SULFUR DYNAMICS IN DECOMPOSING FOREST LITTER - RELATIONSHIP TO INITIAL CONCENTRATION, AMBIENT SULFATE AND NITROGEN

Sulfur in decomposing forest litter was fractionated into soluble SO2-4, soluble C-bonded S, insoluble ester-S and insoluble C-bonded S. In a I yr litterbag study, initially high amounts of soluble SO2-4 and soluble C-bonded S in Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] litter decreased to < 10% of initial mass in 4 months. Insoluble C-bonded S increased to 1.2 times its initial mass, similar to the dynamics of insoluble organic-N. Insoluble C-bonded S in red alder (Alnus rubra Bong.) litter was initially three times higher than in Douglas-fir. It decreased to 84% of initial mass in 1 yr, and insoluble organic-N also decreased. In a second litterbag study, SO2-4 in solutions (0 or 4mg S 1-1) applied for 1 yr to decomposing Douglas-fir foliage had no effect on retention of insoluble C-bonded S, and initial concentration of insoluble C-bonded S had no effect on the amount remaining after 1 yr. In a laboratory experiment, NH4NO3 applied in solutions (28 or 112 mg N1-1) to Douglas-fir litter caused formation of both insoluble organic-N and insoluble C-bonded S. Sulfate application (32 mg S 1-1) enhanced the formation of insoluble C-bonded S only in the high-N treatment. These experiments demonstrated that leaching of initial soluble SO2-4 and soluble C-bonded S contributes significantly to S release from decomposing forest litter. Insoluble C-bonded S is the most abundant S fraction retained in decomposing litter ; it can be formed or released during decomposition and can be affected by N dynamics. © 1990.