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 reserved at the most degraded arable land.