Nitrogen in particle size fractions of soils incubated for five years with 15N-ammonium and 14C-hemicellulose |
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| Authors: | B T CHRISTENSEN L H SØRENSEN† |
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| Institution: | Askov Experimental Station, Vejenvej 55, DK-6600 Vejen;Agricultural Research Department, Rim National Laboratory, DK-4000 Roskilde, Denmark |
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| Abstract: | Four soils with a range of clay and silt contents were incubated for 5 a with 15N-labelled (NH4)SO4 and 14C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The 15N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated. |
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