Tropical Cropping Systems Research Articles

Short-term effects of crop rotation, residue management, and soil water on carbon mineralization in a tropical cropping system

The purpose of this study was to investigate the short-term effects of maize (Zea mays)-fallow rotation, residue management, and soil water on carbon mineralization in a tropical cropping system in Ghana. After 15 months of the trial, maize–legume rotation treatments had significantly (P < 0.001) higher levels of potentially mineralizable carbon, C0 (μg CO2–C g−1) than maize–elephant grass (Pennisetum purpureum) rotations. The C0 for maize–grass rotation treatments was significantly related to the biomass input (r = 0.95; P = 0.05), but that for the maize–legume rotation was not. The soil carbon mineralization rate constant, k (per day), was also significantly related to the rotation treatments (P < 0.001). The k values for maize–grass and maize–legume rotation treatments were 0.025 and 0.036 day−1 respectively. The initial carbon mineralization rate, m0 (μg CO2–C g−1 day −1), was significantly (P < 0.001) related to the soil water content, θ. The m0 ranged from 3.88 to 18.67 and from 2.30 to 15.35 μg CO2–C g−1 day−1 for maize–legume and maize–grass rotation treatments, respectively, when the soil water varied from 28% to 95% field capacity (FC). A simple soil water content (θ)-based factor, fw, formulated as: \(f_{\text{w}} = \left[ {\frac{{\theta - \theta _{\text{d}} }}{{\theta _{{\text{FC}}} - \theta _{\text{d}} }}} \right]\), where θd and θFC were the air-dry and field capacity soil water content, respectively, adequately described the variation of the m0 with respect to soil water (R2 = 0.91; RMSE = 1.6). Such a simple relationship could be useful for SOC modeling under variable soil water conditions.

Soil carbon dynamics and residue stabilization in a Costa Rican and southern Canadian alley cropping system

Agroforestry systems can play a major role in the sequestration of carbon (C) because of their higher input of organic material to the soil compared to sole crop agroecosystems. This study quantified C input in a 19-year old tropical alley cropping system with E. poeppigiana (Walp.) O.F Cook in Costa Rica and in a 13-year old hybrid poplar (Populus deltoides × nigra DN-177) alley cropping system in southern Canada. Changes in the level of the soil organic carbon (SOC) pool, residue decomposition rate, residue stabilization efficiency, and the annual rate of accumulation of SOC were also quantified in both systems. Carbon input from tree prunings in Costa Rica was 401 g C m−2 y−1 compared to 117 g C m−2 y−1 from litterfall at the Canadian site. In southern Canada, crop residue input from maize (Zea mays L.) was 212 g C m−2 y−1, 83 g C m−2 y−1 from soybeans (Glycine max L.) and 125 g C m−2 y−1 for wheat (Triticum aestivum L.), and was not significantly different (p < 0.05) from the sole crop. The average yearly C input from crop residues in Costa Rica was significantly greater (p < 0.05) in the alley crop for maize (134 g C m−2 y−1) and Phaseolus vulgaris L. bean crops (35 g C m−2 y−1) compared to the sole crop. The SOC pool was significantly greater (p < 0.05) in the Costa Rican alley crop (9536 g m−2) compared to its respective sole crop (6143 g m−2) to a 20 cm depth, but no such difference was found for the southern Canadian system. Residue stabilization, defined as the efficiency of the stabilization of added residue (crop residues, tree prunings, litterfall) that is added to the soil C pool, is more efficient in southern Canada (31%) compared to the alley cropping system in Costa Rica (40%). This coincides with a lower organic matter decomposition rate (0.03 y−1) to a 20 cm depth in Canada compared to the Costa Rican system (0.06 y−1). However, the average annual accumulation rate of SOC is greater in Costa Rica (179 g m−2 y−1) and is likely related to the greater input of organic material derived from tree prunings, compared to that in southern Canada (30 g m−2 y−1) to a 20 cm depth.

Potential of Carbon Accumulation in No‐Till Soils with Intensive Use and Cover Crops in Southern Brazil

The area under no-till (NT) in Brazil reached 22 million ha in 2004-2005, of which approximately 45% was located in the southern states. From the 1970s to the mid-1980s, this region was a source of carbon dioxide to the atmosphere due to decrease of soil carbon (C) stocks and high consumption of fuel by intensive tillage. Since then, NT has partially restored the soil C lost and reduced the consumption of fossil fuels. To assess the potential of C accumulation in NT soils, four long-term experiments (7-19 yr) in subtropical soils (Paleudult, Paleudalf, and Hapludox) varying in soil texture (87-760 g kg(-1) of clay) in agroecologic southern Brazil zones (central region, northwest basaltic plateau in Rio Grande Sul, and west basaltic plateau in Santa Catarina) and with different cropping systems (soybean and maize) were investigated. The lability of soil organic matter (SOM) was calculated as the ratio of total organic carbon (TOC) to particulate organic carbon (POC), and the role of physical protection on stability of SOM was evaluated. In general, TOC and POC stocks in native grass correlated closely with clay content. Conversely, there was no clear effect of soil texture on C accumulation rates in NT soils, which ranged from 0.12 to 0.59 Mg ha(-1) yr(-1). The C accumulation was higher in NT than in conventional-till (CT) soils. The legume cover crops pigeon pea [Cajanus cajan (L.) Millsp] and velvet beans (Stizolobium cinereum Piper & Tracy) in NT maize cropping systems had the highest C accumulation rates (0.38-0.59 Mg ha(-1) yr(-1)). The intensive cropping systems also were effective in increasing the C accumulation rates in NT soils (0.25-0.34 Mg ha(-1) yr(-1)) when compared to the double-crop system used by farmers. These results stress the role of N fixation in improving the tropical and subtropical cropping systems. The physical protection of SOM within soil aggregates was an important mechanism of C accumulation in the sandy clay loam Paleudult under NT. The cropping system and NT effects on C stocks were attributed to an increase in the lability of SOM, as evidenced by the higher POC to TOC ratio, which is very important to C and energy flux through the soil.

Mycorrhizas and tropical soil fertility

Major factors that constrain tropical soil fertility and sustainable agriculture are low nutrient capital, moisture stress, erosion, high P fixation, high acidity with aluminium toxicity, and low soil biodiversity. The fragility of many tropical soils limits food production in annual cropping systems. Because some tropical soils under natural conditions have high biological activity, an increased use of the biological potential of these soils to counter the challenges of food production problems is proposed. Most plant species (including the major crops in the tropics) form beneficial associations with arbuscular mycorrhizal (AM) fungi. These fungi could be the most important and poorly understood resource for nutrient acquisition and plant growth in agriculture. This review treats the role of AM fungi in enhancing physical, chemical, and biological soil quality. It focuses on the roles of AM in maintenance and improvement of soil structure, the uptake of relatively immobile elements, both macronutrients (phosphorus) and micronutrients (zinc), the alleviation of aluminium and manganese toxicity, the interactions with other beneficial soil organisms (nitrogen-fixing rhizobia), and improved protection against pathogens. Mycorrhizal associations enable a better use of sparingly soluble phosphorus pools, thereby increasing the efficiency of added phosphorus fertilizer and of the large relatively immobile phosphorus pools. Mycorrhizal management through agroforestry, reduced soil disturbance or crop rotation, is often a better option than mycorrhizal inoculation, considering the problems and costs of large-scale inoculum production. Research directions that are needed to increase understanding of mycorrhizal associations in tropical cropping systems and to increase mycorrhizal benefit are indicated.

A daily multi-layered water balance model to predict water and oxygen availability in tropical cropping systems

Abstract. A daily multi-layered water balance (DAMUWAB) approach has been formulated with modest input data on daily climatic records, slope gradient, physical soil properties and basic crop characteristics, for predicting the water and oxygen availability in freely drained soils of semiarid as well as subhumid tropical cropping systems. In this study, a simple one-dimensional root extension model is coupled to a water-uptake model that differentiates the uptake capacity of roots according to their depth. It has an intermediate level of detail between functional, 10-day period single-layered water balance (DESIWAB) approaches for land evaluation purposes, and the descriptive, daily water balances used in many crop growth models. Modelling capacity, functionality and expandability of DAMUWAB surpass the performance of DESIWAB. Water and oxygen shortages occurring in each compartment were found to limit water uptake and transpiration. Sensitivity analysis and comparison of DAMLWAB with the worldwide operational DESIWAB approach proved successful for model validation in the tropics and subtropics.

A daily multi-layered water balance model to predict water and oxygen availability in tropical cropping systems

Abstract. A daily multi-layered water balance (DAMUWAB) approach has been formulated with modest input data on daily climatic records, slope gradient, physical soil properties and basic crop characteristics, for predicting the water and oxygen availability in freely drained soils of semiarid as well as subhumid tropical cropping systems. In this study, a simple one-dimensional root extension model is coupled to a water-uptake model that differentiates the uptake capacity of roots according to their depth. It has an intermediate level of detail between functional, 10-day period single-layered water balance (DESIWAB) approaches for land evaluation purposes, and the descriptive, daily water balances used in many crop growth models. Modelling capacity, functionality and expandability of DAMUWAB surpass the performance of DESIWAB. Water and oxygen shortages occurring in each compartment were found to limit water uptake and transpiration. Sensitivity analysis and comparison of DAMLWAB with the worldwide operational DESIWAB approach proved successful for model validation in the tropics and subtropics.

The Role of Particulate Organic Matter in Phosphorus Cycling

In tropical cropping systems, a better understanding of P transformations and short-term P cycling during decomposition of incorporated residues is pertinent. The primary objectives of this study were (i) to assess P immobilization in decomposing residues of sorghum (Sorghum bicolor (L.) Moench.) and crotalaria (Crotalaria juncea L.), (ii) to establish the role of soil fungi in the process, and (iii) to determine the contribution of P immobilization in explaining differences in soil P availability from plant residues in two weathered soils (Ultisol and Alfisol). Phosphorus associated with particulate organic matter (POM-P) was measured at different residue decomposition stages. Irradiated and nonirradiated residues were utilized to evaluate the origin of fungal colonization of particulate organic matter (POM). Results indicate a significant release of P from the residues at early stages of decomposition. An average of 93 and 76% of the initial POM-P was released after 5 d of decomposition in the Ultisol and Alfisol, respectively; and no significant differences were found between the residue types. After this initial 5-d period, significantly higher values of POM-P were observed for sorghum (5.8 and 7.9 mg P kg−1) as compared with crotalaria (1.9 and 2.8 mg P kg−1) in the two soils, suggesting a higher P immobilization in decomposing sorghum residue. Phosphorus immobilization represented up to 30% of the P in the residues and it could account for observed differences in P availability in the amended soils. A correspondence between patterns of P accumulation and fungal colonization of POM suggested that soil fungi might be responsible for this P immobilization. Further research is required however, to identify the mechanisms regulating this fungal colonization and P immobilization of plant residues in tropical acid soils.

The Role of Particulate Organic Matter in Phosphorus Cycling

In tropical cropping systems, a better understanding of P transformations and short‐term P cycling during decomposition of incorporated residues is pertinent. The primary objectives of this study were (i) to assess P immobilization in decomposing residues of sorghum (Sorghum bicolor (L.) Moench.) and crotalaria (Crotalaria juncea L.), (ii) to establish the role of soil fungi in the process, and (iii) to determine the contribution of P immobilization in explaining differences in soil P availability from plant residues in two weathered soils (Ultisol and Alfisol). Phosphorus associated with particulate organic matter (POM‐P) was measured at different residue decomposition stages. Irradiated and nonirradiated residues were utilized to evaluate the origin of fungal colonization of particulate organic matter (POM). Results indicate a significant release of P from the residues at early stages of decomposition. An average of 93 and 76% of the initial POM‐P was released after 5 d of decomposition in the Ultisol and Alfisol, respectively; and no significant differences were found between the residue types. After this initial 5‐d period, significantly higher values of POM‐P were observed for sorghum (5.8 and 7.9 mg P kg−1) as compared with crotalaria (1.9 and 2.8 mg P kg−1) in the two soils, suggesting a higher P immobilization in decomposing sorghum residue. Phosphorus immobilization represented up to 30% of the P in the residues and it could account for observed differences in P availability in the amended soils. A correspondence between patterns of P accumulation and fungal colonization of POM suggested that soil fungi might be responsible for this P immobilization. Further research is required however, to identify the mechanisms regulating this fungal colonization and P immobilization of plant residues in tropical acid soils.

Influence of Soil Moisture and Fertilizer Potassium on the Vegetative Growth of Mungbean (Vigna radiata L. Wilczek) and Cowpea (Vigna unguiculata L. Walp)

Tropical food legumes are grown in a wide range of environments, and water stress is considered the principal environmental factor limiting growth and yield. Potassium fertilizer mitigates the impact of water stress in plants. However, the benefits of potassium in overcoming stress in tropical food legumes have not been investigated in comparative studies. The purpose of this study was to determine the benefits of potassium in overcoming water stress in mungbean and cowpea, two important tropical food legumes with different adaptabilities to soil moisture regimes. The experiment carried out under controlled conditions placed emphasis on vegetative growth and selected physiological parameters. The impact of potassium was different in the two legumes grown at optimal and suboptimal soil moisture. Potassium increased shoot growth of mungbean to a greater extent than in cowpea under suboptimal moisture conditions. The roots of cowpea showed a greater response to potassium fertilizer than in mungbean under suboptimal soil moisture. The plant water relations and photosynthetic rates of mungbean were improved to a greater extent by potassium under suboptimal soil moisture than those of cowpea. Although differences were observed in the responses of the vegetative growth of these species to moisture and potassium, in overall terms potassium promoted growth of both species when subject to suboptimal soil moisture. While field studies are required to validate the results, the application of potassium fertilizer can be considered a significant factor in overcoming soil moisture stress in these legumes commonly grown in tropical cropping systems.

Symbiotic Performance of Herbaceous Legumes in Tropical Cover Cropping Systems

Increasing use of herbaceous legumes such as mucuna (Mucuna pruriens var. utilis [Wright] Bruck) and lablab (Lablab purpureus [L.] Sweet) in the derived savannas of West Africa can be attributed to their potential to fix atmospheric nitrogen (N2). The effects of management practices on N2 fixation in mucuna and lablab were examined using 15N isotope dilution technique. Dry matter yield of both legumes at 12 weeks was two to five times more in in situ mulch (IM) than live mulch (LM) systems. Land Equivalent Ratios, however, showed 8 to 30% more efficient utilization of resources required for biomass production under LM than IM systems. Live mulching reduced nodule numbers in the legumes by one third compared to values in the IM systems. Similarly, nodule mass was reduced by 34 to 58% under LM compared to the IM systems. The proportion of fixed N2 in the legumes was 18% higher in LM than IM systems. Except for inoculated mucuna, the amounts of N fixed by both legumes were greater in IM than LM systems. Rhizobia inoculation of the legumes did not significantly increase N2 fixation compared to uninoculated plots. Application of N fertilizer reduced N2 fixed in the legumes by 36 to 51% compared to inoculated or uninoculated systems. The implications of cover cropping, N fertilization, and rhizobia inoculation on N contributions of legumes into tropical low-input systems were discussed.

Risk and Returns of Diversified Cropping Systems under Nonnormal, Cross-, and Autocorrelated Commodity Price Structures

This study analyzes the risks of diversified tropical cropping systems that combine cocoa, plantain, and tree-crop components in different proportions versus traditional monocultures. A technique for modeling the expected values, variances, and covariances of correlated time-series variables that are autocorrelated and nonnormal (right or left skewed and kurtotic) is applied to simulate commodity prices. The importance of using simulated cumulative density functions (cdf's) which reflect the most important characteristics of the stochastic behavior of prices for analyzing risk and returns of diversified agricultural systems is demonstrated. The analysis priovides evidence in favor of diversified cocoa-plantain-Cordia agroforestry system technologies versus the traditional monocultures.

Root-Knot Nematode Management in Tropical Cropping Systems with Organic Products

ABSTRACT Several products acceptable for organically-certified produce were evaluated in the field for their ability to control root-knot nematode, Meloidogyne javanica, in tropical cropping systems. In the first evaluation, tomato yield and nematode reproduction were recorded in plots treated with Nematrol (a ground sesame product) at 129, 256, 516, or 774 kg/ha (115, 230, 460, and 690 lb/ac); composted chicken manure at 11.2 or 22.4 T/ha (5 or 10 ton/ac); or blood meal at 8.3 or 12 T/ha (2.7 or 5.4 ton/ac). Plots treated with fenamiphos (7 or 14 g a.i./m row; 0.08 or 0.15 oz/ft) or left untreated were included as controls. Tomato yield was greatest in the 14 g (0.15 oz) a.i. fenamiphos-treated plots and lowest in the untreated plots. Nematrol at all rates gave better control than the untreated. Blood meal at the high rate may have been toxic to the tomatoes. In a second experiment, root-knot nematode control in basil with Nematrol (258 kg/ha; 230 lb/ac) or Clandosan (4.5 T/ha; 2 ton/ac) was compared to treatment with 1,3-dichloropropene (255.4 kg/ha; 227 lb/ac) and an untreated control. Final basil shoot biomass was greatest in the 1,3-dichloropropene treated plots followed by the Nematrol and Clandosan treatments. The untreated basil yielded the least of all treatments. Alternative products for root-knot nematode control were not as effective as the standard nematicides used as comparisons. However, yield was usually numerically greater than no treatment at all. The alternative controls for root-knot nematode may need to be combined with other tactics to reduce nematode damage.

Food Production, Conservation of Crop Plant Biodiversity and Environmental Protection in the Twenty-First Century: The Relevance of Tropical Cropping Systems

It is now clear that crop production technology and improvement programmes which work in temperate climates for large-scale intensive monoculture, cannot simply be transferred unmodified to the tropics. Tropical agriculture is dominated by small-scale extensive farming systems. Meeting the food and agricultural needs in the region therefore depends on the improvement and diversification of the cropping systems. Tropical cropping systems contribute to the sustainability of ecosystems, enhance the conservation and utilization of local crop diversity for food and agriculture, and take advantage of locally variable conditions. However, tropical agriculture has been seriously threatened by the importation of exotic crop varieties and the diminution of local ones, by the use of heavy machinery and agrochemicals, and by population growth. In order to further enhance sustainable production, more attention should be given to economically and ecologically viable adaptive research and crop variety improvement for mixed systems.

Mineral N dynamics in bare and cropped Leucaena leucocephala and Dactyladenia barteri alley cropping systems after the addition of 15N‐labelled leaf residues

SummaryIn tropical cropping systems with few external inputs, efficient management of mineral N derived from added organic residues is essential for the proper functioning of the system. We studied the dynamics of mineral nitrogen (N) in the top 100 cm of soil with a system of tensiometers and suction cups after applying 15N‐labelled Leucaena leucocephala and Dactyladenia barteri residues to bare and cropped microplots installed in the respective alley cropping systems, and followed the fate of the N for two maize‐cowpea rotations (1992 and 1993).Fifty days after applying the residues (DDA), 20% of the added residue N was found in the soil profile of the bare Leucaena treatment, and 5% under Dactyladenia, compared with 5% and 1%, respectively, where cropped. All values decreased to about 1% after 505 days. In the cropped soil, no mineral N derived from the residues was lost by leaching during the first 6 weeks.As the maize grew, the soil profile was gradually depleted of nitrate to near Zero in the Dactyladenia treatment, whereas during the cowpea season the amount of nitrate N increased to 36 kg N ha−1 for the Leucaena treatment, and 26 kg N ha−1 for the Dactyladenia treatment. The soil of the bare microplots contained substantially more nitrate N (98 and 47 kg N ha‐1 during the first year on average, under Leucaena and Dactyladenia, respetively) than that of the cropped microplots, except during the 1993 cowpea season. Nitrate residing in the subsoil (80–100 cm) in the bare treatments was not readily leached to deeper soil. The risk of losses of native mineral N was greatest during the first 50 DAA and to a lesser extent during the cowpea seasons. Improved management of the hedgerows could increase the potential of the hedgerow trees to recycle mineral N.

Soil Organic Matter Dynamics after Addition of Nitrogen‐15‐Labeled Leucaena and Dactyladenia Residues

AbstractQuantification of the movement of residue N through various biologically meaningful soil organic matter (SOM) fractions is essential in low‐input tropical cropping systems for the development of management practices aimed at optimizing N‐use efficiency. The flow of N derived from 15N‐labeled leucaena [Leucaena leucocephala (Lam.) de Wit] and dactyladenia [Dactyladenia barteri (Hook f ex Oliv.) Engl.] leaf residues was followed through the SOM of different particle‐size fractions during 858 d. A bioassay study with maize (Zea mays L.) was carried out to assess the availability of residue‐derived N (RDN) incorporated in the particle‐size fractions. At 53 d after residue application (DAA), most of the RDN was found in the fraction between 0.250 and 2 mm (O250) in the leucaena treatment and in the fraction &gt;2 mm and the surface litter (O2000 + SL) in the dactyladenia treatment. The RDN content of all fractions &lt;0.250 mm was significantly higher in the leucaena than in the dactyladenia treatment up to 471 DAA. The proportion of the total amount of RDN recovered in the fractions &lt;2 mm shifted toward the smaller particle‐size classes near the end of the experiment. Highly significant (P &lt; 0.001) relationships between RDN present in the particulate organic matter (POM) and uptake of RDN by maize indicated the relatively high availability of RDN in the POM. Fractionating the SOM pool into different particle‐size classes yielded useful information on the relative contributions of the different SOM fractions to N turnover and availability. This information is needed in identifying SOM pools with direct relevance to plant nutrition in low‐input cropping systems.

Recovery of Leucaena and Dactyladenia Residue Nitrogen‐15 in Alley Cropping Systems

AbstractQuantification of the fate of residue N is essential in low‐input tropical cropping systems for the development of management practices that optimize N‐use efficiency. The recovery of N from 15N‐labeled leucaena [Leucaena leucocephala (Lam.) de Wit] and dactyladenia [Dactyladenia barteri (Hook fex Oliv.) Engl.] leaf residues was followed in the soil, crop, and hedgerow of the respective alley cropping systems during three maize (Zea mays L.) and two cowpea (Vigna unguiculata L. Walp. subsp. unguiculata) seasons. More residue N was recovered in the top 5 cm of soil in the leucaena than in the dactyladenia microplot during the first 471 d after residue application (DAA). The first maize crop recovered 8.6 and 5.2% of the N from leucaena and dactyladenia, respectively, while the two subsequent crops recovered &lt;1%. The cowpea plants in the leucaena and dactyladenia microplots contained 0.5 and 1.1% of the residue N in the first harvest. Sixteen and 9% of the residue N was recovered by the leucaena hedgerow in the first and second pruning following residue application. The dactyladenia hedges recovered maximally 3.1% of the residue N in a single pruning. The total N recovery in the leucaena microplots was ≈90% at 120 and 471 DAA, compared with 56 and 35% in the dactyladenia microplots. At 858 DAA, 62% of the added leucaena N was accounted for, compared with 25% of the dactyladenia N. Residue quality was shown to have a major impact on the dynamics of applied residue N in alley cropping systems and will be an important factor in deciding which residue‐supplying plant species to integrate into similar cropping systems.

Agricultural intensification, soil biodiversity and agro-ecosystem function in the tropics: the role of mycorrhiza in crops and trees

Symbiotic association between plant roots and certain fungi is referred to as mycorrhiza. Mycorrhizal fungi provide a greater absorptive surface than root hairs and thus help in the absorption of the relatively immobile ions in soil such as phosphate, copper and zinc. In addition, mycorrhizal plants have greater tolerance to toxic metals, to root pathogens, to drought, to high soil temperature, to adverse soil pH and to transplant shock. The two main types of mycorrhiza in tropical ecosystems are: (1) ectomycorrhiza and (2) arbuscular mycorrhiza (AM). Ectomycorrhizae mainly occur in tropical pines, Caesalpiniaceae, Dipterocarpaceae and Myrtaceae. AM occur in most of the agricultural and horticultural crops and several tropical tree species. Tropical cropping systems are established on areas previously occupied by species-rich forest ecosystems. This brings about a drastic change in the biodiversity of mycorrhizal fungi. Further it can be generalized that modern, high-input agricultural practices generally are detrimental to mycorrhizal fungi, while the low-input sustainable agriculture methods enhance the population of mycorrhizal fungi. More studies are needed to understand the role of mycorrhizal fungi in soil aggregation. Non-destructive methods have to be developed for studying mycorrhizal biodiversity in natural ecosystems.

Book Reviews: Nitrogen Fixation in Tropical Cropping Systems.

Book Reviews: Nitrogen Fixation in Tropical Cropping Systems.

Reviews.

Books reviewed in this article: Abscisic Acid: Physiology and Biochemistry. Ed. by W. J. Davies and H. G. Jones Plant Breeding in the 1990s. Ed. by H. T. Stalker and J.P. Murphy Nitrogen Fixation in Tropical Cropping Systems. By K. E. Giller and K. J. Wilson Vascular-Streak Dieback of Cocoa. By P. J. Keane and C. Prior Coniferous Forest Ecology, from an International Perspective. Ed. by N. Nakagoshi and F. B. Golley The Biology of Vines. Ed. by F. E. Putz and H. A. Mooney Algal Cell Motility. Ed. by M. Melkonian Growth and Reproductive Strategies of Freshwater Phytoplankton. Ed. by Craig D. Sandgren Spores of the Pteridophyta. By Alice F. Tryon and Bernard Lugardon Practical Taxonomic Computing. By R. J. Pankhurst.

Contribution of soil organisms to the sustainability and productivity cropping systems in the tropics

Research to identify appropriate means of sustaining tropical soil productivity has intensified during the last 2 decades and promising technologies such as minimum tillage, herbaceous and tree based systems, and improved fallows have been identified. This paper discusses the contribution of soil microorganisms to the sustainability of such cropping systems in the tropics. A favourable environment for biological activity is promoted by management practices which: (1) maintain continuous vegetational cover of the soil and extensive perennial root growth; (2) encourage the return of plant litter to the soil; (3) support a diversity of plants (preferably and including legumes) and thence of litter inputs; (4) minimize soil disturbance (e.g. by tillage). It is concluded that although the importance of biological processes for soil fertility conservation is increasingly recognised there is still little understanding of these processes in tropical cropping systems.