[1] Stream aquifer hydrology and nitrate removal were studied, over a period of 2 years, in an unsaturated riparian zone, bounded by an intermittent Mediterranean stream, (Fuirosos, northeastern Spain). The riparian groundwater system is characterized by drastic hydrological changes and by mixing of stream water with hillslope groundwater. The hillslope groundwater flowed through a medium with low hydraulic conductivity (9.6 10 À3 < k s < 0.1 m d À1 ) and low specific discharges (1.7 10 À3 < q hll < 15 10). In contrast, stream water infiltrated through the near stream porous medium with relatively high hydraulic conductivity (4.8 < k s < 19 m d À1 ) and variable specific discharges (i.e., 0.03 < q st < 1.5 m d À1 ). An intense and short stream discharge period occurred in autumn, when stream water infiltrated a maximum of 10 m into the riparian zone. Nitrate concentration and nitrate removal spatial rates (h NO3 ) showed wide spatial heterogeneity. Higher nitrate concentrations (3.4 NO 3 -N mg L À1 ) and effective nitrate removal (h NO3 = 0.098 ± 0.04 m À1 ) were found in the deep groundwater of hillslope zone associated to low water fluxes. In contrast, in the stream edge zone (with higher water fluxes), nitrate release predominated over depletion (h NO3 = À0.13 ± 0.04 m À1

superficial fluxes, litter dynamics is an important factor to determine whether the pool of particulate organic Mediterranean riparian zones can experience severe drought perimatter in the forest floor may act as a source or sink ods that lead to low soil moisture content, which dramatically affects for nitrogen (Greenway, 1994). Several studies have their performance as nitrate removal systems. In the Mediterranean riparian zone of this study, we determined that N 2 O emission was coincided in pointing to the importance of leaves in practically nil. To understand the role of forest floor processes in relation to the total litter (leaf litter ϭ approximately nitrogen retention of a Mediterranean riparian area, we studied leaf 80% of the total litter) (Meentemeyer et al., 1982; litter dynamics of two tree species, London planetree [Platanus ϫ Sharma and Ambasht, 1987; Cañ ellas and San Miguel, acerifolia (Aiton) Willd.] and alder [Alnus glutinosa (L.) Gaertn.], 1997; Gallardo et al., 1998). Furthermore, although for two years, along with soil nitrogen mineralization rates. Annual there are studies on the effect of climatic factors on the leaf litter fall equaled 562.6 Ϯ 10.1 (standard error) g dry wt. m Ϫ2 , distribution and transfer of mineral nutrients to the soil 68% of which was planetree and 32% of which was alder. The temporal through litterfall (Bray and Gorham, 1964), little is distribution of litterfall showed a two-peak annual cycle, one occurring in midsummer, the other in autumn. Planetree provided the major known about how strongly dry periods may affect leaf input of organic nitrogen to the forest floor, and the amount of planlitter dynamics and further, the consequences on nitroetree leaves remaining on the forest floor was equivalent to approxigen retention capacity of the system. On the other hand, mately four years of stock. Leaf litter decomposition was three times studies on denitrification conducted in arid and semiarid higher for alder (decay coefficient [k] ϭ 1.13 yr Ϫ1 ) than for planetree areas concluded that when soil moisture content is well (k ϭ 0.365 yr Ϫ1 ). Mineralization rates showed a seasonal pattern, with below field capacity, N 2 O fluxes are low (Mummey et the maximum rate in summer (1.92 mg N kg Ϫ1 d Ϫ1 ). Although the al., 1994; Jorgensen et al., 1998). No previous studies forest floor was an important sink for nitrogen due to planetree leaf on how the conjunction of those mechanisms may affect accumulation, 7.5% of this leaf litter was scoured to the streambed the nitrogen retention capacity of riparian soils under by wind. This loss was irrelevant for alder leaves. Due to the litter quality, the forest floor of this Mediterranean riparian forest acts as Mediterranean climate seem available. This paper dea nitrogen sink. velops this approach in a Mediterranean riparian forest composed of two main tree species characterized by a very different litter quality: alder, with soft, N-rich leaves Departamento Ecología, Universidad de Barcelona, Avd. Diagonal as nitrogen mineralizat...

No abstract

Mediterranean riparian zones can experience severe drought periods that lead to low soil moisture content, which dramatically affects their performance as nitrate removal systems. In the Mediterranean riparian zone of this study, we determined that N2O emission was practically nil. To understand the role of forest floor processes in nitrogen retention of a Mediterranean riparian area, we studied leaf litter dynamics of two tree species, London planetree [Platanus x acerifolia (Aiton) Willd.] and alder [Alnus glutinosa (L.) Gaertn.], for two years, along with soil nitrogen mineralization rates. Annual leaf litter fall equaled 562.6 +/- 10.1 (standard error) g dry wt. m(-2), 68% of which was planetree and 32% of which was alder. The temporal distribution of litterfall showed a two-peak annual cycle, one occurring in midsummer, the other in autumn. Planetree provided the major input of organic nitrogen to the forest floor, and the amount of planetree leaves remaining on the forest floor was equivalent to approximately four years of stock. Leaf litter decomposition was three times higher for alder (decay coefficient [k] = 1.13 yr(-1)) than for planetree (k = 0.365 yr(-1)). Mineralization rates showed a seasonal pattern, with the maximum rate in summer (1.92 mg N kg(-1) d(-1)). Although the forest floor was an important sink for nitrogen due to planetree leaf accumulation, 7.5% of this leaf litter was scoured to the streambed by wind. This loss was irrelevant for alder leaves. Due to the litter quality, the forest floor of this Mediterranean riparian forest acts as a nitrogen sink.