Kim, K., A. K. Guber, M. Rivers, and A. N. Kravchenko. 2020. Contribution of decomposing plant roots to N2O emissions by water absorption. Geoderma 375:114506.
The “sponge effect”, or water absorption by incorporated plant leaf residues, was recently identified as one of the mechanisms that drives activity in microbial hotspots. We explored the presence of the sponge effect in plant root residues, and its role in root decomposition and associated N2O and CO2 emissions. Young soybean (Glycine max) plants were grown in microcosms with two soil materials dominated by (i) large (>30 μm Ø) and (ii) small (<10 μm Ø) pores. After termination, the microcosms with the decomposing roots were incubated at 50% and 75% water-filled pore space (WFPS) soil moisture levels. Root decomposition, water absorption by the decomposing roots, and water redistribution were quantified using X-ray computed micro-tomography (μCT), including dual-energy scanning. The results demonstrated occurrence of the sponge effect in young, in-situ grown soybean roots and sharp gradients in the distribution of the added liquid within ~150 µm distance from the decomposing roots. At 50% WFPS the large pore soil emitted 185% more N2O than the small pore soil; and, during the first 5 days of incubation, more N2O than the large pore soil at 75% WFPS. This finding indicates that the decomposing roots acted as hotspots of N2O production, potentially due to sponge effect and associated anoxic conditions. Our study suggests that the interactions between pore characteristics and soil moisture can play a significant role in defining the contribution of detritusphere, specifically, decomposing young roots, to soil biogeochemical processes, including microbial activity and denitrification dynamics.
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