SPP 1685: Ecosystem Nutrition: Forest Strategies for Limited Phosphorus Resources

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Phosphorus is a limited resource and there is increasing debate regarding the principles of tight P recycling. Forest ecosystems show commonly high P use efficiencies and the processes behind this phenomenon are still unresolved. The members of the priority programme SPP 1685 have applied and refined the concept of ecosystem nutrition, which is based on the integration of results obtained from different spatial and temporal scales and from different disciplines, to unravel these processes. The studies conducted in the first phase, were directed to the identification of processes, interactions and feedbacks associated with the P nutrition of temperate forest ecosystems. We tested the hypothesis that plant and microbial communities established at P rich sites follow a P acquiring strategy introducing P from primary minerals into the biogeochemical P cycle. With decreasing P supply by the parent material, the strategy changes into tight P cycling to sustain the P demand of the forests. The analyses of five beech forest ecosystems on silicate rock representing a P geosequence with different parent materials and thus different total P stocks (160 ¿ 900 g P m-2; down to 1m soil depth) were adjusted to test this hypothesis. These analyses were linked to additional experimental approaches used by individual projects. Spatial scales from micrometers up to the catchment scale and temporal scales from seconds to millions of years have been covered by individual projects. In addition, many methods have been developed or adjusted to the specific requirements related to the challenging analysis of P in forest ecosystems, where P concentrations are in part extremely low. These methodological developments provide promising tools also for ongoing analyses planned for the second phase of the priority programme. Valuable information regarding the P-status of soils and its spatial heterogeneity, the response of plants, plant roots and microbes, and regarding the P fluxes within the ecosystems has been gathered in the first phase of the priority programme. In fact, many of the studies provide evidence for intense P acquiring of beech forests at P- rich sites and tight recycling of P at P- poor sites. Changes in the diversity, activity and spatial distribution soil organisms and in the P-uptake efficiency of plant roots and the transfer of P within plants seem to be the processes behind these different strategies. By using these strategies the organisms change the soil properties, for example the turnover rate of the forest floor, the quality of soil organic matter and the spatial distribution and speciation of P in soils as well as P fluxes. The experience and developments gained from the first phase provide unique opportunities to address still open questions regarding controls and drivers of P strategies, relations between the P dynamics on different temporal and spatial scales or between P and OC dynamics, and human impact on P nutrition.