Denitrification plays a critical role in mitigating anthropogenic nitrate (NO₃^–) accumulation in ecosystems. The isotopic composition of NO₃^– (δ¹⁵N and δ¹⁸O) serves as a powerful tracer for identifying N sources and transformation processes. Denitrification often superimposed on the isotope effects of NO₂^– oxidation, resulting in parallel enrichment of δ¹⁵N- and δ¹⁸O-NO₃^– (Δδ¹⁸O:Δδ¹⁵N trajectory) that causes them to be either below or above 1. This study compared the Δδ¹⁸O:Δδ¹⁵N trajectory during denitrification, functional genes (narG, napA, and nxrA), and carbon sources from metabolites in the Δδ¹⁸O:Δδ¹⁵N trajectories below or above 1 in unsaturated zones. The results revealed that NO₃^– reduction was more important for variation in the Δδ¹⁸O:Δδ¹⁵N trajectory because the difference in isotope effects (¹⁵ε_{NO3 reduction} and ¹⁸ε_{NO3 reduction}) between the two Δδ¹⁸O:Δδ¹⁵N trajectory groups was significant, whereas the difference in isotope effects (¹⁵ε_nxr and ¹⁸ε_nxr) upon NO₂^– oxidation was not. Carbon sources in the group with Δδ¹⁸O:Δδ¹⁵N trajectories below 1 facilitated more efficient electron production to promote NO₃^– reduction because of their low molecular weight and simple structure. Conversely, the lower electron production efficiency due to the high molecular weight and complex structures of carbon sources in the group with Δδ¹⁸O:Δδ¹⁵N trajectories above 1 downregulated the expression of the three functional genes (narG, napA, and nxrA). The group with Δδ¹⁸O:Δδ¹⁵N trajectories below 1 showed significantly higher levels of ¹⁵ε_{NO3 reduction}, ¹⁸ε_{NO3 reduction}, NO₂^– oxidation ratio, and copy numbers of narG, napA, and nxrA genes compared to the other group, revealing that NO₃^– reduction at the cellular level was more active in the former group. This study elucidated the integrated influence of isotope effects, NO₃^– reductase and NO₂^– oxidoreductase activities, and carbon sources from metabolites. These findings are significant for understanding the Δδ¹⁸O:Δδ¹⁵N trajectories of N cycling in terrestrial ecosystems and support groundwater conservation by improving carbon supplementation approaches that stimulate denitrification, with Δδ¹⁸O:Δδ¹⁵N trajectories serving as effective tracers for assessing denitrification performance in terrestrial environments.

Deforestation is one of the most serious environmental problems facing humankind. It continues to escalate rapidly across many regions of the world, thereby deteriorating the forest soil quality. This has prompted a large number of field-based studies aimed at understanding the impacts of deforestation on soil properties. However, the lack of comprehensive meta-analyses that utilized these studies has limited our deeper understanding of how different soil properties, including the soil organic carbon (SOC) pool, respond to deforestation. To address this critical knowledge gap, we conducted a meta-analysis of 144 studies to explore the impacts of deforestation on soil chemical, physical, and biological properties, with special emphasis on the long-term changes in SOC, such as concentrations, stocks, and sequestration. The results revealed that deforestation significantly decreased soil organic matter, electrical conductivity, and base saturation by 52%, 50%, and 98%, respectively. While deforestation increased soil total nitrogen content and decreased available phosphorus content by 51% and 99%, respectively, it resulted in slight decreases in some chemical properties, including soil pH (1%) and base cations (1%–13%). Deforestation significantly increased bulk density by 27% and soil erosion by 47%, but significantly decreased soil aggregate stability by 39% and saturated hydraulic conductivity by 63%. Soil microbial biomass C and N concentrations and enzyme activities were significantly decreased as a consequence of deforestation. Soil biological properties were much more affected by deforestation than soil physical and chemical properties. Regarding the SOC, the land use conversion from forest to pasture significantly increased SOC concentrations, stocks, and sequestration rates (11%–13%), whereas the land use conversions from forest to both plantation and cropland significantly decreased SOC concentrations, stocks, and sequestration rates (10%–43%). This observed decline in SOC accumulations decreased with increasing years after deforestation. The SOC dynamics following deforestation were predominantly regulated by microbial biomass concentrations, dehydrogenase activity, soil erosion, saturated hydraulic conductivity, aggregate stability, as well as concentrations of total organic carbon, total nitrogen, total phosphorus and organic matter. The present meta-analytical study provides compelling evidence that deforestation can induce profound changes in soil characteristics, including soil C contents, and has significant implications for soil health sustainability and climate change mitigation.