From leaf to soil: &amp;lt;i&amp;gt;n&amp;lt;/i&amp;gt;-alkane signal preservation, despite degradation along an environmental gradient in the tropical Andes

Boris Jansen,William D Gosling,Milan L Teunissen Van Manen,Francisco Cuesta,Susana León-Yánez

doi:10.5194/bg-17-5465-2020

Abstract

Abstract. The relative abundance of n-alkanes of different chain lengths obtained from ancient soils and sediments have been used to reconstruct past environmental changes. However, interpretation of ancient n-alkane patterns relies primarily on modern plant wax n-alkane patterns measured from leaves. Little is still known about how n-alkane patterns, and environmental information therein, might be altered during the process of transfer from leaves into soil. We studied the n-alkane patterns extracted from leaves, necromass, and soil samples from an altitudinal gradient in the tropical Andes to clarify if the n-alkane pattern, and the local environmental information reflected, is altered as the plant source material degrades. We considered the (dis)similarity between n-alkane patterns in soil, necromass, and leaves and specifically explored whether a temperature and/or precipitation signal is reflected in their n-alkane patterns. The n-alkane patterns showed degradation in soil as reflected by a reduced carbon preference index (CPI). The lower CPI in soils as compared to leaves and necromass was significantly correlated with temperature and precipitation along the transect, most likely because of increased microbial activity under warmer and wetter conditions. Despite degradation, all sample types showed a systematic shift in longer vs. shorter n-alkanes when moving up the transect. Further examination revealed the systematic shift correlated with transect temperature and precipitation. Since transect vegetation is constant along the transect, this would appear to indicate the recording of a climatic signal within the n-alkane patterns that is preserved in the soil, albeit that the correlation was weaker there. The study results warrant further research into a possible underlying causal relationship that may lead to the development of n-alkane patterns as a novel palaeoecological proxy.

Highlights

Making accurate reconstructions of past environments is important and urgent, as these can inform how modern environments could respond to current climatic and land use changes (Cronin, 2014)
The similarity between the overall n-alkane patterns as well as the average chain length (ACL) in leaf and soil samples shows that such degradation did not obliterate the n-alkane patterns
The fact that the ACL and ratio metrics correlated significantly with the temperature and precipitation gradient along the transect in all sample types shows that the environmental information stored in the n-alkane patterns was preserved along the journey of the n-alkanes from leaf to soil

Summary

Introduction

Making accurate reconstructions of past environments is important and urgent, as these can inform how modern environments could respond to current climatic and land use changes (Cronin, 2014). Interpretations of past environmental change are commonly obtained from fossil pollen, charcoal, and molecular proxies extracted from sediments and soils (Smol et al, 2001). To help improve these reconstructions of past environments and environmental change, the development of molecular proxies such as plant wax n-alkanes has seen much research attention over the last two decades (Jansen and Wiesenberg, 2017). The n-alkanes produced by plants transfer to the soil and sedimentary records, where they are preserved and from which they can be extracted (Jansen and Wiesenberg, 2017). The retention of fossil nalkanes in soils and sediments make n-alkanes a promising proxy to help reconstruct environmental change

Methods

Results

Conclusion