|dc.description.abstract||Soils are the largest terrestrial store of carbon (C) and changes in this store of C can impact on soil quality and atmospheric CO2 concentrations. Research on C budgets at paddock to national scales has focused most attention on the processes of respiration and photosynthesis in determining the net loss or gain of carbon from an ecosystem. However, leaching of dissolved organic carbon (DOC) is a potentially important component of the carbon budget that is rarely measured when developing carbon budgets, and as a consequence, is often estimated or excluded. Much of the literature indicates that while DOC leaching is important, the loss of DOC from the terrestrial ecosystem may only be small. In the vasose zone DOC that is leached may be adsorbed on to soil and stabilised or may be mineralised, effectively preventing it from leaching from the ecosystem.
The objectives of this thesis were to determine if DOC leaching from the soil of a dairy farm was an important contribution to the carbon budget. To measure this, soil leachate was collected from five paddocks using 100 suction cup lysimeters. These were installed within the footprints of two eddy covariance towers on a dairy farm in Waharoa, Waikato, New Zealand. In general samples were bulked over paddocks, with 10 mL of water from each suction cup contributing to the overall bulked sample. Water extracted from the suction cups was analysed for DOC, total nitrogen, and nitrate. DOC concentration measurements were coupled to the volume of water draining through the soil. The volume of drainage was obtained from a water balance model using measurements of evaporation and precipitation. Leaching from the soil started in mid-May continuing through till mid-November. The total amount of water draining through the soil for the year was calculated to be 990 mm, with a mean concentration of 4.5 ± 0.8 mg L-1 (mean ± SE). The mass of DOC leached was 38 ± 4 kg C ha-1 yr-1 (mean ± SE). The concentration of DOC showed no monthly variation, while the mass of DOC showed a strong seasonal trend, with the greatest mass of DOC leaching during the wet winter period. Ultimately the main driver of DOC leaching at this site was the volume of water draining through the soil, because DOC concentration changed very little.
In order to understand the suite of processes that influence the fate of DOC the subsoil, internal cycling process including mineralisation and sorption of DOC were investigated in the laboratory. Results showed that DOC leached to a depth of 0.65 m could be mineralised by soil microbes lower in the profile, converting it to CO2. The total C respired over a week (12.81 µg CO2-C-1 g soil-1) was 11 times greater than the C added (1.18 µg C-1 g soil-1). In a repeat of the same study the amount of CO2 respired was 25 times greater than the addition of DOC. Additionally sorption experiments indicated that the concentration of DOC lost to the groundwater would be less than the concentration of DOC measured at 0.65 m. Soil water solution with a concentration of 7 mg L-1 DOC mixed with subsoil had a 50% reduction in concentration when shaken for four hours with Te Puninga soil. Similar results were found in the Piarere soil with a 34% reduction in DOC concentration. In contrast when both soils were shaken with DOC (4 mg L-1) in a second experiment, there was a small amount of net desorption. There was potential for the soils at this site to reduce the concentration of DOC leached from 0.65 m, through adsorption of DOC onto the soil. Subsequently sorption would have caused a reduction in the DOC mass lost. While results from laboratory studies were variable it was clear that both sorption and mineralisation in subsoils will moderate leaching losses of DOC to groundwater.
In the context of a paddock scale C budget, where the atmospheric exchange of C through respiration and photosynthesis was about -880 kg C ha-1yr-1, leaching of 38 kg C ha-1 yr-1, represents 4.5% of the total exchange. Compared to the net ecosystem carbon budget, which included farm inputs and outputs, of a similar intensive grazed system, DOC leaching is equal to 3-15% of the total. However as DOC leaching at 0.65 m does not accurately represent a leaching loss from the system, as sorption and mineralisation can further alter the mass leached, the contribution of DOC loss through leaching to the carbon budget is comparably small and does not represent a significant component of the C budget at this site.||