Impacts of exogenous potassium and sodium ions on soil aggregates and organic carbon mineralization in Loessial soil

Objectives Evaluating the effects of monovalent cations on aggregate stability and soil organic carbon mineralization could improve the rational use of organic supplements in arable land.

Methods We spiked a gradient of potassium (K⁺: 120, 200, 280, 370, 540 mg/kg), sodium (Na⁺: 90, 180, 270 mg/kg), and sodium at a specific potassium level (based on adding 370 mg/kg K⁺, adding 0, 90 and 180 mg/kg Na⁺ ) to soil subjected to synthetic nitrogen and phosphate fertilizers for 30 years and incubated the soils at 25℃ for 105 days. And the treatment without potassium and sodium ions was used as the control. We separated the soil aggregates at the end of the incubation, measured soil organic carbon (SOC) mineralization dynamics, and modelled SOC mineralization with a double exponential model.

Results The proportion of 2–0.25 mm and >2 mm aggregates in all treatments was higher than 0.25–0.053 mm aggregates and <0.053 mm fraction, accounting for 42.0%–52.7% and 26.0%–38.9% of the total. K⁺, Na⁺ or both treatments tended to decrease the proportion of >2 mm aggregates while increasing the proportion of 2–0.25 mm aggregates and <0.053 mm fraction. Consequently, the mean weight diameter (MWD) and geometric mean diameter (GMD) were reduced, especially for treatments spiked with a sodium gradient at a specific potassium level. The treatments with either K⁺ or Na⁺ addition had no significant effect except that adding 120 mg/kg K⁺ and 180 mg/kg Na⁺ reduced the amount of organic carbon mineralization significantly. For treatments spiked with K⁺ and Na⁺, the cumulative mineralization of organic carbon decreased with increasing Na⁺ concentration by 1.2% to 22.3%. The double exponential model predicted the mineralization dynamics of organic carbon. The simulation results showed that all treatments spiked with K⁺, Na⁺ or both increased the mineralization of activated carbon pool (Ca) and decreased stable carbon pool (Cs) except for K⁺ 540 mg/kg treatment. It reduced the activated carbon pool’s mineralization rate constant (Ka). Simultaneously, all treatments increased the proportion of Ca in total organic carbon mineralization (Ra) and decreased that of Cs (Rs). Redundancy analysis showed that the cumulative mineralization of organic carbon was significantly and positively correlated with GMD and negatively correlated with cation ratio of soil structural stability (CROSS), sodium adsorption ratio (SAR), and exchangeable sodium percentage (ESP).

Conclusions The addition of potassium and sodium ions affects soil aggregates and reduces the latter’s MWD and GMD values. Therefore, the mineralization of stable organic carbon is inhibited, and the mineralization of active organic carbon is increased. This may be why soil organic matter content is not increased due to potassium fertilizer application.