Diverse Soil Carbon Dynamics Expressed at the Molecular Level

Diverse Soil Carbon Dynamics Expressed at the Molecular Level

Abstract The stability and potential vulnerability of soil organic matter to global change remain incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on soil organic matter dynamics in a temperate and subalpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids imply that they reflect a highly stable carbon pool. Furthermore, marked radiocarbon variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different soil organic matter pools. Extremes in soil organic matter dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding soil organic matter dynamics in ecosystems potentially vulnerable to global change.

Plain Language Summary Soil carbon forms the largest amount of organic carbon stored on land. In the context of climate change, it is important to know how stable the carbon in this large reservoir is. In this paper we try to attain a better understanding of the stability of soil carbon in a warm and cold area by looking at specific molecules, soil lipids. By measuring the age of these molecules, using radiocarbon, and comparing it to all environmental information, we attain more insight into the soil carbon stability. We found that the molecules show a wide range of ages, indicating they reflect a wide range of sources. These molecular markers may constitute a cleaner method to assess carbon stability than methods that were used previously. They can also indicate the contribution of extremely old, fossil, carbon derived from carbon-holding rocks. Altogether, this paper presents a new approach to tackle soil carbon stability and showcases new insights gained from this approach.

One. Introduction

Soil organic matter constitutes the largest terrestrial reservoir of organic carbon, and with ongoing climate and land use change it is essential to attain a better understanding of its stability and dynamics, particularly with respect to the most stable carbon pools. However, the inherently complex nature of soil organic matter has confounded attempts to assess potential responses to global change, with timescales of terrestrial carbon turnover remaining one of the largest sources of uncertainty in climate model predictions. Furthermore, while much effort has focused on surface soils, knowledge of the dynamics of the deep soil carbon pool remains particularly elusive despite its key importance in the carbon cycle. In order to address and disentangle the complex sources and processes contributing to soil organic matter behavior, operationally defined soil carbon pools are often separated, assuming that they shed light on stabilization mechanisms.

The value of combining radiocarbon measurements on bulk soil organic matter and on specific operationally defined fractions for assessment of carbon pools has been previously demonstrated. Radiocarbon constitutes a uniquely powerful tool because it allows for the assessment of carbon dynamics on decadal to millennial timescales owing to the bomb spike and natural radioactive decay, respectively.

Though fraction-specific data yields improved insights into the dynamics of specific pools as compared to only bulk measurements, it has drawbacks. For instance, in supposedly labile pools, very stable material, charcoal, can be present. It has been hypothesized that increased temperatures accompanying climate change may lower activation energies necessary for organic matter breakdown, promoting destabilization of previously recalcitrant organic carbon. Previous radiocarbon analysis of aliphatic hydrocarbon fractions revealed that these compounds are consistently older than bulk organic carbon, suggesting that they reflect a slowly cycling, passive, carbon pool in soils. Long-chain n-alkyl lipids, such as C twenty-six plus n n-alkanoic acids and C twenty-five plus n n-alkanes, in soils and aquatic sediments dominated by terrestrial inputs are thought to be exclusively derived from higher plant leaf waxes and may serve as diagnostic markers for stable mineral-bound fractions of soil organic matter due to their hydrophobic characteristics. In contrast, shorter-chain homologues may derive from different biological sources that reside in or trace other soil organic matter pools. For example, short- and medium-chain carboxylic acids, C sixteen to C twenty-two, may derive from plant, microbial, or root inputs. Radiocarbon measurements of soil lipid compound classes by Rethemeyer et al. have demonstrated the potential of this class of lipid biomarker in identifying soil organic matter source material.

Recent work has illustrated the utility of specific biomarker compounds as indicators of soil organic matter compositional alteration under global change, and as tracers of large-scale export of terrestrial organic matter from river drainage basins. With the advent of compound-specific radiocarbon analysis, the potential exists to probe soil organic matter dynamics at the molecular level. Overall, there is growing recognition of the potential of biomarkers in soil, carbon, studies, including the insights that can be gained from compound-specific radiocarbon dating. Furthermore, there is a clear need for an improved understanding of relationships between diagnostic marker compounds and different operationally defined or mathematically modeled soil organic matter pools. In this study, we examine the radiocarbon signatures of lipids, including n-alkanes and n-alkanoic, fatty, acids, in soils from two forest ecosystems. The data are assessed within a framework of ancillary information, including existing and new bulk-and fraction-specific radiocarbon data. We seek to gain a better understanding of carbon dynamics in both top and deep soils, as well as to explore the potential of biomarkers as tracers of specific carbon pools.