Re: Describing N behavior
E. Ann Clark, Associate Professor (ACLARK@CROP.UOGUELPH.CA)
Fri, 24 Feb 1995 13:03:32 EDT
Nancy Grudens Schuck - nice contribution; nice insight. If organic
matter were in fact such a pollution-prone entity, then how would
soil N ever accumulate? I am reminded of one of the Rothamsted
studies where, after years of continuous annual cropping to wheat -
which predictably drug down soil OM and N levels - the researchers
just stopped cropping entirely and left the plots to back to
wilderness" (their word). Over an 80 year period, they monitored
soil N levels and found a linear rate of increase in soil N equal to
about 45 kg N/ha/year (roughly; this very striking graph is in one of
the chapters in Stevenson 1982 Nitrogen in Agricultural Soils). This
suggests a very tight system as far as N retention goes, because all
of this N was arriving naturally - no fertilizer, manure, etc..
What seems to make our arable systems "N leaky" - apart from the
application of N itself - is mechanical cultivation or disruption,
which impacts on many microbially-responsive aspects, including soil
T, moisture, aeration, etc. Maybe that, more than the source of N,
should be the focus of systems and operations designed to enhance N
retention/cycling instead of loss? There is a nice chapter by
Woodmansee in a book whose name I've forgotten, about 10 years old,
which details the attributes of natural systems which enhance
nutrient retention. Very insightful. Thanks again, Nancy, for your
contribution to the dialogue. Ann
ACLARK@crop.uoguelph.ca
Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON N1G 2W1
Phone: 519-824-4120 Ext. 2508
FAX: 519 763-8933