This is the conclusion of CSIRO scientists Andrew Noble and Peter Randall based on field and laboratory studies conducted on behalf of the RIRDC/LWRRDC/FWPRDC Joint Venture Agroforestry Program (with the support of the Vincent Fairfax Family Foundation). The research, which has been published in the scientific literature, has broken new ground in defining the potential of trees to counteract this damaging phenomenon.

The nature of soil acidification

Soil acidification may be thought of as the progressive lowering of soil pH. It is a natural process that occurs slowly during the weathering of soils, but it can be accelerated by agricultural practice. The most acidifying systems are those where large amounts of biomass are harvested and removed from the land, where the recycling of nutrients is inefficient and nitrate is allowed to leach from the soil, and where ammonium-based fertilisers are used.

Acid soils have a number of undesirable characteristics. At low pH, soil minerals such as aluminium and manganese are mobilised in the soil solution at levels that can be toxic to plants. Conversely, fertility declines as soils acidify: essential nutrients such as phosphate and molybdenum become less available, while leaching may deplete cations such as calcium, magnesium and potassium.

Can trees help alleviate soil acidification?

According to Noble and Randall, trees could have several effects:

Noble and Randall set about testing the first three of these processes (the fourth being examined in a separate project). Their work had five components:

Results

The researchers found large differences in ash alkalinity between species. In general, the native species tested – mainly eucalypts and acacias – had lower nutrient contents and ash alkalinities than the exotic species. Melia azedarach (white cedar) was a notable exception: this species, a native of Australia’s east coast, had the highest ash alkalinity of all species tested. An exception amongst the exotics were Quercus (oak) species, which had very low alkalinity.

Leaf litter as neutraliser of soil acidity

As expected, the addition of lime to an acid soil increased pH. All leaf litters also raised pH above the control, even at the lowest application rate (Table 1). However, in all cases except that of Melia azedarach, successive increments of litter had progressively less effect. The least effective species in raising soil pH at the highest application rate were all exotic: Q. palustris, Q. robur and Liquidambar styraciflua.

The capacity of leaf litter to complex aluminium ions

It is well known in agriculture that the addition of organic mulches and manures to acid soils can reduce levels of phytotoxic aluminium and increase crop yield by binding or ‘complexing’ the aluminium ions to organic molecules.

Noble and Randall tested the ability of tree leaf litter to perform the same function, thereby causing similar reductions in phytotoxic aluminium. As with ash alkalinity, they found that there were considerable differences between species. M. azedarach, for example, was much less efficient than Castanea sativa (chestnut) at complexing aluminium in a non-labile (or non-active) state. Nevertheless, as the researchers point out, this effect may be temporary, disappearing once the responsible organic molecules decompose. An increase in pH something that Melia azedarach is capable of bringing about may have a more lasting effect.
 

Table 1: The influence of lime and leaf litter material from selected tree species on soil pHCa after an 8 week incubation at field moisture capacity.
 

Lime or litter source	Application rate (t/ha)	pHCa
Lime	0.5	3.69
	1.0	4.03
	2.0	4.45
	4.0	5.26
Melia azedarach	5	3.95
	10	4.24
	20	4.68
	40	5.64
Populus nigra	5	4.10
	10	4.34
	20	4.55
	40	4.88
Eucalyptus globulus	5	3.95
	10	4.22
	20	4.47
	40	4.69
Acacia mearnsii	5	4.05
	10	4.01
	20	4.13
	40	4.45
Pinus radiata	5	4.04
	10	4.18
	20	4.32
	40	4.40
Quercus robur	5	3.83
	10	3.95
	20	4.17
	40	4.34
Liquidambar styraciflua	5	3.86
	10	3.88
	20	4.02
	40	4.08

Comparing soil acidification in the field

Another way of testing the effect of trees on soil acidification is to compare soils on adjacent sites, one of which is planted with trees and the other used for pasture. Noble and Randall made comparisons of this nature for six tree plantations:

Results showed that the pattern of distribution of cations – such as Ca2+ and Mg2+ and pH with depth in the soil is different under tree plantations than under adjacent pasture. With the exceptions of some parts of the cork oak plantation, the pH in the tree plantation was always higher than outside.

This difference could be interpreted in several ways. It could mean that pH has increased under the trees, it could mean that pH has declined in soils dedicated to pasture over the life of the plantation, or it could be a combination of both effects. The researchers say that while the cessation of agricultural production in the planted areas would have slowed acidification, the possibility that trees might have actually raised pH and the levels of cations on the surface was supported by observations at the M. azedarach site. There, control samples taken from the grazed pasture and a roadside (which would have had limited or no use as pasture) away from the trees were similar to each other but different from the samples taken under the trees.

Factors affecting ash alkalinity of trees

Early on in their study, Noble and Randall had noticed variation in alkalinity in different individuals of the same tree species. So they set out to determine whether factors such as seasonality and soil might influence the alkalinity of litter. To address the first factor, they sampled individuals from three species at different times of the year, measuring calcium content and from this calculating excess cations, which in turn gave an estimate of ash alkalinity. They found that both calcium and excess cations increased over the period November to April for all three species.

The effect of soil was tested in a series of glasshouse trials. Taken together, say the researchers, these trials indicate that the supply of soil nutrients can have a substantial effect on cation excess and thus ash alkalinity of leaves. Accordingly, litter from the same species may differ in its effect on soil acidity depending on where it is grown.

Summing it all up

But there are some provisos. First, any reduction in soil acidity in the surface layers of the soil occurs at the expense of increasing acidity elsewhere in the soil profile. This is a potential problem where subsurface acidity is an issue, but it may be entirely beneficial in soils where roots can take up cations from a neutral or alkaline B horizon.

Another issue is the fate of the biomass. If litter is burnt and the ash is blown or washed from the site, or if timber and especially the more nutrient rich leaves and bulky material is removed from the site, the net affect will be further soil acidification and a loss of soil fertility.

To help in species selection, Noble and Randall have proposed an index to classify the potential of species for their long term effect on soil acidification. This, they hope, will provide a framework for decisions about the desirability of planting trees in particular soils and for matching tree species to sites – something they say is essential for the sustainable management of agroforestry systems.