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 Hawaiiansoils that varied in nutrient availability and mineralcontent to examine what regulates the flux ofdissolved organic carbon (DOC), nitrogen (DON) andphosphorus (DOP) from the surface layer of tropicalsoils. We examined DOM fluxes in a laboratory study from N, P and N+Pfertilized and unfertilized sites on soils that rangedin age from 300 to 4 million years old. The fluxesof DOC and DON were generally related to the % Cand % N content of the soils across the sites. Ingeneral, CO₂ and DOC fluxes were not correlatedsuggesting that physical desorption, dissolution andsorption reactions primarily control DOM release fromthese surface horizons. The one exception to thispattern was at the oldest site where there was asignificant relationship between DOC and CO₂flux. The oldest site also contained the lowestmineral and allophane content of the three sites andthe DOC-respiration correlation indicates arelationship between microbial activity and DOC fluxat this site. N Fertilization increased DON fluxes by50% and decreased DOC:DON ratios in the youngest,most N poor site. In the older, more N rich sites, Nfertilization neither increased DON fluxes nordecreased DOM C:N ratios. Similarly, short termchanges in N availability in laboratory-based soil Nand P fertilization experiments did not affect the DOMC:N ratios of leachate. DOM C:N ratios were similar tosoil organic matter C:N ratios, and changes in DOM C:Nratios with fertilization appeared to have beenmediated through long term effects on SOM C:N ratiosrather than through changes in microbial demand for Cand N. There was no relationship between DON andinorganic N flux during these incubations suggestingthat the organic and inorganic components of N fluxfrom soils are regulated by different factors and thatDON fluxes are not coupled to immediate microbialdemand for N. In contrast to the behavior of DON, thenet flux of dissolved organic phosphorus (DOP) and DOMC:P ratios responded to both long-term P fertilizationand natural variation in reactive P availability. There was lower DOP flux and higher DOM C:P ratiosfrom soils characterized by low P availability andhigh DOP flux and narrow DOM C:P ratios in sites withhigh P availability. DOP fluxes were also closelycorrelated with dissolved inorganic P fluxes. PFertilization increased DOP fluxes by 73% in theyoungest site, 31% in the P rich intermediate agesite and 444% in the old, P poor site indicating thatDOP fluxes closely track P availability in soils.