Weathering by mycorrhizal fungi defines a threshold for nutrients in ecosystems along an aridity gradient

The Earth's surface is in constant change due to biotic and abiotic processes. During the last decades awareness arose that these biotic and abiotic processes might intensely interfere. Biogenic weathering, the acceleration of mineral weathering by autotroph-symbiont couples fuelled by photoassimilates for the sake of an equilibrated nutrient supply of involved biota, potentially drives denudation rates at ecosystem level. Our experiment aimed to examine how aridity affects biogenic weathering. The study was conducted along a gradient in Chile from humid to hyperarid climate (Atacama Desert), where photoassimilate production is increasingly limited by water stress. We hypothesize that biogenic weathering would cease if a threshold between element loss from denudation and energy demand for additional nutrient element mobilization by biogenic weathering is crossed, as competition between life for these elements becomes less intense when water supply limits biomass growth increasingly. We buried mesh bags containing freshly broken minerals, including biotite, muscovite and apatite along the gradient in Chile on granitic bedrock. Unexpectedly and in contrast to our initial hypothesis, we found that mineral weathering rates driven by mycorrhizal fungi under arid conditions were even proportionally higher, indicating a comparatively higher investment of photoassimilates into biogenic weathering by desert plants than by mediterranean, suggesting an adaptive mechanism. Additionally, biogenic weathering occurred at constant rates over a depth of up to 2.3 m, illustrating the constant mining of mycorrhizal fungi, irrespective of overall biological activity along the soil profile. The relative importance of biogenic weathering in arid climates furthermore points towards a fundamental function of biogenic weathering beyond nutrient mobilization by suggesting a regulatory role in overcoming long periods of missing soil water that prevent nutrient exchange from the soil matrix.