption peak intensities of 780-800 cm-1 in farmland soils reduced. Based on principal component analysis, the FTIR variation in vineyard soil was larger than the other two soils, the same to spring and summer soils.This study showed that during the decomposition, the decomposition rates and components were closely related to water condition, C/N ratio and type of crop residues, and soil temperature and precipitation. LMWOA concentrations were also influenced by land uses. Heat values were strongly influenced by land uses and soil temperature. While structural changes were mainly affected by the type of straw and C/N ratio.