How Plants and Microbes Share Nitrogen in Alpine Heath

Plants draw the resources they require from the soil, with nitrogen near the top of the list. A fertile garden bed contains plenty of it, but supplies become scarce high on a cold mountainside.

It might seem inevitable that roots and the microbes clustered around them would battle for every last trace. But when a research team investigated this apparent conflict on a mountain heath, it found almost the reverse.

A quiet truce

Nitrogen sustains virtually all growing life, from the tiniest microbe to the hardiest mountain shrub, yet cold, sparse soils never provide enough for everyone. Scientists have been trying to understand this shortage for decades.

Dr Ellen Fry, a research technician at the University of Manchester, took a team to alpine heath - the low, wind-exposed vegetation that survives at high altitude - to observe what happened. They added a traceable form of nitrogen to the soil and followed its path.

They used an unusually heavy nitrogen form, rare enough to be easily identified in laboratory measurements. When it appeared later in a leaf, root or group of microbes, the researchers could tell what had taken it up.

Two forms of nitrogen

The labelled nitrogen revealed a clear division. Rather than pursuing the same nitrogen, plants and soil microbes each selected a different chemical form.

Plants favoured the simpler options: ammonium and nitrate, the inorganic forms found in garden fertiliser.

After plants absorbed this nitrogen, it travelled from their roots into their shoots, where it accumulated during the weeks that followed.

Microbes, by contrast, preferred more complex organic molecules, particularly amino acids, which are protein building blocks.

This pattern supports suggestions from an earlier study, although few researchers had observed the separation occurring in actual mountain soil.

The molecules plants leave behind

The research also explored less familiar ground. Scientists had questioned whether plants could bypass the intermediary stage and take complete organic molecules directly from the soil, using amino acids as microbes do.

That was largely not the case in this alpine heath. The researchers found little evidence that plants took up the larger organic molecules themselves.

Instead, microbes seem to break down these molecules first, releasing the simpler nitrogen compounds that plants subsequently absorb.

This sequence offers important insight into how the system operates, although the team is cautious not to draw claims beyond the evidence.

Nitrogen on the move

The process was far from static. Nitrogen taken up by a plant did not remain in its roots; it moved rapidly through the plant’s tissues, reaching the shoots within days of absorption.

Microbes also kept nitrogen supplies in motion, processing organic matter and continuously altering the material available to plants.

This ongoing turnover - the everyday process of nitrogen cycling - determines how much of the nutrient comes within reach of plant roots.

Soil microbes exert substantial influence. At certain times, they retain a large proportion of the nitrogen available in soil within their cells, only returning it when they die and decompose.

Some plants draw more nitrogen

Plants did not respond in identical ways. The dominant, faster-growing species - those already outcompeting neighbouring plants above ground - also absorbed the greatest amounts of nitrogen below ground.

This points to another layer of competition alongside the relationship with microbes: competition among plants themselves. The way species contend for the same nutrient can affect which ones prosper, as other grassland studies have shown.

There is a straightforward logic to this. Rapidly growing plants need more material to produce new leaves and stems, meaning the most demanding growers become the largest feeders and extend their existing advantage.

Higher stakes in poor soil

Alpine and heathland soils are harsh environments: cold and persistently low in nutrients. In such impoverished ground, even a minor shift in nitrogen movement can affect which plants persist and which gradually disappear.

These habitats are also among those being changed most quickly by climate change, as warmer air accelerates soil chemistry.

Understanding that plants and microbes use separate nitrogen forms gives researchers a more precise basis for forecasting whether a warming heath remains intact or starts to break down.

“This work helps us understand how plant and microbial communities share limited resources,” said Fry.

She sees the research as a means of understanding cooperation under pressure.

What this changes

Before this research, the view that heath plants rely mainly on nitrogen processed by microbes was a well-supported suspicion.

There is now field evidence for it: plants and microbes use distinct chemical forms and, for the most part, avoid competing directly.

That certainty gives ecologists a firm foundation for further work.

Models that forecast mountain landscape responses to warming can now represent plants and microbes as partners with separate functions, rather than competitors drawing from a single shared supply.

The same insight may help inform less damaging approaches to managing poor soils and preserving the mixture of species that sustains these ecosystems.