Harrowing for weed control: Impacts on mineral nitrogen dynamics, soil aggregation and wheat production

Tillage with a spring tine harrow has become a recommended mechanical weeding technique for cereal crops. In this study, the impact of its use on soil mineral N content, soil aggregation and spring wheat (Triticum aestivum L.) production was investigated. The experiment was performed during 2 successive years (2005–2006) on a clay loam and on a silty loam. The two-main plot treatments consisted of a wheat crop subjected or not to intensive harrow use in a weed-free production system. Two N fertilizer treatments (mineral fertilizer and dry granular poultry manure) were also included as subplots within these main treatments and compared to a non-fertilized control. Harrowing had significant and variable effects on soil NO3− contents in the 0–5 cm soil layer. Slightly higher NO3− contents (average difference of 3.2 kg NO3− ha−1) were measured in the harrowed treatments than in the undisturbed plots in the clay loam soil in 2006. However, significantly lower mineral N contents were observed in the harrowed treatments than in the undisturbed plots in the clay loam soil in 2005 and in the silty loam soil in 2006. This apparent N immobilization amounted to 19 kg NO3− ha−1 in the clay loam soil in 2005 (for both fertilizers) and 30 kg NO3− ha−1 in the silty loam soil in 2006 (only in mineral fertilizer plots) after the successive harrowing treatments. In all cases, data of the last sampling dates in the fall indicated that residual NO3− content was not affected by the treatments. Overall harrowing had a minor decreasing and transient effect on the mean weight diameter (MWD) of soil aggregates while the dry poultry manure tended to increase MWD. The harrowing treatment had no significant effect on wheat, grain N uptake and yield. In conclusion, harrow use had variable impacts on soil NO3− content and a minor decreasing effect on the MWD of soil aggregates. Of note, significant apparent mineral N immobilization was observed on a few sampling dates following the harrow treatments.