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Impact of Tillage Management and Landscape on Nitrogen Availability in Cereal-Fallow Cropping Systems

Jowkin, V. 1997. Ph.D. Dissertation. Dept. Soil Science, University of Saskatchewan, Saskatoon, SK

Abstract

The purpose of this study was to examine the long- and short-term effects of adopting no-till fallow management practices on soil nitrogen (M 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. To gain insight into spatial and temporal variations in amount and supply rate of NO3 over a fallow season in the initial years of imposition of chemical (no-till) fallow, landscape scale studies were conducted in 1994 and 1995 in a field with an undulating topography in the Brown soi1 zone in southwestern Saskatchewan near Central Butte. The bioavailability of mineral N was assessed by 1) in-field measurement of anion exchange membrane sorbed NO3 (AEM-NO3) and 2) extraction of soil samples with 2M KCl solution or water. 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 soi1 disturbance in chemical fallow does not appear to result in reduced N mineralization hm 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 KC1-NO3 concentration and AEM-NO3 supply rate within the Landscape varied hm 1.8 pg/g to 23 pg/g and 1 pg/cm2/2 week to 53.4 pg/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 treatment, 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) mp N uptake and yield and ii) SN 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 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. Furthermore, shoulder positions under chemical fallow had about 32% higher grain yield and N uptake compared to shoulders under conventional fallow, reflecting greater soi1 moisture and NO3 availability. The A-value and the AEM-NO3 supply rate were not significantly different between fallow systems. indicating that N availability to a crop is not likely to be greatly affected in the initial years after switching to no-till fallow in these soils. Instead this study showed that landscape position has a more pronounced impact on N tum over over the fallow season and N availability to the subsequent crop than a change in tillage practice.