duce terms such as soil functions, soil quality, soil health, soll resilience or biomantle, that are very much in use have the clear objective of introducing criteria ...
Preserving Soil Quality and Soil Biodiversity M. C. Lobo Rz J. J. Ibáñez, eds. IMIA - CSIC Madrid, 2003
SOIL FUNCTIONS, SOIL QUALITY OR SOIL HEALTH. SCIENTIFIC, METAPHORICAL OR UTILITARIAN CONCEPTS IN SOIL SCIENCES A. GARCÍA ÁLVAREZ J. J. IBÁÑEZ A. BELLO Centro de Ciencias Medioambientales, CSIC. Serrano 115 dpdo., 28006 Madrid, Spain
1. Introduction During the second half of the twentieth century there has been an unprecedented increase of soil degradation caused by human activity. This phenomenon is not new as humans have interfered with ecosystems since the Neolithic; they have deforested a large part of the territories in which they settled therefore causing soil loss. All through history there has been a manifest increase of soils with very little development that is known as leptosolitation (Ibáñez et al. in this book). Nevertheless, the erosion processes and soil loss, pollution, soil sealed by infrastructures, etc. have increased alarmingly during the past fifty years, specially affecting to the agricultural systems. In this context of increasing worry due to the loss of a resource that is essential for the production of food and which is only renewable on a very long term, has caused the emergence of some concerned social movements, among which the most noticeable ones are those who thrive towards the goals of sustainable development, terms comed at the United Nations Conference on Environment and Development, also known as the Earth Summit, hosted in Rio de Janeiro 1992. Sustainable development proposes a balanced economic growth, in which natural resources are exploited with prudence, and attempts to conciliate this growth with the preservation of the environment. The main problems to be confronted are the significant increaset of deforestation, the intense use of agrochemicals in agriculture, industrial progressive increase of residue production; for all these use the soil as their final destination. This is why it is no surprise those scientists, especially those who work in the soil sciences have taken the situation on board and are studying alternatives that may solve the problem. Nevertheless, there is one question that arises among soil scientists, regarding our present knowledge of soil and whether we are able to find solutions to these problems. During the past few years new concepts have appeared with the intent to deal with the relations of soil and agriculture or the environment. The objective of many of these con-
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cepts is to make an input into a new theoretical basis with which to confront the obvious, and gradually more accepted, complexity of the soil as a system. Nevertheless, many of these newly comed neologisms are only a new image for old knowledge (Bello et al. 2002), while they are others that are a substitute for old postulates, as is the case of the soil quality paradigm (Sojka & Upchurch 1999). The principies that attempt to introduce terms such as soil functions, soil quality, soil health, soll resilience or biomantle, that are very much in use have the clear objective of introducing criteria based on the information obtained from scientific knowledge, facilitating politicians' and managers work in all that is related to soil conservation and rehabilitation. From this perspective soil is considered an essential resource and therefore, most of the researchers and financial resources for studies in this area only taken into account under the following considerations: Production: I ncrease biomass renewal. Sustainability: Do not overbear soil's productive capacity. Environtnental impact: Avoid degradation and loss. Often it is forgotten that in order to study soil in depth, the `applied knowledge" perspective should be kept in the background. In order to avoid the threat of a possible bias that could take place if this type of knowledge were to be taken into account on its own. The discovery of radioactivity among a group of elements in our planet did not take place because the research was directed towards its use as a source of energy to be applied in the medical field or the construction of nuclear bombs. This is why, different specialists in soil sciences present a variety of arguments, criticising and objecting to the present day proliferation of conceptual neologisms, that they consider to have a teological foundation and their existence, in many cases, is unjustified and void of progress towards scientific knowledge. In this paper we intend to make a critical analysis of the tendencies in vogue that try to orientate soil science research, its in put, as well as its conceptual and methodological constraints, that causes the inadequate use of this emerging terminology which is now appearing in soil studies. 2. The
soll theory as conceptual reference
Soil is a self-organised system in space and time that possesses a great structural as well as a functional complexity. This is due to the great diversity of its components (abiotic and biotic ones) and to the processes that take place within the soil itself. Like all complex systems found in nature (Simon 1977) soil organisation is established within a hierarchical model, in which the step from a lower level to a higher one in the hierarchy presupposes the apparition of emerging properties that cannot be explained only by elements that compose the inferior hierarchical level. A fundamental feature that should not be forgotten is that soil is the recipient of a positive flux of chemical energy (+A E), which is originated in necromass that is brought in by the epigaeous' subsystem. In other words, soil behaves like a sink of energy that is used to close all the cycles of the matter of earth's ecosystems, as a result of the decomposition and mineralsation of organic matter. 152
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All of the aboye could be the reason why there is no rigorous definition of soil (Ibáñez et al. 2000), and that the definitions that can be found to date are biased and linked to the historical perception of their time. The study of soil has been traditionally associated to the agronomic paradigm, which explains why since its conception it has a strong productivist projection, thus converting soil sciences, especially pedology, in applied disciplines with very little projection towards basic research. But soil has also been the object of study of other disciplines; this has created other concepts that have materialised in different models, which are, often, unconnected. Dumansky (1993) identifies five models and Ibáñez et al. (2000) extend this list with three additional models, which are the following:
I. Soil as a ‚natura! body: A model established by the founder of pedology V. Dokuchaev, in which soil is considered a natural entity that varies discreetly in space and time. lis diversity is conditioned by factors like the climate, the relief, lithology and living beings. 2. Soil as a substrate for vegetation development: Soil is the substrate of crops, pastures and forests; therefore special attention is paid to the properties related to its fertility or the behaviour of water. From an agronomic perspective this model attempts to give recommendations for soil management. 3. Soil as a geological entity: It considers soil as a geological entity whose origin is due to the superficial alteration of lithologic matter caused by the climate and organisms. 4. Soil as a structural mantle: This model bases its interests in the mechanical features of soil and uses resistance, plasticity, porosity, infiltration or compaction as relevant properties. lt is related to soil technology and is very much used by engineers or geotechnical specialists 5. Soil as a water transmitter mande: It considers soil as an integrated element of the hydrological cycle that absorbs, stores, and transports water into terrestrial's ecosystems. 6. Soil as a component of the ecosystem: Soil is considered a subsystem of the terrestrial ecosystems or as an ecosystem in itself. Upon this model soil is studied through its interaction with all the other components of the ecosystem; its nutrient cycles, its energy fluxes and the architecture of its trophic levels 7. The holistic model of the pedosphere: This model has been proposed a few years ago by Ibáñez and Garcia Álvarez (1991). It considers the soil from a global perspective of being the boundary with other elements of the biosphere. Pedosphere, includes soil to the regolith, represents a geomembrane of the ten-estrial model that has its own organisational features. It is an open, complex, poliphasic, and polifunctional System that interact with other systems within our planet's framework (lithosphere, hydrosphere, biosphere, etc.). It also introduces a joint analysis of soil and terrestrial model. 153
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8. Soil as a component of the terrestrial surface systems: Proposed by Phillips (1999), it introduces the concept of geoecosystem as synonymous to earth surface system. This perspective includes all natural structures that are part and participate in earth's surface processes, among which soil would be included as a subsystem, which is part of the geoecosystem. In these models one can analyse the historical evolution of soil as a concept. In the older ones one can see the limitations of knowledge as well as their fragmented assessments based only on some properties of the soil system. The most recent ones (models 7 & 8) intend to broach the complexity and clarify soil's structure from a seif-organised hierarchical perspective in an attempt to identify the key elements at each level of the hierarchy. At the time they allow for elements of connection between the different levels of the system (for example, the knowledge of the relation thai might exist between the ß-glucosidase dynamics and the accumulation of organic matter in a peat bog). These last models allow for the inclusion of soil within the architecture of earth's surface's System and presuppose a differentiated alternative to the first models that emerge from a strictly agronomic or technical approach while having a clear immediate practical application intent. 3. Neologisms emergence in soil sciences In order to describe the concepts for sustainable land use and soil resilience Blum and Aguilar Santelises (1994) establish what they call soil functions, divided into the two following groups: Ecological functions: •
Biomass production (food, fibre and energy)
•
A reactor that filters, buffers, and transforms matter in order to protect the environment, the groundwater and the food chain from pollution.
•
Biological habitat and genetic reserve of many plants, animals and organisms that would be protected from extinction.
Functions linked to human activities: •
Physical environment which is the support of industrial and technical structures as well as socio-economic activities such as habitation, industrial development, transport systems, recreation or location of residues.
•
Source of raw materials that provide water, clay, sand, minerals, etc.
•
element of our cultural heritage, that contains paleonthological and archaeological treasures that are important in order to understand the history of the earth and of mankind. An
The use of the term function applied to properties or services offered by soll could be confusing to anyone who intends to apply it with rigour within a scientific discipline. 154
Soil fünctions, soil quality or soil health. Scientific, tnetaphorical or utilitarian concepts in soil sciences
Indeed, the abiotic as well as the biocenosis components of edaphic environment selforganise themselves in space and evolve in time. They are conditioned by the other environmental factors that are present in a specific context. To attribute the function of genetic reserve of plants and other organisms or to being a source of raw materials, would be equivalent to acknowledging that soil activity only makes sense in so far as it is of use to human kind, which is a utilitarian and teleological perspective. On the other hand, if we compare the soll functions with the other theoretical models previously described, most of all, those that also have a practical and finalist trait, we can see how the latter were already included in most of the conceptual elements that are among soil's function. Thus, the production of biomass is included and extended in model 2; the soil function as a reactor is also contemplated, amongst others, in models 4 and 5; the biological habitat function is one of the main points of model 6. The functions that are linked to human activity are also included, in varying degrees, in the abo ye mentioned models and are even the object of preferential study in other scientific disciplines. Nevertheless, soil functions have been adopted with certain self-complacency by researchers and environmentalists, that have expanded its application and thus, creating the emergence of a new terminology that been developed over these past ten years. One of the most used terms is that of soil quality, even tough soil health or soil resilience is also often used. Soil quality is an intuitive concept that, under different names, has been used since the old days to refer to the perception of different soil qualities for crops. In any case this concept has been traditionally linked to some features that do concern the management and productivity of agricultural soils ("rich soil", "light soil", etc.). But in recent years the concept of soll quality has been closely linked to soll functions (Warkentin 1995, Doran & Safley 1997, Karlen et al. 1997, Andrews et al. 2002) The concept of soil quality, which is being modified in time (Warkentin 1995), lacks a precise definition. Among the most quoted definitions found in the literature, Doran & Parkin' (1994) definition of soil quality is worth mentioning as: "Soil's capacity to function, within the boundaries of the ecosystem and the use of earthisoil while maintaining the quality of the environment and protnoting the health of plants, animals and humans." This definition has more nuances than a previous one that appeared in the June 1995 edition of the Agronotny News, that states that soll quality is " its capacity to function" (Karlen et a/.1997). Such a simplistic definition could be considered as equivalent to saying that the quality of bacteria, fungus, nematode or collembola, is lis "capacity to live", this goes without mentioning that biologists also have great difficulty in defining what is life. Strictly linked to this definition of soil quality is the need to make a quantitative evaluation. Karlen et al. (1997) propose a indicator's measure to compare it with known or desired qualities. Thus, they attempt to answer two questions: i) how does the soll work? and ii) which indicators are appropriate to make an evaluation? But surely there are many questions to answer such as: can a single indicator or a restricted set of indica155
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tors explain the global functioning of soil? Does the state of an indicator always have (he same interpretation? Which is and what is (he meaning of the conditions of reference with which the indicator is to be compared? These are some questions yet to be answered. It is widely accepted by the majority that the measure of soil quality can be established with adequate indicators, which would be surrogates of essential processes (physical, chemical and biological ones) that take place in soil. These indicators should be sensitive to (he detection of space and time differences, establishing (bus a clear cause-effect relation (Smyth & Dumanksi 1995). From these indexes a soil quality index (SQI) could obtain in order to know (he state of the soil. This proposal of new indices seems to be frequent in modern day scientific literature and it varies according to (he perception that different authors have when identifying relevant indicators. As knowledge of soil biological processes increase, the amount of published indices also increase. These include microbiological or enzymatic activity, based on basal respiration parameters, microbial biomass, (he contents of ATP or t he activities of enzymes such as (he phosphomonoesterases, I3-glucosidase or arysulphatase (Jiménez et al. 2002). Other authors have proposed more general indicators, like Doran & Parkin (1994) who include six elements:
SQ = f (SQE1, SQE2, SQE3, SQE4, SQE5, SQE6) In
which each element corresponds
SQE1 = Food and
ubre
to:
production
SQE2 = Erosivity SQE3 = Underground water quality SQE4 = Surface water quality SQE5 = Air quality SQE6 = Food quality If we recall the state factor equation proposed by Jenny (1941) to establish the driving forces of soll formation: S
= f (el, o, r, p, t...)
We can see the close analogy that exists between the two expressions (hat although they do not constitute a mathematical algorhitm, still try to formalise with apparent rigour a mathematical function with the intervention of elements among which it is very difficult to establish horizontal connections (within the same hierarchical level), for they express themselves in different time-space scales. Some authors acknowledge the difficulty in finding a straight forward measure for soil quality and consider the concept an umbrella under which the connections of different physical, chemical and biological parameters can be examined and integrated (Karlen et a/.1997). Other authors are concerned with the difficulties that exist in obtaining conclusions from a determined index when comparing different regions (Warkentin 1995).
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To all of the aboye, we must add the methodological heterogeneity used in determining with a same variable or factor that differs according to the observation scales adopted, and includes the 'taste' criteria of researchers. Consequently, the evaluation of soil quality has been the object of different proposals that include different indices and pedological parameters. Nevertheless, it seems that there is a consensus on the need to avail a minimum data set (MDS) that allows quantify soil quality. In Table 1 has collected the most used data and the relation that exists between the analysed variables and to soil functions to which is associated as an indicator. Table I. Minimum data set (MDS) of physical, chetnical and biological variables for soil quality determination (Doran ciad Parkin 1994, Larson and Pierce 1994 modified). Soll quality indicators
Physical properties Texture Soil and rooting depth Infiltration and bulk density Field capacity and water retention Chemical properties Organic matter (Total organic C y NI) Soll pH Electrical conductivity Extractable N, P, and K Biological properties Microbial biomass (C y N) Potential N mineralisable (anaerobic incubation) Soil respiration, water content, and temperature
Related soll functions Water and nutrient retention and transmission Potential production estimated Potential lixiviation and erosivity Transmission and erosivity
Soil fertility and stability Chemical and biological activity thresholds Microbial and vegetation activity thresholds Available nutrients for plants Potential microbial catalic
Soil productivity Microbial activity measure
Some authors have recently considered that soil quality indicators and indexes should be selected according to the functions that are to be the object of study and the management objectives defined by the system (Andrews et al. 2002), even though this procedure could favour a proliferation of indicators and indices that would saturate scientific literature, thus transforming research papers into desinformation. Something similar happened in the seventies within the environmental field, indices to measure diversity and water quality proliferated to such extremes that every author proposed their own induce (Washington 1984). In time, only a few of these have survived. lt is also of interest to mention that within this field, that although soil biodiversity is much more important in quantity than in the aerial environment, papers on soil quality hardly take into account the biodiversity of soil. The concept of soil quality has been strongly criticised from a variety of areas in soil sciences. Some of these argue against this concept, amongst which Sojka and Upchurch (1999), high light the following: 1) lt is based on regional agricultural analysis and is largely associated to a type of soil (Mollisols). 157
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11)
Soll quality assessment employs a great variety of measures and empirical perceptions as well as subjective criteria. It includes social and economical values of doubtful scientific validity.
iv) The term is ambiguous and not an objective scientific attribute. v) It is conceived on the basis of its final use and is generally applied to agricultural soils within a context that presumes to be sustainable. v1)
It creates confusion between what are soil properties and soil services when using the so-called "functions".
Some authors admit to the subjectivity of the term (Warkentin 1995) and the complication of its definition and assessment. (Doran & Safley 1997). Nevertheless, it is used increasingly, and sometimes, with an effect that is the opposite of what some authors aim to achieve, which is to improve the collaboration among different disciplines that are all part of soil science.
Soll health is a term that is used in many occasions as a synonym to soll quality (Warkentin 1995, Doran & Safley 1977, Karlen eta!. 1997), although it refers solely to descriptive and qualitative properties. This is why its use is more extended among farmers (Doran & Safley 1977), who refer to more or less intuitive values to make the distinction between "healthy" and "unhealthy", as opposed to the term soil quality which is in much more use among scientists (Karlen et al. 1977). Soil health reveals the idea of a live and dynamic organism that functions holistically (Doran & Safley 1997), although there is no precise definition of the term and there are strong controversies over it in the literature, which is probably the consequence of the arbitrariness of value judgements that are inevitably associated to this concept. A healthy soll would be the one that lacks physical, chemical or biological limitations for the development of plant life from an agronomic perspective. Therefore acid, saline and sodic soils or those who suffer from episodes of flooding (pseudogley) should be considered "unhealthy"? Although they are very frequent in nature and "work" within the imposed limits of the environmental factors that affect them. Also, an inadequate management may ease the surge of plagues and illnesses that could eventually disappear at the moment that an adequate management is re-introduced (rotations, ploughs and so forth.). Thus, frequently it would be considered that soil management marks the boundaries between a healthy or unhealthy soil. The impossibility to measure soil's health directly is generally acknowledged this is why the system's components or processes should be assessed (Elliot 1997). Yet there are some authors who suggest the need to look for a group of syndrome indicators that would make soil's lack of health more obvious (Rapport eta!. 1997). In spite of everything and with some recklessness, the Agricultural Research Service of the USDA has developed a Soil Health Kit and proposes its use as a tool to evaluate soll's health and quality (USDAARS 1999). This kit includes the estimated variables found in Table 2 and proposes the procedures for its application. 158
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Table 2. Parameters included in the Soil Health Kit (USDA-ARS 1999).
Physical
Chemical
Biological
Bulk density Infiltration rate Water content Aggregates stability Stability of soll fragments in water
Electrical conductivity pH Nitrates Water quality
Soll respiration Earthworms number
The term resilience was also introduced during the seventies within the realm of ecological studies. It was an attempt to develop a general theory linked to biologic ecosystems' stability and diversity. Stability has often been defined in many occasions as the absence of a variation within the structure of an ecological system, and it can be identified from a set of element's values through which the system establishes a specific equilibrium called domain or basin of attraction (Hill 1987). The unbalance of this equilibrium due to a disturbance has been analysed within the context of ecosystems' stability, in which resilience is a term used to evaluate soil's response when confronted with a disturbance. Resilience has been defined as the capacity a system has to recover its initial state after suffering a disturbance. The application of this concept has created many controversies among different groups of researchers that have proposed terms like elasticity, amplitude, hysteresis or malleability in order to introduce nuances to the former definition (Hill 1987). The concept of resilience has only recently started to be applied to the soll system (Blum 1994), where it is defined as the capacity of soll to recover its structural and functional integrity after a disturbance (Herrick & Wander 1998), or as its capacity to return to a new dynamic equilibrium after a disturbance (Blum & Aguilar Santelises 1994). In any case, both definitions are analogous to the one that was first mentioned in the context of ecosystems' stability. Nevertheless, there has been similar controversies to those previously mentioned, over the past three decades within the ecology field and in which some authors have proposed the definition for soll resilience as its capacity to resist changes that are provoked by a disturbance (Rozanov 1994), this coincides with the previous definition of resistance (Hill 1987). Soll resilience has been associated with soil quality as the soil funtion's recovery (Seybold et (71. 1999) and it would depend on biological communities, the climate, soll management, the type and intensity of disturbance as well as the observation scales space-time. In order to determine soil resilience three types of approaches have been proposed: (i) a direct measure of soil's recovery, (ii) the identification and quantification of soil's integrity mechanisms that contribute to its resilience after a disturbance, (iii) specific property measures to be used as indicators of the recovery mechanisms (Seybold et al. 1999) In the following table we can see a proposal of potential indicators for soll resilience. 159
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Table 3. Potential indicators for soil resilience (Seybold et al. 1999).
Physical
Chemical
Biological
Soll structure Microaggregates Soll water Retention and transmission properties
CEC Soll pH Exchangeable cations Soll organic matter content Transformations Nutrient-supplying capacity
Rooting depth Soll biodiversity Soil fauna activity Microbial activity
Many of these indicators are the same or are very similar to those that are proposed for soil quality or soil health, even though in this case soil biodiversity is explicitly included, it was not taken into consideration in the previous tables. In any case, soil resilience is beginning to be applied with a different perspective, this is perhaps a consequence of the multiplicity of perceptions with which the concept of soil stability is approached, and this concept should be narrowed down within the terms that we have established in order to avoid diminishing its efficiency when trying to establish considerations of a general nature. 4. Current perspectives
in
soil studies
The proliferation of neologisms within soil sciences is probably due to two elements that have two different causes but are presently developed with a synergetic behaviour within these disciplines. On one hand, there is the increase of concern for soll degradation and loss, while, on the other hand, there is an incessant increase of the awareness of soll sciences' complexity as an object of study. Within this context, when scientific activity seemingly corresponds to the search for a theory that encompasses all, the observation and experiments that have taken place in the past to date are frequently forgotten. From the perspective of scientific knowledge, there are attempts to provide solutions to a very serious social and economic situation that must confront the progressive loss of such an essential resource as soll. But seemingly a solution is sought by reducing the complex structural and functional edaphic environment to a single algorithm, in this the reductionist models fall when confronted with small variations of the scenarios conditions. The disturbances to which soll is subjected to, create different answers when considering soil's special dimension, but one can also observe different effects on that same soil which varies according to at which moment the disturbance takes place. An episode of intense rain on dry soil, close to the wilting point and with a high hydrophobicity has totally different consequences than if the soil is to field capacity. These biological variables of soll react speedily when faced with a disturbance, for there is generally a decrease of final values on populations, activities or concentrations in comparison to their initial values. Nevertheless, there can also be significant increases, like when pathogenic populations in agrosystem increase. This could be due to either inadequate management or to the elimination of key elements of the soll's functioning. The evolution of biotic variables does not have the same behaviour either, even when fac160
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ing the same type of disturbance. Thus, a study of the impact of forest fires in the edaphic environment in which more than fifty edaphic variables were analysed, show a contradictory interpretation if the criteria used to evaluate soil quality or health were applied. The soils of a burnt area were analysed in a Maritime pine (Pinus pinaster) forest partially burned nine months before. There was also a subzone differentiated that had already suffered from another previous fire (rebumed). It was compared to a third zone that had not suffered from the effects of fire. The most remarkable differences among the physical and chemical variables (Table 4) show an increase of pH and of C-organic in the re-burned zone (An increase of quality?). If we consider some biological and biochemical variables that were analysed (Figs. 1 & 2), the interpretation could also be biased depending on which variables were included.
Table 4. Physical and chemical variables in a burned forest of Pinus pinaster Sample
Unburned Burned Re-burned
(%)
Silt (%)
Clay (%)
78,0 76,4 74,4
15,8 16,2 17,8
6,2 7,4 7,8
Sand
pH Organic C (%) (H 20) 5,5 5,8 6,1
Total N
C/N
(%)
Ratio
0,27 0,27 0,25
21,1 20,7 26,4
5,7 5,6 6,6
Ca++
3,0 5,0 6,0
Mr Na 1{' meq / 100 g of soil 2,3 1,9 2,1
0,17 0,17 0,17
0,26 0,26 0,26
Upon seeing the previous results one can well ponder on which parameters should quality soil be determined and in which situation (unburned or burned) is there a healthier soil. There are also other questions that arise such as: are these variables useful when determining the degree and effects of a disturbance? Would the same behaviour be observed in other soil types or in different climatic conditions? Which would be the evolution of the variables observed and in what state will they be in fifty or a hundred years time? These questions have difficult answer that cannot be found in a rigid or reductionist approach.
Re- burned
Re-burned
Burned
Burned Unburne
Unburned 0,0
5,0
10,0
15,0
20,0
d1 0,0
25
''' 1"RW" 5,0 10,0 15,0 LN (n+1)
LN (n+1)
•Fungi
• Actinomycetes
, 20,0
•Total mimflom
1/
Storch hydrolyzers
Figure 1. Systenzatic and fünctional groups
•
Protein hydrolyzers
• Anmonifie7s1
of edaphic nzicroflora. 161
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The burned zone had a considerable increase of the total microflora and to a lesser extent, so did the actinomycetes. On the other hand, there was a significant decrease of fungi population in the re-burned zone. The microorganism's functional groups did not reveal important differences, with the exception of the protein hydrolysers that tend to increase in tue burned zone. The analysis of enzymatic activity differed according to which enzymes were taken into consideration. The activity of the phosphates (acid, neutral and alkaline phosphomonoesterases) decreases significantly in the burned zone and specially in the re-burned one. This same behaviour can be observed in the (3-glucosidase and amylase activities, although in the case of xylanase or the invertase the opposite happens and it is in the burned zone that activity increases.
180 160 300
140
250
120
200
100 80
150
60
100
a-GlucosIdase Xyianase
53
Invertase
4;2
4Z7
Amylase
40
Acid phosphatase Neutral phosphatase
20
o t006 — ere 6
Alkaline phosphatase s)eree, e-e»
Figure 2. Enzymatic activities of C and P cycles.
Upon seeing the previous results one can well ponder on which parameters should quality soil be determined and in which situation (unburned or burned) is there a healthier soil. There are also other questions that arise such as: are these variables useful when determining the degree and effects of a disturbance? Would the same behaviour be observed in other soil types or in different climatic conditions? Which would be the evolution of the variables observed and in what state will they be in fifty or a hundred years time? These questions have difficult answer that cannot be found in a rigid or reductionist approach. When once again taking into consideration soil's complexity and the hierarchy of its structural and functional components (that vary according to the scenario in question), and confronted with the demands for solutions to the problems of degradation, the proliferation of neologisms seems to converge towards seeking a "theory of all" that allows for the explanation of all these phenomena's that take place in the edaphic environment. Some of these neologisms are based on perceptions that have a clear finalist projection (to increase or maintain production), others are subject to metaphoric realms or rescue ambiguous concepts from other disciplines. 162
Soil functions, soil quality or soil health. metaphorical or utilitarian concepts in soil sciences
But in order to increase our knowledge of the soil it is necessary to conciliate observation with theory and set boundaries to the latter in its present dimension. It is estimated that we know that less than ten percent of edaphic's biodiversity and many aspects of its functional mechanisms are still unknown to us. On the other hand, many determinations of edaphic variables only analyse the most superficial layers and usually ignore what happens at a depth beyond twenty or thirty centimetres. In the former circumstances, the search for algorhitms, equations or systems of equations with a universal projection that can describe the soil System as a whole, is subjected to numerous restrictions. It is very probable that the process of self-organisation in the edaphic environment could be conditioned by a group of variables, that are different according to their situation; the spatial and temporal ones. This is why it is fundamental to conserve referential attributes of different soil types that would allow for the comparison with situations in which human intervention has modified their initial conditions. The constitution of 'soll reserves' (See chapter of Ibáñez et al. in this book) would be the equivalent to the conservation of a fossil register, that allows for palaeontologists to recognise initial elements in the different types of evolution of live beings. In a near future soil sciences should continue their labour of observation and analysis that allows for the construction of a theory, more suitable to the complexity of the scenery and the harbouring of hierarchies, and therefore allows for generalisations and the construction of a coherent holistic model. References Andrews, S. S., Karlen, D. L. and Mitchell, J. P. (2002). A comparison of soil quality indexing methods für vegetable production systems in Northern California. Agriculture, Ecosystems and Environment 90, p. 25-45. Bello, A., Ibáñez, J. J. and Garcia Álvarez, A. (2002). El suelo en agricultura ecológica. Manejo de un ente vivo. V Congreso de la SEAE — I Congreso Iberoamericano de Agroecología. Gijón, España, September 16-21 (in press). Blum, W. E. H. (1994). Soil resilience. General approaches and definition. Transactions of the 15th World Congress of Soil Science (2a). ISSS, Acapulco, p. 233-237. Blum, W. E. H. and Aguilar Santelises, A. (1994). "A concept of sustainability and resilience based on soil functions". In: Greenland, D. J. and Szabolcs, I. (Eds.). Soil Resilience and Sustainable Land Use. CAB International, Wallingford, p. 535-542. Doran, J. W. and Parkin, T. B. (1994). "Defining and assessing soil quality". In: Doran, J. W., Coleman, D. C., Bezdicek, D. F. and Steward, B. A. (Eds.). Defining Soil Quality für a Sustainable Environment. Soil Science Society of America Special Publication n°35. SSSA and ASA, Madison, Wisconsin, p. 3-21. Doran, J. W. and Safley, M. (1997). "Defining and assessing soil health and sustainable productivity". In: Pankhurst, C. E., Doube, B. M. and Gupta, V. V. S. R. (Eds.). Biological lndicators of Soil Health. CAB International, Wallingford, p. 1-28. Dumansky, J. (1993). Strategies and opportunities für soil survey information and research. ITC Journal 1993-1, p. 36-41. 163
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