Soil organic matter pools and carbon-protection mechanisms in aggregate classes influenced by surface liming in a no-till system

The stabilisation of soil organic matter (SOM) is the result of the simultaneous action of three mechanisms: chemical stabilisation, biochemical stabilisation and physical protection. The objectives of this study were (i) to evaluate carbon-protection mechanisms in different SOM pools in soil aggregates and (ii) to identify the association of Ca2+ with total organic carbon (TOC) under the influence of surface liming in a medium-textured Oxisol in a long-term experiment under no-till system (NTS) in southern Brazil (25 degrees 10' S. 50 degrees 05' W). The treatments consisted of application of zero or 6 tons ha(-1) of dolomitic lime on the soil surface in 1993 and a reapplication of zero or 3 tons ha(-1) of dolomitic lime in 2000 to plots with or without previous lime application. Soil samples collected at depths of 0-2.5, 2.5-5, 5-10 and 10-20 cm were separated into seven aggregate classes. In each of these classes, TOC, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) were analysed. The 8-19 mm sized aggregates from the 0-2.5 cm layer were assessed by energy-dispersive X-ray spectroscopy (EDS) for the elemental analysis of carbon (C) and calcium (Ca). The liming caused an accumulation of TOC in the aggregates, mainly at a depth of 0-2.5 cm. The aggregates from soils treated with lime had a higher mean weight diameter (MWD) that resulted in the accumulation of TOC, especially in the 8-19 mm aggregate class, that was linear and closely related with C input (R-2 = 0.99). The proportion of large aggregates in the treatments with lime was closely correlated with the TOC content of the whole sample. The largest dose of lime (9 tons ha(-1)) resulted in higher TOC, POC and MAOC values, mainly in the 8-19 mm aggregate class. The elemental analyses for C and Ca revealed similar spectra between them for the surface-liming treatments in the clay fraction found in the centres of the 8-19 mm aggregates. The surface application of lime to NT fields provided greater stability and protection of the intra-aggregate C, presumably due to Ca2+ acting as a cationic bridge between OC and the kaolinite in the clay fraction. (C) 2011 Elsevier B.V. All rights reserved.