Differences In Decomposing Process Between Different Parts Of Rice And Wheat Straws In Flooded Soil

With the development of agricultural production and stable elevation of food crop yield in recent years, annual output has exceeded 700 million t in China. Due to the strengthening of community environmental awareness, open-field burning of crop straws is effecitvely controlled and direct returning of crop straw to the field after harvest has been widely adopted. Massive input of crop straws may produce profound effect on physio-chemical and biological processes that affect the growth, especially the early development, of succeeding crops. Deeper understanding of straw decomposing process after returing is therefor of great importance in crop straw returning pratice and management. The straws derived from cereal crops including wheat and rice are usually consist of stalk and sheath which differ in structure and chemical properties. However, investigation on the difference in decomposing processes between stalk and sheath of wheat and rice straws has been in scarce yet. In this study, rice and wheat straws were used as material and they were respectively divided into two parts, stalk and sheath, which were incubated under fooled soil condition. Weight loss, change of the contents of soluble sugar, hemicellulose, cellulose and lignin in remaining straw materials and dynamics of DOC, NH4+ andpH in soil solution were monitored at different periods during incabation. Then the difference of decomposing processes between the two parts of rice and wheat straws and the relation with nitrogen addition were summerized.The main results were as follows:(1) The proportions of stalk and stheath in wheat and rice straws were different. The proportions of the weight of stalk and sheath to total weight in rice straw were 37% and 63%, while those in wheat straw were 56% and 44%, respectively. In rice straw, the stalk had lower C/N than sheath, and the opposite trend was found in wheat straw. Compared with sheath, stalk contented higher cellulose and lower hemicellulose and lignin in both rice and wheat straws. No obivious difference of soluble sugar contents between stalk and sheath were observed.(2) Under flooded soil condition, all four materials used in the study decomposed fast in early stage of incubation and slowly in late stage. However, the decomposition rate of wheat sheath was the highest among all the straw materials while that of wheat stalk was the lowest.The decomposing rates of four straw materials declined with incubation time but the decomposing rate of wheat sheath dropped in the biggest extent. Through 60 days incubation, the decomposition rates (cumulative weight loss rates) were in order of wheat sheath> rice sheath> rice stem> wheat stem. Upon completion of the same decaying progress, wheat sheaths spent the shortest time. These data indicated that due to the sheath fast decaying, wheat straw had a higer decomposing rate in the early period after flooding, but its stalk was decomposed relatively slowly, thus the overall decomposing process was slower than that of rice straw. In this study, nitrogen application enhanced overall decomposing process of stalk and sheath of wheat straw but inhibited that of rice straw. However, the initial decomposing rates of all straw materials could be promoted by nitrogen addition.(3) After 60d incubation, the cumulative decomposing rates of soluble sugar for rice stalk and sheath were 80.3% and 76.1%, respectively, while those for wheat stalk and sheath were 64.2% and 72.4%, respectively. The decomposing rates of soluble sugar in rice stalk and sheath were higher than those in wheat stalk and sheath. In terms of decoposing speed, the decomposition process of soluble sugar in wheat sheath was the fastest, while that in wheat stalk was the slowest among tested four materials, which was in accordiance with overal decomposition characteristics of straw materials. The time needed for completing the same definite decaying process of soluble sugars s in wheat sheath was also the shortest. The dynamic decomposing process of hemicellulose and lignin in straw materials were similar. After 60d decomposition, cumulative of decaying rate of hemicellulose and lignin in wheat stalk was relatively low, only 38.5% and 28.4%, respectively.The initial decomposing speed of hemicellulose and lignin in wheat sheath was higher, but declined more quickly. Hemicellulose and lignin in wheat straw decomposed mainly in the early stage, while those in rice straw had relatively high speed in late stage. However, the decomposition characteristics of cellulose in stem and sheath showed different trend with hemicellulose and lignin. The initial decomposing speed of cellulose in rice stalk was fastest, with sharpest decline, while that wheat stalk was slowest, with relatively mild deciline. These indicated that the decomposition of cellulose in rice straw mainly happend in the early stage, while the decomposition of cellulose in wheat straw sustained longer. Nitrogen application inhibited the decomposition of hemicellulose of rice stalk and sheath, the cellulose of rice stalk, wheat stalk and wheat sheath, and the lignin of rice stalk, rice sheath and wheat stalk, but promoted the decomposing of hemicellulose of wheat stalk and sheath, cellulose of rice sheath and lignin of wheat sheath.(4) The decomposition of straw materials elevated soil DOC levels, showing a downward trend after the first rise. The highest peak of DOC was observed in the treatments with the addition of rice stalk, while the lowest peak of DOC appeared in wheat stalk treatments.The pH in soil fluctuated between 6.4-8.4, and at the end of the incubation,pH level of the soil solution was in the order of rice stalk, rice sheath> wheat stalk,wheat sheath. In the treatments added with rice stalk and wheat sheath, the concentration of NH4+in soil solution was relatively high in early 15d after flooding, and stable in later stage, In wheat straw treatments, the concentration of NH4+ in soil solution increased slowly, but its concentration was much lower than that in the other three treatments. Nitrogen addition reduced the peak of soil DOC in rice stalk and wheat sheath treatment in the early period, and elevated the soil concentrations of NH4+.