Carbon contents and respiration rates of aggregate size fractions under no-till and conventional tillage

The purpose of the present study was to evaluate the long-term (14 yr) effect of no-till (NT) compared with conventional tillage (CT) on the distribution of dry sieved aggregate size fractions, their carbon (C) and nitrogen (N) contents and respiration activity. Soil samples were taken from a long-term (14 years) tillage experiment with a rotation of 6 cash crops, 4 years pasture and another 6 cash crops in the sandy plains region of semiarid central Argentina, on an Entic Haplustoll. Sampling was carried out at 0.06 m intervals to 0.18 m depth, with 4 replicates per tillage treatment. Bulk density (BD), C and N were determined on air dried samples. Dry aggregate size distribution (fractions: >4, 1–4, and <1 mm) and C and N contents of these size fractions were determined. Samples of fractions and of undisturbed soil were incubated and respiration was measured by trapping evolved carbon dioxide (CO2) in NaOH. The NT soil had 25% more macroaggregates in surface 0.06 m samples than CT, while at 0.06-0.12 and 0.12-0.18 m, these increased by 38 and 17% respectively. This indicated that the effect of NT on aggregate size distribution extended towards deeper soil regions. The intermediate aggregate fraction of surface soil under NT had 8.2 g kg−1 more C than under CT, while for >4 and <1 mm aggregates the differences between tillage systems were smaller (6.8 and 7.6 g kg−1 respectively). The data confirmed our hypothesis with respect to higher C accretion in intermediate aggregates under NT, approaching values of a permanent pasture soil. Large aggregates did not sequester more C than small aggregates, as would be expected according to the conceptual model of aggregate hierarchy. At 0.06-0.12 m depth these differences in C concentrations were less pronounced. Thus, although the effect of tillage on aggregate size distribution extended below the upper 0.06 m, C sequestration occurred mainly in this depth interval. While intermediate aggregates represented 12 and 15% of total C under NT and CT respectively, their contribution to soil respiration was19 and 21% for the same treatments. This would indicate that C losses from soil through mineralization are mostly associated with intermediate aggregate size.