N2O and CH4 emission and CH4 consumption in a sugarcane soil after variation in nitrogen and water application

Nitrous oxide (N2O) and methane (CH4) exchanges between the soil and the atmosphere are influenced by management practices currently used in agriculture. This study was conducted to determine whether N2O and CH4 emission and CH4 consumption in a trash blanketed sugarcane soil could be influenced by changed N fertiliser practice and variation in soil water content. Microplots were placed in rows of sugarcane and urea and ammonium sulphate (both 99 atom % 15N excess) applied to the soil surface, urea as both a split application of 80 kg N ha−1 and a full rate of 160 kg N ha−1 and ammonium sulphate at 160 kg N ha−1. Sugarcane trash (15 t ha−1) was applied to the surface of all microplots. Water was then added to all microplots to achieve water contents of 80% water-filled pore space (WFPS) in half the plots and saturation in the other half. Initially, the application of urea at 160 kg N ha−1 to microplots at 80% WFPS resulted in greater emission of (14N+15N)–N2O than was recorded from the split urea and the ammonium sulphate treatments at both moisture regimes. A mean value of 47 g N2O–N ha−1 d−1 was measured for this treatment compared with 18.5 and 15.2 g and 4.9 and 7.7 g N2O–N ha−1 d−1 for the split urea and ammonium sulphate waterlogged and 80% WFPS treatments respectively. Greater emissions of (14N+15N)–N2O were measured following 132 mm rainfall in February, with mean values of 214 and 143 g N2O–N ha−1 d−1 being recorded for the waterlogged treatments of the split urea and ammonium sulphate treatments respectively. A decrease in the 15N–N2O emission from the split urea treatment was the only difference recorded when comparing the initial (14N+15N)–N2O and 15N–N2O emissions from all treatments. 14N–N2O appeared to be the major gaseous N product from this treatment initially. Total emission of 15N–N2O ranged from 105 to 453 g N2O–N ha−1, which represented between 25.3 and 23.4% of the total amount of N2O evolved. CH4 emission occurred from all microplots fertilised with urea whereas CH4 consumption was measured in plots fertilised with ammonium sulphate only. CH4 emission ranged from 297 to 1005 g CH4–C ha−1 while CH4 consumption ranged from 442 to 467 g CH4–C ha−1. The application of ammonium sulphate and using a split application of urea initially decreased N2O emissions but the delay in nitrification obtained from changing the type and rate of application of these fertilisers resulted in greater N2O emissions later in the season when the soil moisture regime had changed. CH4 emission and consumption were not affected by these management changes.