Evaluation of laboratory-based measures of soil mineral nitrogen and potentially mineralizable nitrogen as predictors of field-based indices of soil nitrogen supply in potato production

Accurate estimation of soil nitrogen (N) supply in the field is required to optimize fertilizer N management and to minimize environmental N losses in humid environments. Laboratory-based measures of N availability were evaluated as predictors of field-based indices of soil N supply within potato production systems. Pre-plant soil samples (0-15 cm) were collected from a series of forty treatments in established potato trials located in Atlantic Canada and Maine, USA. Total plant N uptake at topkill with no fertilizer N applied (PNU0N), PNU0N plus soil mineral N to 30 cm depth at harvest and relative yield were considered as field-based indices of soil N supply. The potentially mineralizable N (N-0) was determined by aerobic incubation at 25 degrees C and periodic leaching for 24 weeks. A series of laboratory-based measures of soil N supply were measured in soil samples. Pre-plant soil nitrate or total mineral N at 0-30 cm depth was the best single predictor of PNU0N (r=0.67 and 0.71, respectively) and relative yield (r=0.58 and 0.61). The ultraviolet absorbance of 0.01 M NaHCO3 extract at 205 nm (NaHCO3-205) was suitable as a predictor of PNU0N and relative yield in each growing season, however, the relationship between this parameter and PNU0N or relative yield varied somewhat among years. A combination of pre-plant mineral N plus N mineralized in the first 2 weeks period of incubation after re-wetting is proposed as a more robust measure of N availability compared with use of mineral N alone.