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 <0.01 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.
Mark Hansen were instrumental with data management and estimation.Cassandra Olson contributed substantial time and her field knowledge to invasive species analysis. Dacia Meneguzzo aided with fragmentation analyses.Daniel Twardus and Robert Hoehn helped with the analysis and interpretation of Indiana' s urban forest data. Various individuals contributed their time and constructive comments during meetings and manuscript review: Burnell
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