Differences In Decay Process Between Wheat And Rice Straws And Their Components Under Flooded Soil Condition

The annual output of crop straws in China is over6×108t, and recycling of such huge amount of crop straws is of great importance not only in sustainable development of agriculture but in maintenance of healthy eco-environment as well. At present, one of the most rational ways to recycle crop straws is to directly incorporate them into agricultural field after the harvest of previous crops. Long-term returning of crop straws into the fields have been proved by many studies to be favorable to the elevation and maintenance of soil fertility. However, returning of crop straws in large amount may affect physically, chemically and biochemically the early development of succeeding crops in crop rotation system. The detrimental impacts of straw retuning to crop development may link to the decay process of straws in soil. Furthermore, different crop straws may show different decay processes under similar soil condition, due to the diversity in components and other properties. In this study, wheat and rice straws, which are the dominant straws in wheat-rice and rice-rice rotation systems, were used as materials, and an incubation experiment was carried out to investigate the differences in decay process between the two straws and their components under flooded soil condition. Straw and soil solution were sampled at0,1,3,5,7,10,15,20,25,30,40,50and60d after incubation started. Straw weight and the contents of cellulose, hemi-cellulose, lignin and soluble sugar of the straw samples were determined. pH, dissolved organic carbon (DOC) and NH4+concentration in soil solution were also measured. Then the decay process in terms of weight lose, decay speed, ace μ mulative decay rate and time requirement for completing decay stage were analyzed and compared between wheat and rice straws. The results obtained showed as follows:(1) There existed apparent difference in overall decay process between wheat and rice straws used in this study. Compared with that of rice straw, the initial decay speed of wheat straw was higher but declined more rapidly at early period of incubation, which led to higher residual weight and lower accumulative decay rate at the end of incubation. Furthermore, the time required for wheat straw to complete definite decay process was shorter than that for rice straw. These results indicated that wheat straw had a shorter active decay period while active decay in rice straw can maintain longer. The decay process of two straws was affected by N addition in that N addition elevated initial decay speed of wheat straw and overall decay rate of both rice and wheat straws, but such enhancement was more apparent in rice straw treatment.(2) The decay processes of cellulose and hemicellulose were close to overall decay process of corresponding straws. The decay speed of cellulose and hemicellulose of wheat straw was higher in early period of incubation but declined more quickly than that of rice straw. Therefore, cellulose and hemicellulose in rice straws decayed more at the end of incobation. In this study, there was no apparent difference in decay process of liglin and soluble sugar between wheat straw and rice straw. N addition can elevate the initial decay speed of cellulose and hemicellulose and this effect was more apparent in wheat straw treatment. However, lignin decay was delayed by N supply especially in rice straw treatment.(3) The DOC level in soil solution increased in straw treatments. Compared with that in rice straw treatment, peak concentration of DOC in wheat straw treatment was lower but appeared earlier. A peak NH4+concentration in soil solution appeared at early period of incobation in all treatments, then NH4+concentration declined more rapidly in wheat straw treatment than that in rice straw treatment. Addition of straws also decreaed pH of soil solution, and the effect was more apparent in wheat straw treatment.