How Resilient is the Seagrass Nitrogen Cycle to Moderate Nutrient Enrichment?

Seagrass meadows are highly productive systems that act as important nitrogen (N) sinks in the coastal zone. However, this service can be impacted by anthropogenic nutrient enrichment, which can increase the release of the greenhouse gas nitrous oxide. Anthropogenic climate change and coastal development will make moderate nutrient enrichment more commonplace in the future. Unlike high nutrient enrichment, our understanding of the impact of this moderate nutrient enrichment on N pathways in seagrass meadows is limited. In this study the water column above Australian seagrass patches (Cymodocea serrulata) was experimentally enriched in situ with N-P-K fertilizer, delivering 0.12 g N m^-2 day^-1 for 75 days. From days 35 to 75 of nutrient enrichment, an in situ ¹⁵N pulse-chase experiment was conducted to compare N uptake and capture by seagrass, epiphytes, and sediment within nutrient-enriched and ambient patches. Simultaneously, the effect of nutrient enrichment on dissimilatory N pathways was measured using three incubation methods: sediment slurries, intact sediment cores, and in situ benthic chambers. ¹⁵N-labelling indicated that moderate nutrient enrichment enhanced N uptake by epiphytes (median increased from 9.9 ± 1.5% to 19.9 ± 5.2% of ¹⁵N label) and lowered N storage in belowground tissue (median decreased from 5.9 ± 0.5% to 3.9 ± 1.9% of ¹⁵N at experiment end). Slurry incubations revealed that the potential denitrification rate in sediment was enhanced by nutrient enrichment. However, the more representative, intact sediment core and in situ incubations showed no change in denitrification rate due to nutrient-enrichment. Furthermore, denitrification was of minor importance in both the core and in situ incubations, while dissimilatory reduction of nitrate to ammonium (DNRA) was the dominant NO₃^- consuming pathway regardless of nutrient treatment. Moderate nutrient enrichment did not alter the rate of nitrous oxide production in the sediment, nor did it increase nitrous oxide flux from the sediment to the water. Our findings support the idea that seagrass functions as a buffer against nutrient enrichment, preventing drastic changes to dissimilatory N pathways. While moderate nutrient enrichment does not induce additional nitrous oxide release, it does decrease the long-term N storage efficiency of seagrass meadows.