Hainnaux, Guy

Horizon / Pleins textes

9 
T. Rosswall (ed). 1980 
Nitrogen Cycling in West African Ecosystems 
REPORT OF THE WORK GROUP 
ON AGROECOSYSTEMS IN THE WET HUMID TROPICS 
G .  Hainnaux (rapporteur) 
Introduction 
The nitrogen cycle is described for agrosystems on ferralitic soils subject to average L-- 
saturation with rainfall between 1400 and 1800 mm. 
The concept of an agrosystem is not used here, as is generally the case, to describe an 
area geographically limited by definite borders. In fact, considering the scarcity of in- 
formation available on the repartition of different rotation crops, it seemed preferable to 
consider the sequence in time of crops on one plot of land. Consequently the agrosystem 
is defined with respect to crop rotation. 
- a cropping phase 
- a regeneration phase or fallow. 
The relative importance and nature of each phase vary significantly according to  
production systems and their level of intensification. Therefore, it was considered appro- 
priate to describe two types of agrosystems illustrating respectively: 
- traditional systems 
- improved systems 
For each of these, only the cropping phase was the subject of a quantification essay. 
In most cultivation systems in use, the rotation comprises two phases: 
Characterization of the agrosystems 
Traditional systems 
These systems range from intensive cropping systems where fallows are absent or very 
short in compound farms and areas of high population density to more itinerant long 
bush or forest fallow systems. The nitrogen content of the soil tends towards an equi- 
librium which is determined by the relation between the losses occurring for the most 
part during the cropping phase, and the additions mainly during the fallow phase. 
No application of fertilizer is generally made but on compound farms, in areas of more 
or less permanent cultivation, much use is made of household refuse, small animals pen 
manure, mulch and sometimes compost. The supply of nitrogen to plants is mainly 
dependent on the mineralization cycle of the organic reserves of the soil, which is itself 
related to the climatic cycle. 
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The planting of associated crops allows a better use of the environmental potentials 
(water, mineral elements and light energy) and limits the effects of climatic uncertainties 
and other factors threatening the crops. 
The productivity of these systems, although not generally high, is essentially depend- 
ent on the length of the period of fallow, which determines the extent of replenishment 
of fertility lost during the cropping phase. 
Improved systems 
In these systems, the productivity increase is obtained through the simultaneous improve- 
ment of a whole set of farming techniques which, apart from the use of fertilizers, affect 
essentially: 
- the choice of selected plant varieties 
- the timing and density of the sowing 
- the tillage of the soil 
- the battle with predators. 
inputloutput relationships. 
The intensification of production factors aims at an economic optimization of the 
Method of presenting the results 
The method used is the one defined by Frisse1 (1977). The systems studies were restricted 
to two compartments: 
- a plant compartment (P) 
- a soil compartment (S) 
each characterized by the following fluxes which traverse it: 
- inputs+ X 
- transfers Y + X 
These fluxes, grouped on one side as supplies and on the other side as removals, allow 
the calculation of a balance and the characterisation of the system's state. They are ex- 
pressed in kg ha-' and represent a yearly average. 
In most cases, in order to integrate the variability of the data from the literature, it 
was judged preferable to give a range of values rather than an average. This implies a cer- 
tain lack of precision in the calculation of the balances. 
- outputs Y -+ 
List of fluxes studied 
In the plant compartment 
- Supplies 
(i) inputs by seed or seedling + P 
(ii) transfer by net uptake from soil S + P 
(iii) input by uptake from the atmosphere + P 
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- Removals 
(i) transfer by seed for sowing P + S 
(ii) output by primary product P + 
(iii) transfer by plant residues P + S 
(iv) output by burning P + 
In the soil compartment 
- Supplies 
(i) input by waste + S 
(ii) input by addition of fertilizer + S 
(iii) input by non-symbiotic N-fixation -+ S 
(iv) input by dry and wet deposition + S 
(v) transfer from plant residues P + S 
- Removals 
(i) output by denitrification and volatilization S 4 
(ii) output by leaching S 4 
(iii) output by erosion and run off S + 
(iv) transfer by plant uptake S -+ P 
Significance of the extreme values 
In the case of traditional systems, the highest values correspond to the cultivation phase 
immediately after the reclamation of the land. This period is characterized by an excess 
of supply over demand. This excess is partly lost due to the inadequacy of the farming 
techniques used. The lowest values correspond to  crops grown several years after the re- 
clamation of the land, when the fertility decreases. 
In the case of improved systems, the range of values represent different levels of inten- 
sification. The lower values correspond to the implementation of the agricultural prac- 
tices most commonly popularized and spread in rural areas, whereas the higher values 
relate to results obtained at experimental farms where the whole set of farming methods 
can be applied in conditions of maximal efficiency. 
Results 
The results are summarized in Table 1. 
Comments on the plant compartment 
The average quantity of nitrogen used yearly by plants has been estimated to: 
- 
- 
This nitrogen originates generally from the soil with the exception of leguminous species 
40 to 80 kg ha-' in traditional systems, 
100 to 200 kg ha-' in improved systems. 
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Table 1.  Example of nitrogen balance for a tropical agrosystem (kg ha’’ yr-I). The 
figures mentioned are based either on: compilation from the literature, extra- 
polations or calculations. 
Identification of fluxes Traditional systems Improved systems 
Plant compartment 
Supplies 
input by seed or seedlings 
transfer by net uptake from soil 
input by uptake from the 
atmosphere 
Total supplies 42 to 85 102 to 205 
Removals 
transfer by seed for sowing 
output by primary product 
transfer by plant residues 
output by burning 
Total removals 
2:::;s 1 t: 6f:::20 1 100 
5 to 10 40 to 80 to 
11 to22 80 - - 200 
42 to 85 102 to 205 
Balance O O 
Soil compartment 
Supplies 
input by waste 
input by addition of fertilizer 
input by non-symbiotic N-fiiation 
input by dry and wet deposition 
transfer from plant residues 
Total supplies 
<5  <5  
< 10 < 10 
10 to 20 
5 to 10 
15 to 45 
O 50 to 100 
10 to 20 
40 to 80 
100 to 265 
Removals 
output by denitrification and 
volatilization 
output by erosion and run-off 
output by leaching 
transfer by plant uptake 
Total removals 
< 20 <50 
<60 
5 to 50 
33 to 66 
< 60 
50 to 150 
83 to  166 
38 to 196 133 to 426 
Balance -23 to -151 -33 to -161 
418 
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which partly use atmospheric nitrogen. The quantity thus fured through symbiotic fura- 
tion has been estimated to: 
- 20 to 40 kg ha-' in traditional systems. The quantity is of less importance when the 
soil is well provided with mineral nitrogen, as a high content of the latter has a 
lowering effect on the efficiency of symbiotic furation. 
40 to 80 kg ha-' in improved systems, for which the increase of'furation when the 
level of intensification is raised originates partly from the use of selected species with 
high furation ability, and partly from the suppression of other limiting factors than 
nitrogen. 
- 
At the rotation level, the relative importance of nitrogen originating the soil compared 
to nitrogen from the atmosphere depends thus on the relative importance of the legumi- 
nous crop in comparison to other crops. 
The results mentioned in the table above were calculated assuming the relation to be 
1/2, i.e., assuming a succession of three crops, one of them being leguminous. 
As to the relative importance of the fractions removed through harvest and the frac- 
tions that can be transferred by plant residues, they have been estimated to be 60 % and 
40 % respectively of the total quantity used by the plants. 
In traditional systems the recovery can be limited by the practice of burning, which 
can affect up to 70 % of the total residues from farming. 
Comments on the soil compartment 
Fertilizers are used only in improved systems. The rates most frequently recommended 
are in the magnitude of 50 kg ha-'. As mentioned above, the aim is to obtain an eco- 
nomic optimum. In less advanced systems inputs are preferably reserved primarily for 
cash crops, and secondly for cereals. 
In more intensive systems the rates can be significantly raised and reach 100 to 200 kg 
ha-'. The cumulative effect of large inputs, most frequently in the form of ammonia 
sulphate, causes an acidification of the soil. 
The coefficient of fertilizer uptake by plants, which is higher at low rates than at high 
rates, does not exceed 50 %. 
The recovery of plant residues constitutes an essential practice for the maintenance of 
fertility and the equilibrium of humus balance. Moreover, it allows the reincorporation 
into the soil of a fraction of the nitrogen taken up from the atmosphere. 
The table shows that in the soil compartment, the total inputs remain insufficient to 
compensate for the losses. In traditional systems the inputs are inferior to the amounts 
absorbed by plants alone. The deficit results in an impoverishment of the soil that could 
only be slowed down by the introduction of a sufficiently long period of fallow, which 
would allow furation and transfer to  the surface of a fraction of the subsoil nitrogen. 
Conversely, in improved systems, the supplies are in excess compared to the absorp- 
tion by plants. Nevertheless, they are necessary for the maintenance of the system's 
productivity as they result in a significant increase of losses, mainly through leaching, 
and hence a deficit which increases with increasing supplies. 
The maintenance of these systems is dependent on the adoption and use of farming 
techniques apt to increase the efficiency of fertilizers by reducing losses (e.g., by splitting 
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up the inputs). Besides, the introduction of an improved fallow or a rotation cultivation 
of fodder based on leguminous crops may constitute a possible solution to the main- 
tenance of a satisfactory nitrogen balance. 
Discussion 
The method of presentation adopted, which exhibits each compartment as a "black box'' 
illustrates inadequately the whole set of interactions that rule the dynamics of a soil- 
plant system as well as the processes set in action. For instance, little information is 
available on the prospection,of soil profies of plant rooting and on profies of root 
activity "in situ". The role of deep roots remains to be defined. This is all the more 
important as erosion phenomena are intensive. 
Regarding the plant compartment, which is the best known, some uncertainties re- 
main. In particular, the contribution from biological fixation to the nitrogen cycle is only 
measured under particular circumstances which makes it difficult to extrapolate and 
evaluate its significance in the field. The emphasis was placed on these shortcomings, and 
research focused on their removal must be given priority. 
In the soil compartment little information has been assembled concerning the internal 
nitrogen cycle (mineralization - immobilization). The knowledge of the impact of farm- 
ing techniques on this cycle constitutes, however, an essential element of a better control 
of the conditions for nitrogen balance. The use of 15N could be a fruitful method for 
these studies. 
On the other hand, if the intensification results in an opening of the nitrogen cycle, 
the available data are, at the current state of knowledge, insufficient for an exhaustive 
and reliable evaluation. In particular, for losses, the variability as well as the number of 
data cause difficulties. There are several reasons for this. Some are of methodological 
nature (differences in experimental measurement devices), while others are probably due 
to  inadequate identification of the processes. 
Therefore, if the data obtained by different authors should be used for the setting up 
of a complete balance, it seems indispensable to have access to a certain number of re- 
ferences which take into account the whole complex (climate - soil - plant - farming 
techniques). For this reason, it is recommended that a network of reference information 
regarding measurement results can be created. 
Finally, it is advisable to once more state that the undertaken quantification essay re- 
lates to a single plot and does not take into consideration the redistributions which may 
occur in the atmosphere. The consequence of this is that deficiences are overestimated in 
the balance. Therefore, measurements ought to be made on the level of whole geo- 
graphical units such as, e.g., basin slopes. 
References 
Frissel, M.J. 1977. Cycling of nutrients in agricultural ecosystems. - Agro-Ecosystems 4: 1-354. 
' 420 
iehdik = Nitrogen ---- _ _ _ _  Cycling -I- in -__ 
West L______II African _- ..___--.-. ---------" Ecosystems --L___. 
Proceedings of a w 
T. Rosswall (Editor) 
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Nitrogen cycling in West african ecosystems

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