Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 190

Fig. 3.12.2. (a) NH 4 + and (b) NO 3 − concentrations (mg N kg − 1 dry soil) in soil from an undrained

site ( ) treated with 1000 mg of 14 C-labelled

glucose C kg − 1 dry soil and 200 mg of NH 4 + -N kg − 1 dry soil incubated aerobically for 97 days at
22

) and sites drained for 1 ( ), 5 ( ) and 8 years (

w v q r

C. Bars are 1 SD .

° ±

sample remained < 50 mg NO 3 − -N. The NO 3 − concentrations in the glu-
cose-treated soil showed a decrease in the first 3 days, but increased there-
after in all soils; the largest increase was found in the soil drained for 8 years
(Fig. 3.12.2b).

Discussion

Production of CO 2 and 14 CO 2

The undrained soil and soil drained for 1 year, with an electrolytic conduc-
tivity of 40 and 80 dS m −1 , respectively, showed the smallest percentage of
organic C mineralized, suggesting that the salts inhibited the microbial
activity. The decrease in CO 2 production from 2.1 to 0.89 g C kg −1 after 90
days reported by Pathak and Rao (1998) with an increase in electrolytic
conductivity in the soils from 1 to 97 dS m −1 , was much larger than we
found. However, they added salts to soil (NaCl and CaCl 2 ) so the micro-
organisms may not have been adapted to osmostic and/or specific ion stress
and this might have inhibited their activity.
The percentage mineralized glucose C in the soil drained for 8 years
was similar to values reported by Bremer and Kuikman (1994), but it was
less in the other investigated soils. The differences in percentages of miner-
alized glucose C pointed to a possible effect of large salt concentrations on
metabolic processes, but possible effects of other soil characteristics, such
as pH, cannot be excluded.