Losses and biogeochemical cycling of soil organic nitrogen with prolonged arable cropping in the South African Highveld – evidence from D- and L-amino acids SONJA BRODOWSKI 1 , WULF AMELUNG 1,3, *, INGO LOBE 1,3 and CHRIS C. DU PREEZ 2 1 Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany; 2 Department of Soil Science, University of the Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa; 3 Current address: Department of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 12, 10587 Berlin, Germany; *Author for correspondence (e-mail: wulf.amelung@ tu-berlin.de; phone: þ49-30-314-73521; fax: þ49-30-314-73548) Received 14 October 2002; accepted in revised form 17 July 2003 Key words: Amino acid enantiomers, Cultivation time, Grassland soils, Nitrogen cycle, Particle-size fractionation, Soil organic nitrogen Abstract. We know little about the mechanisms that cause rapid losses in the soil organic N pool during cropping. As the analysis of amino acid enantiomers can provide insight into both the fate of microbial N and the ageing of cells in the environment, we used this technique as a tool to examine how the pool of protein-bound N in subtropical Plinthosols responds to increasing duration of arable cropping. The samples comprised bulk soils (0–20 cm) and clay fractions from each of three agro-ecosystems in semiarid South Africa; the sites have been cropped for periods varying from 0 to 98 years. The amino acid enantiomers contributed 34% to the total N content. With increasing number of years a piece of land has been cropped, the amino acid concentrations declined bi-exponentially to about 30% of their initial level in the native grasslands. Changes of the remaining soil protein-N pool were indicated by alterations in the D-content of individual amino acids. As the years of arable cropping increased, the proportions of D-alanine and D-glutamic acid increased relative to the respective L-enantiomers. This was attributed to an accumulation of N in residues of bacterial cell walls. In contrast, the D=L-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land. Introduction Nitrogen is the most common limiting macronutrient in crop production (Jenkinson 1981). To ensure sustainable N supply for plants, it is therefore necessary to un- derstand the effects of cropping duration on soil N reservoirs (Paustian et al. 1997). Such understanding is especially important for the coarse-textured soils of South Africa with low N-supply capacities compared to other soils in tropical and sub- tropical climates (Du Toit et al. 1994; Du Preez and Du Toit 1995; Feller and Beare 1997). As in other regions the semiarid ecosystems loose N rapidly upon prolonged cropping (Dalal and Mayer 1987; Lobe et al. 2001). As protein-bound nitrogen builds up a major proportion of N in soils (Sowden 1977; Stevenson 1982; Knicker and Ko ¨gel-Knabner 1998), analysis of amino acids # 2004 Kluwer Academic Publishers. Printed in the Netherlands. Biogeochemistry 17–42, 2004. 71: