Relative nitrogen mineralization and nitrification in soils of two contrasting hardwood forests: Effects of site microclimate and initial soil chemistry

Abstract

This study determined the relative rates of soil nitrogen mineralization and nitrification in neighbouring mixed oak and mixed mesophytic forest stands on sandstone-derived soils in Neotoma Valley, south central Ohio, U.S.A. The soils of the northeast-facing mesophytic stand had higher pH, moisture, and extractable nutrient levels while those of the southwest-facing oak stand were deeper and had a higher C: N ratio. To evaluate the relative importance of initial soil chemical status and microclimate in controlling nitrogen transformation rates, the polyethylene buried-bag method was modified to include reciprocal transplants of bags between sites. Nitrogen mineralization was greater at the mesophytic site most of the year, though the rates were not different between sites during a very dry September. Total annual N mineralization per gram of soil and per m 2 of forest floor were 208 μg N g −1 dry soil and 5604 mg N m −2, respectively. The comparable rates for the mixed oak site were 125 μg N g −1 and 2209 mg N m −2. Approximately 67% of monthly mineralized N was nitrified at the mesophytic site while less than 10% was nitrified at the mixed oak site. Differences in both mineralization and net nitrification rates between sites correlated positively with ammonium pool size and moisture, and negatively with nitrate pool size, while percent nitrification was positively correlated with pH and negatively with C: N ratio. In contrast, seasonal patterns of N transformation within a site were strongly correlated with soil temperature and moisture, and relatively unaffected by soil chemical factors. The rate of N mineralization and relative nitrification in transplanted bags did not differ from those of bags incubated at their native site. Thus, within this watershed, spatial variations in nitrogen dynamics were more closely related to differences in soil chemistry than to microclimate, while seasonal patterns showed just the reverse.