Human impacts and aridity differentially alter soilNavailability in drylands worldwide

Abstract

AbstractAimsClimate and human impacts are changing the nitrogen (N) inputs and losses in terrestrial ecosystems. However, it is largely unknown how these two major drivers of global change will simultaneously influence theNcycle in drylands, the largest terrestrial biome on the planet. We conducted a global observational study to evaluate how aridity and human impacts, together with biotic and abiotic factors, affect key soil variables of theNcycle.LocationTwo hundred and twenty‐four dryland sites from all continents exceptAntarctica widely differing in their environmental conditions and human influence.MethodsUsing a standardized field survey, we measured aridity, human impacts (i.e. proxies of land uses and air pollution), key biophysical variables (i.e. soilpHand texture and total plant cover) and six important variables related toNcycling in soils: totalN, organicN, ammonium, nitrate, dissolved organic:inorganicNandNmineralization rates. We used structural equation modelling to assess the direct and indirect effects of aridity, human impacts and key biophysical variables on theNcycle.ResultsHuman impacts increased the concentration of totalN, while aridity reduced it. The effects of aridity and human impacts on theNcycle were spatially disconnected, which may favour scarcity ofNin the most arid areas and promote its accumulation in the least arid areas.Main conclusionsWe found that increasing aridity and anthropogenic pressure are spatially disconnected in drylands. This implies that while places with low aridity and high human impact accumulateN, most arid sites with the lowest human impacts loseN. Our analyses also provide evidence that both increasing aridity and human impacts may enhance the relative dominance of inorganicNin dryland soils, having a negative impact on key functions and services provided by these ecosystems.