Combined inorganic/organic fertilization enhances N efficiency and increases rice productivity through organic carbon accumulation in a rice paddy from the Tai Lake region, China

The effect of fertilization practices on crop productivity and C storage of cropland soils has been a key focus of research into sustainable agriculture and global change. We present results from a long-term fertilization trial in a rice paddy in the Tai Lake region, China and report variation of rice yields and N efficiency with organic carbon accumulation under different fertilization regimes. The fertilization treatments were no fertilizer application (NF), application of chemical fertilizers only (CF), combined application of chemical fertilizers and pig manure (CFM), and straw return (CFS), respectively since 1987. The rice paddy had been consistently cultivated with double cropping of rice (Oryza sativa) and rape (Brassica campestris) under minimum tillage. The yields of rice grain and rape seeds were recorded each year. Topsoil samples from 0 to 5cm and 5 to 15cm were collected after rape harvest in 2005 and soil organic carbon (SOC) contents and properties of microbial activity were determined. Significant differences in average rice yield, but not rape yield, were observed between the fertilization treatments. A higher and more stable yield of rice was found under CFM and CFS than under CF. Since 1987, there has been a prominent topsoil C accumulation in a range of 0.1-0.4tha⁻¹ yr⁻¹, being greater under CFS and CFM than under CF. Comparing between the fertilized plots, grain productivity and C accumulation was enhanced by 21% and 24%, and 72% and 103% under CFM and CFS compared to CF, respectively. Increased rice productivity was coincident with an increased organic C accumulation rate under fertilization. The coupled effect of increased rice yield and C accumulation may be attributable to the enhanced microbial activity, which was found much higher under combined fertilization. N use efficiency was higher under combined fertilization (by 12.6% and 39.0% for CFM and CFS, respectively) compared to inorganic-only fertilization, meaning that less inorganic N fertilizer would be required for the same level of production, thereby potentially saving C emissions from fertilizer manufacture. This study suggests a win-win effect of combined inorganic/organic fertilization on soil organic carbon accumulation and crop productivity in rice fields through increasing N efficiency possibly by enhanced microbial activity. Well-managed, combined organic/inorganic fertilization could both enhance C storage in soils, and reduce emissions from N fertilizer use, while contributing to high crop productivity in agriculture.