Effects of human activities on nitrogen cycling in Mediterranean streams: contrasts between nitrate and ammoniun dynamics

Human activities have increased dissolved inorganic nitrogen (DIN) availability and modified the relative proportion of DIN as NO3- or NH4+ in running waters. Because nitrogen (N) is an essential element for in-stream biota, increases in DIN availability derived from human activity can alter biological N demand as well as dominant biogeochemical uptake pathways. Ultimately, this may have relevant implications for in-stream nutrient cycling and downstream transport. The general goal of the present Thesis was to understand how in-stream DIN uptake is affected by changes in DIN availability and speciation induced by human activities. We hypothesized that DIN uptake in streams will be influenced by the relative availability of NO3- and NH4+ because biotic assimilatory uptake demand and dissimilatory uptake processes differ between the two DIN species. Results from this Thesis indicated that microbial communities developing on inorganic streambed substrata (biofilms) mostly rely on NO3- as an N source, likely due to the general higher ambient availability of NO3-. However, while NH4+ concentration only accounted for 6% of total DIN (sum of NO3- and NH4+), biofilm assimilation of NH4+ accounted for 27% of total DIN uptake (sum of NO3- and NH4+ assimilatory uptake), suggesting preference for NH4+ with respect to NO3-. Biofilm assimilatory N uptake response to acute (i.e., hours) and chronic (i.e., weeks) enrichments in either NO3- or NH4+ were different. Acute NO3- enrichments did not enhance or even decreased biofilm assimilatory uptake of this DIN species, whereas biofilms responded in conformity to Michaelis-Menten kinetics to acute increases in NH4+ concentration. These results suggest that biofilm N uptake was already at saturation under ambient NO3- concentration, whereas it was below saturation in relation to ambient NH4+ concentration. In contrast to acute enrichments, chronic enrichments of both DIN species had a general inhibitory effect on biofilm N assimilatory uptake, being more relevant for assimilatory uptake of NO3- in those treatments enriched in NH4+. The most plausible explanation for this result was that chronic NH4+ enrichments favour the development of nitrifiers which have a high demand of NH4+ to support their growth. Besides biofilms developing on inorganic substrata, other in-stream uptake compartments (PUCs) may also contribute to assimilate in-stream DIN. In general PUCs had higher uptake rates for NH4+ than for NO3-, and biofilms developing on detritic compartments such as fine benthic organic matter (FBOM), leaves and wood accounted for the largest part of total assimilatory uptake at whole-reach scale. Dissimilatory uptake pathways associated with NO3- (i.e., denitrification and DNRA) have a low incidence on total whole-reach NO3- uptake, whereas those associated to NH4+ (i.e., nitrification) account for a remarkable fraction of total NH4+ uptake. Moreover, the study streams were clearly more efficient taking up NH4+ than NO3- regardless of the ambient DIN concentrations. However, likely due to the higher NO3- availability, uptake fluxes of this DIN species tended to be higher than those of NH4+. This pattern was further supported by results from an extensive literature survey across streams worldwide. Overall, results from this Thesis show that increases in each of the two DIN species will result in distinct responses of in-stream N uptake capacity and related biogeochemical pathways. In general, streams are more reactive for NH4+ whereas NO3- reaching the streams tends to be transported downstream without much processing.La actividad humana ha aumentado las concentraciones de nitrógeno inorgánico disuelto (NID) en los ríos. Además, la proporción relativa de las dos formas principales de NID (nitrato y amonio) depende del tipo de actividad humana en la cuenca (urbana, agrícola, industrial). El objetivo de esta Tesis ha sido el estudio de los efectos del incremento de NID sobre la captación de nitrato y amonio en ríos. Los resultados indicaron que las comunidades microbianas (biofilms) que se desarrollan en los sustratos inorgánicos del lecho de los ríos mostraron preferencia por el amonio pero asimilaron principalmente nitrato debido a la mayor disponibilidad de este. Los biofilms respondieron de forma positiva a incrementos puntuales de amonio, mientras que la respuesta frente a incrementos de nitrato fue nula o negativa. Incrementos sostenidos de la concentración de nitrato y amonio provocaron una inhibición de las tasas de asimilación de N de los biofilms. La inhibición más relevante se observó en las tasas de asimilación de nitrato en aquellos tratamientos enriquecidos con amonio, probablemente debido a la proliferación de microorganismos nitrificantes que se caracterizan por tener una alta demanda de amonio. En los experimentos a escala de tramo, los biofilms que crecen sobre sustratos orgánicos del lecho de los ríos presentaron las mayores tasas de asimilación de NID siendo más altas para el amonio que para el nitrato. Una parte importante de la captación de NID en los ríos se produjo a través de procesos no asimilatorios tales como la desnitrificación y la nitrificación. La recopilación de estudios mostró que las tasas de captación son muy variables entre ríos y que las concentraciones de las dos formas de NID podrían controlar estas tasas. En general, se observó que los ríos presentan mayor demanda de amonio, no obstante, las tasas de captación fueron parecidas para las dos formas de NID. Los resultados de esta Tesis muestran que los ríos tienen una capacidad importante de procesar los aportes de amonio procedentes de su cuenca mientras que en el caso del nitrato esta capacidad se ve reducida y tiende a ser exportado aguas abajo