I hope I can add a few useful ideas to R Nighs interesting comments
on
sawdust, soil ecology and biological N fixation...
My understanding is that a significant feature of the inhibition effect
of mineral N on N fixation by rhizobium is the presence of NO2- (nitrite).
Nitrite is known to bind with and deactivate plant growth
regulator indoleacetic acid (IAA). IAA is a communicator compound produced
by rhizobium in the proximity of legume roots which stimulates root hair
curling. Root hair curling is an initial
step in the infection of legume roots by rhizobium. I believe that
research has shown that the presence specifically of nitrite greatly
inhibits legume root nodulation.
Nitrite is an intermediate product in the process of nitrification
(ammonium ---> nitrate) which is normally only present in very low
concentrations in soil but I think that only trace amounts of nitrite will
strongly inhibit nodulation. Conditions which preferentially interfere
with the second
half of the nitrification process (NO2- ---> NO3-) leading to a
build up of soil nitrite should be especially detrimental to legume root
nodulation.
Once legume roots are fully nodulated my recollection is that they are less
sensitive to the presence of nitrite or other sources of mineral N in the
soil... thus N fertilizer is especially detrimental to legume N fixation
during the nodulation period and less so afterwards...
I am not sure if the nitrite mechanism described above would be relevant
to Azotobacter inhibition by mineral N.
My recollection is that free living N fixers consume approximately 35+ lbs
of C for each lb of N fixed while symbiotic N fixers consume about
6-12 lbs of photosynthate C per lb of N fixed. Thus symbiotic N fixation
is much more efficient...
Suppose azotobacter fixed 50 lbs of N in a Cuban soil at a cost of 1750
lbs of C or 3000+ lbs of OM... this is a very serious consumption of
organic matter... considering all the other heterotrophs competing for the
C substrate, 50 lbs of N fixed seems rather high...
When I was growing vegetables commercially I once had access to a huge
amount of horse bedding that was primarily sawdust...
I did a qualitative experiment (no official data was collected) where I
spread the bedding about 1-2 inches thick and then transplanted broccoli
through the mulch... many of the broccoli did very poorly... but some
of them did well... I dug up some of the plants and observed that the
plants that had been set deeply...i.e their roots were transplanted in
well below the soil/bedding interface, did well... the plants which were
not set as deeply did poorly... very few weeds came through the bedding...
I think that the amount of N immobilization from surface applied sawdust
is relatively low... N immobilization is a function of available C to
available N rather than the total C to N ratio of a substrate. Surface
applied bedding is going to release its available C relatively slowly and
should not have nearly as dramatic an immobilization effect as sawdust
tilled into the soil.
Its probably too late to plant a winter legume (I have forgotten where the
original poster is located) but I would guess that a fall
surface application of the bedding onto soil first broadcast seeded to
hairy vetch might work well... The hairyvetch N should minimize
concerns about intense N immobilization come spring...
Joel Gruver
U of MD
Soil Quality Research
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