Old organic matter in temperate forest soils is very nitrogen-rich

Abstract. Soil organic matter (SOM) is important for Earth’s climate regulation and primary production. Here we tested the hypothesis that old SOM fractions are rich in organic nitrogen given the high affinity of organic nitrogen compounds to interact with mineral surfaces. For this purpose, we studied temperate broadleaf and coniferous forest soils located in Sweden. We used ramped thermal fractionation to obtain persistent SOM fractions, and we evaluated the persistence using ¹⁴C. The results show that old SOM fractions in the temperate forest soils are very nitrogen-rich, indicating that organic nitrogen compounds persist for a long time in soil. A larger proportion of the total organic nitrogen than of the total organic carbon was found in the old SOM fractions, suggesting that organic nitrogen compounds decompose on average more slowly than nitrogen-free organic compounds. The size of the old SOM fractions was correlated with the soil clay content but did not differ significantly between broadleaf and coniferous forests, suggesting that association with minerals rather than forest type or organic matter quality affect the persistence of SOM. This study has important implications for element cycling as it demonstrates that nitrogen in temperate forests persists for a long period in SOM.