Nutrient capture from the subsoil nutrient pumping

Nutrient capture by trees from the subsoil can include nutrients released by weathering of primary minerals and also nutrients leached from the topsoil that are then recycled by the trees. Capture of newly weathered nutrients is restricted to relatively young soils where weatherable minerals still occur within the reach of tree root systems, including colluvial or alluvial soils with irregular nutrient distribution with soil depth.

Despite the prominence of nutrient pumping by trees as a theoretical concept in agroforestry, the actual importance of the process in different ecosystems and agroecological situations is not very well documented. Studies on tree crops in seasonally dry tropical climates have shown that nutrient uptake from the subsoil increases when the surface soil dries out (Comerford et al., 1984), and the process could, therefore, be expected to be important in savanna areas. However, in West African savannas, Kessler and Breman (1991) assume that nutrient pumping by trees is of limited importance, because most trees present in the landscape are too shallow rooted (despite the occurrence of some very deep-rooted species), and many soils are either too shallow or too dry and nutrient-poor at depth to make nutrient pumping a useful strategy. Other authors, in contrast, have stressed the importance of deep-rooting trees with wide distributions in the region. Acacia Senegal, Acacia tortilis, Faidherbia albida and Azadirachta indica were all found to root down to the water table at from 16 m to 35 m depth in sandy soils in Senegal (Leakey et al., 1999) and there is evidence of large nitrate reserves at depth in groundwaters in arid zones (Edmunds and Gaye, 1997). Use of water from the deeper subsoil (>2 m depth) during the dry season has been demonstrated for some woody species in the Brazilian cerrado savanna (Jackson et al., 1999). In a savanna in Belize, on the other hand, pines (Pinus spp.) had tap roots, but other trees were shallow-rooted, and nutrient enrichment under their canopy could not be explained by nutrient recycling from the deeper subsoil (Kellman, 1979). Not surprisingly, where exotic tree species, selected for fast above-ground growth, have been grown with crops in seasonally dry environments, the trees have generally been shallow-rooted and competitive with associated crops (Sinclair, 1996). Clearly, nutrient capture at depth in natural and managed semiarid environments appears to be very site and species specific and it makes sense to look for complementarity in natural associations between tree and herbaceous species on particular site types to find candidate species for use in agroforestry.

The recycling of leached nutrients from the subsoil by deep tree roots could be a relevant process in humid and subhumid regions, where nitrate derived from mineral fertilizer or organic sources is rapidly leached out of the topsoil together with nutrient cations such as calcium and magnesium during the rainy season (see Section 7.1). Soils with a high percentage of kaolinite and oxides of iron and aluminium, such as Oxisols and Ultisols, or soils of volcanic origin that are rich in allophane can have a significant anion exchange capacity that enables them to retain nitrate by sorption to the mineral phase. Anion exchange capacity (and nitrate sorption) increases with decreasing pH and decreasing organic matter content of the soil. They are, therefore, usually negligible in the topsoil and are most pronounced in acidic subsoil horizons. Nitrate sorption isotherms, describing the increase of nitrate sorption with increasing concentration of nitrate in solution, for different depths of Amazonian and Kenyan Oxisols can be found in Cahn et al. (1992) and Hartemink et al. (1996), respectively.

By slowing the downward movement of nitrate, anion sorption increases the probability that part of this nitrate is taken up by deep roots and returned to the topsoil through litterfall, prunings or leaching from the tree crowns. Nitrate accumulations have been reported from different tropical regions under annual crops (Cahn et al., 1993; Weier and Macrae, 1993; Hartemink et al., 1996; Jama et al., 1998) and also under a coffee plantation in Kenya (Michori, 1993, cited in Buresh and Tian, 1998), multistrata agroforestry and tree crop monocultures in central Amazonia (Schroth et al., 1999a, 2000a) and a runoff agroforestry system in semiarid northern Kenya (Lehmann et al., 1999a) (see also Box 8.1). Trees in humid tropical regions have often been seen as shallow-rooted, but it is now established that their roots can reach many metres deep (Nepstad et al., 1994; Canadell et al., 1996), provided that compact, acid or otherwise unfavourable subsoil conditions do not impede deep root development.

Studies in western Kenya showed that planted fallows of fast-growing trees (Calliandra calothyrsus, Sesbania sesban) produced root length densities of >0.1 cm cm-3 to below 1.5 m soil depth and reduced soil nitrate at 0-2 m depth by 150-200 kg ha-1 within 11 months after their establishment (Jama et al., 1998). They thereby recycled considerable amounts of subsoil nutrients, which could be made available to subsequent crops (or crops on neighbouring fields) through application of the tree biomass, either directly or after use as animal fodder. Factors that influenced these very promising results were certainly the fast growth of the trees, the high planting density of 1 m x 1 m, and the fact that the soils presented no obstacles for tree root development (Jama et al., 1998). They thus illustrate the considerable potential of planted fallows, fodder banks and fuelwood plantations for nutrient recycling, but further research is necessary on the efficiency of trees in subsoil nutrient capture under other environmental and management conditions. Where tree spacing is wide, as in many tree-crop associations, tree root growth in the subsoil may be less than in closely spaced fallows because there is less intraspecific root competition in the topsoil. Also, the efficiency of widely spaced trees in nutrient recycling from the subsoil may be limited to a certain area close to the trees to which the deep tree roots have access (Fig. 8.1). Therefore, the evaluation of spatial patterns of root distribution and activity must be part of studies of nutrient leaching and recycling. Many tropical soils also

Fig. 8.1. Distribution of mineral nitrogen (nitrate-N + ammonium-N, means and s.e.) in the soil of an oil palm (Elaeis guineensis) plantation in central Amazonia at different tree distances, reflecting the restricted lateral root extension of the palms at depth (reproduced with permission from Schroth et al., 2000a). For corresponding root distribution data see Fig. 12.1.

Fig. 8.1. Distribution of mineral nitrogen (nitrate-N + ammonium-N, means and s.e.) in the soil of an oil palm (Elaeis guineensis) plantation in central Amazonia at different tree distances, reflecting the restricted lateral root extension of the palms at depth (reproduced with permission from Schroth et al., 2000a). For corresponding root distribution data see Fig. 12.1.

present impediments to tree root growth in the subsoil and hence nutrient pumping, such as hardened, very nutrient-poor or very acidic horizons (Kessler and Breman, 1991). Furthermore, there are indications that regular and frequent shoot pruning, which is a common management practice for trees in agroforestry, can reduce the rooting depth of trees, either through changes in root system architecture (van Noordwijk et al., 1991a), or through a general reduction in root biomass. Such effects of shoot pruning have been found to be tree species specific (Jones et al., 1998). All these factors need to be taken into consideration in future research on subsoil nutrient capture in agroforestry.

Box 8.1. Nitrate profiles as indicators of nitrogen leaching and recycling.

Leaching of nitrate and its recycling from the subsoil through deep-rooting plants are particularly relevant processes in agroforestry because nitrogen availability often limits crop yields, and nitrate leaching contributes to cation loss and soil acidification and may be a source of groundwater contamination (see Sections 5.2 and 7.1). The easiest way of obtaining information about nitrate leaching and recycling is usually by collecting soil samples from different depths and extracting the nitrate as described in Section 5.2. Shepherd et al. (2000) compared the nitrate

Nitrate-N (mg kg-1 soil)

Nitrate-N (mg kg-1 soil)

Fig. 8.2. The distribution of nitrate-N under seven land-use types and plant species on smallholder farms in Kenya illustrates the tighter nutrient cycles in systems with trees and perennial crops compared with annual maize (reproduced with permission from Shepherd et al., 2000).

Continued

Box 8.1. Continued.

distribution to 4 m depth under different land-use types, including annual and perennial crops, trees and spontaneous fallow, in smallholder production systems in Kenya and found the largest accumulations of nitrate in the subsoil under maize (Fig. 8.2). Although such information does not allow quantification of nitrate losses from the different systems, the results demonstrate the advantage of integrating trees and perennial crops in annual cropping systems for closer nutrient cycles.

Using a similar approach, Jama etal. (1998) compared five tree species with respect to their ability to use nitrate that had accumulated in the subsoil during previous maize crops in Kenya as a criterion for their value in improved fallows. Nitrate profiles under different tree crops and a cover crop in a multistrata agroforestry system in Amazonia showed that the leguminous cover crop contributed more to nitrate leaching than three out of four tree crops in the system (Schroth et al., 1999a). Information on such small-scale patterns of nitrate distribution can help to optimize the design and management of complex cropping systems by showing where in a system nutrients are used inefficiently and where additional plants could be added or fertilizer rates reduced. Ideally, such information would be combined with quantitative nutrient balances for the whole system, as obtained, for example, in catchment studies.

Nitrate profiles can also provide useful information about the soil volume from which crops or trees take up nutrients. The extension and shape of the rooting zone of trees, especially in the subsoil, is important information for evaluating their potential role in nutrient cycling in agroforestry systems. For example, for functioning as a safety net for nutrients leached from soil in which associated annual crops are growing (see Section 7.1), tree root systems need a sufficient lateral extension in the subsoil under the crop rooting zone (van Noordwijk et al., 1996). Information on tree root distribution in the subsoil is difficult to collect through root studies and is, therefore, hardly ever available. However, by studying the spatial patterns of nitrate distribution in the soil, information about the lateral root extension at depth and the spatial patterns of nitrate leaching can be inferred with relatively little effort, as shown in Fig. 8.1. A similar approach can be used to analyse the influence of scattered trees, boundary plantings and hedgerows on subsoil nutrients in agroforestry associations (Mekonnen et al., 1999).

Repeated measurements of the nitrate distribution in the soil in the same area, for example at the beginning and the end of a cropping season or a fallow period, provide information on temporal changes of nitrate concentrations at different soil depths, and these may be related to nitrate leaching and uptake. Hartemink et al. (1996, 2000) showed that nitrate in the subsoil (50-200 cm) of two Kenyan soils decreased under Sesbania sesban and weed fallows, but not under maize and concluded that subsoil nutrients and water were used more efficiently by the fallows than by the crops.

Nitrate profiles do not usually allow measurement of nitrate fluxes in a strictly quantitative sense, because several processes which simultaneously influence nitrate concentrations at a given soil depth cannot easily be distinguished. Nitrate accumulations in the subsoil may be the result either of leaching of nitrate from the topsoil, or of in situ mineralization of nitrogen, or of a combination of both processes. Weier and MacRae (1993) measured the nitrogen mineralization rates at different depths in an Australian soil where nitrate was known to accumulate in the subsoil below shallow-rooted crops. They concluded that most of the subsoil nitrate had been leached from the topsoil, although in situ mineralization of nitrogen could make a contribution to the nitrate accumulation in the subsoil under black gram (Vigna mungo). Similar information from other sites would be helpful in the interpretation of nitrate profiles.

Decreases in subsoil nitrate content may also be caused by several processes, which may occur simultaneously. First, nitrate may be taken up by the vegetation, such as by deep-rooting trees. This requires that roots extend to the respective soil depth which should be checked in studies of nitrate capture (Jama et al., 1998). Secondly, during periods with downward water movement, nitrate may be leached to greater soil depths. Thirdly, nitrate may be lost through denitrification in the subsoil, a process that could itself be influenced by the presence of plant roots, since denitrification has been found to be limited by the availability of carbon rather than denitrifying microorganisms in certain subsoils (McCarty and Bremner, 1993). However, studies of denitrification in tropical subsoils and the potential influence of deep tree roots are not available.

Nitrate profiles can thus provide highly relevant information that can help to improve the nutrient cycles of agroforestry systems. However, because of the methodological problems outlined above they should usually be interpreted in largely qualitative terms. The analysis of nitrate profiles is greatly facilitated by complementary measurements of vertical and horizontal patterns of nitrogen mineralization, root distribution, soil water dynamics, and nitrogen uptake by the vegetation.

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  • gerard
    What is nutrient pumping of trees?
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