Allelopathy in saline agricultural land: Vegetation successional changes and patch dynamics

In reclamation fields of salt-affected wasteland, five plant communities colonized the undisturbed land, represented byCynodon dactylon, Desmostachya bipinnata, Prosopis juliflora, Sporobolus arabicus, andSuaeda fruticosa. Kallar grass (Leptochloa fusca), a highly salt tolerant plant when cultivated, shared dominance withCynodon, Desmostachya, andSporobolus in 15-month-old fields, whereasPolypogon was the only dominant species in 30-month-old kallar grass fields. Through successional stages, soil pH, salinity, sodicity, and Na, K, Ca + Mg significantly decreased due to leaching. Electrical conductivity successively changed from 13.0 to 3.0 to 1.0, while soil total nitrogen, NH4 nitrogen, NO3 nitrogen and available P significantly increased. In high-density kallar grass fields, six weed species appeared only in well-defined patches and radially eliminated or reduced kallar grass growth. Many soil factors, such as pH, EC, NH4 nitrogen, NO3 nitrogen and available P analyzed in patch vegetation soils, were mostly either comparable or significantly better than those of surrounding kallar grass fields. On the other hand, aqueous extracts of all six invading species and kallar grass significantly reduced kallar grass seed germination to varying degrees. Further, decaying leaf powder of allelopathically suspected species significantly reduced kallar grass biomass, which varied from species to species and in most cases corresponded with field data of kallar grass in patch vegetation. It should be strongly pointed out that allelopathic behavior discussed in patch dynamics was in areas where soil saline-sodic conditions had improved greatly (e.g., EC = from 13.0 to only 1.0) due to kallar grass plantation. Further,Suaeda appeared to be a poor competitor when soil conditions improved for other species as well, and it could not capitalize on its evolutionary strategic trait of performing well in saline-sodic conditions. To our knowledge, this is the first report indicating that allelopathy may be a factor in determining growth and distribution of plants in saline or sodic soils.