The Carbon Quandary: Essential and Detrimental - Sustainable Design: The Science of Sustainability and Green Engineering - page 287

that trees are central in the figure. At their base is detritus, where microbes are
breaking down complex molecules. Nutrients in the soil are transported to
the tree's cells by the roots' capillary action, and gases are transpired through
leaves back to the atmosphere.

NUTRIENT LOSS

Trees play an important role in holding nutrients in sinks. Nitrate can be a
good indicator of nutrient loss in an ecosystem (see Table B7.3). Combinations
of trees and ground cover can result in significantly different nutrient loss than
that from trees alone. One recent study indicated that considerably higher
losses of nitrate occur early in the growing season in row crops than in tree
plots. With time, the nitrate loss in these plots was reduced to levels similar to
that in tree plots. Similar results were obtained with ammonium nitrate and
phosphorus losses and are explained by the fertilization regimen. *

Table B7.3 Average Nitrate Loss (g ha − 1 ) for Five Selected Months by Plant Cover from
Limestone Valleys of the Tennessee Valley Region a

Trees

Month Corn Switchgrass Without Cover Crop With Cover Crop

May 275 477 1 41

July 4 44 9 2

September 1 0 11 3

November 5 12 1 0

January 6 0 2 0

a The soils are moderately to severely eroded Decatur silty clay loam, undulating phase, with slopes
averaging 2.5 to 3%. The area has been under cultivation for at least the past 15 years.

Table B7.3 also demonstrates the diversity of plants in nutrient storage.
However, growing trees may not provide the initial protection expected
previously. If erosion protection is required, the use of either switchgrass
or trees with cover crops is recommended. Trees can be grown successfully
with a cover crop between the rows if care is taken to keep the tree row itself free

* T. H. Green, G. F. Brown, L. Bingham, D. Mays, K. Sistani, J. D. Joslin, B. R. Bock,
F. C. Thornton, and V. R. Tolbert, “Environmental impacts of conversion of cropland to
biomass production,” Proceedings of the 7th National Bioenergy Conference: Partnerships to Develop
and Apply Biomass Technologies , Nashville, TN, September 15-20, 1996. Note that both corn
and switchgrass plots were fertilized the first year, while fertilization of the tree plots was delayed
until the second year.