Climate resilient cropping systems are required to adapt to the increasing threats of climate change projected for Southern Africa and to better manage current climate variability. Conservation agriculture (CA) has been proposed among technologies that are climate-smart. For a cropping system to be labelled “climate-smart” it has to deliver three benefits: a) adapt to the effects of climate and be of increased resilience; b) mitigate climate effects by sequestering carbon (C) and reducing greenhouse gas emissions (GHG); and c) sustainably increase productivity and income. Research on smallholder farms from Southern Africa was analysed to assess if CA can deliver on the three principles of climate-smart agriculture. Results from Southern Africa showed that CA systems have a positive effect on adaptation and productivity, but its mitigation potential lags far behind expectations. CA systems maintain higher infiltration rates and conserve soil moisture, which helps to overcome seasonal dry-spells. Increased productivity and profitability were recorded although a lag period of 2–5 cropping seasons is common until yield benefits become significant. Immediate economic benefits such as reduced labour requirements in some systems will make CA more attractive in the short term to farmers who cannot afford to wait for several seasons until yield benefits accrue. The available data summarizing the effects of CA on soil organic C (SOC) and reductions in greenhouse gases, are often contradictory and depend a great deal on the agro-ecological environment and the available biomass for surface residue retention. There is an urgent need for more research to better quantify the mitigation effects, as the current data are scanty. Possible co-interventions such as improved intercropping/relay cropping systems, agroforestry and other tree-based systems may improve delivery of mitigation benefits and need further exploration.
Tag: biomass
Ecosystem fertility and fallow function in the humid and subhumid tropics
The regeneration of natural vegetation (fallowing) is a traditional practice for restoring fertility of agricultural land in many parts in the tropics. As a result of increasing human population and insufficient fertilizer inputs, the ecosystem fertility functions of traditional fallows must now be improved upon via the use of managed fallows. Interactions between vegetation and soil determine nutrient losses and gains in crop—fallow systems and are influenced by fallow species, patterns and rates of biomass allocation, and crop and fallow management. Nutrient losses occur through offtake in crop harvests during the cropping phase and through leaching, runoff, and erosion in the cropping phase and the initial stage of fallows $#x2014; when nutrient availability exceeds nutrient demand by vegetation. Gains in nutrient stocks in later stages of fallow are generally more rapid on soils with high than low base status due to greater quantities of weatherable minerals and lack of constraints to N2 fixation, deep rooting, and retrieval of subsoil nutrients by fallow vegetation. On low base status soils (exchangeable Ca 5 yr) are needed for recovery of cation stocks and crop performance on low base status soils. On both soils, however, residual benefits of fallows on crop yields usually last less than three crops.
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 ha1) than acacia (mean = 2.6 Mg ha1). 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.
Alley cropping of maize with calliandra and leucaena in the subhumid highlands of Kenya: Part 2. Biomass decomposition, N mineralization, and N uptake by maize
A major challenge in developing agroforestry approaches that utilize tree-leaf biomass for provision of N to crops is to ensure synchrony between the N released from decomposing prunings and N demand by crops. A study was conducted in the subhumid highlands of Kenya to assess the rate of decomposition and mineralization of soil-incorporated Calliandra calothyrsus Meissner (calliandra) and Leucaena leucocephala (Lam.) de Wit (leucaena) tree biomass and maize roots (Zea mays L.) both in an alley cropping and a sole cropping system. The amount of mineralized N peaked four weeks after planting (WAP) maize in all the treatments during both seasons of 1995. Cumulative mineralized N at week 20 ranged from 114 to 364 kg N ha1 season1, the absolute control treatment giving the lowest and the prunings-incorporated treatments giving the highest amounts in the two seasons. Total N uptake by maize, ranging from 42 to 157 kg ha1 season1, was lowest in the ‘alley-cropped, prunings-removed’ treatments, and highest in the ‘non alley-cropped-prunings-incorporated’ treatments. The apparent N recovery rate by maize was highest in the fertilizer applied treatments in the two seasons. Decomposition rate constants (kD) ranged from 0.07 to 0.21 week1, and the rates among the different plant residues were as follows: leucaena < calliandra < maize roots. Nitrogen release rate constants (kN), ranging from 0.04 to 0.25 week1, followed a similar pattern as the rate of decomposition with leucaena releasing the highest amount of N followed by calliandra and lastly by maize roots
Biomass burning fuel consumption rates: a field measurement database
Landscape fires show large variability in the amount of biomass or fuel consumed per unit area burned. Fuel consumption (FC) depends on the biomass available to burn and the fraction of the biomass that is actually combusted, and can be combined with estimates of area burned to assess emissions. While burned area can be detected from space and estimates are becoming more reliable due to improved algorithms and sensors, FC is usually modeled or taken selectively from the literature. We compiled the peerreviewed literature on FC for various biomes and fuel categories to understand FC and its variability better, and to provide a database that can be used to constrain biogeochemical models with fire modules. We compiled in total 77 studies covering 11 biomes including savanna (15 studies, average FC of 4.6 t DM (dry matter) ha1 with a standard deviation of 2.2), tropical forest (n = 19, FC = 126 ± 77), temperate forest (n = 12, FC = 58 ± 72), boreal forest (n = 16, FC = 35 ± 24), pasture (n = 4, FC = 28 ± 9.3), shifting cultivation (n = 2, FC = 23, with a range of 4.0–43), crop residue (n = 4, FC = 6.5 ± 9.0), chaparral (n = 3, FC = 27 ± 19), tropical peatland (n = 4, FC = 314 ± 196), boreal peatland (n = 2, FC = 42 [42–43]), and tundra (n = 1, FC = 40). Within biomes the regional variability in the number of measurements was sometimes large, with e.g. only three measurement locations in boreal Russia and 35 sites in North America. Substantial regional differences in FC were found within the defined biomes: for example, FC of temperate pine forests in the USA was 37 % lower than Australian forests dominated by eucalypt trees. Besides showing the differences between biomes, FC estimates were also grouped into different fuel classes. Our results highlight the large variability in FC, not only between biomes but also within biomes and fuel classes. This implies that substantial uncertainties are associated with using biome-averaged values to represent FC for whole biomes. Comparing the compiled FC values with co-located Global Fire Emissions Database version 3 (GFED3) FC indicates that modeling studies that aim to represent variability in FC also within biomes, still require improvements as they have difficulty in representing the dynamics governing FC.
Adoption, Profitability, Impacts, and Scaling up of Agroforestry Technologies in Southern African Countries
Because of a mix of agroecological factors (incessant drought, low soil fertility, environmental degradation) and other man-made problems (illiteracy, unfavorable development policies), the southern African region faces several challenges including worsening poverty, food insecurity, low income base, and more recently HIV=AIDS pandemic. Low soil fertility is identified as one of the greatest biophysical constraints to increasing agricultural productivity (Bekunda et al., 1997; Sanchez, 1999). The degradation of soils is caused by a breakdown of the traditional production systems resulting from shortening of fallow periods due to population pressure (Kwesiga et al., 1999). With the collapse of the erstwhile government support for the use of mineral fertilizer (e.g., through subsidies and distribution channels), in the 1990s, the ability of most smallholder farmers to purchase the same level of mineral fertilizers was reduced because the input became unaffordable to them. In addition, many countries in southern Africa are landlocked, thus increasing the cost of transporting fertilizer from the ports. Howard and Mungoma (1996) estimated that the use of mineral fertilizer fell by 70% following an increase in the cost of the inputs. The subregion also faces a rapid degradation of the miombo woodland, shortage of fodder, and decreasing access to fuelwood supplies (Kwesiga and Beniest, 1998). For example, Chidumayo (1987) estimated that Zambia alone loses ~200,000 ha of forests per year. Some of the key avenues for overcoming food insecurity and rural poverty in southern Africa include reversing soil fertility depletion, intensifying and diversifying land use with introduction of high-value products, and facilitating an appropriate policy environment for the smallholder farming sector. Although mineral fertilizer is still one of the best options for overcoming land depletion and increasing food production, the majority of the smallholder farmers are unable to afford and apply the fertilizers at the recommended rates and at the appropriate time because of high cost and delivery delays (Kwesiga et al., 2003; Akinnifesi et al., 2006). Low-cost technologies are needed on a scale wide enough to improve the livelihood of these farmers. This will require the adoption of new approaches to agriculture and rural development. Agroforestry has proven to be one of such approaches. For the past 15 years, farmers and researchers from different national and international institutions led by the International Centre for Research in Agroforestry (ICRAF), otherwise known as the World Agroforestry Centre, have been combining their expertise and resources to develop agroforestry technologies and options to address some of these challenges facing smallholder agricultural production and the environment in the subregion. The different types of agroforestry technologies address specific human and environmental needs in southern Africa. These include fertilizer tree systems for replenishing soil fertility, rotational woodlots for solving fuelwood problems, fodder banks to supplement feed for livestock, and indigenous fruit trees for improving nutrition during the seasonal hunger periods and enhancing the preservation of indigenous plant genetic materials.
Agronomic and economic evaluation of organic and inorganic sources of phosphorus in Western Kenya
Manys oils in the highlands of East and Central Africa are depleted of soil nutrients, particularly P. Ouro bjective was to comparec attle manure, Calliandra calothyrsus Meissner leaf biomass, and triple superphosphule (TSP) as sources of P for maize (Zea mays L.), both individually and as mixtures of organic (manure or calliandra) and inorganic (TSP + urea) sources. Field experiments were conducted on a Kandiudalf at two sites in western Kenya. Net benefits were computed as the difference between the value of additional maize yield accruing from nutrient inputs and the associated additional costs. Maize grain yield was 0.6 Mg ha1 for application of urea without P. Application of 10 kg P ha1 as organic, inorganic, and mixtures of organic and inorganic sources significantly increased maize yield. Grain yield for manure at least equaled and sometimes exceeded (P 0.05) yield for calliandra and TSP + urea. Net benefits in U.S. dollars (USD) for two seasons were highest for manure spot placed in the planting bole (293 USD ha1), broadcast manure (255 USD ha1), and broadcast TSP + 44 kg urea-N ha1 (313 USD ha1 at P = 30 kg ha1 and 98 USD ha1 at P = 10 kg ha1). Net benefits for calliandra leaf biomass were highest (136 USD ha1) when biomass was valued at cost of production and integrated with TSP, such that it provided all the N for maize and TSP provided the additional P not supplied by calliandra. Cailiandra valued at its opportunity cost as a protein supplement for dairy cattle was not an economic source of P. Sensitivity analyses suggest that organic materials most suitable for use as P sources have high P content and low cost of production.
Waste recovered by-products can increase growth of grass-clover mixtures in low fertility soils and alter botanical and mineral nutrient composition
The effectiveness of four by-products (biogas digestate, pot ale, rockdust and wood ash) as fertilisers of a perennial ryegrass (Lolium perenne)–red clover (Trifolium pratense) mixture in terms of biomass production, botanical composition and macro- and micronutrient concentrations was tested in an outdoor pot trial. This was carried out over two growing seasons using two inherently low-fertility soils used for forage production. Macro- and micronutrients (N, P, K, Ca, Mg, Co, Cu, Mn, Mo and Zn) relevant for crops and livestock were determined in soils and plants. All the by-products increased overall biomass production and affected nutrient concentrations of the individual plant species to varying degrees. In addition the competitive balance between grass and clover was altered leading to different botanical composition in the different treatments and consequently differences in the nutrient concentrations of the species mixture. Changes were due to the nutrients applied in the by-products per se and/or to changes in the soil chemistry caused by the by-products. The results suggest a potential to enhance agricultural productivity through improved production and quality of forage on less fertile land by matching of by-products and soil properties.
Agroforestry innovations for soil fertility management in sub-Saharan Africa: prospects and challenges ahead
Improving soil fertility is a key entry point for achieving food security, reducing poverty and preserving the environment for smallholder farms in sub-Saharan Africa. Given the high cost of inorganic fertiliz – ers, an integrated approach that combines promising agroforestry technologies – particularly improved fallows and biomass transfer – with locally available and reactive phosphate rock – such as the Min – jingu of northern Tanzania – is described in this chapter. Leguminous tree fallows of several species can accumulate significant amounts of nitrogen in their leaves in the short duration (from 6 months to 2 years). Incorporating these leaves into the soil before planting can increase crop yields several-fold. Improved fallows can also contribute to the control of weeds (including Striga hermontheca ) and pro – vide wood for energy and for staking climbing crops. Some of the species also have fodder value that can improve manure quantity and quality. For biomass transfer, use of Tithonia diversifolia is the most promising because of its high nutrient content and rapid rates of decomposition. This plant is now being used more widely for high-value crops such as vegetables. To facilitate the scaling up of these fertility options, future research and development needs to address recommended application rates, impacts at both farm and landscape levels, and the method by which high-value trees, crops and livestock can be intensively farmed to provide a natural progression out of poverty.
Organic residues affect phosphorus availability and maize yield in a Nitisol in western Kenya
The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into the soil at 5 Mg ha-1 for six consecutive seasons in 3 years and responses compared with those following the application of 120 kg N ha-1, 0 kg P ha-1 (0P); 120 kg N ha-1, 10 kg P ha-1; and 120 kg N ha-1 25 kg P ha-1 as urea and triple superphosphate (TSP); K was supplied in all treatments. Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Calliandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tithonia compared to 0P, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton treatment. P recovered in the above-ground biomass and resin P, immediately after the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35-77%) treatments than in the inorganic fertilizer treatments (21-27%). The P content of organic residues, and the soluble C:total P ratio, were the main residue parameters predicting soil P availability and maize yield. All organic residues used in this study can replace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of Croton, Lantana and Tithonia.