干旱区生态环境知识资源中心

We compared spatial patterns of N (NO3--N and NH4+-N) and Cl- variability in 2 reaches with different hydrological regimes (permanent vs intermittent) of a typical Mediterranean semiarid stream under different hydrological conditions (baseflow, high flow, and drought). We hypothesized that stream-flow intermittency would affect N spatial variability. We expected higher spatial variability of solute concentrations in intermittent reaches than in permanent reaches because local environmental conditions (advection, water residence time, temperature, biotic properties, sediment-water interactions, or environment redox condition) change through space as flow discharge does. We expected spatial heterogeneity of solute concentrations to change more through seasons, as do stream hydrological conditions (high flood periods and drought), in the intermittent than in the permanent reach. We also expected seasonal variability to be higher in the intermittent than in the permanent reach. We used the coefficient of variation (CV) to measure the spatial and seasonal variability of solute concentration. We used a null model to test whether the differences in CVs among reaches were statistically significant. NH4+-N was the most spatially variable form of N in the intermittent reach. NO3--N and Cl- were seasonally more variable in the intermittent reach than in the permanent one. We propose that NO3--N and Cl- temporal variations are generated mainly by seasonal changes in advection, as shown by the synchrony between solute concentrations and rainfall. However, we propose that NH4+-N seasonal variation is generated mainly by the variation of biotic (uptake and dissimilatory reduction) and abiotic drivers (geochemical processes, such as chemical sorption-desorption) over time as hydrological conditions change. We also analyzed the effect of hydrological conditions on N spatial variability. We predicted that N spatial variability would decrease after rainfall and would increase during drought periods, especially in the intermittent reach. The results obtained in the intermittent reach fulfilled this prediction. However, the increase in runoff after the spring and autumn rainfall in the permanent reach increased the spatial variability in both N forms and Cl-. For our scale of comparison (5-8 km, 13 mo), NO3--N and Cl- were more variable spatially than seasonally in the permanent reach, whereas they were more variable seasonally than spatially in the intermittent reach. NH4+-N concentrations in both reaches were more variable seasonally than spatially. Our study emphasizes the potential effects of changes in climate regimes on the spatial and seasonal variability of N and, ultimately, on stream ecosystem functioning.