Plant–soil feedbacks mediated by humus forms: A review

Abstract

6The present review was undertaken to add more information on the place taken by humus forms in 7 plant-soil interactions. Three questions were asked: (i) are humus forms under the control of plant-soil 8 relationships, (ii) are humus forms the main seat of these relationships, and (iii) can humus forms 9 explain interactions between aboveground and belowground biodiversity. Some conflicting views 10 about humped-back models of species richness may be resolved by considering a limited number of 11 stable humus forms (here considered as ecosystem strategies) which should be treated separately rather 12 than in a single model. Mull, moder and mor pathways are each characterized by a fine tuning between 13 aboveground and belowground communities, the humus form (including litter) being the place where 14 resonance between these communities takes place, both in functional and evolutionary sense. 15 16

Humusica 1, article 2: Essential bases—Functional considerations

“…Following it, the more horizons there are, the more the living soil is organized, and the more it allows species to coexist. However, the higher level of biodiversity is actually achieved by Mull, not Moder or Mor (Ponge, 2003(Ponge, , 2013. Here the notion of stability takes place and should prevail when speaking about biodiversity.…”

“…Thus, even if their direct impact on weight loss is far less than that of the microflora, their impact on litter disappearance from the ground surface and incorporation into underlying mineral matter is of paramount importance. Both microbial and faunal litter-processing chains are tightly interconnected and benefit from each other (Ponge, 2013). In Figure 7 we present the annual bi-phasic cycle of litter (nOL horizon) in a temperate broadleaved forest.…”

“…-ecological conditions: cold climate, and/or base-poor siliceous substrate, poorly degradable litter (rich in resins, phenolics, thick cuticle, C/N > 40); -dominant actors of biodegradation: fungi (mostly mycorrhizal, review in Ponge, 2003Ponge, , 2013 and other non-faunal processes; -actors' action: very slow biodegradation (> 7 years), highest carbon content in organic horizons; -pH water of E or AE or A horizon: < 4.5; -diagnostic characters (morpho-functional result of specific biological activities):…”

“…Here the notion of stability takes place and should prevail when speaking about biodiversity. It is generally acknowledged that biodiversity first increases and then decreases with the level of disturbance according to a hump-shaped curve (Ponge, 2013). As most real objects, humus systems are the result of trade-offs (compromises) between independent, opposed forces (the socalled "laws", which are true only in theory, within a given set of conditions, rarely fulfilled in the real world, to the exception of man-made "devices").…”

“…In Mor the conservation of nutrients and energy is ensured by the soil, not by the vegetation. Moder is an intermediate between these two opposite strategies, allowing some level of nutrient and energy circulation between soil and vegetation, although at a lower level than Mull (Ponge, 2013). However, the all-round champion of compromises is Amphi.…”