Response of soil aggregate-associated potassium to long-term fertilization in red soil

Abstract

Potassium (K) deficiency is commonly observed during crops grown in the red soil (which is a typical Plinthosol based on the IUSS guidelines) of China, but few studies have examined soil aggregate-associated K. In a long-term field experiment (initiated in 1986), the following treatments were applied: no fertilizer (CK), nitrogen and phosphorus fertilizers (NP), NP and K fertilizers (NPK) and a combination of manure and NPK (NPKM). After 30 years of fertilization, the nonexchangeable K (Nonex-K), and exchangeable K (Ex-K) contents or stocks in most aggregates varied among different treatments. Compared with NP treatment, the Nonex-K contents in the aggregate fractions of >2-, 1–2-, 0.5–1-, 0.25–0.5- and 0.053–0.25-mm under NPKM treatment were increased by 40.57%, 40.78%, 42.71%, 40.82% and 55.43%, respectively. The Nonex-K contents in 0.5–1-, 0.25–0.5-, 0.053–0.25- and <0.053-mm aggregates of the NPK treatment were 29.17%, 31.63%, 43.48% and 35.42% higher than those of the NP treatment, respectively. The Ex-K contents in all aggregates of the NPKM treatment were significantly (p < 0.05) higher than those of the other treatments (CK, NP and NPK). Compared with the NP treatment, the Ex-K contents in 1–2-, 0.5–1-, 0.25–0.5- and 0.053–0.25-mm aggregates of NPKM treatment were significantly increased by 30.30%, 32.65%, 33.67% and 33.33%, respectively. Moreover, the Ex-K stocks in the >2-, 1–2-, 0.5–1- and 0.053–0.25-mm aggregates of NPKM treatment were significantly increased by 74.36%, 123.63%, 44.88% and 37.47%, respectively, compared with those of the NPK treatment. Furthermore, a random forest model showed that K stocks in the >2-, 1–2- and 0.5–1-mm aggregates were the main factors affecting the uptake of K by maize. The relationships between the uptake of K by maize and K stocks in the >2-, 1–2- and 0.5–1-mm aggregates could be fitted by linear equations. Therefore, the long-term combination of chemical fertilizers with manure improved K contents and stocks for most aggregate sizes in red soil, especially Ex-K. We found that the turnover rate of K in the >2-mm aggregates was faster than in the other aggregates (1–2- and 0.5–1-mm) through slopes of linear regressions, allowing the soil to meet the requirements for crop K uptake.