FEATURE
matter decomposition , and overall soil health . However , soil respiration is the primary pathway through which carbon dioxide ( CO 2
) is released from the soil to the atmosphere within the carbon cycle . Combustion of aboveground biomass may lead to the deposition of charred organic material , ash , and partially burned plant residues , which can serve as a source of easily decomposable carbon for soil microorganisms . This increased availability of labile carbon substrates can enhance microbial activity and subsequent respiration rates .
Selective grazing and biomass variation in annually burnt and unburnt grasslands
Grassland burnt annually demonstrated lower total standing biomass accumulation ( after grazing ) over the growing period after removal from grazing ( 114 kg / ha ) compared to sites left unburnt ( 802 kg / ha ). Following prescribed burns , cattle are concentrating on a small firebreak which is then subjected to abnormally high grazing pressure relative to the larger adjacent unburnt area . Grazing exclusion chambers placed in the treatment sites were used to assess the effect of grazing on biomass accumulation over the growing season . With the exclusion of grazing , there were marginal differences in biomass between grassland treatments burnt annually ( 6180 kg / ha ) and those left unburnt ( 5626 kg / ha ), suggesting annually burnt grasses were selectively and heavily grazed .
Grassland digestibility
Grasslands subjected to annual burning showed lower levels of aboveground biomass lignin ( 5.28 %), compared to grasslands left unburnt ( 6.33 %). Grazing by livestock can selectively remove lignin-rich plant species or plant parts ,
Key References while annual burning can further contribute to lignin breakdown . Additionally , grasslands burnt annually demonstrated lower levels of aboveground biomass fibre ( 44.23 %) in contrast to grasslands left unburnt ( 48.88 %). Moreover , grasslands that underwent annual burning exhibited greater levels of aboveground biomass moisture content ( 5.38 %) compared to grasslands left unburnt ( 2.22 %). Grazing and fire defoliation can stimulate regrowth and increase the water content in the aboveground biomass due to the presence of younger , more succulent plants , however grazing intensity on the unburnt areas is much too low to have those similar effects to prescribed fire .
The consequence of lower lignin and fibre percentages as well as higher moisture contents in frequently burnt sites results in improved forage quality and increased palatability for livestock . Increased moisture content further improves the palatability and intake of grazed vegetation for livestock . However , given the alternative of grasslands left unburnt , livestock are more likely to select these frequently burnt firebreaks , resulting in selective overgrazing , as was seen in accumulated biomass between the growing seasons .
Conclusion and consequences
Management practices , such as controlled grazing and prescribed burning play a crucial role in maintaining the health and productivity of these ecosystems by impacting their dynamics and functioning . Annual burning led to narrower C : N ratios , indicating a higher carbon concentration relative to nitrogen in the soil . The absence of significant differences in soil organic carbon ( SOC ) stocks between burnt and unburnt grasslands suggests the redistribution of organic matter compensating for carbon losses . Grasslands subjected to annual burning exhibited lower levels of lignin and fibre , and higher moisture content in the aboveground biomass . These factors resulted in improved forage quality and palatability for livestock grazing . However , the lower lignin and fibre content , along with higher moisture content , can lead to selective overgrazing and degradation of preferred plant species as was observed in the aboveground biomass gain after grazing effects . Here , significantly lower total standing biomass after grazing in annually burnt sites reflected the excess removal of aboveground biomass and reduced resource availability for plant growth . Grazing exclusion chambers confirmed selective and heavy grazing in annually burnt grasslands , further affecting biomass accumulation .
We emphasise the importance of balancing burning and grazing practices in mesic grassland management to ensure sustainable nutrient cycling , carbon storage , and forage availability for livestock grazing . Correct application of multiple disturbance regimes that regard the ecological and cultural importance of grazing and fire are crucial for the conservation and restoration of grassland ecosystems . Research at this site is ongoing and further experimental outcomes from this project are in progress .
Acknowledgements
Funding for this research was made available by Red Meat Research and Development South Africa ( RMRDSA ). Special thanks to Richard Bergdorf ( ICFR ) for laboratory availability , and John Tedder for assistance in building grazing enclosures .
1 . Wilcox , B . P ., Fuhlendorf , S . D ., Walker , J . W ., Twidwell , D ., Wu , X . B ., Goodman , L . E ., Treadwell , M . and Birt , A ., 2022 . Saving imperiled grassland biomes by recoupling fire and grazing : a case study from the Great Plains . Frontiers in Ecology and the Environment , 20 ( 3 ), pp . 179-186 . https :// doi . org / 10.1002 / fee . 2448
2 . Johnson , L . C . and Matchett , J . R ., 2001 . Fire and grazing regulate belowground processes in tallgrass prairie . Ecology , 82 ( 12 ), pp . 3377-3389 . https :// doi . org / 10.1890 / 0012-9658 ( 2001 ) 082 [ 3377 : FAGRBP ] 2.0 . CO ; 2
15 Grassroots Vol 23 No 2 July 2023