P.A. Sanchez and C.A. Palm
Pedro A. Sanchez is Director General of the International Centre for Research in Agroforestry (ICRAF), Nairobi, Kenya. Cheryl A. Palm is Senior Scientific Officer for ICRAF's Tropical Soil Biology and Fertility Programme, Nairobi, Kenya.
An examination of the role of agroforestry in nutrient cycling in different ecosystems, with a focus on the two main nutrients, nitrogen and phosphorus, in smallholder maize based systems of Africa.
One of the main tenets of agroforestry is that trees enhance soil fertility, the capacity of the soil to provide essential nutrients for plant growth. There is often confusion between key terms such as nutrient inputs, nutrient outputs, nutrient balances, nutrient cycling and nutrient capital. They all refer to a soil-plant system, usually at the scale of the farmer's field. Nutrient inputs are additions originating from outside the system, such as nitrogen fixed from the air by legumes or the use of chemical fertilizers. Animal manures are inputs if the manure was produced outside the system Nutrient outputs are those that leave the system through crop harvest removals, soil erosion, leaching, gas volatilization and other processes. The nutrient balance is the difference between nutrient inputs and outputs. Nutrient cycling refers to the transfer of nutrients already in the soil plant system from one component to another, for example the release of nitrogen from soil organic matter as ammonium or nitrate and its subsequent uptake by plants.
Other processes involved in nutrient cycling are the return of crop residues such as stover back to the soil; manure and urine deposited by cattle in the system; the incorporation of leguminous green manures into the soil and the transfer of nutrients from trees to crops in agroforestry systems through prunings, leaf drop or root decomposition. The fewer the nutrient losses from the system, the fewer the inputs needed from outside the system to balance the budget. The nutrient capital refers to the soil reserves of nutrients that will be released gradually over a time scale of years or decades. This article first examines the role of agroforestry in nutrient cycling in different ecosystems and then focuses on the two main nutrients, nitrogen and phosphorus, in smallholder maize-based systems of Africa.
Natural tropical forest ecosystems: in equilibrium
The existence of nearly closed nutrient cycles between a mature, tropical moist forest and the soil on which it grows has been recognized and studied for about 60 years (Hardy, 1936; Vitousek and Sanford, 1986). Nutrient inputs from atmospheric deposition, biological nitrogen fixation and weathering of primary soil minerals are in balance with nutrient losses owing to leaching, denitrification, runoff and erosion. Nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca) as well as sulphur (S) and micro-nutrients are absorbed by forest roots and returned to the soil via the decomposition of litter and roots as well by throughfall and stemflow.
Tropical humid forests accumulate huge quantities of nutrients in their vegetation as the forest grows, with a mature forest reaching steady state values of 700 to 2 000 kg N. 30 to 150 kg P and 400 to 3 000 kg K, Mg or Ca per hectare. The soil also contains large quantities of nutrients. The efficient cycling of nutrients from the soil to the biomass and back to the soil makes possible the lush tropical forest growing on acid, relatively infertile soils of the humid tropics, as long as there are no major biomass removals from the system. Improper harvesting of forests results in major disruptions of this process because large quantities of nutrients are removed from the system and nutrient cycles are disturbed.
Agricultural systems: in disequilibrium
Agricultural systems differ from natural systems in one fundamental aspect: there is a net output of nutrients from the site via crop harvest removals. This nutrient removal can result in net negative balances if nutrients are not replaced. Nutrient depletion can be offset by fertilizers, manures from outside the field and other nutrient inputs. This is generally the case in commercial farms of the developed world where such additions, coupled with the recycling of crop residues, have resulted in major nutrient accumulations. Such nutrient buildups, however, sometimes result in the pollution of groundwaters and in algal blooms in streams.
The magnitude of nutrient mining as a result of crop harvests in Africa is huge. Net losses of about 700 kg N. 100 kg P and 450 kg K per hectare during the past 30 years have been estimated for 100 million ha of cultivated land. Smaling's (1993) studies on nutrient balances throughout Africa lead the authors to conclude that soil fertility depletion is the fundamental biophysical reason for the declining per caput food production in smallholder farms in Africa (Sanchez et al., 1995). Nutrient mining in Africa, therefore, contrasts sharply with nutrient capital buildups in temperate regions.
Agroforestry systems
One fundamental principle of sustainability is to return to the soil the nutrients removed through harvests, runoff, erosion, leaching, denitrification and other loss pathways. Can agroforestry help? Following is a synopsis of two reviews (Palm, 1995; Sanchez, 1995), of which some results are encouraging and others disappointing.
Agroforestry, the growing of trees with crops and/or livestock on the same piece of land, is believed to promote a more efficient cycling of nutrients than agriculture. This hypothesis is based partially on studies of the efficient cycling of nutrients from litter to trees in natural ecosystems, and the assumption that trees in agroforestry systems will likewise transfer nutrients to intercropped plants. This is supported by observations of higher crop yields near trees of Faidherbia albida in the Sahel and where trees have been recently removed, as in the case of bush and tree fallows. Trees, therefore, can have an effect on soil fertility; however, one must also consider the relative importance of other factors such as soil structure, soil organic matter and the competition for light, water and nutrients before expecting a positive effect on soil fertility in a particular system.
Two key principles set agroforestry systems apart from agricultural or forestry systems: competition and complexity. They in turn determine two desirable properties: profitability and sustainability. All four combine biophysical and socioeconomic issues. The biophysical bottom line of agroforestry is how to manage the competition between the tree component and the crop and/or livestock components for light, water and nutrients to the benefit of the farmer. Although agroforestry systems have been classified in a myriad different ways, there are two functionally different types, simultaneous and sequential systems.
Simultaneous agroforestry is where the tree and the crop components grow at the same time and sufficiently close to each other to allow competition for light, water or nutrients. Examples of this type are alley cropping (hedgerow intercropping), contour hedges, parklands, boundary plantings, home gardens and several silvipastoral systems. Simultaneous systems can vary greatly in the relative proportions of trees and crops and in their spatial arrangement. Spatially mixed systems such as coffee and cacao plantations include upper-storey trees that provide nutrients, from litter fall and periodic prunings, and shade to the lower storey tree crops. Spatially zoned systems include hedgerow intercropping where annual crops are grown in alleys between rows of trees that are pruned periodically to provide nutrients to the crops in the alleys.
Competition for nutrients is maximized in simultaneous agroforestry systems, particularity short-duration systems such as alley cropping. The available literature, comprising more than 100 alley cropping experimental data sets ranging from semiarid to humid climates in widely different soils, has recently been analysed. Many factors affect alley cropping performance: the choice of tree and crop species, the alley width, the pruning regime, biomass production, the number of crop cycles, the time and frequency of prunings, tillage, fertilization, weed dynamics, etc. The overall conclusion appears to be clear: the chances that alley cropping will work are limited and very site-specific because, in most cases, the trees' competition for water and nutrients is likely to exceed the fertility benefits from the leguminous mulch additions.
Sequential agroforestry systems are those where the maximum growth rates of the crop and the tree components occur at different times, even though both components may have been planted at the same time and are in close proximity. Examples of this type are shifting cultivation, improved fallows, taungya and some multistrata systems. Competition for growth resources is minimized in sequential agroforestry because the peak demands for light, water and nutrients occur at different times for each component.
In tropical ecosystems, both natural and derived, nitrogen and particularly phosphorus frequently limit production. The following discussion focuses on these two most limiting nutrients, using maize as the reference crop.
Inputs
Trees can provide nitrogen inputs in agroforestry systems by two processes: biological nitrogen fixation (BNF) and deep nutrient capture. Although the magnitude of BNF is methodologically difficult to quantify, overall annual estimates are in the order of 150 kg N per hectare (Giller and Wilson, 1991). Empirical evidence, such as the presence of active nodules of leguminous species of the Papilionaceae and Mimosaceae families, indicate that BNF can supply considerable nitrogen inputs to crops via litter in soils that are sufficiently well supplied with phosphorus. This is a definite nutrient input. There is also ample evidence that non-fixing trees, including several species of Cassia (recently renamed Senna), accumulate as much or more nitrogen in their leaves than nitrogen-fixing legumes, presumably because of their greater root volume and ability to capture nutrients (Garrity and Mercado, 1994). It is, however, important to note that these non-fixing trees are only cycling nitrogen, not adding inputs to the system.
Deep nutrient capture is the uptake of nutrients by tree roots at depths where crop roots are not present. It can be considered an additional nutrient input in agroforestry systems because such nutrients are leached as far as the crop is concerned. They become an input on being transferred to the soil via tree litter decomposition.
An exciting discovery was made recently in western Kenya regarding subsoil nitrate in nitrogen-deficient nitisols (red alfisols and oxisols with a high iron content), where Hartemink et al. (in press) detected nitrate levels in the order of 120 kg N per hectare at subsoil depths of 50 to 200 cm. The authors also found that Sesbania sesban fallows depleted this pool, thus capturing a resource that was unavailable to maize. The source of this nitrate pool is believed to be the result of the mineralization of topsoil organic nitrogen, which is relatively high in these soils, followed by the leaching of nitrate from topsoil layers. The nitrate anions are then held in the subsoil by positively charged clay surfaces. What the trees have done, in effect, is to expand the volume of soil used by a considerable amount.
Outputs
A typical maize crop in smallholder African farms yields less than 1 tonne per hectare of grain and requires a plant accumulation of less than 40 kg N per hectare. A 4 tonne per hectare maize crop requires 100 kg N per hectare while a 7 tonne crop requires 200 kg N (Sanchez, 1976). Two-thirds of this nitrogen is accumulated in the grain and will be exported on harvest. Much of the remainder, located in the stover, may not be cycled back to the soil because it is frequently fed to livestock outside the system and manure is seldom brought back to the field where the maize was grown. Other losses through soil erosion, leaching and denitrification provide a similar nitrogen output to that of grain harvest removals in the Kisii District of Kenya (Smaling, 1993).
Balance
A negative annual balance of 112 kg N per hectare was calculated by Smaling in the Kisii District, where nitrogen inputs totalled 55 kg and nitrogen outputs 167 kg per hectare. This is considered typical of much of Africa.
Cycling
The release of nitrogen from soil organic matter may contribute most of the 40 kg N per hectare taken up by the average maize crop of 1 tonne per hectare. Considering the variable use of nitrogen fertilizers and the very limited returns of crop residues to the soil, most of the internal cycling in smallholder maize-based systems of Africa is likely to be due to the decomposition of soil organic matter.
Can agroforestry provide for nitrogen requirements? The nitrogen content of 4 tonnes of leguminous leaf mulch dry matter ranges from 60 to 150 kg per hectare (Palm, 1995). This mulch input rate was also recorded in leguminous fallows in Chipata, Zambia, where maize responds strongly to nitrogen fertilizers, but most farmers can no longer afford their use since the elimination of fertilizer subsidies. Two-year-old Sesbania sesban fallows doubled maize yields over a six-year period in comparison with continuous unfertilized maize production (Kwesiga and Coe, 1994). This was accomplished in spite of two years without crop production while the S. sesban was growing. A cost-benefit analysis also shows about twice the cumulative net benefit of S. sesban fallows over unfertilized maize. S. sesban fallows added 128 kg N per hectare to the maize crop, probably mostly from BNF and recycling. The fertilizer recommendation in that area is 112 kg N per hectare, slightly lower than the nitrogen input provided by the S. sesban fallow.
Appropriate sequential agroforestry systems such as S. sesban fallows appear to be able to replace fertilizer nitrogen applications when maize yields are about 4 tonnes per hectare. At high yield levels comparable with those of commercial farms in the industrialized countries (about 7 tonnes per hectare), organic nitrogen inputs are likely to be insufficient and must be supplemented with inorganic fertilizers. The interaction between organic and inorganic sources of nutrients is essentially a new subject of research in the tropics. Very little is known about it because previous research mainly compared one source against the other.
It appears, therefore, that improved fallow and some alley cropping systems can provide a nitrogen input sufficient to meet low levels of maize yields. The, degree to which this nitrogen is actually taken up by the crop depends on a variety of other factors, including the decomposition rate of organic mulches.
Replenishing nitrogen capital
The recovery of leguminous leaf nitrogen incorporated into the soil by the crop (10 to 30 percent) is generally lower than the recovery from nitrogen fertilizers (20 to 50 percent). Organic inputs, however, have an important advantage over inorganic fertilizers in sustainability terms. Much of the remaining 70 to 90 percent of the applied organic nitrogen not utilized by crops is incorporated into active and less active pools of soil organic matter because these mulches also provide a source of carbon. Soil microorganisms need a carbon substrate for growth; they also utilize the nitrogen from organic inputs, resulting in the formation of soil organic nitrogen. Furthermore, part of the nitrogen bound in the more recalcitrant organic inputs will also build up soil organic nitrogen.
Inorganic fertilizers do not contain such carbon sources; therefore, most of the fertilizer nitrogen not used by crops is subject to leaching and denitrification losses, while much of the nitrogen released from organic inputs and not utilized by crops could build soil organic nitrogen capital as well as having an important role in building up soil moisture capacity.
The slow accumulation of soil organic nitrogen with agroforestry or other organic inputs is likely to make a difference in terms of long-term sustainability. This strategy is not new and has been used for centuries in temperate agricultural regions, with crop rotations and winter leguminous cover crops. What is new is the potential to do something similar in the tropics with low input systems that suit smallholder farmers' perspectives. The potential for improved nitrogen management needs to be quantified in agroforestry systems by measuring processes such as mineralization, immobilization, denitrification, volatilization and leaching, along with changes in the soil organic nitrogen pools in systems combining organic and inorganic sources of nitrogen.
Inputs and cycling
Agroforestry, however, cannot supply most of the phosphorus required by crops. Leguminous mulches and green manures applied at a realistic rate of 4 tonnes per hectare provide 8 to 12 kg P per hectare. This is about half the phosphorus requirement of a maize crop yielding 4 tonnes of grain per hectare, which accumulates 18 kg P. Therefore, phosphorus is often the critical nutrient in agroforestry and other low external-input systems. Inorganic sources of phosphorus must be applied to agroforestry systems in soils depleted of this element. The strategy is to cycle all the available organic sources first, including manures, and supplement the difference with phosphorus fertilizer inputs. Combinations of organic and inorganic sources of phosphorus may result in a more efficient use of nutrients.
The deep capture of phosphorus is likely to be negligible because of the very low concentrations of available phosphorus in the subsoil. Many agroforestry systems do accumulate phosphorus in their biomass and return it to the soil via litter decomposition, but this is cycling and does not constitute an input from outside the system. However, through cycling, some less available inorganic forms of phosphorus in the soil may be converted into more available organic forms.
Outputs
The two main nutrient loss pathways for phosphorus are harvest removals and soil erosion. While the former is a desirable outcome, the latter is environmentally dangerous since, when eroded, a phosphorus-enriched topsoil can cause eutrification of surface waters. Fortunately, there are well-proven biological erosion control options, such as contour leguminous hedges (Kiepe and Rao, 1994).
Replenishing phosphorus capital
The phosphorus in soils of sub-Saharan Africa is being depleted at a fast rate. Agroforestry cannot be expected to provide additional phosphorus to most farming systems. Therefore, an initiative is being considered by the World Bank and other development agencies to replenish phosphorus through large applications of fertilizers. It is possible to replenish phosphorus capital in soils with a high phosphorus-fixation capacity (usually identified by their red clayey topsoils). Large applications of rock phosphates or other phosphorus fertilizers could replenish the phosphorus capital of these soils after being fixed and then gradually released by way of desorption from the oxide clay surfaces to plants during the next five to ten years. This is being considered as part of a new approach: investing in natural resource capital. One of the problems with this approach, however, is the need to add acidifying agents to the rock phosphates in order to facilitate their dissolution in most phosphorus depleted African soils, which have pH values of about 6. The decomposition of organic inputs may produce organic acids which can help acidify rock phosphate, and this may be the way to overcome the problem.
In conclusion, appropriate agroforestry systems can maintain or even restore nitrogen fertility through BNF, deep nitrate capture and several cycling mechanisms. Phosphorus fertility, however, cannot be replenished by agroforestry alone, but it can perhaps be made more available through cycling. For long-term production, agroforestry systems must include the addition of phosphorus and, in many cases, of nitrogen fertilizers as well in order to reverse nutrient depletion and ensure the efficient use of resources.
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