Above- and belowground linkages of a nitrogen and phosphorus co-limited tropical mountain pasture system – responses to nutrient enrichment

Little is known about how N and P co-limited ecosystems respond to single nutrient enrichment. This work assesses the susceptibility of above- and belowground ecosystem components and of their linkages in an N and P co-limited pasture to N- and P-enrichment. We tested if the plants’ responses can be explained by the concept of serially linked nutrients introduced by Agren (Ecol Lett 7:185–191, 2004). In this concept, the control of the growth rate by one nutrient is assumed to depend on the control of a different cellular process by another nutrient. We investigated the responses of shoot and root biomass and C:N:P stoichiometry of the grass Setaria sphacelata (Schumach.) to moderate N, P, and N + P application over 5 years. In addition, the effects of nutrient enrichment on soil nutrient pools, on arbuscular mycorrhizal fungi (AMF) as well as on microbial biomass, activity, and community structure (phospholipid fatty acids: PLFA) were tested. In order to evaluate the importance of different factors explaining microbial responses, we applied a likelihood-based information-theoretic approach. The application of N + P increased aboveground grass biomass (+61 %). Root biomass was stimulated by P-treatment (+45 %). Grass C:N:P stoichiometry responded by altering the P-uptake (P-treatment) or by translocating P from shoot to root (N-treatment). In particular, root C:N and C:P stoichiometry decreased in P- and in N-treatment. Extractable fractions of soil C, N, and P were significantly affected by nutrient enrichment. P application increased the biomass of Gram-positive bacteria (+22 %) and the abundance of AMF (+46 %), however, results of the IT-approach suggested indirect effects of nutrient enrichment on microbes. The responses of the N and P co-limited pasture to particular nutrient enrichment support the concept of serially linked nutrients. The present study provides evidence for the fundamental importance of P for controlling resource allocation of plants in responses to nutrient enrichment. Resource allocation of the grass rather than direct effects of nutrient additions drives changes in AMF, microbial biomass, community structure, and activity.