NAMASTE

Soils play a critical role in carbon (C) sequestration, a key strategy for mitigating climate change. Forest soils are particularly important, storing nearly 40% of global terrestrial organic carbon. Belowground inputs, such as roots, are more effective at stabilizing soil organic carbon (SOC) than aboveground inputs like leaf litter. SOC is essential for soil health, nutrient cycling, and ecosystem productivity.

SOC storage depends on a balance between plant-derived carbon inputs and microbial losses, a balance increasingly disrupted by human activities such as warming and nutrient imbalances. This challenge is especially pronounced in nitrogen-limited forest soils, where the ability to retain nitrogen and respond to increased N availability is crucial for maintaining carbon stocks. Historically, nitrogen (N) has been the most limiting nutrient for plant growth, particularly in ectomycorrhizal forests. However, anthropogenic activities have dramatically increased reactive N deposition, surpassing natural sources and causing eutrophication, acidification, and ecosystem shifts.

Although N addition influences carbon cycling, its effect on SOC storage remains unclear. Chronic N saturation can inhibit soil organic matter (SOM) decomposition, while N fertilization often shifts carbon allocation from roots to shoots, reducing belowground inputs and altering microbial dynamics. These changes may destabilize SOM through priming or slow decomposition, enhancing SOC accumulation. Soil fauna further influence organic matter transformation, microbial communities, and mineral weathering, adding complexity to C sequestration processes.

Recent frameworks view SOC as a continuum of biopolymers transformed by microbes and stabilized via interactions with minerals and aggregates, including mineral-associated organic matter (MAOM) and particulate organic matter (POM). Both pools are critical for C sequestration, and N enrichment may enhance microbial efficiency in forming MAOM under stable pH conditions.

The NAMASTE project builds on two long-term N addition experiments (10 years) in contrasting forests: Cansiglio (Fagus sylvatica, high N deposition, near saturation) and Prades (Quercus ilex, low N deposition, N-limited soils). The central hypothesis is that soil nutrient status shapes belowground inputs and biological communities, influencing root traits, microbial processes, and soil fauna activity. These interactions drive microbial necromass and organic residues, ultimately promoting carbon accrual.

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