Impacts of N Fertilizer Addition to Soil C and N Dynamics in Conventional and Organic Farming Systems.

Soil microaggregates have been implicated as the primary factor for the stabilization of soil carbon in agricultural soils. We hypotheses activities that have an effect on soil microaggregates, such as long term farming management systems, have a considerable effect on soil C and N dynamics.  Soils from plots under organic and conventional farming system for 10 years were collected from the Long Term Agricultural Research Station (LTRAS) in Davis, CA.  Baseline soil chemical (pH, NH₄, NO₃), and physical (particle size and bulk density) characteristics were determined. Water-stable aggregates of the soils and were isolated and measured for total C (TC), and  total N (TN); total inorganic C (TIC) and total inorganic N (TIN) after hypochlorite removal of organic matter; and total organic C (TOC) and total organic N (TON) by difference. Organic matter composition was also determined via subtractive diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). Sub samples of the soils from the two farming systems were also used in laboratory microcosms to investigate the weekly effect of added labeled ¹⁵N fertilizer on soil microaggregate C and N dynamics and composition over a period of 28 days. The samples were subjected to the previous analyses. Results show comparatively higher TC, TOC, TN, and TON in the microaggregates from the organic plots. FTIR analysis showed different organic matter composition between the microaggregates of the two faming systems. The addition of the urea fertilizer resulted in increased differences between the TC and TN of the two treatments and also resulted in changes in the microaggregate organic matter composition.