Greenhouse Gas Emissions from Paddy Soils Respond to Different Crop Root Residues and N Fertilizer Types

Greenhouse gas emissions from paddy soils respond differently to different combinations of crop root residues and N forms. An incubation experiment was carried out to explore the effect of four crop residues (milk vetch, ryegrass, winter wheat, and rape) and four nitrogen treatments (without fertilizer, urea, (NH4)2SO4, and KNO3) on CH4, CO2, and N2O emissions in a paddy soil. Except in KNO3 application treatments, CH4 emissions of milk vetch residue treatments were significantly higher than those of the rest residue treatments. In the presence of milk vetch and ryegrass residues, urea application significantly increased CH4 emissions in comparison to treatments without fertilizer. Urea significantly promoted CO2 emissions, whereas (NH4)2SO4 and KNO3 significantly inhibited CO2 emissions at all root residue treatments. Urea did not increase N2O emissions, but (NH4)2SO4 and KNO3 promoted N2O emissions at all residue treatments. In addition, KNO3 had more effects on the increase of N2O emissions than (NH4)2SO4 in milk vetch-amended soils. Urea addition had no effect on global warming potentials, and (NH4)2SO4 and KNO3 addition significantly increased global warming potentials at all residue treatments except KNO3 + winter wheat residue combination. Our results indicated that urea application had no additive effect on global warming when root residues were left in paddy soils, whereas (NH4)2SO4 and KNO3 application could increase the risk of global warming.