Effects of Reduced and Conventional Tillage on Weed Communities: Results of a Long-Term Experiment in Southwestern Spain

ABSTRACT: An important drawback in adopting minimum tillage (MT) and no-tillage (NT) techniques is the frequently observed weed shift promoting adapted species and achieving poorer weed control. These changes can be detected best with long-term experiments, and results might differ depending on soil characteristics and the local flora. The objectives of this work were to evaluate the effect of reduced tillage on weed seed distribution in the soil profile and to identify possible consequences on weed diversity on a long-term experiment maintained during 24 years in Seville (Spain) with three tillage systems: NT, MT and conventional tillage (CT) including moldboard plow on a vertisol. For this purpose, soil seedbanks at 0-8 cm and 8-16 cm depths were enumerated in autumn 2005 and in-field emerged plants in autumn 2005 and winter 2006. Shannon diversity index (H) and evenness (J’) were calculated for seedbank and aboveground weed communities. Total weed seed density was highest for NT and lowest for CT. Some big-seeded species, such as Chrozophora tinctorea L., showed highest seed density in CT. NT increased the relative density of Amaranthus blitoides S. Watson seeds in the seedbank and the abundance of emerged plants of Malva parviflora L., Anagallis arvensis L. and Picris echioides L. Overall, MT led to a less diverse seedbank in the 0-8 cm depth of soil than CT. The frequent drought-induced deep fractures in the expandable clay soil caused natural tillage, which probably resulted in fewer differences in weed seed and seedling densities among tillage treatments compared to what might be expected in other soil types.

摘要：采用少耕法（minimum tillage, MT）与免耕法（no-tillage, NT）的主要弊端之一，是常出现杂草群落演替现象：即促进适生杂草繁衍，同时削弱杂草防控效果。这类杂草群落变化最适宜通过长期定位试验进行监测，且研究结果可能因土壤特性与本地植物区系的差异而有所不同。本研究的目的为：一是评估减耕对土壤剖面中杂草种子分布的影响；二是基于西班牙塞维利亚开展的24年长期定位试验，明确减耕对杂草多样性的潜在影响。该试验设置了三种耕作模式：免耕法（NT）、少耕法（MT）与常规耕作（conventional tillage, CT，配备翻耕犁），试验地块为变性土（vertisol）。为此，本研究于2005年秋季对0~8 cm与8~16 cm土层的土壤种子库进行计数，并于2005年秋季及2006年冬季调查田间出苗的杂草植株。分别针对土壤种子库与地上杂草群落，计算香农多样性指数（Shannon diversity index, H）与均匀度指数（J’）。结果显示，杂草种子总密度以NT处理最高，CT处理最低；部分大粒种子物种（如斑籽草 (Chrozophora tinctorea L.)）的种子密度在CT处理中最高。NT处理提升了土壤种子库中铺地苋 (Amaranthus blitoides S. Watson) 种子的相对密度，同时增加了小花锦葵 (Malva parviflora L.)、琉璃繁缕 (Anagallis arvensis L.) 与硬毛毛连菜 (Picris echioides L.) 的出苗植株数量。总体而言，在0~8 cm土层中，MT处理的土壤种子库多样性低于CT处理。由于该膨胀性黏土受干旱诱导形成深层裂隙，产生了自然耕作效应，因此相较于其他土壤类型的同类试验，本研究中不同耕作处理间的杂草种子与幼苗密度差异相对更小。