Compaction influences N2O and N2 emissions from 15N-labeled synthetic urine in wet soils during successive saturation/drainage cycles.

Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of complex soil environmental and biochemical regulation of urine-N transformations in wet soils is needed to predict N2O emissions from grazing and also to develop mitigation technologies. Soil aeration, gas diffusion and drainage are key factors regulating N transformations and are affected by compaction during grazing. To understand how soil compaction from animal treading influences N transformations of urine in wet soils we applied pressures of 0, 220 and 400 kPa to repacked soil cores, followed by 15N-labelled synthetic urine and then subjected the cores to three successive saturation-drainage cycles on tension tables from 0 to 10 kPa. Compaction had a relatively small effect on soil bulk density (increasing from 0.81 to 0.88 Mg m-3), but strongly affected the pore size distribution. Compaction reduced the total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30-60 µm and 60-100 µm pores and increasing the proportion of micropores (30 µm). Rates of urine-N transformations, emissions of N2 and N2O, and the N2O to N2 ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation-drainage cycle, production of both N2O and N2 was low (< 0.4 mg N m-2 hr-1), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N2 at all compaction rates. By the third cycle, with increasing availability of mineral-N substrates, N2O was the dominant product in the uncompacted (max = 4.70 mg N m-2 h-1) and 220 kPa compacted soil (max = 7.65 mg N m-2 h-1) with lower amounts of N2 produced, while N2 was produced in similar quantities to N2O (max = 3.11 mg N m-2 h-1) in the 400 kPa compacted soil. Reduced macroporosity in the most compacted soil contributed to more sustained N2 and N2O production as the soils drained. In addition, compaction affected the rate of change of pH and DOC, both of which also affected the N2O to N2 ratio. Denitrification during drainage and re-saturation may make a large contribution to N2O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N2O and N2 emissions.