Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 188

Materials and Methods

The experimental site is located in the former lake Texcoco in the valley of
Mexico City at an altitude of 2240 m above sea level with a mean annual
temperature

C and mean annual precipitation of 705 mm
(Beltrán-Hernández et al ., 1999). Soil was sampled on 27 March 1998 at
random from the 0-15 cm layer of four different plots: undrained soil and
soils drained for 1, 5, and 8 years. The undrained site and the site drained
for 1 year had an 80% cover entirely of Distichlis spicata , a grass adapted
to high salinity. The site drained for 5 years was completely covered with
D. spicata and that drained for 8 years had a diverse flora of several
Gramineae, Cacti and Compositae. Characteristics of the different plots
can be found in Beltrán-Hernández et al . (1999). The soil was passed
through a 5-mm sieve and dried for up to 2 days until a 40% water-holding
capacity (WHC) was obtained. The soil was conditioned for 6 days in
drums treated with 100 ml of distilled H 2 O to avoid desiccation and
containing a beaker with 100 ml of 1 M NaOH to trap any CO 2 evolved.
Fifty-four sub-samples of 50 g of soil from each site were added to
120 ml glass flasks. Half of the sub-samples were treated with a solution
containing

of 16 °

Ci kg −1 soil) and (NH 4 ) 2 SO 4 , and
the remainder were treated with an equal amount of distilled H 2 O. The
amount of water added resulted in a soil moisture content of ~50% WHC,
and the amounts of C and N as glucose and NH 4 + were ~1000 and
200 mg N kg −1 soil. Three flasks were chosen at random from each
treatment. A 20 g aliquot of soil was extracted for inorganic N with 80 ml of
0.5 M K 2 SO 4 to provide zero time samples. The samples were shaken
for 30 min and filtered through Whatman No. 42 paper; the NH 4 +
concentration was determined by distillation with MgO (Bremner and
Keeney, 1966), and the NO 3 − concentration was measured colorimetrically
(APHA AWWA WPCF, 1989).
The glass flasks were placed in 945 ml glass jars treated with 10 ml
of distilled H 2 O and containing one vessel with 20 ml of a 1 M NaOH
solution to trap CO 2 evolved. The jars were sealed to be airtight and stored
in the dark for 97 days at 22

14 C-labelled glucose (~40

µ

C. After 1, 3, 7, 14, 28, 42, 69 and 97 days,
three jars were selected at random from each treatment, opened, the vessel
containing NaOH removed and the NaOH solution titrated with
appropriate concentrations of H 2 SO 4 . The soil was removed from three
flasks and 20 g was extracted with 80 ml of 0.5 M K 2 SO 4 solution. The
samples were shaken for 30 min and filtered through Whatman No. 42
paper and the inorganic N was measured as described for zero time
samples. All remaining flasks were opened, aired for 10 min to avoid
anaerobicity, sealed and incubated further.

°