Legacy effect of elevated CO2 and N fertilization on mineralization and retention of rice (Oryza sativa L.) rhizodeposit-C in paddy soil aggregates

Yuhong Li,Mouliang Xiao,Zhenke Zhu,Kazuyuki Inubushi,Hongzhao Yuan,Rongzhong Ye,Yangwu Deng,Tida Ge,Anlei Chen,Jinshui Wu

doi:10.1007/s42832-020-0066-y

Abstract

Rhizodeposits in rice paddy soil are important in global C sequestration and cycling. This study explored the effects of elevated CO2 and N fertilization during the rice growing season on the subsequent mineralization and retention of rhizodeposit-C in soil aggregates after harvest. Rice (Oryza sativa L.) was labeled with 13CO2 under ambient (400 ppm) and elevated (800 ppm) CO2 concentrations with and without N fertilization. After harvest, soil with labeled rhizodeposits was collected, separated into three aggregate size fractions, and flood-incubated for 100 d. The initial rhizodeposit-13C content of N-fertilized microaggregates was less than 65% of that of non-fertilized microaggregates. During the incubation of microaggregates separated from N-fertilized soils, 3%–9% and 9%–16% more proportion of rhizodeposit-3C was mineralized to 13CO2, and incorporated into the microbial biomass, respectively, while less was allocated to soil organic carbon than in the non-fertilized soils. Elevated CO2 increased the rhizodeposit-13C content of all aggregate fractions by 10%–80%, while it reduced cumulative 13CO2 emission and the bioavailable C pool size of rhizodeposit-C, especially in N-fertilized soil, except for the silt-clay fraction. It also resulted in up to 23% less rhizodeposit-C incorporated into the microbial biomass of the three soil aggregates, and up to 23% more incorporated into soil organic carbon. These results were relatively weak in the silt-clay fraction. Elevated CO2 and N fertilizer applied in rice growing season had a legacy effect on subsequent mineralization and retention of rhizodeposits in paddy soils after harvest, the extent of which varied among the soil aggregates.

Highlights

In rice (Oryza sativa L.) paddy soil, rhizodeposition is an important source of soil organic carbon (SOC) (Kuzyakov, 2002)
This study explored the effects of elevated CO2 and N fertilization during the rice growing season on the subsequent mineralization and retention of rhizodeposit-C in soil aggregates after harvest
During the incubation of microaggregates separated from N-fertilized soils, 3%– 9% and 9%–16% more proportion of rhizodeposit-13C was mineralized to 13CO2, and incorporated into the microbial biomass, respectively, while less was allocated to soil organic carbon than in the non-fertilized soils

Summary

Introduction

In rice (Oryza sativa L.) paddy soil, rhizodeposition is an important source of soil organic carbon (SOC) (Kuzyakov, 2002). Rhizodeposits consist of root exudates, sloughed-off cells, mucilage, tissues, cell lysates, and root debris (Kuzyakov and Larionova, 2005; Gregory, 2006), accounting for over 10% of photosynthesized C (Jones et al, 2009; Nguyen, 2009). They contribute 1.5- to 3-fold more C than shoots (Hooker et al, 2005) and represent approximately 26% of the total belowground C input (Liu et al, 2019b). Recent studies have reported significant increases in SOC in paddy soil (Pan et al, 2004; Wu, 2011), largely due to the practice of residue incorporation and enhanced belowground C inputs (i.e., root and rhizodeposits) (Lu et al, 2002, 2003)

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