Nitrogen transformations revealed by isotope dilution in an organically fertilized hybrid poplar plantation

Abstract

Measuring nitrogen (N) transformations from organic fertilizers can help in selecting applications rates that provide sufficient soluble N to promote tree growth in short-rotation plantations. The objective of this study was to determine how organic fertilizers (papermill biosolids, liquid pig slurry) affected microbially-mediated N transformations in soils. Soil samples were collected from a hybrid poplar plantation before fertilization, 1 month after fertilizer application and at the end of the growing season. Net N mineralization and nitrification were evaluated during a 28 d laboratory incubation, while gross N transformations were assessed using a 15N isotope dilution technique. Pig slurry application increased soil ammonium (NH4-N) and nitrate (NO3-N) concentrations within 1 month, while papermill biosolids increased soil NH4-N and NO3-N concentrations at the end of the growing season. Gross N consumption rates were greater than gross N production rates. The NH4-N and NO3-N consumption rates were positively correlated with labile carbon and microbial biomass. The gross nitrification rate was 18 to 67% of the gross mineralization rate but 30% or less of the gross NH4-N consumption rate, indicating that NH4 consumption was overestimated by the isotope dilution technique. We conclude that N cycling in this hybrid poplar plantation was characterized by rapid consumption of plant-available N following N mineralization and nitrification.