| Abstract | With increased competition for land and water from the urban and industrial sectors and high population growth in the major rice producing nations, the possibility for expanding area under rice-based farming systems is limited. Use of marginal, coarse-textured soils of high permeability has increased over time for production of both upland and lowland irrigated rice to meet the demand for food
and fibre to support growing populations. In irrigated rice fields with fine-textured soil, leaching losses of N are usually low because of low permeability. However, in
highly permeable coarse-textured soils, the losses of N through leaching of nitrate-N (NO3-N) and other processes can be substantial. Information on water and nitrogen
dynamics for rice crop on coarse-textures soils is limited. Since models can provide an insight of the interrelationships between various components of a complex system, the overall aim of this research is to improve crop growth simulation capability for a range of water and nitrogen management strategies for rice-based cropping systems in tropical environment. The specific objectives of this research project are: 1. to examine variation in water use productivity in lowland rice-based cropping systems without significant effects on yield;
2. to explore the nitrogen dynamics in rice-rice-legume crop sequences on a typical coarse-textured soil of lowland cropping systems in the tropics;
3. to calibrate and validate a farming system model that can be used to simulate growth, yield, nitrogen uptake, nitrogen and water dynamics in the above ricebased
cropping systems. To achieve these objectives, field experiments were conducted at the Research Station of Assessment Institute for Agricultural Technology (BPTP) NTB Lombok Indonesia (08°35′N, 116°13′E, 150 m elevation) on a sandy loam soil using rice-rice-legume crop rotation over two years (2007-2009). The experiment was laid out in a randomised split-plot design with water management treatments
(continuously submerged and alternate submerged and non-submerged, hereafter referred to as CS and ASNS, respectively) as main plot and N-fertiliser rates (0, 70
and 140 kg N ha-1) treatments as subplot with three replications. Plant and soil samples were collected at four main phenological stages during rice growth period
(tillering, panicle initiation, flowering and harvesting). Plant samples were measured for dry biomass and total-N. Soil samples were taken within 0-100 cm depth from
four soil layers (0-20, 20-40, 40-70 and 70-100 cm) and each sample was analysed for ammonium-N (NH4-N), NO3-N, total-N, and organic carbon (OC). Legumes (peanut and soybean) were sown immediately following the second rice crop in each calendar year. The experimental design was similar to rice by replacing CS and ASNS treatments with peanut and soybean, respectively and reducing N-fertiliser
application rates to 0, 12 and 24 kg N ha-1. Crop and soil samples were collected at three main phenological stages of legume (maximum vegetative, flowering and harvesting) and analysed as for the rice crop. Data of field experiment were used to parameterise, calibrate and validate the APSIM-Oryza model. The results indicated that biomass, yield and N-uptake of rice were not significantly different between ASNS and CS. Any increase in yield and N-uptake was largely due to increased N-fertiliser application. Average irrigation water saved with ASNS varied in the range of 36% to 44% when compared with CS irrigation treatment. Furthermore, average water productivity in the ASNS treatment was 52%
higher than for the CS irrigation treatment. Considering these results as typical for well-drained soils with deep ground water tables, ASNS practices can make considerable water-saving without substantial yield reduction in irrigated lowlands of eastern Indonesia. Furthermore, yield of both peanut and soybean crops following
the second rice crop were not affected by N fertiliser rates. The implication of this study is that the farmers should consider ASNS as a water saving technology in this
region of study and should not consider applying N-fertiliser for peanut and soybean crops when it follows the second rice crop. Seasonal variation in soil nitrogen and carbon in lowland rice-based cropping systems indicated significant effects of N-fertiliser treatments on NH4-N and NO3-N
concentration in soil, but only on a few occasions for irrigation treatments. For example, NO3-N concentration in soil under ASNS treatment was higher than the CS
treatment during panicle initiation and flowering stages in the later part of the rice growing seasons. Since rice prefers ammonium over the nitrate form of N, increased
nitrate concentration during the periods of non-submergence in ASNS irrigation treatment could have adversely affected N-uptake by rice. However, no significant
difference in N-uptake was observed between CS and ASNS possibly because of the small magnitude of NO3-N concentration differences between these irrigation
treatments. . Since floodwater is another useful source of N for the rice crop, measurements in this experiment showed NH4-N concentration in soil and floodwater to be mostly higher than NO3-N concentration that allowed adequate Nuptake. Organic carbon as an indicator of soil organic matter and overall soil fertility was not affected by irrigation and N fertiliser treatments during the experiment. During the legume season, increased rates of N-fertiliser application increased NH4-N and NO3-N concentration at various soil depths throughout crop growth. Increased concentration of available forms of N as a result of increased level of N-fertiliser applied to legumes decreased the number and weight of root nodules
on some occasions. Since increased N-fertiliser application increased N-uptake and seed N-uptake but not yield, N-fertiliser application is not recommended for legumes
in this region on the basis of improved crop quality. The APSIM-Oryza model was mostly able to capture the variable effects of water and N management strategies on crop growth, nitrogen and carbon dynamic in
soil, and the dynamics of ponded water depth under anaerobic and aerobic soil conditions in the rice-rice-legume crop sequence as practiced in the tropical region of
eastern Indonesia. The model satisfactorily simulated crop variables such as biomass, yield, leaf area index (LAI) and N-uptake. The model also satisfactorily simulated
the variation of water depth during rice growth period. However, the simulation of N dynamics and floodwater (ponding) in the ASNS irrigation treatment need further
improvement. The APSIM-Oryza model provided an operational and a promising modelling framework to test future cropping practices and improve making farm decisions to develop more sustainable and effective lowland rice-based farming
systems. This thesis has produced a dataset to calibrate and evaluate the model performance by capturing the dynamics of various forms of nitrogen and daily
ponded water depth for water limited rice-based cropping systems. More extensive field experimental testing is needed to increase confidence with the widespread use
of this model. |
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