Reactivity of hydroxyproline-rich glycoproteins and their potential as biochemical tracers of plant-derived nitrogen

The amino acid hydroxyproline (Hyp) has potential for tracing plant N because Hyp-rich glycoproteins (HRGPs), structural proteins in the cell walls of plants, are its only major biochemical source in soil. The reactivity of Hyp in tissue from three vascular plants (black mangrove leaves, smooth cordgrass and cypress needles) was investigated in a 4 yr decomposition study to test its suitability as a tracer of plant N. The reactivity was similar to that of bulk plant N in cordgrass and cypress needles, but Hyp was more resistant to decomposition in mangrove leaves. This was likely due to increased cross liking between peptides in HRGPs from dicotyledons vs. other plants. These cross links involve stable diphenyl ether and biphenyl linkages that may protect the HRGP from degradation. The results suggest that Hyp can provide a quantitative estimate of the contribution of plant residues to the detrital N pool in systems dominated by gymnosperm and monocotyledon inputs. A mixing model using litter input representative of boreal forest vegetation demonstrated the potential of Hyp for tracing plant N in soil.