干旱区生态环境知识资源中心

Most nutrient-spiraling studies have focused on estimates of gross uptake (U-gross), which show that streams take up dissolved inorganic nutrients very efficiently. However, studies based on estimates of net uptake (U-net) emphasize that streams tend to be at biogeochemical steady state (i.e., U-net approximate to 0), at least on a time scale of hours. These findings suggest that streams can be highly reactive ecosystems but remain at short-term biogeochemical steady state if U-gross is counterbalanced by release (R), a process that remains widely unexplored. Here, we propose a novel approach to infer R by comparing U-net and U-gross estimated from ambient and plateau concentrations obtained from standard short-term nutrient additions along a reach. We used this approach to examine the temporal variation of R and its balance with U-gross in 2 streams with contrasting hydrological regime (i.e., perennial vs intermittent) during 2 years. We focused on the spiraling metrics of NH4+ and soluble reactive P (SRP), essential sources of N and P in stream ecosystems. R differed substantially between the 2 streams. The perennial stream had a higher proportion of dates with R > 0 and a 2x higher mean R than the intermittent stream for both nutrients. Despite these differences, the magnitude of R and U-gross tended to be similar for both nutrients within each stream, which lead to U-net approximate to 0 in most cases. A notable exception occurred for SRP in the intermittent stream, where R tended to be higher than U-gross during most of the winter period, probably because of desorption of P from stream sediments. Together, our findings shed light on the contribution of release processes to the dynamics of nutrient spiraling and support the idea that streams can be active ecosystems with high spiraling fluxes while simultaneously approaching short-term biogeochemical steady-state.