The influence of Mediterranean riparian zones on stream nitrogen dynamics: A catchment approach.

Abstract

Riparian areas are recognized to be natural filters of nitrogen (N) because they can substantially diminish the N delivery from terrestrial to aquatic ecosystems. However, understanding the influence of riparian zones on regulating N export from catchments is still challenging, mainly because stream water chemistry integrates biogeochemical processes co-occurring within upland, riparian, and fluvial ecosystems. The present dissertation aims to explore the influence of Mediterranean riparian zones on regulating both stream hydrology and catchment N exports, by combining empirical and modelling approaches at different temporal and spatial scales.

Findings obtained from plot experiments show that the studied Mediterranean riparian soils acted as hot spots of soil microbial N supply within the catchment because they exhibited considerably higher net N mineralization (NNM) and net nitrification (NN) rates than upland oak and beech soils. This difference was attributed to larger stocks of N-rich leaf litter and permanent moist conditions in the riparian soils. Furthermore, soil microbial processes in the riparian site showed a distinct climatic sensitivity than in upland sites, which ultimately led to different temporal patterns of soil N cycling. Soil moisture was the major driver of NNM and NN in upland forests, while both temperature and precipitation shaped soil N dynamics in the riparian zone. Therefore, both upland and riparian soils exhibited pulses of NNM and NN following spring rewetting events, though summer temperatures only stimulate microbial activity at the riparian site. Riparian microbial pulses contributed > 25% to annual rates of NNM and NN; and coincided with increases in stream N loads. These results suggest that Mediterranean riparian soils may become important sources of nitrate (NO3-) to streams under future warming scenarios.

Additionally, findings obtained from catchment-scale studies show that our Mediterranean riparian zone exerted a strong control on stream hydrology during the vegetative period. Riparian evapotranspiration (ET) influenced the temporal pattern of stream discharge and riparian groundwater elevation across daily and seasonal scales. Further, the influence of riparian ET on stream hydrology increased from headwaters to the valley bottom, where stream hydrological retention was prominent. Nonetheless, such stream hydrological retention wasn’t accompanied by a decrease in catchment N exports, likely because low flow conditions, relatively warm conditions, and large stocks of N-rich leaf litter within the streambed enhanced in-stream NO3- release in summer. Conversely, in-stream photoautotrophic NO3- uptake was the major controlling factor of stream N dynamics in spring, when high light inputs favored gross primary productivity (GPP) prior to riparian canopy closure. As it occurred for summer nitrification, the influence of GPP on stream N dynamics increased along the stream continuum. At the valley bottom, in-stream photoautotrophic activity drop midday stream N concentration by 13% and reduced catchment NO3 exports by 10%. Finally, during the dormant period, we found minimal evidences of either NO3 uptake or release in the riparian zone. Mass balance calculations at the whole-reach scale showed that both riparian groundwater inputs and in-stream processes contributed to longitudinal changes in stream NO3 concentrations, and thus, both sources of variation were necessary to understand stream water chemistry along the stream. Together, these results suggest that the high bioreactivity of streams ecosystems can influence stream N dynamics at the catchment scale, and even screen the potential buffer capacity of riparian zones as observed for this Mediterranean catchment. Overall, findings gathered in the present dissertation question the idea that Mediterranean riparian zones are efficient N buffers, and stress that an integrated view of upland, riparian, and stream ecosystems is essential for advancing our understanding of catchment hydrology and biogeochemistry.

Durant l'última dècada, les activitats antropogèniques han doblat el nitrogen disponible, provocant nombrosos problemes ambientals. En un context de conca, els boscos de ribera tenen la capacitat de reduir els excessos de nitrogen que els hi arriben dels ecosistemes terrestres adjacents i, per tant, poden ser fonamentals per determinar la concentració de nitrogen al riu i l'exportació d'aquest nutrient aigües avall. L'objectiu d'aquesta tesi és analitzar la influència dels boscos de ribera sobre els recursos hídrics i la dinàmica del nitrogen en una conca mediterrània. Durant l'estudi, els sòls de ribera exhibiren unes taxes netes de mineralització i nitrificació clarament superiors als boscos de capçalera (alzinar i fageda), les quals foren atribuïdes als estocs de fullaraca i a l'alta humitat dels sòls riparians. A més, els sòls de ribera foren importants fonts de nitrat al riu, especialment quan les altes temperatures o les fortes precipitacions provocaren pics d'activitat microbiana que contribuïren > 25% en termes anuals. Hidrològicament parlant, la vegetació ripariana tingué un paper clau sobre els recursos hídrics de la conca, perquè regulà el cabal fluvial a diferents escales temporals, i promogué la pèrdua d'aigua del riu cap a la ribera en el fons de vall. Tanmateix, la ribera no retingué nitrogen durant el període de retenció hidràulica, així com tampoc durant la resta de l'any. Per contra, el baix cabal i les altes temperatures incentivaren la nitrificació a la llera del riu durant l'estiu. A més, durant la primavera, l'augment de llum previ al desenvolupament de la capçada de ribera afavorí la producció primària bruta en el riu, la qual provocà clares fluctuacions diàries en les concentracions de nitrat i reduí un 10% les exportacions d'aquest nutrient aigües avall. Conjuntament, els resultats d'aquesta tesi no només qüestionen la idea que les zones de ribera són filtres naturals de nitrogen en regions mediterrànies, sinó que manifesten la importància d'una visió integradora de tots els components del paisatge per tal per tal d'avançar en el nostre coneixement sobre la hidrologia i la biogeoquímica a escala de conca.