Soil Reclamation and Resource–Microbe–N2O Relationships Across Contrasting Saline Soil Contexts

Soil reclamation can reduce salinity and restore soil fertility, but these improvements may not predict how nitrogen (N) is partitioned between microbial retention and potential gaseous loss. We compared reclaimed and unreclaimed soils in two coastal field settings in Hainan, China: Wenchang, representing a lower-initial-salinity context based on the electrical conductivity of its unreclaimed soil measured in a 1:5 soil-to-water extract (EC1:5 = 1.72 dS m−1), and Danzhou, representing a higher-initial-salinity context (EC1:5 = 5.53 dS m−1). These contexts represented distinct field locations rather than experimentally imposed salinity levels. Soil carbon (C) and N fractions, microbial biomass, extracellular enzyme activities, N-cycling functional genes, and potential nitrous oxide (N2O) production during a standardized 48-day incubation were measured. Reclaimed soils had lower salinity and greater C and N resources than their unreclaimed counterparts at both sites, but standing microbial N pools and potential N2O responses differed. At Wenchang, microbial biomass N (MBN) and the MBN: mineral N ratio were lower in reclaimed soils, whereas cumulative potential N2O–N production was slightly higher (+3.0%). At Danzhou, MBN and the MBN: mineral N ratio increased, while cumulative N2O–N production declined by 40.3%. The archaeal-to-bacterial ammonia monooxygenase gene-abundance ratio increased in reclaimed soils at both sites, whereas the relative representation of nitrite-reductase and N2O-reductase genes shifted differently between sites. These site-specific patterns indicate that similar improvements in salinity and soil resources can coincide with contrasting standing microbial N pools and potential N2O-production outcomes. Because initial salinity was confounded with location and reclamation history, the observed differences cannot be attributed exclusively to salinity; instead, they highlight the importance of considering the broader edaphic and management context when evaluating reclamation outcomes.