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CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration Acid buffering capacity of forest litter
from some important plantations and natural forest in South China
Li
Zhi’an Zou bi Cao Yusong (South China Institute of Botany, Chinese Academy of
Sciences. Guangzhou 510650, China) Abstract: Litter can influence the acidity of the environment, depending on the litter quality. Some crop residues are used to amend the acidity of the soil. Previous research suggests nearly instant response of soil acidity to litter addition. Within days, the soil acidity change after addition of the litter. Rapid response implies that it’s a chemical process, which indicates that the litter can counteract the acid rain if the litter is able to raise the acidity. However, long term response of adding litter to soil is often different from the initial response as the microbiological processes take effects. This study evaluates the chemical buffering capacity of litter to acid or base, which were sampled from both plantation (6 species ) and natural forests (12 species) in south China. The plantation species are mostly used as pioneer species for vegetation reconstruction on degraded lands in this region. In
south China, plantations have been established on a large scale
during last two decades. However, numbers of the most used species
is often no more than a dozen though many species are adopted. Pinus
elliotii and some other pines are the main species used as
pioneer species, covering over half of the newly made plantation. A.mangium
and A.auriculaiformis are
leguminous species, well adaptive exotic species and steadily
increasingly used species in this region. E.
citriodora is another adaptive exotic species having a
relatively long history of introduction from Australia. It has been
largely cultivated as a wood chip source. S.superba
is a native species which can be found in the climax vegetation of
the region. Planting the species is also a widely accepted practice.
Another species Q.acutissima
doesn’t seem to grow well in this region. It’s selected as a
comparison. These materials were got from Heshan long term
ecological station which situated in 112o54' E, 22o41'
N. We
set up 10 litter traps under each of the plantations and collected
the litter monthly. The litter during the dry season (autumn), which
ensured the least leaching by rain, was used to assess the acid
buffering capacity. Litter
from six plantations was cut into ~2 mm pieces. Fifty gram of sample
was soaked for 24 hours in 500ml dilute H2SO4
of pH either 2.7, 3.5 or 5.0, then filtered. The procedures then
repeated further four times with the residue. All extracts were
stored in refrigerator and analyzed for pH, conductivity, and K, Na,
Ca, NO3-, PO43- concentration.
These ions are often considered as the most possible inorganic
factors causing acidity change in plant materials. It
was found that the pH values of extracts of A.mangium
and A.aurifulaeformis litter were much higher than those from
other species. Consequently the buffering capacity of the extracts
was further analyzed by titration. Acids were used for the high pH
extracts (two legume species) and bases for the low pH extracts. Evergreen broadleaf mixed forest has been considered as typical climax model of plant community succession in this region. Succession can be accelerated with the help of mankind by planting species from the climax vegetation. The structure, the composition, and all the materials in such a vegetation have been used as a vegetation prototype. In this context, 12 species are employed from natural forest as a comparison to the plantations. The materials were got from Dinghushan natural reserve situated in 22o10’N and 112o34’E, not far from Heshan station It’s found that the litter of A.mangium and A.auriculaiformis had very high pH values of above 6, with highest pH of 6.5 for A.mangium. Extracts of litter with acid water prepared with H2SO4 (pH5.0, pH3.5 and pH2.7) had unchanged pH of above 6. Other species of either plantations or natural forest all had litter pH values similar to the acidity of soil where they grow. They are generally below pH 4, with lowest pH 3.4 found in E.citriodora. All pH values maintained relatively unchanged in spite of solutions of different acidity. pH values also had little change between different consecutive extractions. The results show that forest litter had high acid buffering capacity. Electric conductivity of litter extracts is very similar among different litters, indicating the all have similar total salt content, but the ion composition of litter extract is different from each other, which probably mainly account for the variation of pH in litter extracts. Inorganic ions could not fully explain the counter effect of litter to the changing environment acidity. However, high Na+ and low NO3- are mainly responsible for the high pH of litter of two legumes, especially NO3- content. Principal Component Analysis shows that first component, which explains about 50% of the total variation, has main loading of NO3-, PO43- and Ca2+ and second component has main loading of K and Na. The PCA ordination figure show that all 6 plantation species widely scatter from each other along the first axis, while not diverge much along the second axis. Among the ions of first component, NO3- exerts greater effect on the acidity, compared to PO43- and Ca, because NO3- is strong acid ion chemically. NO3- is the first factor explaining the current pH pattern of litter extract. It is estimated that the litter produced by two legumes could raise rain acidity by 0.1~0.4 pH units when the rain pass through the litter layer on the ground. Thus, it might be a counter-balanced factor to acid rain in this region. In the
natural forests, the litter has weighted mean pH value of 3.90 of 12
species. Such a low pH could be a driving force for soil (pH 4.19)
to become more acid. The lowest pH is 3.19 of G.oblongifolia
and highest 4.76 of T.odorum. G.oblongifolia
with very low litter pH is not suitable for pioneer species in
restoring the degraded land. It could form a soil habitat of low
acid, which prevents the invasion of other species and retard the
natural succession. Large divergence of litter pH from soil
acidity toward base direction could be an important factor for T.odorum
to be endangered.
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