Fall Application of Animal Manures and Overwinter Soil N Transformations – Friend or Foe?.

Soil N transformations and losses can occur at significant rates throughout winter in cold agricultural areas. Fall application of animal manure is common practice in these areas, but the fate of fall-applied N is still unclear. It is currently thought that liquid manure (which contains 40-80% of its total N as ammonia) bear a great risk of environmental N loss, whereas solid manures may actually benefit from fall application as the N immobilization phase occurs before planting of the next crop in the spring. The best period for fall application, and the use of nitrification inhibitors to retain manure N in the soil during the non-growing season are still a matter of debate. We carried out a factorial experiment for two consecutive years, on a sandy loam and a silty clay loam. The treatments included liquid and solid dairy manure, applied in early (mid-September) or late (early November) fall. The treatments with liquid manure were tested with and without the nitrification inhibitor Nitrapyrin. Barley was grown on all plots in the next spring. All applied manures were labelled with ¹⁵N to allow tracking of N in the soil-plant system. Preliminary results indicate that the liquid manure induced greater barley yields than solid manure on both soil types, whatever the timing of application, except in the silty clay soil in 2013-14 when early application of solid manure resulted in barley yields similar to those with the liquid manure. For the liquid manure, the late application and the addition of the nitrification inhibitor resulted in greater yields than the early application without inhibitor, especially in the sandy soil. The effect of manure type, application timing, and nitrification inhibitor on the temporal dynamics of soil mineral N and on the recovery of manure N in the soil-plant system will be presented.