Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Abstract

Soil samples were collected from the litter, humus and five mineral soil layers to a depth of 50 cm in acid, but highly productive, Norway spruce forests in S Sweden and E Denmark for determination of net N mineralization and potential nitrification. The samples were sieved while still fresh and incubated at a constant temperature (15°C) and soil moisture for 74-117 days with periodic subsamplings. Net N mineralization rates, expressed per g organic matter or per g of total N, decreased with increasing depth. Net N mineralization in the ten soils studied ranged from 35 to 105 kg N ha -1 yr -1, of which the organic horizons contributed 32-74%. Nitrate formation patterns were variable. Almost no nitrification could be detected at pH (H20) values lower than 4.0. Nitrate was formed in humus layers with pH values of 4.0-4.5, but the nitrification was never complete. By contrast, the nitrification was almost complete at a depth of 10-50 cm, where the pH (H20) was 4.1-4.5. Addition of CaCO3 stimulated nitrification in the humus layer, indicating the presence of acid-sensitive nitrifiers, while nitrification in the mineral soil was sometimes stimulated and sometimes inhibited by the addition of CaCO3. Tests with the acetylene block method showed that nitrification, when it occurred, was autotrophic in the humus layer but less easy to characterize in the mineral soil. Thus, further studies are needed to characterize the nitrifier organisms and identify the factors regulating the activity of these organisms. Actual nitrification rates in the field could not be determined, but our findings showed that the B horizon in acid forest soils has a great potential for nitrification. Because N deposition does not appear to decrease in Scandinavian forests, we conclude that an ammonium surplus in the forest floor followed by a downward transport of ammonium to the nitrifying subsoil can increase the risk of nitrate leaching in the future.

Comparison of soil nitrogen dynamics under beech, Norway spruce and Scots pine in central Germany

“…The net N mineralization rates in LOf 1 and Of 2 layers of our coniferous stands were similar to values of N-saturated coniferous forests in areas with a relatively high N deposition (Tietema et al 1992;Vervaet et al 2002). The net N mineralization rates in mineral soils (0-10 cm) in our study, however, were very low (Tietema et al 1992;Persson and Wiren 1995). The net N mineralization in the studied mineral soils was consistent with the suggestion that the net N mineralization in sandy soils is correlated to the total N content (Groot and Houba 1995).…”

“…A high variability of net nitrification has usually been reported in acid forests (Ste-Marie and Pare 1999;Laverman et al 2000). Persson and Wiren (1995) found that the net nitrification rates in L and FH layers in ten spruce stands varied from 0 to 6.2 mg kg )1 day )1 , and this depended largely on site conditions. We expected that soils under coniferous stands would have low net nitrification, because of the low soil reaction (Ste-Marie and Pare 1999) and high C/N, which may favor the immobilization of NO 3 -N by microorganisms (Johanssen 1995;Devito et al 1999).…”

Emission of nitrous acid from soil and biological soil crusts represents an important source of HONO in the remote atmosphere in Cyprus

“…Oswald et al (2015) studied soil samples from a boreal forest in Finland and observed HONO emissions below the detection limit. However, those samples had very low nutrient contents and were highly acidic (pH ≈ 3), for which microbial activity is supposed to be low (Fierer and Jackson, 2006;Persson and Wirén, 1995;Ste-Marie and Pare, 1999;Šimek and Cooper, 2002). Similarly, Laufs et al (2017) did not find correlations between HONO fluxes and temperature or humidity measured in the field and concluded that HONO sources other than biological soil emissions must have dominated.…”

Effects of root zone trenching on soil nitrogen dynamics in Japanese cedar and cypress plantations

“…However, the nitrification of ammonium remained very small in the litter layer and incomplete in the uppermost mineral soil. The nitrification rate depends mainly on the availability of ammonium and the existence and activity of nitrifiers (Robertson 1982;Persson and Wirén 1995). The steep gradient in net nitrification within the soil profile of the trenched cypress site was probably a result of the different availability of organic C originated mainly from fine roots (Table 2), which controls the microbial immobilization of ammonium.…”