Nitrogen‐induced acidification, not N‐nutrient, dominates suppressive N effects on arbuscular mycorrhizal fungi

Abstract

Arbuscular mycorrhizal fungi (AMF) form symbiosis with most terrestrial plant roots, obtaining photosynthates in return for mineral nutrients. Ecological theories based on the economics of trading partnership predict that nutrient enrichment would suppress AMF. Experimental results from nitrogen (N) and phosphorus (P) additions, however, were highly variable, and the underlying mechanisms remain unclear. Here we show distinct AMF responses to soil N:P stoichiometry manipulations via gradients of long-term N and P additions in a Mongolian steppe. A complementary experiment with an acid addition gradient was designed to help tease apart the effect of N-induced acidification from N nutrient. AMF root colonization and extraradical fungal biomass progressively decreased along the P gradient under two distinct host plant species, suggesting a carbon (C)-P tradeoff. In contrast, low to moderate N inputs increased both AMF parameters, corresponding to the increasing N:P ratio. Yet, high N inputs reduced AMF colonization and biomass, and the magnitudes of N-led inhibition were similar to those under acid additions that induced comparable changes in soil pH. Structural equation modeling further showed that while soil N:P stoichiometry primarily controlled the effect of P addition on AMF, N-induced soil acidity overtook the N:P stoichiometry under high N inputs and dominated the effects of reactive N on AMF. In addition, AMF community composition in roots was more dependent on host plants and unresponsive to changes in soil nutrients. We further proposed a comprehensive framework that integrates biological and geochemical effects of reactive N and P inputs on AMF. Together, these results indicate that while the C-P tradeoff controls P suppression of AMF, N-induced acidification dominates the N inhibition. Our findings suggest that incorporation of geochemical impacts of N and P inputs would facilitate modeling efforts to project mycorrhizal impact on plant interactions and soil C balance under future nutrient enrichment scenarios.

Nitrogen Addition and Arbuscular Mycorrhizal Fungi Beta Diversity: Patterns and Mechanisms

“…The microbial biomass carbon and nitrogen were found to have a collaboratively opposite relationship with ammonium nitrogen and nitrate nitrogen (by negatively correlating with overall AMF community composition, structure, and nestedness). We confirmed a strong relationship between soil AMF and pH (Pan et al, 2020), but it may also be influenced by other surroundings and explained more by the partitioning of the Simpson and nestedness components.…”

Soil priming effect and its responses to nutrient addition along a tropical forest elevation gradient

“…Such elevational patterns of P‐addition and NP‐addition effects on priming can be explained by the elevational patterns of available P content and stoichiometric ratios of EOC and available N to available P (Figure S4). The available P content, EOC:available P, and available N:P have been widely used to indicate relative P availability for soil microbes (Fujita et al, 2017; Han, Feng, et al, 2020; Pan et al, 2020; Schleuss et al, 2020), and a higher P content and lower EOC:available P and available N:P ratios represent a higher soil P availability. The positive relationships of P‐addition effect on priming with available P, EOC:available P, and available N:P suggested that P availability for soil microbes is an important regulator controlling the intensity of P‐addition effect on priming.…”

“…C enzyme , carbon-acquisition enzyme (i.e. (Fujita et al, 2017;Han, Feng, et al, 2020;Pan et al, 2020;Schleuss et al, 2020), and a higher P content and lower EOC:available P and available N:P ratios represent a higher soil P availability. The positive relationships of P-addition effect on priming with available P, EOC:available P, and available N:P suggested that P availability for soil microbes is an important regulator controlling the intensity of P-addition effect on priming.…”

Global negative effects of nutrient enrichment on arbuscular mycorrhizal fungi, plant diversity and ecosystem multifunctionality

“…The response of soil pH was positively correlated with the responses of AMF diversity and abundance to nutrient addition and was an important predictor for the response of AMF diversity and abundance. Previous researchers found that nutrient addition‐induced soil acidification can inhibit AMF colonization and mycelial development (Postma et al ., 2007; Sheldrake et al ., 2018; Pan et al ., 2020). Shifts in soil pH could directly impose a physiological constraint on AMF, which reduces the growth of some taxa when the soil pH falls outside a certain range and change in AMF communities (Kohout et al ., 2015; Xu et al ., 2017).…”