Robert G. Qualls

Fluxes of Dissolved Organic Nutrients and Humic Substances in a Deciduous Forest

We evaluated the importance of dissolved organic matter as a vehicle for the movement of N and P from the canopy and the forest floor into the mineral soil of a deciduous forest. We also examined the origin and nature of dissolved organic matter from the forest floor to see whether it was simply soluble plant material or highly humified matter. The average annual output from the forest floor in the form of dissolved organic matter was 18, 28, and 14% of the input in solid litterfall for C, N, and P, respectively. In throughfall, about half of the dissolved N and P was organic. But, in solution percolating from the forest floor, 94% of the N and 64% of the P was organic. Leaching from the forest floor was not a source of inorganic N and P for the mineral soil. Instead, the forest floor was a sink for the removal of these inorganic nutrients delivered in throughfall. Microbial immobilization was the most likely explanation for much of the inorganic nutrient removal. In contrast, the forest floor was an abundant contributor of N and P to the mineral soil in the form of dissolved, and possibly particulate, organic matter. Much of the dissolved organic matter entering the A horizon originated from the upper (Oa and Oe horizon) forest floor, but it was modified in several respects compared to the original soluble material. The solution percolating from the forest floor over most of the year was much richer in nitrogen, contained a much larger proportion of hydrophilic acids, and contained a much smaller proportion of carbohydrate-rich hydrophilic neutrals, than did the original water- extractable material in autumn litter. However, the fresh autumn litter did contain a similar proportion of soluble hydrophobic acids that resembled dissolved humic substances in several respects. Most of the flux of nitrogen from the forest floor to the A horizon was carried by humic substances and highly colored hydrophilic acids.

Comparison of the behavior of soluble organic and inorganic nutrients in forest soils.

Abstract The mechanisms by which potentially soluble inorganic nutrients are retained and eventually recycled within ecosystems have been characterized by many studies. Substantial amounts of potentially soluble organic nutrients are also released as plants grow, die, and decompose. In this study, a conceptual model was developed which shows the pools and significant processes generating and consuming dissolved organic matter and the associated nutrient elements. Experiments showed that the pool of potentially soluble organic matter in both mineral and organic horizons was much larger than the amount dissolved by any individual leaching event. Adsorption had the effect of buffering concentrations of dissolved organic matter in both mineral and organic horizons. The sorption equilibrium also slows the leaching rate of potentially soluble organic matter. By increasing the residence time of potentially soluble organic matter on solid surfaces, sorption results in a much greater time of exposure of soluble organic matter to decomposers while in the sorbed state. Literature sources indicate a bimodal distribution of decay coefficients of dissolved organic C and N with a refractory fraction dominating. A set of hypotheses comparing the factors controlling retention of inorganic vs. organic nutrients was developed. These hypotheses related to: sources of dissolved nutrients; properties of molecules controlling behavior; biological removal from solution; non-biological removal from solution and major factors allowing loss from the system. The major factor allowing loss of organic nutrients from the ecosystem is hydrologic short circuiting or absence of a mineral soil horizon rich in Fe and Al oxyhydroxides whereas, in the case of inorganic nutrients, it is hydrologic short circuiting or absence of a root network in addition to geochemical factors.

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Water draining from the Everglades marshes of southern Florida containshigh concentrations of dissolved organic C (DOC), N (DON), and in somelocations, P (DOP). These dissolved organic nutrients carry over 90% of the Nand organic C, and about 25% of the P exported downstream in the Everglades.Ourobjectives were to describe the most important aspects of the origin and fateofdissolved organic matter (DOM) in the Everglades, and to describe the processescontrolling its concentration and export. Concentrations of dissolved organicnutrients are influenced by local plant production, decomposition, and sorptionequilibrium with peat. The drained peat soils of the Everglades AgriculturalArea and the more productive marshes of the northern Everglades produce some ofthe highest concentrations of DOC and DON in the Everglades watershed. Inportions of the marshes of the northern Everglades, P enrichment was correlatedwith higher local DOC and DON concentrations and greater production of solubleplant matter. Microbial degradation of Everglades DOM was very slow; less than10% of the DOC was lost after 6 months of incubation in the laboratory andsupplements of inorganic nutrients failed to speed the decomposition. Exposureto solar radiation increased the subsequent decay rate of the remaining DOC(25%in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7%of the DON, and degraded about 50% of the humic substances over 21 days insterile porewater samples and thus degraded DOM faster than microbialdegradation. The humic substances appeared to inhibit biodegradation of theother fractions of the DOC since hydrophilic organic acids decomposed fasterwhen isolated from the humic substances.The fate of DOC and DON is closely linked as indicated by a generally narrowrange of C/N ratios. In contrast, high concentrations of DOP were associatedwith P enrichment (at least in pore water). The DOC was composed of about 50%humic substances, 33% hydrophilic acids, and 15% hydrophilic neutralsubstances,typical of DOC from other environments, despite the fact that it originatesfroma neutral to slightly alkaline peatland. Despite high exports of DON (3.9g m−2 y−1 from one area), themarshes of the northern Everglades are a sink for DON on a landscape scale. Theagricultural fields of the Everglades Agricultural Area, however, exported netquantities of DON. High concentrations of DOC desorbed from the agriculturalsoils when water with no DOC was added. Sorption experiments indicated thathighconcentrations of dissolved organic matter flowing into the marshes from theEverglades Agricultural Area could suppress the further desorption ofadditionalsoluble organic matter through physicochemical mechanisms. While biologicalfactors, plant production and microbial decomposition are important inproducingpotentially soluble organic nutrients, physicochemical sorption equilibria,hydrology, and degradation by solar radiation are also likely to control theexport of this material on the landscape scale.

FORMS OF SOIL PHOSPHORUS ALONG A NUTRIENT ENRICHMENT GRADIENT IN THE NORTHERN EVERGLADES

About 60 MT y−1 of P from agricultural runoff have flowed into an area of the northern Everglades marshes of Florida since the late 1960s, creating a nutrient enrichment gradient. The objectives of this study were to determine (i) in what forms the added and native P have been stored over the past 30 years and (ii) whether forms that are resistant to recycling are the main forms of P storage. The peat soils were sampled at 18 stations along a gradient from the source of nutrient inflow in February, May, August, and December and were then subjected to sequential extraction of P. In general, the P concentrations along the gradient decreased to relatively low concentrations about 8 to 10 km into the interior of the marsh. Concentrations of humic organic P, residual insoluble organic P, Ca-bound P, exchangeable inorganic P, and Fe/Al-bound inorganic P (only at the 20 to 25-cm depth) were all correlated negatively with distance from the nutrient input, i.e., they were 2 to 4 times higher in the enriched area. In contrast, concentrations of microbial biomass P in the surface soil were not correlated with distance from the nutrient input although variability was great. Because more peat is accreting in the enriched area, all forms of P are accreting faster (1.8 to 8 times faster, depending on the form) in the enriched area. Deposition of organic P in the peat was the most important process storing excess P from the nutrient inflows. Formation of Cabound P in these neutral to slightly alkaline (pH 7.2 to 7.9) peat soils was also an important mechanism of P deposition at the stations closest to the phosphorus input. The excess P enrichment has permeated most components of the marsh system, including organic and inorganic forms as well as rapidly and slowly cycling forms.

Effects of increased atmospheric CO2, temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.)

Root exudation has been hypothesized as one possible mechanism that may lead to increased inputs of organic C into the soil under elevated atmospheric CO2, which could lead to greater long-term soil C storage. In this study, we analyzed exudation of dissolved organic C from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO2 (35.0 and 70.0 Pa) × 2 temperature (26° and 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. We also analyzed the decomposition rates of root exudate to estimate gross rates of exudation. Elevated CO2 did not affect root exudation of organic C. A 4 °C increase in temperature and N fertilization did, however, significantly increase organic C exudation rates. Approximately 60% of the exudate decomposed relatively rapidly, with a turnover rate of less than one day, while the remaining 40% decomposed more slowly. These results suggest that warmer climates, as predicted for the next century, may accelerate root exudation of organic C, which will probably stimulate rapid C cycling and may make a minor contribution to intermediate to more long-term soil C storage. However, as these losses to root exudation did not exceed 1.2% of the net C fixed by Robinia pseudoacacia, root exudation of organic C appears to have little potential to contribute to long-term soil C sequestration.

Adaptive responses of Lepidium latifolium to soil flooding: biomass allocation, adventitious rooting, aerenchyma formation and ethylene production

Abstract Lepidium latifolium, perennial pepperweed, is an exotic crucifer that has spread explosively in recent years in wetlands and riparian areas of the western United States. Adaptive responses of L. latifolium to different durations of 0, 3, 7, 15, 30 and 50-day soil flooding treatment were investigated. Biomass allocation, adventitious rooting, aerenchyma development and ethylene production in plants were measured. Compared with controls maintained at −20 kPa soil matric water potential, flooding stress reduced total biomass of L. latifolium. After 7 days of flooding, the total biomass and root/shoot ratio of flooded plants were significantly less than those of unflooded controls. The number of adventitious roots on the stem base increased with the duration of flooding. Root porosity was much higher in the flooded plants than in the unflooded controls after 3 days of treatment and rose to 43% after 50 days. Ethylene production in roots was higher in flooded plants than in the control throughout the 50-day duration of flooding and peaked at 7 days. The reduction in the root/shoot ratio, adventitious rooting, and aerenchyma development in flooded L. latifolium are important contributions to flood tolerance. L. latifolium resembled species adapted to standing water conditions in terms of having an initially high porosity, but it resembled species adapted to either saturated or occasionally flooded habitats in terms of the degree of increase in root porosity under flooded conditions. However, in growth of biomass, L. latifolium was more like plants that do not grow in mostly saturated conditions. Thus, L. latifolium appears to be a plant that exhibits plasticity to tolerate or survive saturated conditions, but not to grow well under these conditions. This may be an adaptation to arid or semiarid riparian habitats where spring flooding and summer drought are characteristic.

Retention of soluble organic nutrients by a forested ecosystem

We document an example of a forested watershed at the Coweeta HydrologicLaboratory with an extraordinary tendency to retain dissolved organic matter(DOM) generated in large quantities within the ecosystem. Our objectives weretodetermine fluxes of dissolved organic C, N, and P (DOC, DON, DOP,respectively),in water draining through each stratum of the ecosystem and synthesizeinformation on the physicochemical, biological and hydrologic factors leadingtoretention of dissolved organic nutrients in this ecosystem. The ecosystemretained 99.3, 97.3, and 99.0% of water soluble organic C, N and P,respectively, produced in litterfall, throughfall, and root exudates. Exportsinstreamwater were 4.1 kg ha−1yr−1of DOC, 0.191 kg ha−1 yr−1 ofDON, and 0.011 kg ha−1 yr−1 ofDOP. Fluxes of DON were greater than those of inorganic N in all strata. MostDOC, DON, and DOP was removed from solution in the A and B horizons, with DOCbeing rapidly adsorbed to Fe and Al oxyhydroxides, most likely by ligandexchange. DON and DOC were released gradually from the forest floor over theyear. Water soluble organic C produced in litterfall and throughfall had adisjoint distribution of half-decay times with very labile and veryrefractory fractions so that most labile DOC was decomposed before beingleachedinto the mineral soil and refractory fractions dominated the DOC transportedthrough the ecosystem. We hypothesize that this watershed retained solubleorganic nutrients to an extraordinary degree because the soils have very highcontents of Fe and Al oxyhydroxides with high adsorption capacities and becausethe predominant hydrologic pathway is downwards as unsaturated flow through astrongly adsorbing A and B horizon. The well recognized retention mechanismsforinorganic nutrients combine with adsorption of DOM and hydrologic pathway toefficiently prevent leaching of both soluble inorganic andorganic nutrients in this watershed.

Soil formation and organic matter accretion in a young andesitic chronosequence at Mt. Shasta, California

The objectives of this work were to study rates of increase in allophane concentration, specific surface area changes, and organic matter accretion in a young andesitic chronosequence. We sampled the 0–10-, 10–20-, 30–40-, 70–80-, and 140–150-cm depths of 77-, 255-, 616-, and approximately 1200+-year-old soils and analyzed them for allophane, ferrihydrite, specific surface area, cation exchange capacity, soil pH, and C and N concentrations. Allophane concentrations increased at rates up to a maximum of 0.14 g kg−1 year−1, and concentrations are up to 68 times higher in the oldest than in the youngest soil. During the same time ferrihydrite concentrations increased only by a factor of 2.3. The specific surface area that could be attributed to allophane was only 2–4% in the youngest soil but was 41–97% in the oldest soil. Carbon and N stocks increased linearly with soil age over the first ∼600 years with rates of 139 kg C ha−1 year−1 and 5.3 kg N ha−1 year−1, respectively. After about 600 years, accretion rates were lower. Increases in allophane concentrations lead to increased cation exchange capacity in the soil. Our results indicate that the ability of the soil to retain nutrients improved with soil development.

Anaerobic metabolism in the roots of seedlings of the invasive exotic Lepidium latifolium

Lepidium latifolium is an invasive exotic crucifer that is widely distributed in riparian zones and wetlands. In this study, anoxic carbohydrate metabolism and post-anoxic injury in the roots of L. latifolium seedlings were examined. A significant increase in the activity of the fermentative enzymes alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) in roots occurred under anoxia and increased with the duration of anaerobic treatment during 7 days. However, pyruvate decarboxylase (PDC) and cytochrome c oxidase (CCO) activity was maintained at relatively stable levels under anaerobic and aerobic conditions. Soluble protein concentration in anoxic roots was two to three times that in aerobic roots throughout 7 days of anoxia. The concentration of the fermentation product ethanol in roots was two times greater under anoxia than under aerobic conditions. The concentration of lactate was much smaller than that of ethanol, but the trend was similar to that of ethanol. There was no significant difference in the concentration of malate between aerobic and anaerobic conditions for 9 days. Superoxide dismutase (SOD) activity in roots was two to three times higher under anoxia than under aerobic conditions throughout 7 days, but this increase in SOD activity decreased slightly with the duration of anoxia. Compared with aerobic conditions, the concentration of malondialdehyde (MDA), an indicator of free radical damage, increased by two to three times under anoxia. Two days after L. latifolium seedlings were returned to aerobic conditions, the concentrations of ethanol and MDA in roots were still significantly higher under the previously anoxic treatment than under continuously aerobic conditions, while no significant difference in enzyme activities or in concentrations of lactate and malate was found between treatments. The metabolism of L. latifolium roots under anoxia is characterized by the concurrent activity of both fermentative pathways and aerobic metabolism. Roots of L. latifolium have metabolically adaptive strategies to anoxia, but there is evidence of oxidative stress under anoxia and of post-anoxic injury from free radicals upon re-exposure to air. Results suggest that L. latifolium exhibit a mixture of characteristics typical of hydrophytic, facultative, and anoxia intolerant species.

A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature

N-fixing trees facilitate the growth of neighboring trees of other species. These neighboring species benefit from the simple presence of the N fixation symbiosis in their surroundings. Because of this phenomenon, it has been hypothesized that a change in atmospheric CO2 concentration may alter the role of N-fixing trees in their environment. It is thought that the role of N-fixing trees in ecosystems of the future may be more important since they may help sustain growth increases due to increased CO2 concentration in nitrogen limited forests. We examined: (1) whether symbiotically fixed N was exuded from roots, (2) whether a doubled atmospheric CO2 concentration would result in increased organic N exudation from roots, and (3) whether increased temperature or N availability affected N exudation from roots. This study analyzed exudation of dissolved organic N from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO2 (35.0 and 70.0 Pa) × 2 temperature (26 or 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. Trees with no other source of N except N fixation exuded about 1% to 2% of the fixed N through their roots as dissolved organic N. Increased atmospheric CO2 concentrations did not, however, increase N exudation rates on a per gram belowground biomass basis. A 4 °C increase in temperature and N fertilization did, however, significantly increase N exudation rates. These results suggest that exudation of dissolved organic N from roots or nodules of N-fixing trees could be a significant, but minor, pathway of transferring N to neighboring plants in a much more rapid and direct way than cycling through death, decomposition and mineralization of plant residues. And, while exudation rates of dissolved organic N from roots were not significantly affected by atmospheric CO2 concentration, the previously observed ‘CO2 fertilization effect’ on N-fixing trees suggests that N exudation from roots could play a significant but minor role in sustaining increases in forest growth, and thus C storage, in a CO2 enriched atmosphere.