Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils

Background/Question/Methods

The mobilization of nitrogen (N) from soil organic matter in temperate forest soils is controlled by the microbial production and activity of extracellular enzymes. The exudation of carbon (C) by tree roots into the rhizosphere may subsidize the microbial production of extracellular enzymes and increase the access of roots to N. The objective of this research was to investigate whether the stimulation of extracellular enzyme activity in the rhizosphere compared to surrounding bulk soil (i.e., rhizosphere effect) differs between tree species that form associations with ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. We conducted this research at the Morgan Monroe State Forest in Southern IN.  Monthly, over the 2011 growing season, we measured the rhizosphere effects on enzyme activity and N cycling in soils from plots dominated by either ECM or AM trees. Further, in July 2011, we girdled a subset of these plots to examine whether ECM and AM trees differ in the impact of reducing root C inputs on rates of C and N cycling in the rhizosphere and bulk soil.

Results/Conclusions

We found that the rhizosphere effect on proteolytic, chitinolytic and ligninolytic enzyme activities was greater in ECM soils than in AM soils. In particular, higher rates of proteolytic enzyme activity increased the availability of amino acid-N in ECM rhizospheres relative to the bulk soils. Experimental girdling led to a larger decline in enzyme activity and rates of N cycling in the rhizosphere and bulk soil of ECM trees than AM trees. In both ECM and AM soils, however, there has yet to be a decline in soil respiration. The results of this study contribute to the growing evidence that temperate forest tree roots, in particular ECM roots, can enhance soil-N cycling and extracellular enzyme activity through the allocation of C to the rhizosphere.  The larger decline in enzyme activity in response to girdling in ECM soils than in AM soils suggests that global changes that alter the flux of C belowground will have a larger impact on N cycling in ECM than in AM stands.