The amount of organic material in the soil that comes from fallen leaves is much much lower than most people intuitively think.
Written by : Werner Hendriks
One question we are regularly asked
Because of our urge to clean-up everything, the fallen leaves do not get the chance to compensate for the loss of organic matter in the soil. One question about urban tree planting we receive regularly is: Whether the soil organic matter becomes too low when trees are surrounded by pavement and leaves can no longer decompose in the soil in which the tree grows.
In many natural situations, such as a forest, much of the fallen leaves become part of the soil (mulch layer). But the proportion of soil organic matter that comes from fallen leaves is much lower than most people intuitively think.
The total content of soil organic matter (SOM) in a natural soil comes from three sources:
– Organic waste on the soil surface; fallen leaves and animal droppings.
– Dead organic matter in the soil; Roots, soil biology that die and decompose.
– Exudates; Liquid carbohydrates that flow out of the living root tips into the soil.
The largest contributors to the soil organic material is the tree root system. The fine tree roots but even more importantly in the contribution of SOM are the mycorrhiza associated with the tree roots. And this is only one of the many important functions of mycorrhiza. It is therefore of great importance that this mycorrhiza (and the entire soil biology), should deserve much more attention, especially for those trees surrounded by pavement. Mycorrhiza can save the urban trees.
Many urban trees in streets, parking lots and squares in our cities grow in small tree openings (often 150×150 cm and sometimes even smaller). How does this affect the soil organic matter underneath the pavement? The answer; it depends largely on the soil type and compaction rate, to what extent does the underlaying soil offers an ideal living environment for soil biology. Therefor the real question should be: How does pavement affect the soil conditions to accommodate roots growth and mycorrhiza /soil biology?
For example, Godbold et al (2006) studied the carbon (C) uptake in soil organic matter in relation with leaf litter, roots and mycorrhiza. The results come from a three-year study in a poplar forest in Italy, which indicated that 62 percent of the carbon in the soil organic matter came from mycorrhiza. The comment was made that this proportion is probably higher. The explanation for the high proportion is the fact that the fungal hyphae of mycorrhiza have a very fast turnover rate of only nine days. This is much faster than that of leaves, which are converted once a year, and fine roots, which are converted about three times a year.
If we know the relative influences of fine roots and the associated mycorrhiza versus leaf litter on the formation of soil organic matter, what can we expect as soon as the pavement obstructs the supply of leaf litter into the soil? Knowing many natural environments such as forests, the content of soil organic matter increases until the climatic climax community is reached. This also occurs at places in nature where the fallen leaves don’t get the chance to incorporate back into the soil the tree is growing in (trees against steep cliffs etc.), the soil organic matter also increases in such places.
Also looking at experiences of successful urban tree planting projects over the past decades (first urban tree planting solutions were built in the late 1960s, suspended pavement systems in Boston US, Charlotte US etc.). These projects show the content of soil organic matter has also stabilized and the soil biology is flourishing. Less successful projects often show poor soil biology, often accompanied by a decrease in soil organic matter.
Conclusion: Leaf litter is not needed to maintain soil organic matter content. Everything indicates that soil biology plays the key role in maintaining a healthy soil organic matter content underneath pavement.