Measurement and modelling-induced discrepancies in the long-term contribution of root and added biomass to carbon sequestration in a permanent grassland soil

Dr Mohammad Ibrahim Khalil1, Dr Dario A. Fornara2, Dr Bruce Arthur Osborne1

1UCD SBES and CRAES-EI, University College Dublin, Dublin 4, Ireland, 2Agri-Food & Biosciences Institute (AFBI), Belfast, United Kingdom

International efforts focused on environmentally-friendly agricultural production often place a particular emphasis on soil organic carbon (SOC) as it contributes to improved soil health and sustainable development goals. The direct quantification  of SOC remains a complicated challenge due to large spatial and temporal variability, as well as sampling-associated errors. Modelling approach can minimize the large-scale variability of SOC and identify whether an ecosystem is either a source or sink of atmospheric CO2 and its potential to offset greenhouse gas emissions. For a temperate grassland soil, managed with inorganic fertilizer and animal slurry, the SOC density (ρ) and its annual change (ΔSOCρ) over 45 years simulations using the Denitrification-Decomposition (DNDC95) model were compared with measured values. The measured data for  SOCρ at 0-15 cm depth for unfertilized and urea-fertilized fields (73-77 t-C-ha-1) were significantly higher, relating to a larger contribution from plant roots, than the simulated values (54-55). Despite some variations, SOCρ was greater with cattle amendments (88-99 vs. 66-116 t-C-ha-1) than with  pig slurry (75-78 vs. 55-69). The simulated values correlated significantly well with the measured values  (R2=0.66). The model-estimates revealed increased C sequestration with increasing added-C. Regardless of treatments, the measured and simulated sequestration rate was 0.46±0.06 and 0.37±0.01 t-C-ha-1-yr-1, respectively. The variations in simulated-SOCρ could be explained by differences in added-C (62%), rainfall (15%) and air temperature (11%). Sensitivity tests demonstrated that SOCρ increased with increasing bulk density, inherent SOC concentration and clay fraction (R2 = 0.77-0.99). The ΔSOCρ decreased with bulk density and SOC (R2 = -0.99) and increased with clay fraction and pH (R2 = 0.89-0.97). These findings imply that a new SOC-equilibrium had not been reached in over 45 years. The DNDC95 could provide an accurate representation of the key drivers but predict smaller contribution of roots to SOC build-up.


Dr. M. Ibrahim Khalil is a Sr. Environmental Scientist and Modeler attached with University College Dublin, Ireland, and leading a multi-disciplinary research group (Climate-Resilient Agri-Environmental Systems, CRAES). Dr. Khalil is an agricultural graduate with honours, double masters and PhD in environmental soil science, completed with a Belgian scholarship. He was an awardee of IAEA traineeship and did post-doctoral research with the prestigious fellowships awarded by the Royal Society (UK), Alexander von Humboldt Foundation (Germany) and Japan Society for the Promotion of Science (Japan). Dr. Khalil  has been leading and coordinating a large number of externally-funded research projects, and published more than 150 scientific papers in peer-reviewed journals/book chapters/proceedings; member of editorial board and reviewer  for international journals. He has expertise in the key research areas of Biogeochemistry of Carbon and Nitrogen Cycles, Monitoring, Modelling and Mitigation of Greenhouse Gases/trace gases and SOC density/stocks; Climate Change and Adaptation.


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