Journal
SOIL BIOLOGY & BIOCHEMISTRY
Volume 88, Issue -, Pages 178-188Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.soilbio.2015.05.022
Keywords
Compaction; Denitrification; Nitrous oxide; N-15; Porosity; Urine
Categories
Funding
- New Zealand Agricultural Greenhouse Gas Research Centre and Plant & Food Research's Land Use Change and Intensification programme (Core)
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Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of the 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 targeted 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 N-15-labeled 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 both total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30-60 mu m and 60-100 mu m pores and increasing the proportion of micropores (<30 mu m). Rates of urine-N transformations, emissions of N-2 and N2O, and the N2O to N-2 ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation-drainage cycle, production of both N2O and N-2 was low (<0.4 mg N m(-2) h(-1)), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N-2 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 soils (max = 7.65 mg N m(-2) h(-1)) with lower amounts of N-2 produced, while N-2 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 N-2 and N2O production as the soils drained. In addition, compaction affected the rate of change of soil pH and DOC, both of which affected the N2O to N-2 ratio. Denitrification during drainage and re-saturation may make a large contribution to soil N2O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N2O and N-2 emissions. (C) 2015 Elsevier Ltd. All rights reserved.
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