The interactive effects of organic amendments, fertilizer and gypsum on the formation of soil aggregates and storage of carbon in a dispersive sodic-subsoil

Dr Yunying Fang1, Dr Bhupinder Pal  Singh1, Dr Roger Armstrong2, Dr  Ehsan Tavakkoli3

1NSW Department of Primary Industries, Menangle, Australia, 2Agriculture Research, Department of Economic Development, Jobs, Transport and Resources, Horsham, Australia, 3NSW Department of Primary Industries, Wagga Wagga, Australia

The deep placement of amendments such as the integrated inputs of crop residues, fertilizer nutrients (i.e., nitrogen and phosphorus) and gypsum to ameliorate physicochemical constraints in sodic subsoils have received much attention. Organic and inorganic amendments affect soil structure (e.g., aggregation), as well as soil carbon (C) storage. However, there are knowledge gaps on the impacts of amendments on formation and stability of soil aggregates in sodic subsoils, the linkages between soil aggregation and decomposition process of organic amendments (OAs), as well as the role of resources’ C-nutrient stoichiometry on the C distribution and storage within aggregates. To address these knowledge gaps, we conducted a 270-day laboratory incubation experiment (20°C). A sodic subsoil collected from under C3-vegetation (δ13C-SOC: –24‰; soil organic C content: 5.8 g C kg-1 soil) was uniformly mixed with C4-vegetation derived OAs (δ13C: –14‰; including sorghum stubble and bagasse at application rate of 6.2 g C kg-1 soil), with or without gypsum (7.2 g kg-1 soil) and nutrients. A wet-aggregate sieving procedure was used on days 31, 90 and 270 to obtain four soil fractions: large macro-aggregates (2–6.5 mm), small macro-aggregates (0.25–2 mm), micro-aggregates (0.053–0.25 mm), and silt-clay (< 0.053 mm). Total C and OA-derived C in the aggregate-sizes have been analyzed. After 90 days, the input of OAs (with or without exogenous nutrients and gypsum) increased soil aggregation (of both macro- and micro-aggregates), with the greatest increase (6–7 folds) when a combination of OAs, gypsum and exogenous nutrients was applied. The preliminary results showed that the increased microbial biomass and activities might have enhanced the association of OAs with soil minerals, leading to the improvement of soil aggregation. The dissolution of Ca2+ from gypsum has also further enhanced the formation and stability of macro-aggregates. Our study provides insights on the role of decomposition process of OAs on soil aggregate formation, including aggregate-associated C storage, in a dispersive sodic-soil.

Keywords: gypsum, carbon isotope, soil constraints, soil amelioration, microbial biomass

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