FEATURE
2016 ). Their research in the Serengeti demonstrated that endophytic diazotrophs living in the roots of Themeda triandra , the dominant grass in the area , were fixing labelled gaseous 15 N , and that grass-associated N fixation could rival or even surpass that of a common legume in the Serengeti . Biomolecular techniques confirmed the presence of the nifH bacterial gene that codes for nitrogenase . Nitrogenase is the enzyme that breaks down the strong triple bond between two gaseous nitrogen atoms ( N 2
) and converts them into ammonia ( NH 3
), a form of nitrogen that can be utilized by living organisms .
The nitrogenase gene has also been found associated with the roots and even inside leaves and stems ( borne by endophytic bacteria ) in several other wild grass species . Therefore , there should be no reason why mesic grasses in southern Africa should not also be obtaining significant amounts of fixed N from root or even leaf and stem diazotrophs .
Figure 2 . The response to mineral nitrogen addition ( relative to unfertilised controls ) of the four study grasses , which are hypothesised to harbour the nitrogenase gene ( nifH ) in amounts inversely related to the magnitude of their response to soil N . ns = p > 0.05 .
Figure 3 . The mean (± se ) relative copy number of the nifH gene associated with the roots of four grasses after an old burn and in newly-burned mesic grassland in Pietermaritzburg . A . junciformis = Aristida junciformis ; E . curvula = Eragrostis curvula ; T . leucothrix = Tristachya leucothrix ; T . triandra = Themeda triandra .
the topic of N-fixation in grasses while writing a university essay . I discovered that certain grasses , such as rice and Paspalum notatum , are capable of fixing N in wet , aerobic soils . However , I was unable to find any studies reporting ANF occurring in drier , mixed-species natural grasslands . Leading journals in grassland ecology and management , including the African Journal of Range and Forage Science , have also remained almost completely silent on the topic . However , in 2016 , Mark Ritchie and his team reported the first convincing example of ANF taking place in an African grassland ( Ritchie and Raina
We recently embarked on research to investigate whether the nitrogenase ( nifH ) gene occurs on or inside the topsoil roots of four local grasses as this would indicate the presence of diazotrophs and a potential for grass ANF ( Morris et al . 2022 ). Four grasses were selected with varying N needs , ranging from Aristida junciformis , which apparently requires very little nitrogen , to Eragrostis curvula , which thrives with added fertiliser N . We hypothesized that the nitrogenase gene would be most abundant in grasses that can dominate low-N soil and absent or scarce on the roots of the most N-responsive species ( Figure 2 ). Using the same primers as the Serengeti study , we employed quantitative polymerase chain reaction ( qPCR ) to amplify , detect , and quantify the nifH gene ' s relative copy number .
The results of our study were surprising . The nifH gene was found to occur , in widely varying amounts , in all 80 root samples and not just with the root of grasses that thrive in depauperate soils . Sequencing confirmed that the amplified gene was similar to that of the nitrogenase gene found in a well-known diazotroph species in legume nodules and on some prairie grass roots . Contrary to expectation , there was no significant difference in the mean relative copy number of the N-fixing gene between the four grass species tested , indicating that they all had the potential to host diazotrophs and to probably acquire fixed nitrogen .
When we conducted a comparison of the gene copy numbers in the " old " and " new " swards of the study field , where one half had been burned fifteen
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