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Microbial P limitation in tropical forest soils could be overestimated: Insight from a sorption experiment and a meta-analysis

The prevailing paradigm for soil microbial activity in tropical forests is that microbial activity is limited by phosphorus (P) availability, and thus exogenous P addition stimulates organic matter decomposition. This idea has been testified by studies demonstrating that experimental P addition accelerates soil respiration. Contrary to this conventional view, we hypothesize that the increased rates of soil microbial respiration could be due to the release of organic material from the surface of soil minerals when P is added, because P competes with organic C for binding sites in soil particles. Here we performed a sorption experiment in a tropical evergreen forest in southern China, where P addition had previously been reported to stimulate soil respiration but suppressed leaf litter decomposition. P addition to soils significantly increased dissolved organic carbon (DOC) content, which was extracted immediately after P addition and under a cold temperature where microbial activity was suppressed. This result can explain why P addition stimulated soil respiration but not litter decomposition in our study site. Namely, P addition abiotically elevated microbially-available C through the release of organic matter from the soil mineral surface. We also conducted a meta-analysis using data obtained in forest ecosystems, demonstrating that previous studies have consistently reported that P addition led to higher response ratios of soil microbial respiration than litter decomposition. Our findings suggest that the prevailing paradigm (i.e., soil microbial activity in tropical forests is limited by P availability) might require re-evaluation.

Termite mound as nutrient hot-spots in savannah with emphasis in P cycling and the potential use of mounds as soil amendment

Termites are an important component of pedofauna and are mainly distributed in subtropical and tropical areas. Their main effect on ecosystems is linked to the construction of tunnels, galleries, mounds, and nests. Termites induce strong changes in the physical-chemical and biological properties of the soil, after and through the processes of decomposition of the organic matter and formation of biogenic structures. At sites with abundant termite populations, galleries and foraging holes enhance soil porosity and infiltration rates, thus reducing soil bulk density. Termite activities result in nutrient accumulation in mounds; therefore, abundant termite populations could play an important role in controlling nutrient cycling in savannahs, where nutrients, particularly phosphorus (P), can often be a limiting factor. Regarding the high nutrient concentration accumulated in termite mounds, authors have claimed that parts of termite mounds could have potential as fertilisers for cultivated soils, and indeed, the use of termite materials for soil improvement is an extended practice in rural, poor, indigenous communities of Africa and Asia. This paper reviews the published data on the accumulation of the nutrients, mainly P in the soil of termite mounds in comparison with the none modified soil, and evaluates the potential use of termite biostructures in soil improvement. While it is true that in greenhouse experiments and in home orchards it is possible to observe the benefit of termite mound treatments, the implementation of such practices on a larger scale is prevented by the low ratio (by weight) of termite nests with respect to the total weight of the soil, as well as by the relatively long rate of renewal of termite mounds once destroyed. However, the use of large structure of Macrotermes, appears to be justified in a low-input agro-ecological scheme to promote the enhancement of termite-mediated ecosystem services.

Reintroduction of threatened digging mammals influences soil microbial communities differently along a rainfall gradient

Ecosystem engineers influence co-existing species indirectly, through their modification of habitat conditions, so the loss of these species may have broad consequences for ecosystems globally. Digging mammals alter soil via soil turnover, habitat modification and mycophagy. However, we have a limited understanding of their impacts in different environments. In a continent-scale study spanning 3000 km across southern Australia, we asked whether reintroductions of native digging mammals affect soil microbial communities in the soil matrix outside of their diggings, and if those impacts depend on the environmental context? We used high through-put sequencing analysis of bacterial and fungal environmental DNA to measure soil microbial diversity and community structure inside and outside digging mammal reintroduction areas at five reserves along a rainfall gradient from 166 to 877 mm per year, covering arid, semi-arid and temperate systems. Bacterial observed richness was not different inside and outside of the reserves; in contrast, fungal richness was higher in reserves, but only in arid and semi-arid environments. Fungal saprotrophs were more abundant in reserves: the mixing of soil layers mediated by digging mammals might therefore enhance decomposition. However, crust-forming microbes and ectomycorrhizal fungi were lower in abundance inside reserves, likely due to the disturbance and the altered soil nutrients that resulted from digging activity. Impacts of digging mammals varied among ecosystems which highlights the need for managers to consider the ecological context of reintroductions of ecosystem engineers when restoring for ecological functions.