Soil Architecture of a Loess Soil as Influenced by Long-Term Fertilization.

Agricultural fields not receiving application of organic matter are at long-term risk of decreasing in quality for crop production. We sampled undisturbed soil cores in the plough layer of the >100 yrs field experiment in Bad Lauchstädt, Germany, with a gradient in organic carbon (OC) ranging from 1.5 % to 2.4 % as induced by different fertilization strategies: three levels of animal manure (AM: 0, 20 and 30 t per hectare every second year) each combined with and without addition of mineral fertilizer (MF). Gas diffusivity (D_p/D₀), air permeability and the effective air-filled pore volume were measured at several matric potentials and added to the standard information on water- and air-filled pore volume (eps) derived from the soil water characteristic.

Soil porosity increased with increase in OC as reflected in higher volumes of water- and air-filled pores in the range of matric potentials between -4 to -1000 hPa. Taking eps = 0.1 m³m^-3 and D_p/D₀ = 0.005 as critical thresholds for soil aeration the long-term non-fertilized soil did not reach satisfactory conditions for plant growth until drained to approximately -1000 hPa, while the corresponding figure for the treatment with 30 t/ha manure was about -100 hPa. The specific gas diffusivity was highest for the high AM soil, indicating rather linear pores while the soils without AM showed the lowest ratios, suggesting more tortuousity. The specific air permeability displayed similar trends, the high AM and non-fertilized soils having the highest and lowest pore organization/ continuity respectively.

Analyses on several suggested soil-architectural parameters including equivalent pore diameter and number of pores for gas transport, percolation threshold for diffusive and convective gas transport, and the additional-structural-induced (ASI) gas diffusion coefficient, and their relations to soil fertilization strategy and OC-level will be presented and discussed.