Incorporating Carbon Saturation Concepts and Nitrogen Retention Theory.

Recent advances in soil C saturation concepts have increased our understanding of soil C storage and mineralization without explicit links to N retention and saturation theories.  Here, we exploit a soil texture gradient to test hypotheses that link soil organic C saturation with soil ¹⁵N retention and nitrification. At our site, mineral-associated organic matter (MAOM) concentrations in the silt + clay particle fraction (g MAOM g silt + clay^-1) were negatively correlated with the fraction of NH₄-N transferred to MAOM during a 3d in situ incubation (R = -0.82), but positively correlated with potential net nitrification (R = 0.76).  Moreover, NH₄-N transfer to MAOM was negatively correlated with potential net nitrification (R = -0.74).  MAOM is considered to be resistant to mineralization.  Carbon saturation theory demonstrates that the proportion of new C inputs that can be stabilized in MAOM decreases in proportion to the amount of C already present in the fraction; C inputs not stabilized in MAOM are susceptible to rapid mineralization.  By demonstrating that NH₄-N stabilization in MAOM is similar to C stabilization in MAOM and associated with nitrification, we extend soil C saturation theory to mineral N and link it with N retention and saturation theories.  Incorporating the organic matter retention capacity of fine mineral particles into N saturation theory will improve predictions of N saturation rates and resolve inconsistent relationships between soil organic matter, texture, N mineralization, and N retention.