Differences in responses of ammonia volatilization and greenhouse gas emissions to straw return and paddy-upland rotations.

Paddy-upland rotation and/or straw return could improve soil structure and soil nutrient availability. Different previous crops (wheat and/or oilseed rape) and straw return methods (straw mulching and/or returning) might increase soil organic carbon (C) and total nitrogen (N) content, and further affected the ammonia (NH) volatilization, nitrous oxide (NO), and methane (CH) emissions. A comparison study was carried out in a located field experiment started from 2014 in Central China, aiming to exam seasonal and annual NH, NO, and CH emissions under the wheat-rice (WR) and oilseed rape-rice (OR) rotations. Three treatments were chosen, i.e., (i) no chemical N fertilizer application (PK), (ii) chemical nitrogen-phosphorus-potassium combination (NPK), and (iii) chemical NPK with straw returning (NPK+St). We found that after 3 years of cultivation, treatment with straw return increased soil total N content and organic C by 15.57% and 17.11% on average as compared with the NPK treatment, respectively. Straw return did not generate additional NH and NO losses during the rice season after improving soil fertility. However, CH emissions increased by 45.35% on average after straw return in summer. In winter, straw return increased NH, NO, and CH emissions by 70.12-85.23%, 16.93-22.97%, and 7.18-9.17%, respectively. The stimulation of NH volatilization mainly occurred in the topdressing stage. Compared with WR rotation, OR rotation had no significant effect on NH and CH emissions, and the change of NO emission might be related to the increase of soil C and N pools. The retention of residues in the process of straw decomposition may be the main factor leading to the difference of gas emission between the paddy-upland rotation and straw return.