Nutrient and mineralogical control on dissolved organic C, N and P fluxes and stoichiometry in Hawaiian soils

We measured DOM fluxes from the O horizon of Hawaiian soils that varied in nutrient availability and mineral content to examine what regulates the flux of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) from the surface layer of tropical soils. We examined DOM fluxes in a laboratory study from N, P and N+P fertilized and unfertilized sites on soils that ranged in age from 300 to 4 million years old. The fluxes of DOC and DON were generally related to the % C and % N content of the soils across the sites. In general, CO₂ and DOC fluxes were not correlated suggesting that physical desorption, dissolution and sorption reactions primarily control DOM release from these surface horizons. The one exception to this pattern was at the oldest site where there was a significant relationship between DOC and CO₂ flux. The oldest site also contained the lowest mineral and allophane content of the three sites and the DOC-respiration correlation indicates a relationship between microbial activity and DOC flux at this site. N Fertilization increased DON fluxes by 50% and decreased DOC:DON ratios in the youngest, most N poor site. In the older, more N rich sites, N fertilization neither increased DON fluxes nor decreased DOM C:N ratios. Similarly, short term changes in N availability in laboratory-based soil N and P fertilization experiments did not affect the DOM C:N ratios of leachate. DOM C:N ratios were similar to soil organic matter C:N ratios, and changes in DOM C:N ratios with fertilization appeared to have been mediated through long term effects on SOM C:N ratios rather than through changes in microbial demand for C and N. There was no relationship between DON and inorganic N flux during these incubations suggesting that the organic and inorganic components of N flux from soils are regulated by different factors and that DON fluxes are not coupled to immediate microbial demand for N. In contrast to the behavior of DON, the net flux of dissolved organic phosphorus (DOP) and DOM C:P ratios responded to both long-term P fertilization and natural variation in reactive P availability. There was lower DOP flux and higher DOM C:P ratios from soils characterized by low P availability and high DOP flux and narrow DOM C:P ratios in sites with high P availability. DOP fluxes were also closely correlated with dissolved inorganic P fluxes. P Fertilization increased DOP fluxes by 73% in the youngest site, 31% in the P rich intermediate age site and 444% in the old, P poor site indicating that DOP fluxes closely track P availability in soils.