Restoration of soil microbes and organic matter through tropical reforestation

Abstract

Soil organic matter (SOM) is of global importance as it represents a greater carbon pool than atmosphere or terrestrial vegetation and it strongly regulates primary productivity. SOM is thus integral to global climate change mitigation and food security. Recent advances reveal a pivotal role of soil microbes in both SOM formation and decomposition, placing them at the nexus of global biogeochemical cycles. However, the soil microbial traits that best promote SOM formation and persistence, and the environmental conditions that best promote such traits, remain poorly characterised – particularly in the tropics. In this thesis, I evaluated the interplay between microbial function and composition, vegetation, and SOM dynamics in tropical soils. My experimentation spanned laboratory and field study scales in two continents: (i) soil carbon cycling was examined following manipulations of soil microbial composition and function under controlled laboratory conditions in microcosms, (ii) the responses of soil microbes and SOM were compared in decadalold high diversity rainforest restoration plantings and reference soils under pasture and old-growth rainforest in tropical northeast Australia, and (iii) tropical tree monoculture and mixed species plantings in the Philippines were assessed for restoration of soil microbial traits and SOM. Several insights were gained that advance knowledge on SOM dynamics in the tropical context. Soil microbial substrate use efficiency, an indicator of SOM formation potential, changed significantly with microbial composition, increasing with greater fungal dominance. Stable SOM under Australian rainforest restoration plantings was unchanged circa two decades after plantation establishment, with no sign of recovery towards reference old- growth rainforest levels, which coincided with similarly stagnant microbial recovery. Soil microbial composition explained most of the variation in SOM across land uses in the Philippines, where SOM was also slow to recover, and microbial composition in turn correlated strongly with aboveground plant composition. The results thus indicate that (i) microbial composition can have a direct influence on efficiency of formation of SOM precursor material (microbial residues), and (ii) slow microbial recovery with reforestation coincides with slow SOM recovery. In combination with previous research suggesting that microbial traits are major determinants of SOM formation and persistence, the results prompt me to speculate that reliable restoration of stable SOM through tropical reforestation may often be constrained by limited restoration of the soil microbial community. This in turn may require more comprehensive restoration of aboveground plant community composition, or, potentially, upon active manipulation of soil microbial communities to circumvent long lag times that may prevent effective restoration of soil function. Future developments in forest restoration and climate mitigation efforts may require a shift towards integrating the soil microbial community. A deliberate and nuanced examination of soil microbial communities is needed to clarify their role in soil recovery, with a focus on designing and testing more effective interventions to overcome barriers to the recovery of degraded land.