The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 256

K.B.McTiernanet al. Dissolved Organic Carbon Losses from Grazed Grasslands

4.5

Dissolved Organic Carbon
Losses from Grazed
Grasslands

K.B. M C T IERNAN 1 , S.C. J ARVIS 1 , D. S CHOLEFIELD 1
AND M.H.B. H AYES 2

1
Institute of Grassland and Environmental Research, NorthWyke
Research Station, Devon EX20 2SB, UK; and 2 Chemical and
Environmental Sciences, Foundation Building, University of
Limerick, Ireland

Introduction

Soil organic matter (SOM) is a complex mixture of senescent residues of
plants, animals and microorganisms through to highly humified material
hundreds of years old. These varied materials are broken down simulta-
neously, but at different rates, with most of the products of degradation
being more susceptible to dissolution in soil water than the original
material. Dissolved organic matter (DOM) provides an important energy
substrate for microbial communities to undertake transformation processes
(e.g. for denitrification; Burford and Bremner, 1975) and also controls the
availability of nutrients and trace metals to plants and microorganisms
through formation of complexes (Stevenson, 1994). Environmental
implications of DOM are associated with the transport of elements and
compounds that are potential pollutants. For example, DOM has been
shown to contribute to the mobility of some metals (e.g. Temminghoff
et al ., 1997), polycyclic aromatic hydrocarbons (e.g. Johnson and Amy,
1995) and nutrients such as N and P (Qualls and Haines, 1991). Addi-
tionally, knowledge of the potential fluxes of soluble organic C in different
soils is essential to improve global C budgets (Han and Thompson, 1999).
Several studies have reported on DOM leaching from forest soils (e.g.
McDowell and Likens, 1988) and on DOM in surface waters (see
Thurman, 1985 for references). Hope et al . (1994) calculated that, for most
catchments, the annual flux of organic C (OC) in streams varied from 10 to
100 kg C ha −1 year −1 . However, it is not known whether exports of OC
from managed grasslands fall within this range and the extent of variation