Table_1_The Bacterial Community Structure and Microbial Activity in a Traditional Organic Milpa Farming System Under Different Soil Moisture Conditions.DOCX

Agricultural practices affect the bacterial community structure, but how they determine the response of the bacterial community to drought, is still largely unknown. Conventional cultivated soil, i.e., inorganic fertilization, tillage, crop residue removal and maize (Zea mays L.) monoculture, and traditional organic farmed soil “milpa,” i.e., minimum tillage, rotation of maize, pumpkin (Cucurbita sp.) and beans (Phaseolus vulgaris L.) and organic fertilization were sampled. Both soils from the central highlands of Mexico were characterized and incubated aerobically at 5% field capacity (5ü) and 100% field capacity (FC) for 45 days, while the C and N mineralization, enzyme activity and the bacterial community structure were monitored. After applying the different agricultural practices 3 years, the organic C content was 1.8-times larger in the milpa than in the conventional cultivated soil, the microbial biomass C 1.3-times, and C and N mineralization 2.0-times (mean for soil incubated at 5ü and FC). The dehydrogenase, activity was significantly higher in the conventional cultivated soil than in the milpa soil when incubated at 5ü, but not when incubated at FC. The relative abundance of Gemmatimonadetes was larger in the conventional cultivated soil than in the milpa soil in soil both at 5ü and FC, while that of Bacteroidetes showed an opposite trend. The relative abundance of other groups, such as Nitrospirae and Proteobacteria, was affected by cultivation technique, but controlled by soil water content. The relative abundance of other groups, e.g., FBP, Gemmatimonadetes and Proteobacteria, was affected by water content, but the effect depended on agricultural practice. For soil incubated at FC, the xenobiotics biodegradation and metabolism related functions were higher in the milpa soil than in the conventional cultivated soil, and carbohydrate metabolism showed an opposite trend. It was found that agricultural practices and soil water content had a strong effect on soil characteristics, C and N mineralization, enzyme activity, and the bacterial community structure and its functionality. Decreases or increases in the relative abundance of bacterial groups when the soil water content decreased, i.e., from FC to 5ü, was defined often by the cultivation technique, and the larger organic matter content in the milpa soil did not prevent large changes in the bacterial community structure when the soil was dried.

农业耕作措施会影响细菌群落结构，但耕作措施如何决定细菌群落对干旱胁迫的响应，目前仍未得到充分阐明。本研究采集了两种土壤：一是常规耕作土壤，即采用无机施肥、翻耕、移除作物残体以及玉米（Zea mays L.）单作的耕作模式；二是传统有机耕作土壤“米爾帕（milpa）”，即采用少耕、玉米-南瓜（Cucurbita sp.）-菜豆（Phaseolus vulgaris L.）轮作并配施有机肥的耕作模式。采自墨西哥中部高地的两种土壤均经过特性表征，并分别置于5%田间持水量（5ü）与100%田间持水量（FC）的有氧环境中培养45天，期间监测土壤碳氮矿化、酶活性以及细菌群落结构。经过3年的不同耕作措施处理后，米爾帕土壤的有机碳含量较常规耕作土壤提升1.8倍，微生物生物量碳提升1.3倍，碳氮矿化速率（取5ü与FC培养条件下的均值）提升2.0倍。在5ü培养条件下，常规耕作土壤的脱氢酶活性显著高于米爾帕土壤，但在FC培养条件下则无此差异。芽单胞菌门（Gemmatimonadetes）的相对丰度在5ü与FC培养条件下，均表现为常规耕作土壤高于米爾帕土壤；而拟杆菌门（Bacteroidetes）的相对丰度则呈现相反趋势。其他菌群如硝化螺旋菌门（Nitrospirae）与变形菌门（Proteobacteria）的相对丰度受耕作措施影响，但同时受土壤含水量调控。另有部分菌群如FBP、芽单胞菌门与变形菌门的相对丰度受土壤含水量影响，但其影响效果取决于耕作措施类型。在FC培养条件下，米爾帕土壤中外源异物生物降解与代谢相关功能的相对丰度高于常规耕作土壤，而碳水化合物代谢功能则呈现相反趋势。研究表明，耕作措施与土壤含水量均对土壤特性、碳氮矿化、酶活性以及细菌群落结构及其功能具有显著调控作用。当土壤含水量从FC降至5ü时，细菌类群相对丰度的升降幅度通常由耕作措施决定；即便米爾帕土壤的有机质含量更高，其细菌群落结构在土壤干燥过程中仍会发生显著变化。