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Water-dispersible clay in bare fallow soils after 80 years of continuous fertilizer addition
Geoderma 200-201 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
Geoderma journal homepage: www.elsevier.com/locate/geoderma
Water-dispersible clay in bare fallow soils after 80 years of continuous fertilizer addition R. Paradelo a,⁎, F. van Oort b, C. Chenu a a b
AgroParisTech, UMR 7618, Bioemco, Équipe Matières Organiques des Sols, F-78850 Thiverval-Grignon, France INRA, UR 251 Pessac, RD-10, F-78026 Versailles Cedex, France
a r t i c l e
i n f o
Article history: Received 6 April 2012 Received in revised form 16 January 2013 Accepted 18 January 2013 Available online 20 March 2013 Keywords: Soil structure Long-term fertilization Water dispersible clay
a b s t r a c t The 42 plots of the Versailles long-term bare fallow experiment offer exceptional opportunities to study the impacts of fertilizers and amendments on soil physical properties. In this experiment, continuous annual applications since 1928 of 16 different treatments – including nitrates, phosphates, manure, ammonium, calcium, and potassium salts – have led to strongly diverging physical and chemical properties in the soil's surface layer. In this work, the proportion of water dispersible clay (WDC), which is considered a good indicator of soil structural stability and sensitivity to crusting and erosion, was determined in all plots of the experiment. The results showed the influence of the valence of cations and pH on soil structure and clay dispersability. Application of sodium or potassium salts deteriorated the degree of soil aggregation, with increased bulk density and WDC values, whereas amendment with manure or calcium salts ameliorated soil aggregation, with decreasing bulk density and WDC. The strong acidification of the soils receiving ammonium-based fertilizers was another factor related to the increase of water dispersible clay, and therefore the deterioration of soil structure. This study emphasizes the role of potassium and ammonium, associated with fertilizer additions, on the deterioration of soil structure and on the increase of the risk of erosion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The impact of farming practices on soil and water quality is currently of worldwide concern. The potential impacts of fertilizers on physical soil characteristics hold a particular relevance, since the modification of these characteristics may lead to a significant loss of soil fertility, as well as problems of soil physical degradation and increment of soil erodibility and erosion (Bronick and Lal, 2005; Haynes and Naidu, 1998). The application of common fertilizers influences the physical properties of soils due to the contrasting effect of cationic elements on the stability of soil particles (Bohn et al., 1979). Although the impacts of monovalent or divalent cations on soil structure are well documented in the literature, they often refer to studies conducted on different soils and under different pedoclimatic conditions. In contrast, the observation of such impacts at a field scale, on comparable soils, and over the long term, as a consequence of the addition of fertilizers, is much less well documented. In this sense, the long-term bare fallow experiment of Versailles (France) offers exceptional conditions to study the effects of continuous chemical applications, without interference of plant growth. This experiment was established in 1928 with the aim of determining the effect of fertilizers
⁎ Corresponding author at: AgroParisTech, Bâtiment EGER, 78850 Thiverval-Grignon, France. Tel.: +33 1 30 81 52 84; fax: +33 1 30 81 54 97. E-mail addresses: [email protected], [email protected] (R. Paradelo). 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.01.014
and common soil amendments on the structure of silty-loam soils (Burgevin and Henin, 1939). After 80 years of annual application of fertilizers and amendments, striking visual aspects characterise the surface of the different plots, raising questions about the influence of fertilizers on the stability of soil aggregation. One way to study soil aggregation stability is determining the water-dispersible clay (WDC) fraction, which refers to clay that can be easily dispersed by water. Since it represents the suspended solid fraction transported over the greatest distances by runoff, due to the low sedimentation rate of clay-sized particles, it is a key criterion when assessing risks of surface sealing and crusting, and soil erosion by water (Brubaker et al., 1992; Calero et al., 2008). In addition, soils affected by salts, in particular by Na salts, present conditions where clays are easily dispersed by water (Sumner, 1993). Therefore, WDC has been used to study structural stability of saline and sodic soils (Nelson et al., 1998), as well as of burnt soils, where a saline ash layer may develop (Mills and Fey, 2004). The size of this fraction depends on several soil properties such as the content and nature of clay (Oster et al., 1980; Seta and Karathanasis, 1996), of aggregating agents such as organic matter (Barral et al., 1998; Nelson et al., 1998) or Fe and Al oxides (Goldberg et al., 1990; Shaw et al., 2003), of soluble salts (Emerson, 1971), as well as on the ratios of exchangeable cations, notably the Na/Ca pair (Rengasamy and Marchuk, 2011). Soil management practices also influence clay dispersability, in particular via impacts on organic carbon contents and aggregation (Burt et al., 2001; Chenu et al., 2000a; Rhoton, 2000; Shaw et al., 2002).
R. Paradelo et al. / Geoderma 200-201 (2013) 40–44
In the present work, we study the differences in clay dispersability in the soils of the Versailles long-term bare-fallow experiment, with the objective of assessing the long-term impact of repeated addition of fertilizers and amendments on soil structure. Considering their initial comparable clay, organic matter and iron oxides contents, and clay mineralogy (Pernes-Debuyser et al., 2003), the soils of the Versailles site are expected to improve our knowledge about the factors controlling clay dispersability. 2. Materials and methods The Versailles 42-plot long-term bare-fallow experiment was established in 1928 at the Institut National de la Recherche Agronomique (INRA) site at Versailles (France), to study the evolution of soil physical properties and composition without the influence of plant inputs (Burgevin and Henin, 1939). The experimental site is located in the Paris Basin, with a mean annual temperature of 12.4 °C and a precipitation of 700 mm. The soil is a silty loam Luvisol (FAO), developed on aeolian loess, which characterises large areas of north-western Europe. Soil properties in 1929 (determined in 1999 in archived samples) were the following: a particle size distribution with 21.7% sand (0.05–2 mm), 58.6% silt (2–50 μm) and 19.7% clay (b 2 μm), a pH in water of 6.4 and a cation exchange capacity (cobaltihexamine method) of 15.3 cmol kg−1 (Pernes-Debuyser and Tessier, 2002). Clay mineralogical composition includes mainly disordered illite–smectite mixed-layer minerals, with minor amounts of illite-mica and kaolinite (PernesDebuyser et al., 2003). Nowadays, apart from the manure plots, organic carbon contents are low and predominantly composed of stable carbon (Barré et al., 2010). The experiment consists of 42 plots (2×2.5 m) maintained under bare fallow, including 15 duplicates of mineral fertilizer/amendment treatments, one duplicate of organic manure applied annually since 1928 and 10 unamended reference plots. All plots are turned over with a spade, twice a year (spring and autumn) to a depth of 25 cm, and kept free from vegetation by hand weeding and herbicide treatment. The treatments assayed in the 42 plots of the experiment, which follow the design shown in Fig. 1, are described in Table 1. These include predominantly inorganic treatments, with different sources of N and
41
P, and a variety of cations (Ca, Mg, Na, K, NH4), organic manure amended plots, as well as reference plots without treatment. For our study, we used samples collected in 2008, i.e. 80 years after the beginning of the experiment. Sampling occurred in April, before the spring amendments and digging. Three intact soil cores with a known volume were sampled in each plot at 0–20-cm depth. Samples were air-dried and stored in sealed desiccation flasks prior to analyses. Bulk density (BD), pH and water dispersible clay were determined for all replicate samples. The pH was measured in a 1:2.5 soil:water extract (AFNOR, 2005). Bulk density was determined directly on the soil cores. Water dispersible clay was determined following the method described by Barral et al. (1998), with some modifications. For the determination, 10 g of air dry soil was shaken with 50 mL of deionized water for 1 h in 100-mL plastic flasks. Suspensions were allowed to settle for 8 h at 20 °C. A 20-mL aliquot was taken at a depth of 10 cm with a Robinson pipette, transferred to weighed aluminium capsules, dried for 48 h at 105 °C and weighed to determine the amount of water dispersible clay, which was expressed as g clay 100 g−1 of oven-dried soil (105 °C). The importance of the selection of the method for dispersing clay is not negligible, because the results of the determination are method-dependent, being a function of the energy input (which time of shaking is an expression of). We selected this method among the several existing procedures because most results in the literature suggest that short shaking times would be more suitable to give evidence for modifications of WDC contents induced by differences in management or tillage (Barral et al., 1998; Chenu et al., 2000a). This is so because the amount of clay which is rapidly dispersed is linked to clay dispersability and surface area of aggregates, i.e., aggregate size. On the contrary, longer shaking times break up aggregates, and hence, results will depend less on structure, and more on factors such as soil type, clay mineralogy, organic matter content, etc. (Kay and Dexter, 1990; Nelson et al., 1998). Significance of differences between physical parameters of the amended and the reference plots, as well as correlation analyses, was run using the R statistical package for MacOSX (R Development Core Team, 2011). Assessment of differences between treatments was hampered by the initial design of the original 42 plots, including only two replicates per treatment. Plots are small so that the replicate cores are not truly independent. Therefore, we decided to assess
Fig. 1. Experimental plan of the 42 plots of the long term bare fallow experiment of Versailles.
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R. Paradelo et al. / Geoderma 200-201 (2013) 40–44
Table 1 Description of the treatments employed in the Versailles 42-plot long term bare fallow experiment. Fertilizer/amendment
Rate
Plots
Type of treatment
None
–
Reference
Ammonium sulphate Ammonium phosphate Ammonium chloride Dried blood Ammonium nitrate Sodium nitrate Calcium nitrate Manure Apatite Superphosphate Fly ash Calcium carbonate Calcium oxide Potassium chloride Potassium sulphate Sylvinite
150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 100 Mg ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg CaO ha−1 yr−1 1 Mg CaO ha−1 yr−1 150 kg K2O ha−1 yr−1 150 kg K2O ha−1 yr−1 150 kg K2O ha−1 yr−1
1, 9, 11, 13, 21, 22, 30, 32, 34, 42 2, 19 3, 14 7, 15 9, 18 6, 20 4, 17 5, 16 10, 12 28, 33 27, 38 24, 35 31, 39 26, 40 23, 37 25, 41 29, 36
N N, P N N N N, Na N, Ca N P P P, Ca Ca Ca K K K, Na
differences between each plot by a two-tails t-test, using a mean and variance (the latter of which was determined using the three pseudo-replicates). Due to heterogeneity in carbon contents, the two blocks of the trial were treated separately. The plots of each treatment were compared to the reference plots in the corresponding block of the experiment: this is, the plots receiving nitrogen amendments were compared to the five reference plots in the same block of the experiment (Fig. 1, plots 1–21), whereas the plots receiving K, Ca, or P amendments, were compared to the five reference plots in their block (plots 22–42). 3. Results and discussion The visual aspect of the experimental plots indicates that 80 years of continuous addition of amendments have led to significant different soil surface properties. Plots receiving manure, calcium carbonate or calcium oxide still maintained an aggregated soil structure at the time of sampling. All the other plots presented a surface aspect of disaggregated structure, especially those receiving sodium or potassium salts, which presented a continuous surface crust. The rates of surface crust development were quantified by Bresson and Boiffin (1990), who observed that the plots receiving sodic fertilizers had a much stronger tendency to form a crust after a rain event, as compared to plots receiving manure or calcium compounds. Bulk density values measured in 2008 (Fig. 2) were consistent with visual surface aspects. All the plots receiving sodium or potassium amendments have increased their bulk density with respect to the reference plots. Addition of ammonium fertilizers either decreased or had no effect on bulk density, the same as phosphate fertilizers. In contrast, the amendment with fly ash, calcium salts, and manure led to decreased bulk densities. This reduction was particularly marked for the manureamended plots, which present the lowest observed BD values. Although bulk density measurements represent a comprehensive parameter for assessment of soil structure, it provides no direct information on the soil's susceptibility to erosion. Determination of WDC is considered to provide more insight on the aspect of structure stability. Here, 80 years of fertilizer application produced significant differences in WDC in most of the plots (Fig. 3). All treatments with potassium or sodium salts produced increments in WDC concentrations with respect to reference plots, as did some of the ammonium salts (sulphate and phosphate). Increased WDC values in plots receiving potassium and/ or sodium salts are ascribed to increased amounts of monovalent cations in the surface of soil particles, which favour dispersion of clay
Fig. 2. Bulk density of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
particles, and therefore have a destabilizing effect on clay structures (Bohn et al., 1979; Tessier, 1991). Similar impacts are expected to be produced by the ammonium fertilizers, because if monovalent NH4+ ions accumulate in soils in large amounts, they become a dominant exchangeable cation and, like Na +, favour dispersion of soil colloids (Haynes and Naidu, 1998). When applied at high rates and for soil conditions unfavourable to nitrification, ammonium fertilizers enhance soil dispersion, surface crusting and reduction of the water infiltration rates (Aldrich et al., 1945; Fox et al., 1952; Pillsbury, 1947). Furthermore, such ammonium applications lead to a reduction in soil pH which, in turn, inhibits nitrification and favours accumulation of applied NH4+ in the soil (Haynes and Naidu, 1998). The rest of the N fertilizers, as well as phosphate amendments and fly ash, did not produce significant changes in WDC. On the contrary, amendment with manure and calcium carbonate led to reduced WDC values. In those plots receiving calcium carbonate, Ca+2 likely favours clay flocculation, thus increasing clay stability and strengthening clay– clay bonds in aggregates, resulting in lower WDC concentrations. Also, the low WDC amounts for the manure plots are related to an effect of incorporation of organic matter, known to have an essential role for
Fig. 3. Water-dispersible clay concentrations of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
R. Paradelo et al. / Geoderma 200-201 (2013) 40–44
Fig. 4. Relationship between water-dispersible clay and bulk density in the 42 plots. The values for the Pearson's correlation coefficient (R2) and significance (p) are shown.
soil structure stabilization through the formation of physically stable clay–organic matter associations (Chenu et al., 2000b). Globally, the results of WDC determinations showed a similar trend to that observed for bulk density data. Yet, the structure destabilization for plots receiving potassium salts was more evident in WDC data than in bulk density data. Also, when we compare the data of clay dispersability and soil structure (Fig. 4), disregarding the organic manure amended plots, a significant positive correlation was found between these two variables. Differences observed in clay dispersability in relation to the different chemical amendments are also likely associated with soil pH, which varied significantly between plots, with values ranging from 3.5 to 8.7 (Fig. 5). The soils of the experiment have naturally acidified over the last 80 years, as illustrated by pH values of reference plots, which decreased from an initial value of 6.4 (Burgevin and Henin, 1939) to values between 5 and 6 in 2008, ascribed to a large extent to the soil's lowered buffering capacity due to mineralization of organic carbon. This acidification of the reference plots is consistent with about 25% of exchangeable Al3+ on the soil's exchange complex (Pernes-Debuyser et al., 2003). Continuous application of ammonium fertilizers led to a significant additional soil acidification, with all plots receiving these fertilizers (ammonium
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Fig. 6. Relationship between water-dispersible clay and pH in the 42 plots (the plots of the potassium or manure treatments, marked as full circles, were not used for the correlation). The values for the Pearson's correlation coefficient (R2) and significance (p) are shown.
phosphate, sulphate, chlorhydrate, nitrate, and dried blood) having currently pH values under 4. These strongly acidified plots nowadays display large proportions of exchangeable Al3+ (up to >90%), resulting from destruction of the finest clay fraction (Pernes-Debuyser et al., 2003). On the contrary, plots receiving manure, fly ash or calcium salts (CaO, CaCO3) had strong pH increases, with current values higher than 8. Applications of potassium salts, nitrates or phosphates were without effect on pH in regard to the reference plots. Overall, the range of pH values of the plots is wide enough to expect an effect on clay dispersability. In fact, with the exception of the soils having received K treatments, which seemed to be particularly effective in destabilizing structure without modifying pH, a good negative correlation was found between pH and WDC (Fig. 6). This presumably reflects the progressive replacement of Ca and Mg by ammonium in exchange positions at low pH, which increases clay dispersability (Haynes and Naidu, 1998). Such correlations demonstrate the potential negative effect of excessive acidification due to fertilizers on soil structure and susceptibility to erosion. 4. Conclusion After 80 years of continuous addition of a variety of fertilizers and amendments, the soils of the Versailles bare fallow experiment have developed significant differences in bulk density, water dispersible clay and pH, which are directly related to the addition of the fertilizers/amendments. Here it is shown the long-term impact of the valence of cations and pH on structure and clay dispersability. Water-dispersible clay determinations gave insight in the differences in the structure on the plots, showing the amelioration of structure due to manure and calcium carbonate, and its degradation caused by sodium and potassium salts. Observed differences in clay dispersability were related, not only to the destabilizing effect of monovalent cations, but also to differences in pH induced by the amendments, in particular to the strong acidifying effect of ammonium fertilizers. The results reveal the strong potential of potassium salts for destabilizing clays and affecting negatively soil structure, a fact that has received little attention to date. In a larger sense, this study highlights the potential impact of fertilization upon long-term soil and water quality through the modification of soil physical properties and structure, which can lead to increased risk of soil erosion.
Fig. 5. pH of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
Acknowledgements This work was supported by an INSU EC2CO programme. Dr. R. Paradelo acknowledges the financial support of the Spanish Ministry
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for Science and Technology. The authors acknowledge the help of Jean Pierre Pétraud and Daniel Billiou for sampling and for bulk density measurements. We also would like to express our gratitude to two anonymous reviewers for their useful comments and editorial suggestions which improved the comprehension of the manuscript.
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Goldberg, S., Kapoor, B.S., Rhoades, J.D., 1990. Effect of aluminium and iron oxides and organic matter on flocculation and dispersion of arid zone soils. Soil Science 150, 588–593. Haynes, R.J., Naidu, R., 1998. Influence of lime, fertilizer and manure applications on soil organic matter content and physical conditions: a review. Nutrient Cycling in Agroecosystems 51, 123–137. Kay, B.D., Dexter, A.R., 1990. Influence of aggregate diameter, surface area, and antecedent water content on the dispersability of clay. Canadian Journal of Soil Science 70, 655–671. Mills, A.J., Fey, M.V., 2004. Frequent fires intensify soil crusting: physicochemical feedback in the pedoderm of long-term burn experiments in South Africa. Geoderma 121, 45–64. Nelson, P.N., Baldock, J.A., Oades, J.M., 1998. Changes in dispersible clay content, organic carbon content, and electrolyte composition following incubation of sodic soil. Australian Journal of Soil Research 36, 883–897. Oster, J.D., Shainberg, I., Wood, J.D., 1980. Flocculation value and gel structure of sodium/ calcium montmorillonite and illite suspensions. Soil Science Society of America Journal 44, 955–959. Pernes-Debuyser, A., Tessier, D., 2002. pH effect on soil properties: long time experiment of 42 plots in Versailles (in French). Revue des Sciences de l'Eau 15, 27–39 (special). Pernes-Debuyser, A., Pernes, M., Velde, B., Tessier, D., 2003. Soil mineralogy evolution in the INRA 42 plots experiment (Versailles, France). Clays and Clay Minerals 51, 578–585. Pillsbury, A.F., 1947. Factors influencing infiltration rates into Yolo loam. Soil Science 64, 177–181. R Development Core Team, 2011. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria (http://www.R-project.org/). Rengasamy, P., Marchuk, A., 2011. Cation ratio of soil structural stability (CROSS). Soil Research 49, 280–285. Rhoton, F.E., 2000. Influence of time on soil response to no-till practices. Soil Science Society of America Journal 64, 700–709. Seta, A.K., Karathanasis, A.D., 1996. Water dispersible colloids and factors influencing their dispersibility from soil aggregates. Geoderma 74, 255–266. Shaw, J.N., Truman, C.C., Reeves, D.W., 2002. Mineralogy of eroded sediments derived from highly weathered Ultisols of central Alabama. Soil and Tillage Research 68, 59–69. Shaw, J.N., Reeves, D.W., Truman, C.C., 2003. Clay mineralogy and dispersibility of soil and sediment derived from Rhodic Paleudults. Soil Science 168, 209–217. Sumner, M.E., 1993. Sodic soils — new perspectives. Australian Journal of Soil Research 31, 683–750. Tessier, D., 1991. Behaviour and microstructure of clay minerals. In: De Boodt, M.F. (Ed.), Soil Colloids and Their Associations in Aggregates. : Nato Advanced Series. Plenum Press, New York, pp. 387–415.
Contents lists available at SciVerse ScienceDirect
Geoderma journal homepage: www.elsevier.com/locate/geoderma
Water-dispersible clay in bare fallow soils after 80 years of continuous fertilizer addition R. Paradelo a,⁎, F. van Oort b, C. Chenu a a b
AgroParisTech, UMR 7618, Bioemco, Équipe Matières Organiques des Sols, F-78850 Thiverval-Grignon, France INRA, UR 251 Pessac, RD-10, F-78026 Versailles Cedex, France
a r t i c l e
i n f o
Article history: Received 6 April 2012 Received in revised form 16 January 2013 Accepted 18 January 2013 Available online 20 March 2013 Keywords: Soil structure Long-term fertilization Water dispersible clay
a b s t r a c t The 42 plots of the Versailles long-term bare fallow experiment offer exceptional opportunities to study the impacts of fertilizers and amendments on soil physical properties. In this experiment, continuous annual applications since 1928 of 16 different treatments – including nitrates, phosphates, manure, ammonium, calcium, and potassium salts – have led to strongly diverging physical and chemical properties in the soil's surface layer. In this work, the proportion of water dispersible clay (WDC), which is considered a good indicator of soil structural stability and sensitivity to crusting and erosion, was determined in all plots of the experiment. The results showed the influence of the valence of cations and pH on soil structure and clay dispersability. Application of sodium or potassium salts deteriorated the degree of soil aggregation, with increased bulk density and WDC values, whereas amendment with manure or calcium salts ameliorated soil aggregation, with decreasing bulk density and WDC. The strong acidification of the soils receiving ammonium-based fertilizers was another factor related to the increase of water dispersible clay, and therefore the deterioration of soil structure. This study emphasizes the role of potassium and ammonium, associated with fertilizer additions, on the deterioration of soil structure and on the increase of the risk of erosion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The impact of farming practices on soil and water quality is currently of worldwide concern. The potential impacts of fertilizers on physical soil characteristics hold a particular relevance, since the modification of these characteristics may lead to a significant loss of soil fertility, as well as problems of soil physical degradation and increment of soil erodibility and erosion (Bronick and Lal, 2005; Haynes and Naidu, 1998). The application of common fertilizers influences the physical properties of soils due to the contrasting effect of cationic elements on the stability of soil particles (Bohn et al., 1979). Although the impacts of monovalent or divalent cations on soil structure are well documented in the literature, they often refer to studies conducted on different soils and under different pedoclimatic conditions. In contrast, the observation of such impacts at a field scale, on comparable soils, and over the long term, as a consequence of the addition of fertilizers, is much less well documented. In this sense, the long-term bare fallow experiment of Versailles (France) offers exceptional conditions to study the effects of continuous chemical applications, without interference of plant growth. This experiment was established in 1928 with the aim of determining the effect of fertilizers
⁎ Corresponding author at: AgroParisTech, Bâtiment EGER, 78850 Thiverval-Grignon, France. Tel.: +33 1 30 81 52 84; fax: +33 1 30 81 54 97. E-mail addresses: [email protected], [email protected] (R. Paradelo). 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.01.014
and common soil amendments on the structure of silty-loam soils (Burgevin and Henin, 1939). After 80 years of annual application of fertilizers and amendments, striking visual aspects characterise the surface of the different plots, raising questions about the influence of fertilizers on the stability of soil aggregation. One way to study soil aggregation stability is determining the water-dispersible clay (WDC) fraction, which refers to clay that can be easily dispersed by water. Since it represents the suspended solid fraction transported over the greatest distances by runoff, due to the low sedimentation rate of clay-sized particles, it is a key criterion when assessing risks of surface sealing and crusting, and soil erosion by water (Brubaker et al., 1992; Calero et al., 2008). In addition, soils affected by salts, in particular by Na salts, present conditions where clays are easily dispersed by water (Sumner, 1993). Therefore, WDC has been used to study structural stability of saline and sodic soils (Nelson et al., 1998), as well as of burnt soils, where a saline ash layer may develop (Mills and Fey, 2004). The size of this fraction depends on several soil properties such as the content and nature of clay (Oster et al., 1980; Seta and Karathanasis, 1996), of aggregating agents such as organic matter (Barral et al., 1998; Nelson et al., 1998) or Fe and Al oxides (Goldberg et al., 1990; Shaw et al., 2003), of soluble salts (Emerson, 1971), as well as on the ratios of exchangeable cations, notably the Na/Ca pair (Rengasamy and Marchuk, 2011). Soil management practices also influence clay dispersability, in particular via impacts on organic carbon contents and aggregation (Burt et al., 2001; Chenu et al., 2000a; Rhoton, 2000; Shaw et al., 2002).
R. Paradelo et al. / Geoderma 200-201 (2013) 40–44
In the present work, we study the differences in clay dispersability in the soils of the Versailles long-term bare-fallow experiment, with the objective of assessing the long-term impact of repeated addition of fertilizers and amendments on soil structure. Considering their initial comparable clay, organic matter and iron oxides contents, and clay mineralogy (Pernes-Debuyser et al., 2003), the soils of the Versailles site are expected to improve our knowledge about the factors controlling clay dispersability. 2. Materials and methods The Versailles 42-plot long-term bare-fallow experiment was established in 1928 at the Institut National de la Recherche Agronomique (INRA) site at Versailles (France), to study the evolution of soil physical properties and composition without the influence of plant inputs (Burgevin and Henin, 1939). The experimental site is located in the Paris Basin, with a mean annual temperature of 12.4 °C and a precipitation of 700 mm. The soil is a silty loam Luvisol (FAO), developed on aeolian loess, which characterises large areas of north-western Europe. Soil properties in 1929 (determined in 1999 in archived samples) were the following: a particle size distribution with 21.7% sand (0.05–2 mm), 58.6% silt (2–50 μm) and 19.7% clay (b 2 μm), a pH in water of 6.4 and a cation exchange capacity (cobaltihexamine method) of 15.3 cmol kg−1 (Pernes-Debuyser and Tessier, 2002). Clay mineralogical composition includes mainly disordered illite–smectite mixed-layer minerals, with minor amounts of illite-mica and kaolinite (PernesDebuyser et al., 2003). Nowadays, apart from the manure plots, organic carbon contents are low and predominantly composed of stable carbon (Barré et al., 2010). The experiment consists of 42 plots (2×2.5 m) maintained under bare fallow, including 15 duplicates of mineral fertilizer/amendment treatments, one duplicate of organic manure applied annually since 1928 and 10 unamended reference plots. All plots are turned over with a spade, twice a year (spring and autumn) to a depth of 25 cm, and kept free from vegetation by hand weeding and herbicide treatment. The treatments assayed in the 42 plots of the experiment, which follow the design shown in Fig. 1, are described in Table 1. These include predominantly inorganic treatments, with different sources of N and
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P, and a variety of cations (Ca, Mg, Na, K, NH4), organic manure amended plots, as well as reference plots without treatment. For our study, we used samples collected in 2008, i.e. 80 years after the beginning of the experiment. Sampling occurred in April, before the spring amendments and digging. Three intact soil cores with a known volume were sampled in each plot at 0–20-cm depth. Samples were air-dried and stored in sealed desiccation flasks prior to analyses. Bulk density (BD), pH and water dispersible clay were determined for all replicate samples. The pH was measured in a 1:2.5 soil:water extract (AFNOR, 2005). Bulk density was determined directly on the soil cores. Water dispersible clay was determined following the method described by Barral et al. (1998), with some modifications. For the determination, 10 g of air dry soil was shaken with 50 mL of deionized water for 1 h in 100-mL plastic flasks. Suspensions were allowed to settle for 8 h at 20 °C. A 20-mL aliquot was taken at a depth of 10 cm with a Robinson pipette, transferred to weighed aluminium capsules, dried for 48 h at 105 °C and weighed to determine the amount of water dispersible clay, which was expressed as g clay 100 g−1 of oven-dried soil (105 °C). The importance of the selection of the method for dispersing clay is not negligible, because the results of the determination are method-dependent, being a function of the energy input (which time of shaking is an expression of). We selected this method among the several existing procedures because most results in the literature suggest that short shaking times would be more suitable to give evidence for modifications of WDC contents induced by differences in management or tillage (Barral et al., 1998; Chenu et al., 2000a). This is so because the amount of clay which is rapidly dispersed is linked to clay dispersability and surface area of aggregates, i.e., aggregate size. On the contrary, longer shaking times break up aggregates, and hence, results will depend less on structure, and more on factors such as soil type, clay mineralogy, organic matter content, etc. (Kay and Dexter, 1990; Nelson et al., 1998). Significance of differences between physical parameters of the amended and the reference plots, as well as correlation analyses, was run using the R statistical package for MacOSX (R Development Core Team, 2011). Assessment of differences between treatments was hampered by the initial design of the original 42 plots, including only two replicates per treatment. Plots are small so that the replicate cores are not truly independent. Therefore, we decided to assess
Fig. 1. Experimental plan of the 42 plots of the long term bare fallow experiment of Versailles.
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Table 1 Description of the treatments employed in the Versailles 42-plot long term bare fallow experiment. Fertilizer/amendment
Rate
Plots
Type of treatment
None
–
Reference
Ammonium sulphate Ammonium phosphate Ammonium chloride Dried blood Ammonium nitrate Sodium nitrate Calcium nitrate Manure Apatite Superphosphate Fly ash Calcium carbonate Calcium oxide Potassium chloride Potassium sulphate Sylvinite
150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 150 kg N ha−1 yr−1 100 Mg ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg fertilizer ha−1 yr−1 1 Mg CaO ha−1 yr−1 1 Mg CaO ha−1 yr−1 150 kg K2O ha−1 yr−1 150 kg K2O ha−1 yr−1 150 kg K2O ha−1 yr−1
1, 9, 11, 13, 21, 22, 30, 32, 34, 42 2, 19 3, 14 7, 15 9, 18 6, 20 4, 17 5, 16 10, 12 28, 33 27, 38 24, 35 31, 39 26, 40 23, 37 25, 41 29, 36
N N, P N N N N, Na N, Ca N P P P, Ca Ca Ca K K K, Na
differences between each plot by a two-tails t-test, using a mean and variance (the latter of which was determined using the three pseudo-replicates). Due to heterogeneity in carbon contents, the two blocks of the trial were treated separately. The plots of each treatment were compared to the reference plots in the corresponding block of the experiment: this is, the plots receiving nitrogen amendments were compared to the five reference plots in the same block of the experiment (Fig. 1, plots 1–21), whereas the plots receiving K, Ca, or P amendments, were compared to the five reference plots in their block (plots 22–42). 3. Results and discussion The visual aspect of the experimental plots indicates that 80 years of continuous addition of amendments have led to significant different soil surface properties. Plots receiving manure, calcium carbonate or calcium oxide still maintained an aggregated soil structure at the time of sampling. All the other plots presented a surface aspect of disaggregated structure, especially those receiving sodium or potassium salts, which presented a continuous surface crust. The rates of surface crust development were quantified by Bresson and Boiffin (1990), who observed that the plots receiving sodic fertilizers had a much stronger tendency to form a crust after a rain event, as compared to plots receiving manure or calcium compounds. Bulk density values measured in 2008 (Fig. 2) were consistent with visual surface aspects. All the plots receiving sodium or potassium amendments have increased their bulk density with respect to the reference plots. Addition of ammonium fertilizers either decreased or had no effect on bulk density, the same as phosphate fertilizers. In contrast, the amendment with fly ash, calcium salts, and manure led to decreased bulk densities. This reduction was particularly marked for the manureamended plots, which present the lowest observed BD values. Although bulk density measurements represent a comprehensive parameter for assessment of soil structure, it provides no direct information on the soil's susceptibility to erosion. Determination of WDC is considered to provide more insight on the aspect of structure stability. Here, 80 years of fertilizer application produced significant differences in WDC in most of the plots (Fig. 3). All treatments with potassium or sodium salts produced increments in WDC concentrations with respect to reference plots, as did some of the ammonium salts (sulphate and phosphate). Increased WDC values in plots receiving potassium and/ or sodium salts are ascribed to increased amounts of monovalent cations in the surface of soil particles, which favour dispersion of clay
Fig. 2. Bulk density of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
particles, and therefore have a destabilizing effect on clay structures (Bohn et al., 1979; Tessier, 1991). Similar impacts are expected to be produced by the ammonium fertilizers, because if monovalent NH4+ ions accumulate in soils in large amounts, they become a dominant exchangeable cation and, like Na +, favour dispersion of soil colloids (Haynes and Naidu, 1998). When applied at high rates and for soil conditions unfavourable to nitrification, ammonium fertilizers enhance soil dispersion, surface crusting and reduction of the water infiltration rates (Aldrich et al., 1945; Fox et al., 1952; Pillsbury, 1947). Furthermore, such ammonium applications lead to a reduction in soil pH which, in turn, inhibits nitrification and favours accumulation of applied NH4+ in the soil (Haynes and Naidu, 1998). The rest of the N fertilizers, as well as phosphate amendments and fly ash, did not produce significant changes in WDC. On the contrary, amendment with manure and calcium carbonate led to reduced WDC values. In those plots receiving calcium carbonate, Ca+2 likely favours clay flocculation, thus increasing clay stability and strengthening clay– clay bonds in aggregates, resulting in lower WDC concentrations. Also, the low WDC amounts for the manure plots are related to an effect of incorporation of organic matter, known to have an essential role for
Fig. 3. Water-dispersible clay concentrations of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
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Fig. 4. Relationship between water-dispersible clay and bulk density in the 42 plots. The values for the Pearson's correlation coefficient (R2) and significance (p) are shown.
soil structure stabilization through the formation of physically stable clay–organic matter associations (Chenu et al., 2000b). Globally, the results of WDC determinations showed a similar trend to that observed for bulk density data. Yet, the structure destabilization for plots receiving potassium salts was more evident in WDC data than in bulk density data. Also, when we compare the data of clay dispersability and soil structure (Fig. 4), disregarding the organic manure amended plots, a significant positive correlation was found between these two variables. Differences observed in clay dispersability in relation to the different chemical amendments are also likely associated with soil pH, which varied significantly between plots, with values ranging from 3.5 to 8.7 (Fig. 5). The soils of the experiment have naturally acidified over the last 80 years, as illustrated by pH values of reference plots, which decreased from an initial value of 6.4 (Burgevin and Henin, 1939) to values between 5 and 6 in 2008, ascribed to a large extent to the soil's lowered buffering capacity due to mineralization of organic carbon. This acidification of the reference plots is consistent with about 25% of exchangeable Al3+ on the soil's exchange complex (Pernes-Debuyser et al., 2003). Continuous application of ammonium fertilizers led to a significant additional soil acidification, with all plots receiving these fertilizers (ammonium
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Fig. 6. Relationship between water-dispersible clay and pH in the 42 plots (the plots of the potassium or manure treatments, marked as full circles, were not used for the correlation). The values for the Pearson's correlation coefficient (R2) and significance (p) are shown.
phosphate, sulphate, chlorhydrate, nitrate, and dried blood) having currently pH values under 4. These strongly acidified plots nowadays display large proportions of exchangeable Al3+ (up to >90%), resulting from destruction of the finest clay fraction (Pernes-Debuyser et al., 2003). On the contrary, plots receiving manure, fly ash or calcium salts (CaO, CaCO3) had strong pH increases, with current values higher than 8. Applications of potassium salts, nitrates or phosphates were without effect on pH in regard to the reference plots. Overall, the range of pH values of the plots is wide enough to expect an effect on clay dispersability. In fact, with the exception of the soils having received K treatments, which seemed to be particularly effective in destabilizing structure without modifying pH, a good negative correlation was found between pH and WDC (Fig. 6). This presumably reflects the progressive replacement of Ca and Mg by ammonium in exchange positions at low pH, which increases clay dispersability (Haynes and Naidu, 1998). Such correlations demonstrate the potential negative effect of excessive acidification due to fertilizers on soil structure and susceptibility to erosion. 4. Conclusion After 80 years of continuous addition of a variety of fertilizers and amendments, the soils of the Versailles bare fallow experiment have developed significant differences in bulk density, water dispersible clay and pH, which are directly related to the addition of the fertilizers/amendments. Here it is shown the long-term impact of the valence of cations and pH on structure and clay dispersability. Water-dispersible clay determinations gave insight in the differences in the structure on the plots, showing the amelioration of structure due to manure and calcium carbonate, and its degradation caused by sodium and potassium salts. Observed differences in clay dispersability were related, not only to the destabilizing effect of monovalent cations, but also to differences in pH induced by the amendments, in particular to the strong acidifying effect of ammonium fertilizers. The results reveal the strong potential of potassium salts for destabilizing clays and affecting negatively soil structure, a fact that has received little attention to date. In a larger sense, this study highlights the potential impact of fertilization upon long-term soil and water quality through the modification of soil physical properties and structure, which can lead to increased risk of soil erosion.
Fig. 5. pH of the different plots as a result of the addition of different amendments (error bars represent 95% confidence intervals). Significant differences for the treatments within each block in the t-test are indicated above the bars (ns: not significant at a p-value of 0.05; ⁎ significant at a p-value of 0.05; ⁎⁎ significant at a p-value of 0.01; ⁎⁎⁎ significant at a p-value of 0.001).
Acknowledgements This work was supported by an INSU EC2CO programme. Dr. R. Paradelo acknowledges the financial support of the Spanish Ministry
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for Science and Technology. The authors acknowledge the help of Jean Pierre Pétraud and Daniel Billiou for sampling and for bulk density measurements. We also would like to express our gratitude to two anonymous reviewers for their useful comments and editorial suggestions which improved the comprehension of the manuscript.
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