Stable isotope ratios of nonexchangeable hydrogen in organic matter of soils and plants along a 2100-km climosequence in Argentina : New insights into soil organic matter sources and transformations?

Abstract The quantitative contribution of shoot and root organic matter (OM) to the soil carbon (C) stock is still unknown, mainly because of methodological restrictions. The novel measurement of the nonexchangeable hydrogen (H) stable isotope ratio ( δ 2 H n ) in bulk OM provides new opportunities to investigate the sources of soil C and its climate-dependent transformations. Our objectives were to test whether (I) there are systematic differences between δ 2 H n values of aboveground and belowground plant OM and (II) δ 2 H n values of litter and soil OM relate to climate and plant OM source δ 2 H n signals. We determined δ 2 H n values of bulk shoot, root, litter and demineralized soil OM from 20 sampling sites along a 2100-km climosequence from the Argentinean Pampas to the Patagonian steppe. The δ 2 H n values of shoot and litter OM correlated closely with the aridity index ( r  = −0.83, p r  = −0.78, p δ 2 H n values of root and soil OM showed a close correlation with modeled mean annual δ 2 H values of local precipitation ( r  = 0.91, p r  = 0.97, p r  = 0.80, p r  = 0.88, p δ 2 H n values of shoot and root OM differed markedly (no linear correlation) most likely because of biosynthetic exchange of C-bound H with ambient water in the roots. δ 2 H n values of root and demineralized soil OM, however, were closely correlated ( r  = 0.91, p δ 2 H n signature of soil OM is best explained under the assumption that root OM is the predominant source of soil OM, rather than shoot and litter OM.