Additive effects of organic and inorganic amendments can significantly improve structural stability of a sodic dispersive subsoil

Sodic dispersive subsoils pose significant constraints to agricultural productivity and sustainability in arid and semiarid regions. Gypsum and organic amendments (OAs) have shown the potential to improve soil structure and fertility and may address constraints associated with these dispersive soils. However, the mechanistic linkages between the quality of OAs, and the dynamics of soil structural stability (i.e., soil aggregates), particularly under the interactive impacts of gypsum and exogenous nutrients, remain elusive. We quantified aggregate stability (i.e., wet mean weight diameter, WMWD) and the accumulation of OA-carbon (C) in different aggregate-classes in a dispersive clay subsoil (δ¹³C −24.5‰) with or without the application of OAs (δ¹³C −12.8 to −14.3‰), gypsum and/or exogenous nutrients (nitrogen (N) and phosphorus (P)) over the 270 days incubation period. The OAs include sorghum stubble and sugarcane bagasse that were low C quality (e.g., C:N:P ratio of 328:6.2:1 and 850:17.3:1, respectively) and less decomposed (e.g., low alkyl-C/o-alkyl-C ratio), and sugarcane mill mud that was high C quality (C:N:P ratio of 17:1.4:1) and more decomposed status. The results suggest that the quality of OAs is likely to control the dominant mechanisms of soil aggregation: (i) a quick top-down formation of aggregate hierarchy (~31 days) in the low C quality and less decomposed OAs; and (ii) a slow bottom-up formation of aggregate hierarchy (≥90 days) in the high C-quality and more decomposed OA. Over time, lowering the C-nutrient stoichiometric ratio of OAs (via exogenous nutrients) decreased aggregate stability by 30%, owing to accelerated breakdown/loss of OAs (i.e., by 16%). The exchangeable Ca²⁺ (from gypsum) was positively correlated to WMWD and water stable aggregates (>0.05 mm), which were not influenced by OAs or nutrients. In this dispersive clay subsoil, a long-term positive effect on soil aggregation occurred when improving either biochemical properties (e.g., microbial biomass and soil organic C) or chemical properties (e.g., soil dispersion, exchangeable sodium percentage, and pH), and an additive effect appeared when both properties were changed. This study provides critical information on the effects of OAs with variable C qualities, gypsum, and nutrients on structural stability with insights into strategies of sodic subsoil amelioration and soil C storage.