Fallow Species Research Articles

Maintaining the Conservation Value of Shifting Cultivation Landscapes Requires Spatially Explicit Interventions

Fallow vegetation within landscapes dominated by shifting cultivation represents a woody species pool of critical importance with considerable potential for biodiversity conservation. Here, through the analysis of factors that influence the early stages of fallow vegetation regrowth in two contrasting forest margin landscapes in Southern Cameroon, we assessed the impact of current trends of land use intensification and expansion of the cultivated areas, upon the conservation potential of shifting cultivation landscapes. We combined the analysis of plot and landscape scale factors and identified a complex set of variables that influence fallow regrowth processes in particular the characteristics of the agricultural matrix and the distance from forest. Overall we observed a decline in the fallow species pool, with composition becoming increasingly dominated by species adapted to recurrent disturbance. It is clear that without intervention and if present intensification trends continue, the potential of fallow vegetation to contribute to biodiversity conservation declines because of a reduced capacity, (1) to recover forest vegetation with anything like its original species composition, (2) to connect less disturbed forest patches for forest dependent organisms. Strategies to combat biodiversity loss, including promotion of agroforestry practices and the increase of old secondary forest cover, will need not only to operate at a landscape scale but also to be spatially explicit, reflecting the spatial pattern of species reservoirs and dispersal strategies and human usage across landscapes.

Role of tao (Belotia mexicana) in the traditional Lacandon Maya shifting cultivation ecosystem

For centuries, the Lacandon Maya have farmed the forest while also preserving and regenerating it. The Lacandon manage their fallow by planting certain tree species, and removing volunteer trees that are not optimal for soil fertility. This study focused on tao (Belotia mexicana), one of the Lacandon tree fallow species, and its impact on the soil as it matures in the secondary forest. The effect of tao on soil fertility was evaluated using the following soil fertility parameters: phosphorus, carbon, nitrogen, earthworm density, pH, and soil moisture. Results were compared using a split-plot analysis. Soil C:N ratios decreased with age of tao, indicating an improvement of litter quality over time. Soil extractable phosphorus decreased with age of tao and increased with distance from tao, which suggests that tao is depleting phosphorus. These results provide an introduction for further analysis into how native trees enhance soil fertility in the Lacandon system.

Effect of Fallow Species and Fallow Duration on Soil Fertility, Biomass and White Yam (Dioscorea rotundata Poir) Production

ABSTRACT Field experiments were conducted between 2003 and 2006 to study the effect of fallow species and fallow duration on biomass production, soil nutrient contents, weed control and yam production in Umudike, South Eastern Nigeria. The experiment was laid out as a split plot in a randomized complete block design (RCBD), with three replications. The main plot treatments were duration of fallow (1 and 2 years) and the sub plot treatments were eight legume cover crops (Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamacrista rotundifolia, Mucuna georgia, Mucuna ghana, Pueraria phaseoloides and Stylosanthes guanensis) in comparison with natural fallow and natural fallow plus inorganic fertilizer (NPK Mg 12:12:17:2) at 400 kg ha−1. The yam variety used was Dioscorea rotundata (cv. Nwopoko). Biomass production was significantly higher in Stylosanthes guanensis than in the other cover crop species except Aeschynomene histrix at 6 and 12 months after planting (MAP). Stylosanthes guanensis also gave higher biomass than natural fallow at 3 and 4 MAP. Soil P and K were significantly higher after Mucuna ghana than after other legumes and natural fallow. Soil N and P were significantly higher with one than with 2 years of fallow. Weed biomass was consistently lowest in Stylosanthes guanensis than in other legume cover crops except with Chamacrista rotundifolia, Mucuna ghana and Pueraria phaseoloides at 4 MAP and Aeschynomene histrix at 12 MAP. Yam tuber yields obtained with Centrosema brasilianum after 2 years of fallow was significantly higher than the yield obtained with each of the other fallow species and duration except with Aeschynomene histrix after one-year fallow and Centrosema brasilianum, Mucuna georgia and Mucuna ghana after two-year fallow.

Xerothermic vegetation of fallow lands in western Małopolska

In 2005-2011 the occurrence of xerothermic vegetation on fallow lands located on hills near Trzebinia and Jaworzno was studied. The Braun-Blanquet releves were performed on SE, S and SW slopes. Well-drained brown loamy soils developed on shallow sands on Triassic dolomites, lime-stone and Myślachowice conglomerate dominating at the sites. Investigations were also conducted on amid-field balks and fallow lands where in 2005 moderate agricultural activity was resumed. In general, superficial ploughing has not eliminated plant cover, but only disturbed turf and uncovered soil seed bank of former cultivation weeds.Fields 10-15 years after they had been abandoned, were dominated by the Dauco-Picridetum hieracioidis xerothermic ruderal association, with considerable share of Malva alcea and Verbascum species (V densiflorum, V. lychnitis, V. phlomoides and V. thapsus) in younger fallows. These communities were characterized by occurrence of xerothermic grasslands and warm thickets taxons from Festuco-Brometea and Trifolio-Geranietea sanguinei classes, i.a.: Achillea pannonica, Allium scorodoprasum, Bromus erectus, Centaurea stoebe, Cerinthe minor, Filipendula vulgaris, Fragaria viridis, Gentianella ciliata, Phleum phleoides, Valeriana angustifolia, Veronica spicata, Viola hirta and Thesium linophyllon.Additionally, on ploughed fallows in gaps of turf numerous annual and biennial segetal as well as ruderal species were observed. In many places thermophilous segetal association of Papaveretum argemones developed, in which a significant share of Avena fatua, Camelina microcarpa subsp. sylvestris, Consolida regalis, Lithospermum arvense and Papaver rhoeas was noted in addition to Arabidopsis thaliana, Papaver argemone and Veronica triphyllos.The oldest fallows (20-25 years after abandonment), amid-fields steep slopes and balks were covered by Geranio-Peucedanietum cervariae associations. Their physiognomy was formed by Agrimonia eupatoria, Brachypodium pinnatum and Origanum vulgare, sometimes also by Geranium sanguineum, Peucedanum cervaria and P. oreoselinum. This vegetation was in spatial contact with thermophilous thickets of Berberidion alliance with Cornus sanguinea, Crataegus monogyna, Prunus spinosa, Rhamnus cathartica, and Rosa canina.On xerothermic post-agricultural areas specific stable communities formed by thermophilous species from meadows, ruderals, grasslands and thickets occurred. A new phenomenon of recent years, related to EU benefits - superficial ploughing of fallows, not followed by regular cultivation - favours segetal vegetation. Weeds communities are regressing in Europe as an effect of modern agriculture. Post-agricultural secondary habitats are important elements that should be taken into account in studies of appearance and protection of xerothermic plants.

Occurrence of Summer Fallow Weeds within the Grain Belt Region of Southwestern Australia

Field surveys were conducted on 319 sites of the Western Australian grain belt in 2006 to determine the occurrence and distribution of summer fallow weed species. Sites were located across five growing season regions (north, north central, central, south central, and south) and three annual rainfall zones (high, medium, and low). A total of 51 species (or species groups) from 18 families were identified, with the large majority of species (35%) belonging to the Poaceae family. The most prevalent species found, being present at more than 10% of all sites, were wheat, “melons” (weedy watermelon and paddymelon), rigid ryegrass, capeweed, clover, mintweed, wild radish, fleabane, windmill grass, and rolypoly. Correspondence analysis revealed that the north, central, and southern regions of the grain belt could be predominately segregated according to dominant weed species occurrence; however, no segregation by rainfall zone was apparent. This study has given an overview of summer fallow weed occurrence in the Western Australian grain belt and highlights those weed species that are common and yet lack sufficient research into their ecology and management.

Improved Crotalaria cover crop fallow system for sustainable maize production in the northern Guinea Savanna agroecological zone of Ghana

An on-station trial was carried out at the Research Farm of the Faculty of Agriculture, University for Development Studies, Tamale, in the northern Guinea Savanna agroecological zone of Ghana. The study compared different seeding rates of leguminous cover crops, inorganic fertilization, and a combination of the two in a small-scale cereal-based production system. The experiment was conducted for 2 years within a 3-year fallow system to investigate the use of Crotalaria ochroleuca and Crotalaria retusa as 1 and 2 years' fallow systems. Crotalaria ochroleuca seed fallow at 50 kg ha-1 was more efficient in above-ground biomass production and nutrient accumulation than Crotalaria retusa seed fallow at 50 and 100 kg ha-1 of cover crop regrowth. The above-ground biomass production and nutrient accumulation by the Crotalaria species at 100 kg ha-1 of seeds was higher than at 50 kg ha-1 of seeds. The results showed that C. ochroleuca and C. retusa can accumulate higher soil nutrients at higher seed rates. The average grain yield increase for all the treatments between 2001 and 2002 were 4.6 and 3.0 per cent for "Dobidi" and "Okomasa" at 100 kg ha-1 of Crotalaria seeds, respectively. Grain yields from the Crotalaria fallow species plus fertilizer performed better, followed by fertilizer treatment and Crotalaria fallow systems. Two years' Crotalaria fallow systems at 100 kg ha-1 were more profitable than at 50 kg ha-1.

Effects of Medicago sativa, Taraxacum officinale and Bromus sterilis on the density and diversity of Collembola in grassy arable fallows of different ages

Abstract The density and diversity of Collembola of nine grassy arable fallows of different ages were investigated in a factorial design with the factors ‘plant species’ (legume: Medicago sativa, herb: Taraxacum officinale, grass: Bromus sterilis) and ‘age class’ (2–3, 6–8 and 12–15 years) including the random effect ‘site’ (1–9). In May 2008, four plots were selected randomly at each fallow. Within each plot five M. sativa, T. officinale and B. sterilis plants were extracted with their associated soil using steel cylinders. The material from each plant species was used for extraction of Collembola and for determination of environmental parameters. Thus, the new aspect of the present study compared to other field studies investigating the relationships between plant and Collembolan communities is the focus on the “micro-scale”, investigating the Collembolan communities of the soil associated with single plants. We found that species richness and density of total and euedaphic Collembola were significantly higher in B. sterilis than in T. officinale samples with the M. sativa samples being intermediate. Fine-root feeding euedaphic Collembola particularly benefited from the higher amount of fine roots in B. sterilis samples. We also discovered that the age of the fallows had no significant influence on the number of Collembolan species and the density of the Collembolan groups. Notably, however, species of the epedaphic genera Lepidocyrtus and Sminthurinus were associated with the 12–15 year-old fallows and presumably benefited from the high number of plant species in the old fallows. Finally, canonical correspondence analysis (CCA) indicated that the soil organic matter content and the microbial biomass, both potential food sources for many Collembolan species, were important structuring forces for the Collembolan communities.

Do Mixed‐Species Legume Fallows Provide Long‐Term Maize Yield Benefit Compared with Monoculture Legume Fallows?

The deliberate planting of fast‐growing N2–fixing legume monoculture species in rotation with cereal crops can be an important source of N for soil fertility replenishment. We hypothesized that mixed‐species fallows have a higher potential of giving long‐term residual benefits in terms of biomass, nutrients, and quality of residuals leading to long‐term nutrient supply to postfallow maize (Zea mays L.) crops. To test these hypotheses, two experiments were established in farmers' fields on very fine Kandiudalfic Eutrudox soils with monoculture and mixed‐species fallows. Treatments included: sesbania [Sesbania sesban (L.) Merr.], crotalaria (Crotalaria grahamiana Wight and Arn.), pigeonpea [Cajanus cajan (L.) Millsp.], siratro [Macroptilium atropurpureum (DC.) Urb.], and calliandra (Calliandra calothyrsus Meissn.) as monoculture‐species fallow and mixture fallows of sesbania + crotalaria, sesbania + pigeonpea, sesbania + siratro, or sesbania + calliandra compared with continuous maize cropping with or without N fertilizer, and natural weed fallow. Total aboveground biomass ranged from 4.1 to 20.5 Mg ha−1 for monoculture and 7.8 to 23.3 Mg ha−1 for mixed‐species fallows. Recyclable fallow biomass N ranged from 70 to 313 kg ha−1 and there was a positive interaction in some mixtures leading to increased N accumulation. Postfallow maize yields for fallows over five cropping seasons were 161–272% or 61–103% higher when compared with continuous maize without or with N fertilizer, respectively. Long‐term postfallow effects on maize yield were linearly related to the amount of recycled fallow N yield. Thus, choice of fallow species to mix should be primarily driven by a better risk management strategy and an increased basket of multiple products and services.

Degrading uplands in the rainforest region of Madagascar: Fallow biomass, nutrient stocks, and soil nutrient availability

Soil fertility restoration depends on natural fallows in the slash-and-burn system of eastern Madagascar. In the Beforona-Vohidrazana study zone, none of the fallow species are able to withstand the slashing, burning and cropping frequencies of 3–5 years. Eventually soils are abandoned for agriculture. Along the degradation sequence, this study quantifies fallow biomass, nutrient stocks and soil nutrient availability of four dominant fallow species Trema orientalis, Psiadia altissima, Rubus moluccanus, and Imperata cylindrica. At 3 years, the shrubs Psiadia and Rubus were more productive (11–14.4 t/ha aboveground biomass or AGB) than the tree species Trema (8.5 t/ha). Only after 5 years did tree productivity (24.7 t/ha) exceed that of shrubs (17–20 t/ha). Imperata’s biomass stagnated at 5.5 t/ha after 3 years. A sharp decline in fallow productivity was observed with advancing fallow cycles after deforestation. While Psiadia produced highest AGB in the second fallow cycle (C2) being 100%, C1 achieved 89% of that, C3 74%, and C4 only 29%. With the ability to propagate vegetatively and to accumulate important amounts of nutrients in roots, Rubus and Imperata, both exotic and invasive species, showed improved adaptation mechanisms towards frequent disturbances compared to the two indigenous species Trema and Psiadia. Available soil nutrients P, K, Mg were highest under forest and declined rapidly with increasing fallow cycles. Ca and pH rose momentarily in the first fallow cycle before declining with advanced soil use. Al concentrations increased steadily with time. As lengthened fallow periods are not practical, there is a need to intensify upland systems based on improved nutrient cycling, targeted inputs, fire-less land management, and land use diversification. Allowing regrading tree and bush fallows to accumulate biomass (above- and belowground carbon) will significantly improve Madagascar's greenhouse gas mitigation contribution.

Screening of Seven Plant Species for Short‐Term Improved Fallows in the Humid Forest Zone of Cameroon

Seven short‐fallow plant species were evaluated for their aboveground biomass production, nutrient accumulation, and weed suppression potential in Nkolbisson, Cameroon. The fallow species included Arachis pintoi, Vigna radiata, Desmodium intortum, Centrosema pubescens, Indigofera hirsutus, Indigofera spicata, and Pueraria phaseoloides. The experimental design was a randomized complete block with four replications and seven treatments (fallow species). Twelve months after planting, highly significant differences (p < 0.01) were observed among fallow species for phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations, respectively, in plant tissue. Similarly, highly significant differences (p = 0.0013) occurred among fallow species for aboveground biomass production, which ranged between 3.12 t ha−1 (I. spicata) and 9.16 t ha−1 (A. pintoi). Potassium only showed highly significant differences among plant species for nutrient accumulation (p = 0.0001). However, A. pintoi, C. pubescens, I. hirsutus, and P. phaseoloides yielded substantial amounts of recycled N (>100 kg ha−1 year−1) that can benefit the subsequent crop. For small‐scale producers, these species are likely to supply adequate nitrogen for cereal crops. A total of 26 weed species belonging to 15 botanical families were encountered in the various plots. Weed dry weight differed significantly (p = 0.02) among the fallow plots and ranged from 0.12 t ha−1 (in P. phaseoloides plots) to 2.32 t ha−1 (in A. pintoi plots). Results of this study suggest that there are a wide range of plant species that could be used in improved fallow technologies aimed at improving nutrient‐degraded soils and suppressing weeds.

Coppicing improved fallows are profitable for maize production in striga infested soils of western Kenya

Strigahermonthica (striga) weed is a major threat to crop production in sub-Saharan Africa, and short duration improved fallow species have recently been found to reduce the effects of this weed because of their ability to replenish soil nitrogen. The objective of this study was to compare the efficacy and profitability of coppicing improved fallow species (Gliricidia sepium [gliricidia], Leucaena trichandra [leucaena] and Calliandra calothyrsus [calliandra]) and non-coppicing species (Sesbania sesban [sesbania], Mucuna pruriens [mucuna], and Tephrosia vogelii [tephrosia]), in controlling striga. Natural fallow and a sole maize crop were included as control treatments. The fallow treatments were split into two and either fertilized with N or unfertilized. The results showed that coppicing fallows produced higher biomass than non-coppicing fallows. For example, Callindra (coppicing fallow species) produced 19.5 and 41.4 Mg ha−1 of leafy and woody biomass, respectively after four cumulative harvests as compared with Sesbania (non-coppicing species), which produced only 2.3 and 5.9 Mg ha−1 leaf and woody biomass, respectively. Improved fallows reduced striga population in proportion to the amount of leafy biomass incorporated into the soil (r = 0.87). N application increased cumulative maize yield by between 15–28% in improved fallow systems and by as much as 51–83% in the control treatments. Added total costs of the coppicing fallows did not differ significantly from those of the non-coppicing fallows and control treatments. However, the added net benefits of the coppicing fallows were significantly higher (US$ 527 for +N and 428 for −N subplots; P < 0.01) than those of the non-coppicing fallows (US$ 374 for +N and 278 for −N), and the least for the control treatments. The most profitable fallow system was Tephrosia with net added benefits of US$ 453.5 ha−1 season−1 without N, and US$ 586.7 ha−1 season−1 with added N.

Low impact of climate change on subalpine grasslands in the Swiss Northern Alps

AbstractWhile phenological shifts and migration of isolated species under climate change have already been observed on alpine summits, very few studies have focused on community composition changes in subalpine grasslands. Here we use permanent plots monitored since 1954 and precisely located phytosociological censuses from 1970 to study compositional changes of subalpine grasslands in two distinct regions of the Swiss Northern Alps. In both areas, warming trends during the monitoring period were associated with changes in land management (abandonment of goat and sheep pasturing or grazing replaced by mowing). Old and recent inventories were compared with correspondence analyses (CA). Ecological indicator values, community‐affinities and biological traits of the species were used to infer the factors responsible for triggering the observed changes. In both regions, subalpine grasslands were stable with smaller changes than have previously been observed in alpine environments. Only a few species appeared or disappeared and changes were generally limited to increasing or decreasing frequency and cover of certain taxa. At one site, grazing abandonment favored fallow species. Some of these species were located at their upper altitudinal distribution limits and may have spread because of rising temperatures. In both areas, declining species were predominantly alpine and low‐growing species; their decline was probably due to increased competition (e.g., shadow) with more vigorous subalpine taxa no longer limited by grazing. We conclude that vegetation communities can respond rapidly to warming as long as colonization is facilitated by available space or structural change. In the subalpine grasslands studies, changes were mainly driven by land management. These communities have a dense vegetation cover and newly arriving herbaceous species preferring warmer conditions may take some time to establish themselves. However, climate disturbances, such as exceptional drought, may accelerate community changes by opening gaps for new species.

An evaluation of nutritional constraints on sweet potato (Ipomoea batatas) production in the central highlands of Papua New Guinea

Sweet potato (Ipomoea batatas) is the staple food crop in the highlands of Papua New Guinea (PNG). Declining crop productivity, however, appears to be threatening the sustainability of sweet potato-based farming systems within the region, a probable cause being the exhaustion of soil nutrient reserves in continuously cultivated sweet potato gardens. To assess the extent of the problem, a survey of sweet potato gardens was conducted across four of the highlands provinces and information on soil and crop variables was obtained for old gardens (cultivated over many seasons) and new gardens (newly brought into cultivation) on soils of volcanic and non-volcanic origin. Crop leaf nutrient data collected in the survey were interpreted using the Diagnosis and Recommendation Integrated System (DRIS), to try to identify the main nutritional constraints on tuber production in different garden types on soils of volcanic or non-volcanic origin. The results suggested that K deficiency was the primary cause of poor crop production in almost a third of sweet potato gardens, but was more of a problem in old gardens than in new. Phosphorus deficiency was also a problem on volcanic soils, and S deficiency on non-volcanic soils. These latter deficiencies, however, were at least as prevalent in new gardens as in old. Important factors contributing to K and S depletion from garden systems were the removal of K and S-rich vines from cultivation areas, the shortening of fallow periods and the burning of weed and crop residues, the latter releasing S (SO2) to the atmosphere. Correction of K and S deficiencies may require the recycling of old vines back to sweet potato cultivation areas and the adoption of a zero-burn policy for fallow management. Correction of P deficiency may necessitate the use of P-accumulating fallow species, e.g. wild Mexican sunflower (Tithonia diversifolia), to extract the P fixed by sesquioxide and allophanic minerals

Interactions between root growth, slope and soil detachment depending on land use: a case study in a small mountain catchment of Northern Laos

Roots modify the properties of soil in their immediate vicinity. Individually, fine roots (<1 mm in diameter) have little effect on soil properties, but this is offset by the fact that they make up most of a plant’s total root length. Roots growing near the soil surface may influence soil detachment. Slope conditions are also known to influence root growth. A field study conducted in a small watershed of Northern Lao People’s Democratic Republic (PDR) during the 2005 rainy season assessed putative interactions between shallow fine roots, slope angle and soil detachment under three land uses: shifting cultivation, fallow and tree plantations. We used auger sampling and root windows to measure root length density and 1-m2 microplots to monitor water infiltration, runoff and soil detachment. Annual crops and plantation trees did not explore shallow soil horizons as thoroughly as fallow species. Under both crop and fallow, RLD in the top 5 cm decreased as slope increased. This pattern could be linked, either as a cause or a consequence, to slope-related changes in infiltration regimes. In contrast, no clear relation between fine root development and soil detachment was found.

Improved legume tree fallows and tillage effects on structural stability and infiltration rates of a kaolinitic sandy soil from central Zimbabwe

Improved legume tree fallows have great potential to increase soil organic carbon (SOC), aggregate stability and soil infiltration rates during the fallowing phase. However, persistence of the residual effects of improved fallowing on SOC, aggregate stability and infiltration rates, under different tillage systems in Zimbabwe is not well documented. The relationships between SOC, aggregate stability and infiltration in fallow-maize rotation systems are also not well documented. We therefore evaluated effects of tillage on SOC, aggregate stability and infiltration rates of a kaolinitic sandy soil during the cropping phase of an improved fallow-maize rotation system. Plots that were under legume tree fallows ( Sesbania sesban; Acacia angustissima), natural fallow (NF) and under continuous maize during the previous 2 years were divided into conventional tillage (CT) and no-till (NT) subplots soon after fallow termination, and maize was cropped in all plots during the following two seasons. Aggregate stability was investigated using water stable macroaggregation index ( I ma), water dispersible clay (WDC) and using the mean weight diameter (MWD) after different wetting procedures. Infiltration rates were determined using simulated rainfall at intensity of 35 mm h −1 on 1 m 2 plots. Soil organic carbon was significantly higher ( P < 0.05) under fallows than continuous maize. For the 0–5 cm depth SOC was 11.0, 10.0, 9.4 and 6.6 g kg −1 for A. angustissima, S. sesban, NF and continuous maize, respectively, at fallow termination. After 2 years of cropping SOC was 8.0, 7.0, 6.1 and 5.9 g kg −1 under CT and 9.1, 9.0, 8.0 and 6.0 g kg −1 under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. Aggregate stability was significantly greater ( P < 0.05) under fallows than under continuous maize and also higher under NT than under CT. The macroaggregation index ( I ma) for the 0–5 cm depth was 466, 416, 515 and 301 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination, decreasing to 385, 274, 286 and 255 under CT and 438, 300, 325 and 270 under NT, for A. angustissima, S. sesban, NF and continuous maize, respectively, after 2 years of cropping. Percent WDC was also significantly lower ( P < 0.05) in fallows than in continuous maize, and for the 0–5 cm it was 11, 10, 8 and 17 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination. After 2 years of cropping WDC (%) was 12, 14, 15 and 17 under CT and 10, 12, 12 and 16 under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. MWD also showed significantly higher ( P < 0.05) aggregate stability in fallows than in continuous maize. Water infiltration rates were significantly greater under fallows than continuous maize but these declined significantly during the cropping phase in plots that had been fallowed. In October 2000, infiltration rates in the A. angustissima and NF plots were above 35 mm h −1 as no runoff was observed. Steady-state infiltration rates were 24 mm h −1 in S. sesban and 5 mm h −1 for continuous maize after 30 min of rainfall simulations. After 2 years of cropping infiltration rates remained above 35 mm h −1 in A. angustissima plots, but declined to 18 and 8 mm h −1 for NF, CT and NT respectively and 12 mm h −1 for S. sesban, CT and NT. It is concluded that legume tree fallows improved SOC, aggregate stability and infiltration rates, but these benefits accrued during fallowing decreased significantly after 2 years of cropping following the termination of fallows. The decrease in SOC and aggregate stability was higher under CT than NT. Coppicing fallows of A. angustissima were the best long-term fallow species when integrated with NT as improved soil physical properties were maintained beyond 2 years of post-fallow cropping.

Response of Sesbania sesban (L.) Merr. to rhizobial inoculation in an N-deficient soil containing low numbers of effective indigenous rhizobia

Sesbania sesban (L.) Merr. (sesbania) is a fast growing N2-fixing widely used as an improved fallow species by smallholder farmers in eastern and southern Africa to restore fertility of their N-deficient soils. In order to establish the need for inoculation, the population of sesbania rhizobia in soil collected from a site where the species is intended for introduction was assessed using the most probable number (MPN) plant infection assay. The MPN of sesbania rhizobia was low (21, 6–81 fiducial limits at P=0.05 g−1soil) but with N2-fixation potential comparable to sesbania inoculant strain KFR 651. Evaluation of an indigenous sesbania rhizobial isolate GSS 1 from the MPN assay in potted field soil showed that it was more effective than strain KFR 651 in terms of plant growth and shoot dry matter (DM) accumulation at 9 and 12 weeks after planting, respectively. Total shoot N content was also higher for plants inoculated with isolate GSS 1 than inoculant strain KFR 651 and uninoculated control treatments 12 weeks after planting. These results demonstrate that it is better to inoculate with effective indigenous than exogenous rhizobia where the need for inoculation has been established.

Residual effects of fallows on selected soil hydraulic properties in a kaolinitic soil subjected to conventional tillage (CT) and no tillage (NT)

Improved fallows have been used to reduce time required for soil fertility regeneration after cropping in low input agricultural systems. In semi-arid areas of Southern Africa, Acacia angustissima and Sesbania sesban are among some of the more widely used improved fallow species. However the residual effects of improved fallows on soil hydraulic properties during the cropping phase is not known. The aim of this study was to quantify the residual effects of fallows and tillage imposed at fallow termination on soil hydraulic properties (infiltration rates, hydraulic conductivity and soil porosity) during the cropping phase. Treatments evaluated were planted fallows of Acacia angustissima, Sesbania sesban and natural fallow (NF) and continuous maize as a control. Steady state infiltration rates were measured using a double ring infiltrometer and porosity was calculated as the difference between saturated infiltration rates and tension infiltration measurements on an initially saturated soil. Unsaturated hydraulic conductivity (Ko) and mean pore sizes of water conducting pores were measured using tension infiltrometer at tensions of 5 and 10 cm of water on an initially dry soil. While there was no significant difference in steady state infiltration rates from double ring infiltrometer measurements among the fallow treatments, these were significantly higher than the control. The steady state infiltration rates were 36, 67, 59 and 68 mm h -1 for continuous maize, A. angustissima, S. sesban and NF respec- tively. Tillage had no significant effect on steady state infiltration rate. Pore density at 5 cm tension was significantly higher in the three fallows than in maize and varied from 285-443 m -2 in fallows, while in continuous maize the pore density was less than 256 m -2 . At 10 cm tension pore density remained significantly higher in fallows and ranged from 4,521-8,911 m -2 compared to 2,689-3,938 m -2 in continuous maize. Unsatu- rated hydraulic conductivities at 5 cm tension were significantly higher in fallows than in con- tinuous maize and were 0.9, 0.7, 0.8 cm and 0.5 cm h -1 for A. angustissima, S. sesban, NF and

Influence of slash-and-burn farming practices on fallow succession and land degradation in the rainforest region of Madagascar

Abstract Slash-and-burn agriculture (tavy) is the major cause of upland degradation and deforestation in eastern Madagascar. Upland degradation studies are largely based on floristic loss and have ignored the link to agriculture, the main activity on the uplands. The objectives were to analyze jointly with the Betsimisaraka farmers how slash-and-burn practices influence fallow species succession, and how current fallow/cropping regimes influence agricultural productivity and upland degradation dynamics. The study was conducted in the Vohidrazana/Beforona area, located at the margins of the Ankeniheny-Zahamena rainforest corridor. Semi-structured interviews were conducted in 9 villages resulting in 96 individual and 22 group interviews. These were complemented by 212 historical field plot inventories on 32 farms. The researchers’ and farmers’ knowledge systems were treated equally and complemented each other in the joint analysis. Over the last 30 years, fallow periods decreased from 8–15 years to 3–5 years. Hence, fallow vegetation is changing within 5–7 fallow/cropping cycles after deforestation from tree (Trema orientalis) to shrub (Psiadia altissima, Rubus moluccanus, Lantana camara) to herbaceous fallows (Imperata cylindrica and ferns) and grasslands (Aristida sp.), when land falls out of crop production. This sequence is 5–12 times faster than previously reported. The frequent use of fire is replacing native species with exotic, aggressive ones and favors grasses over woody species, creating treeless landscapes that are of minimal productive and ecological value. Unlike most discussions during the past 20–30 years that refer to fallow duration per se as a measure to soil recovery, our results show that fallow periods need to increase in length with each additional fallow/cropping cycle after deforestation in order to restore the soils to a similar level of productivity. Already with the third cycle after deforestation, vegetation starts to degrade quickly in parallel with agricultural productivity decline. The Betsimisaraka's fallow knowledge is very rich. Fallows are characterized by species life form, species composition, vegetation appearance, cycles after deforestation, and agricultural potential. Distinct fallow types are easily identifiable in the field and inform on the critical threshold, below which soils are lost to farming. Clear management guidelines go along with each fallow type. We recommend upland agricultural intensification and diversification based on improved soil fertility through optimized organic and inorganic inputs and fire-less land management that encourages the re-establishment of nutrient stocks. If young farmers perceive a real opportunity in agricultural intensification, the migration towards the forest borders to pursue tavy might be halted.

RESIDUAL EFFECTIVENESS OF MINJINGU PHOSPHATE ROCK AND FALLOW BIOMASS ON CROP YIELDS AND FINANCIAL RETURNS IN WESTERN KENYA

In western Kenya, severe nutrient depletion, especially that of nitrogen and phosphorus, has drastically reduced crop yields over the past two to three decades. The potential effects of P application, with a single direct application of Minjingu phosphate rock (MPR) at 20, 40 and 60 kg P ha−1, and fallow biomass in terms of nutrient replenishment on maize and bean yields were investigated on N and P deficient soils of western Kenya for six cropping seasons (three years). The agro-forestry shrubs tested were one season (six months) Crotalaria grahamiana and Tephrosia vogelii followed by maize monocrop. These fallow species were intercropped with beans in the 2000 long rains season to provide a food benefit to the farmer. Soil samples were taken at the end of each season and analyzed for soil pH and extractable P. The results showed that soil available P increased significantly following application of MPR. Significant increases in bean yields were obtained when 60 kg P ha−1 as MPR were applied, and contributed to a 260 % increase above the control. Significant maize grain yield increases were obtained when MPR was applied alone or in combination with fallow biomass as compared with treatments with either no external nutrient addition (control) or with fallow biomass alone in all seasons. The 60 kg P ha −1 MPR rate gave the highest cumulative maize grain yields (9.6 t ha−1) over the five consecutive maize growing seasons, followed by 40 kg P ha−1 (8.8 t ha−1). The residual benefits of MPR at modest rates of application (60 kg P ha−1) were found to persist in the soil for only three cropping seasons. Thereafter, there was a steady decline in soil chemical properties (pH and available P), grain yields and net benefits. The study demonstrated the need for frequent additions of P especially in the fourth consecutive season to ensure sustained availability of P, favorable pH, and increased crop yields and net benefits on the nutrient-depleted soils of western Kenya.

A comparative study on zero tillage with bulldozing as land preparations on oil palm growth, and nutrient status after 5 years of planting

A comparative study on zero tillage and bulldozing, as land preparations for oil palm seedling transplanted into the field was conducted at Ayip Eku Oil Palm Estate between 1993 and 1997. The experimental site was a five-year fallow land in which Panicum maximum (Guinea grass) and Centresoma pubescence were dominant fallow species. Sprouted oil palm nuts were obtained from the Nigerian Institute for Oil Palm Research (NIFOR) and established in a single stage nursery and used for the experiment. Number of leaves per palm, palm height and leaf area (LA) determined after 12 months of field planting, were significantly higher in the zero tillage plots than the bulldozed plots. Zero tillage exerted a more significant (P Keywords : zero tillage, bulldozing, crop growth and nutrient status after 5 years Global Journal of Agricultural Sciences Vol. 4(2) 2005: 149-154