N2O AND CH4 FLUXES IN SOIL INFLUENCED BY FERTILIZATION AND TRACTOR TRAFFIC

The effects of fertilization and tractor traffic on N2O emission and CH4 uptake in agricultural soil were studied in a field trial with different fertilization and soil compaction. The soil was a well-drained sandy loam and the crop rotation was rich in ley and legumes. The fertilization treatments were: NPK fertilizer (140 kg NH4NO3-N ha-1); cattle slurry (CS) (189 kg total N ha-1), CS (81 kg total N ha-1), and an unfertilized treatment. The soil was experimentally compacted by two passes with a tractor, wheel by wheel, shortly before fertilization. Gas fluxes at the soil surface were measured by the soil cover method. Soil air at a depth of 7-12 cm was sampled through stationary soil air samplers. Concentrations of N2O in soil air were more than seven times higher in compacted, NPK-fertilized soil than in any other treatments. Maximum concentrations (1900 mul N2O l-1) were observed shortly after periods with heavy rain. The accumulated N2O emissions from the NPK-fertilized treatment (4 June-8 July) corresponded to 5.3% of added NH4NO3-N in compacted soil, and 3.9% in uncompacted soil. Fertilization with cattle slurry equivalent to 81 kg total N ha-1 gave an N2O emission corresponding to 3.1% of added NH4-N in uncompacted soil and 2.7% in compacted soil. Increasing levels of cattle slurry resulted in a reduction in N2O emission per kg NH4-N added. The accumulated CH4 uptake (4 June-8 July) in the soil was 9.7 mg CH4 M-2 in unfertilized and uncompacted soil. It was reduced by 52% by soil compaction, 50% on average by fertilization and 78% by soil compaction and fertilization combined. Fertilization with NH4NO3 or cattle slurry resulted in similar effects.