Effects of landscape modification on coastal sediment nitrogen availability, microbial functional gene abundances and NO production potential across the tropical-subtropical gradient.

Wetland sediment is an important nitrogen pool and a source of the greenhouse gas nitrous oxide (NO). Modification of coastal wetland landscape due to plant invasion and aquaculture activities may drastically change this N pool and the related dynamics of NO. This study measured the sediment properties, NO production and relevant functional gene abundances in 21 coastal wetlands across five provinces along the tropical-subtropical gradient in China, which all had experienced the same sequence of habitat transformation from native mudflats (MFs) to invasive Spartina alterniflora marshes (SAs) and subsequently to aquaculture ponds (APs). Our results showed that change from MFs to SAs increased the availability of NH-N and NO-N and the abundance of functional genes related to NO production (amoA, nirK, nosZ Ⅰ, and nosZ Ⅱ), whereas conversion of SAs to APs resulted in the opposite changes. Invasion of MFs by S. alterniflora increased NO production potential by 127.9%, whereas converting SAs to APs decreased it by 30.4%. Based on structural equation modelling, nitrogen substrate availability and abundance of ammonia oxidizers were the key factors driving the change in sediment NO production potential in these wetlands. This study revealed the main effect patterns of habitat modification on sediment biogeochemistry and NO production across a broad geographical and climate gradient. These findings will help large-scale mapping and assessing landscape change effects on sediment properties and greenhouse gas emissions along the coast.