Application of crop straw with different C/N ratio affects CH4 emission and Cd accumulation in rice (Oryza sativa L.) in Cd polluted paddy soils

Abstract

Return straw to field is a common practice for straw utilization. However, effects of crop straw with different C/N ratio incorporation on the CH₄ emission and Cd phytoavailaility in Cd-contaminated paddy soils have not been simultanously evaluated. Here, we investigated the impacts of rice straw (RS) and pea straw (PS) incorporation on CH₄ emission and rice Cd available in heavily Cd polluted soil (HP) and lightly polluted soil (LP) through a pot experiment. Results showed that RS and PS significantly increased CH₄ emission in the two soils (p ​< ​0.05). CH₄ emission in PS treatment were greater than that in RS treatment. PS with lower C/N ratio favored to increase soil DOC and SOM (p ​< ​0.05), and promote dominant methanogens of Methanobacterium uliginosum and uncultured Methanocellales archeaon growth (p ​< ​0.05), which mainly contributed to higher CH₄ emisison. The significant influences of straw application on soil chemical parmeters subsequently affected soil different Cd fractions (p ​< ​0.05). Specifically, straw significantly decreased water soluble ​+ ​exchangeable Cd and manganese oxides Cd, but significantly increased other Cd fractions in HP soil (p ​< ​0.05); whereas there were nearly opposite trendancies in LP soil. Thus, roots Cd was mainly determined by soil soluble ​+ ​changeable Cd in HP soil, while it was more likely affected by other Cd fractions except for the soluble ​+ ​exchangeable Cd in LP soil, thereby reduced Cd transport from roots to stems in both soils. Greater effects of PS were displayed in inhibiting rice growth and reducing organs Cd than RS in HP soil, while higher efficiency of RS treatment on improving rice growth than that of PS was found in LP soil (p ​< ​0.05). The results can provide a basis for scientific straw returning in Cd contaminated paddy field, achieving safe rice production and reducing carbon emission.