Journal of Conference Abstracts

Evidence for a Preferential Bioavailability of Amino Acid N Over Non-amino Acid N in the O Horizon of a Podzol

Lars Johnsson¹ (Lars.Johnsson@mv.slu.se), Dan Berggren¹ (Dan.Berggren@mv.slu.se) & Ola Kårén² (Ola.Karen@mv.slu.se)

¹ Department of Soil Sciences, Swedish University of Agricultural Sciences, PO Box 7014, S-750 07 Uppsala, Sweden.

² Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences,
PO Box 7026, S-750 07 Uppsala, Sweden.

Hydrolyzable amino acids may contribute 30 - 50 % to the soil organic N pool (Stevenson, 1982). In laboratory experiments ectomycorrhizal fungi have been found to be able to utilise a range of organic com-pounds such as proteins, peptides and amino acids (Abuzinadah and Read, 1986 a,b; Finlay et al., 1992). This direct N-uptake of organic N would then represent a way for plants to short-circuit the mineralisation step of decomposition by directly absorbing amino acids (Abuzinadah et al., 1986). In the present investigation we investigated changes in (1) amounts of hydrolyzable (6 M HCl) individual amino acids (including glucos- and galactosamine) and in (2) the amounts of hydrolyzable amino acid N (aa N) associated with pyrophosphate extractable organic matter (humic acids, fulvic acids and hydrophilic substances) caused by a growing Norway spruce plant (Picea abies, L. Kast).

A microcosm experiment was designed where Norway spruce (Picea abies, L. Karst) plants were grown for 145 days in soil material from the Oa horizon from a Haplic Podzol. Two N treatments, N0 (control) and N2 (60-120 kg N ha^-1 y^-1 from 1967 to 1990) were included. The soils were sampled in a long-term nitrogen fertilisation experiment in Stråsan, central Sweden (Tamm, 1991). Classification of mycorrhizal roots into morphotypes was performed on fresh roots. To identify some common morphotypes PCR-RFLP analysis was used.

The percentage fine roots colonized by mycorrhizal fungi was high (>93 %) and did not differ between N0 and N2. All the 16 amino acids and the 2 amines determined in soil hydrolysates decreased during the experiment period in both N treatments. The total amount of hydrolyzable aa N decreased significantly by 14 % for N0 and 10 % for N2. No significant change was found for non-amino acid N (non-aa N; total organic N - amino acid N). No change in the amount of total organic N (TON) or hydrolyzable aa N was found in the Na₄P₂O₇ (pH 7)-extractable fractions (humic acids, fulvic acids and hydrophilic substances).

This study suggests that hydrolyzable amino acids contain the fraction of organic N that is most easily mineralised and/or taken up by spruce plants colonized by mycorrhizal fungi. In contrast, hydrolyzable aa N extractable by Na₄P₂O₇ seems not to be easily mineralised and/or available for plant uptake. Due to the expected great influence of mycorrhiza on mineralisation and plant uptake of N and the fact that almost all roots in the field are colonized with mycorrhiza we argue that the results from this study are relevant for field conditions.

References

Abuzinadah, R. A., Finlay, R. D. & Read, D. J. New Phytologist 103, 495-506 (1986).

Abuzinadah, R. A. & Read, D. J. New Phytologist 103, 481-493 (1986a).

Abuzinadah, R. A. & Read, D. J. New Phytologist 103, 507-514 (1986b).

Finlay, R.D., Frostegård, Å. & Sonnerfeldt, A.-M. New Phytologist 120, 105-115 (1992).

Stevenson, F. J. Humus Chemistry. (ed Stevenson, F.J.) (John Wiley & Sons, New York 1982).

Tamm, C. Ecological Studies 81. (Springer-Verlag, New York, 1991).