Risposte dei processi biologici del suolo all'elevata [CO2] atmosferica e all'aggiunta di azoto in una piantagione di Populus spp
Author(s)
Lagomarsino, Alessandra
Date Issued
March 15, 2006
Type
Doctoral Thesis
Abstract
This thesis was realized in the frame of the POPFACE-EUROFACE project “an integrated research to determine the functional responses of a cultivated, agro-forestry system, namely a multiclonal poplar plantation, to actual and future atmospheric CO2 concentrations”. FACE (Free Air CO2 Enrichment) technology was used to expose the poplar plantation to elevated atmospheric CO2 concentrations.
The work focused on the impact of elevated [CO2] on soil biological processes and on their role in i) sustaining the increase in plant production and ii) determining the C storage in soil.
The effect of elevated [CO2] on soil is mediated by plants input, from litter and rhizodeposition, and changing quality and quantity of these inputs will affect the soil biota and its metabolism, that in turn can create a feedback on the atmospheric [CO2]. Furthermore, the soil nutrient status might have a main role in determining the direction of these processes, and the study of interactions between elevated [CO2] and N fertilization was included in this work.
The main conclusions of this thesis were the following:
- The total nitrogen in soil decreased under elevated [CO2] and the consequent plants-microbes competition for nitrogen favored microbial N immobilization. The decrease of nitrification and mineralization rates under elevated [CO2] resulted in fact in a lower N-NO3- availability in soil. Although the negative effect of [CO2] enrichment on total N soil content acted similarly in fertilized and not fertilized soil, the decrease of nitrates under elevated [CO2] was partly compensated by the positive interaction with the fertilization treatment. A more intense microbial mineralization activity induced in fact a N-NO3- increase in FACE fertilized soil, more evident in P. nigra.
- The increase of several extracellular enzymes indicated a greater soil nutrient requirement under elevated [CO2]. This resulted in a general enhancement of soil biological activity in order to maintain an adequate supply of nitrogen, sulphur and phosphorus in soil. The greater enzymatic synthesis was indeed related to a general enhancement of microbial processes mediated by plants activity, as confirmed by the different responses of poplar clones.
- Soil cation exchange capacity (CEC) increased during the 2nd rotation cycle, because of the positive impact of the plantation on soil organic matter. Under elevated [CO2] the effect was more pronounced, and was sustained by the higher C/N ratio of labile soil organic matter, the greater root and fungal biomass, and the greater rhizosphere activity.
- Elevated [CO2] resulted in a large increase of all labile C fractions and on the C/N ratio of labile fractions, indicating a lower quality of easily decomposable compounds.
- The recalcitrant C fraction and the total organic C were unaffected by the treatment. The lack of significant changes was indeed related to the difficulty of measuring small changes against the background C-content of the soil in the relative short term of the experiment, and did not exclude longer-term modifications.
- The extra C made available for microbes in elevated [CO2] induced an increase in microbial biomass C sustained by a less energetically expensive metabolism, reflecting more a cell enlargement process than a real microbial proliferation. The relationship between microbial and soluble C suggested in fact that at least part of plants C input was immobilized and used for microbial growth, instead that respired and released back in the atmosphere. Elevated [CO2] induced a preferential stimulation of fungi because of the increase in quantity but decrease in quality of plants input, which favoured a more efficient microbial population in converting substrate into biomass.
- Fertilization treatment did not induce changes in microbial population size, but influenced the fungal community composition and lowered the fungal species richness. Moreover the microbial N immobilization was favoured only in June, during the fertilizer application, therefore lowering the microbial biomass C/N ratio and sustaining in the short-term fast growing microbial communities and higher metabolic rates.
- The interaction between [CO2] enrichment and N addition selected different fungal communities without lowering the species richness, and the utilization of added C compounds was higher in FACE fertilized soil, where the combined C and N availability allowed higher metabolic rates, particularly in P. nigra.
- As general mean, the autotrophic and the heterotrophic components of soil respiration contributed similarly to soil CO2 efflux, and the root-derived CO2 was on average the 48% of the total. Soluble C supply, by means of root and litter production, played a main role in determining soil respiration rates in the plantation, as well as yearly fluctuations.
- The large increase of soil C losses under elevated [CO2] was sustained during years, and after the coppicing a much larger relative stimulation of soil respiration did occur due to the greater availability of roots and decomposable substrates.
- The increase of soil CO2 emissions from both auto- and heterotrophic components was related to the enhancement of plants photosynthetic activity, to the increase in belowground productivity, and to the consequent greater flux of fixed C from plants to soil. The consequent increase of rhizospheric activity, comprehensive of roots and rhizo-microbial respiration, caused higher soil CO2 effluxes in FACE soil.
- Greater root biomass and labile C compounds availability induced higher heterotrophic respiration rates only in the rhizosphere, as demonstrated by the lack of [CO2] enrichment effect on microbial potential respiration in the absence of a direct root influence. A lower quality of labile C pools and the consequent changes in microbial communities selected a more efficient microbial biomass, dominated by fungal populations and characterized by a slower metabolism.
- From these evidences we conclude that the elevated [CO2] enhancement of autotrophic and heterotrophic respiration was more than balanced by the greater C input from plants to soil, and that the increase of labile C fractions was the net result of these processes, suggesting a positive trend of C storage in FACE soil in the longer-term.
Additional information
Corso di dottorato in Ecologia forestale
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