Tillage-induced N mineralization and N uptake in winter wheat on a coarse sandy loam

Summary

The potential of adjusting soil N mineralization by soil tillage in the growing season was investigated in winter wheat on a coarse sandy loam. In two succeeding years, wheat was sown at 25 cm row spacing on plots where four different organic matter (OM) management strategies regarding pig slurry application, straw incorporation and catch crop growing had been applied for the previous 13 years. Soil tillage was carried out in spring with a row cultivator, a PTO-driven row rotovator or a brushweeder. Soil inorganic N content was determined in tilled and undisturbed soil during four weeks after tillage. In the first experimental year, gross nitrogen transformation rates were measured by 15N pool dilution techniques in row rotovated soil and in undisturbed soil at two of the four OM levels. The three types of soil tillage caused higher inorganic N levels in both years with an extra 7-9 µg g-1 soil in the tilled soil than in the undisturbed reference soil. The N uptake in the above-ground wheat biomass harvested in spring did not respond to tillage or to the increased inorganic N content in soil. There was no effect of tillage on N transformation as measured by 15N pool dilution. The higher inorganic N content in disturbed soil in the field was therefore assumed to be mainly caused by reduced crop N uptake rather than an increased N mineralization. In the first year, the grain yield of the mature winter wheat was lower after row cultivation than after rotovation and brushweeding but the grain yield after soil tillage was not significantly different from the undisturbed soil. The N uptake in grain was 4-5 kg N ha-1 higher after rotovation and brushweeding compared to undisturbed soil. In the second year, the grain yield was highest after brushweeding whereas there was no difference in yields between the two other tillage techniques and the undisturbed soil. Soil tillage carried out in the second year did not influence grain N uptake. The N concentration in wheat grain was unaffected by soil tillage in both years. No interactions between tillage and organic matter management were observed. Thus, in regularly ploughed soil as studied here, soil tillage in the growing season does not seem to have a potential for significantly enhancing N availability or improving synchronization between crop demand and N mineralization.