Winter cover crops effects on soil organic carbon and soil physical quality in a Typical Argiudoll under continuous soybean cropping

ABSTRACT The massive adoption of no-tillage (NT), along with the simplification of the cropping sequences has led to physical and chemical degradation of soils. To recover degraded soils, cover crops have been proposed as an alternative to increase soil organic carbon (SOC) and to improve soil physical quality (SPQ). This study aimed (i) to determine the content of SOC and its physical and chemical fractions at different layers and positions, in a soil with a soybean crop under NT with and without winter cover crops, and (ii) to determine SPQ indicators in a soybean crop under NT with and without winter cover crops. Measures and samples were made on a field experiment in a typical Argiudoll of the Argentinean Pampas. Soil organic carbon, coarse and fine particulate organic carbon (POCc and POCf), mineral associated organic carbon (MOC), fulvic acids (FA), humic acids (HA), and humins (H) were determined. Soil physical quality indicators determined were: soil bulk density and total porosity from field samples, and saturated hydraulic conductivity, water-conducting macro and mesoporosity, and total porosity connectivity from field water infiltration data. After eight years, cover crops did not cause any observable change in whole SOC content, but significant differences were observed for some SOC fractions. Humic acids and POCc had 40 and 25 % increases, respectively, in the cover crop treatment. Mineral associated organic carbon and H decreased by 9 and 7 % in cover crop treatment. Soil physical quality did not improve after eight years of cover crops. This can be related to degradation processes after 20 years of soybean monoculture under NT, and to the low ability of Argiudolls to recover from physical degradation.摘要 免耕（No-tillage, NT）的大规模应用与种植序列简化共同引发了土壤理化退化。为修复退化土壤，覆盖作物被提出作为提升土壤有机碳（Soil Organic Carbon, SOC）含量、改善土壤物理质量（Soil Physical Quality, SPQ）的替代方案。本研究旨在达成两项目标：（i）在免耕种植的大豆田（分别搭配与不搭配冬季覆盖作物）中，测定不同土层及位点的土壤有机碳及其理化组分含量；（ii）测定上述两种免耕大豆种植体系下的土壤物理质量指标。本研究在阿根廷潘帕斯草原的典型黏化软土（Argiudoll）田间试验点开展采样与测定工作。 本研究测定的指标包括：土壤有机碳、粗颗粒有机碳与细颗粒有机碳（Coarse and Fine Particulate Organic Carbon, POCc与POCf）、矿质结合态有机碳（Mineral Associated Organic Carbon, MOC）、富里酸（Fulvic Acids, FA）、胡敏酸（Humic Acids, HA）及胡敏素（Humins, H）。测定的土壤物理质量指标包括：基于田间样品获得的土壤容重与总孔隙度，以及基于田间水分入渗数据获得的饱和导水率、导水大孔隙与中孔隙度及总孔隙连通性。 经过8年定位试验后，覆盖作物处理对土壤总有机碳含量未产生可观测的变化，但部分SOC组分出现显著差异：胡敏酸与粗颗粒有机碳在覆盖作物处理组中分别提升40%与25%；矿质结合态有机碳与胡敏素则分别下降9%与7%。8年覆盖作物种植并未改善土壤物理质量，这可能与免耕条件下20年大豆连作引发的土壤退化过程，以及黏化软土自身物理退化修复能力较弱有关。