Organic carbon mineralization characteristics in rhizosphere and bulk soil of rice under organic fertilization modes in yellow paddy fields

Objectives Study of the effects of organic fertilization on soil organic carbon mineralization characteristics could elucidate the soil carbon pool change and stable mechanism of yellow paddy fields.

Methods The long-term experiment of fertility and fertilizer efficiency of Guiyang was established in 1994, with mono-rice planting system. In 2021, rhizosphere and bulk soils of rice were collected from the five treatments for the determination of soil total and active carbon fraction contents. Including the control (no fertilizer, CK), balanced application of chemical fertilizer (NPK), 25% and 50% organic fertilizer nitrogen instead of chemical fertilizer nitrogen (0.25MNPK, 0.5MNPK) and single organic fertilizer (M). A indoor incubation experiment was conducted using the soils for determination of organic carbon mineralization amount and kinetic mineralization parameters.

Results 1) Compared with NPK, organic fertilizer treatments (0.25MNPK, 0.5MNPK, M) increased soil organic carbon (SOC) in rhizosphere and bulk soil by 26%−43% and 24%−32%, and microbial biomass carbon (MBC) in rhizosphere soil by 16%−31%, and the MBC in rhizosphere soil was 148% higher than bulk soil. Compared with NPK, contents of liable organic carbon (LOC) in bulk soils was significantly increased by 36%−75% under organic fertilizer treatments. Dissolved organic carbon (DOC) content was significantly increased by 54% in bulk soil under 0.5MNPK treatment, and DOC in rhizosphere soil was 10% higher than that of bulk soil. 2) Organic fertilizer treatments significantly increased SOC mineralization in rhizosphere and bulk soils, and the mineralization amount and rate in bulk soil were 30% and 33% higher than in rhizosphere soil, respectively. Compared with CK and NPK, organic fertilizer treatments significantly increased rhizosphere SOC mineralization by 36%−63%, which was proportional to the application amount of organic fertilizer. NPK treatment recorded lower mineralization amount than CK, but its variation with CK was more pronounced in bulk soil. CK elicited the highest cumulative mineralization rate in bulk soil, which was 19%−28% higher than organic fertilizer treatments. 3) The dynamics of cumulative SOC mineralization amount with incubation time fit the first-order kinetic equation. In rhizosphere soils, treatment M increased the cumulative mineralization of organic carbon (C_t), while 0.25MNPK and 0.5MNPK treatments increased potential mineralizable organic carbon (C₀) by 1.44−3.23 times of CK. 0.25MNPK and 0.5MNPK treatments significantly reduced the turnover rate of organic carbon pool per day constant k. Compared with NPK, C_t was significantly increased in bulk soil, C₀ was significantly increased by 0.25MNPK treatment, and k value was significantly reduced under 0.25MNPK and 0.5MNPK treatments. 4) C_t was significantly and positively correlated with SOC, LOC, and DOC contents, but both were significantly and negatively correlated with qMB (SMB/SOC) in rhizosphere and bulk soils.

Conclusions In sum, 25% to 50% organic nitrogen substitution are the preferred fertilization modes for carbon pool management and soil fertilization in yellow paddy fields, for it markedly increases the stability of soil organic carbon, carbon storage, and reduces carbon loss.