Long-term animal manure application promoted biological binding agents but not soil aggregation in a Vertisol

To improve soil aggregation through proper fertilization is very important for enhancing soil quality and crop productivity. However, the response of soil aggregation varies with the fertilization practices. The objective of this study was to determine the effects of long-term application of inorganic fertilizer, straw and manure on water-stable aggregate distribution (>2 mm, 0.25–2.0 mm, 0.053–0.25 mm, and <0.053 mm), soil organic carbon (SOC), glomalin-related soil proteins (GRSP) and microbial biomass carbon (MBC) as the major biological binding agents in a Vertisol. The fertilization experiment established in 1982 is composed of six treatments as follows: no fertilization (Control), balanced inorganic fertilizer (NPK), low and high amount of straw plus inorganic fertilizer (NPKLS and NPKHS), and animal manure (both pig and cattle) plus inorganic fertilizer (NPKPM and NPKCM). Long-term straw and manure fertilization significantly increased SOC, GRSP and MBC (P < 0.05), while the application of the two animal manures also increased dispersing agents like exchangeable Na⁺. Consequently, the straw incorporation promoted the formation of >2 mm macroaggregates significantly (P < 0.05) but the two animal manures did not (P > 0.05). The SOC, GRSP and MBC played an important role in the formation and stabilization of 0.25–2.0 mm aggregates. Our results indicate that animal manure may degrade soil structure due to the high salt content but straw incorporation is a judicious practice for sustainable agriculture in the Vertisol.