FEATURE
months prior and the other half only a few months earlier , we found that the nitrogenase gene was about 60 % more abundant in the newly-burned sward , across all grass species ( Figure 3 ). This result suggests that fire could increase the potential for grasses to recruit and harbour useful diazotrophs . After a fire , it ' s probable that the carbon-rich root exudates from rapidly growing grass plants will stimulate root-associated diazotrophs and other vital soil microorganisms , which in turn would provide N and other plant-growth enhancing substances .
These are exciting and novel findings ( at least for southern Africa ) with potentially important consequences for understanding the nitrogen economy of mesic grassland . However , it is important to interpret the results with some caution .
The presence of the N-fixing gene does not necessarily indicate that N is actively being fixed and taken up by plants as inactive or dead diazotrophs may also have been present in the samples and soil and environmental conditions may not always be conducive to N fixation ( Roley et al . 2019 ).
References
Additionally , the apparent positive impact of fire on the abundance of the N- fixing ( nitrogenase ) gene is based on a comparison of two areas within a single field ; independent spatial replications would be necessary to fully evaluate this fire effect . It would also be necessary to quantify the relation between N fixation rates ( mediated by soil environment ) and gene abundance measured by relative copy number .
Our preliminary results have uncovered a crucial and novel research avenue to better comprehend the nitrogen economy of mesic grassland . Nitrogen fixation by root bacterial diazotrophs is a missing component in our understanding and models of the acquisition , distribution , and recycling of N in grassland . There are many open avenues of investigation , including the following priority research questions :
• Where do grass diazotrophs occur and thrive the most – inside and / or on the surface of grass roots and also within leaves and stems ?
• Which bacterial species are responsible for grass associated N fixation and does the composition of diazotrophs vary between grass species and seasonally ?
• How much N is fixed by diazotrophs , and when ? Seasonal variation in moisture could be a key driver of diazotroph activity .
• How much fixed nitrogen do grasses acquire ? Determining the extent to which plants take up fixed N and the specific tissues in which it is incorporated and utilised can only be definitively demonstrated and quantified using 15N tracer methods .
• How does soil N status , extant or through fertiliser application , affect N fixation and what other soil chemical and physical characteristics influence the size , composition , and activity of grass-associated diazotroph assemblages ?
• What are the short- and long-term positive or negative effects of burning on grass diazotrophs ?
Finally , and most importantly , how much fixed nitrogen is introduced to a mesic grassland per hectare each year and does this ‘ new ’ N help stimulate and sustain the post-burn flush and seasonal production of N-rich foliage in mesic grassland ? This is a critical issue that we aim to address .
1 . Boddey RM , Urquiaga S , Alves BJ , Reis V . 2003 . Endophytic nitrogen fixation in sugarcane : present knowledge and future applications . Plant & Soil 252 : 139-14 . https :// link . springer . com / article / 10.1023 / A : 1024152126541
2 . Findlay N , Manson A , Cromsigt JP , Gordijn P , Nixon C , Rietkerk M , Thibaud G , Wassen MJ , Beest MT . 2022 . Long-term frequent fires do not decrease topsoil carbon and nitrogen in an Afromontane grassland . African Journal of Range & Forage Science 39 : 44-55 . https :// doi . org / 10.2989 / 10220119.2021.2016966
3 . Morris CD , Ramesar DR , Burgdorf RJ . 2022 . Associative nitrogen fixation could be common in South African mesic grassland . African Journal of Range & Forage Science Online : https :// doi . org / 10.2989 / 10220119.2022.2145615
4 . Ritchie ME , Raina R . 2016 . Effects of herbivores on nitrogen fixation by grass endophytes , legume symbionts and freeliving soil surface bacteria in the Serengeti . Pedobiologia 59 : 233-241 . https :// doi . org / 10.1016 / j . pedobi . 2016.09.001
5 . Roley SS , Xue C , Hamilton SK , Tiedje JM , Robertson GP . 2019 . Isotopic evidence for episodic nitrogen fixation in switchgrass ( Panicum virgatum L .). Soil Biology and Biochemistry 129:90-98 . https :// doi . org / 10.1016 / j . soilbio . 2018.11.006
11 Grassroots Vol 23 No 2 July 2023