The impact of cattle on the land can be broken down into grazing, hoof action, and inputs (mostly manure and urine, but also dead animals in the wild). For more details on raising cattle see How we Care for our Cattle.
[The following is excerpted from Julius Reuchel's Grass-fed Cattle (2006).]
Context: Nutrient Cycling
"In order for nutrients to be cycled back to the earth quickly, the microbes that decompose dead plant and animal remains must be active... in dry regions the microbes work efficiently only in the moisture zone below the soil surface. Until dead material can make contact with the soil, it remains untouched by these microbes. Nutrients from these dead organic remains are not recycled back to the soil for future plants; instead, they disappear into the atmosphere through oxidation [producing greenhouse gasses since they are mostly carbon-based compounds] or are broken down by the wind, ultraviolet sunlight, and physical weathering until they blow away as dust." "The decomposition microbes need the right balance of moisture and temperature to survive and work efficiently." (ibid, page 12.)
"Animal grazing plays an important role in ecosystems: animals eat grass before it can become old, dry, and unpalatable. Periodic grazing maintains grass in its growth (or vegetative) stage, during which the plant roots spread out, much as they do in a lawn. As the grass extends across the soil, it becomes an insulating layer that shields the earth from direct heat, which in turn helps to retain moisture. When the rains finally come, the carpet of live grass and dead grass litter slows the water runoff, giving it more time to be absorbed by the soil. The more water is absorbed, the more water is stored and the longer it will take the soil to dry out after the rains. It sounds like an ideal arrangement because it is. Grazing animals and grass are a perfect match; they coevolved twenty-four million years ago to take advantage of each other's best traits." (ibid, page 12.)
"Animal feet knock over the dead plant material, driving it into the ground so it contacts the microbes in the moist soil. Looking closely at the feet of the majority of animals that make up the great herds (including cattle), we can see that most have two toes on each foot. As they step, and especially as they step violently, these flexible toes twist and flex, particularly at the front edges of the hooves, where they are the sharpest. The sharp hooves slice up the dead plant material as they push it down into the moist soil, where the active microbes are waiting for lunch, and also fracture the ground, allowing rainfall to penetrate easily through the hard crust on the soil surface. Plant material that has been trampled further slows rainfall runoff, and the depressions left by the animals' hooves create little pools to hold water." (ibid, page 12.)
"After dead [sic] plant material has passed through the digestive tract of a grazing animal, the finely chewed and partially digested material becomes much easier for soil microbes to digest. Manure piles also provide a perfect moist environment in which soil microbes can flourish. This is why cattle, bison, and so many other grazing animals have such seemingly inefficient digestive processes:... partially digested plant remains play a vital role in feeding the soil microbes, which keep the soil fertile." (ibid, page 13.)
"Plant digestion in the soil has a great deal in common with plant digestion in the rumen of the cow; in fact, they are intimately linked in a healthy grazing environment. In the first step of plant digestion in the cow, grass must pass through the rumen, the first chamber of the cow's stomach, which contains a host of microorganisms responsible for fermenting and breaking down grass's tough cellulose structure so the cow can extract nutrients from the cellulose as it passes through the rest of the digestive tract. Many of these rumen microorganisms are the same as those that live in the soil, where they are responsible for the decomposition of plant materials, likewise breaking down or "digesting" the tough cellulose structure of dead plant remains to extract nutrients from them for the use of future plants and other soil microorganisms." (ibid, page 13.)
"The soil can support only as many decomposition microbes as it can feed from dead plant remains in the earth. Very little dead plant material is accessible to the soil microorganisms until grazing animals trample the plants and return partially digested plant remains to the soil via their manure. Every time manure falls onto the ground, an enormous number of rumen microorganisms are carried with it and are incorporated into the soil by the animals' trampling hooves. Because these microorganisms are the same as the soil microbes responsible for plant decomposition, this flood of rumen microorganisms boosts soil microorganism populations at the very instant when passing herds' migration provides a surge of dead plant remains. Thus, plant decomposition can take place quickly within the limited time frame available." (ibid, pages 13 and 14.)
"Without the bison, the plains left behind by the receding ice-age glaciers would have slowly turned to brush and forest, which recycle nutrients and build organic matter at rates far slower than grassland under the influence of a migratory grazing herd" (ibid, page 9).