Experimental Protocol

DIRT assesses how rates and sources of plant litter inputs control the long-term stability, accumulation, and chemical nature of soil organic matter in forested ecosystems. To do this, scientists have established treatment plots in several locations world wide where long term litter manipulations are conducted. Although each site has slight variation in treatment due to location specific constraints, the treatment design is as follows:

Experimental Design

Treatment	Method
Control	Normal litter inputs are allowed.
No Litter	Aboveground inputs are excluded from plots.  Conifer sites are screened, deciduous plots are swept in fall.
Double Litter	Aboveground leaf/needle inputs are doubled by adding litter removed from NO LITTER plots.  All plots receive the same litter additions.
Double Wood	Aboveground wood inputs are doubled by adding large shredded wood pieces equal to the C content of the Double Litter additions.
No Roots	Roots are excluded with impenetrable barriers extending from the soil surface to the top of the C horizon.
No Inputs	Aboveground inputs are prevented as in NO LITTER plots; Belowground inputs are prevented as in NO ROOTS plots.

D-DIRT (Dryland DIRT)

Heather Throop coordinates the D-DIRT network — please contact her for more information. We’d love to have more D-DIRT sites join the network! heather (dot) throop (at) asu (dot) edu

Most of the DIRT sites are in closed canopy, mesic forests. The DIRT protocol has been modified for dryland sites where vegetative cover is a mixture of matrix of perennial grasses, bare areas, and shrubs. This difference in vegetation leads to a different experimental design for manipulating aboveground inputs, and also the opportunity to tease apart the impacts of herbaceous and woody plant inputs with separate shrub and intercanopy plots. Due to extremely deep root profiles, no root manipulations are performed.

D-DIRT sites manipulate aboveground litter in two different vegetation patch types: shrubs and shrub intercanopy areas (grass or bare-dominated areas, depending on the study site). Shrub plots are centered around individual shrubs, with the average size of shrub canopies dictating the size of shrub plots in that site. Intercanopy plots are the same size as shrub plots. Plot perimeters are fenced to limit litter transport off plots. An annual litter manipulation is performed during the non-growing season (timing differs among sites depending on phenology).

Treatment	Method
Shrub No Litter	All standing dead is clipped annually and all standing dead/shrub litter is removed.
Shrub Control	All standing dead is clipped annually. Standing dead and shrub litter is redistributed evenly within each control plot.
Shrub Double Litter	Standing dead/shrub litter from the shrub no litter plots is divided among the shrub double litter plots and distributed evenly.
Intercanopy No Litter	All standing dead is clipped annually and all standing dead/litter is removed.
Intercanopy Control	All standing dead is clipped annually and redistributed evenly within each control plot.
Intercanopy Double Litter	Standing dead/litter from the intercanopy no litter plots is divided among the intercanopy double litter plots and distributed evenly.

Grass-DIRT  (DIRT protocol for grasslands)

The DIRT experiment can also be applied to grasslands, and in many ways, Grass-DIRT may be the most effective tool for understanding controls on soil C sequestration in grasslands.  The protocol is based on an experiment begun by Francis Hole in Wisconsin prairies.

Treatment	Method
Unmanipulated control	No treatment applied
Control	Senescent vegetation is cut at the end of the growing season and spread evenly over the surface of the plot.  This controls for the cutting effect that is mandated in the other treatments.
No AG litter	Senescent vegetation is cut at the end of the growing season and is removed, weighed, and saved.  Grasses are cut so as not to harm roots or   regrowth.
No Roots	No plant growth is allowed within the plots through frequent clipping and/or herbicide.  Above-ground litter from NO AG LITTER plots is added to each plot.
No Inputs	No plant growth is allowed within the plots; herbicide is used sparingly   as vegetative growth demands.  This is the same as a bre/fallow treatment common in agricultural studies.
Fertilizer?	Although this is not a standard treatment, all treatments here can be crossed with a fertilizer addition treatment as well to assess the role of mineral nutrition on soil C sequestration.