19 Managing Fertility and Nutrient Cycles through Trees in Southern Africa

Paramu L. Mafongoya, Elias Kuntashula, and Gudeta Sileshi Q1 World Centre (ICRAF), Zambia

CONTENTS 19.1 Fertilizer Trees and a Typology of Fallows ...... 274 19.1.1 Use of Non-coppicing Fertilizer Trees...... 274 19.1.2 Use of Coppicing Fertilizer Trees...... 275 19.1.3 Mixed-Species Fallows ...... 276 19.1.4 Biomass Transfer Using Fertilizer-Tree Biomass...... 276 19.2 Mechanisms for Improved and Health...... 279 19.2.1 Biomass Quantity and Quality ...... 279 19.2.2 Biological Nitrogen Fixation and N Cycles ...... 279 19.2.3 Deep Capture of Soil Nutrients ...... 280 19.2.4 Soil Acidity and Phosphorus ...... 280 19.2.5 Soil Physical Properties...... 281 19.3 Effects on Soil Biota...... 282 19.4 Sustainability of Fertilizer Tree-Based Land Use Systems ...... 285 19.5 Discussion...... 286 Acknowledgments ...... 287 References ...... 287

Low soil fertility is increasingly recognized as a fundamental biophysical cause for declining food security among small-farm households in sub-Saharan Africa (SSA) (Sanchez et al., 1997). Because maize is the staple food crop in most of southern Africa, it will be our focus in this chapter. In 1993, SSA produced 26 million metric tons of maize on approximately 20 m ha; approximately 54 million metric tons is expected to be needed by 2020. Meeting this maize production goal will depend on sustaining and improving soil fertility levels that have been declining in recent years. Soil fertility is not the only significant constraint; lack of appropriate, high-quality germplasm, unsupportive policies, and inadequate rural infrastructure also limit maize production. However, protecting and enhancing soil fertility is the most basic requirement for achieving production goals. As discussed in Chapters 40 and 41, even controlling the parasitic weed Striga hinges on this fundamental factor. In most cases, nitrogen is the main nutrient that limits maize productivity, with phosphorus and potassium being occasional constraints. Although inorganic

273 274 Biological Approaches to Sustainable Soil Systems are used throughout the region, the amounts applied are seldom sufficient to meet crop demands due to their high costs and uncertain availability. Most countries in southern Africa have formulated fertilizer recommendations for all their major crops, sometimes with regionally specific adaptations. However, the amount of fertilizer used in southern Africa is very small in comparison to other parts of the world. For most smallholders, fertilizer use averages as low as 5 kg ha21 year21 (Gerner and Harris, 1993). While the need for increasing the availability of soil nutrients in southern Africa is quite apparent, increasing their supply is very challenging. A high-external-input strategy cannot rely on standard fertilizer-seeds-credit packages without addressing other requirements for successful uptake of Green Revolution technologies, including reliable irrigation, credit systems, infrastructure, fertilizer manufacture and supply, and access to markets. Most African conditions differ starkly from those in the prime agricultural regions of Asia. Approaches that produced successes in Asia are not readily transferable to the African continent. Considering the acute poverty and the limited access to mineral fertilizers in SSA, therefore, an ecologically robust approach of promoting “fertilizer trees” is discussed here. This is a product of many years of agroforestry research and develop- ment by the International Center for Research on Agroforestry (ICRAF), now called the World Agroforestry Center, working with various partners in eastern and southern Africa.

19.1 Fertilizer Trees and a Typology of Fallows Improved fallows involve the deliberate planting of fast-growing species, usually woody tree legumes, referred to here as fertilizer trees, for the rapid replenishment of soil fertility. Improved fallows were not a major area for research during the Green Revolu- tion due to its focus on eliminating soil constraints by use of mineral fertilizers. Biological approaches to soil fertility improvement began to receive attention in connection with the articulation of a second soil-fertility paradigm based on adaptability and sustainability considerations (Sanchez, 1994). Research on fertilizer trees had begun increasing from the mid-1980s, so by the mid-1990s they had growing justification in research (e.g., Kwesiga and Coe, 1994; Drechsel et al., 1996; Rao et al., 1998; Snapp et al., 1998). Large-scale adoption of fertilizer trees by farmers is now taking place across southern and eastern Africa. A more general consideration of fallows is presented in Chapter 29.

19.1.1 Use of Non-coppicing Fertilizer Trees Non-coppicing species do not resprout and regrow when cut at the end of the fallow period, typically after 2 years of growth. Non-coppicing species include Sesbania sesban, Tephrosia vogelii, Tephrosia candida, Cajanus cajan, and Crotalaria spp. Since the work of Kwesiga and Coe (1994) on Sesbania fallows, much has been learned about the performance of improved fallows using tree species that do not coppice. There has been extensive testing of various species and fallow length on-farm to determine their impact on maize productivity and to assess the processes that influence fallow performance. The performance of Sesbania and Tephrosia under a wide range of biophysical conditions is shown in Table 19.1. Trials at Msekera Research Station, Zambia, have shown that natural regeneration of Sesbania fallows is possible through self-reseeding, but it is highly erratic. Improved fallows of 2-year duration using either Tephrosia or Sesbania significantly increased maize yields well above those of unfertilized maize, the most common farmer practice in the region. While it was true that fertilized maize usually performed better than improved Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 275 fallows in most cases, this required a greater cash outlay, so improved fallows could be more profitable. The problem demonstrated in these trials was that the residual effects of these improved fallows on maize yield declined after the second year of cropping (Table 19.1). In a third year of cropping, maize yields following fallow were similar to those of unfertilized maize. The marked decline of maize yields two or three seasons after a non-coppicing fallow is probably related to depletion of soil nutrients and/or to deterioration in soil chemical and physical properties.

19.1.2 Use of Coppicing Fertilizer Trees Coppicing species include Gliricidia sepium, Leucaena leucocephala, Calliandra calothyrsus, Senna siamea, and Flemingia macrophylla. Fallowing with a coppicing species, in contrast to a non-coppicing species, shows increases in residual soil fertility beyond 2–3 years because of the additional organic inputs that are derived each year from coppice regrowth that is cut and applied to the soil. An experiment was established in the early 1990s at Msekera Research Station to examine these relationships. These plots have now been cropped for 9 years during which time both maize yields and coppice growth were monitored. The species evaluated showed significant differences in their coppicing ability and biomass production, with Leucaena, Gliricidia, and Senna siamea having the greatest coppicing ability and biomass production, while Calliandra and Flemingia performed poorly. The trends in maize yields have been tracked over nine seasons. In the plots with Sesbania fallow, while maize yields were high for the first three seasons, they then declined to the same level as control plots. Flemingia and Calliandra showed low maize yields over all years. There were no significant differences in maize grain between Gliricidia and Leucaena fallows over the seasons. The effects of different fallow species on maize yield can be explained partly by the different amounts of biomass added and the quality of the biomass and coppice regrowth. Species such as Leucaena and Gliricidia, which have good coppicing ability, produce large amounts of high-quality biomass with high nitrogen content and low contents of lignin and polyphenols, thereby contributing to higher maize yields (Mafongoya and Nair, 1997; Mafongoya et al., 1998). While Sesbania produces high quality biomass, its lack of coppice regrowth means that it cannot supply nutrients for an extended period of cropping. Species such as Flemingia, Calliandra, and Senna siamea, on the other hand, produce low- quality biomass, high in lignin and polyphenols and low in nitrogen. Their use as fallow species leads to N immobilization and reduced maize yields.

TABLE 19.1 Effect of Fallows on Maize Grain Yield Across 18 Locations in Zambia

Maize Grain Yield (t ha21) Land Use Year 1 Year 2 Year 3 Sesbania sesban fallow 3.9 1.7 1.1 Tephrosia vogelii fallow 2.4 0.8 0.9 Traditional grass fallow 1.1 0.7 0.7 Unfertilized maize 1.0 0.7 0.6 LSD 0.8 0.6 0.6

Source: Authors’ data. 276 Biological Approaches to Sustainable Soil Systems

Both Gliricidia and Leucaena have shown good potential as coppicing fallows. Over 9 years of cropping, cumulative maize yield of these fallows is greater than unfertilized maize, maize grown after Sesbania, and traditional grass fallow. Continuous nutrient replenishment is achieved by applying the coppice regrowth as mulch to the soil. This trial will be continued for another three seasons to test the sustainability of coppicing fallows in terms of nutrient budgets such as for NPK. On-farm trials have already been established to evaluate responses more widely and to screen more coppicing fallow species.

19.1.3 Mixed-Species Fallows Improved fallow practices using shrub legume species such as Sesbania have become popular agroforestry systems for soil fertility management in southern Africa and western Kenya. Large increases in maize yields have been reported following short-duration fallows of 9–24 months with single species. Sesbania has been the main focus for these improved fallows given its ability to provide large amounts of high-quality biomass and fuel wood. Dependence upon a few successful fallow species has revealed some drawbacks, however. Sesbania is susceptible to root nematodes and the Mesoplatys beetle. The introduction of any new species can lead to an outbreak of new pests and diseases, as was observed with Crotalaria grahamiana in western Kenya (Cadisch et al., 2002). Thus, there is an urgent need to diversify the fallow species and types offered to farmers. Mixing species with compatible and complementary rooting or shoot-growth patterns in fallow systems should lead to more diverse systems and maximize growth and resource utilization above- and belowground. Sowing herbaceous legumes under open- canopy tree species can increase the use of photosynthesis radiation by the whole canopy and thus enhance the system’s primary production. Mixing shallow-rooted species with deep-rooted species can enhance the soil-water and nutrient-uptake zone within the soil profile. More important, it enhances the utilization of subsoil nutrients such as the nitrate that is otherwise lost through leaching. Mixing species in fallows may also reduce the risks with fallow establishment, e.g., if one species is susceptible to water stress, diseases or pests, another can survive and even prosper. Obtaining multiple products from mixed fallows as well as increasing the biodiversity of the system makes the whole system more robust. We have assessed a variety of mixed fallows of tree legumes or tree legumes with herbaceous legumes to test these hypotheses. Mixing a coppicing fallow species such as Gliricidia sepium with a non-coppicing species like Sesbania (Chirwa et al., 2003) significantly increased maize yields compared to single- species fallows (Table 19.2). However, mixtures of non-coppicing species did not increase maize yield compared to sole species (Table 19.3). Mixing coppicing and non-coppicing species reduces the level of subsoil nitrate, and we found that it reduces Mesoplatys beetles (Sileshi and Mafongoya, 2002). We have found also that mixing Gliricidia, Tephrosia, or Sesbania with herbaceous legumes such as Mucuna or Dolichos reduces tree growth, and hence maize yield. Such mixtures also lead to a build-up of the Mesoplatys beetle, which can cause more damage (Sileshi and Mafongoya, 2002).

19.1.4 Biomass Transfer Using Fertilizer-Tree Biomass Traditionally, resource-poor farmers in parts of Southern Africa have collected leaf litter from secondary forest, called miombo, as a source of nutrients for their crops. In the long term, this practice is not sustainable because it mines nutrients from the forest ecosystems in order to enhance soil fertility in croplands. Also, the miombo litter is of low quality and may immobilize N instead of supplying N immediately to the crop (Mafongoya and Nair, 1997). An alternative means of producing high-quality biomass is through the Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 277

TABLE 19.2 Maize Grain Yield (t ha21) From 3-Year Coppicing Mixed-Fallow Species Treatments at Msekera, Eastern Zambia Species 2003 2004

Fertilized maize 5.9 3.4 Acacia angustissma (34/88) 3.7 1.3 Acacia angustissma þ Sesbania sesban 4.6 2.2 Gliricidia sepium (Retalhuleu) 4.1 2.9 Gliricidia sepium þ Sesbania sesban 4.6 2.7 Gliricidia sepium þ Tephrosia vogelii 3.3 2.1 Leucaena diversfolia 3.6 1.5 Leucaena diversfolia þ Sesbania sesban 4.3 2.0 Sesbania sesban 3.9 1.9 Tephrosia vogelii 4.3 2.6 Tephrosia vogelii þ Sesbania sesban 4.3 2.0 Traditional grass fallow 2.5 1.3 Unfertilized maize 1.7 1.4 SED: 0.5 0.8 F probability , 0.001 , 0.05

establishment of on-farm “biomass banks” from which the biomass is cut and transferred to crop fields in different parts of the farm. In western Kenya, for example, the use of Tithonia diversifolia, Senna spectabilis, S. sesban, and Calliandra calothyrsus planted as farm boundaries, woodlots, and fodder banks has proven to be beneficial as a source of nutrients for improving maize production (Palm, 1995; Palm et al., 2001). A study by Gachengo (1996) found that Tithonia green biomass grown outside a field and transferred into a field was quite effective in supplying N, P, and K to maize, equivalent to the amount of commercial NPK fertilizer recommended. In some cases, maize yields were higher with Tithonia biomass than with commercial mineral fertilizer. Biomass transfer using fertilizer-tree species is a more sustainable means for main- taining nutrient balances in maize and vegetable-based production systems, as the tree leafy materials are able to supply to the soil N (Kuntashula et al., 2004). Synchrony between nutrient release from tree litter and crop uptake can be achieved with well-timed

TABLE 19.3 Maize Grain Yield (t ha21) From 2-Year noncoppicing Mixed- Fallow Species Treatments at Msekera, Eastern Zambia Species 2002 2003

Maize with fertilizer 4.7 4.3 Tephrosia vogelii þ Cajanus cajan 4.7 2.0 Sesbania sesban þ Tephrosia 4.4 1.3 Sesbania sesban þ Cajanus cajan 4.0 1.8 Tephrosia vogelii alone 3.9 1.6 Sesbania sesban alone 3.4 1.0 Cajanus cajan alone 2.7 0.9 Maize without fertilizer 1.3 0.4 SED 0.9 0.4 F probability , 0.001 , 0.001 278 Biological Approaches to Sustainable Soil Systems biomass transfer. The management factors that can be manipulated to achieve this are litter quality, rate of litter application, and method and time of litter application (Mafongoya et al., 1998). Biomass transfer technologies requires more labor for managing and incorporating the leafy biomass, however. If used for the production of low-value crops such as maize, the higher maize yield from biomass-transfer technologies may not be enough to compensate for the higher labor cost. Most economic analyses have concluded that it is unprofitable to invest in biomass transfer when labor is scarce and its cost is thus high. However, when prunings are applied to high-value crops like vegetables, the technology becomes profitable (ICRAF, 1997). This practice has been found quite suitable for vegetable production in dambo areas of southern Africa (Kuntashula et al., 2004). Dambos are shallow, seasonally or permanently waterlogged depressions at or near the head of a natural drainage network, or alternatively they can occur independently of a drainage system. All together, dambos serve approximately 240 million ha in all of sub- Saharan Africa (Andriesse, 1986), of which 16 million ha are in southern Africa. Though dambos are extremely vulnerable to poor agricultural practices, rising population pressure has caused their agricultural use to become increasingly important (Kundhlande et al., 1995). Without applying fertilizers or cattle manure, smallholder farmers cannot produce vegetables successfully in dambos that are degraded due to their continuous cultivation for over 25 years (Raussen et al., 1995). Inorganic fertilizer is not always available to smallholder farmers, and cattle manure is accessible only to those with animals. This calls for alternatives such as biomass transfers for fertilizing vegetables in dambos of southern Africa. Additional results of such evaluations are given in Section 29.3.2.1. Farmer participatory experiments conducted in 2000–2004 by Kuntashula et al. (2004) have shown that biomass transfer using Leuceana leucocephala and Gliricidia sepium is tenable for sustaining vegetable production in dambos. In addition to increasing yields of vegetables such as cabbage, rape, onion, tomato, and maize grown after vegetable harvests, biomass transfer has shown potential to increase yields of other high-value crops such as garlic (Table 19.4). Our studies suggest that biomass transfer has greatest potential when (a) the biomass is of high quality and it rapidly releases nutrients, (b) when the opportunity costs of labor are low, (c) when the value of the crop is high, and (d) when the biomass does not have other, valued uses apart from being a reliable source of nutrients.

TABLE 19.4 Selected Vegetable Yields (t ha21) in Dambos Using Inorganic Fertilizers or Organic Inputs From Manure or Tree Leaf Biomass in Chipata District, Zambia Green Maize Green Maize Yield After Yield After Cabbage Yield Onion Onion Yield Cabbage Garlic Yield Treatments (n 5 31) (2000) (t ha21) (n 5 12) (2001) (t ha21) (n 5 6) (2004)

Manure 10 t þ 1/2 rec. 66.8 11.6 96.0 11.7 9.1 fertilizer Recommended fertilizer 57.6 8.4 57.1 10.4 7.2 Gliricidia sepium (12 t) 53.6 12.4 79.8 17.3 — Gliricidia sepium (8 t) 43.1 10.9 68.3 14.9 10.3 Leucaena leucocephala 2 12 t 32.6 - - - - 13.0 - - Nonfertilized 17.0 6.4 28.1 7.8 4.2 SED 5.3 2.06 11.2 3.04 1.2 F probability , 0.001 , 0.001 , 0.05 , 0.05 , 0.05

- -, treatment not evaluated. Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 279

19.2 Mechanisms for Improved Soil Fertility and Health 19.2.1 Biomass Quantity and Quality The success of maize crop rotations with fertilizer trees depends very much on processes for pruning biomass and on their nutrient yields. Analysis of maize yields across several sites with different fertilizer trees shows that maize yield is most closely correlated with the N content of prunings, with rainfall, and with the quantity of biomass applied. Low and insufficient biomass yields, combined with low quality of prunings in most instances, have contributed to frequent low performance of the technology. The low production of biomass for pruning may result from the use of unsuitable species, poor tree growth due to low soil fertility, soil acidity, moisture stress, or poor management of the species. Work carried out for many years has shown how organic decomposition and nutrient release are affected by the levels of polyphenol, lignin, and nitrogen content of the organic inputs (Mafongoya et al., 1998). Recently, we have also found that maize yields after fallows with various tree legumes were negatively correlated with the (L þ P) N ratio and positively correlated with recycled biomass. Fallow species with high N, low lignin, and low polyphenols such as Gliricidia and Sesbania gave higher maize yields compared to species such as Flemingia, Calliandra, and Senna. This work has shown that it is not the quantity of polyphenols that is critically important, but rather their quality as measured by their protein-binding capacity (Mafongoya et al., 2000). Legume species for improved fallows can be screened for their suitability based on the above characteristics.

19.2.2 Biological Nitrogen Fixation and N Cycles

The contribution of leguminous trees to crop yield through N2 fixation is well recognized, although not all legumes fix N2. Numerous nonleguminous species have N fixed in their roots and root zones through associations with N-fixing bacteria (Chapter 12). Nitrogen fixation alley cropping systems in the humid and subhumid zones of Africa has been reviewed by Sanginga et al. (1995). There has been little work carried out quantifying N2 fixation by trees in southern Africa, however. Such analysis has been difficult due to constraints in the methodologies for measuring the N2 fixed. A series of multi-location trials has been set up to measure the amount of N2 fixed by different tree genera and provenances using the 15N natural abundance method. The data on percent N derived from atmospheric N2 fixation (Ndfa) shows high variability among species and varieties of the same species. Greater variation was also recorded for the same species across different locations. So the measurement task is still a challenging one. Sanginga et al. (1990) found that the Ndfa ranged from 37 to 74% for different varieties of Leucaena leucocephala. The initial data show a huge potential of trees to fix N2 and increase N inputs in N-deficient . In future analysis we will focus on factors responsible for the variability in N2-fixation across sites and on how to optimize N2 fixation under field conditions. An estimated value of the level of inorganic N in soil before a cropping season begins is an accepted test for assessing prospective soil productivity. Results of studies in Southern Africa show that preseason inorganic N can also be an effective indicator of the N that is plant-available after fallow with different species (Barrios et al., 1997). Studies we conducted at 18 locations in eastern Zambia have indicated that in a tropical soil with a pronounced dry season, total preseason inorganic N (i.e., NO3 þ NH4) is more closely 2 related to maize yield (R ¼ 0.62; b ¼ 0.27, se ¼ 0.03) than to preseason NO3 alone. While large amounts of NH4 can accumulate during a dry season, it may not be nitrified when 280 Biological Approaches to Sustainable Soil Systems the soil is sampled at the beginning of the rainy season. We have concluded that preseason inorganic N is a relatively rapid and simple index that is related fairly well to maize yield on N-deficient soils, and hence it can be used to screen fallow species and management practices.

19.2.3 Deep Capture of Soil Nutrients The retrieval and cycling of nutrients from soil below the zone exploited by crop roots is referred to as nutrient pumping (Van Noordwijk et al., 1996; also Chapters 20 and 21). Soil nutrients not accessible to annual crops such as maize can be extracted by perennial trees through deep capture. The distributions and density of roots, the demand of plant for nutrients, and the distribution and concentration of plant-extractable nutrients and water will influence deep capture of nutrients by fertilizer trees (Buresh et al., 2004). Deep capture is favored when perennials have a deep rooting system and a high demand for nutrients, when water or nutrient stress occurs in the surface soils, and/or when considerable extractable nutrients or weatherable minerals occur in the subsoil (Buresh and Tian, 1997). These conditions were observed in eastern Zambia where nitrate accumulated in the subsoil during periods of maize growth, and fertilizer trees grown in rotation with maize could then effectively retrieve the nitrate in the subsoil that had been “lost” to maize. Intercropping rather than rotating fertilizer trees with crops appears to improve the long-term efficiency of nutrient use in deep soils. When perennials such as G. sepium are intercropped with maize, they remain always present in the agroecosystem compared with non-coppicing trees such as S. sesban. In a mixed fallow, Gliricidia provides a safety- net function to reduce nitrate leaching. In the Sesbania-maize rotation, there is no active perennial legume. Therefore, nitrate leaches into deep soil below the effective rooting depth of maize. Intercropping with fertilizer trees such as Gliricidia may thus be more effective for pumping of soil nutrients than a Sesbania-maize rotation. In base-rich deep soils of Msekera, eastern Zambia, there is potential for subsoil accumulation of highly mobile cations such Ca, Mg, and K, due to the weathering of minerals and leaching of cations that accompany NO3 leaching in fully fertilized maize crops without any trees present. The introduction of Gliricidia with maize rotation has a great potential for deep capture of Ca and Mg compared to continuously fertilized monoculture maize.

19.2.4 Soil Acidity and Phosphorus Acidic soils cover approximately 27% of the land in tropical Africa. Acidic soils are characterized by low pH, deficiencies of phosphorus, calcium, and magnesium, and toxic levels of aluminum. This is why finding strategies that offset soil acidity and low P availability is so important. Here, we discuss how agroforestry systems can address these two related constraints. In Chapter 37, there is a more detailed consideration of such a strategy, focused in Western Kenya. Lime application is the most widely used remedy for high acidity in countries such as in Brazil and U.S.A., but it is financially prohibitive for resource-poor farmers in southern Africa and cannot be considered a viable solution to the problem. Numerous laboratory experiments have recorded increased soil pH, decreased Al saturation, and improved conditions for plant growth as a result of the addition of plant materials to acid soils such as tree prunings, which also supply base cations such as Ca, Mg, and K. The value of tree prunings as a “liming” material for acid soils is related in their cation content (Wong et al., 2000). There is evidence from field experiments (see Wong et al., 1995) that the lateral transfer of alkalinity can be achieved by pruning pure stands of agroforestry trees and applying their pruned biomass to a maize crop. Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 281

Several mechanisms contribute to the increase in soil pH through such measures (Wong and Swift, 2003). These processes depend on the ash alkalinity of the organic inputs and on organic anion content. Leguminous materials are particularly useful in this respect because they have high ash alkalinity and offer the benefit of N2 fixation, while providing cash-limited farmers with inexpensive biological means of liming acid soils without having to buy costly inorganic lime. In small-scale farming systems in Africa, crop harvesting removes almost all of the P accumulated by cereal crops (Sanchez et al., 1997). In agroforestry systems, root systems may account for as much as 80% of the primary production. Application of plant biomass as green mulch can contribute to P availability, either directly by releasing tissue P during decomposition and mineralization (biological processes) or indirectly by acting on chemical processes that regulate P adsorption-desorption reactions. Soil organic matter contributes indirectly to raising P in soil solution by complexing certain ions such as Al and Fe that would otherwise constrain P availability. Decomposing organic matter also releases anions that can compete with P for fixation sites, thus reducing P adsorption. In agroforestry development, we have focused on the enhance- ment of the use-efficiency of soil P, i.e., on increasing the amount of biomass production for a set amount of P, as a more cost-effective means of improving P availability to crops. The more extensive root systems that trees and shrubs have compared to crops increase the exploration of larger soil volumes which results in enhanced P uptake. Recycled tree biomass is an important source of available P (Jama et al., 1997). Perennial tree species also produce organic anions. However, this production has not been as well studied as that achieved by annual crops (Grierson, 1992). As a result of mycorrhizal "infections," trees can readily produce organic anions that increase P availability through chelation or solubilization mechanisms (Chapter 9). Of these mechanisms, reactions involving metal chelates are most important in tropical acid soils (Gardner et al., 1992). Plant-microbial mechanisms that enhance P bioavailability can be incorporated into tropical agroforestry in the same way that N2-fixing trees have been integrated in agroforestry systems to increase N availability. However, much more research is needed on factors that control organic anion release from tree roots, their longevity in the soil, and different effects on P mobilization in different soils. Other strategies for managing tropical acid soils and increasing their availability of phosphorus are considered in Chapters 23 and 37.

19.2.5 Soil Physical Properties The ability of trees and biomass from trees to maintain or improve soil physical properties has been well documented. Alley-cropping, for example, can definitely improve the soil physical conditions on alfisols (Hullugalle and Kang, 1990). Plots alley-cropped with four hedgerow species showed lower soil bulk density, higher porosity, and greater water infiltration rates compared with a no-tree treatment (Mapa and Gunasena, 1995). Tree fallows can also improve soil physical properties due to the addition of large quantities of litter fall, root biomass, root activity, biological activities, and roots leaving macropores in the soil following their decomposition (Rao et al., 1998). In our studies, we have seen that Sesbania fallow increases the percentage of water- stable aggregates with a diameter .2 mm compared with continuous maize cultivation without fertilizer. After 6 months of cropping, the decrease in water-stable aggregates was highly significant under Sesbania (18%) compared with a traditional grass fallow, which did not lose its aggregate stability. A decrease in aggregate stability was more pronounced under Sesbania followed by maize without fertilizer compared with pigeon pea (Cajanus cajan) followed by maize with fertilizer (Chirwa et al., 2004). Under a Sesbania fallow, 282 Biological Approaches to Sustainable Soil Systems the improvement in soil structure was evident, as reflected by the results from our time-to- runoff studies. Time-to-runoff after fallow clearing followed this order: traditional grass fallow .Sesbania . fertilized maize (Phiri et al., 2003). After one season of cropping, time- to-runoff decreased in all treatments, except that the traditional grass fallow maintained longer time-to-runoff, reflecting its good maintenance of aggregate stability. Through rainfall simulation studies, Nyamadzawo et al. (2005) evaluated the effects of improved fallows on runoff, infiltration, and soil and nutrient losses under improved fallows. Tree fallows of Sesbania and Gliricidia mixed with Dolichos increased infiltration rates significantly compared with continuously fertilized maize plots (Nyamadzawo et al., 2005). Tree fallows also significantly reduced soil loss compared to no-tree plots. That fertilizer trees improve soil physical properties is seen from measured increases in infiltration rates, increased infiltration decay coefficients, and reduced runoff and soil losses. However, these benefits are short-lived and decline rapidly during the first year of cropping where non-coppicing species are used. This is consistent with an increase in soil loss in the second year and a decrease in infiltration rates as well. Mixing a coppicing species like Gliricidia with a herbaceous legume like Dolichos maintains high infiltration rates and reduced soil loss over 2 years of cropping (Mafongoya et al., 2005). In agro- forestry as in other we see repeated advantages of polycropping over use of single species.

19.3 Effects on Soil Biota Soil biological processes, mediated by roots, flora, and fauna, are an integral part of the functioning of natural and managed fallows (Sanginga et al., 1992; Adejuyigbe et al., 1999). As discussed in the preceeding chapter, this plays a key part in regulating the productivity of ecosystems (see also Sanginga et al., 1992). Among the soil biota essential in soil processes in agroforestry, probably the most important ones are the so-called ecosystem engineers, e.g., termites, earthworms, and some ants, and the litter transformers including millipedes, some beetles, and many other soil-dwelling invertebrates. Sileshi and Mafongoya (2005) compare the population of various soil macro-invertebrates under maize grown in an agroforestry system and monoculture maize. In five separate experiments conducted at Msekera and Kalunga, the number of invertebrate orders per sample and the total macrofauna recorded were higher under maize grown in coppicing fallows than under fully fertilized monoculture maize (Figure 19.1a and b). Similarly, the population density of total macrofauna (all individuals per square meter) under maize grown in coppicing fallows was higher than those under fully fertilized monoculture maize in all experiments at Msekera. Earthworm, millipede, and centipede populations under maize grown in coppicing fallows were also higher than under monoculture maize. Millipedes were absent from monoculture maize at both Msekera and Kalunga sites during most of the sampling periods. At Msekera, the population density of beetles was also higher under legume fallows compared to monoculture maize. Clearly, what is grown aboveground affects the flora and fauna belowground. We also noted differences according to fertilizer-tree species used for fallows. Cumulative litter fall, tree leaf biomass, and resprouted biomass under the respective legume species appeared to influence macrofauna populations. Macrofauna diversity (number of orders) was positively associated with total recycled biomass. The litter biomass under the tree species at fallow termination also influenced populations of beetles and earthworms. The tree-leaf biomass incorporated into the soil at fallow Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 283

500 (4a) Fallow 450 Monoculture maize

400

350

300

250

200 Macrofauna density 150

100

50

0 2000-3M 2000-3K 92-3 97-3 99-3 Experiment

4.5 (4b) 4 3.5 3 2.5 2

Orders/sample 1.5 1

0.5 0 2000-3M 2000-3K 92-3 97-3 99-3 Experiment

FIGURE 19.1 Macrofauna density (number of all individuals per square meter), and number of orders per sample under improved fallows and fully fertilized monoculture maize in various experiments. 2000-03M ¼ Experiment established in 2000 at Msekera; 2000-3K ¼ Experiment established in 2000 at Kalunga; 92-3 ¼ Experiment established in 1992; 97-3 and 99-3 ¼ experiments established in 1997 and 1999, respectively, at Msekera. termination was positively correlated with populations of beetles, earthworms, and millipede in the wet season. Among the fallows species, litter transformer populations were higher under G. sepium, which produced good quality organic inputs. On the other hand, a higher population of ecosystem engineers was found under trees that produce poor quality organic inputs (Sileshi and Mafongoya, 2005). The soil under fertilizer trees also harbors plant pathogens and soil insects that can adversely affect the crop and trees in agroforestry. Among the major soil pests of fertilizer trees are plant parasitic nematodes (Meloidogyne and Pratylenchus spp.) and termites. Root knot nematodes seriously affect the planting of S. sesban, pigeon pea (Cajanus cajan), and T. vogelii in southern Africa (Karachi, 1995; Shirima et al., 2000). In Tanzania, Meloidogyne infections were consistently highest when tobacco was planted after 2 years 284 Biological Approaches to Sustainable Soil Systems of S. sesban fallow (Shirima et al., 2000). So plants’ interactions with the soil systems that support them can have some undesirable effects. Although termites are generally essential ecosystem engineers in fallows, some are also crop pests. In parts of Kenya, Tanzania, Zambia, Malawi, Zimbabwe, and South Africa, 20–30% of preharvest loss in maize is said to be due to termites (Nkunika, 1994; Munthali et al., 1999; Riekert and Van den Berg, 2003; Van den Berg and Riekert, 2003). Termites are estimated to affect maize production on approximately 80,000 ha in the arid north and northwestern parts of South Africa (Riekert and Van den Berg, 1999). Few, if any, effective methods exist to control termite species that have subterranean nests such as Microtermes and Odontotermes (Van den Berg and Riekert, 2003). In a study conducted in eastern Zambia, Sileshi and Mafongoya (2003), recorded lower termite damage (% lodged plants) on maize planted after T. vogelii þ pigeon pea, S. sesban þ pigeon pea, and pure S. sesban compared with maize grown after traditional grass fallow. Monoculture maize grown after traditional grass fallow had approximately 11 and 5 times more termite damage compared to maize grown after T. vogelii þ pigeon pea and S. sesban þ pigeon pea, respectively. In another set of experiments, Sileshi et al. (2005) monitored termite damage on maize grown in coppicing fallows. Those studies showed that fully fertilized monoculture maize and maize grown in S. siamea and F.macrophylla fallows suffered higher termite damage compared to maize grown in G. sepium and L. leucocephala. Soil-dwelling insects such as white grubs (Schizonycha spp.) and snout beetles (Diaecoderus sp.) also affect trees and associated crops. Larvae of Diaecoderus spp. develop in the soil, and these later attack maize roots. Emerging adults also attack pigeon pea, Crotalaria grahamiana, Gliricidia sepium, and Tephrosia vogelii. Adult populations building up on these legumes during the fallow phase later infest maize plants (Sileshi and Mafongoya, 2003). In an experiment involving pure fallows and mixtures of these legume species, the density of Diaecoderus beetles was found to be significantly higher in maize planted after S. sesban þ C. grahamiana compared with maize planted after traditional grass fallow. The population of snout beetles was significantly positively correlated with the amount of nitrate and total inorganic nitrogen content of the soil and with cumulative litter fall under fallow species (Sileshi and Mafongoya, 2003). Other soil biota can play vital roles in controlling plant pathogens and soil-dwelling insect pests. Kenis et al. (2001) and Sileshi et al. (2001) reported some natural enemies of pests that affect fallow species. The entomopathogenic nematode, Hexamermis sp., the braconid wasp Perilitus larvicida, the carabid beetle Cyaneodinodes fasciger, and ants Tetramorium sericeiventre and Pheidole sp., all live in the soil and are natural enemies of M. ochroptera (Kenis et al., 2001; Sileshi et al., 2001). Most of these natural enemies were found to be more abundant in the improved fallows compared to monoculture maize. Although it is known that soil biota are the major determinants of soil processes and that pest management is an integral part of crop production, studies on soil biota have rarely been undertaken in conjunction with the design of agroforestry practices in southern Africa. The few studies cited above have shown that fertilizer trees can increase the diversity and function of soil biota compared to a continuously cropped, fully fertilized monoculture maize. This subject is discussed further in Chapter 41. Maintaining active soil invertebrate communities in soils could considerably improve the sustainability of cropping systems through regulation of the soil process at different scales of time and space. To increase the activity of natural enemies and reduce pest problems, fallow management practices that provide habitat, cover, and refuge for natural enemies and reduce build-up of pestiferous species need to be adopted. As experience and knowledge increase, we expect that in the future, routine fallow manage- ment practices will be manipulated to meet pest management objectives (Sileshi and Kenis, 2001). Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 285

19.4 Sustainability of Fertilizer Tree-Based Land Use Systems Improved fallows with Sesbania or Tephrosia have been shown to give subsequent maize grain yields of 3–4 t ha21 without any inorganic fertilizer addition. Palm (1995) showed that organic inputs of various tree legumes applied at 4 t ha21 can supply enough nitrogen for maize grain yields of 4 t ha21. However, most of these organic inputs could not supply enough phosphorus and potassium to support such maize yields over time. The question for sustainability is: can improved fallows potentially reduce soil stocks of P and K over time while maintaining a positive N balance? To answer this question we have conducted nutrient balance studies on improved fallow trials at Msekera Research Station. These plots were maintained under fallow-crop rotations for 8 years. The studies on nutrient balances addressed the following questions: (1) Can nutrient balances be used as land quality indicators? (2) Can they be used to assess soil fertility status, productivity, and sustainability? (3) Can they be used as a policy instrument for deter- mining the types of fertilizers to be imported or distributed to farmers? The nutrient balance studies considered the nutrients added through leaves and litter fall, which were incorporated after fallows as inputs. The nutrients in maize grain harvested, in maize stover removed, and in fuelwood taken away at end of the fallow period were then considered as nutrient exports. For all the land use systems, there was a positive N balance in the 2 years of cropping after the fallow. Fertilized maize had the highest N balance due to the annual application of 112 kg N ha21 for the past 10 years. Unfertilized maize had lower balances due to low maize grain and stover yields over time. The tree-based fallows had a positive N balance due to BNF and deep capture of N from depth. These results are consistent with those of Palm (1995) showing that organic inputs can supply enough N to support maize grain yields of 3–4 t ha21. However, we note that in the second year of cropping, the N balance became very small. This is consistent with our earlier results which showed a decline of maize yields in the second year of cropping after 2-year fallow. The large amount of N supplied by fallow species could be lost through leaching beyond the rooting depth of maize. Our leaching studies have shown substantial inorganic N at some depths under maize after improved fallows. This implies that if cropping goes beyond 3 years after fallowing, there will be a negative N balance. Thus, the recommendation of 2 years of fallow follow- ed by 2 years of cropping is supported by both N balance analyses and maize grain yield trends. Most of the land use systems showed a positive P balance. This can be attributed to low off-take of P in maize grain yield and stover. However, it should be noted that this site had a high phosphorus status already. The trees could have increased P availability through the secretion of organic acids and increased mycorrhizal populations in the soil. These issues are under investigation at our site. In general, we have observed positive P balances over 8 years. However, this result needs to be tested on-farm where the soils are inherently low in P. Most land-use systems showed a negative balance for K. For tree-based systems, Sesbania showed a higher negative K balance compared to pigeon pea. This is attributed to the higher fuelwood yield of Sesbania with subsequent higher export of K compared to pigeon pea. The higher negative K balance for fully fertilized maize is due to higher maize and stover yields which export a lot of potassium. This implies that the K stocks in the soil were very high and that K mining has not reached a point where it negatively affects maize productivity. However, in sites with low stocks of K in the soil, maize productivity may become adversely affected. 286 Biological Approaches to Sustainable Soil Systems

Overall, the tree-based fallows maintained positive N and P balances. However, on low- P-status soils, a negative P balance would be expected. There was a negative K balance with most land-use systems. It can be hypothesized that as improved fallows are scaled up on depleted soils on farmers’ fields, the K and P balances would be, or become, negative. This has implications for fertilizer policy. In Zambia, a mixture called “compound D” containing N, P, and K is the currently imported basal fertilizer for maize. If farmers adopt improved fallows on a wider scale, these will meet their N requirement for maize. Where there is K and P deficit, farmers may not need to buy “compound D” because N is adequately supplied by fallows since they need only K and P as nutrients to supplement their N from the fallow. This may require a shift in government policy on the type of fertilizer imported. There is an urgent need to conduct nutrient budget analyses at a landscape level on farmers’ fields to test the validity of our findings.

19.5 Discussion This chapter has described the progress that has been made during the past decade in research efforts to understand the mechanisms involved in how fertilizer trees work. A large amount of knowledge has been generated. However, some aspects of improved fallows have received little evaluation, and these will be highlighted here. Work on improved fallows has focused on just a few genera of fertilizer trees such as Sesbania, Tephrosia, Crotalaria, and Gliricidia. Further work is needed to identify more species for improved fallows. Given a large number of potential genera and species of legumes, the selection process could be accelerated by creating a database containing information on fallow performance in relation to environmental factors such as rainfall, soil type and chemistry, and incidence of pests and diseases. Our recent trials across sites have shown a great potential for Tephrosia candida as an alternative species to Sesbania and T. vogelii, and equally for Leucaena collinsii and Acacia angustissima as alternative coppicing fallow species to G. sepium. The biophysical limits of improved fallows need to be assessed and extended to facilitate scaling up with minimum research efforts. Simulation modeling, both as a tool for research and for extrapolation, has potential for integrating research results, identifying key components or processes that merit greater research attention, and also ecozones where appropriate fallow species and management techniques have a good chance of success. Agroforestry land-use systems have been reported to have large potentials to sequester soil carbon. However, there are few studies, if any, in southern Africa that have measured C sequestration in improved fallows. The relationship between increased soil aggregation and carbon storage also needs further research. As noted earlier, the interaction of pests with soil fertility is gaining attention due to wider interest in scaling-up of improved fallows. So far, most research efforts have concentrated on insect pests and nematodes. Equally important for farmers, however, are plant diseases and weeds. Little effort has been invested in these issues. With scaling-up across many ecozones, the incidence of new pests and diseases is likely to increase. This means there will be a need to monitor pests and diseases with farmers to determine which economic pests need to be dealt with in a concerted research program. Such work is now beginning in southern Africa. Many of the species currently used in improved fallows are prolific seed producers. If not managed well, these species can become invasive weeds and become a menace to natural ecosystems such as the miombo woodlands. To date there has been no concerted research effort to determine the invasiveness of introduced fertilizer-tree species. There is Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 287 an urgent need to use current models to predict the potential of new species to become invasive, while at the same time studying the reproductive biology and design management practices that will mitigate potential invasions of natural ecosystems. Research during the last decade has established the main mechanisms explaining how improved fallows work. Despite significant progress in biophysical research in improved fallows in southern Africa, the application of that scientific knowledge by small-scale farmers is still minimal. The main challenge now is to increase the generation of viable and acceptable fallow options that can make improved fallows more productive so that they markedly increase the income and food security of small-scale farmers. Future research issues on biomass transfer will involve the residual effect of low- and high-quality biomass, combinations of organic and inorganic sources of nutrients, the effects of biomass banks on nutrient mining, agronomic research of biomass transfer possible with different leguminous species, and economic analysis of the systems.

Acknowledgments The authors are grateful to the Swedish International Development Agency (SIDA) and Canadian International Development Agency (CIDA) for their continued financial support for agroforestry research for over 10 years.

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Q1 Please provide city. Q2 Kindly update Mafongoya et al. (2005).