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Org. N loss was Re:Soil Quality Attributes




Thought I'd respond to your question about organic vs.
conventional N sources.  I did my PhD on exactly that
question.  So, while its only one project, in one location
(central Pennsylvania), I did do a lit. review on the
subject and found *no evidence* for the conventionally held
point of view.  In fact, there's been little research
designed to test this theory.  I think it is a reasonable
idea that if mineralization occurs after crop harvest the
nitrate is at risk for leaching losses, but there is little
research out there that supports this argument.  My
research, in corn cropping systems, no till and till,
alfalfa in rotation, dairy manure or ammonium nitrate
supplying N, found no evidence that the organic sources
of N increased N leaching risks.  In fact, the corn
following alfalfa treatment had lower fall soil nitrate
levels than corn supplied the economic optimum rate of
ammonium nitrate (and grain yields in these two treatments
were the same and high - 10 Mg/ha on average all three years
of the study).  Guess I ought to get around to writing that
up, huh?!  

The issue of taking land out of a pasture (ley) is
different.  Lots of nitrate can be released and lost in this
case.  But as you say, organic management is more than just
*one* practice, and the *system* of covering the land all
year helps to prevent N losses that may occur through
mineralization of organic N during periods of no crop
uptake.

Reasons for no N mineralization after crop harvest in
systems supplied with organic N?  My best guess is that in
Central Penn., fall is dry, crops have pulled all the
moisture out of the soil, low microbial activity.  Once
precip. begins in Nov/Dec,soils too cold to support
mineralization/nitrification.

The project I was working in also directly measured N
leaching in these systems with lysimeters at 1 meter depth
(ie below corn rooting depth) and corroborate my findings. 
That work has continued at Penn State.  The project is now 6
years old.

By the way, the argument that your colleagues present is
also strongly argued by British soil scientists....but I
could find no studies to support the view in the British
literature, either.  Curious, isn't it?

Laura


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