Reactive nitrogen retention and flushing along a soil texture gradient

Background/Question/Methods Most ecosystems retain a majority of reactive N inputs, transforming reactive mineral N into relatively nonreactive stable organic N.  Mechanistic explanations for these observations focus on C-dependent processes, and in particular wide C:N ratios.  However, in some ecosystems C-dependent mechanisms leave unexplained variation in reactive N retention.  The potential for soil texture to explain variation in temperate forest N retention remains largely unexplored.  Soil texture may help to explain N retention through two mechanisms: Negatively charged clay particles can promote the adsorption and physical protection of NH₄.  In contrast, coarse soil texture can promote rapid soil water flow, allowing reactive N to bypass plant, microbial and SOC sinks. Using a small forested catchment containing large gradients in soil texture and SOC, we provide evidence for interaction between these mechanisms. 

Results/Conclusions

Tracer ¹⁵NH₄ and ¹⁵NO₃, applications show a positive correlation between silt content and ¹⁵NH₄ transfer to the insoluble soil organic N pool, but no correlations between ¹⁵NH₄ or ¹⁵NO₃ transfer to the insoluble soil organic N pool and SOC.  Consistent with these data, soil solution NO₃ and hydraulic conductivity are positively correlated with sand content.  Coupled lysimeter and soil moisture data show that in sandy soils only, NO₃ is rapidly transported from surface soils to subsoils during intense precipitation events.  These data suggest that in silty soils, low hydraulic conductivity and high transfer of NH₄ to the insoluble organic pool prevent the vertical export of NO₃ and promote ecosystem N retention_.