Impact of crop protection on nitrogen utilisation and losses in winter wheat production

A 3-year (2002–2004) field experiment investigated the effects of disease and insect attack on nitrogen dynamics and losses during cultivation of winter wheat. Three treatments providing different degrees of crop protection were studied on a silty clay soil in south-western Sweden, in three consecutive wheat crops that were continually inspected for pests and diseases. A field with a history of cereal-dominated crop sequences was chosen to increase the possibilities of disease incidence. Nitrogen leaching was measured directly in tile-drained field plots equipped with individual collectors for drainage water. Ammonia emissions from the wheat stands were measured in one replicate plot during the first two growing seasons. Nitrogen accumulation and distribution in plants were investigated by sampling the crop at different stages of development and analysing different plant parts for total nitrogen content. Soil mineral nitrogen was determined within the 0–90 cm soil layer in early spring, at yellow ripeness and in November. Grain yield and grain nitrogen use efficiency were always significantly larger and mean residual soil mineral nitrogen levels (at maturity and in November) significantly lower in treatments with crop protection. At maturity, total N concentration in straw was significantly higher in the treatment without crop protection. Mean nitrogen leaching tended to be greater in the treatment without crop protection but the differences were not statistically significant. Ammonia emissions of wheat were very small (0.1–0.3 kg N ha−1) in all treatments and could thus be neglected. The better N use efficiency with crop protection was probably due to a combination of larger N amounts in above-ground plant parts and better N translocation to grain when diseases were reduced.