FEATURE
Mesic Grasses probably get their Nitrogen fix from root diazotrophs
Craig Morris
Current Address : Agricultural Research Council – Animal Production ( ARC-AP ), c / o School of Life Sciences , University of KwaZulu-Natal , Private Bag X01 , Scottsville 3209 , Pietermaritzburg , South Africa . Email Address : morris @ ukzn . ac . za
T he vibrant green flush of fresh grass
that emerges after a burn at the end of winter is a welcome sight for farmers , agrostologists , and myriad hungry herbivores alike . The proteinrich new growth , fuelled by soil nitrogen and spring rains , provides essential fodder for livestock , wild ungulates , small mammals , and numerous arthropods .
How has this ongoing cycle of renewed growth been sustained in mesic , productive , subtropical grasslands for millennia ? Nitrogen is available for plant growth from mineralised ‘ old ’ N recycled by microbes from soil organic matter and animal excreta . Rainfall deposition of nitrogen oxides ( LNOx ) generated by lightning does contribute some new nitrogen into the nitrogen cycle and legumes , generally scarce in mesic grassland , probably add a little more . However , in grasslands and other ecosystems , mass balance methods have often revealed a nett shortage of nitrogen available for new growth each year because losses of N can be higher than presumed inputs plus available soil stocks . Large amounts of nitrogen are exported from grassland by the offtake of livestock and through volatilization of plant material during a fire ; recurrent burning inevitably diminishes available soil N stocks , except in the most organic-rich soils ( Findlay et al . 2022 ).
In the late 1950s , the discovery of active nitrogen fixing bacteria ( known , along with other N-fixing microbes such as archaea , as diazotrophs ) in the rhizosphere , leaves , and stems of some varieties of sugarcane in Brazil ( Boddey et al . 2003 ) raised the intriguing possibility that grasses may be able to directly obtain fixed N from diazotrophs . Further research revealed that such associative nitrogen fixation ( ANF ) can contribute to the growth of some cultivated pasture grasses and switchgrass , which is grown for biofuel ( as briefly discussed in Morris et al . 2022 ). This leads to the question : Could ANF be widespread in local mesic grasslands , providing an ongoing source of nitrogen into the ecosystem ?
Around 40 years ago , I first encountered
Figure 1 . Grasslands produce a flush of new green growth in spring after a burn .
09 Grassroots Vol 23 No 2 July 2023