Journal of Environmental Quality 32:472-479 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Ecosystem Restoration
Biowaste Effects on Soil and Native Plants in a Semiarid Ecosystem
F. Martíneza,
G. Cuevasa,
R. Calvob and
I. Walter*,a
a Dep. of the Environment, Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), Apartado de correos 8111, Madrid 28080 Spain
b Biometry Service, Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), Apartado de correos 8111, Madrid 28080 Spain
* Corresponding author (walter{at}inia.es)
Received for publication May 21, 2002.
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ABSTRACT
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Many soils of the Mediterranean region with a semiarid climate are subjected to progressive degradation as a result of water erosion. Biosolids and municipal solid wastes (MSW) were surface-applied once at three rates (40, 80, and 120 Mg ha-1) to different plots in a degraded semiarid ecosystem. The study was conducted to determine the effects of such applications on soil chemical properties and native vegetation over a three-year period. Soil N, P, and K initially increased with increasing biowaste application rates, but then decreased over time. Levels of Zn and Cu were higher in MSW than biosolid-treated plots, and increased in both years after application. Concentrations of soil Cd, Pb, Ni, and Cr did not change as a result of biowaste amendment in the study period. The growth of native plants was enhanced by the addition of biowastes. Total plant canopy and plant biomass increased significantly and remained higher in all treatments than in the control plot over the three-year period. The species richness of native plants decreased with increasing biowaste rates. Differences in the development of native plant communities between treatments were observed, and were more remarkable three years after biowaste application. Tissue N, P, K, Zn, and Cu levels increased with the biowaste application rate, but concentrations of tissue Pb, Cd, Ni, and Cr did not increase significantly. Biowastes applied at the rate of 80 Mg ha-1 gave rise to the most favorable soil and native vegetation results while avoiding environmental risks.
Abbreviations: MSW, municipal solid wastes
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INTRODUCTION
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THE AGRONOMIC USE OF ORGANIC WASTES such as biosolids and municipal solid waste (MSW) has increased because of the need to lower disposal costs, to recycle nutrient elements in the soil–crop system, and to offset the decreasing organic matter content of soils. In contrast to inorganic fertilizers, urban organic wastes have high organic matter content and can be used as a source of slow-release N and P. Recently, urban wastes have been used in the restoration of native rangelands (White et al., 1997; Pierce et al., 1998; Rostagno and Sosebee, 2001). When applied to agricultural land, these organic amendments are generally incorporated into the soil, whereas in rangeland restoration they are surface-applied only once to avoid disturbing soil and vegetation. The extensive use of organic wastes on disturbed lands has been slow to develop, mainly because of the cost of transport from the source of production, yet semiarid degraded soils offer ideal sites where these type of waste can be beneficially recycled (Sabey et al., 1990).
Most soils in areas with Mediterranean climate are undergoing degradation mainly due to water erosion. The use of biosolids and MSW for rehabilitating degraded ecosystems is an alternative to their mere disposal. In arid and semiarid ecosystems, biosolids can improve the low fertility of soils. High organic content can improve soil physical, chemical, and biological properties, and consequently speed plant establishment. This has a feedback effect, leading to the addition of more organic carbon from plant residues and the development of root systems in the soil (García et al., 1992; Giusquiani et al., 1995; Nortcliff, 1998), which may contribute to minimizing runoff and thereby mitigating water erosion (Harris-Pierce et al., 1995; Moffet, 1997).
In semiarid ecosystems, the probability of nutrients or toxic elements leaching to ground water is low. Semiarid Mediterranean soils are generally calcareous and have a high pH, which favors the immobilization of most heavy metals. In addition, plant growth could be stimulated following the application of biosolids to such soils because of increased availability of essential micronutrients (O'Connor et al., 1983).
The results of Aguilar et al. (1994), who studied a semiarid grassland in the southwestern USA to see how different quantities of biosolids affected plant response and soil chemistry, showed that a once-only surface application of biosolids at 22.5 to 45 Mg ha-1 significantly increased plant production and ground cover without causing undesirable levels of potentially hazardous elements (including heavy metals) in either soils or plant tissues.
The aim of this study was to determine the effects of different application rates of biosolids and municipal solid wastes on a semiarid native grassland over a three-year period. The objectives were to (i) determine the effect of waste application on soil chemical properties and heavy metal concentrations; (ii) evaluate plant canopy cover and production, as well as the total nutrient and heavy metal concentration of native plant species; and (iii) evaluate the effect of waste treatments on the development of the native plant community over three years.
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MATERIALS AND METHODS
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Site Description and Experimental Procedures
In April 1997, anaerobically digested biosolids and the composted organic fraction of a municipal solid waste coming from home garbage (MSW) were surface-applied to a degraded semiarid soil sited 35 km southeast of the city of Madrid, Spain. The study design was a completely randomized block in which four sub-blocks were horizontal to the slope (range: 8–12%) of the terrain. Each sub-block contained seven 3- x 20-m plots (with a 1-m buffer zone between plots and 3 m between sub-blocks) assigned at random within the sub-block to one of the following treatments: 0, 40, 80, and 120 Mg ha-1 of biosolids and 40, 80, and 120 Mg ha-1 of MSW (dry weight) applied once in 1997. The climate of the study area is Mediterranean, belonging to semiarid type with an annual mean precipitation of 350 mm. The mean annual temperature is 16.8°C. The soil is described as Lithic Xerorthents according to the USDA Soil Conservation Service (1975). The average compositions of the organic urban wastes applied, as well as numerous chemical and physical properties of the soil before treatment, are shown in Table 1. The plant canopy cover was scarce (<40%) and was mainly composed of an herbaceous mixture of plants and slowly growing, low shrubs. Degradation processes included loss of grass cover and an increase in bare ground.
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Table 1. Average chemical composition of biosolids and municipal solid wastes (MSW) (dry-matter basis) and pretreatment soil characteristics.
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Soil Sampling and Analysis
Twelve composite soil samples were taken from each plot at depths of 0 to 15 cm with a 5-cm-diameter bucket auger in March 1997 (before treatment), and then again every March from 1998 to 2000. Wastes that did not incorporate to the soil were previously separated. The samples were air-dried, and then ground to pass a 2-mm sieve. All 28 soil samples from each of the four sampling dates were analyzed for chemical properties with standard soil test laboratory procedures of the Ministry of Agriculture, Fisheries and Food (1994). The pH (1:2.5 soil and water) was determined by a glass electrode. Electrical conductivity was measured by a conductivity cell in a soil to water ratio of 1:5. Total N was determined on a Kjeldahl digest. Available P was determined by Olsen's procedure following extraction with 0.5 M sodium bicarbonate. Available K+ was extracted with 1 M ammonium acetate solution (pH 7) in a 1:20 soil to extracting solution ratio and measured by inductively coupled plasma emission spectroscopy (ICP). Organic matter was analyzed by the Walkley–Black procedure. The concentrations of soil heavy metals were determined in a DTPA (diethylentriaminepentaacetic acid) extract (Lindsay and Norvell, 1978) with ICP. The methods used for the chemical analysis of the biowastes have been previously described by Walter et al. (1989).
Plant Sampling and Analysis
The vegetation was sampled in May of 1998, 1999, and 2000 to obtain the percentage total canopy cover, species richness, and total aboveground plant biomass production. Chemical composition of the vegetation was also determined. Total plant canopy cover and species richness were estimated in 16 individual microplots of the different treatment plots (0.22 m2) along a random permanent transect. About 40 plant species were identified during the study period (Table 2).
Aboveground plant biomass was also randomly measured at the same time by harvesting (hand cutting) the total vegetation of six 0.22-m2 quadrats. Plant tissue samples were washed in distilled water, oven-dried at 60°C for 48 h, weighed, and then ground to 0.1 mm before chemical analysis. They were then analyzed for total nutrient and heavy metal concentrations by digestion with concentrated nitric acid followed by ICP. Total N was determined by the Kjeldahl method (Ministry of Agriculture, Fisheries and Food, 1994).
Statistical Analysis
Analysis of variance (ANOVA) was used to study the results of soil chemical analyses. Tukey's multiple range test at P 
0.05 was used for the comparison of means. The same tests were also used to analyze aboveground biomass production, plant canopy cover, and plant tissue nutrient and heavy metals results. When data were not normally distributed, the Box and Cox (1964) diagnostic procedure was used to select the most appropriate transformation. Univariate (ANOVA, Kolmogorov–Smirnov test of goodness of fit) and multivariate analyses (stepwise discriminant analysis, canonical discriminant analysis, M. De Box test) were used to analyze the richness of species in the native plant communities. All these analyses were undertaken with BMDP7M and CANDIS programs from BMDP Release 7 (BMDP Statistical Software, 1992).
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RESULTS AND DISCUSSION
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Soil Chemical Properties and Heavy Metal Concentration
Generally, the levels of plant nutrients were higher in the biowaste-treated plots than in the control (Tables 3 and 4). Organic C levels did not significantly change with biosolid amendment over the three years (Table 3). Soils often respond to nitrogen additions with further increases in the mineralization of indigenous soil organic matter (Woods et al., 1987). In the short term, this results in a decrease in soil organic C. It takes three to four years to observe any positive effects of biosolid applications on soil organic C (White et al., 1997). Organic C levels begin to change in the second year after MSW amendment. Significantly higher organic C levels were found only with the highest rate in 1998 and with all MSW rates in the third year after application (Table 4).
Application of the biosolids at the intermediate and high rates caused the electrical conductivity (EC) to change significantly in the second year. After the first year, significant differences were also found for the intermediate and high MSW applications. However, soil EC levels with both types of treatments showed that soluble salts were not sufficiently concentrated to inhibit plant growth (Brofas et al., 2000). The EC in the MSW treatments was slightly higher than in the same-rate biosolid treatments. Soil pH did not change significantly as a result of biowaste application (Tables 3 and 4).
After the third year, the available P remained significantly higher in the waste-amended soils than in the unamended soil (Table 3), but not as high as in the first year, due mainly to P fixation by CaCO3. After the first year, available P concentrations in the intermediate and high biosolid treatments were about 10-fold more than the control value (Table 3). White et al. (1997) and Brofas et al. (2000) also reported a remarkable increase in soil-available P after the application of biosolids. The rise in soil-available P in MSW treatments was less than in the biosolid treatments. In the first year, no significant differences in NaCO3H-extractable P were noted among the different application rates, but significant differences between these and the unamended soil were found in the second and third years (Table 4).
Soil-available K was higher in the MSW than in the biosolid treatments. The values decreased in the last year in both types of treatments. Soil-available K contents were low and did not increase significantly with biosolid treatments compared with the control (Table 3). Soil-available K increased with the MSW treatments (Table 4) over the three years of the study, with significant differences found among treatments. These results agree with the K content of the biowastes applied.
Total soil nitrogen did not change significantly as a result of biosolid application in any of the three years of the experiment (Table 3), while in the plots that received the highest rate of MSW, soil total N began to increase significantly in the third year (Table 4). The two forms of inorganic N, which comprise the available N pool at the time of collection, initially increased with biowaste application. Inorganic N concentration in the biosolid treatments (Table 3) showed the greatest increase in the first two years after the application of the intermediate and high rates. In contrast, in the MSW treatments, a significant increase in inorganic N was found in the first year after application with the intermediate and high rates, but no differences were observed in the second and third years (Table 4). In both biosolid and MSW treatments, NO3–N concentrations were higher than NH4–N concentrations. Soil NO3–N levels were relatively high. Therefore, biosolid application rates of more than 80 Mg ha-1 could pose a potential surface water contamination hazard in times of significant runoff.
Concentrations of DTPA-extractable Cd and Cr were below the method's detection limits (<0.10 mg Cd kg-1 and <0.08 mg Cr kg-1) in all treatments. Concentrations of DTPA-extractable Ni did not change significantly as a result of biowaste application (Tables 5 and 6). This is important since heavy metals limit biowaste application. Similar results were obtained by Fresquez et al. (1990), who reported that levels of DTPA-extractable soil Cd did not increase significantly as a result of biosolid amendment after the first three growing seasons. The DTPA-extractable Zn and Cu concentrations significantly increased in MSW plots (Table 6). Levels of Zn and Cu increased over the three years of the experiment. Concentrations of DTPA-extractable Zn and Cu in the biosolid treatments did not significantly change until the second year (Table 5), when higher Zn and Cu levels were found for the intermediate and high biosolid application plots compared with the control. In the third year, only the high biosolid application increased Zn and Cu concentration. Extractable Pb concentrations in both biowaste treatments did not significantly change over time. The highest DTPA-extractable Zn and Cu concentrations encountered in both amended soils were still far below the levels considered phytotoxic (Tiedemann and Lopez, 1982).
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Table 6. Concentration of DTPA-extractable heavy metals from unamended and municipal solid wastes (MSW)-amended soils.
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Native Vegetation Properties
With the exception of the highest rate of both wastes in the first year, the total percentage plant cover increased significantly with all the biowaste treatments for the three years (Table 7). Total canopy cover showed a significant decrease at the high rate for both biowastes during the first year. Partial biowaste decomposition was observed in the first year, which seems to indicate that some remained on the soil surface, perhaps acting as a physical barrier to plant emergence.
Total aerial plant biomass production increased in the three years of the study in response to the application of both types of biowaste (Table 7). These increases can be attributed directly to a rise in soil fertility (Tables 3 and 4). However, improvements in biological and physical properties cannot be discounted. During the first year, the low and intermediate biosolid rates produced significantly greater dry matter yields than the control or the highest rate (Table 7). Although the 120 Mg ha-1 biosolid rate produced a total plant biomass 548 g m-2 greater than the control for the first year, a combination of different factors (physical, biological, and chemical) associated with this application may have partially limited plant yields, at least when compared with the other biosolid treatments. Total plant production in the second and third year (1999 and 2000) increased as the rates of biosolids increased, probably because the inhibitor effects observed in the first year decreased over time. Total plant production obtained with either biowaste was lower in the second year (1999) compared with the first and the third (1998 and 2000). The decrease in plant biomass production in the second year in all treatments was probably due to a lower average mean annual precipitation.
Compared with the unamended treatment, the application of both biosolids and MSW increased the macronutrients of the native vegetation to concentrations adequate for plant growth (Tables 8 and 9). In all three years, the N, P, and K contents of tissues from plants in treated plots were significantly higher than in those from control plots. As expected, the concentrations of Cr, Cd, Ni, and Pb in plant tissue did not increase significantly over the three years as a result of the waste applications. Tissue Cr was below the method detection limit (<0.08 mg kg-1) and no significant differences in tissue Pb, Cd, and Ni were found (Tables 8 and 9). On the contrary, the concentrations of plant tissue micronutrients, such as Zn and Cu, increased with biowaste application rates. The Zn and Cu contents in the MSW applications were adequate for plant growth, while the Zn and Cu levels with the biosolid applications were lower and could even be considered Zn and Cu deficient.
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Table 9. Selected elemental composition of plant tissue from unamended and municipal solid wastes (MSW)-amended soils.
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A remarkable change in the native plant communities was observed since the first year after the surface application of either biosolids or MSW, with a reduction in perennial species and an increase in annual plants (Fig. 1)
. This is similar to that reported by Biondini and Redente (1986), who indicate that plant diversity decreases in the presence of high nutrient concentrations. No similar pattern was found, however, by Pierce et al. (1998), who reported that annual plant species never represented more than 3% of total canopy cover: perennial grasses, mid-seral shrubs, and forbs remained the dominant species in a semiarid shrubland following biosolid treatment. In the third year of the study, the change in richness in plant species remained. This effect was greater in the biosolid than in the MSW treatments, possibly due to greater organic matter decomposition and therefore greater quantities of plant-available N. With the surface application of biowastes, N was first available in the upper few centimeters of the soil surface. Under these conditions, annual plants may be in a better position than perennial species, which have deeper root systems.
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Fig. 1. Native plant community evolution in the study period. B, biosolids treatment; C, control treatment; MSW, municipal solid wastes treatment. Can1 and Can2 are characterized by the most representative species or groups of species selected that discriminate the different groups (treatments).
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Different plant species or groups were selected (Table 2) and used as variables in the statistical analysis. The Box and Cox (1964) diagnostic procedure was used to select the most appropriate transformation (square root). Canonical and stepwise discriminant analyses were undertaken to evaluate the study treatments and to establish the characteristic variables (species) of each. Univariate and multivariate analyses showed very significant differences (P < 0.0001) among species in relation to treatment applied.
For the stepwise analysis, the plots were grouped together according to their treatments (control, biosolids, and MSW), irrespective of application rate differences. Linear discriminant analysis involves an algorithm in which mathematical functions (linear combinations of the variables) are constructed in such a way that the differences among the established groups are maximized. The more representative species were selected each year as the best feature for discrimination of the different treatments applied. The species selected were, for the first year (1998), Cruciferae, rocketsalad [Eruca vesicaria (L.) Cav.], Gramineae 2, and corn poppy (Papaver rhoeas L.); for the second year (1999) Gramineae 2, white plantain (Plantago albicans L.), corn chamomile (Anthemis arvensis L.), Mediterranean stork's bill [Erodium malacoides (L.) L'Hér. ex Ait.], and bristly hawkbit (Leontodon hispidus L.); and for the third year (2000) the species were Spanish thyme (Thymus zygis L.), rocketsalad, Spanish heal-all [Cleonia lusitanica (L.) L.], and frostweed asperum (Helianthemum asperum Lag. Dunal). With these selected species it is possible to classify at 100% the control and the MSW treatments for the first two years, and the biosolid-treated plots at 81 and 93% for the first and second year (1998 and 1999), respectively. For the last year (2000) the discriminant function gave 100% correct classification rates for control and biosolids and 83% for MSW.
These results show that the biowaste-treated plots showed different native plant community development to the unamended plots, and also that differences are found in plant species richness between the two biowaste treatments. In addition, canonical discriminant analysis was undertaken to complement the results obtained, and again all treatments were correctly classified. This analysis takes into account all the species selected. The squared canonical correlations obtained for 1998 were 0.86 and 0.63 for the first and second function (Can 1 and Can 2), respectively. In the second year (1999), the squared canonical correlations were 0.87 and 0.71 for the same functions. In the last year, values of 0.94 for the first function and 0.82 for the second function were obtained. The correlations between the canonical variable and the original variables, called canonical structures, were used in conjunction with plots of discriminant canonical functions to aid interpretation of group differences. The plots obtained are shown in Fig. 1. For 1998, the biosolids treatments were clearly separated from the MSW and control treatments, positioned at the negative side of the first canonical function (Can 1). This is mainly due to the presence of Gramineae 1 and 2, Cruciferae, and Mediterranean stork's bill. The second canonical function (Can 2) separated the control treatment from the MSW treatments. White plantain and Spanish thyme are the species that distinguish control treatment, while rocketsalad distinguishes MSW treatments. For 1999, the control treatment was clearly separated from the other treatments at the positive side of the first canonical function, mainly due to the presence of Spanish thyme, white plantain, and legumes. The second axis (Can 2) separated the biosolids treatments (Gramineae 2) from MSW treatments (white plantain, Mediterranean stork's bill, and Gramineae 1). Finally, for the last year, the separation of the groups (treatments) was more remarkable. The control treatment was positioned at the extreme positive side of the first canonical function, clearly separated from the biosolids and MSW treatments. High positive canonical structure values were observed for Spanish thyme and germander (Teucrium spp.); both species are, therefore, important in distinguishing the control treatment. The biosolids treatments were separated from the MSW treatments since the former were positioned at the negative side of the second canonical function, while the latter were at the extreme positive side of the same axis (Can 2). Gramineae 2, Mediterranean stork's bill, common vipersbugloss (Echium vulgare L.), and Thistle 2 were the characteristic variables of the biosolids treatments and white plantain, germander, frostweed asperum, Spanish heal-all, and Mediterranean lineseed [Bellardia trixago (L.) All.] were the characteristic variables for the MSW treatments.
The results clearly show that the different treatments define the different native plant communities that develop. There were sufficient quantities of nutrients from the biowaste amendments to sustain the proper conditions for rapid growth and the dominance of annual species. This resulted in competitive exclusion of other species and, consequently, reduced species richness. Total herbaceous production was greater in the plots that received biosolids than in the MSW-treated plots, and different plant species were found on the plots thus treated (Fig. 1). This is probably due to the higher soil nutrient content, especially in N and P, provided by biosolid treatment. Plant tissue N and P contents in the biosolids treatments were also higher than those of plants growing in MSW plots (Tables 8 and 9). The results obtained are consistent with the accepted hypothesis that a stimulus factor (such as nutrient availability) decreases species diversity while stress factors increase diversity (Huston, 1979). In other words, species with optimal growth at higher nutrient levels grow rapidly and out-compete other species.
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CONCLUSIONS
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The data available from the present three-year study indicate that surface application of both biosolid and MSW to a degraded semiarid ecosystem increases the level of essential nutrients to plants, without increasing soil DTPA-extractable heavy metals, such as Cr, Cd, Pb, and Ni.
Although biowaste amendment decreased species richness, plant canopy cover and aerial biomass production increased significantly and remained higher by the third year of the study. These effects were greater in the biosolid than in the MSW treatments. This could be due to greater decomposition of organic matter and therefore an increase in plant-available N with the biosolid treatment (MSW organic matter is more stable). Similarly increased levels of macronutrients (N, P, and K) and micronutrients (Cu and Zn) were found in the plant tissues grown in the plots that had received biowaste amendment.
A remarkable change in the native plant communities was seen with a reduction in perennial species and an increase in annual plants, which persisted for the three years of the study. Thus, if the aim of the degraded ecosystem reclamation project is erosion control, then high rates of biowastes should be applied. On the contrary, if high species richness is the objective, very low rates of biowaste should be applied.
The degraded soils used in this study required biowaste rates between 40 and 80 Mg ha-1 to improve their chemical properties without there being any apparent, accompanying adverse environmental effect. Higher treatment rates may result in significant improvement in many soil properties as well as in plant production, but pose a potential P and N-NO3 contamination hazard to surface waters at times of significant runoff.
Further studies are needed to evaluate the possible changes in soil chemical properties and vegetation growth response in the medium and long term.
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TOP
ABSTRACT
INTRODUCTION
