Tolerance to acid soil conditions of the velvet beans Mucuna pruriens var. utilis and M. deeringiana

Velvet beans, fast growing leguminous cover crops used in the humid tropics, are shallow rooted on acid soils. This might be due to an inherent branching pattern, to an intrinsic toxicity of the acid subsoil or to a relative preference for root development in the topsoil. Such preference could be based on soil chemical factors in the subsoil or on physical factors such as penetration resistance or aeration. In a field experiment with two species of velvet bean (Mucuna pruriens var. utilis and M. deeringiana) all topsoil was removed and plants were sown directly into the acid subsoil. Root development was neither affected by this treatment nor by P fertilization or liming. In the absence of topsoil good root development in the exposed upper layer of subsoil was possible, so the hypothesis of a toxicity per se of the subsoil could be rejected. To test whether poor root development in the subsoil in the presence of topsoil is due to an inherent branching pattern of the plant or to a relative preference for topsoil, a modified in-growth core technique was used. Local topsoil and subsoil and an acid soil with a higher exchangeable Al content were placed in mesh bags at different depths and at different bulk densities, with and without lime and/or P fertilizer. A comparison of root development in mesh bags placed in the topsoil or subsoil showed that position and thus inherent branching pattern is not important. Root development in the subsoil was poor when this soil was placed in a mesh bag in the topsoil, but in an acid soil of much higher exchangeable Al content and higher percentage Al saturation more roots developed. In a second experiment in mesh bags, bulk density of the repacked soil in the range 1.0–1.5 g cm^-3 had no significant effect on root development. P fertilization and a high rate of liming of the soil placed in the mesh bag had a positive effect on root length density. It is concluded that poor root development in the acid subsoil under field conditions is due to a relative preference for topsoil. Al saturation and bulk density of the soil are not directly involved in this preference, but differences in availability of P and Mg or in Ca/Al ratios might play a role.     

Tolerance to acid soil conditions of the velvet beans Mucuna pruriens var. utilis and M. deeringiana

Fast growing, climbing leguminous cover crops such as the velvet beans can be used to reclaim weed-infested, degraded soils in the humid tropics, especially land covered by the grass Imperata cylindrica; they climb over the grass leaves and shade the grass out if their cover lasts long enough. Tolerance of two species of velvet bean to eroded soils was investigated by removing topsoil and directly sowing into the subsoil; plots where topsoil was not removed were used as a control. The response to small amounts of P fertilizer and lime was also tested. Removal of the topsoil resulted in retarded growth of both species, in increased dry matter content of the shoot, in decreased specific leaf area and in increased leaf weight ratio, due to shorter internodes. Six weeks after planting the leaf area index (LAI) was about 1.2 where topsoil was retained, sufficient for a shading effect on Imperata. Where topsoil had been removed, the LAI was only 0.6. Mucuna pruriens var. utilis showed a faster aboveground growth than M. deeringiana; the species did not differ in tolerance to eroded soil. Small amounts of P fertilizer had no significant effect on the growth of both Mucuna species. Shoot: root ratios, on a dry weight basis, were much lower when topsoil had been removed, about 3.7 and 2.4 for M. p. utilis and M. deeringiana respectively, compared to 6.2 and 3.3 where topsoil was retained. Removal of topsoil led to reduced Mg and to increased Al concentrations in roots, and to increased levels of Mn and Al in shoots. In the second year no effect of lime or residual effect of P application was found on growth of Mucuna or Imperata. Removal of the topsoil had little effect on the growth of weeds after the cover crop had been harvested. Due to the high Al tolerance of Imperata, reclamation by Mucuna will be less effective if the topsoil has been lost by erosion.     

Carbon stock assessment for a forest-to-coffee conversion landscape in Sumber-Jaya (Lampung, Indonesia): from allometric equations to land use change analysis

The change in stored carbon (C) stocks was assessed for a 700 km~2 area where forestcover decreased from 60% to 10% in the last 30 years. At the same time, the area under coffee increased from 7% to 70% with a gradual evolution from open "sun coffee" systems to multi-strata "shade coffee" systems that provide a partial compensation for C loss. The use of a generic tropi-cal forest rather than tree-specific allometric equation can lead to substantial (up to 100%) overes-timates of aboveground biomass depending on wood density and tree shape. The shoot: root ratio (biomass) of coffee shifted with age, from the 4∶1 value often assumed for tropical trees to 2∶1.Annual aboveground C stock accumulation rates during the establishment stage after slash-and-burn land clearing were 1, close to 2 or 3.5 Mg C ha~(-1)a~(-1) for sun coffee, shade coffee and fallowregrowth, respectively. Forest remnants, shade coffee and sun coffee had soil C stocks in the up-per 30 cm of the soil that were 79%, 60% or 45%, respectively, of the values expected for primary forest in Sumatra. Total C stock (time averaged, above-0.3 m in the soil) for forest, shade and sun coffee was 262, 82 and 52 Mg C ha~(-1), respectively. In the 1970-1984 period, while forest cover was reduced from 59.5% to 19.7%, the landscape lost on average 6.8 Mg C ha~(-1) a~(-1). In the1984-2000 period forest cover was further reduced to 12.6%, but the landscape lost only 0.39 MgC ha~(-1) a~(-1), as forest loss was partially compensated by an increase in shade coffee systems. Conversion of all current sun coffee to shade coffee systems while protecting the remaining forest,could increase average landscape level C stocks by 10 Mg ha~(-1) over a time frame of say 20 years,or 0.5 Mg C ha~(-1) a~(-1).     

Mycorrhizal infection in relation to soil pH and soil phosphorus content in a rain forest of northern Sumatra

Summary An autoradiographic procedure for the investigation of the uptake of minerals by plant roots and its application to the stimulation of uptake of ferric iron by agrobactin is described.     

Plant functional types and traits as biodiversity indicators for tropical forests: two biogeographically separated case studies including birds, mammals and termites

Multi-taxon surveys were conducted in species-rich, lowland palaeotropical and neotropical forested landscapes in Sumatra, Indonesia and Mato Grosso, Brazil. Gradient-directed transects (gradsects) were sampled across a range of forested land use mosaics, using a uniform protocol to simultaneously record vegetation (vascular plant species, plant functional types (PFTs) and vegetation structure), vertebrates (birds, mammals) and invertebrates (termites), in addition to measuring site and soil properties, including carbon stocks. At both sites similar correlations were detected between major components of structure (mean canopy height, woody basal area and litter depth) and the diversities of plant species and PFTs. A plant species to PFT ratio [spp.:PFTs] was the best overall predictor of animal diversity, especially termite species richness in Sumatra. To a notable extent vegetation structure also correlated with animal diversity. These surrogates demonstrate generic links between habitat structural elements, carbon stocks and biodiversity. They may also offer practical low-cost indicators for rapid assessment in tropical forest landscapes.     

Al avoidance and Al tolerance ofMucuna pruriens var.utilis: Effects of a heterogeneous root environment and the nitrogen form in the root environment

InMucuna pruriens var.utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance ofMucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms (NO₃ ^– and NH₄ ⁺) on the reactions ofMucuna to Al were studied, since in acid soils N is present as a mixture of NO₃ ^– and NH₄ ⁺. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction ofMucuna and P stress is still likely.Abbreviations D_r root diameter - L_pr total root length per plant - L_rw specific root length - NRA nitrate reductase activity - S/R shoot: root ratio     

Root distributions partially explain 15N uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associations may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of (¹⁵NH₄)₂SO₄ at 21.8 atom% ¹⁵N at different soil depths within established hedgerow intercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm of soil in intercrops of maize and trees, root length density (L_rv) of maize was greater than that of Gliricidia sepium trees, which had greater L_rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Gliricidia or maize. These vertical root distributions were related to the pattern of recovery of ¹⁵N placed at different soil depths; more ¹⁵N was recovered by maize and Gliricidia from placements at 5 cm depth than from placements at 45 or 65 cm depth. Peltophorum recovered similar amounts of ¹⁵N from placements at each of these depths, and hence had a deeper N uptake distribution than Gliricidiaor maize. Differences in tree L_rv across the cropping alley were comparatively small, and there was no significant difference (P<0.05) in the uptake of ¹⁵N placed in topsoil at different distances from hedgerows. A greater proportion of the ¹⁵N recovered by maize was found in grain following ¹⁵N placement at 45 cm or 65 cm depth than following placement at 5 cm depth, reflecting the later arrival of maize roots in these deeper soil layers. Thus trees have an important role in preventing N leaching from subsoil during early crop establishment, although they themselves showed a lag phase in ¹⁵N uptake after pruning. Residual ¹⁵N enrichment in soil was strongly related to application depth even 406 days after ¹⁵N placement, demonstrating the validity of this approach to mapping root activity distributions.     

Old tree root channels in acid soils in the humid tropics: Important for crop root penetration,water infiltration and nitrogen management

Under high rainfall conditions on acid soils with shallow crop root systems the rate of N leaching is high. A simple model predicts nitrogen uptake efficiency as a function of the amount of rainfall in excess of evapotranspiration, rooting depth and degree to which N leaching is retarded in comparison with water transport. Field observations on acid soils in S.E. Nigeria and S. Sumatera (Indonesia) showed that this model should be amended to include the role of old tree root channels. Crop roots can follow these channels, which are coated with partly decayed organic matter, into the acid subsoil. Measurements of water infiltration with a Guelph permeameter and a methylene blue dye showed that such channels form the major infiltration sites during rainstorms. Implications for nitrogen use efficiency and cropping pattern are discussed.     

Carbon stock assessment for a forest-to-coffee conversion landscape in Sumber-Jaya (Lampung, Indonesia): from allometric equations to land use change analysis

The change in stored carbon (C) stocks was assessed for a 700 km2 areawhere forest cover decreased from 60% to 10% in the last 30 years. At the same time, the area under coffee increased from 7% to 70% with a gradual evolution from open "sun coffee" systems to multi-strata "shade coffee" systems that providea partial compensation for C loss. The use of a generic tropical forest rather than tree-specific allometric equation can lead to substantial (up to 100%) overestimates of aboveground biomass depending on wood density and tree shape. The shoot:root ratio (biomass) of coffee shifted with age, from the 4:1 value often assumed for tropical trees to 2:1. Annual aboveground C stock accumulation rates during the establishment stage after slash-and- burn land clearing were 1, closeto 2 or 3.5 Mg C ha-1a-1 for sun coffee, shade coffee and fallow regrowth, respectively. Forest remnants, shade coffee and sun coffee had soil C stocks in the upper 30 cm of the soil that were 79%, 60% or 45%, respectively, of the values expected for primary forest in Sumatra. Total C stock (time averaged, above - 0.3m in the soil) for forest, shade and sun coffee was 262, 82 and 52 Mg C ha-1, respectively. In the 1970-1984 period, while forest cover was reduced from 59.5%to 19.7%, the landscape lost on average 6.8 Mg C ha-1 a-1. In the 1984-2000 period forest cover was further reduced to 12.6%, but the landscape lost only 0.39Mg C ha-1 a-1, as forest loss was partially compensated by an increase in shadecoffee systems. Conversion of all current sun coffee to shade coffee systems while protecting the remaining forest, could increase average landscape level C stocks by 10 Mg ha-1 over a time frame of say 20 years, or 0.5 Mg C ha-1 a-1.     

Tolerance and avoidance of Al toxicity by Mucuna pruriens var. utilis at different levels of P supply

Previous laboratory experiments showed that velvet bean Mucuna pruriens is moderately tolerant to the presence of Al (up to 185 µM) in the root environment, but that it only develops a shallow root system in acid soils. Field experiments showed that Mucuna can tolerate acid subsoil conditions in a homogeneous root environment, but avoids subsoil if topsoil is present. Subsequent split-root experiments with a recirculating nutrient solution showed that this subsoil avoidance may be based on an Al avoidance mechanism in the root system. This Al avoidance mechanism, however, was not evident when phosphorus (P) supply to the whole plant was adequate. We thus hypothesized that surface application of P may help to overcome Al avoidance in the subsoil.In a field experiment on an ultisol in Lampung (Indonesia), only a moderate increase in aboveground biomass production was found for a wide range of P application rates, although the soil was low in available P, and the P adsorption isotherm was very steep. An increased P status of the topsoil and an increased P concentration in the aboveground biomass (from 50 to 75 mmol kg^-1) had no effect on root development in the subsoil.