Temporal patterns in the chemistry, flux and molecular characteristics of dissolved organic carbon in drainage water from an upland peat system

摘要

A small peat and ranker catchment upon Great Dun Fell, North Pennines, UK was chosen as a site to monitor the soil water chemistry dynamics of an upland system. The site is a small catchment (approx. 400m[2]) and contains a number of small, naturally occurring peat pools. Two pools were selected (X and Y) and four soil water wells were installed at the site. These were sampled monthly (weather permitting), with bulk samples (25 litres) of X and Y also being taken to be isolated using the macroporous non-ionic resin method. Using the isolation procedure of Leenheer (1981) the DOC was fractionated into humic, fulvic and hydrophilic acids. At a site close to the study catchment monthly rain collections were made and an automatic weather station measured daily average air temperatures and precipitation amounts. Pool samples were measured for [13/14]C, DOC, hydrophilic DOC, pH, A[340] and selected metals and anions. Wells were analysed for DOC, hydrophilic DOC, pH, A340 and selected metals. With rain samples being monitored for A[340], pH and selected metals and anions. The isolated humic substances were characterised for elemental composition (C,H and N), weight average molecular weight, A[340]gC[-1] and functional groups titratable between pH 3-7 and 7-11. Using a version of a dilution gauging experiment soil water flows were measured. Using this data a rainfall run-off model (the hydrological subroutine of CHUM, Tipping, 1996) was fitted to the hydrology of the system. The model produced soil water flows from inputs of daily average air temperature and daily precipitation volumes (measured by the automatic weather station). Using soil water flow rates and DOC concentration measurements it was possible to estimate DOC flux. Patterns in pool, well water chemistry and molecular characteristics of DOC isolates were compared to rainfall, temperature and soil water flow in an attempt to identify processes and factors which mediate DOC production and flux. Using data from the system studied it was possible to draw some conclusions about some of the processes occurring. During the summer, when soils were relatively warm and dry DOC production appeared to be promoted. However, extreme drought restricted DOC production. The majority of DOC flux occurred towards the end of summer or early autumn when elevated rainfall amounts increased soil water flow rates. During the summer DOC became proportionally more hydrophilic and the molecular weight of the humic fraction increased. It was thought that these observations were indicative of increased microbial activity in the soil. Strong drought conditions result in an influx of oxygen into the deeper (usually anoxic) peat layers. This oxidises reduced forms of sulphur already present in the soil producing a large increase in sulphate concentration. The influx of oxygen also alters the decomposition process, producing DOC that is up to 50% less coloured than before. The carbon isotope data indicates that the majority of DOC is modern (i.e. originating from plant material no older than 45 years). This suggests that the majority of DOC production occurs in the very surface layers of the peat profile.