Agriculture Reference
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improved fallow systems. The effects of C : N ratio, lignin and polyphenol
composition on N
2
O emissions were examined during the main period of
flux activity.
Materials and Methods
Experimental design
Samples of 50 g of soil and 50 g of acid-washed silica sand were mixed
thoroughly and placed in leaching tubes (25 cm length; 4 cm diameter),
and 360 mg of dry, chopped prunings (< 7 mm) were incorporated in to
the top 5 cm of the soil-sand mixture. Tubes were maintained at 70% of
the water holding capacity and incubated at 28°C in the dark.
In the first experiment, leaves of
Gliricidia sepium
,
Calliandra calothyrsus
and
Peltophorum dasyrrachis
were incorporated into a sandy clay loam
soil (2.39% C, 0.16% N) from Chitedze, Malawi. Unamended soil-sand
mixtures provided controls. Available soil N and N
2
O emissions were
measured after incorporation. In the second experiment N
2
O emissions
were measured following incorporation of
Sesbania sesban
leaves and
Macroptilium atropurpureum
residues into a clay Oxisol (1.50% C, 0.16% N)
from Kenya. The effect of residue particle size was examined by comparing
emissions from ground and chopped
Sesbania
leaves.
Analyses
Soil for experiment 1 was sampled from replicate tubes that were not used
for gas analysis. Fresh soil was extracted with 1 M KCl in a 1 : 5 ratio of soil
to extractant. The concentrations of NH
4
+
-N and NO
3
−
-N in the filtered
extractant was determined by continuous flow analysis on a Chemlab
Instruments autoanalyser. The chemical composition of material (Table
3.2.1) was analysed according to the methods described by Handayanto
et al
. (1994).
N
2
O fluxes
Tubes used for gas analysis were closed for 1 h with gas-tight lids with a
rubber septum. After the hour closure samples were taken using air-tight
glass syringes and analysed for N
2
O in a Pye Unicam gas chromatograph
fitted with an electron capture detector. Column and detector temperatures
were 50 and 250
°
C, respectively.
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