Agriculture Reference
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having a chemical composition based upon the mean annual composition
of weekly bulked precipitation samples collected in the area (Chapman
et al
., 1997). The NH
4
+
concentration was 0.55 mg N l
−1
and the NO
3
−
concentration was 0.33 mg N l
−1
. Each core received 400 cm
3
of artificial
rain twice a week for 6 weeks. The leachate from each core was collected
24 h after the rain was applied and analysed for pH, N fractions and DOC.
Methods of extraction and chemical analysis
Ammonium, NO
3
−
and DON were extracted from the soil by shaking 10 g
fresh weight of soil for 2 h with 50 ml of distilled water or 50 ml of 0.5 M
K
2
SO
4
. The suspensions were first filtered through Whatman No. 42 paper
before passing through 0.45 µm membrane filters. The soil acidity was
determined by measuring pH in suspension of fresh soil in 0.01 M CaCl
2
at
a sample : solution ratio of 1 : 10 (w/v). Total dissolved N (TDN) in the
water and K
2
SO
4
extracts and soil leachates was measured as NO
3
−
after
oxidation with alkaline potassium persulphate (Williams
et al
., 1995).
Ammonium and NO
3
−
were measured using standard colorimetric meth-
ods. DON was calculated by difference: DON = TDN − (NH
4
+
+NO
3
−
).
Concentrations of DOC were determined using a soluble carbon analyser
(OI Analytical Model 700).
Statistical analyses
Analyses of variance to compare treatments, temperatures and depth were
carried out using the software Genstat (Genstat 5, 1993). A repeated
measures technique was used to test for treatment effects in the leachate
experiment. Skewed data were transformed logarithmically for analysis
of variance, and the arithmetic means of the untransformed data are
presented.
Results
Soil characteristics
The soil used in both experiments contained on average 278 mg C kg
−1
(
11) and 16.3 mg N kg
−1
(
1.2), and the mean C : N ratio was 17.4. At
sampling, the mean moisture content was 44.42% (
±
±
±
0.78) of fresh weight
and the average pH (CaCl
2
) was 4.04 (
±
0.04).
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