Impact of tillage management and landscape on nitrogen availability in cereal-fallow cropping systems
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The purpose of this study was to examine the long- and short-term effects of adopting no-till fallow management practices on soil nitrogen (N) dynamics in southern Saskatchewan. The hypothesis tested was that landscape and environmental conditions have a greater impact on N turnover and availability than elimination of tillage. Assessment of the effect of small-plot long-term (12 yr) zero-till, minimum tillage and conventional tillage practices established on a level landscape in the Dark Brown soil zone near Saskatoon, revealed no significant differences in organic C and N and available N in the top 15 cm. Nutrient uptake and yield of spring wheat was not significantly different among tillage treatments compared. These findings indicate that reduced and no-till fallow have relatively little impact on soil N availability and yield where soil erosion is not a factor. Across the landscape, lack of incorporation of crop residue into the soil appeared to contribute to the same or slightly higher amounts and supply rates of nitrate in the no-till compared to conventional fallow. Lack of soil disturbance in chemical fallow does not appear to result in reduced N mineralization from soil organic matter and crop residue. At the landform scale, footslope (FS) and level (LE) positions exhibited greater fluctuations coupled with higher NO3 availability and supply rate compared to shoulders (SH) over the fallow season. From late May to the middle of August 1994, median KCl-NO3 concentration and AEM-NO3 supply rate within the landscape varied from 1.8 ìg/g to 23 ìg/g and 1 ìg/cm2/2 week to 53.4 ìg/cm2/2 week, respectively. Water soluble organic carbon (WSOC) was measured over the 1995 fallow season to provide an indication of changes in substrate availability and mineralization. WSOC was not influenced by the tillage treatments, but was sensitive to slope positions and environmental conditions. In 1995, the effects of chemical fallow management on N availability to the subsequent cereal crop were assessed by (i) crop N uptake and yield and (ii) 15N tracer technique (A value approach) and in situ burial of anion exchange membrane (AEM) probes. In spring before seeding the moisture content of the surface soil (0-10 cm) was u significantly higher (48% higher) under chemical fallow than conventional fallow as a result of greater residue coverage and associated moisture retention. Slightly higher N uptake and grain yield by spring wheat were observed in the chemical compared to conventional fallow. At the landform scale, FS and LE positions had 57% and 39% higher grain yield respectively, compared to SH positions. Nitrogen uptake was also significantly higher on FS and LE compared to SH as a result of greater water availability and greater N release by mineralization in these slope positions.