Soil Nitrification as Affected by N Fertility and Changes in Forest Floor C/N Ratio in Four Forest Soils

Potential and actual nitrification were compared between a high and low N fertility Inceptisol in Washington State and between two Tennessee Ultisols to investigate the effect of soil N status and changes in forest floor C/N ratio on NO3-N production. Soil and soil solution data were collected (i) prior to treatment, (ii) after doubling the forest floor C/N ratio with sawdust, and (iii) after doubling the forest floor N content by adding urea N to each of the four forest types. Nitrate N production during aerobic soil incubation before and 1 year after treatment demonstrated significantly higher nitrification capacity of the N-rich Washington soil under alder, the stimulatory effect of N addition, and the suppressing effect of C on nitrification. Our study also indicated that the N fertility status was not as different between the Tennessee soils as originally assumed, that both soils in fact behaved similarly to the N-poor Washingon soil, and that these three soils should be considered in the same category of low N fertility sites. Nitrate concentrations in soil and soil solution samples collected in the field generally agreed with the laboratory findings, but differences in NO3− leaching between the untreated soils and between treatments were often less pronounced. Soil solution NO3− concentrations were \u3c0.01 mmol/L in the two Tennessee soils and the N-poor Washington soil, and differed little between them. In soil that was N enriched by N-fixing alder, the average NO3− solution concentration was 0.4 mmol/L. Spring application of urea N caused an immediate and significant increase in NO3− solution concentration in all four soils, but the treatment effect subsided by the end of the 2nd year in all cases. Adding C to the forest floor did not further reduce already low NO3− solution levels in the two Tennessee soils and the N-poor Washington soil. This treatment did not cause a prolonged reduction in NO3− leaching from the N-rich Washington soil, and it was speculated that this was due to belowground N addition from the root system of the N-fixing alder at this site