Soil water conservation and nitrous oxide emissions from different crop sequences and fallow under Mediterranean conditions

• The chemical fallow conserved most soil water followed by cultivation fallow.
• Weedy fallow depleted soil water and resulted in profuse weed growth in next season.
• The enhanced soil moisture due to fallowing did not increase nitrous oxide emissions.
• Fallow with weed management could be an ideal drought adaptation strategy.

Interest in fallowing as the drought adaptation strategy has increased recently due to the occurrence of frequent droughts in cropping areas of Australia. Weed management in a fallow is crucial as it affects the level of soil water and nitrogen conserved by this practice. Increased use of fallow has implications for nitrous oxide emissions as soil moisture is a major determinant of evolution of nitrous oxide. In a two year study, three types of fallow with different weed management were compared with wheat and canola crops in 2010 and the carry-over effects with wheat on all plots in 2011. The weeds in the fallows were managed either by herbicide (chemical fallow) or by tillage (cultivation fallow) and compared to an unweeded fallow (weedy fallow). The chemical fallow conserved most soil water followed by cultivation fallow, with the lowest soil water content in the weedy fallow. In addition, more weeds were observed in the wheat crop following weedy fallow. Low wheat and canola yields were measured in 2010 because of a drought, which also depleted soil water compared to the chemical and cultivation fallows. Therefore, fallow is a potential strategy for increasing crop yield under dry conditions. The results also indicated that weeds had a detrimental effect by reducing soil water content due to lack of timely weed management. To test the compatibility of fallow with climate-change mitigation, the nitrous oxide emissions were compared in the 2011 wheat crop. There was a temporal fluctuation in nitrous oxide emissions with increased rates of emission after top dressing with urea. Importantly, however, the nitrous oxide emission from wheat following fallow was not increased compared with wheat in the continuous cropping sequences. Therefore, this study found that fallowing with good weed control, could be used as a drought adaptation strategy without increasing nitrous oxide emissions in following crops.