STABILIZATION OF RESIDUAL C AND N IN SOIL

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Abstract
Three Manitoba soils varying in clay and organic matter content were mixed with each of four plant residue amendments: (1) a control where no plant or fertilizer materials were added; (2) 14C- and 15N-labelled wheat straw; (3) 14C- and 15N-labelled wheat straw plus 15N-labelled KNO3; and (4) 14C- and 15N-labelled prebloom alfalfa residue. The soils were incubated at 20 °C and 75% field capacity for 90 d. Soil samples were collected at 0, 7, 30, and 90 d of incubation. Two humic acid fractions were obtained from the amended soils. Fraction A was obtained by Na4P2O7 extraction and Fraction B was recovered from the remaining residue by sonication. Significant (P < 0.05) differences were observed in the incorporation of 14C and 15N into both fractions. The effect of clay content on C and N incorporation was most marked in Fraction B: C incorporation in all treatments tended to be higher in the Red River Clay soil than in the Newdale (bottom of the knoll) soil; N incorporation in the straw and alfalfa treatments tended to be higher in the Red River Clay soil but, in the straw-plus-fertilizer treatment, N incorporation tended to be higher in the Newdale (bottom of the knoll) soil. The effect of the form of C and N in the residue was most apparent in the incorporation of N into Fraction B: in all soils the level of 15N in the alfalfa treatment was significantly (P < 0.05) higher than that in the straw plus fertilizer treatment. Key words: Plant residue management, incorporation, incubation, C and N turnover, KNO3 fertilizer