Straw and manure are the main sources of organic materials in China, and rotary tillage is the main way of returning farmland. In this study, the characteristics and driving factors of their decomposition process in typical farmland soils in China were investigated to provide a scientific basis for better utilization of straw resources and rational application of manure.

The landfill experiment of field organic material mixed with soil was carried out on a typical black soil, fluvo-aquic soil, and red soil in China. Four treatments were set up: wheat straw plus soil (WS+Soil), maize straw plus soil (MS+Soil), pig manure plus soil (PM+Soil), and cow manure plus soil (CM+Soil). All the organic materials (8 g carbon) were mixed with 200 g of soil in a nylon mesh bag buried in soil, six samples were taken within one year after landfilling, once from 2 to 2.5 years after landfilling, and once in 3 years, then a total of eight samples were taken. The decomposition differences of organic materials in different mixtures were analyzed. The double exponential equation was used to clarify the proportion and decomposition rate of easy-to-decompose carbon pool and refractory organic carbon pool, and to clarify the fast decomposition rate, slow decomposition rate and accumulated temperature turning point of organic materials in each soil. The random forest model was used to quantify the relative contribution of material composition and environmental factors to the decomposition rate.

The humification coefficient of straw pluil was ranged from 22% to 43%, and that of manure plus soil was ranged from 45% to 58%. The decomposition rate of manure plus soil was slower than straw plus soil, and there was little difference between regions. Besides, the decomposition of organic materials was accelerated from north to south, and the decomposition of straw was faster than that of organic fertilizers. The proportion of decomposable organic carbon poor of straw (70%-87%) was higher than that of organic fertilizer (57%-83%), and the decomposition rate of straw decomposable carbon pool k₁ (1.0%·a^-1-4.9%·a^-1) was higher than that of organic fertilizer k₁ (0.7%·a^-1-1.1%·a^-1). There was little difference in the turning point of accumulated temperature of straw in the three types of soil. Before 3 700℃, it was the stage of rapid decomposition, and then it was the stage of slow decomposition. On the other hand, the difference in accumulated temperature at the turning point of rapid decomposition and slow decomposition of organic fertilizer gradually increased from north to south, and the accumulated temperature at the turning point of organic fertilizer was 2-5 times higher that of straw. Soil accumulated temperature and soil organic carbon content were the main driving factors of mixed soil straw and organic fertilizer, contributing about 17% and 13% of the decomposition rate, respectively. In addition, the main driving factor for mixed soil straw was the ratio of soil carbon to nitrogen, but the main driving factor for mixed soil organic fertilizer was the time.

Straw returning to the field decomposed faster than organic fertilizer, and the high proportion of easily decomposable organic carbon pool with fast decomposition rate of straw was one of the main reasons for the difference. For straw, the ratio of carbon to nitrogen was the main factor affecting its decomposition, and for organic fertilizer, it was time. Therefore, the ratio of carbon to nitrogen of soil and material should be considered to regulate the decomposition of straw, and the long-term effect of organic fertilizer should be fully considered.