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Organic Matter Leaching as a Component of Carbon, Nitrogen, Phosphorus, and Sulfur Cycles in a Forest, Grassland, and Gleyed Soil¹

ABSTRACT

The profiles of a forest (Typic Cryoboralf), grassland (Aridic Haploboroll), and gleyed (Argiaquoll) soil were examined to assess the role of organic matter leaching in determining organic matter composition in genetic horizons of soil profiles. An increasing proportion of NaOH extractable C, N, P, and S was found in the fulvic acid (FA) fraction as depth in the profiles increased, suggesting that fulvic acids produced in biologically active surface horizons have been translocated to B and C horizons by percolating water. The fulvic acid in B and C horizons was highly enriched in N, P, and S relative to the humic acid and fulvic acid in surface horizons, indicating that the organic matter being translocated downward is richer in N, P, and S than fractions not susceptible to leaching. Leaching of nutrient-rich organic matter thus provides a valid explanation for the observed narrowing of soil organic matter C/N, C/P, and C/S ratios with increasing depth. The lowest HA/FA ratios coincided with the depth of CaCO₃ accumulation, revealing a possible role of Ca and other basic cations in precipitation of organic matter from solution. Of the four elements studied, P appeared to be the most susceptible to deep leaching in the organic form. This is believed to reflect the predominance of organic P in the low molecular weight, mobile fraction of soil organic matter. Loss of labile, nutrient-rich organics from surface horizons by leaching may be important in controlling element turnover rates and nutrient distribution within soil profiles. It may also represent a major nutrient export mechanism.

¹ Contribution no. R 501 from Saskatchewan Institute of Pedology, Univ. of Saskatchewan, Saskatoon, Canada S7N OWO.

² Graduate Student, and Professor of Soil Science, respectively.

Received for publication August 25, 1986.

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