Grassroots
Augustus 2017
Vol. 17, No.3
April 2017
Vol. 17, No. 1
OPINION
9
Nitrogen and grass-legume pastures: How does it work?
Bernhard Jordaan
Department of Agronomy, Stellenbosch University
t is common practice for dairy farmers to include a legume component in their grass pastures. Legumes have the ability to
transform atmospheric nitrogen (N), which is not otherwise available for use by plants, into plant-available forms of N. This is achieved via a symbiotic relationship with Rhizobium bacteria, that are capable of harnessing atmospheric N through a process called N fixation. The bacterium induces the formation of nodules (N-fixing organs) on the roots of the legume. Nitrogen is transported from the roots to the rest of the plant where it is incorporated into proteins, the building blocks of vegetative growth. Some of the N produced by N fixation is also released to grasses when legume roots, stems and leaves decompose or via urine and manure from grazing animals.
However, it is unclear how much N is released by the legumes and whether additional N fertilization is required to maintain the grass component. Moreover, it has been found that legumes persist poorly when established as a grass-legume pasture. To understand this limitation and adapt management accordingly, the relationship between N and the grass-legume pasture must be explained.
Nitrogen release and availability
For fixed N to become available to the grass, it must be transferred from the legume to the grass. Nitrogen can be transferred by one of two pathways. Firstly, N can directly be released (excreted) from the nodules. The released N is immediately available for uptake by the grass. Secondly, the N incorporated as plant protein is released once the legume material dies off and starts to decompose. Nitrogen released from decomposed material takes place over time and, therefore, the N is not immediately available for grass uptake.
For a grass-legume pasture, N transfer is generally lowest in the first year after establishment and mainly attributed to direct root release. In the second year, direct root release as well as decomposition contributes to N transfer. Nitrogen transfer can be five times higher in the second year following establishment than the first year. In the second, year of a perennial ryegrass-white clover pasture, N fixation can equate up to 150 to 200 kg N/ha per annum of which 50 kg N/ha per annum can be transferred to the grass component.
The influence of nitrogen fertilisation
Nitrogen fertilisation directly influences the amount of N fixed by the legume. High levels of soil N inhibits N fixation. Therefore, when soil N increases N fixation decreases.
Not only do high rates of N fertiliser have an inhibitory effect on N fixation, it also reduces legume persistence. Under high N levels in soil, grasses rapidly take up N and out-compete
legumes for light and water. Under these conditions grasses dominate the pasture and
legumes persist poorly.
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