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（soil management）与作物轮作（crop rotation）是调控土壤剖面碳氮累积的核心因素，但其对深层土壤的长期影响仍未得到充分阐明，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，旨在探究不同土壤管理模式配合不同冬季覆盖作物，对1米深高黏（黏粒含量72%）土壤中碳氮累积的影响。试验在巴西南部的亚热带氧化土（subtropical Oxisol）中开展，设置两种耕作制度：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（black oat）、黑麦（rye）、普通野豌豆（common vetch）、毛叶野豌豆（hairy vetch）、油用萝卜（oilseed radish）、小麦（wheat）、蓝羽扇豆（blue lupine）及休耕（fallow）处理。试验开展26年后，在0~1.00米土层内的8个分层采集土壤样品，同时采集邻近原生林的土壤样品作为对照。 原生林开垦后，仅经过10年（1976至1986年）的耕作，0.00~0.20米土层的碳储量便下降了45%。试验启动26年后，免耕处理下0.00~0.20米土层的碳、氮储量较常规耕作分别高出13%和20%，其中0.00~0.05米土层的碳氮含量差异最为显著。在0.00~0.20米土层中，与冬季休耕的常规耕作相比，搭配冬季覆盖作物的免耕处理每年多累积0.6和0.06 Mg·ha⁻¹·yr⁻¹的碳和氮。免耕与常规耕作分别恢复了原生林0.00~0.20米土层碳储量的95%和83%。但在0.00~1.00米全土层中，免耕相较于常规耕作对土壤碳累积的促进作用被显著稀释，未观测到免耕的明确效应。此外，各土层的土壤碳氮储量未因冬季覆盖作物种类不同呈现显著差异，这可能与不同覆盖作物的残体输入量相近（3.3~4.9 Mg·ha⁻¹·yr⁻¹）有关。与冬季覆盖作物配合的免耕模式可通过较高的生物量输入，比常规耕作更快地恢复土壤碳氮储量，因此即便在高黏粒含量的氧化土中，该模式也是提升土壤碳氮累积的有效手段。