Effects of profile depth and management on the composition of labile and total soil organic matter

Carbon in deeper soil layers may be more stable than that in surface soil due to differences in source, composition, and environmental conditions. We studied the effects of profile depth on soil organic matter (SOM) composition and stability by comparing soils which had received organic amendments since 1991 to non-amended soils from the same study. we used thermometric analysis and excitation-emission fluorescence spectroscopy to characterize total SOM as well as a highly labile fraction of soil organic carbon (SOC) extracted with water (about 1% of total SOC), a moderately labile fraction extracted by pyrophosphate (5–18% of total SOC), and the resistant fraction remaining in the soil after pyrophosphate extraction. The concentrations of both total and water-extractable C decreased with depth. Amended soils had more total SOC than non-amended soils at the 0- to 15- and 15- to 30-cm depths, but not at the 30- to 45-cm depth, and greater amounts of water-extractable C at all depths. The most thermally labile fraction of SOM in the bulk samples was not affected by either depth or amendment, while the thermally resistant fraction decreased slightly with depth, but was unaffected by amendment. The thermally refractory fraction was unaffected by amendment but increased with depth. Thermal analysis of the residue after sequential extraction confirmed that the sequential extraction procedure removed relatively labile C and left behind more stable organic matter. Both thermal analysis, of soil pre- and post-pyrophosphate extraction, and fluorescence of pyrophosphate-extractable C suggested that C deeper in the profile was more microbially decomposed than surface C