The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 302

warming could have important effects on soil C cycling (Smith et al ., 1999).
Studies have shown that increased soil temperature will affect soil nutrient
cycling in agroecosystems (Buyanovsky et al ., 1986). Also, temperature was
shown to be the climatic factor that most closely explained the rate of soil C
accumulation due to removal of soil tillage in agroecosystems (Potter et al .,
1998). Few studies have examined the interaction of elevated CO 2 with
changes in soil temperature. Our objective was to conduct an incubation
study to evaluate how changes in soil temperature influence C and N
cycling of soil collected from soybean and grain sorghum cropping systems
after 5 years of elevated CO 2 treatment.

Materials and Methods

Soil samples were collected from a 5-year CO 2 enrichment study conducted
in an outdoor soil bin at the USDA-ARS National Soil Dynamics
Laboratory in Auburn, Alabama, USA. The bin was 2 m deep, 7 m wide
and 76 m long, and was filled uniformly with surface soil of a Blanton
loamy sand (loamy, siliceous, thermic Grossarenic Paleudult) that had
been fallow continuously for > 25 years. Fertilizer and lime additions were
used to maintain soil conditions within a normal range for crop production.
To ensure adequate plant growth, fertilizer N was broadcast applied at a
rate of 34 kg N ha −1 to the grain sorghum ( Sorghum bicolor (L.) Moench)
and soybean ( Glycine max (L.) Merr.) crop shortly after planting. An
additional 67 kg N ha −1 was applied to grain sorghum 30 days after
planting. All plots were managed under no-tillage conditions.
This study had a split plot design with main plots of two crop species
and two CO 2 levels as sub-plots replicated three times. Soybean and grain
sorghum were chosen to provide legume and non-legume species that are
widely produced in agroecosystems. Open-top field chambers (Rogers
et al ., 1983) were used to impose CO 2 regimes (365 and 720 ll −1 ). Har-

vests consisted of grain sorghum head and soybean pod removal and
threshing with a plot combine. Plant stalks were cut (15 cm length) using
hedge clippers and uniformly applied to plots.
To determine soil C and N cycling, sieved soil samples were weighed
(25 g dry mass basis) and placed in plastic containers; deionized water was
then added to adjust soil water content (soil water content equivalent to
−

µ

20 kPa at a bulk density of 1.3 mg m −3 ). Sample containers were placed in
sealed glass jars with 20 ml of water (humidity control) and a 15 ml vial
of 1 M NaOH (CO 2 trap), then incubated in the dark at temperatures of
20, 25, and 30

C. Treatment samples were removed after 30 and 60 days.
Carbon dioxide in NaOH traps was determined by titrating excess base
with 1 M HCl in the presence of BaCl 2 . The cumulative CO 2 emissions
after 30 and 60 days incubation were calculated by the difference between

°