Reverse Fertilization

Reverse Fertlization
(Nitrogen Immobilization by Carbon Ammendment)
OSU Graduate Student Jack Averett’s Master’s Thesis Research
2001-2003 (results published 2004)

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with non-prairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast-growing exotics at the expense of slow-growing prairie species characteristic of low N soils. This experiment tested whether reducing N availability through soil carbon amendments (tilled-in sawdust) could be a useful tool in facilitating successful tallgrass prairie restoration.

The basic principle of soil carbon amendment is that druing the time that the sawdust is being decomposed by bacteria and fungi, nitrogen that would otherwise be available to fertilize plants instead in incorportated into the decomoosing organisms’ bodies. Thus the N is immobilized. (The process is sometimes called “reverse fertilization.”

This is the site before any preparation. It is located just west of the Shelter House. (Note: this is the old wooden sheltert house that burned down in March 2005.)

The researchers  added 6 kg/m² sawdust to experimental plots on a portion of the Ohio State Marion Prairie characterized as “an abandoned agricultural field in the Sandusky Plains of central Ohio.” Below, see images of the principal investigator’s pickup truck being loaded with sawdust at a local sawmill, and unloading at the Prairie.

Site preparation incolved uniformly rototilling the ground, and adding sawdust to the treament plots.

Here’s a map of the treatment plots. It shows eight  blocks each with four 2 meter x 2 meter treatment areas randomly assigned as ammended (yellow) ior unammended.

 

 

A seed mix was prepared using wild-collected seed. Each species was weighed and added to the mix to create equalu numbers of seeds. Seeds were broadcast in mid-winter, thus giving them ample time to be naturally stratified (cold treated to overcome barriers to germination).

This treatment increased soil carbon 67% in the upper 15 cm, causing a 94% reduction in annual net N mineralization, and a 27% increase in soil moisture. However, it had no effect on total N or pH.

Here are the plot as seen from the roof of the old shelter house.

…and here is the old shelter house as seen from the research plots.

Field work consisted of recording data in species composition, sampling soill for C and N analysis, and harvesting plants for biomass determinations.

Plant mass after one growing season was reduced  64%% on amended compared to unamended soil but this effect was less for prairie forbs (-34%) than for prairie grasses (-67%) or exotics  -62%). After the 2nd growing season, the only vegetation category that responded significantly to soil amendment was exotics, which were reduced 40 percent. The N concentration of green leaf tissue and of senescent leaf litter was also reduced by the soil carbon treatment in most cases. The researchers concluded that soil carbon amendment imparts these immediate benefits for tallgrass prairie restoration: reduced N availability, slower plant growth, and lower competition from exotic species.

 

Reference: Averett, Jack & Klips, Robert & Nave, Lucas & Frey, Serita & Curtis, Peter. (2004). Effects of Soil Carbon Amendment on Nitrogen Availability and Plant Growth in an Experimental Tallgrass Prairie Restoration. Restoration Ecology. 12. 568 – 574. 10.1111/j.1061-2971.2004.00284.x.