SOLUBLE ANTHRONE-REACTIVE CARBON IN SOILS - EFFECT OF CARBON AND NITROGEN AMENDMENTS

Studies were undertaken to determine the effect of plant residues on levels of soluble sugars measured as anthrone-reactive carbon (ARC) in soils. Three different soils were amended with 10 g kg(-l)of grain sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.] straw, or soybean straw plus 100 mg N kg(-1) as glycine. The treated soils were maintained at 40% water-holding capacity and 30 degrees C and observed for changes in soluble ARC, microbial biomass C, and extractable NO3- and NH4+ at 0, 5, 10, 20, and 30 d. Changes in soluble ARC also were observed in soil treated with 2 g kg(-1) cellulose with or without N added as glycine or as (NH4)(2)SO4. Soluble ARC in soil is initially increased by plant residue addition to soil as a result of the free sugar associated with the plant tissue. Levels of soluble ARC decline rapidly, but grain sorghum and soybean straw plus glycine treatments maintained soluble ARC at levels above the background. Soybean straw alone resulted in an N-limited environment in which soluble ARC declined to background levels after 10 d. Microbial biomass C increased in all treated soils. Cellulose additions to soil increased soluble ARC only in the presence of added N, demonstrating the relation of free sugars to microbial activity. Soluble ARC reflects the level of free sugars associated with plant residues and that made available by microbial activity.