Ecological lessons and applications from one century
of low external-input farming in the
pampas of Argentina
E.F. Viglizzo
a,b,∗, F. Lértora
b, A.J. Pordomingo
a,
J.N. Bernardos
c, Z.E. Roberto
a, H. Del Valle
baINTA, Centro Regional La Pampa, 6300 Santa Rosa, La Pampa, Argentina bCONICET/INTA, Centro Regional La Pampa, 6300 Santa Rosa, La Pampa, Argentina cUniv. La Pampa, Fac. Ciencias Exactas y Naturales, 6300 Santa Rosa, La Pampa, Argentina
Received 7 July 1999; received in revised form 5 January 2000; accepted 15 March 2000
Abstract
Ecology may benefit from long term, large scale experiments on low intensity farming to test theoretical principles and convert them into practical lessons. One century of land conversion in the Argentine pampas, and its effect on critical ecological properties, were analysed and discussed. Land transformation has resulted in significant changes of land use, land cover, energy flow, nutrient dynamics, hydrology, and the trade-offs between productivity, stability and sustainability. The analytical procedure involved the complementary utilisation of different data sources and approaches. The study was focused on large geographical scales: the entire pampas and its five ecoregions. Results were interpreted under the theoretical framework of succession in ecology. The historical conversion of natural grasslands into cultivated grasslands and croplands was not homogeneous, determining a variety of land use and land cover patterns. Due to its higher productivity, much more energy, nutrients and water were mobilised in the rolling pampas than in the other ecoregions. This study provides lessons about how the energy flow, the nutrient dynamics and the hydrological process are modified by land transformation under low external-input conditions. Technical coefficients to be applied in emerging fields of environment administration such as ecological-monitoring, environmental accounting and auditing, agro-ecological certification, land evaluation and allocation, and land management, can also be supplied by this kind of studies. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Low external-input farming; 100-year farming; Ecological applications; Pampas; Argentina
1. Introduction
Ecology is an evolving collection of principles with application to practical fields, such as agriculture, forestry, fisheries, natural lands and medicine.
Ecolo-∗Corresponding author. Tel.:+54-2954-434-222;
fax:+54-2954-434-222.
E-mail address:evigliz@ssdnet.com.ar (E.F. Viglizzo).
gical principles cannot always be tested by experi-mental research because of (a) the large areas and long time periods that need experimental control (Tilman, 1990), (b) the financial and managerial limitations to designing long term treatments for large areas (Car-penter et al., 1995), and (c) the difficulty to find areas with homogenous pre-treatment history, as well as un-treated control areas for reliable comparisons (Likens, 1992). Although surveys are considered acceptable
Table 1
Comparison of farming intensity in three countries that differ in resources and external-inputs usea
Argentina USA The Netherlands
Cropland area (×1000 ha) 35750 189915 924
Average yield (kg/ha) 2257 4383 6641
Annual use of fertiliser (kg/ha) 4 93 748
Annual use of pesticides (kg/ha) 0.40 1.97 10.47
Tractors (number/100 ha) 0.58 2.46 20.58
Irrigated cropland (%) 5 10 59
Cattle (number/km2) 18.79 10.41 130.06
Pigs (number/km2) 1.46 4.97 375.54
aSource: WRI (1990) after various sources.
options to test hypothesis and theories when experi-mentation is not possible, the applicability of poorly tested principles is cause of concern among ecologists (Pienkowski and Watkinson, 1996).
Ecology may benefit from low external-input farm-ing to test principles and deliver practical lessons. The pampas of Argentina represent a large scale, long term, and non-controlled experiment in low input farming that started at the end of the 19th century and lasted until the beginning of the 1990s. Until then, lands had maintained their pristine condition with negligible human intervention. One century of land transformation (Pizarro, 1997; Solbrig, 1997) in different ecoregions (Ghersa et al., 1998) was a valu-able source of variability that has comprised different crop, cattle, and crop-cattle production activities. Low external-input farming refers to systems that use reduced quantities of inputs, such as fertiliser, pesticide, irrigation, high-yielding crops, concentrate feed, machinery, etc. (Matson et al., 1997), that in-volve fossil energy in manufacturing or operation. In comparison with more intensive schemes of USA and the Netherlands, the low input use and crop yields in Argentina before the 1990s are shown in Table 1.
This article provides an overview of one century of land transformation under low input conditions in the pampas, and its consequences on ecological structures like land-use and land-cover, and functions, such as energy flow, nutrient dynamics, hydrological process, and the trade-offs between productivity, stability and sustainability. The study focuses on large geographi-cal sgeographi-cales: the pampas region, and its corresponding five ecoregions. The key expected outcomes of this
investigation are to (a) interpret the ecological dynam-ics of the region, and (b) obtain technical coefficients for application, such as land evaluation, allocation and management.
2. Methods
2.1. Characteristics of the study area
The Argentine pampas is a wide plain with more than 52 million ha of lands suitable for cattle rearing and cropping. Similar to the Great Plains of North America, the area has a relatively short farming his-tory. Both regions remained as native grassland until the end of 19th century and the beginning of 20th century, and then, both were used for cattle and crop production under dryland conditions. In both cases, the generalised application of agronomic practices suitable for humid zones caused severe episodes of soil erosion in semiarid and subhumid areas during the first half of the 20th century (Cole et al., 1989). Examples of this ecological disruption were the ‘dust bowl’ in USA, and a similar process in the west-ern pampas (Covas, 1989; Viglizzo et al., 1991; Lal, 1994).
Fig. 1. Location of the pampas in the Argentinean territory.
well as the nitrogen contents and the granular struc-ture of soils decrease from the eastern humid to the western semiarid lands. Most lands are suitable for cultivation in the Central pampas, although suscep-tibility to wind erosion imposes some limitations to crop production. Problems of salinity, drainage and water erosion limit production in the marginal lands
on a sustained basis, are very rare in the pampas. In a large part of the region, cattle and crop produc-tion activities are combined in different proporproduc-tions according to their susceptibility to environmental constraints (Viglizzo, 1986). Cattle production ranges from steer fattening to cow-calf operations on peren-nial and annual pastures, and native grasslands (Hall et al., 1992). The rainfall regime varies in space and time, determining occasional extreme conditions of droughts and floods over wide areas (Viglizzo et al., 1997).
2.2. Data sources
Different sources of information have been utilised in this study: (1) eight general agricultural censuses of years 1881, 1914, 1937, 1947, 1960, 1969, 1973, and 1988 that comprised the totality of farms; (2) a variety of production and yield statistics published by the Secretary of Agriculture; (3) energy, nitrogen (N), phosphorus (P) and potassium (K) concentration of in-puts and outin-puts as determined by various authors and (4) published accounts of change in flora and fauna. Data on land use and crop yields were analysed for all districts. Land use was expressed in terms of the relative area (%) of crops, pastures and natural grass-lands with respect to the total area devoted to farm-ing activities. The analysis comprised dominant crops, such as wheat (Triticum aestivumL.), maize (Zea mays
L.), sorghum (Sorghum bicolorL. Moench.), linseed (Linum usitatissimum L.), soybean (Glycine max L. Merr.), sunflower (Helianthus annuusL.) and peanut (Arachis hypogaeaL.), and pastures based on annual and perennial grasses, leguminous perennial pastures, and mixed grass-leguminous pastures. Because of the lack of long term data, beef production was estimated from equations for each ecoregion (Viglizzo, 1982) that relate stocking rate (available data) to meat pro-duction per hectare. Simple linear and quadratic re-gression models were used to estimate trends in land use. Average values on land-use and statistic variabil-ity among political departments for each ecoregion are provided in Appendix A.
The complement of different approaches and infor-mation sources was used to reinforce the explanation of ecological processes (Carpenter et al., 1998; Foster et al., 1998; Fuller et al., 1999). Interpretation of results under the framework of basic
ecologi-cal principles was useful to check the coherence of analysis. The theory of ecological succession (Clements, 1916) and its later conceptual evolution (Odum, 1969; Connell and Slatyer, 1977; Clapham, 1983) seem to be particularly suitable to this purpose.
2.3. Land use/land cover
Land use involves both the purpose for which the land is used, and the manner in which the biophysical attributes of the land are manipulated, affecting the structure and function of ecosystems. On the other hand, land cover is the biophysical state of the land surface, which determines the basic structure and functional characteristics of ecosystems (IGBP/HDP, 1995). Different patterns of land use in time and space were estimated by means of the relative oc-cupation of lands by natural grasslands, crops, and introduced pastures in the five-study ecoregions along the century. A land-cover factor was an estimation of the relative (%) seasonal coverage of land with resources (natural grasslands, introduced pastures, crops) during one century of farming in the five-study ecoregions. The difference between 100 and the cover factor value allows the identification of periods of bare soil (Viglizzo et al., unpublished). Maps show-ing changes of land use in different historical periods were elaborated with the use of kriging analysis (Burgess and Webster, 1980a,b), which is a method to estimate areas through an interpolation of geo-graphically referenced points that were not physically sampled.
2.4. Energy model
An energy model proposed by Odum (1975) based on basic input–output relationships, was utilised for the energy analysis. Different sources (Grossi-Gallegos et al., 1985; Reed et al., 1986; Stout, 1991; Conforti and Giampietro, 1997) were used to estimate the energy values of inputs and outputs. The following energy values expressed in MJ ha−1 per
year were used for inputs: (a) incoming solar energy ranged between 55.12 and 59.13×106from cloudy to
transportation, and output transportation, respectively, and (c) energy consumption was 418, 20.3, and 277 as pesticides, seeds, and tractors and machinery, re-spectively. Energy values of outputs (in MJ kg−1)
were estimated at 25.53 for sunflower, soybean and peanut grains, 16.33 for wheat, maize and sorghum, and 13.36 for bovine meat.
An aggregated analysis of inputs, outputs, and output–input relations for solar and fossil energy was computed for different points in time. Fossil energy consumption in typical farming operations and machinery was estimated for the 1940s and the 1980s, and for the five-study ecoregions. The amount of operations was considerably higher during the 1980s. Estimations have been made for perennial pastures, summer and winter grain pastures, sum-mer and winter grain crops, and oil–seed crops. The analysis involved estimations of oil consumption for tillage, agrochemical applications, and input and out-put transportation, as well as the energy utilised to manufacture herbicides, insecticides and genetically improved seeds. Based on literature data (Ehrlich et al., 1977), it was assumed that fossil energy consumption during the 1880s was approximately equivalent to 1% of the estimations made for the 1940s.
2.5. Nutrient balance
Using a simple mathematical model, the balance of N, P and K was estimated by difference between the main sources of gain and loss (Lértora et al., 1998). The respective nutrient content (in g kg−1 of
prod-uct) of wheat, maize, sorghum, linseed, soybean, sun-flower, peanut and meat were 22.9, 16.3, 20.0, 40.8, 58.1, 40.8, 51.2 and 27.0, respectively, in terms of N; 4.3, 3.5, 3.4, 8.0, 6.8, 7.6, 6.1 and 43.1, respec-tively, in terms of P; and 4.9, 3.7, 4.0, 9.8, 11.3, 11.6, 11.3 and 5.9, respectively, for K (Lloyd et al., 1978; NRC, 1978). In the case of N, extraction from soil by crops and cattle production was subtracted from literature-based N input estimation to the soil by legumes in different ecoregions. The following is-sues have been taken into account: (a) the area de-voted to crop production; (b) the area dede-voted to legu-minous perennial pastures; (c) the N lost by outputs (grain and beef) that was closely related to yield and N density in products and (d) the N fixed by
legu-minous, and negligible amounts added by occasional fertilisation.
3. Results and discussion
The pristine pampas can be considered a vast natu-ral grassland ecosystem that have reached a condition of dynamic equilibrium with the surrounding environ-ment along succession processes. Following accepted principles in ecology (Margalef, 1968; Odum, 1969), a condition of dynamic equilibrium in the pampas could be characterised as follows: (a) large accumulation of biomass (mainly from herbaceous species) and nutri-ents (in biomass and soil) due to the predominance of closed natural cycles; (b) high diversity of functional groups comprising different plant and animal species; (c) maintenance of complex trophic networks; (d) con-tinuity of biological processes through out the year; (e) low rate of energy flow and low biomass productivity due to the accumulation of fibrous inert material; (f) high stability of functional groups and (g) high sus-tainability and self-regeneration capacity of the whole ecosystem. All these features have been periodically subjected to normal fluctuations and both, forward and backward movements, in response to environmental variability.
3.1. Structural changes
3.1.1. Land use
Significant shifts in the ecoregional pattern of land use have taken place during the analysed century. Elaborated on national census data, sequential maps in Fig. 2 show the evolution of croplands at the be-ginning (1880s), the middle (1940s), and the end (1980s) of the period. After finishing the so-called conquest of desert in 1879, most of the area remained for decades as a wide natural grassland with little human intervention. Approximately one-half of the region showed <10% of the land cultivated with
Fig. 2. One century of conversion of grazing lands into croplands in the Argentinean pampas.
relatively few years. Thus, no areas completely free of annual crops were recorded during the 1930s, with a crop occupation of lands that ranged between 20 and 60% (Fig. 2b), even in the marginal and fragile western lands. Although an extensive flooding had produced major alterations on land use (especially in the flooding pampas) at the end of the 1980s, the area of annual crops ranged between 40% to more than 60%, being quite evident in the most fertile lands of the rolling, the central and the southern pampas (Fig. 2c).
The conversion of natural grasslands into cultivated grasslands and croplands was not homogeneous in all ecoregions (Fig. 3). Conversion happened very early in the rolling pampas, given that more than 60% of natural lands had been transformed in the 1910s. Only 10% of the land has no agricultural use nowadays. On the other extreme, the flooding pampas has experi-enced the lowest conversion rate. On average, 60% of land remained as modified natural grassland at the end of the 1980s. The other ecoregions showed different degrees of land transformation. With the only excep-tion of the rolling pampas, where the cropland raised steeply between the 1960s and the 1990s, the others had maintained a rather stable cropping area after a wave of rapid increase during the 1920s. Again, with the exception of the rolling pampas, the rest of the zones showed a persistent increase of cultivated
pas-tures, contradicting the belief that croplands had ex-panded all over the pampas since the 1950s, displacing cattle production to the semiarid, marginal lands of the western pampas. Neither crop has expanded all over the region, nor livestock has been removed from better lands.
3.1.2. Land cover
Land use and land cover are interrelated. Because of land transformation, natural habitats have been deeply fragmented with unknown consequences on biodiver-sity. The land cover pattern, which refers to physical attributes of the land surface, was modified in a few decades, especially in the rolling, the central, and the southern pampas, where annual periods of biological recession by the shorter life cycle of crops have in-creased during the century (Fig. 4). This was partic-ularly evident when the wheat–soybean rotation was introduced into the rolling pampas. Land cover and bi-ological disruption have been much less severe in the flooding and the Mesopotamian pampas, where a di-versity of natural and perennial species has persisted until now.
Fig. 3. Land use transformation in five ecoregions of the pampas plain along one century.
only 57 and 77.3% of the species identified by Parodi (1930) in two different locations of the rolling pampas can be found nowadays. But at the same time, approx-imately 10% new species that were not described by Parodi in 1930 have appeared. An important percent-age of the remaining natural species are considered and treated as weeds in the main crops (Ghersa et al., 1998).
From a successional perspective, the anthropogenic disturbance that began at the end of the 19th century pushed the pampas away from their climax condi-tion. Over one century of farming intervention, land-scapes were altered by interacting human-induced and natural disturbance forces that led to mosaics and patches of different successional ages. They rep-resent, in variable degree, a backward movement
of succession to younger seral stages (Krebs, 1972) with major alteration of structure and function. Such backward movement that was induced by man aim-ing at utilitarian objectives has represented, in prac-tice, a simplification of structures and functions that resemble the younger and more productive succes-sional stages of centuries ago. Nowadays, different degrees of human-induced successional regression can be observed all over the region. Extreme cases of over-rejuvenation can be found in the highly sim-plified crop rotation schemes (wheat–soybean) of the rolling pampas, where the energy flow and the pro-ductivity are enhanced, the nutrient and water cycles are disturbed, the long-term accumulation of inert material is inhibited, and the lifetime of the principal biological activities (annual crops, in this case) is short and discontinuous. Since stability and sustain-ability are lost by the disruption of natural processes, a human subsidy is needed to keep the ecosystem viable.
3.2. Change of functional properties
3.2.1. The energy flow and the nutrient dynamics
Two energy sources have been considered in this study: solar and fossil energy. Both types of energy flow through the agro-ecosystem before converting into final products. Most of the incoming energy has been highly degraded into heat and lost after pass-ing through plant and animal metabolic processes. As a consequence, the net amount of energy con-centrated as final product (output) represents only a negligible proportion of the total incoming energy (input).
Fig. 4. Evolution of land cover along the century in five ecoregions of the pampas plain.
but also in their potential response to agricultural pressure.
In agreement with accepted ecological principles, as productivity increased in time, the efficiency of solar energy use increased also, because more product was
Table 2
Analysis of energy inputs, outputs and efficiency through incoming solar radiation and fossil energy (MJ ha−1 per year) consumption
during the period 1880–1990 in the five-study ecoregions of the pampas plaina
Ecoregions Period Incoming solar radiation (×106)
Fossil energy Energy output Output–input relationship (%)
Incoming solar radiation (×10−6)
Fossil energy
Rolling 1880s 57.7 24.5 881.8 15.3 35.99
1940s 57.7 2456.2 27614.1 479.0 11.24
1980s 57.7 6110.8 55548.0 663.0 9.09
Central 1880s 59.1 15.0 534.4 9.0 35.63
1940s 59.1 1522.1 9224.7 156.0 6.06
1980s 59.1 3190.5 16801.0 284.0 5.27
Southern 1880s 55.1 16.4 721.4 13.1 43.99
1940s 55.1 1644.3 5883.0 107.0 3.58
1980s 55.1 3027.0 12027.3 218.0 3.97
Mesopotamian 1880s 57.7 11.0 534.4 9.3 48.58
1940s 57.7 1100.4 6992.5 121.0 6.35
1980s 57.7 1576.5 4919.2 85.3 3.12
Flooding 1880s 55.1 5.1 561.1 10.2 110.02
1940s 55.1 512.5 3740.1 67.9 7.30
1980s 55.1 1141.0 4895.7 88.9 4.29
aReferences: it was assumed that the consumption of fossil energy during the 1880s was equivalent to 1% of total consumption during
the 1940s.
energy efficiency was higher in zones with the greater agricultural potential. It should be noted that, in terms of fossil energy, low input systems in the pampas are more efficient than the intensive farming (Viglizzo, 1984).
Large amounts of N, P and K are required, among other nutrients, by agricultural production. Consider-ing the increasConsider-ing productivity of ecoregions durConsider-ing the century, it is obvious that nutrient cycles have be-come more and more open. However, given that the outflow was higher than the inflow of nutrients under low input conditions, negative balances were unavoid-able in the pampas (Tunavoid-able 3).
According to ecological theory, since nutrient dy-namics an energy flow are interdependent processes, nutrients in ecosystems are mobilised in response to the energy impulse (Odum, 1975). Thus, the larger the energy flow, the greater the recycling and removal of nutrients. In practical terms, higher productivity of agriculture means both, higher rates of flowing energy, and larger amounts of nutrients taken up and carried away from the system. Fossil energy reinforces
nutri-ent dynamics. Spatial and temporal data of Tables 2 and 3 were incorporated into a correlation analysis to compare fossil energy consumption with extraction of N, P, and K. Highly significant (P<0.01) correlation
coefficients were obtained. Coefficients and best fit-ting models were 0.96 (linear), 0.79 (quadratic), and 0.98 (linear) for N, P, and K, respectively, thus, con-firming a parallel performance between energy and nutrient dynamics.
3.2.2. The hydrology
Table 3
Estimation of input, output and balance of nitrogen (N), phosphorus (P) and potassium (K) during the period 1880–1990 in the study ecoregions of the pampas plaina
Ecoregions Period Nutrient (kg ha−1 per year)
Input Output Balance
N P K N P K N P K
Rolling 1880s 2.59 – – 1.69 3.6 0.6 0.9 −3.6 −0.6
1940s 24.40 – – 36.35 7.30 6.40 −11.95 −7.30 −6.40
1980s 75.30 – – 81.39 13.50 19.50 −6.09 −13.50 −19.50
Central 1880s 1.32 – – 1.02 4.20 0.60 0.30 −4.20 −0.60
1940s 39.23 – – 12.43 4.70 2.00 26.80 −4.70 −2.00
1980s 23.21 – – 25.38 10.90 6.70 −2.17 −10.90 −6.70
Southern 1880s 2.08 – – 1.38 3.50 0.50 0.70 −3.50 −0.50
1940s 14.20 – – 9.26 4.10 2.90 4.94 −4.10 −2.90
1980s 18.47 3.1 – 20.74 8.70 5.10 −2.27 −5.60 −5.10
Mesopotamian 1880s 3.02 – – 1.02 1.60 0.20 2.00 −1.60 −0.20
1940s 9.14 – – 8.80 3.00 1.80 0.34 −3.00 −1.80
1980s 4.93 – – 7.49 6.20 2.50 −2.56 −6.20 −2.50
Flooding 1880s 2.88 – – 1.08 2.00 0.30 1.80 −2.00 −0.30
1940s 1.16 – – 5.49 3.60 1.70 −4.33 −3.60 −1.70
1980s 11.2 – – 7.28 5.40 1.40 3.92 −5.40 −1.40
a(−) Means that there was no input by fertilisation.
transformation, a positive correlation between en-ergy flow, nutrient dynamics and water process can be expected. However, cause-effect relations arise a key question that still remains unanswered: does land use influence the hydrological process, or inversely, does the hydrological process drive changes in land use?
Various authors have studied the hydrological pro-cess in the pampas since the beginning of agriculture (Hoffmann, 1988; Forte Lay and Falasca, 1991; Car-ballo and Hartmann, 1996; López Gay et al., 1996; Viglizzo et al., 1997). Although there is variability between ecoregions, regression analysis has shown a decline of precipitation all over the pampas un-til the 1950s, and then an inversion of trends dur-ing the second-half of the century. Durdur-ing the study century, periods of improved hydrological conditions seem to have favoured the conversion of grazing lands into croplands, and vice versa. Viglizzo et al. (1997) have found significant (P<0.05) and highly
signifi-cant (P<0.01) correlations between rainfall
variabil-ity and percentage of cropland for the humid and
subhumid districts of the pampas, respectively, and a non-significant one (P>0.05) for the semiarid dis-tricts. They considered that changes in the rainfall regime have principally explained the variability of land use in the better areas, but the lower quality and water-retention capacity of soils could explain the loss of correlation in the western, semiarid districts. The interaction between rainfall and technology was the main factor explaining land use change in these districts.
the near-surface atmosphere has happened over much of the USA territory in spring and summer. According to this author, the climate shift could be explained by a reduction of (a) the surface roughness, (b) the leaf and stem area index, (c) the stomatal resistance and (d) the increasing of the surface albedo. Bonan (1997) has summarised this view by saying that ‘rain follows the plow’. What was cause, and what was effect in the relationship between land use and hydrology in the pampas? Is there perhaps a mutual reinforcing action?
The analysis of altered functions is a valuable way to learn from ecosystems study. In agreement with ecological theory, the results of this study confirm that the basic functions of energy flow, nutrient dynamics and the hydrological process should not be considered in isolation because they are strongly interrelated. Pio-neer studies by Lotka during the 1920s (Krebs, 1972) have demonstrated that resources in natural systems are mobilised in proportion to the flow of energy. Al-though the hydrological process is mobilised by the global dynamics of energy and gravity, nutrients as well as water are ‘pumped’ by the local flow of energy. When energy flow is enhanced by energy subsidy (pesticide, tillage, irrigation, etc.) or land transfor-mation (conversion of native grasslands into pas-tures or crops), nutrients and water are mobilised at higher rates, determining open cycles that lead to less conservative ecosystems. A noticeable eco-logical regression due to land conversion and use of machinery, pesticides and high yielding varieties without fertiliser application, took place in the pam-pas during one century of farming. More crops and higher yields have increased the extraction of nutri-ents, determining negative mineral balances in areas well-suited to crop production. The models utilised in this study would confirm this, demonstrating that actual farming systems are not sustainable in the long term. The lack of fertilisation has avoided the problem of contamination, but at the cost of nutrient depletion.
Conditions vary in different ecoregions. In the rolling pampas, where annual crops have been the greatest component of land use, the flow of energy and the loss of nutrients are several times greater than in the mixed systems of the central and south-ern pampas, and in the Mesopotamian and flooding pampas, where cattle raising is the principal
activ-ity. In ecological terms, livestock systems resemble the conditions of mature, close-to-climax natural ecosystems, more than the mixed and the cropping systems. Various authors (Odum, 1969; Vitousek and Reiners, 1975; Woodmansee, 1978; Margalef, 1980) have argued that as long as the ecosystem progresses toward mature stages, they become more conservative in terms of energy transfer and nutrient mobilisation. The results in the present study would confirmed this.
3.2.3. Productivity, stability, sustainability
Productivity, stability and sustainability are, at the same time, relevant emergent properties of agro-ecosystems, and an expression of performance in ecology and agriculture. Generally speaking, pro-ductivity is, in natural ecosystems, the biological process that determines a change of weight of plants and animals over a year (Clapham, 1983), while stability is the variation in number of plant and an-imal communities around a dynamic equilibrium point after a normal disturbance (Margalef, 1968). According to Chapin et al. (1998), sustainability is the maintenance of characteristic diversity of major functional groups, productivity, soil fertility, and rates of bio-geo-chemical cycling over disturbance events. Conway (1987) adapted these concepts to agricultural ecosystems. Conway (1987) defined productivity not as a process, but the output of valued product (edible or saleable products, energy, money, etc.) per unit of resource input (e.g., hectare of land) on an annual basis, stability as the small inter-annual oscillation of productivity in response to small, normal variations of the surrounding environment, and sustainability as the lasting, long term maintenance of productiv-ity when subject to major disturbing forces that are infrequent and unpredictable.
(meat and milk), and vice versa. However, this in-verse relationship is not so evident in areas where the environmental conditions (rainfall and soil quality) improve; e.g., towards the eastern pampas (Viglizzo, 1986). Productivity and sustainability were also in-versely related in the long term (Viglizzo et al., 1995). Nitrogen in soil was the factor selected to demonstrate this, which was related to the so-called storage function. Its size in agro-ecosystems depends on the capacity of leguminous pastures to incorpo-rate atmospheric N. Other attributes of soils could have been selected for quantifying the storage func-tion, such as organic matter and structural stability of soils. According to this scheme, the larger the size of the storage function, the greater the long term sustainability of a system (Viglizzo and Roberto, 1998).
The historical conversion of grazing areas into croplands in the pampas, has provided a strong em-pirical evidence of trade-offs between productivity, stability and sustainability under real farming condi-tions. Productivity has increased all over the region at the expense of stability and sustainability, and this effect was particularly noticeable in the semi-arid western lands where climate conditions are more variable (Viglizzo, 1986; Viglizzo et al., 1991). This behaviour was consistent with principles of eco-logical succession: the successive transformation of natural grasslands into pasturelands and croplands represents for ecologists (Odum, 1969) a backward movement to younger, more productive, less stable and less sustainable stages in the ecological succes-sion. Man displaced the system away from mature, less productive, stable and self-sustainable stages in dynamic equilibrium, towards younger and more pro-ductive ones that can render a higher and short-term economic income. Although man can be considered itself an ecological factor, its impact should be anal-ysed in isolation because of human-based utilitarian purposes.
4. Lessons and applications
4.1. Lessons
History has demonstrated that two sequential phases normally explain the increase of food and
fibre production in most regions of the world in re-sponse to growing demands: the first characterised by an increasing allocation of land to agricultural activi-ties, and the second by the intensification of farming on existing lands. The two phases were observed in Europe, North America and Asia (Rabbinge and van Latesteijn, 1992). Nowadays, intensive land use is being reconsidered because of nutrient contamination of soil and water (Carpenter et al., 1998), and recla-mation of land for other purposes, such as landscape conservation, biodiversity protection, recreation, etc. (Van Latesteijn, 1993). Until the beginning of the 1990s, the pampas of Argentina were still undergoing the first phase. The land use/land cover pattern, as well as the energy flow and the nutrient dynamics of this period, appear to be promising attributes to learn ecology from. Useful lessons can be summarised as follows:
1. Although there is a lack of direct evidence, changes in land use/land cover provide indirect means to evaluate the historical fragmentation of habitats, and infer its consequences on bio-diversity. Well-documented evidence (Freemark, 1995; Milne, 1996; Tinker, 1997) has shown the effect of agriculture on habitat fragmentation and biodiversity. Land use and land cover factors in this study provide indirect indicators to iden-tify, quantify and locate temporal and geographic fragmentation.
different patterns of productivity and stability, were in practice an adaptive response of farmers to cyclical environmental conditions (Viglizzo et al., 1997).
3. The case of nutrient dynamics in the pampas is especially interesting. Given that chemical fertili-sation was rarely used due to unfavourable price conditions, contamination by nutrient overloading was negligible. Nutrient depletion of soils was, on the other hand, the principal and more extensive problem. Since nutrient imbalances have accumu-lated over time, fertilisation would had been neces-sary to maintain the system sustainable in the long term. But this did not happen. Negative balances of N, P and K in Table 3 illustrate the magnitude of the problem in different ecoregions and periods. In spite of technology adoption, yields of some crops have reached a plateau towards the 1980s (Viglizzo and Roberto, 1994), raising a concern about the long-term sustainability of agriculture. Investiga-tions have demonstrated that trade-offs between productivity and sustainability measured through nutrient balance, have been evident in the pampas. The nutrient balance offers a powerful indicator to assess sustainability under low input conditions, and even more, because of the high correlation be-tween the input of fossil energy and the output of N, P, and K, the energy analysis also provides a way to elucidate and predict the fate of these criti-cal nutrients.
4.2. Applications
Agro-ecological indicators derived from land use/ land cover, energy flow and nutrient dynamics anal-ysis have a potential value for applying the lessons from ecology. They can contribute to emerging fields of natural resource administration, such as environ-mental monitoring, environenviron-mental accounting, eco-logical certification, land evaluation and allocation, and land administration. Some applications are the following:
1. The historical evaluation of changes in land use, land cover, energy flow and nutrient balance can serve to implement a long-term, indicator-based monitoring system for the pampas. Current condi-tions and trends of critical ecological attributes can be analysed by displaying the monitored variables
across different temporal and geographic scales. A well-founded monitoring system is key to project the agricultural change, and predict its environmen-tal impact. Environmenenvironmen-tal impact evaluation will as well-benefit from such tools.
2. Environmental accounting and auditing represent a forward step that derive from monitoring. Indi-cators from monitored ecosystems can be utilised to estimate changes in the natural resource endow-ment. For example, nutrient gains and losses in the pampas, and their corresponding economic valua-tion, can be estimated from annual balances of N, P and K. Given that they are not converted into money, neither public nor private accounts show the current environmental cost of agriculture. Such es-timation could be done by converting the change of the nutrient endowment into fertiliser-price equiv-alents.
3. Agro-ecological certification is a modern tool for assessing the quality of products and processes in agriculture, that have an increasing commercial value. Trade barriers as well as commercial ad-vantages will be associated with the environmental implication of farming processes. Technical coef-ficients for eco-certification can be obtained from factors that quantify land use/land cover, energy flow and nutrient dynamics of selected areas. Eco-logical auditors can benefit from these coefficients for improving the certification work.
mathemat-ical models that utilise such coefficients, look-ing for balanclook-ing agricultural and environmental goals.
5. Finally, land administrators can also benefit from the presented analytical procedure and results to prevent undesirable environmental outcomes of non-proper land management. The right interpre-tation of historical processes in the pampas can be helpful for identifying environmental risk thresh-olds in different ecoregions, making the future more predictable. For example, cropland expan-sion with inappropriate soil technology during the 1920s and 1930s in the fragile lands of the central pampas, caused a severe ecological collapse dur-ing the 1940s. It is unlikely that such an episode can be repeated because soil management has im-proved, but the current replication of past farming scenarios may call our attention to the appearance of threshold conditions in future.
5. Conclusions
The historical analysis of agro-ecological data from the Argentine pampas suggests that a widespread, human-induced disturbance has led to major shifts in the structure and function of natural ecosys-tems. Lessons from investigations about the eco-logical consequence of changes in land-use and external-input use, can be fruitfully applied to sup-port sensible decisions from policy-makers and managers.
In spite of its extractive condition, during many decades farmers in the pampas used to manage energy and matter not pushing them too far beyond natural flows and cycles, and thus, agricultural productivity was acceptably sustained, and even increased, after one century of low external-input farming. However, because maintenance of such natural flows and cycles is no longer an option under current socio-economic demands, decision-makers need approaches to explore new agro-ecological options and trajectories that can be sustainable on the basis of existing local constraints. Considering that farmers need to maintain or increase their annual income, and Argentina needs to benefit from exports and external exchange, cropland expan-sion and dependence on external inputs will proba-bly increase in the near future, resulting in both a
greater functional alteration and a larger environmen-tal load on agro-ecosystems. The development of a knowledge-based productivity increase that must also be ecologically sound represents a challenge for the next decades.
Agriculture in the pampas has undergone a pro-gressive change in land use from natural condition to native grassland grazing, to introduced pasture grazing, and to rotational-based and even perma-nent crops. Lessons from the past suggest that the cumulative impact of more than one century of cropping under low external-input conditions affect strongly the structure and nutrient endowment of soils, specially in the more productive lands. Given trade-offs between productivity and sustainability, it seems to be critical to investigate how produc-tivity and sustainability can be reconciled in future intensive farming schemes, to minimise the negative impact of environmental contamination and habitat alteration.
The research approach and methods presented here cannot be generalised to assess agro-ecological (un)sustainability under a variety of conditions. However, the results seem to demonstrate that the analysis of land-use and land-cover, energy flow, nutrient dynamics and the hydrological process at larger scales, can provide a realistic theoretical ba-sis for obtaining technical coefficients that, in turn, might find application in (a) agro-ecological account-ing, auditaccount-ing, and certification, (b) land evaluation, allocation and management and (c) intensification alternatives.
Acknowledgements
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