FEM - 22803
Agroforestry Period 2 Academic Year 2012-2013 Wageningen University
The Potential of Agroforestry
for Soil Quality Improvement in European Temperate
Agroecosystems
Literature
Research
Jelte van 't Foort and Vincent Delobel
Prof. Dr. Frans Bongers
MSc Development & Rural
Innovation Forest Ecology and Management Group
Before going into the content of our literature research, we would like to answer three fundamental questions and by doing so, get the purpose and the frame of this paper
clear.
What is Agroforestry?
Agroforestry is an agricultural practice and a scientific field that bridges the worlds of forestry and agriculture. World Agroforestry Center defines agroforestry as “a
deliberate integration of woody components with agricultural and pastoral operations on the same piece of land either in a spatial or temporal sequence in such a way that both ecological and
economic interactions occur between them.’’ In the first section of this paper (Context), we will explain that the pursuit of a broad range of goals and functions can result in the intentional
presence of trees in agroecosystems. In Europe, agroforestry belongs to a set of practices labeled “conservation agriculture” including complex crop rotations, cover crops, crop associations
(Labant, 2012).
What is soil quality improvement?
In organic farming systems, soil fertility is defined as the “ability of a soil to provide the conditions required for plant growth” (Stockdale et al. 2002). In the same vein,
we do not want to focus only on the improvement of the nutrients content of the soil but also on the effects of trees on its structure and its biology. The soil is not only seen as a substrate
for crops but also as a key-component of the ecosystem services supply chain. Thus, Larson and Pierce (1994) suggest the concept of soil quality, “represented by a range of physical, chemical and
biological properties of the soil within its particular environment that together provide a medium for plant growth and biological activity, regulate and partition water flow and storage in the
environment, and, serve as a buffer in the formation and destruction of environmental hazardous compounds”.
Why focus on European Temperate Agroecosystems?
Although the literature on tropical Agroforestry is extensive, we decided to focus on the potential of agroforestry in the agroecosystems of our region, Western Europe. Thus,
although we gathered literature on agroforestry worldwide in order to understand its mechanisms and the various functions of the trees in agroecosystems, we also gathered information about the
characteristics of temperate tree species, climate and soils in our region in order to study the actual potential of this practice in European temperate agroecosystems.
Context of Agroforestry
From the very beginning of the existence of agriculture as a human
social and economic practice, humans have cultivated crops and trees together throughout the globe. In Europe, until the middle ages and in some areas even up to the early twentieth century
(King, 1968), people clear- felled derelict forests for agriculture, used the slash- and burn technique, cultivated their lands for food crops for varying periods on cleared areas and planted, or sowing trees before, along with or after sowing agricultural crops. The shifting
cultivation of early civilizations evolved into a more settled systems of agriculture and involved woodland grazing and silvopasture with transfer of fertility of the soil from woodlands to
cultivated crops via manure. (More on pdf file)
As we explained above, we want to go beyond the strict definition of soil fertility based
on nutrient content and to consider the soil quality in a broader frame. Thus, we want to study the effects of trees on the soil that we consider as a key-component of the ecosystem services
supply chain. The trees have three main types of effects on the soil quality. They influence the soil structure, the soil fauna and flora, and the nutrients content of the soil. In this section,
we will tackle successively these three categories by gathering information on the trees in general. The characteristics of different tree species will be detailed in the next section. These
three subsections have the same structure: we first explain the functions of trees, their effects on the soil properties and the possible benefits for the crops.
The presence of trees in a crop field influences soil structure via two main phenomena. Firstly, the trees bring organic matter to the soil (litter fall, twigs, roots turnover,
pruning by-products); important increases (+4-7%) have been observed in alley cropping systems planted with red alders (Seiter et al., 1999b).
Secondly, the trees develop important root systems that explore and improve notably deep and compacted soil layers that crop roots do not reach (Schroth, 1999). These two phenomena have seven
major effects on soil structure. By increasing the cover of the soil, the presence of the trees limits the effects of wind and rain erosion (Rigueiro-Rodriguez et al., 2009). The increase of
organic matter and the development of root system limits soil compaction (del Castillo et al., 1994), improves the water infiltration in (Chander et al., 1998, Jose et al. 2008) and the porosity
of the soil (Jose, 2009) and increases basal soil respiration (Chander et al., 1998). Moreover, they increase the weight diameter of soil aggregates and so, increase soil stability and decrease
erosion risks in response to rainfall (Gupta et al., 2009). Finally, the trees improve the soil holding capacity for water and nutrients (Jose et al., 2008). These seven effects can be beneficial
for the crops because the roots ability to permeate the soil and the plant capacity to uptake and export water and nutrients from the soil are improved (Jose et al., 2008). Moreover, they can
create better conditions for soil biology.
Different observations of microbial community dynamics in ecosystems characterized by the presence of trees (Lejon et al. 2005, Seiter et al. 1999a) showed a correlation
between the proximity to trees and an increase of both fungal and bacterial active biomass in the top fifteen centimeters of the soil. Moreover, the fungi/bacteria ratio and the number of
earthworms are higher close to the trees because of the increased C-rich organic matter input. A likely conclusion from these observations is that the trees provide physical habitat and
nutrient-rich substrate for fungi and bacteria. Actually, the supply of organic matter in the topsoil creates a better environment for microorganisms in the rooting zone (Jose et al. 2008) but
the latter are essential actors of the biochemical degradation of organic matter. As part of the recycling process, the soil microorganisms convert pruning by-products, litter fall and crop
residues into crops-available and mineralized-form nutrients (Lejon et al. 2005).
Seiter et al. (1999a) and Lejon et al. (2005) focused on the composition of microbial biomass and the types of mycorrhizal
associations. The authors say that although the trees are influenced notably by the acidity, the quality and the quantity of organic matter input and the root exudates that differ between conifer
and hardwood species, they themselves influence the speed of organic matter decomposition and the type of nutrients that are released. Among the microbial, rhizospheric and mycorrhizal organisms
that interact with tree root systems, two distinct groups are particularly relevant in relation to their influence on the nutrient availability: the nitrogen-fixing and phosphorus-solubilising
organisms and/or associations (Schroth 1999); we come back to these two nutrients in the next section.
The presence of trees in the ecosystem can influence the nutrient content of the soil via its impacts on the soil structure and on the soil biology but also due to the
particular functioning of trees. We distinguish three levels of interactions.
First of all, trees can be providers of new inputs for the nutrient cycle. For instance, tree canopies collect atmospheric depositions that are incorporated into the soil when
the leaves fall. Thus, in the forests next to the sea, the Na content in the soil is particularly high (Augusto et al. 2004). Also, deep roots of the trees can capture nutrients and minerals from
the rock and the subsoil; pump it to the canopy so that it is an input in the nutrient cycle (Chander et al. 1998). As well, certain tree species associate with nitrogen-fixing or
phosphorus-solubilising mycorrhizal or rhizospheric organisms and make these nutrients available for the plants of the entire ecosystem (Chander et al. 1998).
Secondly, trees can be active drivers in the nutrient (re)cycling processes. Kho (2008) studied the nitrogen cycle in agroforestry. In such agroecosystems, nitrogen is supplied
to the topsoil by the fertilizer and/or the manure applied in the alleys (Jose et al. 2000), the biological fixation from atmosphere (N-fixing tree species) and the mineralization of organic
matter (which is also influenced by the trees) (notably by root exudates, improved soil conditions and root turnover) (Lehmann et al. 1995). According to Kho (2008), the problem is that nitrates
are highly soluble and easily transported by runoffs and leaching through soil layers. They contaminate the water table and the rivers which make the water unsuitable for drinking and cause
eutrophication of downstream ecosystems. To avoid such seepages, trees are helpful thanks to their “safety net”, i.e. their vast root system that can uptake nutrients in the deeper soil layers
where crop roots cannot (Rowe et al. 1999, Jose et al. 2004, Jose et al. 2008). By actively capturing leached nutrients and pumping them back to the canopy and then, to the topsoil, the trees
decrease the erosion and nutrient losses (Chander et al. 1998), help to maintain nutrient pool and the soil fertility (Young 1997), and improve the efficiency of nutrients use in the
agroecosystems (Rigueiro-Rodriguez et al. 2009).
Finally, some tree roles in the nutrient cycle can be improved by human intervention. As we said below, litter fall and pruning
by-products are major organic matter inputs and their decomposition releases nutrients in the topsoil (Chander et al. 1998, Jose et al. 2008). But what we do with the pruning by-products really
matters to avoid nutrient losses at the ecosystem level (Kho 2008, Seiter et al. 1999b). If they are used as ramial chipped wood or mulch (cf. “Agroforestry practices” section), the nutrients are
recycled in the agroecosystem. According to Kho (2008), in certain conditions and with particular species, tree prunings content in nutrients can meet crops requirements in N, Ca, Mg and
K.
In addition, root turnover can enrich soil with nutrients (Jose et al. 2004) and notably with carbon (Schroth 1999, Gupta et al. 2009) such as trees influence C/N ratio
(Augusto et al. 2004). But the root management practices such as root prunings accelerate the root turnover and enhance the redistribution of nutrients through lateral roots (Chander et al.
1998). In fact, the roots contain a lot of nutrients that travel easily along the whole root system. By pruning the roots, agroforesters induce root decomposition and the release of nutrients in
the topsoil throughout the entire crop field (cf. Root management practices, next section).
Different species have been found to impact the soils in a positive manner. Based on existing literature about
the tree species that are already used in temperate agroforestry, the aim of this section is to find which species characteristics are beneficial for the soil quality (its structure, biology and
nutrients content) and so, for the crops. In the following subsections, we gather information successively about Alder (Alnus), Wild Cherry
(Prunus avium), Common Hornbeam (Carpinus betulus), Walnut (Juglans), White Willow (Salix alba), Poplar (Populus), Lime
(Tilia) and finally Maple (Acer). (More on pdf file)
Agroforestry ecosystems, characterized by the coexistence or succession of trees and crops, are particularly dependent on human action to
remain stable. Their productivity is dependent on the balance between the positive (complementarity) and negative (competition) interactions among their different components (Jose et al. 2004).
Thus, human action is central to regulate these interactions and enhance the productivity toward selected goals by converting physical, chemical and biological processes into beneficial inputs
for crops and wood production. In this part, we aim to gather information about specific “agroforestry practices” related to the improvement of soil quality and below-ground interactions.
The presence of tree roots in the soil can be very beneficial for fertility and nutrient cycling. Schroth (1995 & 1999) lists several
expected benefits: enriching the soil with organic matter, maintaining the soil biomass and improving of nutrient cycling through root production and turnover; reducing leaching losses through
the uptake of mobile nutrients; pumping up nutrients from subsoil layers to the topsoil; improving soil penetrability for crop roots; fixing atmospheric N; creating appropriate conditions for the
development of mycorrhizal and rhizospheric microorganisms (N-fixing and P-solubilising). Nevertheless, tree roots may compete with crop roots for water and nutrients. In order to avoid such
competition and yields decrease, farmers can intervene in such a way that crop and tree roots are complementary and that the soil resources use is enhanced (Schroth, 1999). The nearness between
tree and crops roots can have both positive and negative effects - this is why agroforesters have developed root management practices to play with these interactions. These practices differ in
terms of labour intensity and we distinguish two main groups: those which require intervening directly on the soil and those which rely on species selection.
Tree and crop species selection
(More on pdf file)
Root pruning: trenching and tillage
Besides species selection, agroforesters have developed other root management practices that require more interventions throughout the entire
tree life. These labour intensive practices seek to decrease competition, to benefit from the root turnover and to maintain the positive interactions at the same time. We distinct three
ideal-type practices: trenching, deep ploughing and superficial tillage.
The first practice consists of digging open trenches to separate the alley and the trees in order to reduce root competition and to avoid
yield decrease near the trees. Even if the first goal is often reached, the latter may not follow because this practice negates the positive benefits of roots in nutrient cycling (Schroth, 1995).
This practice can be effective only if the trees are able to develop their roots below the crop rooting zone sufficiently.
Instead of digging open trenches, it is also possible to plough deeply between the trees and the crops for instance, at the beginning of the
cropping season. This practice requires less effort but needs to be repeated several times during the tree’s life. The efficacy of this practice relies on the mineralization of organic matter but
it has also a side effect: ploughing increases soil erosion and so, decreases soil fertility (Schroth et al. 1995). Another problem is that the roots take time to decompose and they are not
immediately available nutriments for crops.
Finally, the superficial tillage destroys the tree roots present in the topsoil just before sowing the crops. Doing so temporarily protects
the crops from tree root competition but does not force the trees to develop their root system exclusively in the lower layers of the soil. Repeating superficial tillage regularly increases the
root turnover (C input into the soil), the release of nutrients from the decomposing tree roots and maintains the “safety net”, i.e. the root system below crop rooting zone that uptakes the
leached nutrients (Schroth, 1995). Enacting superficial and regular tillage avoid erosion phenomena and respond to the need of time for root decomposition.
(More on pdf file)
(More on pdf file)
The aim of this literature research is to evaluate the potential of Agroforestry for soil quality improvement in
European temperate agroecosystems. To do so, on the one hand, we gathered literature on Agroforestry worldwide in order to understand its mechanisms and the various functions of the trees in
agroecosystems. On the other hand, we also gathered information about the characteristics of temperate tree species, climate and soils in order to study the actual potential of this practice in
European temperate agroecosystems.
To conclude, we would like to focus on three main points. First of all, from the several articles we gathered, we can assert that agroforestry practices have an actual
potential in the improvement of soil quality. Indeed, the effects of trees and hedgerows on soil structure, biology and nutrient content are significant. Secondly, humans play a central role in the combination of the biochemical processes, in the improvement of their functioning and in the design of new ecosystems
for sustainable agriculture. In fact, human action is central to regulate the interactions between the different components of agroecosystems and enhance its productivity toward selected goals by
converting physical, chemical and biological processes into beneficial inputs for crops and wood production as well as for other ecosystem services. Finally, the existing literature already
brings evidence that Agroforestry embodies the idea of multifunctional agriculture and constitutes a promising way to value biodiversity. In fact, by mixing species, Agroforestry fulfil several
functions at the ecosystem level. Thus, this single practice can be concretized in various ways, adapted to different ecological and socioeconomic contexts.
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References
(See pdf file)
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