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：在碱性富钙堆肥体系中，磷矿石（phosphate rocks, PRs）中的磷灰石未必能实现有效溶出；但堆肥过程中产生的螯合剂与腐殖化物质，可改变以磷酸一铵（monoammonium phosphate, MAP）为磷源的堆肥的可溶性磷含量与磷形态。上述效应的强弱取决于堆肥料堆中有机废弃物与磷源的配比。本研究旨在评估不同配比下，添加鸡粪、咖啡壳与阿腊萨磷矿石（Araxá PR）、巴约瓦尔磷矿石（Bayóvar PR）或磷酸一铵的堆肥体系中，有机质降解程度、总氮含量、钙含量以及可溶性磷组分的变化规律。试验采用3×4×2因子设计，即组合3种磷源[阿腊萨磷矿石、巴约瓦尔磷矿石与磷酸一铵]，搭配4种配比梯度：磷源占比分别为25%、40%、50%、75%，对应鸡粪占比依次为37.5%、40%、25%、12.5%，咖啡壳占比依次为37.5%、20%、25%、12.5%，各配比混合物经150天堆肥发酵或未堆肥处理。相较于以磷酸一铵为磷源的堆肥，磷矿石基堆肥的总碳与水溶性碳降幅更大，干物质得率更低。添加磷酸一铵的混合物料堆肥，相比磷矿石基堆肥可有效降低氮素损失。无论鸡粪、咖啡壳与磷矿石的配比如何，堆肥发酵均会提升体系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-水溶性磷的组分占比。针对阿腊萨与巴约瓦尔磷矿石，通过上述混合物料堆肥实现磷素溶出并非高效途径。pH高于8且钙含量较高，是解释堆肥中磷矿石磷灰石保持稳定、未被溶出的核心影响因素。将磷酸一铵与鸡粪、咖啡壳按不同比例混合进行堆肥，可降低水溶性磷含量，将物料pH维持在5~7区间，并提升堆肥的氮、磷富集水平。