Losses and Biogeochemical Cycling of Soil Organic Nitrogen with Prolonged Arable Cropping in the South African Highveld: Evidence from D- and L-Amino Acids

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.