干旱区苜蓿和沙打旺单混播下土壤水分利用格局的驱动力分析

大田条件下采用生物替代试验对沙打旺和苜蓿单播及混播的一年生人工草地进行了土壤水分利用动态格局调查研究.结果表明,3种密度下土壤水分随生育期变化相似,均呈下降趋势,但随播种密度增大,不同播种方式平均土壤水分利用效率逐渐增大,而2种牧草平均群体水分利用效率比较为混播下苜蓿>单播苜蓿>单播沙打旺>混播下沙打旺.牧草返青后,在中密度下显示出较宽的资源共享生态位,而低密度和高密度显示出稍窄的资源共享生态位.3种密度下整个生育期内苜蓿的相对竞争力均高于沙打旺,其中平均相对竞争力比较为高密度>中密度>低密度,而且随生育期变化苜蓿的相对竞争力持续增强.苜蓿对沙打旺的相对竞争力与混播处理中苜蓿对沙打旺的群体水分利用效率比有显著正相关.由此可见,在苜蓿与沙打旺一年生混播试验中苜蓿竞争力始终大于沙打旺,其中密度效应和水分利用为两者种间关系调节的重要影响因素.2种牧草混播中苜蓿生产力、群体水分利用效率与苜蓿对沙打旺的竞争优势为性状互利关系.因此,旱生环境中苜蓿与沙打旺的水分利用格局可能由2个物种种间竞争关系所驱动的,水分资源利用状况反应了两者对干旱胁迫环境下适应种间竞争的生活史对策.

Driving mechanism of simultaneous pattern on soil moisture utilization under the monoculture and mixture of Medicago sativa and Astragalus adsurgens grown in an arid region of the Loess Plateau of China

In replacement series field trials the utilization dynamics of soil moisture and inter-specific competition under both monocultures and mixtures of Astragalus adsurgens and Medicago sativa was studied for one year.Soil moisture exhibited a similar trend in all treatments,declining with the growth periods,at three plant densities.However with increased sowing densities the soil moisture utilization rate gradually increased.Meanwhile the water utilization efficiency at the population level declined in the order M.sativa in the mixture,M.sativa in monoculture,A.adsurgens in monoculture and A.adsurgens in the mixture.In the intermediate and high density treatments both legumes showed a significant positive relationship with their water utilization efficiency at a population level.M.sativa and A.adsurgens competed for the same resources under different densities in the post-turning green stages,but the degree depended on the mixture density: For wide and narrow niches in medium density and for high or low densities respectively.In all test stages M.sativa showed a stronger competition than A.adsurgens at all three densities.At a low density,the predominant competition started from the turning green period.The stronger competition from M.sativa to A.adsurgens,increased with the development stages,and was accounted for by the higher ratio of counterpart indices such as aboveground biomass,the absolute growth rate in the above two pastures,with specific responses to the NAS protein and high water use efficiency.Accordingly,the dominant competition in the mixture of M.sativa to A.adsurgens,depended closely and positively on the relative density and soil moisture factors.The stronger competition of M.sativa can be determined by the relationship of mutual promotion between the productivity,especially water utilization efficiency at a population level and the superiority of relative growth.Therefore,the productivity,water utilization efficiency,and inter-specific relationship were possibly the main characteristics in the life history of M.sativa and A.adsurgens,whose water utilization pattern was possibly driven by interspecific relationships like competition and allelopathy.