Effect of 26-years of soil tillage systems and winter cover crops on C and N stocks in a Southern Brazilian Oxisol

ABSTRACT Soil management and crop rotation are key factors in controlling the accumulation of C and N in the soil profile, but their long-term effect remains poorly understood for deep soil layers, especially in subtropical conditions. Using a long-term experiment (26-years), this study aimed to evaluate the effect of different soil management systems associated with different winter cover crops on C and N accumulation in a very clayey (72 % clay) soil up to 1 m deep. Two tillage systems [conventional tillage (CT) and no-tillage (NT)] were cultivated with eight winter cover crops (black oat, rye, common vetch, hairy vetch, oilseed radish, wheat, blue lupine, and fallow) in a subtropical Oxisol from Southern Brazil. Soil samples were taken in eight soil layers up to 1.00 m soil depth after 26 years of experiment and, also from an adjacent native forest. After forest clearing, the C stock in the 0.00-0.20 m soil layer was reduced by 45 % in only 10 years (from 1976 to 1986) of soil tillage. Twenty-six years after the beginning of the experiment, C and N stock in 0.00-0.20 m soil layer were 13 and 20 % higher in NT compared to CT, with the greatest differences in C and N content observed in the 0.00-0.05 m layer. When associated with winter cover crops, NT accumulated 0.6 and 0.06 Mg ha-1 yr-1 more C and N than CT with winter fallow in the 0.00-0.20 m soil layer. No-tillage and CT recovered 95 and 83 %, respectively, of the C stock found in the 0.00-0.20 m layer from the native forest. However, in the 0.00-1.00 m soil layer, the positive effect of NT on soil C accumulation compared to CT was diluted, and no clear effect of NT was verified. Moreover, no difference in winter cover crops on soil C and N stocks were observed in all soil layers, possibly due to their similar residues input (3.3-4.9 Mg ha-1 yr-1). No-tillage associated with high biomass input through winter cover crops promoted a faster recovery of soil C and N stock than in CT and, therefore, is an efficient tool to improve soil C and N accumulation even in Oxisols with high clay content.

摘要 土壤管理与作物轮作是调控土壤剖面碳（C）、氮（N）累积的关键因子，但其对深层土壤的长期效应仍未得到充分阐明，尤其在亚热带环境下。本研究依托一项为期26年的长期定位试验，旨在评估不同土壤管理体系搭配不同冬季覆盖作物，对巴西南部亚热带氧化土（Oxisol，黏粒含量72%）中深度达1米的土层内碳氮累积的影响。试验设置两种耕作体系：常规耕作（conventional tillage, CT）与免耕（no-tillage, NT），并搭配8种冬季覆盖作物：黑燕麦、黑麦、普通野豌豆、毛叶苕子、油料萝卜、小麦、蓝羽扇豆及休闲处理。试验开展26年后，在深度达1.00米的土层中分层采集土壤样品，同时采集邻近原生林的土壤作为对照。结果显示，森林砍伐后仅10年（1976-1986年）的常规耕作便使0.00-0.20米土层的碳库降低了45%。试验开始26年后，免耕处理0.00-0.20米土层的碳、氮库较常规耕作分别提升13%与20%，其中碳、氮含量的最大差异出现在0.00-0.05米土层。当搭配冬季覆盖作物时，免耕处理在0.00-0.20米土层的碳、氮累积量较休闲处理的常规耕作分别高0.6和0.06 Mg ha⁻¹ yr⁻¹。免耕与常规耕作分别恢复了原生林0.00-0.20米土层碳库的95%与83%。然而在0.00-1.00米全土层中，免耕相较于常规耕作的土壤碳累积正向效应被显著稀释，未观测到免耕的明确效应。此外，所有土层中冬季覆盖作物对土壤碳、氮库均无显著差异，这可能与其残体输入量相近（3.3-4.9 Mg ha⁻¹ yr⁻¹）有关。通过冬季覆盖作物实现高生物量输入的免耕，相较常规耕作可更快恢复土壤碳氮库，因此即便在高黏粒含量的氧化土中，也是提升土壤碳氮累积的有效手段。