Short communication Compound speci®c d 15 N- values: amino acids in grassland and arable soils N.J. Ostle a, *, R. Bol a , K.J. Petzke b , S.C. Jarvis a a Institute of Grassland and Environmental Research, North Wyke, Devon EX20 2SB, UK b German Institute of Human Nutrition, Potsdam-Rehbrucke, Bergholz-Rehbrucke, D-14858 Germany Accepted 25 May 1999 Soil nitrogen occurs in a suite of functionally diverse and biologically important forms ranging in complex- ity from NO 3 À /NH 4 + to nucleic acids and heterocyclic compounds, many of which remain unidenti®ed (Paul and Clark, 1989). New analytical approaches are required to elucidate the composition and biological function of SOM N pools (Hopkins et al., 1998). In this exploratory investigation, ion chromatography (IC) and gas chromatography combustion isotope ratio mass spectrometry (GC±C±IRMS) were used to characterise acid-hydrolysable amino acids in soil and plant samples from long-term grassland and arable systems. Results showed that relative concentrations of individual amino acids (IAAs) were similar in soil and plant matter and that speci®c features of soil IAA d 15 N- signatures could be used to dierentiate between management regimes. Acid hydrolysis studies indicate that amino acids comprise 20±60% of the total soil N pool (Stevenson, 1956; Senwo and Tabatabai, 1998). Recent work has con®rmed that amino acids are taken up directly by plants (Nasholm et al., 1998) and represent an accessi- ble source of N for soil microorganisms (Barraclough, 1997). This large dynamic form of organic N is believed to contribute signi®cantly to the annually mineralised SOM pool (Jarvis et al., 1996). Natural abundance 15 N/ 14 N stable isotope ratios of bulk soil, plant tissue and macrofauna samples have been used to clarify the eects of inorganic fertiliser, manuring and other agricultural practices on N cycles (Kohl and Shearer, 1995; Yoneyama, 1996; Neilson et al., 1998). Now, using GC±C±IRMS, it is possible to measure 15 N/ 14 N ratios of individual amino acids at natural abundance amounts (Metges et al., 1996; Metges and Petzke, 1997). Soil and plant samples were collected in January 1997 from the Rowden Moor experimental pasture system at the Institute of Grassland and Environmental Research (IGER), North Wyke, UK. Plots used in this study were conventional N pasture (C-grass), 280 kg N ha À1 yr À1 applied as NH 4 NO 3 on six occasions at 2 week intervals from March each year and zero N pasture (Z-grass), which had received no inorganic N fertiliser. No pesticides were used on either of the grassland treatments. The soil is a clayey nonclacareous pelosol (gleyic cambisol FAO) and con- tained 36% clay, 27% silt and 37% sand. The site has mean annual temperatures and rainfall of 10.58C and 1035 mm, respectively. Both plots were undrained and grazed by beef steers for approximately 8 months of the year (March±October). C-grass and Z-grass pas- tures had been maintained under the present manage- ments for 15 and 30 yr, respectively (Gill et al., 1995). Vegetation in C-grass was dominated by Lolium per- enne (L.) sward interspersed with Holcus lanatus (L.), whereas the Z-grass sward was visibly more diverse and included Trifolium repens (L.) and Agrostis stoloni- fera (L.). Triplicate soil±plant cores (5 cm dia- meter  20 cm deep) were taken at 4 points along a 20 m transect running perpendicular to the prevailing 4± 58 slope (n = 12). Triplicate plant and soil samples (0± 30 cm depth) were taken in October 1992 from winter wheat (Triticum aestivum (L.)) and maize (Zea mays (L.)) experimental plots located at Boigneville, Essone, France (Puget et al., 1995). Both crops had been con- tinuously grown for at least 22 yr under conventional management, ploughing to 30 cm, 12 cm tillage and Soil Biology and Biochemistry 31 (1999) 1751±1755 0038-0717/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(99)00094-2 www.elsevier.com/locate/soilbio * Corresponding author. Present address: ITE ± Merlewood, Grange-Over-Sands, Cumbria LA11 6JU, UK. Fax: +44-15395-34- 705. E-mail address: no@ite.ac.uk (N.J. Ostle)