Agriculture Reference
In-Depth Information
Most plants in cold and temperate regions belong to the C3 group while, in the tropics,
grasses usually belong to the C4 group and trees and shrubs to the C3 group. Organic
matter formed from decomposed plant residues retains the value of its parent plant
with only small changes (Table I.30) and these values may thus be used to determine its
origin. Continuous cultivation of corn (a C4 plant) for 23 years lead to significant changes
in the value of soil organic matter in a soil which had previously received inputs of
solely C3 organic matter (Balesdent
et al.,
1987). The mean value had increased
significantly, with the most marked changes taking place in the coarser (200-2,000
and surface litter) and in the finest fractions (Figure I.46). These differences
have been used to assess the turnover times of organic matter (Cerri
et al.,
1985; Martin
et al.,
1990) and to investigate the origin of organic matter assimilated or transported by
invertebrates (de Niro and Epstein, 1978; Spain
et al.,
1990; Martin
et al.,
1991; Spain
and Reddell, 1996) and are illustrated further in Chapters III.4.3.2.1 and IV.5.3.2.4.
values increase by 0 to 1‰ per trophic level along food chains (Petersen and Fry, 1987).
Stable isotope ratios of other elements may also be useful as source tracers and
process indicators in the same way as those of carbon. The ratio of to (adjusted
to relate the ratios of these isotopes to the air as a standard, values) has been used
in this way and has potential uses in soil-based and related studies as an indicator of
trophic level and other processes (Petersen and Fry, 1987).
values of the tissues of plants possessing nitrogen-fixing symbionts may be used
to indicate the proportional contributions of N fixation to total plant nitrogen supply,
provided that certain other information is available, or may be assumed (Shearer and
Kohl, 1989). In forests,
values of decomposing materials increase with depth in
