Effects of attapulgite-biochar composites on nutrient slow-release in pakchoi field and the growth of pakchoi

Slow-release performance is one of the most important indicators for the quality of fertilizers in fields. However, the common defects of fertilizers cannot fully meet the large-scale production in recent years, such as the low utilization rate, large amount of runoff leaching and nutrient volatilizing loss, as well as the difficult synchronization of nutrient release and crop absorption. In this study, thermal modification treatment was carried out on the high-quality slow-release material attapulgite combined with biochar material at 300℃. A heat-modified attapulgite-biochar composite was prepared to explore the release of nitrogen (N), phosphorus (P), and potassium (K). The biological properties of materials were verified with four seasons of Chinese cabbage. Seven treatments included: fertilizer + attapulgite (AF), fertilizer + thermal modified attapulgite (TAF), fertilizer + biochar (BF), fertilizer + attapulgite + biochar (ABF), fertilizer + thermal modified attapulgite + biochar (TABF), and no fertilizer (CK), where the ordinary fertilizer (F) was used as the control. The results show that: 1) The natural attapulgite was achieved in the better adsorption capacity for the N, P, and K elements after modification at 300℃. The modified attapulgite structure was broken with circular collapse, where there were a lot of gaps and cracks. 2) The combination of attapulgite and biochar enabled the structure of biochar particles from the lamellar to loosely porous, thus effectively preventing the agglomeration of biochar particles to dispersed evenly. Thermally modified attapulgite + biochar shared more channels and deeper pore size, compared with the biochar. As such, more sites were offered for the interaction between nutrient ions and composite materials. The SiO₂, MgFe₄(SO₄)₆(OH)₂(H₂O)_20, and CaMg(CO₃)₂, oxygen-containing groups improved the adsorption efficiency of composite adsorbent. 3) The slow release of nutrients was better achieved in TABF, compared with pure fertilizer. The peak of N and P nutrient release occurred on the 52th, and 42th day, respectively. 4) TABF can be expected to fully meet the nutrient supply requirements in the whole growth period of Chinese cabbage, which was superior to control and pure fertilizer treatment in the phenotypic indexes, quality, and yield. Five slow-release fertilizers significantly increased the utilization rate of NPK fertilizer and agronomic efficiency. TABF was the best, compared with common fertilizers. Therefore, 300℃ of thermal modification significantly improved the adsorption site of attapulgite. The biochar mixture can be expected to obtain the structure of the slow-release composite material with attapulgite-biochar as the core, in order to enhance the interaction with nutrient ions, and then effectively improve the slow-release performance of fertilizer. The finding can provide a strong reference to improve the utilization rate of N, P, and K fertilizers.