The aim of this study was to evaluate i) the different cover crops contribution used in no-tillage system (NT) to biogenic aggregation; and ii) the influence of aggregate formation pathways on the compartmentalization and the soil organic carbon origin. Two areas managed under NT with different implementation times (6 and 18 years, NT06 and NT18, respectively) and cover crops were evaluated, totaling six sampling areas: NT06, millet (NT06MI); NT06, brachiaria (NT06BR); NT06, sunn hemp (NT06SH); NT18, millet (NT18MI); NT18, brachiaria (NT18BR); NT18, and sunn hemp (NT18SH). In each sampling area, five pseudo-replicates were collected in the 0.00-0.05 and 0.05-0.10 m layers. The samples were air-dried and sieved using sieves with 9.7 and 8.0 mm mesh, and the aggregates retained within this interval were selected. The percentage of each type of aggregate (physicogenic and biogenic) was quantified. Total organic carbon (TOC) and the natural abundance of δ13C (‰) were analyzed and the physical fractionations of SOM were performed: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) and density fractionation (free light fraction carbon, FLFC). Physicogenic aggregates were quantified in greater proportion, except for the areas of NT06BR and NT18BR in the 0.00-0.05 m layer. The biogenic aggregates showed the highest contents of TOC, POC, MAOC, FLFC and more negative values of δ13C. The use of grasses, especially Brachiaria spp., as cover plants in NT after 6 and 18 years of adoption favors the formation of aggregates through the biogenic pathway and they influence the compartmentalization and origin of stored organic carbon.

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Conventional and conservation tillage systems can alter soil aggregation and total and labile soil organic matter (SOM) contents. This study aimed to determine the degree of soil aggregation, quantify total carbon (TC), permanganate oxidizable carbon (POXC), light organic matter (LOM), and potentially mineralizable carbon (CO2-C) contents in soils aggregates, and assess soil quality indices at sites under conventional and conservation tillage in the Cerrado region of Minas Gerais State, Brazil. Four experimental areas were analyzed: a area under conventional tillage for 20 years, a area under no-till for 6 years, a area under no-till for 18 years, and a reference area of undisturbed Cerrado vegetation. Soil aggregates retained on 8.0 to 4.0 mm sieves were evaluated for size class distribution and mean weight diameter. TC, POXC, LOM, daily and total CO2-C emissions were also analyzed. These data were used to calculate the C/N ratio and sensitivity, carbon pool, and lability indices. The results of SOM compartments were in agreement with those obtained for the soil aggregation status. Environmental conditions at no-till areas promoted macroaggregate formation and preserved TC and LOM contents, resulting in a high degree of aggregate stability. Soil quality indices were sensitive to identify changes between the reference area and managed areas. Soil aggregates from no-till areas had higher CO2-C emissions and accumulations than those from the conventional tillage area.

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