EFFECTS OF LABILE SOIL CARBON ON NUTRIENT PARTITIONING BETWEEN AN ARCTIC GRAMINOID AND MICROBES

We measured partitioning of N and P uptake between soil microorganisms and potted Festuca vivipara in soil from a subarctic heath in respons e to factorial addition of three levels of labile carbon (glucose) com bined with two levels of inorganic N and P. The glucose was added to e ither non-sterilized or sterilized (autoclaved) soils in quantities wh ich were within the range of reported, naturally occurring amounts of C released periodically from the plant canopy. The aims were, firstly, to examine whether the glucose stimulated microbial nutrient uptake t o the extent of reducing plant nutrient uptake. This is expected in nu trient-deficient soils if microbes and plants compete for the same nut rients. Secondly, we wanted to test our earlier interpretation that gr owth reduction observed in graminoids after addition of leaf extracts could be caused directly by labile carbon addition, rather than by phy totoxins in the extracts. Addition of high amounts of N did not affect the microbial N pool, whereas high amounts of added P significantly i ncreased the microbial P pool, indicating a luxury P uptake in the mic robes. Both plant N and in particular P uptake increased strongly in r esponse to soil sterilization and to addition of extra N or P. The inc reased uptake led to enhanced plant growth when both elements were app lied in high amounts, but only led to increased tissue concentrations without growth responses when the nutrients were added separately. Glu cose had strong and contrasting effects on plant and microbial N and P uptake. Microbial N and P uptake increased, soil inorganic N and P co ncentrations were reduced and plant N and P uptake declined when gluco se was added. The responses were dose-dependent within the range of 0- 450 mu g C g(-1) soil added to the non-sterilized soil. The opposite r esponses of plants and microbes showed that plant acquisition of limit ing nutrients is dependent on release of nutrients from the soil micro bes, which is under strong regulation by the availability and microbia l uptake of labile C. Hence, we conclude, firstly, that the microbial populations can compete efficiently with plants for nutrients to an ex tent of affecting plant growth when the microbial access to labile car bon is high in nutrient deficient soils. We also conclude that reduced growth of plants after addition of leaf extracts to soil can be cause d by carbon-induced shifts in nutrient partitioning between plants and microbes, and not necessarily by phytotoxins added with the extracts as suggested by some experiments.