Fate and transport of lignin in the soil-water continuum

Until recently plants and trees on land absorbed much of the increasing carbon dioxide emissions caused by the burning of coal, oil and gas. However vegetation can only absorb so much carbon and there is also an awareness that global sinks may shrink in the future. This could lead to a runaway positive feedback on global climate with the very serious threat of abrupt climate change. What can we do about it? In addition to cutting emissions we can try to restore and even increase the strength of the terrestrial soil sink. There is more carbon stored in the soil than there is in all the vegetation on earth. This carbon is sourced from cellulose and lignin which are being constantly formed in the cell walls of land plants by fixing carbon dioxide from the atmosphere. The conventional wisdom is that lignin is more refractory than cellulose but both biopolymers can be preserved, albeit often in a degraded state, in soil organic matter (SOM). There is, therefore, a huge potential for soils to absorb or sequester carbon. The Intergovernmental Panel on Climate Change has estimated that 23 to 44 Gt C could be sequestered in agricultural soils by the year 2050. This could be magnified with appropriate land/water management strategies - one of these might be to inhibit losses of dissolved organic matter (DOM) by preventing water losses from soils. But for that approach to be effective a much deeper scientific understanding is required, especially of the biogeochemical aspects of the soil/water continuum. In this project, we will investigate the form lignin takes as it is mobilised in the aqueous phase in soils, as well as the dynamics of its sequential transport from litter to soil to watercourse. Although there have been many studies on lignin in forest and grassland soils, surprisingly little is known about its contribution to DOM even though lignin phenols (the primary products of intermonomer bond breaking in this important aromatic biopolymer) are water soluble. We are particularly interested in the dynamics of the transportation of its degradation products and how differences in the chemistry of different types of plants (and of the soil they produce) affect how quickly DOM is lost from soil and enters into streams or rivers. Our field sites are the experimental sites at North Wyke Research in Devon (including the Rowden experimental field scale lysimeters; a facility of international importance). These provide a variety of soil and vegetation types (sandy v clayey, tilled v non-tilled, new plantation forest v coppice v established forest soil and vegetation types). Using the full range of variation, we will analyse in detail the chemistry of the litter of a wide variety of decomposing plants. We will also examine changes in chemistry and rates of soil accumulation in short cores that provide a historical record of the past hundreds of years or so. In addition, we will track how the chemistry of lignin changes as it is degraded in the litter and is then deposited in the soil where it can be solubilised in whatever form as it enters the water system. The results of these studies will help us to predict ways of minimising carbon losses from this very important terrestrial sink in the face of future climate change.