Multi-stage process of microbe-mediated decay of giant ragweed (Ambrosia trifida) seed following burial in an artificial seed bank system

Authors: Chee Sanford, Joanne; FU, XIANHUI - UNIVERSITY OF ILLINOIS

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2012
Publication Date: 11/1/2012
Citation: Chee-Sanford, J.C., Fu, X. 2012. Multi-stage process of microbe-mediated decay of giant ragweed (Ambrosia trifida) seed following burial in an artificial seed bank system. Soil Biology and Biochemistry. 11:2203-2211.

Interpretive Summary: Reducing soil seed banks of annual weeds remains an important strategy in sustainable crop management. Seed decay is considered one of the main processes affecting seed bank abundance and persistence, however, the (micro-)biological mechanisms involved are not well understood. An artificial seed bank of giant ragweed was examined in two separate year-long studies. Extensive decay of the seed embryo occurred during summer and early fall, and was preceded by a stage of microbial-mediated decay affecting the outer woody bur structure of the seed. Microbe–mediated seed decay leading to loss of seed viability comprised the majority fate in Yr06 (71%) and a lower proportion of the seeds in Yr07 (44%). Approximately 10% of seeds in Yr06 and 34% of seed in Yr07 underwent some degree of decay but remained viable by the end of November. Depth of burial did not significantly affect the extent of seed decay within each year. The rate of seed loss during the summer months in both years was similar, however the total loss of seed differed significantly. The extent of annual seed bank losses depended on distinct microbial activities, and we hypothesize that environmental drivers control these activities, particularly those affecting initial decay processes and especially in the early season where wider extremes and durations of soil temperature and moisture occur. Bacterial populations were significantly different on seeds once decay progressed, however more bacterial populations were more alike in association with early stages of seed decay. The significance of these results demonstrate that while microbe-mediated seed decay is the major mechanism of giant ragweed seed bank depletion, decay likely occurs over multiple stages mediated by different populations of microorganisms. These microorganisms are affected by enviornmental factors that likely include weather and crop management effects. Seeds that undergo some partial decay may not necessarily lead to seed bank loss since viable seed still remain as a portion of the live seed bank for subsequent emergence. Understanding the environmental conditions that promote key microbial populations and their activities that lead to higher proportions of complete seed loss provides an important basis for future seed bank management.

Technical Abstract: Understanding the ecology and dynamics of soil seed banks remains critical for sustainable crop management and control of annual weeds. Seed decay is considered one of the main processes affecting seed bank abundance and persistence, however, the (micro-)biological mechanisms involved are not well understood. This study investigated the seed bank dynamics of a problematic annual weed species, giant ragweed (Ambrosia trifida), and provides a closer examination of the mechanisms of seed decay using an artificial seed bank approach. In two independent year-long studies, defined mixtures of seed-soil were buried in a commercially-managed soybean crop field using small-pore size bags that allowed exposure of natural soil and climate conditions, but facilitated easy retrieval of seed and associated soil microbiota over spatial and temporal spans. The design uniquely provided a uniform baseline microclimate from which true changes in seed status and microbial assemblages could be readily discerned and could be attributed to events that occurred during burial. Seeds were retrieved at different times of the year and categorized according to fate outcomes, namely viable, germinated, lethal decay, and predation damage. Cumulative proportions of seed fates were determined. Soil moisture, temperature, and weather data was recorded throughout the year. T-RFLP analysis of seed-associated bacterial communities was used to compare populational shifts over time and in correspondence to seed fates. Extensive decay of the embryo occurred during summer and early fall, preceded necessarily by sufficient decay of the outer involucre structure of the seed. Microbe–mediated seed decay leading to loss of seed viability comprised the majority fate in Yr06 (71%) and a lower proportion of the seeds in Yr07 (44%). Approximately 10% of seeds in Yr06 and 34% of seed in Yr07 underwent some degree of decay but remained viable by the end of November. There was no significant depth (0-9 cm) effect observed. The rate of seed loss during the summer months in both years was similar (k=0.13%•hydrothermal day-1) and followed typical decay kinetics, however the total loss of seed differed significantly. The extent of annual seed bank losses depended on distinct microbial activities, and we hypothesize that environmental drivers control these activities, particularly those affecting initial decay processes and especially in the early season where the amplitude and duration of soil temperature and moisture vary widely. Significantly higher divergence of bacterial populations occurred on seeds once decay became more extensive, however more cohesive bacterial populations were associated with early stages of seed decay. These results demonstrate that while microbe-mediated seed decay is the major mechanism of giant ragweed seed bank depletion, decay likely occurs over multiple stages mediated by different populations of microorganisms, with their activities affected by enviornmental factors that likely include weather and crop management effects.