N2 fixation and cycling in Alnus glutinosa, Betula pendula and Fagus sylvatica woodland exposed to free air CO2 enrichment

We measured the effect of elevated atmospheric CO2 on atmospheric nitrogen (N2) fixation in the tree species Alnus glutinosa growing in monoculture or in mixture with the non-N2-fixing tree species Betula pendula and Fagus sylvatica. We addressed the hypotheses that (1) N2 fixation in A. glutinosa will increase in response to increased atmospheric CO2 concentrations, when growing in monoculture, (2) the impact of elevated CO2 on N2 fixation in A. glutinosa is the same in mixture and in monoculture and (3) the impacts of elevated CO2 on N cycling will be evident by a decrease in leaf δ15N and by the soil-leaf enrichment factor (EF), and that these impacts will not differ between mixed and single species stands. Trees were grown in a forest plantation on former agricultural fields for four growing seasons, after which the trees were on average 3.8 m tall and canopy closure had occurred. Atmospheric CO2 concentrations were maintained at either ambient or elevated (by 200 ppm) concentrations using a free-air CO2 enrichment (FACE) system. Leaf δ15N was measured and used to estimate the amount (Ndfa) and proportion (%Ndfa) of N derived from atmospheric fixation. On average, 62% of the N in A. glutinosa leaves was from fixation. The %Ndfa and Ndfa for A. glutinosa trees in monoculture did not increase under elevated CO2, despite higher growth rates. However, N2 fixation did increase for trees growing in mixture, despite the absence of significant growth stimulation. There was evidence that fixed N2 was transferred from A. glutinosa to F. sylvatica and B. pendula, but no evidence that this affected their CO2 response. The results of this study show that N2 fixation in A. glutinosa may be higher in a future elevated CO2 world, but that this effect will only occur where the trees are growing in mixed species stands

Biogeochemical cycling in terrestrial ecosystems of the Caatinga Biome

The biogeochemical cycles of C, N, P and water, the impacts of land use in the stocks and flows of these elements and how they can affect the structure and functioning of Caatinga were reviewed. About half of this biome is still covered by native secondary vegetation. Soils are deficient in nutrients, especially N and P. Average concentrations of total soil P and C in the top layer (0-20 cm) are 196 mg kg-1 and 9.3 g kg-1, corresponding to C stocks around 23 Mg ha-1. Aboveground biomass of native vegetation varies from 30 to 50 Mg ha-1, and average root biomass from 3 to 12 Mg ha-1. Average annual productivities and biomass accumulation in different land use systems vary from 1 to 7 Mg ha-1 year-1. Biological atmospheric N2 fixation is estimated to vary from 3 to 11 kg N ha-1 year-1and 21 to 26 kg N ha-1 year-1 in mature and secondary Caatinga, respectively. The main processes responsible for nutrient and water losses are fire, soil erosion, runoff and harvest of crops and animal products. Projected climate changes in the future point to higher temperatures and rainfall decreases. In face of the high intrinsic variability, actions to increase sustainability should improve resilience and stability of the ecosystems. Land use systems based on perennial species, as opposed to annual species, may be more stable and resilient, thus more adequate to face future potential increases in climate variability. Long-term studies to investigate the potential of the native biodiversity or adapted exotic species to design sustainable land use systems should be encouraged