Journal of Conference Abstracts

Does Nitrogen Deposition Affect the Leaching of Dissolved Organic Matter in Spodosols? Results of Measurements in a Long-Term Nitrogen Fertilisation Experiment

Dan Berggren¹ (Dan.Berggren@mv.slu.se), Bo Bergkvist² (Bo.Bergkvist@planteco.lu.se) &
Lars Johnsson¹ (Lars.Johnsson@mv.slu.se)

¹ Dept of Soil Sciences, Swedish Univ. of Agricultural Sciences, Box 7014, S-750 07 Uppsala, Sweden.

² Dept of Ecology, Plant Ecology, Ecology Building, S-223 62 Lund, Sweden.

The deposition of nitrogen (N) will probably continue to be high in many areas in Europe and North America in the foreseeable future. To date, little is known about how this situation will affect the decomposition of soil organic matter (SOM) and the leaching of dissolved organic matter (DOM). We have measured concentrations of dissolved organic carbon (DOC) and the chemical characteristics of DOM in soil solutions collected in a long-term N fertilisation experiment. The field site is located in a Norway spruce forest in central Sweden (Stråsan; Tamm, 1991). The soil has been classified as a Typic Haplorthod. Ammonium nitrate has been added yearly, since 1967, to replicated plots in (average) doses of 36 (N1), 73 (N2) and 102 (N3) kg ha^-1 yr^-1. The NH₄NO₃ application ended in 1990 in N2 and in 1992 in N3, but is still continuing in N1. Measurements were done during the period 13 June to 8 November 1995. Bulk precipitation and throughfall samples were collected by plastic funnels. Gravitational solution leaving the O horizon was collected by zero-tension plate lysimeters. In addition, soil solution was sampled from the bottom of the O and B horizons using tension lysimeters (Rhizon SMS and PRENART, respectively). Samples were collected once every second week.

Samples were analysed for DOC, dissolved organic N, NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, NO₃^-, SO₄^2-and Cl^-. In addition, DOC was fractionated into hydrophobic acids (HoA) and neutrals (HoN), and into hydrophilic acids (HiA), neutrals (HiN) and bases (HiB) according to Leenheer (1981). For a few samples the content of low-molecular acids was analysed (Shen et al., 1996).

The N fertilisation approximately doubled the amount of organic C accumulated in the O horizon. The C/N ratio of SOM decreased significantly, from 39 (Oe, control) to 25 (Oe, N₂). The corresponding changes in the Oa horizon was from 41 to 30. The C/N-ratio trend indicates structural changes of the organic material. Also, the internal N cycling increased considerably by the treatment. Despite these obvious effects of N addition on the N and C dynamics, there was no treatment effect on the amount or composition of DOM in soil solutions collected from the O horizon. The concentration of DOC in soil solutions obtained with the tension lysimeters was significantly higher than in gravitational solutions leaving the O horizon; mean concentrations ranged from 38 to 54 mg L^-1 in tension lysimeters and from 27 to 36 mg L^-1 in zero-tension lysimeters. Organic acids (HoA + HiA) were the dominating DOM components (~90%). Low-molecular organic acids constituted only a minor fraction (¾1%).

Interestingly, we found no correlation between DOC concentrations in solutions sampled with tension lysimeters and DOC concentrations in solutions sampled with zero-tension lysimeters. This suggest that the two lysimeter types to a large extent sampled solution of different origin. Tension lysimeters withdraw 'capillary water' held in the micropores, whereas the zero-tension lysimeters collect the gravitational water leaving the O horizon. The quality of DOM indicates that the gravitational water, at least partly, can be viewed as a mixture of throughfall and 'capillary water'. The composition of DOM in the zero-tension lysimeter solutions was intermediate of that in throughfall and tension lysimeter solutions. The percentage of HoA increased in the following order: throughfall (41) < zero-tension lysimeter (47) < tension lysimeter (58), whereas there was an opposite trend for the percentage of HoN: tension lysimeter (2) < zero-tension lysimeter (8) < throughfall (12).

In conclusion, the results of this investigation lend no support for the hypothesis that an elevated nitrogen deposition affects the quality or quantity of DOM leached from Spodosol O horizons.

References

Leenheer, J.A., Environ. Sci. Technol. 15, 578-587 (1981).

Shen, Y., Obuseng, V., Grönberg, L. & Jönsson, J.A., J. Chromatogr. 725, 189-198 (1996).

Tamm, C.O., Ecological Studies 81, 1-115 (1991).