Relationships between soil quality indicators, redox properties, and bioactivity of humic substances of soils under integrated farming, livestock, and forestry

ABSTRACT Once it is stabilized in the soil, organic matter minimizes limitations of Brazilian Oxisols, such as low cation exchange capacity, low nutrient availability, toxicity due to high aluminum content, and phosphate adsorption. Moreover, humified organic matter fractions are bioactive. It is, therefore, important to evaluate the biostimulant ability of compounds present in soil carbon stocks to develop sustainable technologies for tropical agriculture based on renewable natural resources. The objective of this research was to correlate some soil quality indicators, redox properties, and bioactivity of humic acids isolated from integrated farming, livestock, and forestry systems aiming to understand the mechanisms involved in plant stimulation by humified organic matter. Carbon stocks and their stability were determined from oxidation by dichromatometry and iodometry, respectively. Bioactivity was assessed using yield data of corn indicator plants. The results indicated that when native-like forests were reintroduced instead of pastureland, soil carbon stocks and their stability increased along with overall improvements in soil fertility, chemical and physical properties, and soil biodiversity. The bioactivity of humic substances isolated from soils used in integrated crop, livestock, and forestry management was higher than that of soils derived from pastures or eucalyptus alone.

摘要
有机质在土壤中稳定后，可缓解巴西氧化土（Oxisols）的诸多限制因素，例如较低的阳离子交换容量（cation exchange capacity）、养分有效性（nutrient availability）不足、高铝含量引发的毒性，以及磷酸盐吸附问题。此外，腐殖化有机质组分具有生物活性。因此，评估土壤碳库（soil carbon stocks）中所含化合物的生物刺激能力，对于开发基于可再生自然资源的热带农业可持续技术具有重要意义。
本研究旨在关联农-牧-林复合系统（integrated farming, livestock, and forestry systems）中分离得到的腐殖酸（humic acids）的部分土壤质量指标、氧化还原特性与生物活性，以期阐明腐殖化有机质刺激植物生长的内在机制。碳库及其稳定性分别通过重铬酸钾氧化法（dichromatometry）与碘量法（iodometry）测定。生物活性通过指示作物玉米的产量数据进行评估。
研究结果显示，相较于牧草地，若重新引入类原生森林，土壤碳库及其稳定性会随之提升，同时土壤肥力、理化性质与土壤生物多样性也得到全面改善。从作物-畜牧-林业复合经营管理（integrated crop, livestock, and forestry management）土壤中分离得到的腐殖物质（humic substances），其生物活性高于仅牧草地或单一桉树种植土壤的腐殖物质。