Dissolved organic carbon in soil solution

 

Introduction

Diagram illustrating soil water chemistry equipment installation at Forest Research’s Intensive Forest Monitoring plots (N. Barsoum)

Dissolved organic matter (DOM), usually measured as dissolved organic carbon (DOC), is a significant product of organic matter decomposition in soils.  Transport of DOM from the surface soil is a significant component of the carbon cycle, contributing to C storage in deeper horizons, and being transported to surface waters where it has a number of chemical and biological functions. 

Recent years have seen widespread increases in DOC concentrations in surface waters of the UK and elsewhere, suggestive of a common response to one or more environmental variables. 

Because of its binding properties, DOM also plays a role in the transport, geochemical reactivity and bioavailability of metals and organic compounds.  Dissolved organic matter also contributes significantly to soil nutrient cycling.

Aims

Quantification of DOC concentrations and fluxes in UK topsoils is one of the aims of the PhD research of Sarah Buckingham, a student at the Centre for Ecology and Hydrology and Lancaster University.  She has carried out fieldwork at a number of moorland sites, and combined her results with data from the monitoring programmes of Forest Research (Intensive Forest Monitoring plots – contact N. Barsoum nadia.barsoum@forestry.gsi.gov.uk) and the UK Environmental Change Network.

Annual average DOC concentrations range from less than 10 g m^-3 (= 10 mg l^-1) at the Coalburn and Grizedale Intensive Forest Monitoring plots to 100 g m^-3 at Thetford intensive forest monitoring plot.  Broadly speaking, the concentration is inversely proportional to annual runoff.  Thus, for six of the forested plots, the annual flux is similar, averaging about 10 gC m^-2 yr^-1.  The exception is the Tummel intensive forest monitoring plot, where the flux is appreciably higher. However, at each site the flux varies considerably between them, depending on the runoff.  The low fluxes for 2003 reflect the low rainfall in that year.

Results

• Analysis of the data for a total of 21 forest and moorland sites, leads to the following conclusions.
• Annual average concentrations of DOC in topsoils at 21 moorland and forest sites in the UK, derived from data obtained with tension samplers between 2000 and 2006, ranged from 1.3 to 97.5 g m^-3.  The mean concentration for the 10 moorland sites was 19.5 (standard deviation 15.2) g m^-3 while that for the forest sites was 27.6 (sd 23.3) g m^-3.
• For sites with data for three or more years, inter-annual differences in average DOC concentration were small (average 1.5-fold).  The highest concentrations (often several times the minimum values) occurred during summer.  The concentrations during summer (April to September) are on average 17 % greater than those in winter (October to March).
• If data from two of the sites are ignored, the DOC concentrations are strongly inversely related to run off, approximately consistent with a constant average DOC flux at all sites.  One of the two exceptional sites is the only peatland of the study, while the other is a forest on mineral soil with unusually high DOC concentrations.
• Fluxes of DOC could reliably be estimated by combining the DOC concentrations with rainfall and evaporation data, either by summing results for one- or two-week periods, or simply as the product of mean annual DOC concentration and annual run off.
• The DOC fluxes ranged from 2.2 to 71.9 gC m^-2 yr^-1 over all sites and years, with overall means of 19.2 (sd 13.6) and 12.2 (sd 13.9) gC m^-2 yr^-1 for the moorland and forest sites respectively.  If the two exceptional sites were omitted, the overall mean was 9.1 gC m^-2 yr^-1 with a relatively small standard deviation of 4.9 gC m^-2 yr^-1.
• For all the sites where data for three or more years were available, DOC flux was strongly dependent upon runoff, varying by up to fourfold between years.  On average, 75.5 % of the DOC was exported during the winter period (October to March).

All the sites show an intriguing increase in DOC concentration with time, reminiscent of those in surface waters seen over much longer time periods.  The time period is too short for these to be considered sustained trends, and continued monitoring is needed to establish their possible significance.

Funders and partners

The project is funded by a NERC studentship. Forest Research is a partner in this project, providing DOC data from some of the forested plots.

Contact

Sarah Buckingham’s supervisors are Professor Edward Tipping (CEH) and Dr John Hamilton-Taylor (Lancaster University).

E-mails:  sbuck@ceh.ac.uk,  et@ceh.ac.uk,  j.hamilton-taylor@lancaster.ac.uk