Wednesday, August 5, 2009 - 4:00 PM

COS 67-8: Reactive nitrogen retention and flushing along a soil texture gradient

Michael Castellano1, JP Kaye1, H Lin1, and JP Schmidt2. (1) Penn State University, (2) USDA ARS

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.