【フロー型反応器（stirred, mixed flow reactor）に植物（バナナ、とうもろこし）をセットして溶解実験を行い、植物が玄武岩質岩石の風化に与える影響を検討】
『Abstract
The active role of higher plants in the weathering of silicate
minerals and rocks is still a question for debate. The present
work aimed at providing experimental evidence of the important
role of a range of crop plants in such processes. In order to
quantitatively assess the possible effect of these diverse plant
species on the weathering of a basaltic rock, two laboratory experiments
were carried out at room temperature. These compared the amounts
of elements released from basalt when leached with a dilute salt
solution in the presence or absence of crop plants grown for up
to 36 days. For Si, Ca, Mg, and Na, plants resulted in an increase
in the release rate by a factor ranging from 1 to 5 in most cases.
Ca and Na seemed to be preferentially released relative to other
elements, suggesting that plagioclase dissolved faster than the
other constituents of the studied basalt. Negligible amounts of
Fe were released in the absence of plants as a consequence of
the neutral pH and atmospheric pO2 that were
maintained in the leaching solution. However, the amounts of Fe
released from basalt in the presence of plants were up to 100-
to 500-fold larger than in the absence of plants, for banana and
maize. The kinetics of dissolution of basalt in the absence of
plants showed a constantly decreasing release rate over the whole
duration of the experiments (36 days). No steady state value was
reached both in the absence and presence of banana plants. However,
in the latter case, the rates remained at a high initial level
over a longer period of time (up to 15 days) before starting to
decrease. For Fe, the maximum rate of release was reached beyond
4 days and this rate remained high up to 22 days of growth of
banana. The possible mechanisms responsible for this enhanced
release of elements from basalt in the presence of plants are
discussed. Although these mechanisms need to be elucidated, the
present results clearly show that higher plants can considerably
affect the kinetics of dissolution of basaltic rock. Therefore,
they need to be taken into account when assessing the biogeochemical
cycles of elements that are major nutrients for plans, such as
Ca, Mg, and K, but also micronutrients such as Fe and ‘nonessential’
elements such as Si and Na.』
『要旨
珪酸塩鉱物と岩石の風化における高等植物の能動的な役割は依然として議論のある問題である。本研究は、そのような過程における一群の穀物植物の重要な役割について、実験による証拠を提供するのが目的である。玄武岩質岩の風化において、これらの様々な植物種の可能性のある影響を定量的に評価するために、2種類の室内実験が室温で行われた。36日間にわたって成長した穀物植物が存在する場合としない場合とにおいて、希薄な塩溶液に浸出される時に、玄武岩から放出される元素の量が比較された。Si・Ca・Mg・Naに対して、植物は、ほとんどの場合に1〜5倍の範囲で放出速度の増加を起こした。CaとNaは、他の元素に比べて優先的に放出されると思われ、このことは斜長石が玄武岩試料のほかの構成鉱物よりも速く溶解することを示している。浸出液は中性pHおよび大気pO2に維持されていたため、植物が存在しない場合にはごくわずかの量の鉄が放出されただけである。しかし、植物が存在する場合に玄武岩から放出される鉄の量は、植物の存在しない場合よりも、バナナととうもろこしに対して、100〜500倍も大きかった。植物が存在しない場合の玄武岩の溶解カイネティックスは、実験の全期間にわたり（36日間）、放出速度が変わることなく減少することを示した。バナナ植物が存在してもしなくても、定常状態の値には達しなかった。しかし、バナナ植物が存在する場合に速度は、減少を始める前に、長い期間にわたり（15日間まで）高い初期レベルを維持した。鉄に対して、最大の放出速度は4日後に達成され、この速度はバナナの生長する22日間まで高いままであった。植物が存在する場合の、玄武岩からの元素の放出を促進する可能性のあるメカニズムが議論されている。これらのメカニズムは明らかにされる必要があるが、本結果は明らかに、高等植物は玄武岩質岩の溶解カイネティックスにかなりの影響を与えることを示している。したがって、Ca・Mg・Kのような植物の主要な栄養素だけでなくFeのような微量栄養素そしてSiやNaのような‘不要’元素の生物地球化学循環を評価するときに、これらは考慮される必要がある。』
『1. Introduction
The alteration of rocks at the surface of Earth crust has
long been attributed to the sole meteoric processes, as implicitly
evidenced by the etymology of the word ‘weathering.’ While the
circulating water is certainly a major weathering agent, investigations
on the effects of organic acids on rocks have received great interest
in recent years, in the hope of elucidating the possible role
of these biochemical agents in the weathering of rocks and minerals,
especially in soil environments (e.g., Huang and Keller, 1970;
Schnitzer and Kodama,1976; Razzaghe and Robert, 1979; Tan, 1986;
Barman et al., 1992; Eick et al., 1996a and b). In most of these
studies organic acids were implicitly assumed to originate from
the decomposition of organic matter (i.e., dead parts of living
organisms) by microorganisms such as bacteria and fungi. There
are also numerous research findings which show that microbes can
themselves significantly weather minerals and rocks by excreting
organic acids or siderophores (e.g., Banfield et al., 1999; Brantley
et al., 1999) and possibly other metabolites which influence pH
and redox conditions, as reviewed in details by Robert and Berthelin(1986).
Microbiological activity can thereby be responsible for typical
weathering features as evidenced for glass and olivine by Callot
et al.(1987), for basalt glass by Thorseth et al.(1992), and for
silicates such as feldspars by Jongmans et al.(1997) or olivine
by Banfield et al.(1999).
Although land plants are widespread on the whole surface of the
Earth, there is much less reported evidence of them being directly
responsible for the weathering of rocks (Robert and Berthelin,
1986). Recent papers even showed that whether land plants can
effectively play a significant direct role in the weathering of
rocks and minerals is still a question for debate (Drever, 1994;
Jackson, 1996). This debate, however, focuses on lichens, which
are symbiotic associations of algae and fungi characterized by
fairly low growth rates and nutrient requirements that enable
them to play the role of pioneer vegetation in the colonization
of fresh rocks (Chapin, 1980).
Conversely, other land plants such as higher plants can achieve
very high growth rates, which are associated with large rates
of uptake of water and nutrients. In addition, as their growth
relies on photosynthesis they play a major role in CO2
cycling and contribute to a considerable input of C into the soil
since about 25％ to 60％ of C that is assimilated by the plants
is recovered in the below ground parts of the plants during the
course of their lives (Lambers et al., 1996; Gobran et al., 1999).
About half of this is used for growing roots, the other half being
excreted in the soil as various root exudates including respired
CO2 (Lambers et al., 1996). As Berner(1992,
1999) pointed out, higher plants have thereby played a major role
in the dissolution of Ca and Mg silicates. The C that is released
by roots also stimulates the growth of microorganisms in the peculiar
volume of soil that is surrounding the roots, i.e., the rhizosphere
(Darrah, 1993). In addition to these microbial changes it has
been shown in the recent decades that the roots of higher plants
can also lead to profound changes of the chemical conditions in
the so-called rhizosphere (as reviewed by Darrah, 1993; Marschner,
1995; Hinsinger, 1998).
Uptake of water and inorganic ions (mineral nutrients) at high
fluxes by plant roots is responsible for important mass exchanges
in the rhizosphere that can drastically modify ionic concentrations
in the surrounding liquid phases as reviewed by Hinsinger(1998).
Hinsinger and Jaillard(1993) have shown that the considerable
decrease in K concentration that occurred around absorbing roots
was directly responsible for the weathering of K-bearing phyllosilicates
such as trioctahedral micas that had been reported to occur in
the rhizosphere by several authors (Mortland et al., 1956; Spyridakis
et al., 1967; Hinsinger et al., 1992; Kodama et al., 1994).
The uptake of cations and anions by roots is also directly responsible
for substantial changes in rhizosphere pH (Romheld, 1986; Marschner,
1995; Hinsinger, 1998). Indeed when an excess of cations over
anions are taken up by roots, they excrete protons so as to maintain
their electrical neutrality (Haynes, 1990)., thereby resulting
in a decrease in pH in the outer medium. Such a root-induced acidification
of the rhizosphere has been shown to be responsible for significant
dissolution of Ca carbonates (Jaillard et al., 1991) or phosphates
(Aguilar and van Dienst, 1981; Hinsinger and Gilkes, 1996; Hinsinger
and Gilkes, 1997) and even phyllosilicates (Hinsinger et al.,
1993).
Higher plants can also profoundly affect the redox conditions
in the rhizosphere, which strongly determine the dynamics of Fe-
and Mn-bearing minerals (Uren, 1981; Hinsinger, 1998). Most plants
can reduce Fe (Brown and Ambler, 1973; Bienfait et al., 1983;
Marschner and Romhedl,1994) and should thereby influence the dissolution
of Fe-bearing minerals. Besides, as pointed out earlier, considerable
amounts of C are released by roots as various exudates (Darrah,
1993). These not only feed rhizosphere microorganisms. Some of
them and particularly organic anions such as citrate for instance
(Dinkelaker et al., 1987; Gerke et al., 1994) and nonproteinogenic
amino acids called phytosiderophores (Takagi et al., 1984; Romheld,
1991) can complex Fe and numerous other metals. They can thus
directly affect the dissolution of the solid phases that contain
such metals (Jones et al., 1996; Hiradate and Inoue, 1998). Among
higher plants, the various plant species can considerably differ
in their ability to alter the chemical conditions of their rhizosphere
and, thereby, to adapt to adverse soil conditions (Marschner,
1995): those differences are both qualitative (implied mechanisms)
and quantitative (resulting fluxes). Although the direct effect
of higher plants on the weathering of minerals and rocks has not
been often clearly demonstrated in situ (Gobran et al., 1999),
the abovementioned, recent studies showing the diverse chemical
changes occurring in the rhizosphere suggest that various mechanisms
may be involved.
Evaluating the role of higher plants in the weathering of minerals
and rocks is crucial to a better understanding of biogeochemical
cycles at the surface of the Earth. These processes are a major
source of nutrients for plants and thus need to be better accounted
for in the overall budgets of nutrients in natural ecosystems
and, more critically so in the perspective of sustainable production
of forests (Marques et al., 1997) and agrosystems. In addition,
silicate rocks are now being considered for their potential use
in agriculture as alternative slow-release fertilizers (Barak
et al., 1983; Coroneos et al., 1996) or amendments (Gillman, 1980).
In that respect, too, a better knowledge of how the plants can
affect the kinetics of dissolution of silicate rocks and minerals
is a prerequisite to the assessment of their potential benefit
(Harley and Gilkes, 2000).
The aim of this study was to quantitatively assess the possible
effect of higher plants on the weathering of a basaltic rock,
for a range of plant species. These were selected among diverse
botanical groups. Crop plants were used because of their potentially
high growth rates and correspondingly high requirements for nutrients.
Two laboratory experiments were carried out to compare the amounts
of elements released from basalt when leached with a dilute salt
solution at room temperature in the presence or absence of crop
plants grown for up to 36 days.』
2. Material and methods
2.1. Basalt material
2.2. Plant material
2.3. Experimental set-up and conditions
2.4. Analytical techniques
3. Results
3.1. Analysis of the leachates
3.2. Plant growth
3.3. Plant uptake
3.4. Mass balance of elements in the presence or absence of plants
4. Discussion
4.1. Order of release of elements and congruence of the dissolution
of basalt
4.2. Kinetics of dissolution
4.3. Dissolution rates in the presence or absence of plants
4.4. Possible mechanisms of plant-induced dissolution of basalt
5. Conclusions
Acknowledgments
References