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University of KwaZulu-Natal (2022)

Assessing decomposition and soil carbon sequestration potential of sorghum residues from different cultivars.

Ntonta, Sipho.

Titre : Assessing decomposition and soil carbon sequestration potential of sorghum residues from different cultivars.

Auteur : Ntonta, Sipho.

Université de soutenance : University of KwaZulu-Natal

Grade : Master of Science in Soil Science 2022

Résumé partiel
Decomposition of crop residues may affect soil organic carbon (C) stocks, which are key for soil fertility improvement and mitigation of climate change. Numerous studies focus on decomposition from a single crop type and have given little attention to the difference between crop cultivars and/or plant parts. This opens a gap to conduct a comprehensive analysis of the impact of crop types, cultivars, and their plant parts on residues decomposition in soils. Data from 394 trials from across the world were used to assess cumulative CO2 emissions from residues of 17 crops during 0-30, 30-90, and 90-120 days (i.e., CR30, CR90, and CR120 ; 1- (CR30/120) ratio as C stability index of emission) of decomposition in a meta-study. In a separate incubation study, ground (<2mm) root and shoot residues (0.25g ; n=3) from five sorghum cultivars (AS8, KZ5246, LP4403, OS-POTCH, and MAMOLOKWANE) were mixed with 100 g of soil in a 500 ml sealed plastic container and incubated at 25°C and 50% soil field capacity. The emitted CO2-C was trapped in NaOH, and measurements were made at days 0, 7, 14, 28, 42, 56, 84, and 120. The amount of NH4 +-N and NO3- -N release were also analyzed using a Thermo Scientific Gallery Discrete Autoanalyzer. The decomposition potential of these sorghum cultivars was further assessed based on the percentage weight of residues remaining (DM) from buried litterbags, after 0, 14, 28, 42, 56, 84, 112, and 168 days in a field study. These were then analyzed for total C and N concentrations, using the LECO Trumac auto-analyzer, and lignin concentration using the ANKOM-200 method. Residue quality (i.e., C, N, and lignin concentrations) and/or selected soil properties (texture, pH, soil organic carbon concentration), and climatic conditions (temperature and precipitation) were considered as influencing factors of residue decomposition. Results of the meta-study showed that at all times, leguminous crops exhibited the highest CO2 emissions per gram of C added (1003 mg CO2-C g-1 C after 120 days) followed by grasses (947), oil crops (944), and cereals (846), with the legumes and grasses showing the lowest temporal C stability index of emissions. Maize residues however, emitted the least CO2 (86 at day 30, 275 at 90, and 495 mg CO2-C g-1 C at day 120), followed by two other lignin-rich crops (cotton and sunflower), while the highest emissions were from alfalfa residues. In the incubation experiment, emissions varied between 7.8 and 45 mg CO2-C kg-1 of the initial added C in roots and between 5.9 and 49.9 mg CO2-C kg-1 added C in shoots for OS-POTCH and KZ5246 residues, respectively (120 days).

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