Is Composting a Route to Solubilize Low-Grade Phosphate Rocks and Improve MAP-Based Composts?

ABSTRACT: In alkalinized and Ca-rich composts, solubilization of apatite from phosphate rocks (PRs) is not guaranteed; however, chelating agents and humified substances produced during composting may alter the soluble contents and P forms of monoammonium phosphate (MAP)-based composts. These effects may depend on the proportions of organic wastes and P source used in the compost piles. The aim of this study was to evaluate the effect of composting chicken manure, coffee husk, and Araxá PR, Bayóvar PR, or MAP in different proportions on the organic matter decomposition, total N, Ca contents, and soluble P fractions in the composts. The treatments consisted of a 3 × 4 × 2 factorial, through the combination of three P sources [Araxá PR, Bayóvar PR, and MAP], with four mixtures in the respective proportions: 25, 40, 50, and 75 % of P source with 37.5, 40, 25, and 12.5 % of chicken manure, and 37.5, 20, 25, and 12.5 % of coffee husk, composted or not for 150 days. The composts with PRs showed greater reductions in total C and water-soluble C and lower dry mass yields than MAP-based composts. The use of MAP in mixtures ensured lower N losses compared to composts formulated with PRs. Regardless of the mixture among chicken manure, coffee husk, and PRs, composting increased the pH and total Ca contents and did not alter the fractions of soluble P in water, in citric acid, and in neutral ammonium citrate plus water in the final PR-based composts. Composting of these mixtures was not an efficient route to solubilize P from Araxá and Bayóvar PRs. Values of pH above 8 and high Ca contents were the main factors explaining the stability and non-solubilization of the apatite of PRs in the composts. Composting with MAP, mixed in different proportions with chicken manure and coffee husk, reduced water-soluble P, maintained the pH of the mixtures in the range of 5 to 7, and enriched the composts with N and P.

摘要：在碱性且富钙的堆肥体系中，磷矿石（phosphate rocks, PRs）中的磷灰石溶解并非必然发生；然而堆肥过程中产生的螯合剂与腐殖化物质，可能改变以磷酸一铵（monoammonium phosphate, MAP）为磷源的堆肥的可溶性磷含量与磷形态。这类效应可能取决于堆肥物料中有机废弃物与磷源的配比。本研究旨在评估不同配比下，鸡粪、咖啡壳与阿腊萨磷矿石（Araxá PR）、巴约瓦尔磷矿石（Bayóvar PR）或磷酸一铵堆肥后，堆肥的有机质降解、总氮、钙含量以及可溶性磷组分的变化。
试验采用3×4×2因子设计，组合3种磷源[阿腊萨磷矿石、巴约瓦尔磷矿石、磷酸一铵]，搭配4种不同比例的物料配比：磷源占比分别为25%、40%、50%、75%，对应鸡粪占比分别为37.5%、40%、25%、12.5%，咖啡壳占比分别为37.5%、20%、25%、12.5%，并设置堆肥（堆肥时长150 d）与未堆肥两个组别。
结果显示，以磷矿石为磷源的堆肥，其总碳与水溶性碳降幅更大，干物质产量低于以磷酸一铵为磷源的堆肥。相较于磷矿石基堆肥，添加磷酸一铵的混合料可降低氮素损失。无论鸡粪、咖啡壳与磷矿石的配比如何，堆肥均会提升体系pH与总钙含量，且不会改变最终磷矿石基堆肥中水溶性磷、柠檬酸溶性磷以及中性柠檬酸铵-水混合液溶性磷的组分占比。
将上述混合料进行堆肥，无法有效溶解阿腊萨与巴约瓦尔磷矿石中的磷。pH高于8且钙含量较高，是解释堆肥中磷矿石磷灰石稳定性与难溶解的核心因素。而与不同配比的鸡粪、咖啡壳混合后进行堆肥的磷酸一铵，可降低水溶性磷含量，将混合料pH维持在5~7区间，并提升堆肥的氮、磷养分含量。