Thursday, 19 January 2012

Nitrogen in the savanna biome

Nutrients, along with fire, water and herbivory, are one of savanna ecology's big 4 and we've covered quite a bit about nutrients in the savanna biome in general already on the blog. So far we've mostly covered the issue in a general sense, not focussing on specific nutrients but a new review by Corlie Coetsee and others of the nitrogen cycling in Kruger National Park (available,but sadly not free to view here) made me think it was time we tackled the issue slightly more specifically.

Middens are amazingly rich patches!
Nitrogen, although the most abundant gas in the atmosphere (nearly 80% of air is nitrogen), is one of the three commonest elements limiting plant growth (the others are the P - phosphorus - and K - potassium - of traditional NPK fertilisers). It's vitally important to life, because it's a major component of protein, but most of that nitrogen int he world is in fact useless for plants or animals being what we call inorganic. Before plants or animals can make use of it a chemical transformation needs to occur from the inorganic form, to a form that is bound to hydrogen or oxygen atoms and can be used by plants. The cycling of nitrogen from inert inorganic forms, to useful organic forms is referred to as the nitrogen cycle, and the rate at which conversions happen are can be critical at determining the fertility of soils. The most important process that converts inorganic to organic nitrogen is driven by various types of bacteria, in particular there's a group called Archaea that we now think probably aren't bacteria at all that play an absolutely critical role in this process.

Termite mounds produce fine-scale variation in nitrogen, Serengeti, Aug 2011
Once in the soil, organic nitrogen can be picked up by plants, which may in turn be eaten by animals who defecate and eventually die allowing (mainly) bacteria to recycle the organic nitrogen to the soil. There's some loss in the process (other bacteria are called denitrifying bacteria because they return the organic nitrogen to inorganic forms) so there's no continual build-up of organic nitrogen in the soil and, as we know, it's often limiting to plant growth - add a nitrogen fertiliser to savanna and plants grow better. But it's distribution across the landscape isn't even and this is where it gets interesting - the full range of processes operating in the savannah affect both the fine and large-scale distribution of nitrogen which, in turn, affects the distribution of plants and animals across the landscape.

Grazing lawns are locally high in nutrient, Kruger, June 2011
We've talked a lot about various ways in which nutrients are moved about the landscape and result in different patterns, and I was pleased that I've not missed many of those Coetsee et al talk about. In fact, seeing them all together in one paper made me realise just how complicated the various processes are in savannahs, and it's worth running over them one more time here. Let's start by talking about large-scale patterns that are ultimately related to geology: in Kruger the higher lands regularly have nitrogen washed out of them with nitrogen accumulating at valley bottoms - exactly as we've talked about for broad-leaved woodlands (the same doesn't always hold in Serengeti as there are some particular fertile patches on a few ridges in the western corridor). And the underlying geology itself affects the soil type, which determines how long nitrogen stays in the soil, another process we've talked about at length. So there's the large-scale process they talk about (they might also have mentioned rainfall gradients being important, as rain can wash organic forms of nitrogen from the atmosphere to the ground, with the effect that wetter areas often have more nitrogen deposition than drier areas). But within these areas there are a large number of fine-scale processes that mean that even within low nutrient areas there can be particular hotspots that are, actually, richer and more fertile than the generally higher nutrient areas: termites are particularly important at concentrating nitrogen around termite mounds (we covered termites). Large animals also move them around a lot (especially if they make middens), and if they develop grazing lawns they can encourage grass with higher nitrogen content to keep growing. We've also mentioned the work of dung beetles here being a further process that creates fine-scale variations in nitrogen content in the soil. And we've talked about how nitrogen fixing legumes - all those trees that were formally known as Acacias - make a huge differece. In fact, Coetsee remind me that other trees too increase soil nitrogen levels by pumping the nitrogen they find deep down up to the surface where they can use it, and they also mention the nitrogen hotspots that develop around waterholes thanks to animal droppings, and issue we didn't mention at all when discussing waterholes in the savanna biome. They also talk about the role of fire and note that in general fire frequency doesn't decrease nutrient quantity in the savanna (except, perhaps, in areas with annual fires - which would actually include much of Serengeti), but that in fact through a complex set of interactions it might lead to increased fertility: when a fire happens it can actually mineralise some nitrogen (nearly making it available to plants) and it certainly stimulates grass growth. This grass is actually higher in nitrogen than the old grass, which encourages animals to come and graze on the new shoots. The animals, in turn, bring their dung and fertilise the soil further. So although it might seem that burning off vegetation will result in a loss of nutrient (all that ash blowing away), in fact the evidence from Kruger at least is that it's at worse neutral, at best beneficial.
This Vachellia tortilis will be creating a nitrogen hotspot. Serengeti NP Dec 11

Put together and you have a picture of variation in nutrient quality at a large scale, with fine scale variatoin superimposed to make an incredibly complex picture. I also like the way they show how the savanna big four influence one another - fire changes nutrients, which changes grazing, which further changes nutrients, etc. It's a wonderfully complicated set of processes, and I thoroughly encourage you to read this paper if you want a more detailed overview of nitrogen in the savanna biome!

Main reference:

ResearchBlogging.orgCoetsee, C., Jacobs, S., & Govender, N. (2011). An Overview of Nitrogen Cycling in a Semiarid Savanna: Some Implications for Management and Conservation in a Large African Park Environmental Management, 49 (2), 387-402 DOI: 10.1007/s00267-011-9779-0

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