Investigating Rhizosphere Controls of Soil Organic Matter Dynamics in Forest Soils using a 13C Labelling Approach

Rising atmospheric CO2 concentration may increase plant productivity through the “CO2 fertilization effect”, which may in turn increase the input of carbon (C) to soils through rhizodeposition or plant residues. However, whether this increase in C input to soils results in greater soil C storage is not clear, as the decomposition of different forms of organic matter and the role of the rhizosphere in the decomposition process remain poorly understood. In this thesis, I investigated the interactions between plant C dynamics and soil microbial processes, and how these interactions control C and nutrient cycling in forest soils. I manipulated soil carbon supply from trees to the rhizosphere both in mesocosms and in the field through either canopy shading or soil trenching. This allowed me to investigate the effect of assimilate C supply on the decomposition of 13C-labelled substrates of varying chemical compositions and structural complexities (glucose, straw, fungal necromass or biochar), and their combined effect on soil organic matter (SOM) decomposition. I found that plant C supply to the rhizosphere had no significant effect on the decomposition of substrates. Similarly, the presence of roots and their associated mycorrhizal fungi had no significant effect on litter mass loss. However, it was found that supply of C from plant to the rhizosphere promoted SOM decomposition by up to two-fold in soils amended with substrates. Although, the addition of both simple and complex substrates stimulated the activities of C, N and P- degrading enzymes, I observed that the activities of these enzymes were significantly greater in soils where a labile substrate (glucose) had been added. The increased activities of C-degrading enzymes suggest that microorganisms were C limited, and the input of labile C substrate alleviated C and energy limitation of enzyme production, allowing microbial communities to mobilize nutrients from decomposition of native SOM. This thesis demonstrates that substrate quality influences SOM decomposition, and that increased availability of labile substrates to the rhizosphere may have implications on forest soil C stocks

Rhizosphere carbon supply accelerates soil organic matter decomposition in the presence of fresh organic substrates

Background and aims: Belowground C supply from plant roots may accelerate the decomposition of SOM through the rhizosphere priming effect, but the detailed interaction between substrate quality and rhizosphere C supply is poorly understood. We hypothesize that decomposition of organic matter is enhanced by the combined effect of assimilate C supply to the rhizosphere and substrate amendments. Methods: Birch trees (Betula pendula) planted in experimental mesocosms; half of these trees were shaded to reduce the supply of assimilate C to roots and ECM fungi. Either 13C-enriched glucose, straw, fungal necromass or C4 biochar were subsequently added to each mesocosm. CO2 efflux derived from substrates were separated from that derived from native SOM and roots based on the isotopic composition of total respired CO2. Results: The addition of all substrates increased fluxes in both un-shaded and shaded treatments, with greatest total CO2 efflux observed in soils amended with straw. Increases in un-labelled CO2 were observed to be greater in the presence of belowground C supply than in mesocosms with shaded trees. Conclusions: Turnover of SOM is closely linked to belowground C allocation. The biochemical quality and recalcitrance of litter entering the soil C pool is of critical importance to this priming, as is the interaction with rhizosphere-associated decomposition activity