Lower C sequestration and N use efficiency by straw incorporation than manure amendment on paddy soils

Understanding the effects of external organic and inorganic components on soil organic C (SOC) and fertilizer N use efficiency (NUE, in kg of grain per kg of N applied) is essential for better stewardship of domesticated soils. We collected 106 paired-treatment data points from 28 long-term field fertilization trials of subtropical paddy soils with double-rice cropping systems in China. The effects of chemical fertilizer (NPK), NPK plus straw (NPK + s), and NPK plus manure (NPK + m) on rice yield, SOC density, and NUE were assessed. The greatest SOC sequestration rate was found with the use of NPK + m (0.67 Mg ha−1 year−1), whereas this value was lower with NPK + s (0.48 Mg ha−1 year−1) and NPK (0.30 Mg ha−1 year−1). The soil C sequestration rate decreased with the experimental time, leading to a sequestration period of 43, 65, and 55 years to reach a new equilibrium value of SOC for NPK, NPK + s and NPK + m, respectively. Under the same N input condition, the treatment with N fertilizer proportionally replaced by manure (NPK + m) could enhance both rice yield and NUE by 28% and 27%, respectively, whereas the in situ rice straw incorporation (NPK + s) showed no distinct effect. Additional manure amendment on the basis of existing N fertilizer application rate did not have an effect on both rice yield and NUE. In contrast, additional rice straw incorporation decreased NUE by 24%, even though no distinct change of rice yield was found. Our results indicate that application of chemical fertilizer plus manure, rather than rice straw, to paddy fields is a promising practice to enhance SOC accumulation and improve rice yield, as well as the crop N use efficiency in subtropical rice production of China.