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The physical protection and stabilization of soil organic carbon in soil aggregates are important processes for the mitigation of greenhouse gas emissions through carbon sequestration. Compared with temperate soils, the relationships between management practices, soil aggregation and soil organic matter (SOM) are not well understood in subtropical and tropical soils. The objectives of this study were to assess the influence of tillage system and Ca-amendment on the size distribution of soil aggregates and aggregate-associated organic C and N, as well as to contribute to the understanding of the role of soil organic matter in the aggregation process in a soil with tropical mineralogical characteristics. Our study was conducted on an annual forage agrosystem with a degraded Palexerult soil in SW Spain with mineralogical and morphological characteristics inherited from a pre-Quaternary humid-subtropical climate. The experiment had a split-plot design with tillage (no-tillage vs. traditional tillage) as the main factor and with or without Ca-amendment as the second factor. Aggregate size distribution was dominated by microaggregates (0.053–0.250 mm). Compared with tilled plots, after six years, no-tillage increased the macroaggregates (>0.250 mm) to a depth of 10 cm by about 40%. The relatively low clay content of the surface soil may explain the strong impact of tillage on macroaggregation and the relatively slow recovery after the adoption of no-tillage. In contrast to the tillage system, the Ca-amendment barely influenced aggregation parameters. No-tillage also favored accumulation of organic C and N in all aggregate fractions but no size preference between macro- and microaggregates was observed while Ca-amendment enhanced accumulation only in large macroaggregates. Our results indicate an aggregation hierarchy in these kaolinitic soils, as well as an important role of SOM in enhancing soil aggregation. [ABSTRACT FROM AUTHOR]

In Mediterranean regions, the performance of conservation tillage under water-deficit conditions has been widely reported, but conversely there exists a lack of studies investigating its effects during periods of water excess, which are common in these regions between fall and spring. Our experiment was established in 2005 on a degraded Palexerult from a raña, a continental detritic formation, of western Spain to study the influence of soil tillage and Ca amendments on soil quality. During high precipitation periods, perched water tables can be formed in very flat raña surfaces with the upper limit close to soil surface due to the presence of Bt horizons with low permeability. In the 2010–2011 and 2012–2013 campaigns, when perched water tables developed very close to the soil surface between November and March, we studied the effects of no tillage (NT) vs. tillage (T) on selected soil hydraulic properties in the 0–5 cm soil layer and on crop yield. We observed the lowest bulk density and the highest soil organic matter content (SOM) for this soil layer under NT. As a result of the higher SOM content and higher total porosity as well as the potential to preserve soil biopores under NT, water content at saturation (SAT), plant-available water capacity (AWC), water infiltration and saturated hydraulic conductivity were also higher in the NT plots compared with those under T. During wet periods, the degree of water saturation ( S r ) under T was higher, and sometimes close to saturation. Crop development and yield were strongly affected by near-saturation conditions in the 0–5 cm surface layer, resulting in substantial yield losses under T. [ABSTRACT FROM AUTHOR]

Highlights: [•] Combining no-tillage with a Ca-amendment was the best practice in acid degraded soils. [•] In some cases tillage suppressed the positive effects of liming. [•] Carbon and glomalin-related soil proteins seem to play a role in the alleviation of Al toxicity. [Copyright &y& Elsevier]