Impact of ploughing on soil seed bank dynamics in temporary pools

Summary

The impact of ploughing on soil seed banks of plant communities living in temporary flooded areas in arable fields in northern France was investigated. The quantity, quality (in terms of germination ability) and vertical distribution of seeds were quantified under ploughed or unploughed treatments. A focus of the study was to investigate the impact of ploughing on populations of the ephemeral, semi-aquatic starfruit Damasonium alisma (Alismataceae). This species is a rare annual plant which grows in the muddy margins of pools with seasonal fluctuating water levels.

Study area: The two study ponds (5 km apart) were located on farmland near Paris in the region of ÃŽle-de-France, northern France: one near the village of Écharcon ('E pool') and the other near the town of Fleury-Mérogis ('F pool').

Pool description: Both pools were on similar clayey soils, were round and approximately 50 m² in area in March 2003. The pools generally fill up over winter (30 cm maximum depth during the study) and progressively dry during spring. E pool was at the edge of an annually cultivated arable field; one half in the cultivated area ('Ec') and the other in the uncultivated field margin i.e. not ploughed ('Ew'). F pool was located in an area uncultivated for at least 5 years.

Seed sampling: Ten soil core samples (7 cm diameter; 15 cm depth) were taken from each pool in March 2003, i.e. after autumn ploughing and before 2003 seed production. Each sample was divided into 5 depth layers of 3 cm thickness. Due to soil compaction, the deepest layer (12-15 cm) was not adequately reached at F pool and around the uncultivated half of E pool (therefore not considered further). The samples were sieved and seeds were hand sorted and identified.

Starfruit seeds: From each layer, 150 D.alisma seeds were collected and cultivated in waterproof pots filled with sterilized pool soil. Seeds had been shown to germinate only when submerged and not just in damp conditions. Thus the pots were filled so that the soil surface was under 1 cm of (distilled) water (pH 7). Seedlings were counted after 40 days.

Community seedling dynamics: To study seedling dynamics of the whole plant community, 15 soil cores (20 cm depth; 5 cm diameter) were taken from each part of E pool (Ec and Ew) and from F pool, in March 2003. Each sample was divided into an upper and a lower stratum, of 10 cm thickness. These were spread in large plastic pots placed outdoors, watered to keep the soil surface submerged under 1 cm of water. Seedling emergence was monitored over 6 weeks.

The total number of seeds of all species was similar among pools, however, more were found in the upper soil layers in ‘fallow’ F pool and the unploughed part of E pool; in the ploughed half more seeds were found in lower layers. Seeds found included 10 wetland species (four regionally rare*): *grass-poly Lythrum hyssopifolia, water purslane L.portula, *Juncus tenageia, redshank Polygonum persicaria, *starfruit D.alisma, marsh cudweed Gnaphalium uliginosum, toad rush Juncus bufonius, barnyard grass Echinochloa crus-galli, hairy buttercup Ranunculus sardous and *waterwort Elatine alsinastrum).

Starfruit seeds and germination: The number of seeds in soil sample differed between pools and the two halves of E pool; the number of seeds showed an exponential decrease with depth in F pool and the uncultivated part of E pool (Ew), whereas there was a slight increase with depth in the ploughed part (Ec). Germination also differed among pools according to the depth at which they were extracted. In Ec, seeds from the deeper layers had a higher germination rate than those of the upper layers; conversely, in F pool and Ew, the seeds from the upper layers had better germination. Most of the seeds (56%) were stored in the two deepest soil layers among the four considered in Ec; germination was higher for buried seeds (84%) than for seeds at or near the surface (33.6%).

Community seedling dynamics: Species richness of seeds, seed abundance and germination ability appeared strongly affected by ploughing (Table 1). In the E pool, all the species that emerged in cores from Ec were also present in Ew: L.hyssopifolia, L.portula, J.tenageia, P.persicaria, G.uliginosum, D.alisma. Four species were only found in the uncultivated part: J.bufonius, E.crus-galli, R.sardous and E.alsinastrum. Seedling abundance of species present in both parts of the pool varied according to species (e.g. P.persicaria was more abundant in the cultivated part (170 Ec vs. 15 Ew) J.tenageia was more abundant in the uncultivated part (62 Ec vs.1045 Ew).

More species germinated from the lower layer compared to the upper in Ec, but germination was similar between layers in F pool and Ew.

Conclusions: The authors consider that in this study, soil disturbance by ploughing may have a beneficial effect for conservation of rare semi-aquatic plants through preservation of seed banks and removal of perennial species. Ploughing induced a favourable seed storage effect, accumulating species and individuals in the seed banks that positively influenced community dynamics.


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