Variation in soil carbon storage and emission in different land use types of the Letaba Catchment, Limpopo Province, South Africa

Abstract

The exponential growth in the world population has led to increased greenhouse gas emission. Carbon dioxide (CO2) is one of the major greenhouse gases being emitted mainly through the burning of fossil fuels, solid waste and soil respiration, thus it’s crucial to reduce its emission into the atmosphere. However, the emission of CO2 from agricultural fields have long been overlooked. This study was aimed at attaining a better understanding of the effect of land use type on carbon (C) storage and emission from the soil. The objectives of the research were to (1) determine the variation and effect of season and land use on selected soil properties, soil CO2 emission and soil C storage, to (2) characterise the microbial decomposer communities in the various land use types and determine their impact on CO2 emission and to (3) determine the major soil factors driving the emission of CO2 in the various land use types. The study was conducted in the Letaba catchment in the Limpopo province under the banana plantation, Eucalyptus grandis (planted forest), bush, maize and forest (natural forest) land use types where in each, three plots were set up and C chambers were installed. The CO2 and soil temperature data were measured by the GMP343 probe fitted with infra-red sensor (CO2) and Pt1000 temperature sensor. While the volumetric moisture (measured in %) of the soil was measured using a soil moisture meter. The CO2, soil moisture and soil surface temperature data were measured every two weeks for a whole year. Soil samples were also collected and analysed for chemical (exchangeable acidity, pH, macro and micro nutrients), physical (SOC stocks, MWD, bulk density, infiltration, clay % and SOC %) and biological properties (bacterial and fungal counts). The results showed a significant (p<0.001) influence of season (temperature and moisture) on the emission rate of CO2. The summer season resulted in the highest (0.11 tons/ha/day) emission rates of CO2 as the soil temperature (30.2°C) and soil moisture (19.3%) increased during this season creating conditions which promote CO2 emission. Thus, the winter season accounting for the lowest (0.03 tons/ha/day) soil CO2 emission rates as temperatures (26.4°C) and moisture (7.5%) contents were lower. Higher (0.08 tons/ha/day) CO2 emission rates were observed during autumn than that observed during spring (0.05 tons/ha/day) and winter (0.03 tons/ha/day). This is accounted for by the increased plant litter on the soil surfaces as plants shed their leaves during this season allowing for increased microbial decomposition. The land use type also had a significant impact on the emission rate of CO2 with the rates ranking from highest to lowest in the following order: forest > banana > maize > bush > eucalyptus. The high CO2 emission rate under the forest land use type was due to the high amount of organic matter (OM) and the quality of the OM found on the forest floor as it is easily degradable than that found under the eucalyptus land use type. This was also due to higher root respiration as all areas without trees had standing grass biomass. The land use type affected the soil C storage significantly and the C stocks were ranked from highest to lowest in the following order: forest > eucalyptus > banana > maize > bush. The interaction between the season and land use type displayed significant (p<0.001) impacts on the emission rate of CO2. The eucalyptus land use type had the lowest CO2 emission rate in the autumn and summer while the bush land use type had lower CO2 rates in winter and spring. The highest CO2 emission rates during the spring and summer season was under the forest land use type, while autumn and winter had the highest emission rates under the banana land use type. The results displayed insignificant negative relationships between all soil physical properties (except C stocks and fungal count), with both the C emission and the C storage. In contrast, the C stocks and the fungal count of the soils were the only properties which had significant (p<0.05) relationships with both the storage and the emission of soil C. The main driver of CO2 emission was the soil moisture content under all the land use types, with exception of the banana land use type where soil surface temperature drove CO2 emission. In conclusion, the land use type and season have significant impacts on the storage and emission of C and are largely influenced by the soil surface temperature and moisture content. Also, land use type and seasons interacted to influence soil CO2 emission rates. Thus, an improved understanding of the dynamic relationship between the land use type and C storage and emission as affected by season, soil properties and land management practices can subsequently aid in the mitigation of CO2 emission and thus enhance soil C sequestration, to help reduce climate change.