Polyphosphate is an alternative phosphorus (P) source which can substitute for orthophosphate-based P fertilizers in agriculture. In order to explore the effects of polyphosphate addition on soil P availability, and how pH and temperature affect polyphosphate hydrolysis, an aqueous and a soil incubation experiment were conducted at different pHs (5.0 and 8.2) and temperatures (15, 25 and 35°C); the influence of polyphosphate addition on soil available micronutrients (i.e., iron—Fe, manganese—Mn and zinc—Zn) was also studied. The experiments used three different polyphosphate fertilizers (solid and liquid ammonium polyphosphate and polyphosphoric acid) and two soil types (acid and alkaline). In aqueous incubation conditions, the average amount of phosphate ( ) released from polyphosphate fertilizers at 35°C was 2.7 times greater than at 25°C. The average Olsen P in polyphosphate-treated soil at 35°C was 1.12 times greater than at 25°C, indicating that higher temperature facilitates polyphosphate hydrolysis. Polyphosphate hydrolysis increased as the pH decreased in aqueous solution, but its hydrolysis rate was greater in calcareous soil than in acid soil. Moreover, polyphosphate significantly increased soil available Fe, Mn and Zn concentrations by an average of 14%, 16% and 20%, respectively, relative to orthophosphate fertilizer. In summary, high temperature and low pH favour the hydrolysis of short-chain polyphosphate (aqueous incubation experiment), and polyphosphate significantly increases soil P availability and mobilizes soil micronutrients. The application of polyphosphate can be recommended as a pragmatic P management strategy in agriculture: however, soil temperature and pH should be taken into account when using polyphosphate fertilizers.
On-site monitoring of phosphate concentration has attracted much attention recently for soil fertility management in agricultural products. This study proposes a portable and easy-operate electrochemical microfluidic system with integrated reagent reactions for on-site continuous monitoring of soil phosphate. Polydimethylsiloxane (PDMS) chips with channel layers and PDMS substrates embedded with screen-printed electrodes (SPEs) were prepared using 3-D printed molds. Reagents and samples were pumped into the chips at optimal flow rates of $100~\mu \text{L}$ min $^{-{1}}$ to automatically react, which were used for cyclic voltammetry (CV) electrochemical detection of analytes via carbon black nanoparticles (CBNPs) modified electrodes. After optimizing experimental conditions, including pH value, liquid flow rates, and voltammetry scan rates, the system achieved a linear phosphate concentration range of 5– $100~\mu \text{M}$ with a detection limit (LOD) of $4.2~\mu \text{M}$ ; moreover, the system exhibited excellent resistance to interference long lifespan and excellent phosphate recovery rate from 91.1% to 110.48%. This system has been successfully applied to continuous field phosphate determination in real soil environments. Results elucidated that the system has good practicality and great potential for fine nutrient management in the field.
Microplastics are an emerging threat to soils, but little is known about their effects on soil nitrogen (N) and phosphorus (P) cycling. In this study, a three-month soil incubation experiment has been conducted to analyze the effects of polyethene (PE) and polypropylene (PP) microplastics in sizes of 0–1 mm and 1–5 mm on soil available phosphate, nitrate, and ammonium contents under different fertilization regimes. Soil phosphorus and nitrogen availability were continuously determined in-situ by ion-exchange membrane method during the incubation. Microplastic surface chemical composition and the specific surface area were analyzed by FTIR and BET, respectively. The 16s rRNA sequencing of soil bacterial communities as well as soil pH have been determined after the incubation. The results showed that the presence of microplastics could significantly (P < 0.05) decrease soil available phosphate content from 122.61 mg P L −1 to 63.43 mg P L −1 . The addition of PP microplastics could significantly increase soil available ammonium content from 0.94 mg N L −1 to 1.53 mg N L −1 . Since microplastics had undetectable specific surface area and limited effects on soil microorganisms, adsorption and microorganism alteration functions might not be the main drivers of microplastic effects on soil phosphorus and nitrogen.
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