Soil C Sequestration From No-till to Conservation Plow-Tillage: Role of Protected Soil Niches.

Effective sequestration of atmospheric carbon (C) into agricultural soils is a promising technique to reduce global warming potential. Previous studies have shown that management of nitrogen and crop residues to promote residue decomposition can lead to soil C and nitrogen (N) sequestration. In this study we assess the role of soil aggregates after conversion from no-till (NT) to conservation-plow tillage in the fall with N fertilization (RT+N) under continuous maize rotation in an irrigated field plot. Soil cores were collected in the fall of 2005, 2006, 2007 and 2008 from a continuous maize rotation field, separated into 0-5, 5-15 and 15-30 cm depths, air dried and wet sieved to obtain >250μm, 250-53 μm and <53 μm aggregate size classes. Labile and stable humic acids (MHA and CaHA) and light fraction (LF_1.6) were extracted from the soil aggregates, and analyzed for C and N. Breakdown of  >250μm aggregates and mixing of the 0-5 and 5-15 cm soil layers resulted from the conservation-plow tillage following NT. No changes in the proportion of aggregate sizes were observed in the 15-30 cm soil layer. Cumulative C mass for the first 150 and 300 kg soil m^-2 was 3.49±0.08 and 5.68±0.08 kg C m^-2 in 2005 and 3.52±0.13 and 5.24±0.12 kg C m^-2 in 2008, respectively. The proportion of cumulative C in the first 150 and 300 kg soil m^-2 was reduced over time for the largest aggregates while it increased in the middle and small size aggregates. The reduction in C storage in the largest aggregates is associated with the breakdown of aggregates and loss of the LF pool. The increase in C storage in the middle and small size aggregates with depth is associated to an increase in humic material in these aggregates. Our results indicate that the incorporation of the residues in the soil along with N fertilizer in the conservation-plow system favored the decomposition of the residues to humic materials, while it exposed the more labile short term storage C pools (free and occluded LF in the larger aggregates) to decomposition.