The second contribution, which is still a little bit controversial, is the contribution to soil fertility through nitrogen fixation. We know the crust fixes nitrogen and we know that in some deserts it's very significant. In the Colorado Plateau and in the Great Basin it's been demonstrated through the study of nitrogen isotope ratios that nitrogen fixation is an important source of fixed nitrogen. We have two sources of nitrogen in a desert. One is dry fall coming out of the atmosphere and the other is what is fixed by nitrogen-fixing microbes. It turns out that most of the nitrogen in soil is fixed by nitrogen fixation. It's not dry fall. So this process is very vital to desert ecosystem productivity.

A microbiotic crust also helps the soil retain fine particles: the silts and the clays. One of the first things you note when you find an uncrusted soil is that it has a much coarser texture. Now the significance of this for plant nutrition is that it is the fine materials that provide the chemical safe sites for things like calcium and other micronutrients. So if you lose your fine particles you lose a lot of the micronutrients. Another factor in micronutrient availability to plants is the evidence that cyanobacteria produce chelating agents that will make certain micronutrients more available to organisms. Kim Harper has done some work with Jayne Belnap and some others where he has shown that plants growing in an area that is well crusted have much better plant nutrition. If you look at the nutrient content in their leaves, it is much higher and much better in crusted soils than in areas where the crust has been destroyed. So, even though you may not pick up a significant difference when looking at the micronutrients in the soils, the plants are affected by it.

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