Come spring, when the seedlings go in, bags of fertilizer pile up at the edge of the garden. Nitrogen fertilizer pushes leafy greens along quickly and deepens their color. But a large-scale analysis by forest researchers around the world shows that this same nitrogen sets off changes underground that are more complicated than we assumed.
Hundreds of millions of living things live in soil
Soil is alive. A single gram of it holds hundreds of millions of bacteria, fungi, and protozoa. As these microbes break down fallen leaves and old root matter, they give off carbon dioxide—researchers call this soil respiration. Nitrogen is the nutrient microbes need to build proteins and multiply. With the right amount of it, they stay busy, and the soil turns soft and dark. If earthworms turn up readily in your beds, that's a sign the microbial life down there is doing well.
When nitrogen piles up, something else happens
The trouble starts when airborne nitrogen pollution settles onto the ground year after year. Nitrogen oxides (NOx) from cars and factories mix into rainfall and accumulate in the soil, a process called nitrogen deposition. In an analysis pooling data from forests across dozens of countries, the first increases in nitrogen sped microbial activity up, and soil respiration rose along with it. Past a certain level, though, some forests showed the opposite: soil respiration slowed markedly. The shift was most pronounced in soils that were already nitrogen-rich or well along in acidification.
Slower soil respiration may look like less carbon dioxide escaping, but it isn't a good sign. When decomposition stalls, a forest's nutrient cycling changes, and less is left for tree roots to draw on. Researchers view this point as a fork that is hard to walk back from. Once the makeup of a microbial community breaks down, recovering on its own takes a very long time.
Keeping your garden soil breathing
Nitrogen pollution in the air isn't something a single gardener can stop. But keeping the microbes in your own beds healthy is well within reach.
- Split synthetic nitrogen fertilizer into several small doses across the season. Applying a heavy amount all at once gradually reshapes the microbial community in the soil.
- Use amendments that bring organic matter along with them, such as compost or worm castings. They help maintain microbial diversity.
- Soil that feels hard, or smells sour, can be showing early signs of acidification. Try working in a small amount of lime to bring the pH to around 6.5.
If you've ever felt the heat rising off a compost pile as the leaves break down, you've already met the very thing researchers measure out in the forest: soil respiration. Before you reach for the fertilizer, look at the color of your soil and feel it in your hand. Checking the soil's condition and adjusting how much fertilizer you give—that's the first step in keeping your garden soil breathing.
Reference: a meta-analysis of the relationship between global forest soil respiration and nitrogen deposition, published in an academic journal in 2024.
