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Inadequate soil management such as excessive ploughing is one of the main problems affecting the sustainability of agroecosystems and may exacerbate problems in acid soils. The application of Ca-amendments to ameliorate acid soils and no tillage practices to recover degraded soils are common management strategies. However, few studies have focused on their interactive effects under conditions of variable rainfall. In this study, we assessed the changes in soil properties over a period of seven years in response to no tillage (NT) and tillage, with or without the application of a Ca-amendment (sugar beet foam and red gypsum). Our split-plot experiment, which started in 2005 was carried out in a degraded Palexerult in Southwestern Spain with an annual rainfed forage crop. The Ca-amendment increased soil pH, Ca content and ameliorated Al-toxicity down to a depth of 50 cm, even under NT, due to the relatively high solubility of the amendment. However, after seven years of experiment, soil organic matter variables were enhanced only to a soil depth of 0–5 cm: NT and the Ca-amendment increased total organic carbon by 31% and 25%, respectively, whereas particulate organic carbon was increased to a larger extent (by a factor of 2.5 and 2, respectively). The positive effect of NT on organic matter variables increased with time, whereas the beneficial effect of the Ca-amendment was stronger in the early years of the experiment. Adverse weather conditions, with either excessive precipitation in autumn-winter or scarce precipitation in spring, favoured biomass production under NT. Given that NT mitigated the adverse effects of both water excess and deficit on biomass, it constitutes a valuable tool to combine with Ca-amendment to maintain crop productivity and recover degraded acid soils. This is especially the case under the changing weather conditions of a Mediterranean climate or other climates with increasing occurrences of periods of drought and/or water excess. [ABSTRACT FROM AUTHOR]

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]