Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Changes in organic matter composition of forest soil treated with a large amount of urea to promote ammonia fungi and the abilities of these fungi to decompose organic matter

Organic matter composition (lignin, holocellulose, 50% (v/v) methanol extract, water-soluble carbohydrate (WSC) and phenolics (WSP), petroleum ether extract, and ash) of A₀ layer soil treated with 700 g/m² of urea to promote ammonia fungi was investigated in a Japanese red pine (Pinus densiflora) forest. Nine species of fungi were found during the study period of 18 months after the treatment. Of these, seven species belong to the ammonia fungi. WSC content of the treated soil was lower than that of the control. Methanol extract content increased initially after the treatment, then decreased to below the control level. There were no consistent differences in other components between the treated plot and the control. The abilities to decompose cellulose, lignin, chitin, protein and lipid in 18 strains in 10 species of the ammonia fungi were also screened. Cellulose was not lysed byPseudombrophila deerata, Hebeloma spp. andLaccaria bicolor. Strong lignolytic activity was shown byLyophyllum tylicolor, Coprinus echinosporus andP. deerata. Chitin was decomposed byAmblyosporium botrytis, L. tylicolor, C. echinosporus andHebeloma vinosophyllum. All strains possessed proteolytic and lipolytic activities. Supply of glucose to the culture media resulted in weaker enzyme activities except for lignolytic ability.     

The effect of fire on nutrient losses and cycling in aQuercus coccifera garrigue (southern France)

The effects of burning on plant nutrient budgets and rates of carbonic gas and particulate matter emission during fires were evaluated in aQuercus coccifera (garrigue) shrubland. Nutrient levels were determined in field-collected pre-fire vegetation and combustion residues. The losses (increased elemental transfer) were calculated as the difference between the quantity of an element in the fuel (combustible standing vegetation plus litter) before burning and that present in the postfire residues (ash). Weight losses of elements are correlated with weight losses of burnt plant biomass. The relative order of nutrient losses was: N>C>Na>Ca>P>K>Mg. Estimated losses of N, C and P from combustible plant matter exceeded 98, 97 and 79% respectively. Copious N, C and P volatilization during burning was promoted by high concentrations of these elements in foliage and fine woody biomass of the aboveground vegetation and leaf litter of the garrigue. Elements were principally removed in the smoke. The quantities of gaseous emissions of CO₂, CO and particulate matter produced were estimated.     

A simple technique for producing 13C or 14C-labelled fronds of Lemna gibba and their use in soil incubation investigations

The duckweed Lemna gibba required light and a suitable energy source such as sucrose, glucose or fructose, for maximum growth in culture. The requirement for light was relatively unimportant and the plants grew well in a photon flux density of only 52 μmol m^-2s^-1 PAR. The uptake and incorporation of uniformly labelled ¹⁴C-glucose into fronds was related only to the concentration of the sugar. When incubated with soil, labelled L. gibba behaved in a manner similar to that of labelled ryegrass roots which had been produced by a more elaborate technique using a ¹⁴CO₂ labelled atmosphere. During incubation with soil for 224 days the L. gibba material (specific activity 6133 Bq mg^-1 d. wt) lost 64% of its radioactivity as ¹⁴CO₂ and ryegrass (specific activity 6634 Bq mg^-1 d. wt) lost 49%. Alkaline extracted humic and fulvic acids from soil had specific activities for the L. gibba incubation of 3409 and 407 Bq mg^-1 solid and for ryegrass roots of 4609 and 546 Bq mg^-1 solid respectively. The production of ¹³C or ¹⁴C-labelled L. gibba can be undertaken using only simple equipment producing material the specific radioactivity of which can be controlled by adjusting the activity of the sugar energy source.     

Microbial-feeding nematodes and protozoa in soil: Their effectson microbial activity and nitrogen mineralization in decomposition hotspots and the rhizosphere

Food web studies from a range of ecosystems have demonstrated that the fauna contributes about 30% of total net nitrogen mineralization. This results mainly from the activities of microbial-feeding microfauna (nematodes and protozoa). Microbial and microfaunal activity is concentrated at spatially discrete and heterogeneously distributed organic substrates, including the rhizosphere. The dynamics of microfauna and their effect on nutrient cycling and microbial processes at these sites is reviewed. The potential manipulation of microfauna, either as an experimental tool to further understand soil microbial ecology or as a practical means of managing nutrient flows in agroecosystems, is discussed.     

Nutrient distribution in a Swedish tree species experiment

The influence of four tree species on the distribution of nutrients between different compartments of the ecosystem was examined. In a randomized block (n=3) experiment in south-western Sweden, Ca, Mg and K were determined as exchangeable amounts in the mineral soil and as total amounts in the O+A₁ horizons (topsoil) and in the aboveground tree biomass. N contents were determined in all compartments as well as P contents of the aboveground tree biomass and the topsoil. The four tree species planted were: silver fir [Abies alba Mill.] (AA), grand fir [Abies grandis Lindl.] (AG), Norway spruce [Picea abies L. Karst.] (PA) and Japanese larch [Larix leptolepis (Sieb. och Zucc.) Endl.] (LL). At the age of 35–36 years, the total stemwood production of the most productive species, AG, was estimated at 471 m³ ha⁻¹. In relation to AG, LL had produced 80%, PA 73% and AA 37%. The system totals [aboveground tree biomass total + topsoil total + exchangeable (Ca, Mg, K) or total (N) in the mineral soil] of Ca, K and N did not differ significantly at the 5% level between the investigated species. For Mg, the system total in LL was significantly higher than for the other species. There was an indication that LL and AA contained higher amounts of Ca, Mg, K and N in the topsoil but less in the biomass than did AG and PA (partly significant). In the mineral soil, there were no significant differences in the exchangeable pools of Ca and K, nor in the total amounts of N. The biomass nutrient concentrations generally decreased in the order: AA > PA > AG > LL. At stem or whole-tree harvest, the Ca export per biomass unit would more than double in the case of PA compared to LL. LL also contained less N in the biomass than the other species. However, the N content in the biomass did not differ between the most (AG) and the least (AA) productive species, although the production of dry weight biomass (standing + harvested) of AG had been twice that of AA. It is concluded that the nutrient budget of a managed forest may vary considerably depending on the choice of tree species.     

Estimating regional carbon stocks and spatially covarying edaphic factors using soil maps at three scales

Most estimates of regional and global soil carbon stocks are based on extrapolations of mean soil C contents for broad categories of soil or vegetation types. Uncertainties exist in both the estimates of mean soil C contents and the area over which each mean should be extrapolated. Geographic information systems now permit spatially referenced estimates of soil C at finer scales of resolution than were previously practical. We compared estimates of total soil C stocks of the state of Maine using three methods: (1) multiplying the area of the state by published means of soil C for temperate forests and for Spodosols; (2) calculating areas of inclusions of soil taxa in the 1:5,000,000 FAO/UNESCO Soils Map of the World and multiplying those areas by selected mean carbon contents; and (3) calculating soil C for each soil series and map unit in the 1:250,000 State Soil Geographic Data Base (STATSGO) and summing these estimates for the entire state. The STATSGO estimate of total soil C was between 23% and 49% higher than the common coarse scale extrapolations, primarily because STATSGO included data on Histosols, which cover less than 5% of the area of the state, but which constitute over one-third of the soil C. Spodosols cover about 65% of the state, but contribute less than 39% of the soil C. Estimates of total soil C in Maine based on the FAO map agreed within 8% of the STATSGO estimate for one possible matching of FAO soil taxa with data on soil C, but another plausible matching overestimated soil C stocks. We also compared estimates from the 1:250,000 STATSGO database and from the 1:20,000 Soil Survey Geographic Data Base (SSURGO) for a 7.5 minute quadrangle within the state. SSURGO indicated 13% less total soil C than did STATSGO, largely because the attribute data on depths of soil horizons in SSURGO are more specific for this locality. Despite localized differences, the STATSGO database offers promise of scaling up county soil survey data to regional scales because it includes attribute data and estimates of areal coverage of C-rich inclusions within map units. The spatially referenced data also permit examination of covariation of soil C stocks with soil properties thought to affect stabilization of soil C. Clay content was a poor predictor of soil C in Maine, but drainage class covaried significantly with soil C across the state.     

Longitudinal patterns of dissolved organic carbon concentration and suspended bacterial density along a blackwater river

Dissolved organic carbon (DOC) is the dominant form of carbon in transport in blackwater rivers, and bacteria are the major biological agents of its utilization. This study describes longitudinal patterns in DOC concentration and relates them to suspended bacterial populations in the channel. Concentrations of total DOC, three molecular weight fractions, and bacterial numbers were determined at 12 sites along the Ogeechee River in 1985–1986 and 1989 during periods of low and high discharge. Suspended bacterial populations were compared with DOC concentrations to determine if differences in bacterial abundance were related to longitudinal patterns of DOC concentration. Three distinct longitudinal patterns were observed: (1) The longitudinal pattern followed by both total and intermediate molecular weight DOC concentrations was a linear function of the geographic distance along the river. (2) During low flow conditions, there was a high degree of correspondence between patterns of bacterial numbers and low MW DOC (< 1000 apparent MW). (3) During periods of high discharge, the proportion of high (> 10,000) and intermediate (1000–10,000) MW fractions increased, and there was no longer a clear relationship between bacterial cells and low MW DOC.     

Remote sensing of sea surface and coastal features in the area of the Golfe d'Arguin,Mauritania

The analysis of a series of NOAA AVHRR satellite data recorded between July and September 1986 shows the interaction between northerly and southerly water masses during summer as well as the northern limit of the northward advance of water masses originating from the South Equatorial Current. Two upwelling situations analysed by means of NOAA data show the intrusion of suspended matter from the ocean into the Golfe d'Arguin and illustrate the water circulation within the coastal zone. This circulation pattern is detailed by LANDSAT and SPOT data. A SPOT picture from October 1987 shows the structure of the intertidal and coastal geomorphology.