Bourdon, Emmanuel

Brunet, Didier

Kanchanakool, N.

Poss, Roland

Horizon / Pleins textes

In situ incubation to evaluate inorganic nitrogen dynamics III a cropped
tropical sandy soil
R. Poss', D. Brunet', E. Bourdon l and N. Kanchanakoof
'Institut de Recherche pour le Devel oppement, 911 av. Agropolis, 34032 Montpellier cedex I, France,
corresponding author poss@mpl.ird.fr
2Land Development Department, Phaholyothin Rd, Chatuchak, Bangkok 10900, Thailand
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Abstract
Most inorganic nitrogen in soils onginates from the mineralisation of"organic
matter and plant residues. Inorganic nitrogen dynamics in involved in many major
issues like plant nutrition, global changes and environmental pollution. However, the
understanding of inorganic nitrogen dynamics was long restricted by a lack of suitable
methods under field' conditions. The objective of the paper is to evaluate the in situ
incubation method first introduced by Raison et al. (1987) to study temperate forest
soils under tropical conditions, and to present some results obtained with this method.
This study was conducted in Thailand near Nakhon Ratchasima (15°N, 102°E).
The soil is an Arenic Acrisol in the FAO classification (Nam Phong series in the Thai
soil reference system). Twenty plots (6x8 m) from a factorial block design experiment
with two factors (control/lime and bare/cropped) and 5 blocks were studied for 16
months. The change in total nitrogen with time was determined by taking soil samples
three times before the start of the experiment and once at the end. For each plot twelve
samples were taken in the 0-20 cm layer using a small-diameter auger. Total nitrogen
was determined by the Kjeldahl method on the bulk sample. The pattern of inorganic
nitrogen dynamics was studied using in situ incubation. Every fortnight five capped
PVC tubes, 5 cm in diameter and 20 cm in length, were inserted in each plot to 10 cm,
and five soil samples were collected from the bulk soil and kept refrigerated. The PVC
tubes were collected 2 weeks later, then the samples of each plot were bulked together
and kept refrigerated. Soil water content was determined in the oven at 105.~C, and
inorganic nitrogen was extracted using 1M KCI and analysed by colorimetry. The
difference between inorganic nitrogen at the beginning and at the end of the two-week
periods gave the rate of mineralisation over the period.
One potential problem of the in situ incubation method is the possibility of a
difference in soil water content between the soil inside the incubation tube and the bulk
soil. The results showed that the soil water content in the tube equilibrated with the
bulk soil within a few days after each rain. In case of drought the soil inside the tube
was on average 0.01 m3 m'3 wetter than the bulk soil. These soil water differences may
lead to otential biases difficult to evaluate. In cultivated plots the soil inside the tubes
was not drier than the bulk
soil, indicating that not roots
developed within the tubes.
For both the bare and
the cropped plots the rates of
mineralisation as nitrate and
as ammonium as well were
not different in the control
(pll., 5.3) and in the lime
treatment (pHw 5.9) over the
16-month period.' No
.19.
difference were measured in the inorganic content of the bulk soil either. As a
consequence the calculations were carried out after merging the data from the control
and limed plots.
Bare
-1.4±7.3
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_ Fertilizer addition
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_Bare plots
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Cowpea was grown in May and
June, maize from August to October.
April and July were the onsets of the
two rainy seasons. The peak of
mineralisation of each rainy season
led to a production of less than 20
kgN ha", with no significant
difference between the cropped and
the bare plots (Figure 2). The
amount of nitrate was less than 5
kgN ha-1 under the crop. In one
hand this low amount decreased the
hazard of leaching, but in the other hand the plant had to rely mostly to the rate of
mineralisation (only 0.3 kgN03-N ha-Ion average) to cover its needs.
Even though the in situ incubation technique may lead to overestimated rates of
mineralisation because of a higher soil water content in the incubation tubes, it seems
to be an easy-to-use and robust method to assess many aspects of the dynamics of
inorganic nitrogen in tropical soils.
. 20.
The Second International Conference on
Soil Quality Evolution Mechanism and
Sustainable Use of Soil Resources
Soil Quality,
Environment and Sustainable Agriculture
in Tropical and Subtropical Regions
•
Yingtan, P. R. China
23 - 25 September 2003
Organizer:
Key Basic Research Develop Foundation (973 Project) No. G 19990118
The Institute of Soil Science, CAS, Nanjing, China
Sponsors:
Ministry of Science and Technology
National Natural Sciences Foundation of China
Chinese Academy of Sciences
IRD, France
Institut de recherche
pour le developpernent
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S t .1~JTl ~ The Ministry of Science and Technology
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General information:
Organize Committee
Chair
Prof. Dr. Qiguo Zhao
Dr. Roland Moreau
Institute of Soil Science. CAS
1nstitut de recherche pour le developpernent
Scientific Secretary General
Prof. Yuanqiu He
Dr. Ba Sun
Institute of Soil Science, CAS
Institute of Soil Science. CAS
. I .


Soil quality, environment and sustainable agriculture in tropical and subtropical regions

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