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DIVISION S-7 - FOREST & RANGE SOILS

Soil Solution Response to Nitrogen and Magnesium Application in a Scots Pine Forest

Department of Soil and Water Sciences, Agricultural University of Norway, P.O. Box 5028, N-1432 Ås, Norway

* Corresponding author (live.semb.vestgarden{at}ijvf.nlh.no)

High inputs of N to N-limited forests may disturb the nutrient balance of the plant-soil system. The objective of this study was to quantify the effects of N and Mg applications on soil water chemistry in a 35- to 45-yr-old Scots pine (Pinus sylvestris L.) forest in southern Norway. Fertilizer applications began in 1990 and soil water sampling was carried out in May through October 1997 to 1999. Nine years with annual (one dose) inputs of 30 kg N ha^-1 yr^-1 increased the annual average nitrate N (NO₃–N) leaching below the B horizon to 2.1 kg ha^-1, compared with 0.1 kg ha^-1 in the control plots. An input of 90 kg N ha^-1 yr^-1 resulted in symptoms of N saturation. The soil water concentration of NO^-₃ and ammonium (NH⁺₄) increased, and the annual average leaching of inorganic N from May to October during the years 1997 to 1999 was 15 kg N ha^-1. The N addition significantly reduced the concentrations of dissolved organic carbon and N (DOC and DON), but the contribution of DON to the total N leaching from the plots receiving the highest N dose was still large. Addition of 1.5 kg ha^-1 yr^-1 of Mg increased the DOC and DON concentrations. The reason for this is unknown, and further studies are needed to understand DOC and DON dynamics. Even though applications of high single N doses are not directly comparable to chronic N depositions of small doses in precipitation, the results indicate future consequences to pine forests growing under similar conditions.

Abbreviations: DOC, dissolved organic carbon • DON, dissolved organic N