A Comparison of Methods to Estimate N Mineralization in Soils Amended with Poultry Litter.

Fertilization with poultry litter (PL) may increase the nitrogen (N) mineralization potential (N₀) of the soil. Due to the long time required to measure N₀, short-term biological and chemical indexes have been developed to estimate it. The objective of this study was to quantify the change in N₀ in soils amended with PL and to evaluate the validity of the use of N₀ estimators in PL amended soils.

Four sites receiving long-term (>10 years) fertilization treatments (PL vs. Inorganic) were sampled: Delaware 1, Delaware 2, Iowa and Georgia (11.8%, 14.5%, 29.2% and 39.9% clay respectively). Samples were incubated in an aerobic open system for 6 months with bi-weekly leaching to measure N₀. Different N₀ estimators were evaluated: anaerobic incubation for 7 days (Nan), Illinois soil test analysis (ISNT), extraction with hot KCl,  distillation with NaOH or phosphate-borate (PB) and quantification of N in the particulate fraction (N-PF).

The response of N₀ to N sources was highly associated with soil texture. In Delaware 1 N₀ was not modified, while in Delaware 2, Iowa and Georgia N₀ it was increased by PL fertilization (34, 99 and 166 mg N kg^-1 respectively). Regarding the N₀ estimators, hot KCl and ISNT underestimated N₀ in PL treatments, while PB showed a low association to N₀ (R²=0.58). N-PF presented a high variability. From the evaluated methods, Nan and NaOH are recommended, as they showed the best correlation with N₀ (R²= 0.98 and 0.89 respectively), low variability and were not biased by the fertilization treatments.