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Can farmers in sub-Saharan Africa (SSA) boost crop yields and improve food availability without using more mineral fertilizer? This question has been at the center of lively debates among the civil society, policy-makers, and in academic editorials. Proponents of the "yes" answer have put forward the "input reduction" principle of agroecology, i.e. by relying on agrobiodiversity, recycling and better efficiency, agroecological practices such as the use of legumes and manure can increase crop productivity without the need for more mineral fertilizer. We reviewed decades of scientific literature on nutrient balances in SSA, biological nitrogen fixation of tropical legumes, manure production and use in smallholder farming systems, and the environmental impact of mineral fertilizer. Our analyses show that more mineral fertilizer is needed in SSA for five reasons: (i) the starting point in SSA is that agricultural production is "agroecological" by default, that is, very low mineral fertilizer use, widespread mixed crop-livestock systems and large crop diversity including legumes, but leading to poor soil fertility as a result of widespread soil nutrient mining, (ii) the nitrogen needs of crops cannot be adequately met solely through biological nitrogen fixation by legumes and recycling of animal manure, (iii) other nutrients like phosphorus and potassium need to be replaced continuously, (iv) mineral fertilizers, if used appropriately, cause little harm to the environment, and (v) reducing the use of mineral fertilizers would hamper productivity gains and contribute indirectly to agricultural expansion and to deforestation. Yet, the agroecological principles directly related to soil fertility—recycling, efficiency, diversity—remain key in improving soil health and nutrient-use efficiency, and are critical to sustaining crop productivity in the long run. We argue for a nuanced position that acknowledges the critical need for more mineral fertilizers in SSA, in combination with the use of agroecological practices and adequate policy support. [ABSTRACT FROM AUTHOR]

The document titled "Correction to: Quantifying country-to-global scale nitrogen fixation for grain legumes. I. Reliance on nitrogen fixation of soybean, groundnut and pulses" addresses an error in the original article related to the equation describing the line of best fit in Figure 3. The corrected equation should read: y = 0.0123x2 - 48.576x + 48,003 (r2 = 0.30). The authors are affiliated with institutions in Australia, the Netherlands, Sweden, and the University of New England in Australia. The corrected article can be accessed online for further reference. [Extracted from the article]

The rotation of crops with planted [N.sub.2]-fixing legumes (improved fallows) is a promising agroforestry innovation for replenishing soil fertility in the tropics. We postulated that woody and herbaceous legumes with different rooting and growth patterns could be mixed in improved fallows to maximize utilization of belowground growth resources. We used a method of injecting a solution of [sup.15]N-labeled [(N[H.sub.4]).sub.2]S[O.sub.4] into soil at 0.15- and 1.0-m depths to measure soil mineral N acquisition by sesbania [Sesbania sesban (L) Merr], crotalaria [Crotalaria grahamiana Wight and Arn.], and the understory herbaceous legume siratro [Macroptilium atropurpureum (DC.) Urb.] grown in mixed stands on a Kandiudalfic Eutrudox soil in western Kenya. Crotalaria had the highest root length in the topsoil. Sesbania on the other hand had nearly half its total root length below 0.3 m at 0.3-to 1.5-m depth; sesbania took up more added [sup.15]N than crotalaria and siratro from the 1.0-m depth. Mixed sesbania and crotalaria stands, as compared with growing species in monocultures, increased root length at the 0.3- to 1.2-m depth. Sesbania mixed with siratro was more effective than sesbania mixed with crotalaria in uptake of [sup.15]N at 1.0-m depth but not at 0.15-m depth. At 2 mo after injection, the [sup.15]N was concentrated immediately below the injection point with little lateral movement. This confirmed the utility of the methodology in determining temporal N uptake for species in mixed stands. Our results suggest that opportunities exist for enhanced subsoil N retrieval through the mixing of leguminous species, which can influence root distribution and increase rooting in the subsoil.

Background: We collated estimates of the percentage of legume N derived from atmospheric N2 (%Ndfa) for 14 major grain legumes and then analysed and aggregated the data to derive average values for different crops and regions/countries. The effects of cultivation year and whether data collected from experimental plots were relevant to crops growing in farmers' fields were examined. Scope: A total of 5374 %Ndfa estimates (observations) were sourced, 4205 from field experiments and 1169 on-farm measurements collected from farmer-grown crops. The largest number of reports (82) and %Ndfa estimates (1391) were for soybean. Conclusions: The %Ndfa estimates for each legume species were consistent across years, except for soybean in North America. For some species estimates were also similar across geographic regions. There were no significant differences (P > 0.05) between estimates of %Ndfa derived from experimental plots and farmer-grown legume crops for nine of the 10 crops evaluated. Three distinct groups were identified with statistically-similar average %Ndfa values with associated standard deviations, namely: pigeonpea, faba bean and lupin – 74±11.8 %; groundnut, green and black gram, cowpea, chickpea, field pea, lentil, vetches and Bambara groundnut – 62±13.4 %; common bean – 38±11.1 %. There were three distinct different regional groupings for soybean: Brazil – 78±6.3 %; North America, Argentina, Asia, Africa and Oceania – 61±14.0 %; Europe – 44±13.8 %. Our findings provide more certainty and simplify the challenge of using field-scale measures of legume %Ndfa to estimate country-to-global inputs of fixed N from grain legumes. [ABSTRACT FROM AUTHOR]

Rhizobia are soilborne bacteria that form symbiotic relations with legumes and fix atmospheric nitrogen. The nitrogen fixation potential depends on several factors such as the type of host and symbionts and on environmental factors that affect the distribution of rhizobia. We isolated bacteria nodulating common bean in Southern Ethiopia to evaluate their genetic diversity and phylogeography at nucleotide, locus (gene/haplotype) and species levels of genetic hierarchy. Phylogenetically, eight rhizobial genospecies (including previous collections) were determined that had less genetic diversity than found among reference strains. The limited genetic diversity of the Ethiopian collections was due to absence of many of the Rhizobium lineages known to nodulate beans. Rhizobium etli and Rhizobium phaseoli were predominant strains of bean-nodulating rhizobia in Ethiopia. We found no evidence for a phylogeographic pattern in strain distribution. However, joint analysis of the current and previous collections revealed differences between the two collections at nucleotide level of genetic hierarchy. The differences were due to genospecies Rhizobium aethiopicum that was only isolated in the earlier collection. [ABSTRACT FROM AUTHOR]

East African highland bananas and climbing beans are important crops for food and income in the highlands of Uganda. Intercropping of banana with legume crops is a common practice, yet climbing bean intercropping with perennials has rarely been studied in Uganda. To understand how best to improve the production system, we assessed the effects of pruning of banana leaves on light availability for climbing beans, resulting effects on bean yields and potential differences in shade tolerance between two climbing bean varieties in the eastern and southwestern highlands of Uganda. Measurements of the transmission of photosynthetically active radiation (PAR) through the banana canopy were combined with yield measurements of a local and improved climbing bean variety and with banana pseudostem girth in two seasons (2016A and 2016B). We also compared yields of intercropped with sole-cropped climbing beans. The mean fractions of PAR transmitted through the banana canopy – hence available for beans – were 0.43 on pruned and 0.38 on non-pruned subplots, a significant 15% difference. The improved light availability did not increase climbing bean yield. Although no direct relationship between light interception and bean yields was found, bean yields on the most and least shaded parts of the intercropped fields differed significantly, suggesting that beans do benefit from improved light availability in intercropping. Generally, yields of sole-cropped beans were significantly larger than of intercropped beans, but we could not single out the effects of competition for light, water, and/or nutrients. The bean varieties responded similarly to the pruning treatments. The local variety tended to perform relatively better in intercropping, the improved variety in sole cropping, though differences were not significant overall. Pruning and retention of eight banana leaves over the course of a season did not affect banana pseudostem girths in the mature banana plantations. Although light availability improved, farmers may not expect a major effect on bean yield. Future research may focus on the effects of a lower number of leaves retained, comparing a number of bean varieties for suitability in sole or intercropping, or on other factors influencing the relation between the two crops such as relative plant densities of beans and bananas. [ABSTRACT FROM AUTHOR]

Food-based dietary guidelines (FBDGs) provide guidance to policy makers, the private sector and consumers to redesign food systems and to improve diets of vulnerable populations. As appropriate FBDGs are based on the actual dietary patterns and their costs, it is assumed that the recommended foods are available, affordable and acceptable for the population under study. Using quantitative dietary intake data of young children in rural Northern Ghana, we developed local FBDGs and studied whether these are supported by the diversity and quantity of the production of a household among 329 households. We found that 40% of rural Northern Ghanaian infants and young children were stunted and their nutrient intakes were far below the recommendations: the probability of adequacy for most nutrient intakes was less than 50%. At household level, the developed FBDGs were, on average, unable to sufficiently cover the household requirements for fat (60.4% of recommended nutrient intake (RNI)), calcium (34.3% RNI), iron (60.3% RNI), vitamin A (39.1% RNI), vitamin B12 (2.3% RNI) and vitamin C (54.6% RNI). This implies that even when these FBDGs are fully adopted the requirements for these nutrients will not be met. In addition, the nutrient needs and food needs (according to the developed FBDGs) of a household were only marginally covered by their own food production. The food production of over half the households supplied insufficient calcium (75.7%), vitamin A (100%), vitamin B12 (100%) and vitamin C (77.5%) to cover their needs. The food production of about 60% of the households did not cover their required quantities of grains and legumes and none covered their required quantities of vegetables. Further analysis of the food gaps at district and national level showed that sufficient grains were available at both levels (267% and 148%, respectively) to meet requirements; availability of legumes was sufficient at district level (268%) but not at national level (52%); and vegetables were insufficient at both levels (2% and 49%, respectively). Diversifying household food production is often proposed as a means to increase the diversity of foods available and thereby increasing dietary diversity of rural populations. We found that the diversity of the production of a household was indeed positively related with their food and nutrient coverage. However, the diversity of the production of a household and their food and nutrient coverage were not related with children’s dietary diversity and nutrient adequacy. Our results show that the production of a households does not support the adoption of FBDGs in rural Northern Ghana, especially for vegetables. This suggests that the promotion of FBDGs through nutrition education or behaviour change communications activities alone is insufficient to lead to improvements in diets. Additional strategies are needed to increase the food availability and accessibility of the households, especially that of fruits and vegetables, such as diversification of the crops grown, increased production of specific crops and market-based strategies. [ABSTRACT FROM AUTHOR]

Global support for Conservation Agriculture (CA) as a pathway to Sustainable Intensification is strong. CA revolves around three principles: no-till (or minimal soil disturbance), soil cover, and crop rotation. The benefits arising from the ease of crop management, energy/cost/time savings, and soil and water conservation led to widespread adoption of CA, particularly on large farms in the Americas and Australia, where farmers harness the tools of modern science: highly-sophisticated machines, potent agrochemicals, and biotechnology. Over the past 10 years CA has been promoted among smallholder farmers in the (sub-) tropics, often with disappointing results. Growing evidence challenges the claims that CA increases crop yields and builds-up soil carbon although increased stability of crop yields in dry climates is evident. Our analyses suggest pragmatic adoption on larger mechanized farms, and limited uptake of CA by smallholder farmers in developing countries. We propose a rigorous, context-sensitive approach based on Systems Agronomy to analyze and explore sustainable intensification options, including the potential of CA. There is an urgent need to move beyond dogma and prescriptive approaches to provide soil and crop management options for farmers to enable the Sustainable Intensification of agriculture. [ABSTRACT FROM AUTHOR]

To derive optimal benefits from short-term planted legume fallows, farmers need to make important initial decisions on the establishment and growth performance of specific species in monoculture stands and in mixtures. We hypothesized that species with contrasting growth characteristics could be mixed to optimize aboveground resource capture due to their complementary or compensatory gains in resource acquisition. A selection of tree/shrub/herbaceous legumes planted in monoculture and mixed stands were evaluated for growth, biomass and nitrogen productivity on a Kandiudalfic Eutrudox in western Kenya. Species evaluated included: sesbania (Sesbania sesban (L.) Merr.), crotalaria (Crotalaria grahamiana Wight and Arn.), pigeonpea (Cajanus cajan (L.) Millsp.), and siratro (Macroptilium atropurpureum (DC.) Urb). Total aboveground biomass ranged from 5 to 12 Mg ha−1 (monocultures) and 3.4–8 Mg ha−1 (mixtures). N yield per plant was linearly correlated with total plant biomass (R2=0.95,slope=0.018) across all of the fallows, despite different amounts of leaf and wood with widely differing N concentrations being present with the different species. Biomass and N yield of sesbania was negatively affected by mixing with other species. For instance, sesbania N yield decreased when mixed with pigeonpea (54%) or crotalaria (67%). Crotalaria and pigeonpea established best under relay cropping with maize and emerged to be better competitors in mixtures than sesbania and siratro. Results of this study suggest that mixing species in fallows provide a better risk management strategy through compensatory biomass and nutrient production gains obtained from the strongly competing species. [Copyright &y& Elsevier]

Abstract The natural rhizobial populations of Calliandra calothyrsus , Gliricidia sepium , Leucaena leucocephala and Sesbania sesban were assessed in soils from nine sites across tropical areas of three continents. The rhizobial population size varied from undetectable numbers to 1.8 × 104 cells/g of soil depending on the trap host and the soil. Calliandra calothyrsus was the most promiscuous legume, nodulating in eight soils, while S. sesban nodulated in only one of the soils. Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analyses of the 16S rRNA gene and the internally transcribed spacer (ITS) region between the 16S and 23S rRNA genes were used to assess the diversity and relative abundance of rhizobia trapped from seven of the soils by C. calothyrsus , G. sepium and L. leucocephala . Representatives of the 16S rRNA RFLP groups were also subjected to sequence analysis of the first 950 base pairs of the 16S rRNA gene. Eighty ITS groups were obtained, with none of the ITS types being sampled in more than one soil. RFLP analysis of the 16S rRNA yielded 23 ‘species’ groups distributed among the Rhizobium , Mesorhizobium , Sinorhizobium and Agrobacterium branches of the rhizobial phylogenetic tree. The phylogeny of the isolates was independent of the site or host of isolation, with different rhizobial groups associated with each host across the soils from widely separated geographical regions. Although rhizobial populations in soils sampled from the centre of diversity of the host legumes were the most genetically diverse, soil acidity was highly correlated with the diversity of ITS types. Our results support the hypothesis that the success of these tree legumes in soils throughout the tropics is the result of their relative promiscuity (permissiveness) allowing nodulation with diverse indigenous rhizobial types. [ABSTRACT FROM AUTHOR]

Abstract: Legumes are prized for their seed protein and lipid mass fractions. Since legumes spend up to 4–16% of photosynthesis on each of the rhizobial and arbuscular mycorrhizal (AM) fungal symbioses, it might be expected that positive responses in yield due to rhizobial and AM symbioses are accompanied by decreases in seed protein and lipid mass fractions due to a photosynthate (C) limitation. We performed a meta-analysis of 348 data points from published studies with 12 legume species to test whether yield, harvest index, and seed protein and lipid mass fractions are affected by symbioses. There was a significant increase in yield due to rhizobial inoculation (16% in the field; 59% in pot experiments). There were no responses of yield to AM fungi and rhizobial + AM fungi inoculations in the field (presumably because an AM fungi-free control cannot be ensured), but significant responses in pots (45% with AM fungi; 44% with rhizobial + AM fungi). Rhizobial inoculation improved seed protein mass fraction by 7% in the field; AM fungi increased this parameter by 14% in pots. There were no discernable effects of symbioses on seed lipid mass fraction. Rhizobial symbioses in the field increased harvest index (+5%), but AM fungi did not affect harvest index. In conclusion, increases in yield due to symbioses also resulted in increases in seed protein and constant lipid mass fractions, indicating that legumes are not C-limited under symbiotic conditions. [Copyright &y& Elsevier]

Grain legumes are key components of sustainable production systems in sub-Saharan Africa, but wide-spread nutrient deficiencies severely restrict yields. Whereas legumes can meet a large part of their nitrogen (N) requirement through symbiosis with N 2 -fixing bacteria, elements such as phosphorus (P), potassium (K) and secondary and micronutrients may still be limiting and require supplementation. Responses to P are generally strong but variable, while evidence for other nutrients tends to show weak or highly localised effects. Here we present the results of a joint statistical analysis of a series of on-farm nutrient addition trials, implemented across four legumes in four countries over two years. Linear mixed models were used to quantify both mean nutrient responses and their variability, followed by a random forest analysis to determine the extent to which such variability can be explained or predicted by geographic, environmental or farm survey data. Legume response to P was indeed variable, but consistently positive and we predicted application to be profitable for 67% of farms in any given year, based on prevailing input costs and grain prices. Other nutrients did not show significant mean effects, but considerable response variation was found. This response heterogeneity was mostly associated with local or temporary factors and could not be explained or predicted by spatial, biophysical or management factors. An exception was K response, which displayed appreciable spatial variation that could be partly accounted for by spatial and environmental covariables. While of apparent relevance for targeted recommendations, the minor amplitude of expected response, the large proportion of unexplained variation and the unreliability of the predicted spatial patterns suggests that such data-driven targeting is unlikely to be effective with current data. • A large number of on-farm fertilizer trials show consistent but variable responses to phosphorus. • No consistently positive effect was found for potassium or secondary and micronutrients. • Potassium response shows spatial variation, but its small magnitude and lack of predictability limit the scope for targeted recommendations. [ABSTRACT FROM AUTHOR]

Options that contribute to sustainable intensification offer an avenue to improve crop yields and farmers’ livelihoods. However, insufficient knowledge on the performance of various options in the context of smallholder farm systems impedes local adaptation and adoption. Therefore, together with farmers in southern Mali we tested a range of options for sustainable intensification including intensification of cereal (maize and sorghum) and legume (groundnut, soyabean and cowpea) sole crops and cereal-legume intercropping during three years on on-farm trials. There was huge variability among fields in crop yields of unamended control plots: maize yielded from 0.20 to 5.24 t ha −1 , sorghum from 0 to 3.53 t ha −1 , groundnut from 0.10 to 1.16 t ha −1 , soyabean from 0 to 2.48 t ha −1 and cowpea from 0 to 1.02 t ha −1 . This variability was partly explained by (i) soil type and water holding capacity, (ii) previous crop, its management and the nutrient carry-over and (iii) inter-annual weather variability. Farmers recognized three soil types: gravelly soils, sandy soils and black soils. Yields were very poor on gravelly soils and two to three times greater (depending on the crop) on black soils. Yields were also poor at the end of the typical crop rotation, i.e., after sorghum and millet, and 1.3–1.7 times greater (depending on the crop) after the fertilized crops maize and cotton. We diagnosed a number of cases of technology failure where no improvement in yield was observed with hybrid varieties of maize and sorghum and rhizobial inoculation of soyabean. Regardless of soil type and previous crop, mineral fertilizer improved yields by 34–126% depending on the crop. Targeting options to a given soil type and/or place in the rotation enhanced their agronomic performance: (i) the biomass production of the cowpea fodder variety was doubled on black soils compared with gravelly soils, (ii) the additive maize/cowpea intercropping option after cotton or maize resulted in an average overall LER of 1.47, no maize grain penalty, and 1.38 t ha −1 more cowpea fodder production compared with sole maize. Soil type and position in the rotation, two indicators easy to assess by farmers and extension workers, allowed the identification of specific niches for enhanced agronomic performance of legume sole cropping and/or intercropping. [ABSTRACT FROM AUTHOR]

Abstract: Many farmers in central Mozambique intercrop maize with grain legumes as a means to improve food security and income. The objective of this study was to understand the farming system, and to evaluate the suitability of maize–legume intercropping to alleviate the biophysical and socio-economic constraints faced by smallholder farmers in Ruaca and Vunduzi villages, central Mozambique. To achieve this we characterised the farming systems and measured grain yields, rainfall infiltration, economic returns and acceptability of maize–legume intercrops under different N and P application rates. Two intercropping strategies were tested: (a) an additive design of within-row intercropping in which legume was intercropped with alternating hills of maize within the same row; maize plant population was the same as sole crop maize, and (b) a substitutive design with distinct alternating rows of maize and legume (local practice). Fertiliser treatments imposed on all treatments were: (i) no fertiliser, (ii) 20kgPha−1, (iii) 20kgPha−1 +30kgNha−1, and (iv) 20kgPha−1 +60kgNha−1. Intercrops were relatively more productive than the corresponding sole crops; land equivalent ratios (LER) for within-row intercropping ranged between 1.1 and 2.4, and between 1.0 and 1.9 for distinct-row intercropping. Average maize yield penalty for intercropping maize and pigeonpea in the within-row was small (8%) compared with 50% in the distinct-row design; average (season×fertiliser) sole maize yield was 3.2tha−1. Intercropping maize and cowpea in within-row led to maize yield loss of only 6%, whereas distinct-row intercropping reduced maize yield by 25% from 2.1tha−1 of sole maize (season×fertiliser). Cowpea yield was less affected by intercropping: sole cowpea had an average yield of 0.9tha−1, distinct-row intercropping (0.8tha−1) and the within-row intercropping yielded 0.9tha−1. Legumes were comparatively less affected by the long dry spells which were prevalent during the study period. Response to N and P fertiliser was weak due to poor rainfall distribution. In the third season, maize in rotation with pigeonpea and without N fertiliser application yielded 5.6tha−1, eight times more than continuous maize which was severely infested by striga (Striga asiatica) and yielded only 0.7tha−1. Rainfall infiltration increased from 6mmh−1 to 22mmh−1 with long-term maize–legume intercropping due to a combination of good quality biomass production which provided mulch combined with no tillage. Intercropping maize and pigeonpea was profitable with a rate of return of at least 343% over sole maize cropping. Farmers preferred the within-row maize–legume intercropping with an acceptability score of 84% because of good yields for both maize and legume. Intercropping increased the labour required for weeding by 36% compared with the sole crops. Farmers in Ruaca faced labour constraints due to extensification thus maize–pigeonpea intercropping may improve productivity and help reduce the area cultivated. In Vunduzi, land limitation was a major problem and intensification through legumes is amongst the few feasible options to increase both production and productivity. The late maturity of pigeonpea means that free-grazing of cattle has to be delayed, which allows farmers to retain crop residues in the fields as mulch if they choose to; this allows the use of no-tillage practises. We conclude that maize–legume intercropping has potential to: (a) reduce the risk of crop failure, (b) improve productivity and income, and (c) increase food security in vulnerable production systems, and is a feasible entry point to ecological intensification. [Copyright &y& Elsevier]

Abstract: Rhizobial N2 fixation is a costly biochemical process, which takes 6–14% of current photosynthate (C) from legumes, without compromising grain productivity. In addition to the effects of leaf N nutrition, rhizobial symbiosis could stimulate photosynthesis due to the removal of C sink limitation by nodule activity. To test that hypothesis, we compared the photosynthetic capacity of soybean plants inoculated with two different strains of Bradyrhizobium japonicum (CPAC 390 or CPAC 7), varying in the effectiveness to fix N2, with plants fertilized with NO3 −. Nodulated plants had 14–31% higher rates of photosynthesis and accumulated less starch in the leaves than N-fertilized plants. There was evidence that B. japonicum CPAC 390 had higher carbon costs of N2 fixation compared with CPAC 7, but the increases in carbon costs were accompanied by higher rates of photosynthesis. By applying a biochemical model of leaf photosynthesis, including the limitations of Rubisco activity , electron transport rates (J) and triose-P utilization (TPU), we show that soybean plants adapt their photosynthetic capacity to support the stronger carbon sink created by faster rates of N2 fixation. We observed that plants associated with CPAC 7 (of low effectiveness to fix N2) increased their photosynthesis by removing sink limitation solely (with a constant ) whereas plants associated with CPAC 390 (of high effectiveness to fix N2) increased their photosynthesis by sink stimulation. Based on the model, we propose that sink stimulation is governed by a positive feedback between TPU and Rubisco activation, resulting in an increased . [Copyright &y& Elsevier]

Abstract: The APSIM model was used to assess the impact of legumes on sorghum grown in rotation in a nutrient-limited system under dry conditions in south-western Zimbabwe. An experiment was conducted at Lucydale, Matopos Research Station, between 2002 and 2005. The model was used to simulate soil and plant responses in the experiment. Sequences of cowpea (Vigna unguiculata), pigeonpea (Cajanus cajan), groundnut (Arachis hypogaea) and sorghum (Sorghum bicolor) were used in the rotations. Legumes accumulated up to 130kg of Nha−1 which was potentially available for uptake by sorghum in the following season. The APSIM model predicted total biomass, grain and N yields of the legume phase within the experimental error and performed well in predicting sorghum yield and N supplied in the rotation after cowpea and groundnut. The model generally under-predicted sorghum total biomass and grain yield after pigeonpea. Observed patterns of crop water use, evaporative losses during the dry season and re-charge of soil profile at the start of the rainy season were generally well predicted by the model. An assessment of output on sorghum N and water stresses in the rotation indicated that the legume–cereal rotation is more driven by soil nitrogen availability than water availability even under semi-arid conditions. Further legume–cereal rotation analysis using the model will assist in the understanding of other processes in the rotations in dry environments. [Copyright &y& Elsevier]

There is a need for an improved understanding of nitrogen (N) dynamics in depleted sandy soils in southern Africa. A field experiment was conducted to evaluate the performance of different soil fertility improvement practices on a degraded granitic sandy soil in Zimbabwe. Legumes capable of accumulating large amounts of N through biological N2 fixation and subsoil N capture were tested against soybean/maize rotation, cattle manure fertilization and continuous maize (Zea mays L) with or without fertilizer. Soybean (Glycine max) accumulated 82 kg ha−1 N (seed+stover), while mucuna (Mucuna pruriens) produced 87 kg ha−1 N in its biomass. Soybean fixed 76% of its N, while mucuna fixed 96% of the accumulated N as estimated by the 15N natural abundance method. Although the following maize crop in the second season suffered from drought stress, maize N uptake was 14.8 kg ha−1 following soybean and 16.4 kg ha−1 following mucuna, compared with 5.2 kg ha−1 for the unfertilized maize and 25.6 kg ha−1 for the maize fertilized with N at 90 kg ha−1. Cajanus cajan and Crotalaria paulina added barely 10 kg ha−1 of N through their biomass and had no effect on N uptake by maize. Apparent recovery of the added N by maize was 47% for the fertilized maize, 36% for soybean, 12% for mucuna and 9% for cattle manure. There was very little partitioning of N into grain and uptake was mostly before the onset of the drought. Despite the large differences in added residue N, differences in soil mineral N were only evident up to 4 weeks after the beginning of the rains, after which mineral N concentrations became very small in all treatments due to leaching, rather than crop uptake. By the eighth week after crop emergence, maize root length density had increased to about 0.1 cm cm−3 at the 60–80 cm depth, the rapid increase apparently stimulated by the drought. It was concluded that mineral N available to maize from the residues tested falls short of what is required to sustain high maize yields. In these environments where biomass accumulation in many legumes is restricted by soil biophysical factors (poor nutrient concentrations, acidity, coarse texture), combinations of legume rotations and mineral N fertilization will remain the most viable option for sustainable agriculture. [Copyright &y& Elsevier]

The use of options for sustainable intensification of smallholder farming in sub-Saharan Africa is often limited by knowledge and resource constraints. To address both constraints, we developed and tested an integrated co-learning approach to improve farm level productivity. The approach was tested by differentiating a group of co-learning farmers and a group of comparison farmers in two locations in western Kenya during five seasons. Both groups received a US$ 100 voucher each growing season and the co-learning group also took part in co-learning activities. The integrated co-learning approach was comprised of four complementary elements: input vouchers, an iterative learning process, common grounds for communication, and complementary knowledge. Central to the approach were co-learning workshops before each season. Workshop topics built on topics from previous seasons and on farmers' feedback and researchers' observations. Activities during each season included farm management monitoring, yield measurements and evaluation interviews. This resulted in multiple learning loops for both farmers and researchers. The voucher fostered learning through increased and diversified input use. For instance, intercropped legumes were smothered by the prolific growth of maize resulting from increased fertilizer use. After setting up joint demonstrations, farmers started to use alternative spacing options for intercropping. Building common ground on concepts and processes governing farm system functioning fostered a deeper understanding by farmers on the suitability of options to their farm and by researchers on locally relevant content. Soil fertility gradients was such a concept through which judicious use of fertilizers was discussed. After five seasons, co-learning farmers had a more diverse and cohesive knowledge of their farm than comparison farmers. Co-learning farmers highlighted farm level management options, management of the parasitic weed striga and options for integrated soil fertility management as the most important things they learned. A tangible learning outcome was the continued increase in groundnut and soybean area among co-learning farmers, which led to more diversified maize cropping systems. We attribute these differences to the co-learning process. Our results demonstrate how the integrated co-learning approach changed both knowledge and practices of participating farmers and researchers. The amplifying effects of the four key elements appeared to be important for enabling sustainable intensification of smallholder farming systems. Unlabelled Image • We developed and tested an integrated co-learning approach. • The approach combined input vouchers and iterative learning on sustainable intensification. • Co-learning farmers had a more diverse and cohesive knowledge after five seasons. • Co-learning farmers increased the share of legumes in their cropping system. [ABSTRACT FROM AUTHOR]