In China，the hilly and mountainous areas account for 34.62% of the total cultivated land and 34.20% of the total crop sown area，covering 54.2% of the population，while contributing only 30% to the national GDP. With the overall agricultural mechanization level below 50%，it is urgently needed to accelerate the development of agricultural mechanization in hilly and mountainous areas to build an agricultural powerhouse. Based on the agricultural tillage system zoning in China，this paper conducts a zoning of agricultural mechanization in hilly and mountainous areas，taking into account factors such as the distribution of hilly and mountainous areas. Based on the resource endowment conditions，economic and social conditions and industrial economic benefits，this paper summarizes the development status of agricultural mechanization，analyzes the special scenarios of agricultural machinery application from the aspects of cultivated land conditions，soil and agronomy，and analyzes the constraints and bottlenecks of agricultural mechanization development in combination with the factors of agricultural machinery research and development，manufacturing，promotion and application in hilly and mountainous areas of China. In the context of the application scenario in hilly and mountainous areas，ten key agricultural machinery general technology needs are proposed，such as compact and lightweight specialized engines，lightweight and anti-adhesion and anti-friction materials，and simplified machine structures. Ten applicable agricultural machinery equipment needs，such as efficient and low-cost tillage equipment in sticky soil，seeding equipment for grain，oil，sugar crops，and vegetables in mountainous areas for heavy and sticky soil in gentle slopes，and transplanting equipment for rape and vegetables in mountainous areas for heavy and sticky soil in gentle slopes，are also discussed. Drawing on the experience of agricultural mechanization in hilly and mountainous areas of Japan，Republic of Korea，etc.，this paper puts forward a systematic approach to promoting agricultural mechanization in hilly and mountainous areas，focusing on the integration of good machines，good seeds，good methods，good farmland，and good systems，and the four-in-one approach of research，production，promotion，and use. This approach aims to promote the high-quality development of agricultural mechanization in hilly and mountainous areas.

Exploring the effects of straw returning years on soil aggregates and the distribution of organic carbon and total nitrogen in rice-wheat cropping fields is of great significance for improving its soil quality. Field experiment was conducted in 2010 with 9 treatments: no straw returning (NR), 1 year of straw returning (SR1), 2 years of straw returning (SR2), 3 years of straw returning (SR3), 4 years of straw returning (SR4), 5 years of straw returning (SR5), 6 years of straw returning (SR6), 7 years of straw returning (SR7), and 8 years of straw returning (SR8). The results showed that with the increase of straw returning years, the proportion of >2 mm aggregates increased, the proportion of 0.25-2 mm and < 0.053 mm aggregates decreased, while the proportion of 0.053-0.25 mm aggregates had little change. The contributing rate of total soil organic carbon and total nitrogen was the highest in >0.25 mm macro-aggregates. Contents of soil organic carbon and total nitrogen were significantly positively correlated with the proportion of >2 mm and >0.25 mm aggregates, while negatively correlated with the proportion of 0.25-2 mm, < 0.053 mm and < 0.25 mm aggregates. In conclusion, continuous straw returning has positive effects on improving the soil aggregates fraction and promoting the transfer of soil organic carbon and total nitrogen to large aggregates.

Tillage practices influence the fate of soil organic carbon (SOC) by different intensities of soil disturbance. Few studies have examined soil aggregate structure and carbon pool from rice-wheat fields in eastern China. This study aims to evaluate the influence of tillage methods on aggregate fractions, SOC distribution, carbon storage, and carbon pool management index (CMI). A 5-year field experiment was conducted including 4 treatments: minimum tillage (MT), rotary tillage (RT), conventional tillage (CT), and conventional tillage with residue removed (CT0) in a rice-wheat system. Results showed that MT and straw retention are beneficial to the formation of >2000 μm aggregates. The organic carbon contents at 0~5, 5~10, and 10~20 cm depths increased when using MT, RT, and CT. Since the organic carbon was mainly distributed in macro aggregates, it increased by straw retention regardless of the tillage treatment used. There is a significant (P < 0.05) correlation between aggregates fractions and the content of organic carbon. Organic carbon stock at 0~20 cm depths increases by 15.6%, 14.5%, 13.4%, and -0.95% under MT, RT, CT and CT0, respectively. Compared with CT0, MT reduces the CMI while RT and CT enhance it. The CMI has a significant negative correlation with soil bulk density. Our results indicate that reducing tillage intensity, implementing straw retention, and especially their combined application can be effectively used to improve aggregates fractions and carbon sequestration in rice-wheat fields in eastern China.