FEATURE
Overview of the Soil Carbon status
There were differences observed in soil C : N ratios based on both soil depth ( 0-5 cm , 5-10 cm , 10-15 cm ). The soil C : N ratios were marginally narrower in annually burnt grasslands ( 15:1 ) compared to unburnt grasslands ( 16:1 ), indicating a higher C to N concentration the soil in sites left unburnt . There were no differences in N between grasslands burnt annually and those left unburnt , indicating the changes were predominantly carbon driven . A higher proportion of carbon relative to nitrogen is associated with grass-dominated biomes and decomposition processes within these systems . Grasses tend to have higher carbon content and lower nitrogen content compared to other plants such as legumes and forbs . With the application of prescribed fires , carbon is released through combustion , but highly volatile N may be lost in the form of nitrogen gases . This leads to a reduction in the nitrogen content within the soil organic matter .
Burning of aboveground biomass in frequently burnt sites can result in decreases in the carbon content in the top horizons , contributing to the narrow C : N ratio observed at the top 0-5cm soil horizons . In unburnt sites , the absence of fire allows organic matter , such as dead plant material and litter , to accumulate at the soil surface . Over time , this organic matter undergoes partial decomposition , resulting in a higher proportion of carbon compared to nitrogen . Additionally , the ratios widened with increasing soil depth in the 0-5 cm ( 15:1 ), 5-10 cm ( 15.5:1 ), and 10-15 cm ( 16:1 ) soil horizons . Moving deeper into the soil horizon , organic matter undergoes processes such as vertical translocation and leaching , leading to the redistribution of carbon and nitrogen . These processes can cause nitrogen to become relatively more concentrated in the deeper soil layers , narrowing the C : N ratio with increasing depth .
No statistically significant differences were found between the treatments of annually burnt and unburnt grasslands in terms of SOC stocks . However , significant variation was observed based on soil depth . SOC stocks increased with increasing soil depth , within the 0-5 cm ( 32.1 t C / ha ), 5-10 cm ( 48.6 t C / ha ), and 10-15 cm ( 66.7 t C / ha ) increments . Although frequent burning may lead to carbon losses through combustion , it is possible that the redistribution of organic matter within the soil compensates for these losses . The movement of organic residues from the burned surface to deeper soil horizons , through processes like incorporation by fauna or physical translocation , can help maintain overall SOC stocks .
Soil respiration
Soil respiration exhibited a significant increase of 13 % in sites subjected to annual burning ( 7.24 μmol CO 2 m -2 s -1 ) compared to sites left unburnt ( 5.54 μmol CO2 m-2 s-1 ). Soil respiration is largely driven by microbial activity . Grasslands burnt frequently may modify root exudation patterns which subsequently impact microbial activity and soil respiration rates , suggesting microbial activity is more active in grassland sites burnt frequently . Increased soil respiration indicates greater microbial biomass and activity , which can positively influence nutrient transformations , organic
Figure 4 . Grazing and fire have been integral to the cultural and traditional practices of local communities in South African mesic grasslands . ( Photo : Nicolay and Tedder )
Grassroots Vol 23 No 2 July 2023 14