Soil physical properties and soil organic carbon content in northeast Brazil: long-term tillage systems effects

ABSTRACT Concerns about the negative effects of agriculture on soil physical quality and soil organic carbon (SOC) pools have spurred on the adoption of conservation tillage systems in tropical regions. This study aimed to evaluate the long-term impacts (16th year) of conventional (CT), minimum (MT) and no-tillage (NT) practices and different cover crops (sunn hemp and a bean/millet sequence) on soil physical properties and SOC content of a corn cultivated Ultisol in the northeast of Brazil. Soil bulk density (Bd), soil penetration resistance (SPR), soil aggregation, and total aggregate-associated carbon (C) (4-2 mm) were evaluated. Tillage practices exerted strong control on soil physical properties and total aggregate-C content but were influenced by cover crop species. Minimum tillage presented the lowest Bd, irrespective of cover crop, while NT with bean/millet sequence resulted in the lowest SPR. However, as regards soil aggregation and total aggregate-C, the results indicated that there were no differences between MT and NT, with both systems presenting greater mean weight diameter (MWD) and total aggregate-C content than CT. Total aggregate-C content in the 0.00-0.05 m soil layer in conservation tillage was increased by the adoption of a bean/millet sequence. Increased mechanical disturbance through CT practices was harmful to Bd, soil aggregation and SOC accrual. Overall, more conservative tillage practices and the proper choice of cover crops might yield greater soil quality. Low intensity of soil disturbances due to the adoption of MT favors soil aggregation and the accrual of SOC in weakly structured soils through increases in contact between organic and mineral particles while not causing significant destruction of soil aggregates.

摘要 针对农业活动对土壤物理质量与土壤有机碳（SOC）库的负面影响的担忧，推动了热带地区保护性耕作系统的推广应用。本研究旨在评估长期（第16年）的常规耕作（CT）、少耕（MT）与免耕（NT）模式，以及不同覆盖作物（太阳麻、豆科/粟米轮作）对巴西东北部种植玉米的老成土（Ultisol）的土壤物理性质与土壤有机碳（SOC）含量的影响。研究测定了土壤容重（Bd）、土壤穿透阻力（SPR）、土壤团聚体状况以及4-2 mm粒径团聚体结合态总碳（C）含量。耕作模式对土壤物理性质与团聚体总碳含量具有显著调控作用，但同时受覆盖作物种类的影响。无论搭配何种覆盖作物，少耕模式下的土壤容重均最低；而免耕搭配豆科/粟米轮作时，土壤穿透阻力最低。但就土壤团聚体状况与团聚体总碳含量而言，少耕与免耕之间并无显著差异，且二者的平均重量直径（MWD）与团聚体总碳含量均高于常规耕作。采用豆科/粟米轮作可提升保护性耕作模式下0.00-0.05 m土层的团聚体总碳含量。常规耕作模式下机械扰动强度增加，会对土壤容重、土壤团聚体结构与土壤有机碳累积产生不利影响。总体而言，采用更为保守的耕作模式并合理选择覆盖作物，可有效提升土壤质量。少耕模式通过降低土壤扰动强度，能够通过增加有机颗粒与矿质颗粒的接触，促进结构较弱土壤的团聚体形成与有机碳累积，且不会对土壤团聚体造成显著破坏。