Improved Grass Selections and Their Impact On Soil Carbon Partitioning in Pastoral Systems of the Northern Marianas Islands.

Three improved pasture grasses, Signal (Brachiaria decumbens) Buffel (Cenchrus ciliaris)and Mulato (Brachiaria spp.), , and grasses) were planted in 3X3 m plots in pastures on Guam, Rota, Tinian and Saipan with four replicates. After six months of growth, soil samples were collected from the 4 replicate plots of each grass, and from 4 plots of unmanaged pasture land, which served as the control. The soils were analyzed using the Gochlin soil fractionation method (Citation), which physically separates soil by density, using a 1.6 density sodium polytungstate solution (Na6[H2W12O40]), into three fractions. The first of three fractions, the free light fraction represents organic matter with quick turnover time in the soil. The second fraction, the occluded light fraction, contains organic matter protected within clay aggregates of soil, and has a much longer turnover time. Finally, the dense fraction contains the majority of soil mineral particles, and highly stable organic compounds with turnover time in the range of centuries to millennia. Elemental Analysis of the carbon content, and mass balancing was performed on the free and occluded light fractions of each soil sample. No statistically significant variation between grass treatments and control in the free light fraction was observed. Suggesting that grass type does not affect short term C turnover. However, mass balancing of the occluded fraction revealed elevated masses of C in plots planted to Buffel and Signal grass compared to the control plots indicating enhanced C sequestration. In only six months of treatment, impact on the slowly changing occluded fraction was a surprising result which begs the question, what chemically about these grasses, particularly Buffel grass, could explain such quick decomposition of root organic matter. Further incubation studies using isoptopically labeled sources of Carbon and Nitrogen could allow experimenters to outline a chemical pathyway/mechanism by which these transformations take place.

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See more from this Division: S07 Forest, Range & Wildland Soils
See more from this Session: Soil Carbon, Nutrients, and Greenhouse Gases From Managed Forest and Range Systems

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