Effects of root exudate stoichiometry on CO2 emission from paddy soil

Abstract

Root exudates are a labile source of carbon (C) for microorganisms that can lead to increased CO2 emission. Root exudates can vary in C:N stoichiometric ratio and their impact on microbially driven soil organic matter (SOM) turnover in paddy soils still remains unclear. The objective was to explore the underlying mechanisms involved in SOM decomposition due to root exudate (artificial) addition with three different C:N ratios (10, 20, and 40) during 45 days incubation. Different root exudates C:N ratios were obtained by adding mineral N and exudate components (glucose, oxalic acid, and glutamate) to paddy soil. N-only addition decreased dissolved organic C to limit CO2 emissions, which is an indicative of C sequestration. Conversely, simulated C:N stoichiometric ratios of root exudates significantly increased both microbial activity and metabolism without altering the microbial biomass C:N ratio. However, soil available dissolved organic C to NH4+ ratio decreased by exudates addition. The stoichiometric ratio of key C and N compound degrading enzymes activities increased only with C:N = 10 and remained unchanged with exudates C:N = 20 and 40. The qCO2 values increased with decreasing N-containing compounds in root exudates (i.e. highest CO2 emission was observed under C:N = 40 exudates addition). The results suggest that increasing exudates C:N ratio intensify CO2 emission due to high microbial N demand. Overall result show that root exudates C:N ratio and soil available N co-regulate on CO2 emission, which was controlled by microbial and potential extracellular enzyme activities.