Location of study site.
Land use change (deforestation) has several negative consequences
for the soil system. It is known to increase erosion rates, which affect the
distribution of elements in soils. In this context, the crucial nutrient Si
has received little attention, especially in a tropical context. Therefore, we
studied the effect of land conversion and erosion intensity on the biogenic
silica pools in a subtropical soil in the south of Brazil. Biogenic silica
(BSi) was determined using a novel alkaline continuous extraction where Si
Location of study site.
The terrestrial Si cycle has received increased attention in the past two
decades. Multiple studies show its complexity, with a strong interaction
among primary lithology and weathering, biotic Si uptake, the formation of
secondary pedogenic phases and environmental controls such as precipitation,
temperature and hydrology (Struyf and Conley, 2012). Lithology
controls the primary source of Si through the weathering of silicate
minerals of the bedrock (Drever, 1994). This process provides
Si to the soil solution in the form of monosilicic acid (H
Land use change is a particularly interesting global change driver to address in this context. Dissolution of soil BSi increases immediately after deforestation (Conley et al., 2008), increasing DSi fluxes out of the soil and the ecosystem. However, in the long term, Struyf et al. (2010) showed a decrease in overall DSi fluxes from cultivated land. The conversion from forest to croplands decreases the soil biogenic Si stock, the most important contributor to the easily available Si pool for plants. The decrease in soil biogenic Si stock has been related to two important factors. The first factor is the harvesting of crops (Guntzer et al., 2012; Meunier et al., 1999; Vandevenne et al., 2012). Harvest prevents the return of plant phytoliths to the soil, depleting the phytolith pool. The resulting decrease in DSi availability also reduces the formation of non-biogenic secondary Si fractions (Barão et al., 2014). A thorough analysis separating both biogenic and non-biogenic fractions is crucial in this regard, since traditional extraction procedures to quantify biogenic Si may also dissolve non-biogenic Si fractions. The second factor affecting BSi losses is erosion. In cultivated catchments, preferential BSi mobilization is associated with erosion during strong rainfall events (Clymans et al., 2015). During such events, biogenic Si can represent up to 40 % of the easily soluble Si inputs to rivers (Smis et al., 2011). Clymans et al. (2015) found that Si mobilization did not depend on tillage technique or crop type but solely on soil loss rate due to erosion.
While it is now accepted that cultivation can cause significant changes in soil Si pools and Si fluxes in temperate climates (Keller et al., 2012), the effect of cultivation on (sub)tropical soil Si pools or on soils of volcanic origin is poorly known. Only specific ecosystems, such as rice fields, have been studied (Guntzer et al., 2012) in this regard. Yet, the increasing demand for firewood, timber, pasture and food crops is causing an increase in land conversion to croplands, implying ongoing rapid land degradation in tropical and subtropical forests (Von Braun, 2007; Hall et al., 1993). The aim of our study was to investigate the interactive effects of land use change and terrain slope (as a proxy for erosion) on the distribution of the BSi pool in a subtropical soil system derived from a basaltic parent material. For this purpose, we studied terrestrial Si pools in a natural forest and cultivated land, in gently and steeply sloped locations, applying a recently developed alkaline extraction technique that permits the biogenic and non-biogenic phases to be distinguished.
Diagram of the studied sites and the abbreviations used in the text ordered by ecosystem (F: forest; C: cropland), slope (G: gentle; S: steep), position (T: top; UM: upper middle; LM: lower middle; M: middle; B: bottom) and replicate (R1: replicate 1; R2: replicate 2; R3: replicate 3). Plus signs represent sampling points and yellow circles the selected pits.
The study area is situated near Arvorezinha, in the south of Brazil
(28
The forest site consists of a semi-deciduous forest with
The cropland sites were located in two geographically separated areas, 1.4 km
apart. Deforestation occurred around 50 years ago and they have since
experienced the same historical agricultural practices. Intensive soil
tillage occurred from the time of deforestation to 2003, when a cover
cropping and a minimum tillage practice was introduced
(Minella et al., 2014). The actual soil
tillage is traditional, based on topsoil mixing and making ridges and
furrows. Crops in the gently sloping cropland (maximum 7
Bulk soil samples (
Kopecky ring samples were also collected at each sampled depth. Samples were
weighted before and after drying at 105
One pit per position was selected as a representative pit due to the impossibility of carrying out the novel alkaline extraction analyses on such a high number of samples (297), resulting in a total of 81 samples. The selection avoided pits containing large inclusions (visually) or pits shallower than the other two replicas. The abbreviations and selected pits are shown in Fig. 2.
A portion of the bulk samples was crushed and a subsample was heated at
105 and 1000
Particle size distribution was executed with a Beckman Coulter device
(LSTM-13320) to quantify the sand (2 mm–50
The mineralogy of sand and silt fractions was determined by powder X-ray
diffraction (XRD, Cu Ka, D8). Clay fraction mineralogy was assessed by XRD
after K
All samples from selected pits (
AlkExSi pools or stocks every 10 cm depth (kg Si m
In order to estimate the total biogenic and non-biogenic AlkExSi pools per pit, the sum of all 10 cm depth biogenic and non-biogenic AlkExSi pools of each pit was made.
Once having the biogenic and the non-biogenic AlkExSi pools per pit, averages between the three (for the croplands) or four (for the forests) selected pits were made in order to assign average biogenic and non-biogenic AlkExSi pool values to the slope and to be able to compare AlkExSi pools between different sites. Then, comparisons between the different study sites were made. In order to compare the biogenic and non-biogenic AlkExSi pools from the forests with the croplands, two different methods were considered, taking into consideration that the number of positions along the slope in the forest sites is higher than in the cropland sites (four and three, respectively): Average 1, using all available measurements for the forest (the four positions along the slope) and cropland sites, and Average 2, using a pre-calculated average between upper and lower middle position measurements in the forest sites.
To study the accumulation of biogenic and non-biogenic AlkExSi pools at the
bottom of the slope we have calculated the accumulation (AC) using the pool
in the bottom compared to the summed pools along the slope for the forests
(Eq. 3) and the croplands (Eq. 4). The closer the AC value is to
100 %, the higher the accumulation results.
Results from total element content, particle size, bulk density and TRB
values for selected pits are shown in Tables S2–S4 in the Supplement. The XRD mineralogical
analysis of the bedrock (rhyodacitic volcanic rocks) reveals that sanidine
(feldspar group) is the most abundant mineral (45–55 %), followed by very
fine-grained quartz (
AlkExSi values (mg g
Biogenic and non-biogenic AlkExSi concentrations (mg g
Biogenic and non-biogenic AlkExSi pools (kg Si m
Figure 3 shows the concentrations of biogenic (Si
The biogenic and non-biogenic AlkExSi pools of selected pits at 10 cm intervals are presented in Table S5.
Figure 4 shows the biogenic and non-biogenic AlkExSi pools as a soil profile cut from the top to the bottom of the slope, for the four study sites.
The averages of biogenic and non-biogenic AlkExSi pools per position, land
use and slope are shown in Table 1. As mentioned, other averaged AlkExSi
pools were calculated when comparing forest to cropland (“Average 2” in
Table 1). The pre-calculated average between the upper-middle and lower-middle position was used in the calculation for “Average 2” (Table 1)
(i.e.,
values used for the gentle slope “Average 2” calculation were 16.7 (top),
16.1 (middle) and 6.79 kg m
Biogenic and non-biogenic AlkExSi pools (kg m
While the gentle and the steep slope of the forest showed near-equal
biogenic AlkExSi pools (
In the cropland, results were slightly different. Both AlkExSi pools were
higher on the gently sloped cropland (
When comparing gently sloped forest and cropland (using “Average 2” for
forests), there was only a small difference for biogenic AlkExSi pool
(
On the steep slopes, it was clear that both AlkExSi pools were much lower in
the cropland compared to the forest (
The sum of the AlkExSi pools of selected pits per land use and slope is shown in the Table 1 (“Total (sum)”). The accumulation of the biogenic and non-biogenic AlkExSi pools at the bottom position of each slope is also shown in Table 2. Both steep slopes clearly showed higher accumulation of both pools at the lowest position than the gentle slopes, with the exception of the non-biogenic AlkExSi pool in the steep slope of the cropland.
Pairs showing significant differences are represented with the same letter in Table 1.
One of the most striking observations in our study is the interaction between slope and land use effect. On the steep slope, there is a decrease in AlkExSi pools from forest to cropland. In contrast, the gentle slopes had similar biogenic AlkExSi pools. It is also clear that there is redistribution of biogenic AlkExSi towards the bottom positions of the slope on steeply sloped croplands and forests.
In general, the distribution of biogenic AlkExSi shows the same pattern within each pit: the concentration decreases with depth and highest concentrations are found at the bottom of the slope (with the exception of the gentle slope of the cropland). This agrees with earlier observations on the distribution of BSi along a toposequence in several soil catenas from temperate areas (Saccone et al., 2007). The distribution of non-biogenic AlkExSi shows a complementary pattern. Non-biogenic AlkExSi fractions are rarely present at the top of the profiles but higher concentrations are found in deeper layers. Similar patterns were reported in a study carried out in arkosic sediment soils in California (Kendrick and Graham, 2004) and for temperate Luvisols in Belgium and Sweden (Barão et al., 2014; Vandevenne et al., 2015a). Upon leaching of DSi after BSi dissolution, the DSi infiltrates and reacts to form, for example, secondary clays. It can also be adsorbed onto oxides. The rate of adsorption of DSi by oxides is determined by water infiltration rate, pH, water residence time and weathering intensity (Cornelis et al., 2011; Jones and Handreck, 1963). A large amount of oxides in soil (see “Mineralogy” in Table S4), high DSi supply, strong water infiltration rates and high pH may result in larger concentrations of Si absorbed by oxides. Our studied sites satisfy these conditions with the exception of the pH (4.7–5.9). Uehara and Gillman (1981) suggested that weathered soil systems can result in a desilicated soil enriched in Fe and Al oxides, with pH close to neutral values. Similar processes might occur in our soils, although they are not desilicated, but do show a high weathering intensity.
Biogenic Si concentrations from Vandevenne et al. (2015a) in temperate Luvisols were 1 order of magnitude lower than in our study. The high silica content of the rhyodacite bedrock in our study sites, together with high weathering rates characteristic of tropical and subtropical soils (Drever, 1994), supplies a large amount of DSi to the soil. In addition, weathering stimulated by plants is particularly strong in the tropics (Blecker et al., 2006; Kelly et al., 1998); turnover rates of nutrients are also higher in tropical and subtropical ecosystems than in temperate regions (Alexandre et al., 1997; Derry et al., 2005), due to high water availability and temperature. Meunier et al. (2010) showed that the DSi supply from the dissolution of basalts was 1.8 times higher than the DSi produced from the dissolution of the litter in a Leptosol of La Réunion (Indian Ocean).
For cropland, it is well documented that the harvest of crops exports large amounts of BSi from the system. This generates BSi-depleted systems in the long term (e.g., Vandevenne et al. 2015b). Results from Clymans et al. (2011) in long-term croplands from Sweden showed a BSi pool reduction of 10 % compared to a forested system.
Guntzer et al. (2012) showed the importance of crop rotation in the turnover and accumulation of phytoliths in soil. The accumulation of phytoliths is also influenced by the geochemical stability of phytoliths (Song et al., 2012). However, the crops rotating in both fields are different and have different Si demands. Maize and black oat are known to have high Si content, while tobacco and soy do not (Currie and Perry, 2007; Piperno, 2006). The turnover between maize/tobacco and fallow/black oat on the steep slope might be an explanation for the smaller biogenic AlkExSi pool at this site. Moreover, the higher erosion rate increases the biogenic AlkExSi deposition at the bottom of the steeply sloped cropland. In fact, the TRB in this slope was higher than at any of the other sites (the lower the TRB, the more weathered the soil is, or vice versa – the higher the TRB, the closer the soil is to the composition of the bedrock), suggesting that all weathered material has been already eroded and the saprolite is closer to the surface.
It is interesting to note that a redistribution of biogenic AlkExSi occurs along the slope (Fig. 4). A higher slope degree, and thus higher erosion rate, provokes the loss of material through water erosion and tillage (Govers et al., 1996), transporting material downslope and resulting in an accumulation of the biogenic AlkExSi pool at the bottom of the slope. In the gently sloped sample site, biogenic AlkExSi is more stable at the higher positions of the slope, while in the steep slope it accumulates at the bottom.
The biogenic AlkExSi pool in the gentle slope of the forest was
The biogenic AlkExSi pool was not enriched at the lowest position of the gently sloped forest. This suggests that the physical erosion at this site is low. In the steeply sloped forest, higher erosion rate apparently did provoke the physical loss of biogenic AlkExSi, potentially decreasing the amount of Si recycled by the vegetation. BSi is consequently transported to the bottom of the slope before it can dissolve and be recycled by plants, resulting in an accumulation of BSi at the bottom of the slope (AC of 37 %). However, this apparent effect is not statically confirmed probably due to the strong variability of biogenic AlkExSi pools within the top and the bottom pits in the steep-sloped forest. Larger biogenic AlkExSi pools are also found at the lower-middle position, which suggests that the accumulation of eroded material also occurs at the lower-middle slope. Both (lower-middle and bottom) pits together accumulate the 76 % of the total biogenic AlkExSi pool of the slope. These deposition zones could serve as a location for permanent BSi storage.
The average biogenic AlkExSi pool size followed the sequence FS > FG > CG > CS. Overall, cropland gentle and steep slopes had 10 and 53 % lower biogenic AlkExSi pool, respectively, compared to well-conserved forest. This loss of biogenic AlkExSi has previously been described in other studies. Vandevenne et al. (2015b) showed similar results for temperate Belgian Luvisols, where croplands showed a decrease in total biogenic AlkExSi of 35 % compared to the temperate forest. Results from Clymans et al. (2011) support the same pattern, showing smaller AlkExSi pools in cultivated systems in Sweden. Our results are apparently in contrast with results from Struyf et al. (2010), who showed a large reduction in DSi export after deforestation in croplands deforested > 250 years ago. Nevertheless, the absence of a larger decrease in the gently sloped cropland may indicate that deforestation occurred too recently to see such a decrease, only triggered by harvest. Opfergelt et al. (2010) found phytoliths from the previous forested system in croplands of Cameroon deforested in the early 1950s. However, top and bottom positions do not differ statistically between the cropland and its forest counterpart for any of the slopes. The difference relies only on the mid-positions, where erosion is higher (Doetterl et al., 2015), highlighting the importance of erosion as an added factor, as a consequence of the agricultural tillage (Govers et al., 1996).
A depletion of > 50 % is seen at the steep slope of the cropland compared to its forested counterpart. Although it has been shown that an increase in erosion rate occurs after the conversion from forests to croplands (Vanacker et al., 2014) and this may affect both croplands, Montgomery and Brandon (2002) described how the erosion rate depends directly on the slope and stressed the importance of landslides. The consequence is an increase in the accumulation of biogenic AlkExSi pool at the bottom of the steep slope of the cropland (AC of 67 %).
The present study clearly shows how deforestation may have a strong impact on the silica cycling in subtropical soils under steep slopes, and potentially also on gentler slopes in the long term. The croplands in earlier studies, e.g., Vandevenne et al. (2015b), had usually been cultivated for more than 200 years, and BSi depletion was explained as a result of long-term cultivation. However, the croplands in the present study were deforested 50 years ago, highlighting how fast the biogenic AlkExSi pool can be depleted from the soil system when physical erosion is high.
Our results confirm the importance of using a continuous extraction to
determine BSi pools in soils (Barão et al., 2014).
The non-biogenic AlkExSi fractions would have been determined as BSi if
conventional alkaline extractions, applying only analyses during the linear
phase of the extraction, had been used (i.e., adaptations of the method from
DeMaster, 1981). We acknowledge that some difficulties still
remain when applying the method we have used. The dissolution in NaOH does
not show a true reactivity within soils: the non-biogenic AlkExSi fractions
probably have lower solubility in soils (Ronchi et al.,
2015) or water
(Unzué-Belmonte et al.,
2016) than BSi. Using the Si
The averaged total pool of non-biogenic AlkExSi followed the sequence FS > CG > FG > CS. A study in Belgian Luvisols under long-term cropland management (Vandevenne et al., 2015a) showed a larger non-biogenic AlkExSi pool in the croplands relative to a forested site. The authors explained the result by the fact that the high Si demand from the crops increases the weathering rate of the mineral phases, transforming low-solubility compounds into high-solubility ones (with the caveat that solubility is determined in NaOH). A combination of a relatively short time period since deforestation and the increased demand for Si by the crops compared to forest species could thus explain the larger non-biogenic AlkExSi pool in gently sloping cropland, compared to forests.
However, the non-biogenic AlkExSi pool of the steeply sloping cropland is almost non-existent. As with the biogenic AlkExSi pool, the high Si demand by crops together with the higher erosion rate results in a complete depletion of the non-biogenic AlkExSi pool in the steeply sloped cropland.
The steeply sloped forest showed a larger non-biogenic AlkExSi pool, mainly accumulated at top and bottom positions (Fig. 4). It is clear that the continuous long-term biogenic AlkExSi deposition at bottom positions (apparent also at the lower-middle position) triggers the formation of new non-biogenic AlkExSi phases that correspond with lower TRB values. Weathering degree has previously been correlated to the amount of pedogenic silica accumulation in sedimentary soils (Kendrick and Graham, 2004). Further, clay minerals and Si adsorbed onto oxides were reported by Delvaux et al. (1989) and Opfergelt et al. (2009), respectively, to be largest at most weathered sites in a study carried out in volcanic soils from Cameroon.
We show how slope and land use change have strong interacting effects on the distribution of the AlkExSi pool in a subtropical soil. In general, our study agrees well with earlier findings in temperate climates: landscape cultivation diminishes soil BSi stocks. Even though deforestation occurred only 50 years ago, the biogenic AlkExSi pool in the steeply sloped cropland was only 50 % of the pool in steeply sloped forests. In contrast, on the gentle slopes, no similar depletion was observed. This highlights the importance of erosion strength for the rate of depletion. To our knowledge, almost no studies have included slope as a potential factor (Ibrahim and Lal, 2014). It could therefore also be relevant to include erosion rates in studies of BSi in temperate ecosystems.
The presence of phytoliths from the past in soils helps to reconstruct former vegetation (Kirchholtes et al., 2015; Rovner, 1971). Here, we consider the biogenic Si pool as a single biogeochemical pool that is able to supply readily available DSi for plants. Although the presence of two Si pools within the plant is well documented (Fraysse et al., 2009; Watteau and Villemin, 2001) and different pools may show different solubilities, the higher solubility of phytoliths in soils compared to non-biological solid Si phases has been confirmed by several studies (Fraysse et al., 2006; Lindsay, 1979; Ronchi et al., 2015; Sommer et al., 2013). Moreover, Alexandre et al. (1997) described how 92 % of the BSi in top soil is rapidly recycled, while only 8 % seems to be permanently stored due to a lower turnover.
The silicon and carbon cycles are closely related through the production of phytoliths. A recent study showed a positive relation between soil organic carbon (SOC) and amorphous silica content along a toposequence and along the depth profile (Ibrahim and Lal, 2014). However, a comparison between their results and ours is not possible due to the different methods used to extract the silica fractions. The assumed tight relationship between both elements together with the SOC depletion (reported at 45 %) after 11–50 years of conversion from forest to cropland (Wei et al., 2014) hints at similar mechanisms behind both observations. Some studies have indicated that silica could act as a “carbon protector” through phytolith formation: carbon is occluded within the phytoliths and remains stored until they dissolve (Song et al., 2014). Although there are different opinions regarding this topic (Santos and Alexandre, 2017) some have suggested that atmospheric carbon sequestration could be enhanced through phytolith production and subsequent burial (Li et al., 2013; Parr et al., 2010; Song et al., 2016).
Our study highlights the accumulation of biogenic AlkExSi at deposition zones in croplands. Very little is known on the potential Si sink associated with such deposition zones, as little research has actually focused on Si biogeochemistry in these zones. Deposition of BSi here could be an important sink for Si in the long term. As shown earlier in tidal marshes (Struyf et al., 2007), rapid accumulation of BSi can prevent its complete dissolution, resulting in long-term burial and removal from the global biogeochemical Si cycle.
Particle size distribution can be found under
The authors declare that they have no conflict of interest.
We thank BELSPO for funding project SOGLO (The soil system under global change, P7/24), all the members of the SOGLO project and the University of Santa Maria for their help during field work in Brazil. Dácil Unzué-Belmonte also thanks the Soil System Sciences Division of the European Geoscience Union (EGU) for awarding her the best Outstanding Student Poster Award at the 2014 EGU Assembly. Edited by: Miriam Muñoz-Rojas Reviewed by: two anonymous referees