E. Humphreys

Rice and Water

The Comprehensive Assessment of Water Management in Agriculture (CA) seeks answers to the question of how freshwater resources can be developed and managed to feed the worlds population and reduce poverty, while at the same time promoting environmental security. The CA pays particular attention to rice as this crop is the most common staple food of the largest number of people on Earth (about 3 billion people) while receiving an estimated 24–30% of the worlds developed freshwater resources. Rice environments also provide unique—but as yet poorly understood—ecosystem services such as the regulation of water and the preservation of aquatic and terrestrial biodiversity. Rice production under flooded conditions is highly sustainable. In comparison with other field crops, flooded rice fields produce more of the greenhouse gas methane but less nitrous oxide, have no to very little nitrate pollution of the groundwater, and use relatively little to no herbicides. Flooded rice can locally raise groundwater tables with subsequent risk of salinization if the groundwater carries salts, but is also an effective restoration crop to leach accumulated salts from the soil in combination with drainage. The production of rice needs to increase in the coming decades to meet the food demand of growing populations. To meet the dual challenges of producing enough food and alleviating poverty, more rice needs to be produced at a low cost per kilogram grain (ensuring reasonable profits for producers) so that prices can be kept low for poor consumers. This increase in rice production needs to be accomplished under increasing scarcity of water, which threatens the sustainability and capability to provide ecosystem services of current production systems. Water scarcity is expected to shift rice production to more water‐abundant delta areas, and to lead to crop diversification and more aerobic (nonflooded) soil conditions in rice fields in water‐short areas. In these latter areas, investments should target the adoption of water‐saving technologies, the reuse of drainage and percolation water, and the improvement of irrigation supply systems. A suite of water‐saving technologies can help farmers reduce percolation, drainage, and evaporation losses from their fields by 15–20% without a yield decline. However, greater understanding of the adverse effects of increasingly aerobic field conditions on the sustainability of rice production, environment, and ecosystem services is needed. In drought‐, salinity‐, and flood‐prone environments, the combination of improved varieties with specific management packages has the potential to increase on‐farm yields by 50–100% in the coming 10 years, provided that investment in research and extension is intensified.

Nutrient Uptake and Apparent Balances for Rice-Wheat Sequences. I. Nitrogen

ABSTRACT Nitrogen (N) nutrition of the rice-wheat (RW) systems of the Indo-Gangetic Plain is important for sustaining the regions productivity and food needs. Soil N plays an important role in regulating the supply of N to plants. Monitoring plant concentrations, uptake, and balance of N assist in our understanding of plant and soil N status and in devising N-fertilizer strategies for both individual crops and a cropping system. Field experiments with rice-wheat-mungbean and rice-wheat-maize annual cropping sequences were conducted at Joydebpur, Nashipur, and Ishwordi in Bangladesh, which differ in their soils and climates. The experiments compared three pre-rice treatments (mungbean residues retained, mungbean residues removed, and maize residues removed), supplying each with two fertilizer levels (soil-test based, or STB, and farmers practice, or FP). Zero N (control) treatments were included, with all other nutrients applied as STB or FP. The objectives were to detect N deficiency, if any, in the component crops, and to determine the changes in soil N fertility, plant N uptake, and soil N balance for various RW sequences. There was a significant decrease in mineral N in the topsoil (0–15 cm) of the +N mungbean and maize-residues removed treatments at Ishwordi, and a generally significant but less marked decline under the same treatments at Nashipur. Wheat and maize crops suffered from N deficiency ranging from 33% to 95% each year, at all sites, but deficiency in rice and mungbean was minimal. Annual system-level N uptake across sites ranged from 89 kg ha−1 for the control to 239 kg ha−1 for sequences containing maize with N. There were significant linear relationships between total system productivity (TSP) and annual N application and between TSP and annual system-level N uptake. Considering no N loss through the system, N fertilizer resulted in a positive N balance that ranged between 24–190 kg ha−1 compared with a negative balance of between 40–49 kg ha−1 without it. However, if a 30% N loss was assumed, N balances were reduced to between −37–62 kg/ha−1 for N-containing treatments, and to between −64–55 kg/ha−1 for the control treatments. Further research is needed to understand N depletion and replenishment and to sustain the productivity of the RW system.

Effect of raised beds, irrigation and nitrogen management on growth, water use and yield of rice in south-eastern Australia

To remain economically and environmentally sustainable, Australian rice growers need to be able to readily respond to market opportunities and increase cropping system productivity and water productivity. Water availability is decreasing whereas its price is increasing. Alternative irrigation layouts and water management approaches could contribute to reduced water use and increased irrigation efficiency. This paper reports results for the first crop (rice) in a cropping system experiment to compare permanent raised bed and conventional layouts on a transitional red-brown earth at Coleambally, New South Wales. The performance of conventional ponded rice grown on a flat layout was compared with rice grown on 1.84-m wide, raised beds with furrow and subsurface drip irrigation. In addition, deep and shallow ponded water depth treatments (15 and 5 cm water depth over the beds) were imposed on the rice on beds during the reproductive period. A range of nitrogen (N) fertiliser rates (0–180 kg N/ha) was applied to all treatments. The traditional flat flooded treatment (Flat) achieved the highest grain yield of 12.7 t/ha, followed by the deep (Bed 15) and shallow (Bed 5) ponded beds (10.2 and 10.1 t/ha, respectively). The furrow (Furrow) irrigated bed treatment yielded 9.4 t/ha and the furrow/drip (Furr/Drip) treatment yielded the lowest grain yield (8.3 t/ha). Grain yield from all bed treatments was reduced owing to the wide furrows (0.8 m between edge rows on adjacent beds), which were not planted to rice. Rice crop water use was significantly different between the layout–irrigation treatments. The Flat, Bed 5 and Bed 15 treatments had similar input (irrigation + rainfall – surface drainage) water use (mean of 18.3 ML/ha). The water use for the Furrow treatment was 17.2 ML/ha and for the Furr/Drip treatment, 15.1 ML/ha. Input WP of the Flat treatment (0.68 t/ML) was higher than the raised bed treatments, which were all similar (mean 0.55 t/ML). This single season experiment shows that high yielding rice crops can be successfully grown on raised beds, but when beds are ponded after panicle initiation, there is no water saving compared with rice grown on a conventional flat layout. Preliminary recommendations for the growing of rice on raised beds are that the crop be grown as a flooded crop in a bankless channel layout. This assists with weed control and allows flooding for cold temperature protection, which is necessary with current varieties. Until we find effective herbicides and other methods of weed control and N application that do not require ponding, there is little scope for saving water while maintaining yield on suitable rice soil through the use of beds.

Nutrient Uptake and Apparent Balances for Rice-Wheat Sequences. III. Potassium

ABSTRACT Potassium (K) nutrition of rice-wheat (RW) systems of the Indo-Gangetic Plain (IGP) of South Asia is important because of its role in productivity and the large quantities of this macronutrient that are extracted by such intensive cropping systems. Field experiments on the RW cropping sequence were conducted at three locations in Bangladesh with three soil types. Two fertilizer doses—farmers practice (FP) and soil-test based (STB)—of nitrogen (N), phosphorus (P), K and other nutrients were combined with mungbean or maize as a third crop. The objective of the experiments was to detect K deficiency, if any, in rice, wheat, mungbean, and maize, and to compare the FP- and STB-based sequences in terms of the K nutrition of those crops and the apparent K balance in soil. Frequent K deficiency was observed in rice and wheat at all sites, especially at Ishwordi, while maize was less affected and mungbean not deficient. There was a significant effect of fertilizer on K uptake by maize, mungbean, and rice, but little effect of the retention of mungbean residue on K uptake by crops at any site. Mean annual system-level K uptake was greatest at Ishwordi (126–239 kg ha−1) and least at Joydebpur (64–116 kg ha−1). The majority of K uptake was in straw and the proportion in grain varied little across sites (range: 11%–29%). There were large negative apparent K balances in all treatments at all sites (range: −25–212 kg ha−1), with the greatest at Ishwordi and the smallest at Joydebpur. Soil K balance responded differently to the retention of residues across soils, and positive effects could be observed on clayey soils. Long-term experiments will be required to monitor soil and plant K dynamics under various fertilizer and residue management of crops in RW systems of the IGP.

Nutrient uptake and apparent balances for rice-wheat sequences. II. Phosphorus

ABSTRACT Phosphorus (P) nutrition of the rice-wheat (RW) systems of the Indo-Gangetic Plain of South Asia has become important due to the alternate flooding and drying cycles of this crop rotation. Field experiments on the RW cropping sequence were conducted at three locations of Bangladesh on three soil types. Two fertilizer doses—farmers practice (FP) and soil-test based (STB), containing recommended amounts of P, nitrogen (N), potassium (K), and other nutrients—were compared with mungbean or maize as a third crop. The objective of the experiments was to detect P deficiency, if any, in rice, wheat, mungbean, and maize, and to compare the FP and STB doses of fertilizers in rice-wheat-mungbean and rice-wheat-maize sequences under two mungbean management practices (residue removed or retained) and one maize management practice (residue removed) in terms of P nutrition of those crops and annual system-level P removal and apparent P balance in the soil. The apparent P balance was negative with the FP dose (−1 to −9 kg ha−1 for mungbean sequences at Joydebpur and Nashipur) and there was soil P accumulation under both the STB dose (9–49 kg ha−1) and zero N control (13–50 kg ha−1) across sites. The effect of maize or mungbean as the pre-rice crop on the apparent P balance of various RW sequences was not significant. Phosphorus deficiency occurred at all sites in wheat and maize, and at Ishwordi in rice, suggesting that P fertilizer recommendations need to be revised for RW systems in Bangladesh. The results also suggest that long-term monitoring for P concentration, uptake, and balance would be necessary for improving not only the productivity and sustainability of this system but also the fertilizer P-use efficiency.