Biomarker pigment signatures in Cochin back water system e A tropical estuary south west coast of India

Sedimentary biomarker pigments around Cochin estuary situated in the southwest coast of India were determined by HPLC. Fucoxanthin, an indicator of diatom was observed to be the most abundant carotenoid pigment in the estuary. Dinoflagellate derived carotenoid pigment peridinin was confined in the southern part of estuary and zeaxanthin pigment indicative of cyanobacteria were more found in sites influenced by anthropogenic activities. One compound having close similarity to fucoxanthin was also detected. Alloxanthin (cryptophyceae), chl b (green algae), canthaxanthin, neoxanthin, lutein and peridinin isomer were also detected by spectra and corresponding algal class were identified. The highest concentration of chl a (11.01 mg g 1) found near to the anthropogenic affected area while the lowest chl a (0.65 mg g 1) was recorded in industrial area. Degradation products of chl a, such as pheophorbide and pheophytin were observed and principal mode of mechanism of degradation were derived. Higher pheopigments content than chl a, reflects a density trapping of dead cells and early degradation of phytopigments from grazing activities

Population characteristics and taxonomy of lantern fishes of genus Diaphus (Family Myctophidae) off south west coast of India

Globally most of the conventional fish stocks have reached a state of optimum exploitation or even over-exploitation; efficient utilization of non-conventional resources is necessary to meet the supply-demand gap for protein supply. Mesopelagic fishes can be considered as one such promising resource for the future, if appropriate harvest and post-harvest technologies are developed. Increasing human population and increasing demand for cheaper food fishes has made myctophids a possible potential resource for future exploitation and utilization. Earlier studies indicated the abundance of Diaphus spp. in the eastern and northeastern Arabian Sea. The present study also indicates the dominance of Diaphus spp. in the deep sea trawling grounds of south west coast of India. Commercial viability of the myctophid fishing in the Indian waters has to be worked out. The present catch estimation is based on the Stratified Random Sampling Method from the landing data. As the coverage of sampling area was limited and the gear efficiency was not standardized, the data generated are not precise. A counter check for the estimates is also not possible due to the absence of comparable works in the study area. Fish biomass estimation by acoustics survey coupled with direct fishing would only confirm the accuracy of estimates. Exploratory surveys for new fishing areas to be continued, for gathering the distribution, abundance, biological and ecological data and map the potential fishing ground on a GIS platform and the data should be provided to the commercial entrepreneurs. Generally non-conventional and non-targeted resources are under low fishing pressure and exploitation rates. Low values of fishing mortality and exploitation rates indicate that removal from the stock by fishing was only nominal from the present fishing grounds. The results indicate that the stock is almost at virgin state and remains grossly underexploited. Since the extent of distribution and abundance of the stock in the ecosystem remains to be ascertained, sustainable yield could not be estimated. Also the impact of myctophids harvest, on other commercially important fishes, has to be studied.

Root-induced increases in soil pH and nutrient availability to field-grown cereals and legumes on acid sandy soils of Sudano-Sahelian West Africa

A field experiment with millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata L.) and groundnut (Arachnis hypogeae L.) was conducted on severely P-deficient acid sandy soils of Niger, Mali and Burkina Faso to measure changes in pH and nutrient availability as affected by distance from the root surface and by mineral fertiliser application. Treatments included three rates of phosphorus (P) and four levels of nitrogen (N) application. Bulk, rhizosphere and rhizoplane soils were sampled at 35, 45 and 75 DAS in 1997 and at 55 and 65 DAS in 1998. Regardless of the cropping system and level of mineral fertiliser applied, soil pH consistently increased between 0.7 and two units from the bulk soil to the rhizoplane of millet. Similar pH gradients were observed in cowpea, but pH changes were much smaller in sorghum with a difference of only 0.3 units. Shifts in pH led to large increases in nutrient availability close to the roots. Compared with the bulk soil, available P in the rhizoplane was between 190 and 270% higher for P-Bray and between 360 and 600% higher for P-water. Exchangeable calcium (Ca) and magnesium (Mg) levels were also higher in the millet rhizoplane than in the bulk soil, whereas exchangeable aluminium (Al) levels decreased with increasing pH close to the root surface. The results suggest an important role of root-induced pH increases for crops to cope with acidity-induced nutrient deficiency and Al stress of soils in the Sudano-Sahelian zone of West Africa.

Mechanisms of residue mulch-induced cereal growth increases in West Africa

The use of crop residues (CR) has been widely reported as a means of increasing crop yields across West Africa. However, little has been done to compare the magnitude and mechanisms of CR effects systematically in the different agro-ecological zones of the region. To this end, a series of field trials with millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], and maize (Zea mays L.) was conducted over a 4-yr period in the Sahelian, Sudanian, and Guinean zones of West Africa. Soils ranged in pH from 4.1 to 5.4 along a rainfall gradient from 510 to 1300 mm. Treatments in the factorial experiments were three CR rates (0,500, and 2000 kg ha^-1)and several levels of phosphorus and nitrogen. The results showed CR-induced total dry matter (TDM) increases in cereals up to 73% for the Sahel compared with a maximum of 16% in the wetter Sudanian and Guinean zones. Residue effects on weakly buffered Sahelian soils were due to improved P availability and to a protection of seedlings against wind erosion. Additional effects of CR mulching on topsoil properties in the Sahel were a decrease in peak temperatures by 4°C and increased water availability. These mulch effects on soil chemical and physical properties strongly decreased from North to South. Likely explanations for this decrease are the decline of dust deposition and wind erosion hazards, the higher soil clay content, lower air temperature, and a faster decomposition rate of mulch material with increasing rainfall from the Sahel to the Sudanian and Guinean zones.

Decomposition of and nutrient release from ruminant manure on acid sandy soils in the Sahelian zone of Niger, West Africa

In ago-pastoral systems of the semi-arid West African Sahel, targeted applications of ruminant manure to the cropland is a widespread practice to maintain soil productivity. However, studies exploring the decomposition and mineralisation processes of manure under farmers' conditions are scarce. The present research in south-west Niger was undertaken to examine the role of micro-organisms and meso-fauna on in situ release rates of nitrogen (N), phosphorus (P) and potassium (K) from cattle and sheep-goat manure collected from village corrals during the rainy season. The results show tha (1) macro-organisms played a dominant role in the initial phase of manure decomposition; (2) manure decomposition was faster on crusted than on sandy soils; (3) throughout the study N and P release rates closely followed the dry matter decomposition; (4) during the first 6 weeks after application the K concentration in the manure declined much faster than N or P. At the applied dry matter rate of 18.8 Mg ha^-1, the quantities of N, P and K released from the manure during the rainy season were up to 10-fold larger than the annual nutrient uptake of pearl millet (Pennisetum glaucum L.), the dominant crop in the traditional agro-pastoral systems. The results indicate considerable nutrient losses with the scarce but heavy rainfalls which could be alleviated by smaller rates of manure application. Those, however, would require a more labour intensive system of corralling or manure distribution.

Effects of fertility management strategies on phosphorus bioavailability in four West African soils

Low phosphorus (P) in acid sandy soils of the West African Sudano-Sahelian zone is a major limitation to crop growth. To compare treatment effects on total dry matter (TDM) of crops and plant available P (P-Bray and isotopically exchangeable P), field experiments were carried out for 2 years at four sites where annual rainfall ranged from 560 to 850 mm and topsoil pH varied between 4.2 and 5.6. Main treatments were: (i) crop residue (CR) mulch at 500 and 2000 kg ha^-1, (ii) eight different rates and sources of P and (iii) cereal/legume rotations including millet (Pennisetum glaucum L.), sorhum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). For the two Sahelian sites with large CR-induced differences in TDM, mulching did not modify significantly the soils' buffering capacity for phosphate ions but led to large increases in the intensity factor (C_p) and quantity of directly available soil P (E_1min). In the wetter Sudanian zone lacking effects of CR mulching on TDM mirrored a decline of E_1min with CR. Broadcast application of soluble single superphosphate (SSP) at 13 kg P ha^-1 led to large increases in C_p and quantity of E_1min at all sites which translated in respective TDM increases. The high agronomic efficiency of SSP placement (4 kg P ha^-1) across sites could be explained by consistent increases in the quantity factor which confirms the power of the isotopic exchange method in explaining management effects on crop growth across the region.

Efficient phosphorus application strategies for increased crop production in sub-Saharan West Africa

Comparable data are lacking from the range of environments found in sub-Saharan West Africa to draw more general conclusions about the relative merits of locally available rockphosphate (RockP) in alleviating phosphorus (P) constraints to crop growth. To fill this gap, a multi-factorial field experiment was conducted over 4 years at eight locations in Niger, Burkina Faso and Togo. These ranged in annual rainfall from 510 to 1300 mm. Crops grown were pearl millet (Pennisetum glaucum L.), sorghum (Sorghum bicolor (L.) Moench) and maize (Zea mays L.) either continuously or in rotation with cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). Crops were subjected to six P fertiliser treatments comprising RockP and soluble P at different rates and combined with 0 and 60 kg N ha^-1. For legumes, time trend analyses showed P-induced total dry matter (TDM) increases between 28 and 72% only with groundnut. Similarly, rotation-induced raises in cereal TDM compared to cereal monoculture were only observed with groundnut. For cereals, at the same rate of application, RockP was comparable to single superphosphate (SSP) only at two millet sites with topsoil pH-KCl <4.2 and annual average rainfall >600 mm. Across the eight sites NPK placement at 0.4 g P per hill raised average cereal yields between 26 and 220%. This was confirmed in 119 on-farm trials revealing P placement as a promising strategy to overcome P deficiency as the regionally most growth limiting nutrient constraint to cereals.

Cereal/legume rotations affect chemical properties and biological activities in two West African soils

A more widespread use of cereal/legume rotations has been suggested as a means to sustainably meet increasing food demands in sub-Saharan West Africa. Enhanced cereal yields following legumes have been attributed to chemical and biological factors such as higher levels of mineral nitrogen (Nmin) and arbuscular mycorrhizae (AM) but also to lower amounts of plant parasitic nematodes. This study was conducted under controlled conditions to examine the relative contribution of AM, plant parasitic nematodes and increased nitrogen (N) and phosphorus (P) availability to cereal/legume rotation effects on two West African soils. Sample soils were taken from field experiments at Gaya (Niger) and Fada (Burkina Faso) supporting continuous cereal and cereal/legume rotation systems and analysed for chemical and biological parameters. Average increases in cereal shoot dry matter (DM) of rotation cereals compared with continuous cereals were 490% at Gaya and 550% at Fada. Shoot P concentration of rotation millet was significantly higher than in continuous millet and P uptake in rotation cereals was on average 62.5-fold higher than in continuous cereals. Rotation rhizosphere soils also had higher pH at both sites. For the Fada soil, large increases in Bray1-P and organic P were observed in bulk and rhizosphere soils. Plant parasitic nematodes in roots of continuous cereals were 60–80-fold higher than in those of rotation cereals. In both cropping systems mycorrhizal infection rates were similar at 37 days after sowing (DAS) but at 57 DAS AM infection was 10–15% higher in rotation sorghum than in continuous sorghum. This study provides strong evidence that cereal/legume rotations can enhance P nutrition of cereals through improved soil chemical P availability and microbiologically increased P uptake.

Causes of legume-rotation effects in increasing cereal yields across the Sudanian, Sahelian and Guinean zone of West Africa

On-farm experiments and pot trials were conducted on eight West African soils to explore the mechanisms governing the often reported legume rotation-induced cereal growth increases in this region. Crops comprised pearl millet (Pennisetum glaucum L.), sorghum (Sorghum bicolor Moench), maize (Zea mays L.), cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). In groundnut trials the observed 26 to 85% increases in total dry matter (TDM) of rotation cereals (RC) compared with continuous cereals (CC) in the 4th year appeared to be triggered by site- and crop-specific early season differences in nematode infestation (up to 6-fold lower in RC than in CC), enhanced Nmin and a 7% increase in mycorrhizal (AM) infection. In cowpea trials yield effects on millet and differences in nematode numbers, Nmin and AM were much smaller. Rhizosphere studies indicated effects on pH and acid phosphatase activity as secondary causes for the observed growth differences between RC and CC. In the study region legume-rotation effects on cereals seemed to depend on the capability of the legume to suppress nematodes and to enhance early N and P availability for the subsequent cereal.

Soil organic carbon management for sustainable land use in Sudano-Sahelian West Africa

Judged by their negative nutrient balances, low soil cover and low productivity, the predominant agro-pastoral farming systems in the Sudano-Sahelian zone of West Africa are highly unsustainable for crop production intensification. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less than 1 cmol_c kg^-1 soil, depends heavily on the organic carbon (Corg) content. However, due to low carbon sequestration and to the microbe, termite and temperature-induced rapid turnover rates of organic material in the present land-use systems, Corg contents of the topsoil are very low, ranging between 1 and 8 g kg^-1 in most soils. For sustainable food production, the availability of phosphorus (P) and nitrogen (N) has to be increased considerably in combination with an improvement in soil physical properties. Therefore, the adoption of innovative management options that help to stop or even reverse the decline in Corg typically observed after cultivating bush or rangeland is of utmost importance. To maintain food production for a rapidly growing population, targeted applications of mineral fertilisers and the effective recycling of organic amendments as crop residues and manure are essential. Any increase in soil cover has large effects in reducing topsoil erosion by wind and water and favours the accumulation of wind-blown dust high in bases which in turn improves P availability. In the future decision support systems, based on GIS, modelling and simulation should be used to combine (i) available fertiliser response data from on-station and on-farm research, (ii) results on soil productivity restoration with the application of mineral and organic amendments and (iii) our present understanding of the cause-effect relationships governing the prevailing soil degradation processes. This will help to predict the effectiveness of regionally differentiated soil fertility management approaches to maintain or even increase soil Corg levels.

Multi-site time-trend analysis of soil fertility management effects on crop production in sub-Saharan West Africa

Soil fertility constraints to crop production have been recognized widely as a major obstacle to food security and agro-ecosystem sustainability in sub-Saharan West Africa. As such, they have led to a multitude of research projects and policy debates on how best they should be overcome. Conclusions, based on long-term multi-site experiments, are lacking with respect to a regional assessment of phosphorus and nitrogen fertilizer effects, surface mulched crop residues, and legume rotations on total dry matter of cereals in this region. A mixed model time-trend analysis was used to investigate the effects of four nitrogen and phosphorus rates, annually applied crop residue dry matter at 500 and 2000 kg ha^-1, and cereal-legume rotation versus continuous cereal cropping on the total dry matter of cereals and legumes. The multi-factorial experiment was conducted over four years at eight locations, with annual rainfall ranging from 510 to 1300 mm, in Niger, Burkina Faso, and Togo. With the exception of phosphorus, treatment effects on legume growth were marginal. At most locations, except for typical Sudanian sites with very low base saturation and high rainfall, phosphorus effects on cereal total dry matter were much lower with rock phosphate than with soluble phosphorus, unless the rock phosphate was combined with an annual seed-placement of 4 kg ha^-1 phosphorus. Across all other treatments, nitrogen effects were negligible at 500 mm annual rainfall but at 900 mm, the highest nitrogen rate led to total dry matter increases of up to 77% and, at 1300 mm, to 183%. Mulch-induced increases in cereal total dry matter were larger with lower base saturation, reaching 45% on typical acid sandy Sahelian soils. Legume rotation effects tended to increase over time but were strongly species-dependent.

Phosphorus placement an acid arenosols of the West African Sahel

Phosphorus (P) deficiency is a major constraint to pearl millet (Pennisetum glaucum L.) growth on acid sandy soils of the West African Sahel. To develop cost-effective fertilization strategies for cash poor farmers, experiments with pearl millet were conducted in southwestern Niger. Treatments comprised single superphosphate hill-placed at rates of 1, 3, 5 or 7 kg P ha^−1 factorially combined with broadcast P at a rate of 13 kg ha^−1. Nitrogen was applied as calcium ammonium nitrate at rates of 30 and 45 kg ha^−1. At low soil moisture, placement of single superphosphate in immediate proximity to the seed reduced seedling emergence. Despite these negative effects on germination, P placement resulted in much faster growth of millet seedlings than did broadcast P. With P application, potassium nutrition of millet was improved and seedling nitrogen uptake increased two- to three-fold, indicating that nitrogen was not limiting early millet growth. Averaged over the 1995 and 1996 cropping seasons, placed applications of 3, 5 and 7 kg P ha^−1 led to 72%, 81% and 88% respectively, of the grain yield produced by broadcasting 13 kg P ha^−1. Nitrogen application did not show major effects on grain yield unless P requirements were met. A simple economic analysis revealed that the profitability of P application, defined as additional income per unit of fertilizer, was highest for P placement at 3 and 5 kg ha^−1.

Cereal/legume rotation effects on rhizosphere bacterial community structure in West African soils

The increased use of cereal/legume crop rotation has been advocated as a strategy to increase cereal yields of subsistence farmers in West Africa, and is believed to promote changes in the rhizosphere that enhance early plant growth. In this study we investigated the microbial diversity of the rhizoplane from seedlings grown in two soils previously planted to cereal or legume from experimental plots in Gaya, Niger, and Kaboli, Togo. Soils from these legume rotation and continuous cereal plots were placed into containers and sown in a growth chamber with maize (Zea mays L.), millet (Pennisetum glaucum L.), sorghum (Sorghum bicolor L. Moench.), cowpea (Vigna unguiculata L.) or groundnut (Arachis hypogaea L.). At 7 and 14 days after sowing, 16S rDNA profiles of the eubacterial and ammoniaoxidizing communities from the rhizoplane and bulk soil were generated using denaturing gradient gel electrophoresis (DGGE). Community profiles were subjected to peak fitting analyses to quantify the DNA band position and intensities, after which these data were compared using correspondence and principal components analysis. The data showed that cropping system had a highly significant effect on community structure (p <0.005), irrespective of plant species or sampling time. Continuous cereal-soil grown plants had highly similar rhizoplane communities across crop species and sites, whereas communities from the rotation soil showed greater variability and clustered with respect to plant species. Analyses of the ammonia-oxidizing communities provided no evidence of any effects of plant species or management history on ammonia oxidizers in soil from Kaboli, but there were large shifts with respect to this group of bacteria in soils from Gaya. The results of these analyses show that crop rotation can cause significant shifts in rhizosphere bacterial communities.

Legume rotation effects on early growth and rhizosphere microbiology of sorghum in West African soils

Cereal yield increases in legume rotations on west African soils were the subject of much recent research aiming at the development of more productive cropping systems for the mainly subsistence-oriented agriculture in this region. However, little has been done to elucidate the possible contribution of soil microbiological factors to these rotation effects. Therefore a pot trial was conducted using legume rotation and continuous cereal soils each from one site in Burkina Faso and two sites in Togo where cropping system experiments had been conducted over 4 yrs. All soils were planted with seedlings of sorghum (Sorghum bicolor L. Moench). From 21 days after sowing onwards relative growth rates in rotation soils were higher than in the continuous cereal soils, resulting in between 69 and 500% higher shoot dry matter of rotation sorghum compared to sorghum growing in continuous cereal soils. Across sites rotation soils were characterized by higher pH, higher microbial N and a lower microbial biomass C/N ratio and, with the exception of one site, a higher fungal biomass in the rhizosphere. The bacterial and eukaryal community structure in the soil, assessed by denaturing gradient gel electrophoresis (DGGE), differed between sites. However, only at one site differed the bacterial and the eukaryal community structure in the rotation soil significantly from that in the continuous cereal soil. Although the results of this study confirmed the marked plantgrowth differences between sub-Saharan legume-rotation soils and their continuous cereal counterparts they also showed the difficulties to differentiate possible microbiological causes from their effects.