Carbon isotopic ratios of modern C 3 and C 4 vegetation on the Indian peninsula and changes along the plant–soil–river continuum – implications for vegetation reconstructions

[摘要] The large difference in the fractionation of stable carbon isotopes betweenC 3 and C 4 plants is widely used in vegetation reconstructions, where thepredominance of C 3 plants suggests wetter and that of C 4 plants drierconditions. The stable carbon isotopic composition of organic carbon (OC)preserved in soils or sediments may be a valuable (paleo-)environmentalindicator, based on the assumption that plant-derived material retains thestable carbon isotopic value of its photosynthetic pathway during transferfrom plant to sediment. In this study, we investigated the bulk carbonisotopic values of C 3 and C 4 plants ( δ 13 C) and of organiccarbon ( δ 13 C org ) in soils, river suspended particulatematter (SPM) and riverbed sediments to gain insight into the control ofprecipitation on C 3 and C 4 plant δ 13 C values and to assesschanges in δ 13 C org values along the plant–soil–rivercontinuum. This information allows us to elucidate the implications ofdifferent δ 13 C end-members on C 3  / C 4 vegetation reconstructions.Our analysis was performed in the Godavari River basin, located in the coremonsoon zone in peninsular India, a region that integrates the hydroclimaticand vegetation changes caused by variation in monsoonal strength. The basinhas distinct wet and dry seasons and is characterised by natural gradientsin soil type (from clay-rich to sandy), precipitation ( ∼  500 to 1500 mm yr −1 ) and vegetation type (from mixed C 3  / C 4 to primarily C 3 )from the upper to the lower basin. The δ 13 C values ofGodavari C 3 plants were strongly controlled by mean annual precipitation(MAP), showing an isotopic enrichment of ∼  2.2 ‰ from ∼  1500 to 500 mm yr −1 . Tracing δ 13 C org values from plant to soils and rivers revealedthat soils and riverbed sediments reflected the transition from mixed C 3 andC 4 vegetation in the dry upper basin to more C 3 vegetation in the humidlower basin. Soil degradation and stabilisation processes and hydrodynamicsorting within the river altered the plant-derived δ 13 C signal.Phytoplankton dominated the δ 13 C org signal carried by SPMin the dry season and year-round in the upper basin. Application of a linearmixing model showed that the %C 4 plants in the different subbasins was ∼  7 %–15 % higher using plant end-members based onmeasurement of the Godavari vegetation and tailored to local moistureavailability than using those derived from data compilations of globalvegetation. Including a correction for the 13 C enrichment in GodavariC 3 plants due to drought resulted in maximally 6 % lower estimated C 4 plantcover. Our results from the Godavari basin underline the importance ofmaking informed choices about the plant δ 13 C end-members forvegetation reconstructions, considering characteristics of the regionalvegetation and environmental factors such as MAP in monsoonal regions.

[学科分类] 大气科学