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Detritus Input and Removal Treatments (DIRT)
Last Updated 2011, 2005; Originally posted 2002
Soil organic matter (SOM) is the terrestrial biosphere's largest pool of organic carbon and is an integral part of the global C cycle. Therefore, changes in SOM formation and decomposition due to climate change, land management, disturbance or other factors can feed back to the climate system to either sequester CO2 into organic forms or release it to the atmosphere. Despite its pivotal role in the global C cycle, the relative importance and linkages of the various biological, chemical and physical processes regulating SOM balances are not well understood. Although forests contain more than 3x the soil C (575 x 1015 g) of agricultural lands (180 x 1015 g), forest SOM dynamics and their relation to detrital inputs and soil biotic processing remain poorly understood. Even fundamental relationships, such as between mean annual temperature and SOM turnover rates, are controversial. A major challenge for environmental science is to develop a predictive understanding of how climate and vegetation interact to determine how detritus and soil biota affect SOM formation and stability. Over the years, our informal group of ecologists, biogeochemists and ecosystem modelers has been using a common set of experimental manipulations, referred to collectively as DIRT (Detritus Input and Removal Treatments), to assess how rates and sources of plant inputs control the accumulation and dynamics of SOM and nutrients in forest soils over decadal time scales. The original DIRT treatments consist of chronically altering plant inputs to forest soils by regularly removing surface litter from permanent plots and adding it to others. Our network of DIRT sites now includes four operational temperate forest sites . Core DIRT treatments.Treatments are replicated (n = 3). Plot size is 10 m x 15 m.
TREATMENT METHOD CONTROL Normal litter inputs are allowed. NO LITTER Aboveground inputs are excluded from plots by raking. DOUBLE LITTER Aboveground leaves/needles are doubled by adding litter from NO LITTER plots. DOUBLE WOOD Aboveground CWD inputs are doubled by adding shredded CWD (N.B.: AND, CWT, and SIK sites only). NO ROOTS Roots are excluded by inserting barriers around plots in backfilled trenches NO INPUTS Combined treatments from NO LITTER and NO ROOTS (above).
The basic DIRT treatments are adapted from a study started in 1957 at the University of Wisconsin Arboretum (Nielsen and Hole 1963) that has been maintained through the present. These manipulations allow us to experimentally test our hypotheses concerning detrital control on SOM formation and stability. Because these are manipulations and not observations of existing sites, we can follow specific pools of SOM over time to observe the earliest responders, as well as the most stable pools. We can also assess the role of belowground C inputs (roots, rhizosphere organisms, symbionts) to SOM. DIRT plots have been used to assess ecosystem C sources to total respiration (Bowden et al. 1993), the temperature sensitivity of soil respiration (Boone et al. 1998), sources of DOC in a hardwood forest (Aitkenhead and McDowell 2000), role of litter quality in DOC chemistry (Yano et al. 2005), litter effects on the microbial community (Brant 2005), biotic and abiotic effects of coarse woody debris on soils (Spears and Lajtha 2004) among others.