Wheat straw return can lead to biogenic toluene emissions.

As a common practice in agricultural system, straw return has been reported to release a large number of trace gases and attracted much attention. However, the role of straw return in toluene emission remains poorly understood. In this study we measured the emissions of toluene as well as other 50 volatile organic compounds (VOCs) from wheat straw return for 66 days under flooded and non-flooded conditions, respectively. The results showed that substantial toluene was released from the returned wheat straw particularly under flooded condition, and primarily derived from the secondary product. Toluene emissions from the returned wheat straw were 36.8 and 8.45 mg C/kg, sharing 28.0% and 8.6% of total VOCs released, and over 90% of toluene emissions occurred between days 24-56 and 0-17 under flooded and non-flooded conditions, respectively. The emission rates of toluene were relatively high but decreased sharply at the beginning 2 days, and then was steady until 24 days under the two moisture conditions. After the initial decrease these rose again to form one "peak emission window" between days 24-56 under flooded condition, while these were still very low and steady until the end under non-flooded condition. The toluene emission rates significantly positively correlated with microbial biomass C under flooded condition, but negatively associated with bacteria and fungus number, microbial biomass C, and CO2 flux under non-flooded condition, suggesting that microorganism might play an important role in toluene emissions from wheat straw return. A rough estimate indicated that straw return might be important for biogenic toluene.