UC Santa Barbara

Riparian zones are important hotspots of biogeochemical cycling, yet these systems are stressed by the compounding pressures of altered hydrology, nitrogen-rich agricultural runoff, and opportunistic introduced species. Changes in environmental conditions can favor opportunistic species and disadvantage native woody species. Once established, grasses’ litter inputs and subsequent decomposition may in turn alter watershed biogeochemical cycling through differences in leaf chemistry. However, these outcomes are species- and environment-specific. A shift in the plant community from woody species toward grasses can enhance, reduce, or maintain carbon and nitrogen cycles; further, these cycles may play out differently in riparian soils and in the streams they are adjacent to. I evaluated the biogeochemical interactions of one such species that is now prolific in watersheds throughout the U.S. southwest: giant reed Arundo donax. In experimental settings Arundo inhibited the growth of Salix lasiolepis (willow) under conditions with high N amendments characteristic of intensive upland agricultural runoff. However, Arundo did not reduce willow’s growth in low N amendments or without N amendments (levels representative of upland runoff associated with sustainable farming practices or no agriculture). Arundo was able to reduce willow’s growth under high N conditions by taking up more N and maintaining high photosynthetic N use efficiency as well as decreasing investment in root biomass—perhaps enabling it to allocate excess N to compete for another resource. Once established, collecting field litter and soils showed that Arundo contributed less litter C and N than woody species, but soils beneath Arundo accumulated dissolved organic C (DOC) and N (DON). These soils also hosted a relatively small microbial community and had higher silicate content, suggesting that EOC is stabilized following microbial transformations and/or in silicate complexes. In an aquatic incubation, leachates of Arundo foliage contributed less dissolved DOC and more DON than willow, but this DOC was in fact more extensively decomposed by aquatic microbes during a two-week incubation. High resolution mass spectrometry revealed that Arundo foliage’s low DOC:DON, high aliphatics and peptides and low aromaticity and oxidation state contributed to its bioavailable fraction, yet over longer-term stages of decomposition Arundo foliage required more DON to break down, perhaps to create exoenzymes. Overall, Arundo material contributes a lower quantity of bioavailable DOC than willow. These results indicate that plant community shift towards Arundo could reduce soil and stream respiration, but also reduce bioavailable DOC at the base of these foodwebs.