Effects of climate change on soil phosphorus pools in drylands.A multiscalar perspective

Resumen

Climate change is having significant impacts on biogeochemical cycles, particularly in drylands. Besides that, the global increase in atmospheric nitrogen (N) deposition may destabilize vegetation primary production in terrestrial ecosystems, and phosphorous (P) may become the most limiting nutrient in many of these ecosystems. Hence, clarifying the effects of climate change on the dynamics and availability of P in the soil is essential to forecast its impacts on ecosystem functioning, especially in drylands. However, the effects of climate change and biocrusts on soil P pools in arid and semi-arid ecosystems remain poorly understood. In addition, we do not have sufficient knowledge of the main factors that control soil P pools in coastal dunes ecosystems, which are among the most valued priority conservation areas worldwide and will probably suffer extreme events caused by climate change. Likewise, there are knowledge gaps regarding the interactions between increases in aridity, land-use intensification, and soil P dynamics in drylands. Finally, in the context of increases in aridity driven by climate change, it is essential to have a deeper understanding of how climate and other environmental factors drive P dynamics across terrestrial biomes at a global scale. The main objective of this thesis is to evaluate how major global change drivers (climate change and land-use intensification) will affect the dynamics of the P cycle across terrestrial biomes, with a particular focus on dryland ecosystems. Specifically, we evaluated the role of biological soil crusts, aridity, and livestock grazing on P pools in drylands at different spatial scales: local, regional, and global. Also, I determined the main environmental drivers controlling pools of soil P availability in terrestrial biomes globally. Chapter 1 evaluates how simulated climate change and biocrusts affect P pools in the soil top layer. For this, we used two long-term experiments located in Central (Aranjuez) and Southeast (Sorbas) Spain. Our results highlight the important role of biocrusts in regulating major P pools in dryland soils, and in increasing the resistance of the P cycle to the impacts of simulated climate change. They also show the large impacts of warming on the P pools, with significant increases in major pools, which may be related to both the decomposition of biocrusts tissues and the decrease in the activity of P solubilizing bacteria and fungi responsible for the transfer of mineral to organic P pools. Chapter 2 investigates the combined effects of biological and geochemical drivers on the labile, medium-labile, and recalcitrant P pools along dune ecosystems of the Atlantic coast of the Iberian Peninsula, encompassing a wide aridity gradient. Our results suggest a novel transfer mechanism mediated by microorganisms that transfer medium-lability P forms to the more labile P pool. At the same time, increases in bacterial richness associated with biofilms might be involved in the thickening of the medium-lability P pool in our climosequence. These bacterial-mediated transfers would confer resistance to labile P under a climate change scenario, and reveal the critical role of soil microorganisms as modulators of the P geochemical cycle. Chapter 3 evaluates the joined effects of aridity and livestock grazing pressure on soil P pools along global grazing pressure and climatic gradients using 98 rangelands from all continents except Antarctica. Our findings reveal that both climate change (i.e., increasing aridity) and land-use intensification (i.e., increases in livestock grazing pressure) have a synergistic effect on P availability, increasing concentrations of non-occluded P, which promotes the decoupling of N:P cycles across global drylands. Chapter 4 provides a novel clustering approach to soil P availability across biomes worldwide. Also, the study evaluates the main environmental drivers that determine the spatial distribution and vulnerability to climate change of the soil P pools in terrestrial biomes globally. This study highlights that soil P pools will depend on the biotic factors, ultimately affected by the climate and the amount of soil organic matter, and on geochemical processes influenced by topographic relief. These results also suggest that the predicted increase in temperature caused by ongoing climate change will have a direct negative impact on the reserves of bioavailable P in the short term, limiting primary production in terrestrial ecosystems worldwide. Overall, the findings obtained in this doctoral thesis provide outstanding information about microscale and macroscale mechanisms that control soil P dynamics in terrestrial ecosystems, especially in drylands. Specifically, our results provide novel insights about the increase of soil P availability caused by climate change and land-use intensification, promoting the imbalance between N and P on soils, which will negatively affect dryland ecosystem production. Likewise, they highlight the key role of biocrust and microbial communities control soil P pools in drylands. The information provided at a local and regional scale will be a precious source for the management and conservation strategies of drylands and mitigating the consequences of global change in ecosystem functions. Likewise, this deeper global-scale knowledge about the dynamics of the soil P cycle could be incorporated into Earth system models to understand better the effect of global change on the multifunctionality of ecosystems in the Anthropocene.