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Carbonates interfere with soil aggregation in semi-arid calcareous soils, promoting the stability of macroaggregates and decelerating the decomposition of the organic matter within them. Our aim was to determine the process through which carbonates participate in aggregation. We hypothesised (i) a tendency to accumulate reactive clay minerals via Ca2+ bridging, and (ii) a precipitation of carbonates within aggregates due to dissolution/re-precipitation phenomena. The <250-µm fractions of a Typic Calcixerept (CALC) and a decarbonated Calcic Haploxerept (DECALC) were incubated from the same agricultural field in semi-arid Spain with added maize straw during 28 days. A size-based fractionation was used to separate different aggregates in incubated and field-moist samples, and aggregates were analysed for organic C and maize-derived C, clay mineralogy by X-ray diffraction, and micromorphology in digital images of thin sections. Contrary to the first hypothesis, the two soils showed a similar tendency to accumulate smectite in aggregates, probably because the cation exchange capacity was saturated by Ca2+ in both CALC and DECALC. Macroaggregates showed a less porous structure in CALC than in DECALC due to the accumulation of calcite microcrystals, as formulated in the second hypothesis. We propose that low porosity of macroaggregates is mainly responsible for the slower turnover of organic matter observed in CALC than in DECALC. These results explain the greater concentration of organic C in microaggregates within macroaggregates in field-moist samples in CALC than in DECALC. The different porosity of macroaggregates may also result in differences in physical properties between CALC and DECALC. These observations suggest a different response of calcareous soils in terms of organic matter protection, resistance to erosion, and water storage compared with other soil types in semi-arid lands.

Abstract:Irrigation is being initiated on large areas of traditionally rainfed land to meet increasing global demand for food, feed, fiber and fuel. However, the consequences of this transition on soil quality (SQ) have scarcely been studied. Therefore, after previously identifying the most tillage-sensitive SQ indicators under long-term rainfed conditions, conversion of a research site on a Haplic Calcisol in Navarre, in northeast Spain provided an ideal location to reevaluate those SQ indicators after three years of irrigated management. The Soil Management Assessment Framework (SMAF) was used to test our hypothesis that adopting irrigation could change the sensitivity and importance of non-irrigated SQ indicators. Several soil physical, chemical, and biological indicators along with crop yields were used to evaluate SQ three years after initiating irrigation on a long-term conventional tillage (CT), minimum tillage (MT) and no-tillage (NT) study where either barley (Hordeum vulgare L.) or wheat (Triticum aestivum L.) was being grown. The results confirmed our hypothesis that irrigation would change the relative importance of various SQ indicators and suggested that some SMAF algorithms, such as those used to assess bulk density, needed to be recalibrated for these Mediterranean soils.

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