Detrital Controls on Soil Solution N and Dissolved Organic Matter in Soils: A Field Experiment

• Published in: Biogeochemistry 2005-11, Vol.76 (2), p.261-281
• Main Authors: Lajtha, K, Crow, S.E, Yano, Y, Kaushal, S.S, Sulzman, E, Sollins, P, Spears, J.D.H
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; biodegradation; Biological and medical sciences; coarse wood debris; Coniferous forests; Detritus; Dissolved organic carbon; Dissolved organic matter; dissolved organic nitrogen; Earth sciences; Earth, ocean, space; Exact sciences and technology; forest litter; Forest soils; Fundamental and applied biological sciences. Psychology; immobilization in soil; Leachates; Leaching; Leaf litter; mineralization; nitrates; Nitrogen; Old growth; Old growth forests; organic horizons; Organic nitrogen; organic nitrogen compounds; Organic soils; Respiration; Retention; Roots; Soil chemistry; Soil ecology; soil microorganisms; Soil organic matter; Soil profiles; Soil respiration; Soil solution; Soil water; Soils; Solution chemistry; Sorption; Surficial geology; Synecology; Terrestrial ecosystems
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-005-5071-9

We established a long-term field study in an old growth coniferous forest at the H. J. Andrews Experimental Forest, OR, USA, to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The Detritus Input and Removal Treatments (DIRT) plots consist of treatments that double leaf litter, double woody debris inputs, exclude litter inputs, or remove root inputs via trenching. We measured changes in soil solution chemistry with depth, and conducted long-term incubations of bulk soils from different treatments in order to elucidate effects of detrital inputs on the relative amounts and lability of different soil C pools. In the field, the addition of woody debris increased dissolved organic carbon (DOC) concentrations in O-horizon leachate and at 30 cm, but not at 100 cm, compared to control plots, suggesting increased rates of DOC retention with added woody debris. DOC concentrations decreased through the soil profile in all plots to a greater degree than did dissolved organic nitrogen (DON), most likely due to preferential sorption of high C:N hydrophobic dissolved organic matter (DOM) in upper horizons; percent hydrophobic DOM decreased significantly with depth, and hydrophilic DOM had a much lower and less variable C:N ratio. Although laboratory extracts of different litter types showed differences in DOM chemistry, percent hydrophobic DOM did not differ among soil solutions from different detrital treatments in the field, suggesting that microbial processing of DOM leachate in the field consumed easily degradable components, thus equalizing leachate chemistry among treatments. Total dissolved N leaching from plots with intact roots was very low (0.17 g$\text{m}^{-2}$ $\text{year}^{-1}$), slightly less than measured deposition to this very unpolluted forest (∼0.2 g$\text{m}^{-2}$ $\text{year}^{-1}$). Total dissolved N losses showed significant increases in the two treatments without roots whereas concentrations of DOC decreased. In these plots, N losses were less than half of estimated plant uptake, suggesting that other mechanisms, such as increased microbial immobilization of N, accounted for retention of N in deep soils. In long-term laboratory incubations, soils from plots that had both above- and below-ground litter inputs excluded for 5 years showed a trend towards lower DOC loss rates, but not lower respiration rates. Soils from plots with added wood had similar respiration and DOC loss rates as control soi