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

The large difference in the fractionation of stable carbon isotopes between C$_3$ and C$_4$ plants is widely used in vegetation reconstructions, where the predominance of C$_3$ plants suggests wetter and that of C$_4$ plants drier conditions. The stable carbon isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the stable carbon isotopic value of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the bulk carbon isotopic values of C$_3$ and C$_4$ plants ($\delta ^{13}$ C) and of organic carbon ($\delta ^{13}$ C$_{org}$) in soils, river suspended particulate matter (SPM) and riverbed sediments to gain insight into the control of precipitation onC$_3$ and C$_4$ plants ($\delta ^{13}$ C values and to assess changes in $\delta ^{13}$ C$_{org}$ values along the plant–soil–river continuum. This information allows us to elucidate the implications of different $\delta ^{13}$ C end-members on C$_3$/C$_4$ vegetation reconstructions. Our analysis was performed in the Godavari River basin, located in the core monsoon zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The basin has distinct wet and dry seasons and is characterised by natural gradients in 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 $\delta ^{13}$C values of Godavari 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 $\delta ^{13}$C$_{org}$ values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C$_3$ and C$_4$ vegetation in the dry upper basin to more C$_3$ vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived $\delta ^{13}$C signal. Phytoplankton dominated the $\delta ^{13}$C$_{org}$ signal carried by SPM in the dry season and year-round in the upper basin. Application of a linear mixing model showed that the %C$_4$ plants in the different subbasins was ∼ 7 %–15 % higher using plant end-members based on measurement of the Godavari vegetation and tailored to local moisture availability than using those derived from data compilations of global vegetation. Including a correction for the $^{13}$C enrichment in Godavari C$_3$ plants due to drought resulted in maximally 6 % lower estimated C$_4$ plant cover. Our results from the Godavari basin underline the importance of making informed choices about the plant $\delta ^{13}$C nd-members for vegetation reconstructions, considering characteristics of the regional vegetation and environmental factors such as MAP in monsoonal regions.ISSN:1726-4170ISSN:1726-417