Mathias Becker

Green manure technology: Potential, usage, and limitations. A case study for lowland rice

The growing concern about the sustainability of tropical agricultural systems stands in striking contrast to a world-wide decline in the use of soil-improving legumes. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews recent progress, potential, and limitations of green manure technology, using lowland rice cropping systems as the example.

Stem-Nodulating Legume-Rhizobium Symbiosis and Its Agronomic Use in Lowland Rice

In recent years there has been a great surge of interest and activity in the field of biological nitrogen fixation (BNF). The legume-Rhizobium symbiosis is estimated to account for 40% of the world’s fixed nitrogen. Because the ultimate goal of BNF research is to increase crop yields on farmers’ fields, researchers’ attention must focus mainly on the understanding, improvement, and agronomic use of legume-Rhizobium symbiosis. Rightly so, since the increased cost of energy and the growing demand for N fertilizers are expected to cause dramatic increases in the price of this commodity in the years ahead. Rice (Oryza sativa L.) is the world’s most important food crop as more than 40% of the world’s population depend on rice as a major source of calories. Most of the rice in tropical developing countries is produced in lowland areas and by subsistence farmers whose resource base is so small and whose physical access to supply markets is so limited that they cannot take full advantage of technologies that are overdependent on purchased inputs. The BNF in legumes, used appropriately, reduces production constraints in low-input agricultural systems in the tropics. However, most traditionally used green manure (GM) species cannot tolerate prolonged periods of flooding that occur in most lowland rice-farming systems. A new and promising alternative may be the use of nitrogen-fixing, stem-nodulating legumes.

GROWTH AND N2-FIXATION OF TWO STEM-NODULATING LEGUMES AND THEIR EFFECT AS GREEN MANURE ON LOWLAND RICE

Abstract Two stem-nodulating tropical legumes (Sesbania rostrata and Aeschymmene afraspera) show promise as green manure in lowland rice farming systems. Their performance as biofertilizers for lowland rice was studied at different growth stages, seasons and sites. Both species exhibited fast growth and high N2 fixation and produced high biomass in a short growth period. Due to photoperiod sensitivity, the use of S. rostrata was limited during the short-day period. A. afraspera was found to be less sensitive to photoperiodism. Both species increased significantly the grain yield of rice. In the short-day period A. afraspera seemed superior-its productivity is higher than that of S. rostrata. In the long-day period, the same increase in yield was reached with both species but A. afraspera had lower biomass and was less labor-intensive to incorporate into soil.

Effect of NPK on growth and nitrogen fixation of Sesbania rostrata as a green manure for lowland rice (Oryza sativa L.)

The stem-nodulating tropical legume Sesbania rostrata is a promising green manure species for low input rice-farming systems in lowland areas. However, its success as biofertilizer depends on its biomass production and N2 fixation. Nutrient imbalances and soils low in available nutrients can considerably affect biofertilizer production. Use of mineral N, P, and K fertilizers in growing S. rostrata as biofertilizer for lowland rice was therefore evaluated in pot experiments, and in the fields in Central Luzon, Philippines. Two soils low in Olsen P (3–7.3 mg kg−1) and exchangeable K (0.05–0.08 meq 100g-1) were used. Increasing amounts of N (0, 10, 20, 30, and 40 mg kg-1), P (0, 50, and 100 mg kg-1), and K (0, 100, 200, and 300 mg kg-1) were applied to S. rostrata grown in the greenhouse, whereas small amounts of N, P, and K fertilizers (30, 15, and 33 kg ha-1, respectively) were applied in the field.Mineral N application depressed nodulation and N2 fixation in roots. It however, stimulated nodulation and N2 fixation in stems. Applying 30 kg N ha-1 as urea increased total N accumulation by S. rostrata and yield of the subsequent rice crop (IR64). Applied P and K both stimulated growth, nodulation, and N2 fixation of S. rostrata. Nitrogen accumulation in P- and K-fertilized S. rostrata was about 40% higher than that in nonfertilized green manure. Thus integration of mineral N, P, and K fertilizers in a green manure-based rice-farming system can considerably improve biofertilizer production and increase rice grain yield.

Spatial and temporal variability of surface water pollution in the Mekong Delta, Vietnam.

Surface water pollution in the Vietnamese Mekong Delta (MD) could threaten human, animal and ecosystem health given the fact that this water source is intensively used for drinking, irrigation and domestic services. We therefore determined the levels of pollution by organic pollutants, salts, metals and microbial indicators by (bi)monthly monitoring of canals between November 2011 and July 2012 at 32 sampling locations, representing fresh and saline/brackish environments. The results were compared with national water quality guidelines, between the studied regions and with water quality data from main waterways. Key factors explaining the observed levels of pollution in surface water were identified through principal component analysis (PCA). Temporal variations due to tidal regime and seasonality were also assessed. Based on regression models, the spatial variability of five water quality parameters was visualized using GIS based maps. Results indicate that pH (max. 8.6), turbidity (max. 461 FTU), maximum concentrations of ammonium (14.7 mg L(-1)), arsenic (44.1 μg L(-1)), barium (157.5 μg L(-1)), chromium (84.7 μg L(-1)), mercury (45.5 μg L(-1)), manganese (1659.7 μg L(-1)), aluminum (14.5 mg L(-1)), iron (17.0 mg L(-1)) and the number of Escherichia coli (87,000 CFU 100 mL(-1)) and total coliforms (2,500,000 CFU 100 mL(-1)) in canals exceed the thresholds set by Vietnamese quality guidelines for drinking and domestic purposes. The PCA showed that i) urbanization; ii) metal leaching from soils; iii) aquaculture; and iv) tidal regime explain 85% of the variance of surface water quality attributes. Significant differences in water quality were found due to daily tidal regime and as a result of seasonality. Surface water quality maps for dissolved oxygen, ammonium, ortho-phosphate, manganese and total coliforms were developed to highlight hot-spot areas of pollution. The results of this study can assist policy makers in developing water management strategies and drinking water companies in selecting optimum water extraction locations.

Stem-nodule symbiosis and its unusual properties

Although the phenomenon of stem nodulation was first reported in 1928 in Aeschynomene spp. by Hagerup (8), actual interest arose only after the recent discovery of Sesbania rostrata (4). S. rostrata is a remarkably fast-growing and high N2-fixing legume with a great potential as a source of N for rice (11). This interest has led to the identification of yet another stem-nodulating species, Aeschynomene afraspera, with comparable or higher potential than S. rostrata (3). Stem nodulation, a phenomenon that evolved in nature as a result of adaptation to flooding, is characterized by several unique features: 1) increasing number of infection sites with plant growth, 2) cyclic nature of symbiosis, 3) tolerance for combined N, 4) presence of photosynthetic cells around the nodules, 5) dinitrogen-dependent growth and N2 fixation by stem-nodule rhizobia, and 6) photosynthetic nature of stem-nodule rhizobia. Due to these unusual features, there has been extensive research on stem nodulation recently. This paper reviews the most recent findings on stem nodulation.

Evaluating the use of two contrasting legume species as relay intercrop in upland rice cropping systems

Abstract In the savanna zone of West Africa with a prolonged dry season, the successful growth of legumes as short-season fallow necessitates relay establishment of cover crops into the preceding food crop. Inappropriate choice of legume species and establishment dates may result in severe interspecific competition for light, water and nutrients and a subsequent yield loss in the main crop. In the current study, the highly competitive Cajanus cajan L. and the weakly competitive Stylosanthes hamata Taub. were relay-seeded into two upland rice cultivars with different competitive ability (WAB56-50, WAB450-24-3-2-P18-HB syn. V4 ) at different sowing dates, ranging from 0 to 84 days after rice sowing (DARS). Using a logistic function was adequate to describe rice grain yield, final rice biomass and legume biomass at 1 week after rice harvest as a function of legume sowing date. Rice biomass and grain yield were significantly ( P C. cajan was the most appropriate legume species of the two under study, to suit a relay intercropping system with upland rice, as it produced reasonable amounts of biomass at low levels of rice grain yield loss. This conclusion was confirmed by results on the after-effect of the legumes, where only the improved fallow by C. cajan gave a significant increase in the yield of the subsequent rice crop. Optimum sowing time of C. cajan was between 30 and 35 DARS. The effect of selecting a specific legume was found to surpass that of choosing either one of the two rice cultivars, for which contrasting results were obtained in 2 years of experimentation.

Aeschynomene as green manure for rice

A preliminary investigation was conducted in 1 m2 microplots using two stem nodulatedAeschynomene species,A. afraspera andA. nilotica, to evaluate their beneficial effects as green manure on the yield and the nitrogen content of rice.A. afraspera andA. nilotica yielded 7.5 and 9.2 kg m−2 of fresh matter, (42.3 and 53.2 g N m−2 respectively) in 7 weeks growth. Green manuring increased grain yield by more than 80% over the control and by 45% over rice fertilized with 10 g N-urea m−2. The nitrogen content of grain and straw was significantly higher in treatments with Aeschynomene green manure than in other treatments.

Mapping small wetlands of Kenya and Tanzania using remote sensing techniques

Abstract Although wetlands in Tanzania and Kenya have great potentials for agricultural production and a multitude of uses, many of them are not even documented on official maps. Lack of official recognition has done little in preventing there over utilization. As the wetlands continue to play remarkable roles in the movement of people and terrestrial species in the region, it is important that they are monitored and properly managed. This study was undertaken in Usambara highlands and the Pangani floodplain in Tanzania, the Mount Kenya highlands and Laikipia floodplain in Kenya to map the different types of wetlands in terms of their size, density, spatial distribution and use patterns. Remote sensing techniques and field surveys were adopted, and 51 wetlands were identified in flood plains within the semi-arid and sub-humid lowlands, and inland valleys in the region. The detailed maps generated showed the intensity of wetland use, inland valleys being the most intensively used, and are useful in monitoring changes in wetlands for their effective management. The use of multispatial resolution imagery, combined with field survey and GIS produced satisfactory results for the delineation and mapping of small wetlands and their uses.

Agronomic and economic evaluation of Sesbania rostrata green manure establishment in irrigated rice

Abstract Research on crop establishment methods may improve green manure performance, reduce costs, and increase the adaptability of pre-rice green manure technology in lowland rice-based cropping systems. A two-season field experiment was conducted at the International Rice Research Institute (IRRI) in Los Banos, Philippines in 1991–1992 to compare four establishment practices of Sesbania rostrata green manure (zero tillage, with tillage, relay cropping in rice for 2 or 4 weeks) with four mineral N fertilizer levels (0, 30, 60, and 90 kg urea N/ha) in an intensive irrigated lowland system with three rice crops per year. S. rostrata was grown twice a year during the 43-day dry-wet and wet-dry transition periods between the wet and dry season rice crops. Grain yield potential and fertilizer responsiveness of rice was generally highest in the dry season. On the other hand, S. rostrata growth was more vigorous in the wet season (long-day period) than in the dry season, regardless of establishment method. Green manure N accumulation was lowest with zero tillage (30 and 90 kg N/ha in dry and wet season, respectively) and highest when it was relay-cropped for two weeks (60 and 180 kg N/ha in dry and wet season, respectively). Land preparation for Sesbania ensured best green manure stand (> 100 plants/m 2 vs 20–40 plants/m 2 at no-till establishments) but increased costs of green manuring by US