Felipe G. Sanchez

Impacts of intensive forestry on early rotation trends in site carbon pools in the southeastern US

Abstract The effects of different silvicultural practices on site, especially soil, carbon (C) pools are still poorly known. We studied changes in site C pools during the first 5 years following harvesting and conversion of two extensively managed pine-hardwood stands to intensively managed loblolly pine plantations. One study site was located on the lower Atlantic Coastal Plain in North Carolina (NC) and another on the Gulf Coastal Plain in Louisiana (La). Four different harvesting-disturbance regimes were applied: stem only harvest (SO), whole tree harvest (WT), whole tree harvest with forest floor removal (WTFF), and full amelioration, i.e. whole tree harvest, disking, bedding and fertilization (FA; only in NC). Each harvesting-disturbance regime plot was split and one-half received annual herbicide treatments while the other half received no herbicide treatments. In NC, soil C decreased slightly with WT, and increased with FA, otherwise no significant changes were detected. In La, there was a consistent decrease in soil C content from the pre-harvest value in all cases where herbicides were applied. All treatments caused a reduction in the forest floor C pool in NC. In La, the most intensive treatments also resulted in a decrease in the forest floor C, but to a smaller extent. In contrast, there was no net change in forest floor C with the SO and WT treatments, even though significant amounts of logging slash were added to the forest floor at harvest in the SO plots and not in the WT. Herbicide treatment clearly decreased the C pool of hardwoods and understory, and more than doubled that of planted pines. Carbon accumulation in the planted pines was similar for trees growing in the SO, WT, and WTFF treatments on both the LA and NC sites. The full amelioration treatment (only applied at the NC site) led to a significant increase in C sequestration by the planted pine component. Due to a large amount of voluntary pines, total 5-year pine C pool was highest on the non-herbicided intensive management plots on the NC site, however. The differing response patterns of soil and forest floor C pools between the two sites may be due to their differing drainage-summer rainfall regimes. Our results suggest that while poor drainage-wet summer conditions may be impeding carbon loss from the soil component it may be accelerating the rate of decomposition of the forest floor and slash on the soil surface.

Loblolly pine needle decomposition and nutrient dynamics as affected by irrigation, fertilization, and substrate quality

Abstract This study examined the effects of initial litter quality and irrigation and fertilization treatments on litter decomposition rates and nutrient dynamics (N, Ca, K, Mg, and P) of loblolly ( Pinus taeda L.) pine needles in the North Carolina Sand Hills over 3 years. Litter quality was based on the initial C/N ratios, with the high-quality litter having a significantly ( P k =0.36±0.01 year −1 for the fertilized and irrigated+fertilized plots) than on unfertilized plots ( k =0.26±0.01 and 0.28±0.01 year −1 for the control and irrigated plots, respectively). The decomposing litter was a net sink for P and N and a net source of Mg, Ca, and especially K. Whereas initial substrate quality did not affect decomposition rates, it did affect the rate of release. Compared to the low quality litter, the high quality litter released K at a higher rate, released Mg at a lower rate, and accumulated N at a higher rate. Fertilization decreased the rate of release of Mg and K in high-quality litter and Mg and Ca in low quality litter. In addition, fertilization increased the rate of accumulation of P in both.

Enhancing the soil organic matter pool through biomass incorporation.

A study was installed in the upper Coastal Plains of South Carolina, USA that sought to examine the impact of incorporating downed slash materials into subsoil layers on soil chemical and physical properties as compared with the effect of slash materials left on the soil surface. Two sites were examined which differed in soil textural composition: sandy vs. clay.

Transient nature of rhizosphere carbon elucidated by supercritical freon-22 extraction and 13C NMR analysis

Abstract The region immediately adjacent to established roots of mature trees has been termed the “reoccurring rhizosphere” and it has been hypothesized that organic matter input from fine root turnover, root exudates and sloughing may result in a build up of the soil carbon in this region. The “reoccurring rhizosphere” for first-, second- and third-order roots of select loblolly pines (Pinus taeda L.) were examined on sandy, loamy sand and sandy loam soils. A significant carbon build up next to the root orders was confirmed for the sandy and loamy sand soils. The carbon build up was substantial (55% increase) next to the first-order roots of the sandy soil. However, the sandy loam soil did not display a significant amount of carbon build up next to the root orders. Extraction of the soil samples with supercritical freon-22 showed that the additional carbon in the “reoccurring rhizosphere” was highly soluble. Approximately 60% of the total soil carbon was extracted from the sandy and loamy sand soils, while approximately 40% was extracted from the sandy loam soil. A qualitative comparison of the extracts by liquid state 13 C nuclear magnetic resonance showed that the “reoccurring rhizosphere” region had a higher relative proportion of labile materials (i.e. carbohydrates, proteins, etc.) than the bulk soil. This information coupled with the high solubility in supercritical freon-22 suggests that the carbon build up in the “reoccurring rhizosphere” region of loblolly pines may be transient in nature.

Irrigation, fertilization and initial substrate quality effects on decomposing Loblolly pine litter chemistry

Changes in carbon chemistry (i.e., carbon compound classes such as aromatics, phenolics, etc.) of loblolly pine (Pinus taeda L.) litter were examined during three years of decomposition under factorial combinations of irrigation and fertilization treatments. Cross polarization magic angle spinning 13C nuclear magnetic resonance revealed that total carbon and nutrient concentrations correlated strongly with carbohydrate and O-alkyl carbon concentrations but did not relate well with concentrations of lignin, aromatic and phenolic carbon, or with lignin-related decomposition indices. The best correlations to carbon and nutrient concentrations occurred with the C/N (R2=0.86, P > 0.0001) and alkyl/O-alkyl (R2=0.75, P > 0.0001) decomposition indices. In all situations, the carbon chemistry of the decomposing litter followed the general pattern of accumulation of alkyl and carbonyl carbon with a loss of O-alkyl and methoxy carbon. Only small variations in the aromatic and phenolic carbon concentrations were detected. Since lignin is composed primarily of aromatic and phenolic carbons, the observation that there were only small changes in the aromatic and phenolic carbons of the litter is consistent with the general stability of lignin in these ecosystems. Trends in carbon chemistry during decomposition suggested that fertilization accelerated the decomposition process by about 100% as compared with the control plots. Irrigation also accelerated the decomposition process but to a lower extent (about 62% greater than control plots). Initial litter quality, as defined by the litter C/N, did not have a significant effect on the carbon chemistry of the decomposing litter. This study demonstrated that the decomposition mechanisms were not altered by the treatments but there were important changes in the relative chemistry of the decomposing litter which impacted the rate of decomposition.

Analyzing Water Soluble Soil Organics as Trifluoroacetyl Derivatives by Liquid State Proton Nuclear Magnetic Resonance

Abstract In forested ecosystems, water soluble organics play an important role in soil processes including carbon and nutrient turnover, microbial activity and pedogenesis. The quantity and quality (i.e., chemistry) of these materials is sensitive to land management practices. Monitoring alterations in the chemistry of water soluble organics resulting from land management practices is difficult because of the complexity and low concentration of these compounds. A procedure is described in which the water soluble organics are quantitatively derivatized with trifluoroacetic anhydride and then analyzed by liquid state proton nuclear magnetic resonance (1H NMR). The procedure was applied to sample amounts as low as 0.03 mg from the forest floor and root exudates. The root exudate samples were dominated by aliphatic compounds with relatively few O‐alkyl, olefinic and aromatic compounds. The chemistry of the samples originating from the forest floor differed dramatically with soil texture and treatment combinations.