Depth trends of soil organic matter C:N and 15N natural abundance controlled by association with minerals
Plant residues show carbon:nitrogen (C:N) decreases, 15N isotopic enrichment and preferential loss of labile substrates during microbial decay. In soil profiles, strikingly similar patterns of decreasing C:N and 15N isotopic enrichment with increasing depth are well documented. The parallel trend in organic matter composition with soil depth and during plant residue decay has been used as evidence to suggest that organic products accumulate or develop in the subsoil due to increasing intensity of microbially-driven processing, although no studies to date have verified this. Here, by applying sequential density fractionation, specific surface area, oxalate extractable Fe and Al, C:N and δ15N measures with depth to soils with relatively uniform soil mineralogy (Oxisols), climates and vegetation we show that changes in organo-mineral associations drive subsoil C:N and δ15N and C:N depth patterns more than in situ organic matter decay. Our results provide the first direct evidence that soil depth trends could be driven by mineral association instead of in situ processing.
KeywordsSoil organic matter Natural abundance stable isotopes Mineral soil carbon Soil mineralogy Specific surface area Soil mineralogy Carbon to nitrogen ratio Microbial processing of soil organic matter Sequential density fractionation
We thank Russell Johnson and Dr. Dyke Andreasen for assistance for the elemental and stable isotope analysis. We thank Dr. Douglas Allen Schaefer And Xioming Zou of the Tropical Forest Ecology Laboratory, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences for assistance with sampling soils in the field at the Xishuangbanna site in China and Dr. Phillip Sollins for assistance with sampling at the Susua Site in Puerto Rico. Sequential density fractionation was performed at Oregon State University, Corvallis OR. Soil sample preparation, elemental and isotopic analyses were conducted at the University of California, Santa Cruz and the Western Regional Research Center of the Agricultural Research Center in Albany California.
MGK processed the soil samples and performed elemental and stable isotope analysis, oxalate acid extractions, and XRD on all the bulk soil and density fractions. KL performed the sequential density fractionation separations. MGK and KL designed, executed and interpreted results from two soils examined in the study. AA provided specific surface mineralogy analysis of the samples. MGK wrote the manuscript, to which all authors contributed substantial interpretation, discussion and text.
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