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

Abstract

Rhizodeposits in rice paddy soil are important in global C sequestration and cycling. This study explored the effects of elevated CO 2 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 13 CO 2 under ambient (400 ppm) and elevated (800 ppm) CO 2 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-13 C 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-13 C was mineralized to 13 CO 2 , and incorporated into the microbial biomass, respectively, while less was allocated to soil organic carbon than in the non-fertilized soils. Elevated CO 2 increased the rhizodeposit-13 C content of all aggregate fractions by 10%-80%, while it reduced cumulative 13 CO 2 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 CO 2 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.

Microbial Resource Limitation in Aggregates in Karst and Non-Karst Soils

“…We observed a greater accumulation of MBP and MBC in finer aggregates (<0.053 mm) than in macroaggregates in karst forests (Table 1), which is consistent with the observations by Dorodnikov and Li [68,69]. The accumulation of MBC in finer aggregate fractions is usually due to higher specific surfaces for microbial habitats and greater protections of microorganisms from predation by protozoa or from desiccation, which help in stabilizing microbial residues [69,70] and may cause high levels of C sequestration in karst forests. MBP accumulated more in finer aggregates in both karst (<0.053mm and 0.053-0.25mm) and non-karst (<0.053 mm) ecosystems for the same reason.…”

Root exudates with low C/N ratios accelerate CO2 emissions from paddy soil

“…Plants modify the soil environment either by releasing C from their roots (e.g., root exudates) or by the rapid uptake of nutrients from the soil by their associated microorganisms (Kuzyakov, 2002; Jones et al, 2009; Fisk et al, 2015; Liu et al, 2019; Xiong et al, 2019; Li, Ge, et al, 2020; Li, Yuan, et al, 2020). Across different plant species, around 1%–10% of photoassimilated C is released into the soil as root exudates (Jones et al, 2004; Phillips et al, 2011; Qiao et al, 2014; Yin et al, 2014), which consist primarily of sugars, but can also include phenolics, amino acids, organic acids, and other metabolite derivatives (Haichar et al, 2014; Yuan et al, 2017).…”

“…Several mechanisms have been proposed to explain how root exudates affect the microbial decomposition of soil organic matter (SOM). It has been hypothesized that (i) root exudates provide energy for stimulating SOM decomposition and changing the chemical and physical characteristics of the soil environment (Qiao et al, 2016; Zhu, Ge, Liu, et al, 2018; Zhu, Ge, Luo, et al, 2018; Mehnaz et al, 2019; Du et al, 2020., Liu et al, 2022); (ii) labile C promotes microbial growth, which in turn, increases the N demand and microbial N mining from SOM (Manzoni et al, 2010; Qiao et al, 2016; Zhu, Ge, Liu, et al, 2018; Zhu, Ge, Luo, et al, 2018); and (iii) microbial C and N demand causes community shifts that alter microbial‐mediated C decomposition (Phillips et al, 2011; Wild et al, 2014; Li, Ge, et al, 2020; Li, Yuan, et al, 2020; Xu et al, 2020; Wei et al, 2022). Moreover, the impact of nutrients on CN stoichiometry need to be considered, such as the addition of C substrates like glucose, and the different several levels of N application, which provide a proportion of C that is integrated into the microbial biomass that becomes stabilized as SOM with the cost of CO 2 emission (Creamer et al, 2014; Liu, Ge, et al, 2021).…”

“…Several mechanisms have been proposed to explain how root exudates affect the microbial decomposition of soil organic matter (SOM). It has been hypothesized that (i) root exudates provide energy for stimulating SOM decomposition and changing the chemical and physical characteristics of the soil environment (Qiao et al, 2016;Zhu, Ge, Luo, et al, 2018;Mehnaz et al, 2019;Du et al, 2020., Liu et al, 2022; (ii) labile C promotes microbial growth, which in turn, increases the N demand and microbial N mining from SOM (Manzoni et al, 2010;Qiao et al, 2016;Zhu, Ge, Luo, et al, 2018); and (iii) microbial C and N demand causes community shifts that alter microbial-mediated C decomposition (Phillips et al, 2011;Wild et al, 2014;Li, Yuan, et al, 2020;Xu et al, 2020;Wei et al, 2022).…”

Visualization and quantification of carbon “rusty sink” by rice root iron plaque: Mechanisms, functions, and global implications

“…Complex biogeochemical processes facilitate specific C sequestration mechanisms in paddy soils (Ge et al, 2012; Li et al, 2021; Liu, Ge, et al, 2019). The organic carbon (OC) storage in paddy soils primarily occurs through the combined action of (i) physical protection of aggregates (Li et al, 2020), (ii) sorption by and precipitation on iron (Fe) (oxyhydr)oxides, (iii) chemical stability of the OC molecular structure, and (iv) slow decomposition of OC under anoxic conditions (Wei, Ge, et al, 2021). For example, the physicochemical protection of OC arises through bonding and aggregation with amorphous Fe (oxyhydr)oxides and aluminum oxides (Wissing et al, 2013).…”