Soil nitrification as affected by N fertility and changes in forest floor C/N ratio in four forest soils

Abstract

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.

Nitrification and nitrate reductase activity along a secondary successional gradient

“…Greater competition for ammonium in later stages could have been over-compensated by greater gross input of litter N. The high nitrification as a proportion of mineralized N indicates that direct phytochemical inhibition of nitrification is probably not important in this system. The importance of nitrification and thus nitrate nutrition in temperate forests has also been found by Nadelhoffer et al (1984), Rhia et al (1986) and Van Miegroet et al (1990). The consistent relationship between mineralization and nitrification in all soil types emphasizes the dependence of nitrification on substrate availability in these soils.…”

Controls on inorganic N species transformations and potential leaching in freely drained sub-soils of heavily N-impacted acid grassland

“…For profile A, net nitrification in ammonium-spiked soils showed a significant negative correlation with C:N ratio, as did net mineralization in d.w. spiked soils (Table 3), which was in line with findings of Gundersen and Rasmussen (1990) and Van Miegroet et al (1990) who claimed that net nitrification increased with decrease in soil C:N ratio. However, no such relationships were seen for soil profile B.…”

Regulation of nutrient concentrations in a temperate forest stream: Roles of upland, riparian, and instream processes

“…Segars et al (1986) found that most of the throughfall SRP was removed in the litter layer and that there was little seasonality in soil solution P concentrations in Walker Branch. Soil solution NH,+ and N03-concentrations in Walker Branch are low throughout the year and also show little seasonality (Mulholland et al 1990;Van Miegroet et al 1990). Although soil temperatures are lower in winter (generally So-10°C) compared with summer (20"-22"C, M. A. Huston pers.…”