Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 191

In a previous experiment, we applied 1000 mg of glucose C kg −1 soil. A
significant increase in CO 2 production was found in soil drained for 8 years
compared with the untreated soil, but not in undrained soil or soils drained
for 1 and 5 years (Beltrán-Hernández et al ., 1999). The results found in
the experiment reported herein confirm a difference in mineralization of
organic material added, but not its sequestration, i.e. its availability as
C-substrate for soil microorganisms. The difference between the two
experiments presumably was related to a difference in the time of sampling.
Soil used in the experiment reported herein was much wetter upon
sampling than the soil used by Beltrán-Hernández et al . (1999).
Gutiérrez-Castorena (1997) stated that the amorphous materials provide
the soil with a very high water retention capacity (2.4-5 kg kg −1 ) which
decreases upon drying; presumably also affecting other soil characteristics
and thus the capacity to sequester added organic material. Compared with
the untreated soil, the application of glucose increased the CO 2 production
more than the amount of 14 CO 2 produced. Accelerated decomposition of
unlabelled soil organic matter following the addition of organic material has
often been referred to as a 'priming effect' (Brookes et al ., 1990). These
authors described situations where a true priming effect is caused mainly by
an increased turnover of microbial cell C (after addition of glucose) or by an
increased decomposition of native soil organic matter (after addition of rye
grass). It was difficult to pinpoint what happened in our soil, but it
appeared to be related to an increased turnover of microbial cell C (no data
shown).
Inorganic N
Soils from the former lake Texcoco are characterized by limitations in
inorganic N, and > 150 mg of N kg −1 soil could not be accounted for after
3 days of incubation in the glucose-treated soil. Losses through NH 3
volatilization were < 10 mg of N kg −1 soil (no data shown) so it was
immobilized in the soil microbial biomass. Nitrifiers in soil of the former
lake Texcoco were adapted to elevated salt concentrations, as indicated
by increases in NO 3 − concentrations in the glucose-treated soil without
accumulation of NH 4 + .
It was concluded that sequestration of added organic C as found in
previous experiments was not confirmed, but sod characteristics changed
by drainage affected the decomposition of added organic material.