Analysis and behavior of soluble organic nitrogen in forest soils

Background, aim, and scope A large proportion of soil nitrogen (N; >80%) is present in organic form. Current research on plant N uptake in terrestrial ecosystems has focused mainly on inorganic N such as ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻), while soluble organic N (SON) has received little attention. In recent years, the increasing evidence showing the direct uptake of various amino acids by plants and the predominance of the organic form in N loss by leaching in many forest ecosystems has drawn attention to critically re-examine the nature and the ecological role of soil SON in terrestrial N cycling. However, little is known about the sources and dynamics, chemical nature, and ecological functions of soil SON in forest ecosystems. This paper reviews recent advances in the areas of research on current techniques for characterizing soil SON and the size, nature, and dynamics of soil SON pools in forest ecosystems. Materials and methods The SON represents a significant pool of available and mobile N in forest soils. The SON can be sampled using a number of physical (e.g., suction cup, microdialysis) and chemical (e.g., extraction) methods and analyzed by the wet chemistry method (e.g., persulfate oxidation) and combustion. Chemical and physical fractionation, pyrolysis-mass spectrometry, ¹³C and ¹⁵N nuclear magnetic resonance spectroscopy, and ¹⁵N, ¹³C, and ¹⁴C isotopic techniques have been used for investigating the nature and dynamics of SON in forest ecosystems. Results and discussion The amount of SON in soil may vary with land use type, forest species, management practices, and analytical procedures used. Statistical figures, based on 116 datasets available in the literature, have shown that concentrations of soil SON can range from 1 mg N kg⁻¹ (dry weight basis) to up to 448 mg N kg⁻¹, with an average of 35 mg N kg⁻¹, representing an average of 48% of total soluble N. Soil SON consists of a mixture of structurally diverse N-containing compounds, possibly with the amide N, amino acids, and amino sugars being predominant. The biodegradability of SON largely depends upon its chemical characteristics (e.g., C-to-N ratio). Conclusions Direct utilization of added amino acids by a large number of forest species may present a potentially important new pathway for plant N uptake. However, the evidence of direct utilization of soil native SON by forest plants is still lacking, and the extent of the contribution of SON to forest plants is far from certain. Transformation of soil organic matter into SON, rather than the conversion of SON to NH₄ ⁺/NO₃ ⁻, may be the rate-limiting step, which regulates the overall N cycling in N-limited forest ecosystems. Recommendations and perspectives Future work may well focus on the chemical and biological nature of SON in forest soils, microbial transformation of SON, the nutritional role of the SON pools in the N-limited forest ecosystems, and the potential role of SON in global climate change (e.g., N₂O emission).