ZEF Bonn

58
ZEF Bonn ● Center for Development Research – Annual Report 2001/2002
A ZEF project in Southeast Asia
focuses on the improvement of
nitrogen (N) use efficiency in
rice cropping systems by integration
of the floating water
fern Azolla. In association with
a blue-green algae, this fern
provides considerable amounts
of nitrogen to the system.
gy for soil fertility maintenance was also assessed during this research. It could be
shown that farmers do better to invest the resources for soil fertility replenishment
in areas with productive soils because of their higher economic benefit.
It is widely believed that different land management techniques are required for
each slope section due to natural variability of soil properties and erosion potentials
along the slope. The Water Erosion Prediction Project (WEPP model) was applied to
evaluate the effect of land management methods on soil erosion. This research aims
to quantify the influence of different types of land management on a hillslope. A
detailed soil survey and participatory hillslope mapping were also used to identify
natural changes in soil properties. The constructed model shows that soil conservation
techniques such as residue management and contouring may reduce the risk of
soil erosion by up to 70 % on steep slope sections compared to presently used tillage
practices at the research site.
As part of nutrient balance and soil degradation studies, Cs-137 and Pb-210 techniques
were applied to characterise the long-term erosion rate under different environmental
and land use conditions. During the last two decades, the efficiency and
the value of the Cs-137 were increasingly recognised to estimate spatially distributed
mid-term (10-30 years) soil erosion rates. However, the application of this technique
under tropical conditions has been limited because the amount of radio-nuclide
fallout is low due to atmospheric circulation. Two study sites in Eastern Uganda
show that the level of Cs-137 is indeed low (2-3 Bq kg-1 with a 10 % measurement
error), but that it still opens the possibility to utilise these techniques for soil degradation
studies in the tropics.
The influence of possible technological options for future crop yields has been
modelled using two different approaches; a deterministic crop simulation model
(DSSAT) and an adaptive modelling framework utilising Artificial Neural Networks
(ANN). These two model structures are based on entirely different theories and logical
concepts, but complement each other for the given purposes. The calibrated
models were used for the bioeconomic modelling approaches and will be successively
utilised for a further integration of individual research components.
3.2.2 Nutrient-enhancing mechanisms
One of ZEF's focal research projects is the improvement of nitrogen (N) use efficiency
in rice cropping systems in Southeast Asia by integration of the floating water
fern Azolla. In association with a blue-green algae (Anabaena), this fern provides
considerable amounts of atmospheric nitrogen to the system.
In a Ph.D. study, finished in June 2002, and carried out in collaboration with the
Philippine Rice Research Institute (PhilRice) in Los Baños during the dry season of
1998-99, the use of Azolla to reduce ammonia (NH3) volatilisation and improve the
low N use efficiency of applied urea in lowland rice fields was evaluated for 3 cropping
seasons. The Azolla cover approach was further investigated and verified in farmers'
fields during the wet and dry seasons of 2000-2001. Findings revealed that a
full Azolla cover on the floodwater surface at the time of urea application prevented
the rapid and large increase in floodwater pH associated with urea hydrolysis and
the photosynthetic activities of the algae. In the presence of an Azolla cover, the
mean floodwater pH was reduced by as much as 1.9 pH units, and the maximum
pH value was kept below 8.3. In contrast, in the absence of a cover, floodwater pH
rose above 8.5 and reached a maximum of 10.1. The floodwater temperature was