Controls of nitrogen isotope patterns in soil profiles

Abstract

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (d 15 N) data, patterns of 15 N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15 N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4% in ectomycorrhizal systems and 4.6 ± 0.5% in arbuscular mycorrhizal systems. The 15 N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15 N in soil profiles, fractionation against 15 N during N transfer by mycorrhizal fungi to host plants, leads to 15 N-depleted plant litter at the soil surface and 15 N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15 N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15 N enrichments with depth in less N-limited forests and may account for soil profiles where maximum d 15 N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.

Stable Nitrogen Isotopes in a Forested Watershed in Taiwan

“…This pattern results from several processes involving isotopic fractionation during nitrogen cycling, leading to plant uptake of N that is depleted in 15 N, and because of internal N cycling through mycorrhizal fungi that also results in the transfer of N that is depleted in 15 N to host plants (Hobbie et al 2000;Hobbie and Ouimette 2009). Both of these mechanisms results in the transfer of N which is depleted in 15 N to the soil surface via litterfall.…”

“…Thus, for nitrogen, plants are nearly always isotopically lighter than the soil supporting their growth (Nadelhoffer and Fry 1994). Isotopic fractionation during decomposition, soil food web processing, and denitrification also contribute to this pattern (Hobbie and Ouimette 2009).…”

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

“…For instance, d 15 N measurements have been used to trace pathways of forest N losses in gaseous form (N 2 O and N 2 ) or in leachate [NO 3 -and dissolved organic N (DON)] at both local (Houlton et al 2006;Brookshire et al 2012;Koehler et al 2009) and regional scales (Bai and Houlton 2009). High rates of N loss influence d 15 N in the remaining soil pools in part through the strongly fractionating process of denitrification (Pérez et al 2000) which preferentially consumes 14 N and can yield relatively 15 N-enriched soil nitrate remaining in soil solutions (reviewed in Hobbie and Ouimette 2009). This is a main mechanism whereby a ''loosening'' of the N cycle leads to enriched soil and plant d 15 N values.…”

Impact of a Low Severity Fire on Soil Organic Carbon and Nitrogen Characteristics in Japanese Cedar Soil, Yamagata Prefecture, Japan

“…It can alter the amounts and distribution of available nitrogen as well as δ 15 N ratio due to the change of the soil surface layers' chemical composition and/or the removal of the soil surface layers because of erosion (Hobbie & Ouimette, 2009). The slash and burn practises in high precipitation and heavy snowfall areas of typical Japan Sea mountain forests offer an opportunity to study the impact of low intensity fires on available nitrogen and δ 15 N of soils where Japanese cedar seedlings (Cryptomeria japonica) will be replanted and thus, it is essential to know the nitrogen availability of the soils for a suitable growth of the new seedlings.…”