The Bureau of Economic Geology The University of Texas at Austin Jackson School of Geosciences

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Bureau Seminar, October 15, 2009

Linking Carbon, Nitrogen, and Phosphorus Fluxes in Agroecosystems
Simulation Models: Strengths and Implications of Some Knowledge Gaps

Dr. Armen R. Kemanian

Link for streaming video: available 10.15.09 at 9:55am CST

Carbon plays a pivotal role in the cycling of nutrients in the soil-plant-water-atmospheric continuum. In the soil in particular, the fluxes of nitrogen, phosphorus and carbon are intimately intertwined. The soil organic matter (SOM) is a complex mixture of living organisms, decomposing organic materials, and charcoal, with a varying degree of association with the soil mineral fraction, and with a wide range of turnover rates. Fractionating SOM into components with defined, measurable, and modelable properties has been a challenge for soil biologist and modelers alike. In this presentation the modeling of SOM cycling using a one-SOM pool approach is discussed. This model has been introduced in the model C-Farm and a modified version can be selected when using the model SWAT. Inputs to the SOM pool are from stabilized organic components (humified) from crop residues from aboveground and dead roots, manure or other organic amendments. The fraction of the decomposed inputs that are stabilized as SOM depends on soil texture and in particular on the layer SOM content. The SOM decomposition rate has a strong dependence on the layer SOM – the higher the SOM the higher the decomposition rate. This approach has some resemblance with the concept of “carrying capacity” usually discussed in ecology, and with the mathematical solution to simple sorption-desorption equations. The C:N and C:P ratios of the newly formed SOM depend on the C:N and C:P ratios of the inputs and on the availability of mineral N (nitrates and ammonia) and mineral P. Tillage accelerates the SOM turnover rate of the mechanically disturbed layers with tool-specific effects. The SOM pool does not include explicitly organic matter of pyrogenic origin, and may not represent adequately the properties of SOM in organic soil horizons or seasonally flooded soils. The algorithm does not include a process causing the often observed lower C:N ratio of SOM with depth. Preliminary testing has shown that the model represents appropriately SOM cycling in agricultural soils in temperate climates. Modeling results showing the effect of erosion and SOM decomposition on the carbon balance are discussed in relation to landscape and annual/perennial composition of cropping systems. Future testing and improvements of the model should emphasize tropical soils, subtropical climates with different types of vegetation, and organic horizons.


Department of Geological Sciences
Institute for Geophysics
The University of Texas
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