Soil and mycorrhizal fungi origin influence the growth of Inga edulis seedlings transplanted into degraded pasture

Background/Question/Methods

The degree to which mycorrhizal fungi are locally adapted to environmental conditions is poorly understood yet has important implications for natural and agricultural ecosystems.  If mycorrhizal fungi adapt to local soil conditions including nutrient availability and pH, then transplanting them to novel soil conditions may negatively impact their growth and subsequent services to hosts.  To address this problem, I inoculated Inga edulis seedlings, an agriculturally important tropical tree, with mycorrhizal fungi familiar and foreign to a degraded field site in Costa Rica.  Seeds of I. edulis were raised in the greenhouse in soil from two sources—slopes or valleys—and under four inoculum treatments—slope mycorrhizae, valley mycorrhizae, slope non-mycorrhizal control, and valley non-mycorrhizal control in factorial combination.  After four months, seedlings were transplanted into slope and valley field sites.

Results/Conclusions

Before transplanting, mycorrhizal seedlings were significantly taller than non-mycorrhizal seedlings (p= 0.021).  Additionally, seedlings raised in valley soil benefited more from mycorrhizal inoculation than seedlings in slope soil (p=0.008), suggesting slope soil may represent poorer habitat for mycorrhizal fungi.  Phosphatase enzyme production was significantly greater in slope than valley soil (p=0.014), signifying plants upregulated enzyme production to meet phosphorus demands.  There was no interaction between soil source and mycorrhizal source, indicating mycorrhizae from both slopes and valleys perform similarly in a given soil type.  Following six months in the field, seedlings grew marginally significantly more with mycorrhizae from slopes than from valleys (p=0.065) but the effects of mycorrhizal treatments also depended upon greenhouse soil and planting location (p=0.0037).  Seedlings planted in valley sites but raised in slope soil with slope mycorrhizae exhibited the greatest change in growth.  It is possible that slope nursery soil stimulated seedlings to allocate resources toward acquiring nutrients, in the form of phosphatase enzyme production and mycorrhizal associations.  Once introduced to relatively resource rich valley soils, these seedlings were more effective at capturing available nutrients.  In conclusion, while there is little support so far that mycorrhizae are adapted to a narrow soil niche, there is some evidence that mycorrhizae inhabiting slopes may be more beneficial to their hosts provided the planting area is sufficiently nutrient rich.  Tests of fungal community composition in slope and valley sites are planned.