No abstract

The growth of many pine plantations in the southern United States is limited by soil nutrient availability. Therefore, forest fertilization is a common silvicultural practice throughout the South. Approximately 1.2 million ac of pine plantations were fertilized in 2004. In the last 10 years, considerable advances have been made in identifying the ecophysiological basis for stand growth and the response to fertilizer additions. Nitrogen (N) and phosphorus (P) are the nutrients that most commonly limit growth of southern pine. On wet clay soils in the lower Coastal Plain and on some well-drained soil in the upper Coastal Plain, severe P deficiencies exist. On these soils, P fertilization with 25–50 lb of P per acre at the time of planting produces a large and sustained growth response, on the order of 50 ft3 ac−1 yr−1 (1.5 tn ac−1 yr−1) throughout the rotation. On most other soils in the South, chronic deficiencies of both N and P exist. On these sites, soil nutrient availability often is adequate early in the rotation when tree demand is small. However, around the time of crown closure, N and P frequently become limiting. Fertilization with both N and P in these intermediate aged stands typically increases growth for 8–10 years. The growth response to a combination of 25 lb of P per acre plus 200 lb of N per acre averages around 55 ft3 ac−1 yr−1 (1.6 tn ac−1 yr−1) for an 8-year period. The amount of leaf area in the stand is the main factor determining the current growth rate of the stand and the potential growth response after fertilization. When stand leaf area index is less than 3.5, light capture by the stand is restricted and growth is negatively affected. In many of these stands, fertilization will increase leaf area because of increased soil nutrient availability and thus increase growth. The financial return after fertilization depends on the growth response that occurs, the cost of the fertilizer treatment, and the stumpage value of the timber produced. Using a growth response of 55 ft3 ac−1 yr−1 over 8 years, a fertilizer cost of $90 ac−1, and stumpage values from the first quarter of 2006, the internal rate of return from midrotation fertilization of a loblolly pine plantation with N and P would be approximately 16%.

With an increasing fraction of the world's forests being intensively managed for meeting humanity's need for wood, fiber and ecosystem services, quantitative understanding of the functional changes in these ecosystems in comparison with natural forests is needed. In particular, the role of managed forests as long-term carbon (C) sinks and for mitigating climate change require a detailed assessment of their carbon cycle on different temporal scales. In the current review we assess available data on the structure and function of the world's forests, explore the main differences in the C exchange between managed and unmanaged stands, and explore potential physiological mechanisms behind both observed and expected changes. Two global databases that include classification for management indicate that managed forests are about 50 years younger, include 25% more coniferous stands, and have about 50% lower C stocks than unmanaged forests. The gross primary productivity (GPP) and total net primary productivity (NPP) are the similar, but relatively more of the assimilated carbon is allocated to aboveground pools in managed than in unmanaged forests, whereas allocation to fine roots and rhizosymbionts is lower. This shift in allocation patterns is promoted by increasing plant size, and by increased nutrient availability. Long-term carbon sequestration potential in soils is assessed through the ratio of heterotrophic respiration to total detritus production, which indicates that (i) the forest soils may be losing more carbon on an annual basis than they regain in detritus, and (ii) the deficit appears to be greater in managed forests. While climate change and management factors (esp. fertilization) both contribute to greater carbon accumulation potential in the soil, the harvest-related increase in decomposition affects the C budget over the entire harvest cycle. Although the findings do not preclude the use of forests for climate mitigation, maximizing merchantable productivity may have significant carbon costs for the soil pool. We conclude that optimal management strategies for maximizing multiple benefits from ecosystem services require better understanding of the dynamics of belowground allocation, carbohydrate availability, heterotrophic respiration, and carbon stabilization in the soil.

Organic acids can form stable complexes with metals and therefore can affect metal solubility and speciation. The low‐molecular‐weight aliphatic organic acids extracted in water and present in soil solution from O, A, Bh and Bt horizons from a group of forested Utlisols, Entisols and Spodosols were identified by high performance liquid chromatography (hplc). Oxalic acid was found in all samples and was present generally in the highest concentrations. Oxalate concentrations in soil solution ranged from 25 to 1000 µM and were greater in the Bh and Bt horizon soils than in the A horizon soil. High concentrations of formic acid were also identified in most soils, ranging from 5 to 174 µM in soil solution. Trace amounts of citric, acetic, malic, lactic, aconitic, and succinic acids were detected in some samples. In a greenhouse pot study, the concentrations of low‐molecular‐weight organic acids in the rhizosphere of slash pine (Pinus elliottii Engelm.) seedlings growing in A, Bh, and Bt horizons from an Ultic Haplaquod were compared with the concentrations in the nonrhizosphere soil. The observed concentrations were approximately an order of magnitude greater than in native soils. Oxalate was the only low‐molecular‐weight organic acid identified in the non‐rhizosphere soil. The suite of organic acids identified in the rhizosphere was more complex than in the bulk soil. In the rhizosphere, high concentrations of both oxalate and formate were detected, along with trace amounts of citric, acetic, and aconitic acids. Since oxalate forms stable complexes with Al, the presence of large concentrations of oxalate may affect P availability in these soils.

The ability of organic acids to affect surface and solution reactions of P and Al in soils containing Al‐oxide surfaces is related to their ability to form stable complexes with Al. This study was conducted to determine if the amount of P and Al released following the addition of an organic acid to a Bh horizon soil could be estimated based on the Al stability constant (logKAl) of the organic acid. The release of Al and inorganic P from spodic horizon material increased in the presence of organic acids that form stable complexes with Al. Overall, the logKAl value was a good indicator of the effect of the organic acid on inorganic P and Al release. Among the 16 organic acids studied, release of Al and inorganic P increased exponentially with increasing stability constants. A threshold value of approximately 4.1, however, was required before substantial amounts of inorganic P were released. This value may reflect the stability of P bond to Al‐oxide surfaces in this soil. Salicylic acid was a notable exception to the observed trend in release of both Al and P; despite a large stability constant, little inorganic P or Al was released. Although significant amounts of soluble organic P were released in all the organic‐acid solutions, the amount of soluble organic P released was not related to the stability constant of the organic acids.