ABSTRACT
To investigate the effects of long-term fertilization on the accumulation and availability of heavy metals in reddish paddy soil and to analyze the major influencing factors, soil samples were collected after the later rice was harvested in 2018 from a long-term fertilization field experiment that began in 1984. Six treatments were selected, namely CK (control without fertilization), PK (P and K fertilizer), and NPK (N, P, and K fertilizer), and different proportions of organic fertilizer plus chemical fertilizer (M1NPK: 30%M+70%NPK; M2NPK: 50%M+50%NPK; and M3NPK: 70%M+30%NPK), soil chemical properties, total and available heavy metal contents, and the relationships between the available forms of heavy metals and soil chemical parameters and total heavy metals (THM) were analyzed. The results showed that â long-term fertilization changed the soil chemical properties; compared with those of CK, PK significantly increased the contents of soil available phosphorus (AP) and available potassium (AK), NPK significantly increased the soil organic matter (SOM), cation exchange capacity (CEC), AP, and AK, and the organic fertilizer treatments significantly increased the contents of SOM, CEC, AP, AK, and nitrate (NO3--N). â¡ There were slight variations in the THM contents under the chemical fertilizer treatments (PK and NPK), whereas the organic fertilizer treatments significantly increased the total contents of Cu, Zn, and Cd. ⢠The chemical fertilizer treatments significantly increased the available Cr and As, whereas the organic fertilizer treatments significantly increased the available Cu, Zn, Cr, Cd, As, and Fe. ⣠There were significant positive correlations between the available Cu, Zn, Cr, Cd, As, and Fe and the SOM, CEC, AP, and NO3--N. In addition, the available Zn and Cd were significantly positively correlated with the soil pH, whereas the available Pb was significantly negatively correlated with soil pH, SOM, CEC, and NO3--N. ⤠There were significant positive correlations between the available and total contents of Cu, Zn, and Cd, whereas there were significant negative correlations between the available and total contents of Cr and Fe. ⥠Redundancy analysis showed that SOM and pH accounted for 80.7% and 5.5% of the variation in THM, whereas the soil CEC, AP, and pH accounted for 81.1%, 4.9%, and 3.3% of the variation in the available heavy metals, respectively. ⦠The partial least squares path model analysis showed that the path coefficients of the THM, CEC, and AP on the available state of heavy metals were 0.459, 0.417, and 0.293, respectively. Long-term application of organic manure, such as pig manure, significantly improved the soil chemical properties and affected the availability of heavy metals, and soil CEC and AP may play key roles in regulation.
ABSTRACT
Soil acidification of large areas of paddy fields in southern China has become an important problem in rice production. Therefore, how to ameliorate or remedy the acidifying paddy soil and to exposit its mechanism has important theoretical and practical significance for rebuilding healthy soils and guaranteeing national food security. Although lime has already been extensively used to remedy acidified soils, long-term application of a large amount of lime would not only cause the soil to harden, but also disturb the balance between calcium, potassium and magnesium in the soil. Given the advantages of lower solubility and comprehensive nutrient supply, fertilizer of calcium silicon magnesium potassium (CSMP) may be used as an alternative. The aim of this study was to clarify the functions of CSMP and its effects on soil acidification in paddy fields. A four-year field experiment was conducted to investigate the dynamics of soil pH, exchangeable acidi-ty, exchangeable base cation and available silicon, as well as 0~30 cm pH buffer capacity (pHBC), net base production under CSMP fertilization in the paddy soil. There were five treatments, i.e. CK (traditional fertilization practice of the local farmers), treatment I (CK plus 750 kg·hm-2 CSMP); treatment II (CK plus 1125 kg·hm-2 CSMP), treatment III (CK plus 1500 kg·hm-2 CSMP), and treatment IV (CK plus 1875 kg·hm-2 CSMP). The results showed that the traditional fertilization practice of the local farmers resulted in a decline of soil pH, soil exchangeable base cation and base saturation year by year, but soil exchangeable acid was increased with year. Conversely, CSPM fertlization significantly raised soil pH, with the magnitude of increases positively depending on the number of application times or application rate. Continuous and repeated application of CSMP effectively promoted the accumulation of exchangeable base cation and the consumption of soil exchangeable acid in paddy soil, especially for the accumulation of soil exchangeable Ca2+, Mg2+ and the consumption of soil exchangeable Al3+. Furthermore, the more amount of CSPM application resulted in the more accumulation or consumption, but with relatively slower rate. The exchangeable base cation and alkali released by CSMP contributed 108.8% to the total reduction of soil exchangeable acid, suggesting that it was the main path to reduce soil exchangeable acid. Meanwhile, CSMP application improved soil acidity in paddy field, with the content of available silicon increased year by year and the increase amplitude became larger with the more amount of CSMP application. The traditional fertilization of local farmers resulted in soil acidification, with a acidification rate was 2.86 kmol H+·hm-2·a-1. CSMP application could effectively control soil acidification, producing a lot of alkalinity with net alkalinity production of 9.93-13.82 kmol OH-·hm-2·a-1. CSPM could release Ca2+, Mg2+ and alkali, which would mitigate soil acidification in paddy fields.
