Nitrogen Transfer
Forage legumes contain from 3 to 4% nitrogen that can come from both the soil and air. Legumes are generally grown with grasses in the hope that the legume will provide nitrogen for the grass and thereby eliminate or reduce the need for applied commercial nitrogen fertilizer. Grasses have a fiberous root system that is more efficient at extracting nutrients and moisture from the soil than legumes with a tap root system. Therefore in a grass-legume mixture, the grass will utilize more of the soil nitrogen. As the soil nitrogen level decreases, legumes will obtain more nitrogen from the air and be less competitive with the grass for limited soil nitrogen.
A common misconception is that the nitrogen is released into the soil from the legume roots. Research has shown there is a release of some soluble nitrogen compounds such as amino acids and ammonium from intact legume roots and nodules, but it is an insignificant amount. The primary pathways for nitrogen transfer from the legume to the soil are through grazing livestock and decomposition of dead legume plant material. When legume forage is consumed by grazing livestock, from 80 to 90% of the nitrogen in that forage passes through the animal and is excreted in the urine and feces. Unfortunately about 50% of the nitrogen in the urine is lost through volatilization. Another problem is the distribution of feces and urine on the pasture. With continuous grazing at low stocking rates, much of the animal excreta is concentrated around the water source and under shade trees. Animal excreta distribution is improved with moderate to high stocking rates and with rotational grazing systems where stock density is higher.
The root system and unused leaves and stems of annual legumes die at plant maturity and are decomposed by soil microbes over time. Nitrogen contained in this plant material is released over time and is available to other plants. However, because most of this nitrogen is not available until after the legume dies, only grasses that follow the legume growing season can use it. This is a major nitrogen transfer pathway for cool-season annual legumes overseeded on warm-season perennial grasses because the clover-growing period occurs before the warm-season grass-growing period.
Forage legumes contain from 3 to 4% nitrogen that can come from both the soil and air. Legumes are generally grown with grasses in the hope that the legume will provide nitrogen for the grass and thereby eliminate or reduce the need for applied commercial nitrogen fertilizer. Grasses have a fiberous root system that is more efficient at extracting nutrients and moisture from the soil than legumes with a tap root system. Therefore in a grass-legume mixture, the grass will utilize more of the soil nitrogen. As the soil nitrogen level decreases, legumes will obtain more nitrogen from the air and be less competitive with the grass for limited soil nitrogen.
A common misconception is that the nitrogen is released into the soil from the legume roots. Research has shown there is a release of some soluble nitrogen compounds such as amino acids and ammonium from intact legume roots and nodules, but it is an insignificant amount. The primary pathways for nitrogen transfer from the legume to the soil are through grazing livestock and decomposition of dead legume plant material. When legume forage is consumed by grazing livestock, from 80 to 90% of the nitrogen in that forage passes through the animal and is excreted in the urine and feces. Unfortunately about 50% of the nitrogen in the urine is lost through volatilization. Another problem is the distribution of feces and urine on the pasture. With continuous grazing at low stocking rates, much of the animal excreta is concentrated around the water source and under shade trees. Animal excreta distribution is improved with moderate to high stocking rates and with rotational grazing systems where stock density is higher.
The root system and unused leaves and stems of annual legumes die at plant maturity and are decomposed by soil microbes over time. Nitrogen contained in this plant material is released over time and is available to other plants. However, because most of this nitrogen is not available until after the legume dies, only grasses that follow the legume growing season can use it. This is a major nitrogen transfer pathway for cool-season annual legumes overseeded on warm-season perennial grasses because the clover-growing period occurs before the warm-season grass-growing period.
