Changes in soil nutrient availability will alter net ecosystem carbon storage, but the direction and magnitude of this effect is dependent on the independent responses of plants, their microbial symbionts, and free-living decomposers. We examined how plant-microbe interactions integrate ecosystem carbon, nitrogen and phosphorus cycles in the context of an ongoing stand-level, full-factorial NP fertilization experiment in a regenerating tropical dry forest in Guanacaste, Costa Rica. Changes in plant growth, biomass allocation, and soil biogeochemistry have been monitored for three years.
Results/Conclusions
Addition of phosphorus has doubled soil PO4 and microbial biomass P pools, depressed soil respiration rates, and increased root production by 30%. In contrast, soil N pools have been unaffected by fertilization treatments, and N fertilization did not alter microbial growth, soil respiration, or root biomass. Plant investment in wood and leaves did not respond to addition of either nutrient, likely because a severe drought in the first two years of the study depressed productivity. However, plants did adjust allocation to microbial symbionts (arbuscular mycorrhizal fungi and N-fixing bacteria) in order to maintain balanced stoichiometric ratios of nitrogen and phosphorus. We observed dramatic increases in root nodulation when P was added alone, and decreases in mycorrhizal colonization when N and P were added together. These results demonstrate rapid, pronounced belowground responses to altered nutrient availability, which may translate to changes in overall belowground C storage over longer timescales.