Once in a great while, you come across a scientific paper that is so illuminating and information rich, you keep it on your desktop to reread and refer to over and over again. That is the case for a 2008 review paper by Hans Lambers and his team. I encourage all of you to download this comprehensive review, so you too can refer to it on a monthly basis. When you are asked to write a review paper, you are regarded as an expert in your field-a type of role model for other scientists studying your specific discipline. What these scientists accomplish is nothing short of a timeless paper to be enjoyed for the ages. In this post, I will sum up their review which talks about how nutrients vary with soil age, and what plant and fungal adaptations coalesce with the different stages of soil erosion.
The first main concept to understand is how the two main soil nutrients (phosphorus and nitrogen) enter the system. Inorganic phosphorus is locked up in “parent” material- the rocks resting beneath the soils depth. This means that young soils have the highest amounts of phosphorus, and over time the amount of phosphorus in a given ecosystem decreases as erosion weathers away this vital nutrient. Nitrogen follows a different trend altogether, as newly formed soil has low levels of nitrogen, until nitrogen fixing organisms gain a foothold in the new habitat. Since the organisms that fix nitrogen require phosphorus, nitrogen becomes limiting once phosphorus levels are reduced. This is seen in legumes living in soils with low phosphorus, as they will not nodulate if they are not colonized by arbuscular mycorrhizal fungi (AMF). It would be incredibly interesting to see if these legumes with combined strategies, evolved on young soils (not the youngest soils) where phosphorus has rapidly decreased, while nitrogen loads are still scant.
These patterns of soil nutrient load do not always correspond with the age of the soil. Huge glaciation events like the last ice age rejuvenated relatively old soils of North America and Eurasia, as giant swaths of ice deposited massive amounts of nutrients once receded. Places occurring near the vicinity of flat plains with strong winds are revitalized by wind-blown nutrients. Systems near the base of mountain regions receive nutrients carried by gravity. Habitats with ancient soils in close proximity to the ocean obtain a constant supply of nitrogen and phosphorus by direct marine deposition. So again, soil nutrients are influenced by several processes, but clearly, time is a heavy driver of many species adaptations.
This paper discloses the different functions of both type of mycorrhizal fungus, for AMF and ectomycorrhizae (ECM) are not completely analogous. AMF ‘scavenge’ soils nutrients while ECM both ‘scavenge’ and ‘mine’ for soil nutrients. What this means is that both fungal types with their fine hyphae can maneuver their way into tight pockets of soil to locate and transfer available nutrients to their plant host. However, AMF isn’t very compatible with mobilizing bound up nutrients via enzymatic processes. By contrast, ECM can access nutrients that would otherwise be inaccessible to plants by exuding hydrolyzing chemicals. Because of this, we tend to see ECM plants on more developed soils, where there is a less readily available phosphorus.
In the most ancient soils on the planet found in Australia and South Africa, some plants have evolved a different strategy to acquire bound soil nutrients. Interestingly, these plants do not form symbiotic relationships with fungi; their roots do the scavenging and mining for them. Plants in the families Proteaceae and Cyperaceae have evolved structures called cluster roots, which are densely branched root hairs that release carboxylates. It is these enzymes that mobilize the scant nutrients present in these ancient soils. Symbiosis with fungi may not be useful in these habitats for a few reasons, but some suggest that paying mycorrhizae with plant sugars is too costly. It can also be that mycorrhizae don’t efficiently function due to climatic or regional inferences- too little water, not enough nutrients, or not enough plant production to receive a sugary reward.
It is important to note, that these trends are not a law that plants and fungi abide by. Plants that form cluster roots are also found in newly formed soils, but only in regions with extreme basic or acidic soils because in these pH settings, phosphorus is immobilized. Some plants have completely avoided the nutrient drawbacks present in their soil, by gaining nitrogen and phosphorus through insects. It’s neat to comprehend that insectivorous plants like pitcher plants and sundews have evolved aboveground structures to acquire nutrients, instead of below ground adaptations. Plants and fungi make an attempt to live wherever they are dispersed because, well, that’s what species do. This paper simply goes over the importance of each strategy relative to soil age, not the abundance of species type.
Good papers are like cinematic masterpieces; every time you watch it, you pick up something new. Again, I urge you to download this paper for yourself. In a previous post I explain why temperate regions have the most fungal rich assemblages on the planet. I describe that the lower plant diversity at temperate latitudes increases the continuity of underground hyphal networks, which allows the ecosystem to function more efficiently. An efficient ecosystem transfers more sugars to mycorrhizae which in turn supports more fungal species. I stand by those remarks, but what I can add is that the temperate ecosystems that support the fungal diversity we cherish, may also be a function of soil age. The soil age of these regions is not ancient, and may have the specific loads of nutrients that fungi are fine tuned to function from. The forest floor is a place influenced by so many dynamic factors. It’s amazing to think that the forest floors most important driver may be something not dynamic, but the constant, impending ticking of time.