Improving the Traditional Acacia Senegal-Crop System in Sudan: The Effect of Tree Density on Water Use, Gum Production and Crop Yields

The traditional Acacia senegal bush-fallow in North Kordofan, Sudan, was disrupted and the traditional rotational fallow cultivation cycle has been shortened or completely abandoned, causing decline in soil fertility and crop and gum yields. An agroforestry system may give reasonable crop and gum yields, and be more appealing to farmers. We studied the effect of tree density (266 or 433 trees ha⁻¹) on two traditional crops; sorghum (Sorghum bicolor) early maturing variety and karkadeh (Hibiscus sabdariffa), with regard to physiological interactions, yields and soil water depletion. There was little evidence of complementarity of resource sharing between trees and crops, since both trees and field crops competed for soil water from the same depth. Intercropping significantly affected the soil water status, photosynthesis and stomatal conductance in trees and crops. Gum production per unit area increased when sorghum was intercropped with trees in low or high density. However, karkadeh reduced the gum yield significantly at high tree density. Yields of sorghum and karkadeh planted within trees of high density diminished by 44 and 55% compared to sole crops, respectively. Intercropping increased the rain use efficiency significantly compared to trees and field crops grown solely. Karkadeh appears to be more appropriate for intercropping with A. senegal than sorghum and particularly recommendable in combination with low tree density. Modification of tree density can be used as a management tool to mitigate competitive interaction in the intercropping system.     

Water and nitrogen dynamics in rotational woodlots of five tree species in western Tanzania

The objective of this study was to compare the effects of woodlots of five tree species, continuous maize (Zea mays L.) and natural fallow on soil water and nitrogen dynamics in western Tanzania. The tree species evaluated were Acacia crassicarpa (A. Cunn. ex Benth.), Acacia julifera (Berth.), Acacia leptocarpa (A. Cunn. ex Benth), Leucaena pallida (Britton and Rose), and Senna siamea (Lamarck) Irwin & Barneby). The field experiment was established in November 1996 in a completely randomized block design replicated three times. Maize was intercropped between the trees during the first three years after planting and thereafter the trees were allowed to grow as pure woodlots for another two years. Transpiration by the trees was monitored when they were 3 years old using sap flow gauges. Soil water content was measured using the neutron probe approach between November 1999 and March 2001. Soil inorganic N profiles were measured when the trees were four years old in all treatments. The results indicated that the trees transpired more water than natural fallow vegetation during the dry season. The difference was apparent at a depth of 35 cm soil, but was more pronounced in deeper horizons. The water content in the entire soil profile under woodlots and natural fallow during the dry period was 0.01 to 0.06 cm³ cm⁻³ lower than in the annual cropped plots. This pattern was reversed after rainfall, when woodlots of A. crassicarpa, A. leptocarpa, A. julifera, S. siamea and L. pallida contained greater quantity of stored water than natural fallow or continuous maize by as much as 0.00 to 0.02, 0.01 to 0.04, 0.01 to 0.04, 0.01 to 0.03 and 0.00 to 0.02 cm³ cm⁻³, respectively. Natural fallow plots contained the lowest quantity of stored water within the entire profile during this period. Transpiration was greatest in A. crassicarpa and lowest in L. pallida. All tree species examined were `scavengers' of N and retrieved inorganic N from soil horizons up to 2-m depth and increased its concentration close to their trunks. This study has provided evidence in semi-arid environments that woodlots can effectively retrieve subsoil N and store more soil water after rains than natural fallow and bare soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonal pattern of nitrogen mineralization and soil moisture beneath Faidherbia albida (synAcacia albida) in central malawi

On fertile alluvial soils on the lakeshore plain of Malawi, maize (Zea mays L.) yields beneath canopies of large Faidherbia albida (synAcacia albida) trees greatly exceed those found beyound tree canopies, yet there is little difference in soil nutrients or organic matter. To investigate the possibility that soil nutrient dynamics contribute to increased maize yields, this study focused on the impact of Faidherbia albida on nitrogen mineralization and soil moisture from the time of crop planting until harvest. Both large and small trees were studied to consider whether tree effects change as trees mature.During the first month of the rainy season, a seven-fold difference in net N mineralization was recorded beneath large tree canopies compared to rates measured in open sites. The initial pulse beneath the trees was 60 g N g^–1 in the top 15 cm of soil. During the rest of the cropping cycle, N availability was 1.5 to 3 times higher beneath tree canopies than in open sites. The total production of N for the 4-month study period was 112 g N g^–1 below tree canopies compared to 42 g N g^–1 beyond the canopies. Soil moisture in the 0–15 cm soil layer was higher under the influence of the tree canopies. The canopy versus open site difference grew from 4% at the beginning of the season to 50% at the end of the cropping season.Both N mineralization and soil moisture were decreased below young trees. Hence, the impact of F. albida on these soil properties changes with tree age and size. While maize yields were not depressed beneath young F. albida, it is important to realize that the full benefits of this traditional agroforestry system may require decades to develop.     

Comparative productivity of Prosopis cineraria and Tecomella undulata based agroforestry systems in degraded lands of Indian Desert

Tree-crop interactions were monitored by measuring tree growth characters of Prosopis cineraria L.and Tecomella undulata L.and yields of Vigna radiata(L) in agroforestry systems in degraded lands of Indian Desert.Potential competition for resource between the trees and associated crop was analyzed by measuring soil water contents, soil organic matters and NH4-N at different depths of soil layers i.e., 0-25 cm, 25-50 cm and 50-75 cm in the experimental plots.The plots size were 16 m × 18 m(D1), 20 m × 18 m(D...     

Soil improvement by trees in sub-Saharan Africa

Trees can influence both the supply and availability of nutrients in the soil. Trees increase the supply of nutrients within the rooting zone of crops through (1) input of N by biological N₂ fixation, (2) retrieval of nutrients from below the rooting zone of crops and (3) reduction in nutrient losses from processes such as leaching and erosion. Trees can increase the availability of nutrients through increased release of nutrients from soil organic matter (SOM) and recycled organic residues. Roots of trees frequently extend beyond the rooting depth of crops. Research on a Kandiudalfic Eutrudox in western Kenya showed that fast-growing trees with high N demand (Calliandra calothyrsus, Sesbania sesban and Eucalyptus grandis) took up subsoil nitrate that had accumulated below the rooting depth of annual crops. Sesbania sesban was also more effective than a natural grass fallow in extracting subsoil water, suggesting less leaching loss of nutrients under S. sesban than under natural uncultivated fallows. Nutrient release from SOM is normally more dependent on the portion of the SOM in biologically active fractions than on total quantity of SOM. Trees can increase inorganic soil N, N mineralization and amount of N in light fraction SOM. Among six tree fallows of 2- and 3-year duration on an Ustic Rhodustalf in Zambia, inorganic N and N mineralization were higher for the two tree species with lowest (lignin + polyphenol)-to-N ratio (mean = 11) in leaf litter than for the two tree species with highest ratio (mean = 20) in leaf litter. Trees can also restore soil fauna, which are important for SOM and plant residue decomposition. Some agroforestry trees have potential to provide N in quantities sufficient to support moderate crop yields through (i) N inputs from biological N₂ fixation and retrieval of nitrate from deep soil layers and (ii) cycling of N from plant residues and manures. The cycling of P from organic materials is normally insufficient to meet the P requirements of crops. Sustained crop production with agroforestry on P-deficient soils will typically require external P inputs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of remnant trees on nutrients and fallow biomass in slash and burn agroecosystems in Guinea

Three tree species are traditionally conserved in the traditional slash and burn agricultural system practiced for the production of upland rice (Oryza sativa Linn.) in the Fouta Djallon region of Guinea, i.e. (Parinari excelsa [Sabine], Parkia biglobosa [Jacq.) Benth.], Erythrophleum guinensis [G. Don.]). Sampling a chronosequence of fallow sites indicated that extractable P; exchangeable K, Ca and Mg; cation exchange capacity (CEC) and pH all decreased over the fallow period with patterns that differed with soil depth. Soils under the remnant Parinari excelsa and Parkia biglobosa trees had higher concentrations of organic C; total N; extractable P; exchangeable K, Ca and Mg; total P and Ca; and CEC than did the open microsites. Extractable P; exchangeable K, Ca and Mg; total P and Ca; and CEC were greater under Parinari excelsa than under Erythrophleum guinensis. Intensive measurements of a single mature fallow site showed that the foliar nutrient concentration of the large trees was not an accurate index of which microsite had the greatest accumulation of biomass and nutrients in the aboveground fallow vegetation. The biomass and the total amounts of N, P, K, Ca and Mg in the fallow vegetation were significantly greater for the Parinari excelsa and Parkia biglobosa microsites than for the open microsites. We conclude that although the nutrient status of the soils decreased over the 8-year fallow period, the microsites under the large trees were generally more fertile than the open microsites. However, there were important differences in the effects of the different tree species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil organic carbon and aggregation under poplar based agroforestry system in relation to tree age and soil type

The poplar based agroforestry system improves aggregation of soil through huge amounts of organic matter in the form of leaf biomass. The extent of improvement may be affected by the age of the poplar trees and the soil type. The surface and subsurface soil samples from agroforestry and adjoining non-agroforestry sites with different years of poplar plantation (1, 3 and 6 years) and varying soil textures (loamy sand and sandy clay) were analyzed for soil organic carbon, its sequestration and aggregate size distribution. The average soil organic carbon increased from 0.36 in sole crop to 0.66% in agroforestry soils. The increase was higher in loamy sand than sandy clay. The soil organic carbon increased with increase in tree age. The soils under agroforestry had 2.9–4.8 Mg ha⁻¹ higher soil organic carbon than in sole crop. The poplar trees could sequester higher soil organic carbon in 0–30 cm profile during the first year of their plantation (6.07 Mg ha⁻¹ year⁻¹) than the subsequent years (1.95–2.63 Mg ha⁻¹ year⁻¹). The sandy clay could sequester higher carbon (2.85 Mg ha⁻¹ year⁻¹) than in loamy sand (2.32 Mg ha⁻¹ year⁻¹). The mean weight diameter (MWD) of soil aggregates increased by 3.2, 7.3 and 13.3 times in soils with 1, 3 and 6 years plantation, respectively from that in sole crop. The increase in MWD with agroforestry was higher in loamy sand than sandy clay soil. The water stable aggregates (WSA >0.25 mm) increased by 14.4, 32.6 and 56.9 times in soils with 1, 3 and 6 years plantation, respectively, from that in sole crop. The WSA >0.25 mm were 6.02 times higher in loamy sand and 2.2 times in sandy clay than in sole crop soils.     

Potential of natural and improved fallow using indigenous trees to facilitate cacao replanting in Ghana

Poor establishment, due to loss of soil fertility, weeds and lack of appropriate shade, is a major constraint to replanting cacao on previously used land. Spathodea campanulata, Newbouldia laevis and Ricinodendron heudelotii planted as monospecific improved fallow and Terminalia ivorensis, T. superba and Antiaris toxicaria planted as a multispecies improved fallow and a natural tree fallow were assessed for their potential to facilitate cacao replanting in a randomized complete block design experiment. Simpson and Shannon diversity indices and species richness in the natural tree fallow were 0.6, 1.6 and 20, respectively, at 4 years after trial inception. The Multispecies and the R. heudelotii improved fallows had better height growth, crown development and light transmission characteristics, which are desirable for cacao shade. However, these were not comparable to S. campanulata or the natural tree fallow in terms of improving microsite topsoil pH, % organic carbon and % total nitrogen and site capture. Since optimum fallow period is shortened by growing fast-growing trees, the height growth rate >2.0 m per annum in all the treatments except N. laevis indicates the suitability of these species for improved fallow. The trees species showed different and complementary characteristics and from a standpoint of biodiversity conservation and the future floristic composition of the landscape the natural tree fallow with its diversity of tree species may be recommended as a rehabilitation technique to facilitate the replanting of cacao with a diverse overhead shade.     

Performance of an improved fallow system in the Peruvian Amazon—modelling approach

As traditional slash-and-burn systems with prolonged fallow periods are no longer feasible in most parts of the tropics, improved agroforestry systems have high potential to increase the productivity of farming systems and sustain continuous crop production. Our objective was to assess biophysical and economic performance of planted leguminous tree fallow (using Inga edulis) compared to the traditional slash-and-burn farming system, practiced by farmers on fields infested with noxious weedy grass Imperata brasiliensis around the city of Pucallpa, Peru. An existing agroforestry model SCUAF was used to predict biophysical factors, such as changes in soil characteristics and farm outputs (crop and tree yield). While a cost–benefit analysis spreadsheet, which uses the output from SCUAF and economic data on input/output levels and prices, calculates economic performance of the systems. The Inga fallow system can provide improvements to a range of soil biophysical measures (C, N, P content). This enables higher levels of farm outputs to be achieved (higher cassava yields). However, for smallholders the improved system must be more economically profitable than the existing one. At prices currently encountered, the Inga fallow system is more profitable than the Imperata fallow system only in the long-term. In adopting the Inga fallow system, smallholders will incur lower profits in the first years, and it will take approximately 10 years for smallholders to begin making a profit above that achievable with the Imperata fallow system. Unless smallholders are capable of accepting the lower profitability in first years, they are less likely to adopt the new system.     

Contribution of agroforestry trees to nutrient requirements of intercropped plants

A major tenet of agroforestry, that trees maintain soil fertility, is based primarily on observations of higher crop yields near trees or where trees were previously grown. Recently objective analyses and controlled experiments have addressed this topic. This paper examines the issues of tree prunings containing sufficient nutrients to meet crop demands, the timing of nutrient transfer from decomposition to intercrops, the percent of nutrients released that are taken up by the crop, and the fate of nutrients not taken up by the crop.The amount of nutrients provided by prunings are determined by the production rate and nutrient concentrations, both depending on climate, soil type, tree species, plant part, tree density and tree pruning regime. A large number of screening and alley cropping trials in different climate-soil environments indicate that prunings of several tree species contain sufficient nutrients to meet crop demand, with the notable exception of phosphorus. Specific recommendations for the appropriate trees in a given environment await synthesis of existing data, currently only general guidelines can be provided.Tree biomass containing sufficient nutrients to meet crop demand is not enough, the nutrients must be supplied in synchrony to crop needs. Nutrient release patterns from organic materials are, in part, determined by their chemical composition, or quality. Leguminous materials release nitrogen immediately, unless they contain high levels of lignin or polyphenols. Nonlegumes and litter of both legumes and nonlegumes generally immobilize N initially. There is little data on release patterns of other nutrients. Indices that predict nutrient release patterns will assist in the selection of species for synchronizing with crop demand and improve nutrient use-efficiency.Field trials with agroforestry species ranging in quality show that as much as 80% of the nutrients are released during the course of annual crop growth but less than 20% is captured by the crop, a low nutrient-use efficiency. There are insufficient data to determine how much of the N not captured by the crop is captured by the trees or is in the soil organic matter, the availability of that N to subsequent crops, or how much of that N is lost through leaching, volatilization or denitrification. Longer term trials are needed.     

Biophysical interactions in tropical agroforestry systems

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems. In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positively influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees negatively affects the yields. Trees in boundary plantings compete with crops for above- and belowground resources, with belowground competition of trees often extending beyond their crown areas. The major biophysical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increased crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and micro-climate modification are avoided in the absence of direct tree–crop interactions. Future research on biophysical interactions should concentrate on (1) exploiting the diversity that exists within and between species of trees, (2) determining interactions between systems at different spatial (farm and landscape) and temporal scales, (3) improving understanding of belowground interactions, (4) assessing the environmental implications of agroforestry, particularly in the humid tropics, and (5) devising management schedules for agroforestry components in order to maximize benefits. This revised version was published online in June 2006 with corrections to the Cover Date.     

Crop damage by nematodes in improved-fallow fields in western Kenya

The rotation of leguminous shrubs and crops is being tested on farms and recommended as a means of improving soil fertility and increasing crop yield in eastern and southern Africa, including western Kenya. However, this improved fallow practice may also increase the nematode population in the soil. An experiment was conducted to monitor the effects of plant-parasitic nematodes on crops after improved fallow. Soil was collected from a maize (Zea mays L.)/bean (Phaseolus vulgaris L.) field, a natural fallow, a Crotalaria (Crotalaria grahamiana Wight & Arn.) fallow, a Tephrosia (Tephrosia vogelii Hook. f.) fallow and a Crotalaria — Tephrosia mixed fallow and used to fill plastic pots placed in a shade. Three successive crop cycles of 2 months were tested in these pots using maize and beans, the most important staple foods in western Kenya. In the first cycle, beans grew poorly on the Tephrosia and Crotalaria — Tephrosia soil due to the high incidence of root knot nematodes, Meloidogyne spp., while maize did not suffer any loss. Although the populations of root knot nematodes reduced drastically in the second and third cycles, both maize and beans experienced heavy losses on the soil under improved fallow probably due to the spiral nematodes, Scutellonema spp., which became dominant in the nematode communities. Despite the use of fertilisers (N, P, K), both crops became highly sensitive to spiral nematodes in the third cycle because of the degradation of the soil physical properties. The study showed that the benefits of improved fallows in terms of crop production may be limited by the high number of plant-parasitic nematodes they help develop in the process.This revised version was published online in November 2005 with corrections to the Cover Date.     

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.     

Biomass production of tree fallows and their residual effect on maize in Zimbabwe

The rotation of maize (Zea mays) with fast-growing, N₂-fixing trees (improved fallows) can increase soil fertility and crop yields on N-deficient soils. There is little predictive understanding on the magnitude and duration of residual effects of improved fallows on maize yield. Our objectives were to determine the effect of fallow species and duration on biomass production and to relate biomass produced during the fallow to residual effects on maize. The study was conducted on an N-deficient, sandy loam (Alfisol) under unimodal rainfall conditions in Zimbabwe. Three fallow species — Acacia angustissima, pigeonpea (Cajanus cajan), and Sesbania sesban — of one-, two-, and three-year duration were followed by three seasons of maize. Pigeonpea and acacia produced more fallow biomass than sesbania. The regrowth of acacia during post-fallow maize cropping provided an annual input of biomass to maize. Grain yields for the first unfertilized maize crop after the fallows were higher following sesbania (mean = 4.2 Mg ha^–1) than acacia (mean = 2.6 Mg ha^–1). The increased yield of the first maize crop following sesbania was directly related to leaf biomass of sesbania at the end of the fallow. Nitrogen fertilizer did not increase yield of the first maize crop following one- and two-year sesbania fallows, but it increased yield following acacia fallows. Nitrogen fertilizer supplementation was not required for the first maize crop after sesbania, which produced high-quality biomass. For acacia, which produced low-quality biomass and regrew after cutting, N fertilizer increased yield of the first post-fallow maize crop, but it had little benefit on yield of the third post-fallow maize crop.This revised version was published online in November 2005 with corrections to the Cover Date.     

Carbon sequestration through agroforestry in indigenous communities of Chiapas, Mexico

The importance of agroforestry systems as carbon sinks has recently been recognized due to the need of climate change mitigation. The objective of this study was to compare the carbon content in living biomass, soil (0–10, 10–20, 20–30 cm in depth), dead organic matter between a set of non-agroforestry and agroforestry prototypes in Chiapas, Mexico where the carbon sequestration programme called Scolel’te has been carried out. The prototypes compared were: traditional maize (rotational prototype with pioneer native trees evaluated in the crop period), Taungya (maize with timber trees), improved fallow, traditional fallow (the last three rotational prototypes in the crop-free period), Inga-shade-organic coffee, polyculture-shade organic coffee, polyculture-non-organic coffee, pasture without trees, pasture with live fences, and pasture with scattered trees. Taungya and improved fallow were designed agroforestry prototypes, while the others were reproduced traditional systems. Seventy-nine plots were selected in three agro-climatic zones. Carbon in living biomass, dead biomass, and soil organic matter was measured in each plot. Results showed that carbon in living biomass and dead organic matter were different according to prototype; while soil organic carbon and total carbon were influenced mostly by the agro-climatic zone (P < 0.01). Carbon density in the high tropical agro-climatic zone (1,000 m) was higher compared to the intermediate and low tropical agro-climatic zones (600 and 200 m, respectively, P < 0.01). All the systems contained more carbon than traditional maize and pastures without trees. Silvopastoral systems, improved fallow, Taungya and coffee systems (especially polyculture-shade coffee and organic coffee) have the potential to sequester carbon via growing trees. Agroforestry systems could also contribute to carbon sequestration and reducing emissions when burning is avoided. The potential of organic coffee to maintain carbon in soil and to reduce emissions from deforestation and ecosystem degradation (REDD) is discussed.     

Five native tree species and manioc under slash-and-mulch agroforestry in the eastern Amazon of Brazil: plant growth and soil responses

Throughout the Amazon of Brazil, manioc (Manihot esculenta) is a staple crop produced through slash-and-burn agriculture. Nutrient losses during slash-and-burn can be large and nutrient demand by food crops so great that fields are often abandoned after two years. In recent decades, farmers have reduced the fallow phase from 20 to ~5 years, limiting plant nutrient accumulation to sustain crop yields. Improved fallows through simultaneous planting of trees with food crops may accelerate nutrient re-accumulation. In addition, slash-and-mulch technology may prevent loss of nutrients due to burning and mulch decomposition may serve as a slow-release source of nutrients. This study in Pará, Brazil, in a 7-year-old secondary forest following slashing and mulching of the vegetation, involved two main plot treatments (with and without P and K fertilizers) and two sub-plot treatments (with or without a N₂-fixer Inga edulis). A mixed-culture of trees and manioc was planted in all plots. P and K fertilizer increased tree mortality due to weed competition but growth of surviving trees in four of the five tree species tested also increased as did biomass production of manioc. In the N₂-fixer treatment trends of greater growth and survival of four of five tree species and manioc biomass were also observed. Fertilization increased the biomass of competing vegetation, but there was a fertilizer by N₂-fixer interaction as I. edulis caused a reduction in competing biomass in the fertilized treatment. After one year, fertilization increased decomposition of the mulch such that Ca, Mg, and N contents within the mulch all decreased. In contrast, P and K contents of mulch increased in all treatments. No influence of the N₂-fixer on 0–10 cm soil N contents was observed. Two years after establishment, this agroforestry system succeeded in growing a manioc crop and leaving a well-maintained tree fallow after the crop harvest.     

Effects of black locust on productivity and nitrogen nutrition of intercropped barley

Effects of black locust (Robinia pseudoacacia L.) on productivity and N nutrition of barley (Hordeum vulgare L.) were evaluated under various management regimes (2 soil types, 3 levels of N fertilizer, and 3 cropping systems — barley alone, and barley intercropped with trees pruned or unpruned).Intercropping did not affect productivity and N nutrition of barley in 1988 when trees were small. However, there was a significant yield decline in 1989 as the trees grew bigger. On average, productivity of the sole crop was 8% higher in both soil types. Pruning and mulching moderated the yield reduction compared with the unpruned treatment. Competition for soil moisture was considered a major constraint. Nonetheless, the overall productivity (barley+black locust) from the intercropped treatments was 53% higher than sole cropping.In 1989 and 1990, intercropped barley had significantly higher grain and straw N concentrations (%). In 1989, for example, grain N content was 11% higher than in the sole crop. Removal of trees in 1990 resulted in significant increase in productivity and N content of subsequent barley crop relative to continuous sole cropping. From N nutrition viewpoint, barley from previously intercropped treatments showed superior quality and it had, on average, 23% higher grain N content than the sole crop. This was attributed to N₂-fixation and N return by black locust. It was estimated that black locust contributed about 36 kg N ha^–1 to the system.This study underscores the role black locust is potentially capable of playing in the development of sustainable and low-input agricultural systems in temperate regions. Nonetheless, the study also illustrates the importance of the below and above-ground interactions that occur in intercropped systems and the need for further research in this area.     

Modelling of planted legume fallows in Western Kenya. (II) Productivity and sustainability of simulated management strategies

Improved fallow is a technology that can help to raise agricultural productivity in systems of poor soil fertility and low financial capital. Models, once calibrated, can be used to investigate a range of improved fallow systems relatively quickly and at relatively low cost, helping to direct experimental research towards promising areas of interest. Six fallow crop rotations were simulated using the WaNuLCAS model in a bimodal rainfall setting in Kenya over a 10 year period: (A) alternating fallow and crop seasons, (B) one season fallow followed by three seasons crop, (C) one season fallow followed by four seasons crop, (D–F) 1–3 seasons fallow periods followed by 3–5 seasons crop. The strategies were tested using a number of fallow growth rates, soil clay contents, and rainfall amounts to determine the interaction of fallow rotation and biophysical variables on maize (Zea mays (L.)) yield and sustainability (organic matter, N₂ fixation, leaching). The best simulated fallow strategies doubled maize yield compared to continuous maize over a 10 year period. Across all biophysical treatments strategy A and B of no more than three consecutive cropping seasons and of one consecutive fallow season yielded the most maize. This was because fallow benefits were largely due to the immediate fallow soil fertility benefit (IFB) rather than the cumulative benefit (CFB). The difference in yield between the two strategies was through a balance between (1) their interaction with the biophysical variables affecting accumulation of organic matter, hence increasing soil fertility and (2) the extra intrinsic soil fertility used for maize productivity by the inclusion of more cropping seasons within the rotation. We propose the following conceptual framework to manage fallows for maximum maize yield: when environmental factors are strongly limiting to fallow and crop growth then fallow strategy A would be the best strategy to employ (less risk but more labour) and when factors are less limiting then strategy B would be the best to employ.     

Effect of tree-crop intercropping on a young Populus tomentosa plantation

In order to study the effect of tree crop intercropping on a young plantation of Populus tomentosa in the plains along the Yellow River, field experiments were conducted by observing the growth of the plantation, the nutrient content in leaves, the nutrient and water content in the soil, and the output of crops. The relationship between forest growth and nutrient content in the tree leaves and the soil were analyzed. Results show that tree crop intercropping in young plantations can not only improve soil water content, but also enhance the contents of organic matter and the available nitrogen, phosphorus and potassium in soil resulting in the vigorous growth of the individual trees. Diameter at breast height (DBH) was positively related to the contents of organic matter in the soil, and the contents of N, P and K in the tree leaves had correlation coefficients of 0.967, 0.955, 0.988 and 0.972, respectively. Whole tree leaf area, crown width, number of branches and the mean length of branches in the intercropped plantation (intercropped with watermelon and vegetables, peanut and winter wheat, and soybean) were, respectively, 1.70–3.0 times, 2.22–2.47 times, 1.0–1.41 times and 1.70–2.32 times of those of CK (without intercropping). Diameter at breast height (DBH) and tree height in the intercropped plantation were 50.5%–136.7% and 27%–59.5% higher than those of the CK, respectively. The study also showed that intercropping with watermelon and vegetables proved to have the highest economic return among the treatments adopted. Tree crop intercropping in young plantations is an effective measure to increase forest growth and economic benefit. __________ Translated from Journal of Beijing Forestry University, 2006, 28(3): 81–85 [译自: 北京林业大学学报]     

Productivity of intensified crop—fallow rotations in the Trenbath model

Management of crop—fallow rotations should strike a balance between exploitation, during cropping, and restoration of soil fertility during the fallow period. The Trenbath model describes build-up of soil fertility during a fallow period by two parameters (a maximum level and a half-recovery time) and decline during cropping as a simple proportion. The model can be used to predict potential crop production for a large number of management options consisting of length of cropping period and duration of fallow. In solving the equations, the model can be restricted to sustainable systems, where fallow length is sufficient to restore soil fertility to its value at the start of the previous cropping period. The model outcome suggests that the highest yields per unit of land can be obtained by starting a new cropping period after soil fertility has recovered to 50–60% of its maximum value. This prediction is virtually independent of the growth rate of the fallow vegetation. The nature of the fallow vegetation (natural regrowth, planted trees, or cover crops) mainly influences the crop yield by modifying the required duration of fallow periods. Intensification of land use by shortening fallow periods will initially increase returns per unit land at the likely costs of returns per unit labor. When fallows no longer restore soil fertility to 50% of the maximum, overall productivity will decline both per unit land and per unit labor, unless external inputs replace the soil fertility restoring functions of a fallow.This revised version was published online in November 2005 with corrections to the Cover Date.