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Although low solubility and slow cycling control P circulation in a wide range of ecosystems, most studies that evaluate bioavailability of soil P use only indices of short-term supply. The objective here is to quantify changes in P fractions in an Ultisol during the growth of an old-field pine forest from 1957 to 2005, specifically changes with organic P (Po) and with inorganic P (Pi) associated with Fe and Al oxides as well as Ca compounds. Changes in soil P were estimated from archived mineral soil samples collected in 1962 shortly after pine seedlings were planted, and on six subsequent occasions (1968, 1977, 1982, 1990, 1997, and 2005) from eight permanent plots and four mineral soil layers (0-7.5, 7.5-15, 15-35, and 35-60 cm). Despite the net transfer of 82.5 kg ha(-1) of P from mineral soil into tree biomass and O horizons, labile soil P was not diminished, as indexed by anion exchange resins, and NaHCO(3) and Mehlich III extractants. An absence of depletion in most labile P fractions masks major restructuring of soil P chemistry driven by ecosystem development. During 28 years of forest growth, decreases were significant and substantial in slowly cycling Po and Pi associated with Fe and Al oxides and Ca compounds, and these accounted for most of the P supplied to biomass and O horizons, and for buffering labile soil fractions as well. Changes in soil P are attributed to the P sink strength of the aggrading forest (at 2.9 kg ha(-1) year(-1) over 28 years); legacies of fertilization, which enriched slowly cycling fractions of Po and Pi; and the changing biogeochemistry of the soil itself.

Elevated concentrations of atmospheric CO2 increase plant biomass, net primary production (NPP) and plant demand for nitrogen (N). The demand for N set by rapid plant growth under elevated CO2 could be met by increasing soil N availability or by greater efficiency of N uptake. Alternatively, plants could increase their nitrogen-use efficiency (NUE), thereby maintaining high rates of growth and NPP in the face of nutrient limitation. We quantified dry matter and N budgets for a young pine forest exposed to 4 years of elevated CO2 using free-air CO2 enrichment technology. We addressed three questions: Does elevated CO2 increase forest NPP and the demand for N by vegetation? Is demand for N met by greater uptake from soils, a shift in the distribution of N between plants, microbes, and soils, or increases in NUE under elevated CO2? Will soil N availability constrain the NPP response of this forest as CO2 fumigation continues? A step-function increase in atmospheric CO2 significantly increased NPP during the first 4 years of this study. Significant increases in NUE under elevated CO2 modulated the average annual requirement for N by vegetation in the first and third growing seasons under elevated CO2; the average stimulation of NPP in these years was 21% whereas the average annual stimulation of the N requirement was only 6%. In the second and fourth growing seasons, increases in NPP increased the annual requirement for N by 27-33%. Increases in the annual requirement for N were largely met by increases in N uptake from soils. Retranslocation of nutrients prior to senescence played only a minor role in supplying the additional N required by trees growing under elevated CO2. NPP was highly correlated with between-plot variation in the annual rate of net N mineralization and CO2 treatment. This demonstrates that NPP is co-limited by C availability, as CO2 from the atmosphere, and N availability from soils. There is no evidence that soil N mineralization rates have increased under elevated CO2. The correlation between NPP and N mineralization rates and the increase in the annual requirement for N in certain years imply that soil N availability may control the long-term productivity response of this ecosystem to elevated CO2. Although we have no evidence suggesting that NPP is declining in response to >4 years of CO2 fumigation, if the annual requirement of N continues to be stimulated by elevated CO2, we predict that the productivity response of this forest ecosystem will decline over time.

Using a new approach involving one-time measurements of radiocarbon (14C) in fine (

Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO2 and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO2 partial pressures of 350 and 650 μbar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO2 when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO2 enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO2 enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO2. In seedlings supplied with soil N, elevated CO2 increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf δ15N). While CO2 enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO2 partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO2 will depend greatly on plant and soil nutrient status.

Sulfur (S) cycling in a chestnut oak forest on Walker Branch Watershed, Tennessee, was dominated by geochemical processes involving sulfate. Even though available SO 42- was present far in excess of forest nutritional requirements, the ecosystem as a whole accumulated ∼60% of incoming SO4-S. Most (90%) of this accumulation occurred by SO 42- adsorption in sesquioxide-rich subsurface soils, with a relatively minor amount accumulating and cycling as SO 42- within vegetative components. Organic sulfates are thought to constitute a large proportion of total S in surface soils, also, and to provide a pool of readily mineralized available S within the ecosystem.