SUSCEPTIBILITY OF ORGANIC MATTER OF CHERNOZEMIC Ah HORIZONS TO BIOLOGICAL DECOMPOSITION

Abstract

Samples of five Chernozemic Ah horizons from soils under prairie of predominantly single grass species were incubated at 30 C with moisture maintained at 300 mbars for 74 days with and without uniformly labelled 14C-glucose. The 14CO2 formed during decomposition was collected in NaOH and its activity measured by scintillation spectrometry. Within the Brown soil zone, soils covered by Stipa comata Trin. & Rupr. and Agropyron smithii Rydb. contained considerably more organic matter that was readily decomposable than a soil covered by Bouteloua gracilis (HBK.) Lag. when the cumulative CO2 evolved was expressed in terms of the C in the soil. In comparison with the three Brown soils, the organic matter of a Dark Brown soil covered by Stipa spartea var. curtiseta Hitchc. and a Black soil covered by Festuca scabrella Torr. was found to be very resistant to biological decomposition, as the percentage of C lost during incubation of the latter soils was less than half the percentage mineralized by any of the former soils. Between 80.4 and 91.4% of the added 14C was mineralized as 14CO2 in four of the soils and between 20 and 35% of the remaining 14C was extractable with Chelex-100. In the fifth soil, the Black Chernozemic soil covered by F. scabrella, only 50% of the added 14C was mineralized and only 8% of the remaining 14C was extractable with Chelex-100. The potential susceptibility to biological decomposition of the organic matter of various Chernozemic Ah horizons gave a measure of the proportion of the oxidizable component still present. It thereby helped with the interpretation of the genesis of the whole organic matter formed under different hydrothermal conditions in the field.