Nitrous oxide emission and denitrifier communities in drip-irrigated calcareous soil as affected by chemical and organic fertilizers.

The effects of consecutive application of chemical fertilizer with or without organic fertilizer on soil N2 O emissions and denitrifying community structure in a drip-irrigated field were determined. The four fertilizer treatments were (i) unfertilized, (ii) chemical fertilizer, (iii) 60% chemical fertilizer plus cattle manure, and (iv) 60% chemical fertilizer plus biofertilizer. The treatments with organic amendments (i.e. cattle manure and biofertilizer) reduced cumulative N2 O emissions by 4.9-9.9%, reduced the N2 O emission factor by 1.3-42%, and increased denitrifying enzyme activities by 14.3-56.2%. The nirK gene copy numbers were greatest in soil which received only chemical fertilizer. In contrast, nirS- and nosZ-copy numbers were greatest in soil amended with chemical fertilizer plus biofertilizer. Chemical fertilizer application with or without organic fertilizer significantly changed the community structure of nirK-type denitrifiers relative to the unfertilized soil. In comparison, the nirS- and nosZ-type denitrifier genotypes varied in treatments receiving organic fertilizer but not chemical fertilizer alone. The changes in the denitrifier communities were closely associated with soil organic carbon (SOC), NO3 - , NH4 + , water holding capacity, and soil pH. Modeling indicated that N2 O emissions in this soil were primarily associated with the abundance of nirS type denitrifying bacteria, SOC, and NO3 - . Overall, our findings indicate that (i) the organic fertilizers increased denitrifying enzyme activity, increased denitrifying-bacteria gene copy numbers, but reduced N2 O emissions, and (ii) nirS- and nosZ-type denitrifiers were more sensitive than nirK-type denitrifiers to the organic fertilizers.