Nitrogen mineralization across a climosequence in the Pacific Northwest

Nitrogen fertilizer recommendations could be improved by better estimates of N mineralized (Nmin) from soil organic matter. Application of more or less N than required by the plant can result in detrimental effects on plant nutrition, the environment and producer economics. A range of soils spanning a climosequence (<250 to >700 mm annual precipitation), were sampled, in 20 cm depth increments, to evaluate Nmin along an 80 km transect in northeastern Oregon. Soils were incubated at 25°C with or without addition of wheat residue, for a maximum of 112 d. In 112 d, cropped soils (0–20 cm) mineralized from 28 to 61 mg N kg−1, depending on the annual precipitation at the sampling site. Uncultivated silt loam soils at paired sampling sites, mineralized twice as much N at moderate and high rainfall sites, as did cultivated silt loam soils. Residue addition resulted in net N immobilization, and decreased the total amount of Nmin an average of 33% in 112 d for both cropped and native soils. Net mineralization with depth in dryland native soils was greater than in cropped soils.