Conventional and No-Tillage Effects on the Distribution of Crop Residues and Light Fraction Organic Matter

Tillage management on agricultural soils is important because of its effect on soil organic matter (SOM) dynamics. The decomposition of fresh crop residues to stabilized organic matter by soil microbes results in a continuum of intermediary SOM fractions or pools. The light fraction organic matter (LFOM) represents one of the SOM pools formed in the early stages of decomposition and is distinguished as a potential labile SOM pool which is sensitive to changes in management practices. The objective of this study was to measure the quantity, distribution, and delta C-13 signature of the LFOM pool in farmland soils managed under a conventional tillage (CT) and no-tillage (NT) system. The study uses the delta C-13 natural abundance technique which involved a crop rotation of C-3 and C-4 species on a calcareous Typic Hapludalf soil. The LFOM was obtained using density fractionation, while the organic C, N, and delta C-13 of whole soil and light fraction were measured using high-temperature combustion coupled with isotope ratio mass spectrometry. Our findings showed that there was a significant increase in the light fraction C and N pools in the NT soils compared with the CT soils after 6 yr of NT. There was a higher proportion of corn-derived C in the light fraction in the 0- to 10- and 10- to 20-cm depth of NT soils. The differences in the isotopic signature of the whole soil and SOM fractions also show a preservation of newly derived C in the NT soils compared with the CT soils.