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DIRT: Detrital Input and Removal Treatments

DIRT: Detrital Input and Removal Treatments

Kate Lajtha

May 2005

In 1997, Kate Lajtha, Bruce Caldwell, and Phil Sollins established a long-term field study in an old growth coniferous forest at the H.J. Andrews Experimental Forest to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The central goal of the DIRT project is to assess how rates and sources of plant litter inputs control the accumulation and dynamics of organic matter and nutrients in forest soils over decadal time scales. The project examines processes at multiple levels, across decades and centuries, exploring the intricate interconnections of biology and chemistry that lead to the formation of humic materials over these hitherto unexplored long time spans. Our plots are part of an informal network of similar experimental treatments that span a significant climatic gradient and that encompass both coniferous and deciduous forests, and that vary widely in anthropogenic N loading. Current DIRT sites include Harvard Forest, MA (oak forest, established 1991); Bousson Experimental Research Reserve, PA (deciduous forest, established 1992); Sikfokut Forest, Hungary (turkey oak forest, established 2001), and the Michigan Biological Station, MI (pine forest, established 2004).

We have four main objectives:

  1. to characterize the physico-chemical characteristics of SOM components formed under different detrital input scenarios across these five (and additional) forested ecosystems;
  2. to determine the role of detrital quality and quantity in DOC formation and chemical composition, and the stabilization of DOC components as SOM;
  3. to measure the lability of organic C pools under the different detrital input treatments;
  4. to determine the effects of detrital inputs on the activities of key enzymes responsible for the processing of major C pools.

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:

TREATMENTMETHOD
CONTROLNormal litter inputs are allowed.
NO LITTERAboveground inputs are excluded from plots.
DOUBLE LITTERAboveground leaf/needle inputs are doubled by adding litter removed from NO LITTER plots.
DOUBLE WOODAboveground wood inputs are doubled by adding large shredded wood pieces based on measured input rates of woody debris fall.
NO ROOTSRoots are excluded with impenetrable barriers extending from the soil surface to the top of the C horizon.
NO INPUTSAboveground inputs are prevented as in NO LITTER plots; Belowground inputs are prevented as in NO ROOTS plots.

Results and activities to date:

We have 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. Our results show that 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 narrower 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 m-2 yr-1), slightly less than measured deposition to this very unpolluted forest (~0.2 g m-2 yr-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 other mechanisms, such as increased microbial immobilization, 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 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 soils, suggesting that the additional DOC sorption observed in the field in these soils was stabilized in the soil and not readily lost upon incubation.

Select scientific publications and theses (pdf)