Dria, Karl (Purdue University, 550 Stadium Mall Dr, W. Lafayette, IN, 47907; Phone: 765-494-3274; Fax: 765-496-1210; Email: kdria@purdue.edu)

Dynamics of Biopolymer Turnover in Soil Physical Fractions Following Land-Cover Change in a Subtropical Savanna

T.R. Filley, K.J. Dria *, D.E. Gamblin, J. Liao, T. Boutton, J. Jastrow

Changes in the apportionment of organic carbon and nitrogen among soil physical fractions following land-cover shifts are of critical importance to the debate surrounding the capacity of terrestrial ecosystems to store or release greenhouse gases. For example, the difference between the mean residence times (MRTs) of light particulate organic matter (POM) vs. silts and clays is typically quite large, with silt and clay associated organic matter having the longest MRTs and the greatest likelihood to contribute to long term carbon storage. A few studies in agricultural and forest systems have demonstrated that biopolymer chemistry also varies along physical, as well as density, fractionation gradients. We quantified changes in biopolymer (lignin, suberin and cutin, and hydrolysable amino acids) chemistry of size and density fractionated soil from the Rio Grande Plains of Texas where C4 grasslands (d13C = -14 %) have undergone succession to subtropical thorn woodland dominated by C3 trees/shrubs (d13C = -27 %) over the past 150 years. This natural isotopic distinction was used to determine MRTs of free light organic matter (density less than 1.0 g/cc), macroaggregate (greater than 250 um), microaggregate (53-250 um) and silt+clay (less than 53 um) fractions which were then related to their specific biopolymer chemistries. Our results illustrate that lignin and aliphatic biopolymers (as measured by hydroxyl fatty acids) are apportioned differently among size/density fractions and along the successional chronosequence. Lignin is incorporated into all soil fractions soon after woody encroachment, whereas aliphatic components are slow to be incorporated in the silt and clay fractions. The lignin components that do become associated with silts and clays are, in general, highly oxidized. Differences in foliar chemistry among the plant sources indicate selective movement of leaf cutins into POM, macro- and microaggregate fractions, but not into free or intra-aggregate silts and clays. Selected analyses of silt and clay fractions for hydrolysable amino acids showed differences along the chronosequence, with total hydrolysable amino acids comprising 30-45% of total nitrogen. It is possible that amino and phenolic compounds are tightly bound to the silts and clays (the fractions with the longest MRT) and repel the more hydrophobic and less water soluble cutin and suberin monomers, thereby restricting turnover. These results provide new insights regarding the interactions between soil structure, chemistry, turnover, and preservation of soil organic matter.