Novel Semi-IPN Nanocomposites with Functions of both Nutrient Slow-Release and Water Retention. 2. Effects on Soil Fertility and Tomato Quality

So far, the effects of the semi-interpenetrating polymer network (semi-IPN) composites with functions of both nutrient slow-release and water retention on soil physicochemical properties, yield, and quality of crops have not been studied. In Part 1 of this paper (Song, J.; Zhao, H.; Zhao, G.; Xiang, Y.; Liu, Y. J. Agric. Food Chem.2019, DOI: 10.1021/acs.jafc.9b00888), superabsorbent polymers SAP_WS (grafting wheat straw (WS) to poly­(acrylic-co-acrylamide), which is WS-g-P­(AA-co-AM)) and SAP_HEC (HEC (hydroxyethyl cellulose)-g-P­(AA-co-AM)), and their semi-IPN nanocomposites SI-PSRF/SAP_WS and SI-PSRF/SAP_HEC (formed by chemical bonding of SAP_WS or SAP_HEC with PSRF (NPK-containing polymeric slow-release fertilizer)) were prepared, and their microstructures and degradation performances were systematically studied. In this study, effects of these two nanocomposites on soil physicochemical properties, crop yield, and quality as well as soil fertility, especially the relationships between these effects and the degradation performances of the materials themselves, were investigated by a pot experiment of the tomato. Results show that SI-PSRF/SAP nanocomposites can regulate the pH values of weak alkaline soils close to 7.0. The changes of soil pH values, in our study, are basically synchronized with the degradation rates of SI-PSRF/SAP, the higher the degradation rate of SI-PSRF/SAP, the lower the pH value of the alkaline soil treated. Compared with PSRF+SAP (the simple physically mixed system of PSRF and SAP) and PSRF, during the whole growth period of the tomato, SI-PSRF/SAP treatments have the lowest nitrogen release amounts, 4.74 g for SI-PSRF/SAP_WS and 4.88 g for SI-PSRF/SAP_HEC, the highest nitrogen contents of soils after day 40, and the highest nitrogen contents of plants on day 100, 1.16 and 1.68 g for SI-PSRF/SAP_WS and 1.26 and 1.86 g for SI-PSRF/SAP_HEC. While for PSRF+SAP_WS, PSRF+SAP_HEC, and PSRF, they are 5.16 g, 0.81 g, 0.63 g and 5.26 g, 0.87 g, 0.66 g and 5.17 g, 0.63 g, 0.52 g, respectively. There is a significant positive correlation between the material degradation rates and their nitrogen release amounts in this study, while SI-PSRF/SAP systems have the highest correlation coefficient, 0.950. In addition, compared to the control blank, the SI-PSRF/SAP system significantly increases tomato yield, 270.1% for SI-PSRF/SAP_WS and 301.7% for SI-PSRF/SAP_HEC. Compared with PSRF+SAP, the SI-PSRF/SAP system can make the soil treated become a high-quality soil by influencing the soil pH value, conductivity, cation exchange capacity, and the contents of nitrogen, phosphorus, organic carbon, and active organic carbon, which have significant impact on the soil quality. The chemical-bonded functional nanocomposites with a semi-IPN three-dimensional network structures formed by hydrogen-bonding interactions among functional groups of their components can more efficiently improve soil fertility, increase soil nutrient supply capacity, and promote plants growth and development as well as solve the environmental pollution caused by traditional fertilizers. The technology reported in this paper is simple and feasible for large-scale production of fertilizer with both water retention and nutrient slow-release, even nanofertilizer, which has great application potential.