川中人工纯柏林凋落物分解动态研究

对柏木人工纯林的渊落物有机碳和一些营养元素的分解和释放过程的研究表明,柏木凋落物中有机碳和营养元素的矿化非常缓慢,凋落物有机碳的半减期是33周,氮素矿化半减期是433周,磷矿化半减期是10周,钾矿化半减期是24周,钙矿化半减期是86周,镁矿化半减期是12周。柏木早期生长缓慢、成林困难主要是受氮素不足的制约。在林分改造和营林中适当增加阔叶速生树种,改善凋落物性质．对促进凋落物分解和养分周转、改善土壤养分状况、提高柏木人工林的生态和经济效益是很必要的。     

Mineral nitrogen and microbial dynamics in the forest floor of clearcut or partially harvested successional boreal forest stands

The effects of clearcut and partial harvesting of early-seral trembling aspen plots were compared to conventional clearcut harvesting in mid-seral mixedwood and late-seral conifer plots. Twice a year, for three consecutive years, we assessed mineral N and microbial dynamics in the forest floor of these plots to test three hypotheses related to the higher litter quality of aspen leaves and to the sustained inputs of available C on partially harvested plots: (1) the post-clearcutting mineral N flush and the net [(NO₃^–): (NO₃^– + NH₄⁺)] production ratio (RNI) are higher in aspen plots than in black spruce plots, with intermediate values occurring in mixedwood plots; (2) net N mineralization rates in aspen plots are higher in spring than in autumn; and (3) compared to clearcutting, partial harvesting reduces potential ammonification and nitrification rates. Initial NH₄⁺ and NO₃^– concentrations respectively ranged between 1.7–4.4 and 0.2–1.5 g N kg^–1 N_total, net ammonification and nitrification rates (30 d incubations) respectively ranged between 5.3–17.8 and 0.1–27.6 g N kg^–1 N_total, basal respiration ranged between 20.9–38.9 mg CO₂-C kg^–1 h^–1, and microbial biomass ranged between 6.1–8.7 g C_mic kg^–1. Although clearcutting increased NO₃^– concentrations in aspen plots, the balance of our results did not support our first hypothesis, because NH₄⁺ concentrations increased in conifer plots only, potential ammonification was unaffected by clearcutting, potential nitrification increased in mixedwood plots only, and RNI increased in all plots. In each seral stage, basal respiration, microbial biomass, and metabolic quotient either increased or were unaffected by clearcutting, suggesting that increases in RNI after disturbance were not related to lower microbial immobilisation of NO₃^– due to lower available C. Forest floors in mid-seral mixedwood plots exhibited a distinct combination of mineral N and microbial properties, suggesting that the functional richness of the forest is enhanced not only by the number of species, but also by the diversity of assemblages that are present. Results supported our second hypothesis and showed, furthermore, that net N mineralization in conifer stands is greater in autumn than in spring. Partial harvesting in aspen stands resulted in lower potential mineralization of N and lower RNI, compared to clearcutting. Further lysimetry studies are needed to confirm whether partial harvesting mitigates NO₃^– leaching following disturbance.     

13C natural abundance variations in carbonates and organic carbon from boreal forest wetlands

¹³C natural abundance variations were measured in peat soil and vegetation from two contrasting boreal forest wetlands: an upland watershed basin and a permanently saturated lowland mire. Evidence of methane oxidation was shown in the permanently saturated wetland with δ¹³C values as low as -97 ‰ in carbonate minerals found in floating peat mats. It is postulated that¹³C depleted CH₄ is oxidized in the mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Methane flux measurements during the summer of 1992 showed much lower fluxes in areas with floating peat mats relative to open water. Secondary carbonates in the basin peat have isotope compositions close to the δ¹³C values of the peat organic carbon (-25 ‰), indicating their origin from fermentation and possibly from sulfate-reduction. In the upland basin peat deposits, the δ¹³C_PDB values of organic C were constant with depth, while the permanently saturated mire had localities of¹³C enrichment in deeper layers of the peat. The¹³C enrichment may reflect areas of intense CH₄ production in which¹³C enriched residual substrate is left behind during the production of highly¹³C depleted CH₄.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decomposition and nutrient release temporal pattern of oil palm residues

The decomposition and nutrient release temporal patterns of three oil palm residues used as soil mulch were studied. Empty fruit bunches (EFB; 1000 kg plot^?1), Eco‐mat (processed EFB carpet; 30 kg plot^?1), and pruned palm fronds (180 kg plot^?1) were left to decompose (and sampled monthly) on the soil surface for 8 months. The frond's leaflets had the highest initial concentration for most nutrients, and the frond's rachis and Eco‐mat the lowest. The order of residue quality and rate of residue mass loss were: leaflets > fronds > EFB > Eco‐mat > rachis. EFB however had a higher mass loss rate than the fronds. Residue mass loss and nutrient release rates were faster at the beginning than at the end of the decomposition period. Leaflets released the highest total amount of nutrients (except for K), and rachis the lowest. The fronds released either significantly higher (for N and Ca) or not significantly different (for P and Mg) total amount of nutrients than EFB. Converting EFB into Eco‐mat had resulted in nutrient losses (e.g. N, K and Mg) and a residue quality reduction in Eco‐mat. This study's results would aid in better soil and oil palm fertilisation management.     

The nutrient content and the release of nutrients from fish food and faeces

The fish food and faeces were fractioned into the differentcomponents of phosphorus and nitrogen. There was a rapid release ofphosphorus from the fish food and faeces and a decrease thereafterwhereas ammonium release was slow at first with the rate increasingwith time. Both temperature and pH affected the release of nutrientsfrom fish food and faeces. The release of phosphorus and nitrogen washigher at higher temperatures. The maximum release of phosphorus wasat pH 4.0 whereas nitrogen release was maximum at neutral (7.0) toalkaline (10.0) media.     

Plasticity in photosynthetic response to nutrient supply of seedlings from a mixed conifer‐angiosperm forest

Abstract We measured the plasticity of the response of photosynthesis to nutrient supply in seedlings of the dominant four conifer and broadleaved angiosperm tree species from an indigenous forest in South‐westland, New Zealand. We hypothesized that the response of conifers to differing nutrient supply would be less than the response for the angiosperms because of greater adaptation to low fertility conditions. In Prumnopitys ferruginea (D. Don) de Laub. the maximum velocity of electron transport, J_max, doubled with a 10‐fold increase in concentration of nitrogen supply. In Dacrydium cupressinum Lamb. the maximum velocity of carboxylation, V_cmax, doubled with a 10‐fold increase in phosphorus supply. In contrast, photosynthetic capacity for the angiosperm species Weinmannia racemosa L.f. was affected only by the interaction of nitrogen and phosphorus and photosynthetic capacity of Metrosideros umbellata Cav. was not affected by nutrient supply. The response of the conifers to increasing availability of nutrient suggests greater plasticity in photosynthetic capacity, a characteristic not generally associated with adaptation to soil infertility, thus invalidating our hypothesis. Our data suggest that photosynthetic response to nutrient supply cannot be broadly generalized between the two functional groups.     

不同覆盖方式对底泥内源营养盐释放的控制效果

通过底泥内源营养盐释放控制室内模拟试验,考察了塑料包被、斜发沸石、方解石、石英砂和硝酸钙5种覆盖材料对底泥氮磷释放效率的影响,系统分析了各自优劣程度,为实际环境中不同污染背景水体选择适宜的控制技术提供科学依据.结果表明: 不同覆盖材料对底泥总磷释放的控制效果依次为:塑料包被>硝酸钙>斜发沸石>方解石>石英砂;不同覆盖材料对底泥总氮释放的控制效果依次为:斜发沸石>塑料包被>方解石>石英砂>硝酸钙;不同覆盖材料对底泥硝态氮释放的控制效果依次为:塑料包被>斜发沸石>方解石>石英砂>硝酸钙;不同覆盖材料对底泥铵态氮释放的控制效果依次为:硝酸钙>石英砂>斜发沸石>方解石>塑料包被;温度和底泥内源营养盐释放有对应关系,水样中总磷、总氮和硝态氮浓度会随着温度上升而增加,而铵态氮浓度呈下降趋势.     

Decomposition of soil organic matter from boreal black spruce forest: environmental and chemical controls

Black spruce forests are a dominant covertype in the boreal forest region, and they inhabit landscapes that span a wide range of hydrologic and thermal conditions. These forests often have large stores of soil organic carbon. Recent increases in temperature at northern latitudes may be stimulating decomposition rates of this soil carbon. It is unclear, however, how changes in environmental conditions influence decomposition in these systems, and if substrate controls of decomposition vary with hydrologic and thermal regime. We addressed these issues by investigating the effects of temperature, moisture, and organic matter chemical characteristics on decomposition of fibric soil horizons from three black spruce forest sites. The sites varied in drainage and permafrost, and included a “Well Drained” site where permafrost was absent, and “Moderately well Drained” and “Poorly Drained” sites where permafrost was present at about 0.5 m depth. Samples collected from each site were incubated at five different moisture contents (2, 25, 50, 75, and 100% saturation) and two different temperatures (10°C and 20°C) in a full factorial design for two months. Organic matter chemistry was analyzed using pyrolysis gas chromatography-mass spectrometry prior to incubation, and after incubation on soils held at 20°C, 50% saturation. Mean cumulative mineralization, normalized to initial carbon content, ranged from 0.2% to 4.7%, and was dependent on temperature, moisture, and site. The effect of temperature on mineralization was significantly influenced by moisture content, as mineralization was greatest at 20°C and 50–75% saturation. While the relative effects of temperature and moisture were similar for all soils, mineralization rates were significantly greater for samples from the “Well Drained” site compared to the other sites. Variations in the relative abundances of polysaccharide-derivatives and compounds of undetermined source (such as toluene, phenol, 4-methyl phenol, and several unidentifiable compounds) could account for approximately 44% of the variation in mineralization across all sites under ideal temperature and moisture conditions. Based on our results, changes in temperature and moisture likely have similar, additive effects on in situ soil organic matter (SOM) decomposition across a wide range of black spruce forest systems, while variations in SOM chemistry can lead to significant differences in decomposition rates within and among forest sites.     

The effect of biochar management on soil and plant community properties in a boreal forest

Biochar management has been proposed as a possible tool to mitigate anthropogenic CO₂ emissions, and thus far its impacts in forested environments remain poorly understood. We conducted a large‐scale, replicated field experiment using 0.05‐ha plots in the boreal region in northern Sweden to evaluate how soil and vegetation properties and processes responded to biochar application and the disturbance associated with burying biochar in the soil. We employed a randomized block design, where biochar and soil mixing treatments were established in factorial combination (i.e., control, soil mixing only, biochar only, and biochar and soil mixing; n = 6 plots of each). After two growing seasons, we found that biochar application enhanced net soil N mineralization rates and soil concentrations regardless of the soil mixing treatment, but had no impact on the availability of , the majority of soil microbial community parameters, or soil respiration. Meanwhile, soil mixing enhanced soil concentrations, but had negative impacts on net N mineralization rates and several soil microbial community variables. Many of the effects of soil mixing on soil nutrient and microbial community properties were less extreme when biochar was also added. Biochar addition had almost no effects on vegetation properties (except for a small reduction in species richness of the ground layer vegetation), while soil mixing caused significant reductions in graminoid and total ground layer vegetation cover, and enhanced seedling survival rates of P. sylvestris, and seed germination rates for four tree species. Our results suggest that biochar application can serve as an effective tool to store soil C in boreal forests while enhancing availability. They also suggest that biochar may serve as a useful complement to site preparation techniques that are frequently used in the boreal region, by enhancing soil fertility and reducing nutrient losses when soils are scarified during site preparation.     

Mass loss and nutrient release from decomposing evergreen and deciduous Nothofagus litters from the Chilean Andes

Abstract Leaf litter decomposition experiments were conducted on two deciduous (Nothofagus obliqua (roble)) and Nothofagus pumilio (lenga)) and one evergreen (Nothofagus dombeyi) Nothofagus (Nothofagaceae) species from a single Chilean forest in order to understand how congeneric trees with differing leaf lifespans impact the soil in which they grow. Single‐species litter samples were decomposed in a mixed hardwood forest in Ohio and in a deciduous‐evergreen Nothofagus forest in Chile. In the Ohio forest, the two deciduous species’ litters decomposed at k ≈ 1.00 per year and the evergreen at k ≈ 0.75 per year. In Chile k ranged from k ≈ 0.06 (N. obliqua) to k ≈ 0.23 (N. pumilio) per year. In both experiments, N and P were released faster from the deciduous litters than from evergreen litter. In Ohio, evergreen litter immobilized more N and P for a longer time period than did deciduous litter. As N. dombeyi stands tend to have lower available soil N and P in this particular mixed Nothofagus forest, the increased time of N and P immobilization by N. dombeyi litter suggests a feedback role of the tree itself in perpetuating low N and P soil conditions.     

Variation in nutrient availability and plant species diversity across forb and graminoid zones of a Northern New England high salt marsh

Plant zonation patterns across New England salt marshes have been investigated for years, but how nutrient availability differs between zones has received little attention. We investigated how N availability, P availability, and plant N status varied across Juncus gerardii, Spartina patens, and mixed forb zones of a Northern New England high salt marsh. We also investigated relationships between several edaphic factors and community production and diversity across the high marsh. P availability, soil salinity, and soil moisture were higher in the mixed forb zone than in the two graminoid zones. NH⁺ ₄-N availability was highest in the J. gerardii zone, but NO^– ₃-N availability and mid season net N mineralization rates did not vary among zones. Plant tissue N concentrations were highest in the mixed forb zone and lowest in the S. patens zone, reflecting plant physiologies more so than soil N availability. Community production was highest in the J. gerardii zone and was positively correlated with N availability and negatively correlated with soil moisture. Plant species diversity was highest in the mixed forb zone and was positively correlated with P availability and soil salinity. Thus, nutrient availability, plant N status, and plant species diversity varied across zones of this high marsh. Further investigation is needed to ascertain if soil nutrient availability influences or is a result of the production and diversity differences that exist between vegetation zones of New England high salt marshes.     

Initial responses of rove and ground beetles (Coleoptera,Staphylinidae, Carabidae) to removal of logging residues following clearcut harvesting in the boreal forest of Quebec,Canada

Increased interest in biomass harvesting for bioenergetic applications has raised questions regarding the potential ecological consequences on forest biodiversity. Here we evaluate the initial changes in the abundance, species richness and community composition of rove (Staphylinidae) and ground beetles (Carabidae), immediately following 1) stem-only harvesting (SOH), in which logging debris (i.e., tree tops and branches) are retained on site, and 2) whole-tree harvesting (WTH), in which stems, tops and branches are removed in mature balsam fir stands in Quebec, Canada. Beetles were collected throughout the summer of 2011, one year following harvesting, using pitfall traps. Overall catch rates were greater in uncut forest (Control) than either stem-only or whole-tree harvested sites. Catch rates in WTH were greater than SOH sites. Uncut stands were characterized primarily by five species: Atheta capsularis, Atheta klagesi, Atheta strigosula, Tachinus fumipennis/frigidus complex (Staphylinidae) and to a lesser extent to Pterostichus punctatissimus(Carabidae). Increased catch rates in WTH sites, where post-harvest biomass was less, were attributable to increased catches of rove beetles Pseudopsis subulata, Quedius labradorensis and to a lesser extent Gabrius brevipennis. We were able to characterize differences in beetle assemblages between harvested and non-harvested plots as well as differences between whole tree (WTH) and stem only (SOH) harvested sites where logging residues had been removed or left following harvest. However, the overall assemblage response was largely a recapitulation of the responses of several abundant species.     

Gross nitrogen transformations in soils from uncut and cut boreal upland and peatland coniferous forest stands

Gross and net nitrogen (N) ammonification and nitrification were measured in soils from an uncut and recently cut upland and peatland conifer stand in northwestern Ontario, Canada. Rates of gross total inorganic N immobilization were similar to gross mineralization, resulting in low net mineralization rates in soils from all four upland and peatland conifer stands. Gross ammonification rates were variable but similar in soils from uncut and cut peatland hollows (18–19mgNkg^–1day^–1) and upland forest floor soils (14–19mgNkg^–1day^–1). Gross ammonium ( ) immobilization rates were also variable but similar to ammonification rates. Median gross nitrification rates were within 0–2mgNkg^–1day^–1 in soils from all four upland and peatland cut and uncut stands, although rates were consistently higher for the soils from the cut stands. Large variability in gross nitrification rates were observed in peatland soils, however the highest gross nitrification rates were measured in saturated peatland soils. Net rates remained low in the soils from all four stands due to high nitrate ( ) immobilization and very fast turnover (<0.2 day). Our results suggest that potential losses may be negated by high immobilization in uncut and cut boreal forest stands. This study reveals the potential for the interaction of N production and consumption processes in regulating N retention in upland and peatland conifer forests, and the resilience of the boreal forest to disturbance.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

Decomposition rate and nutrient release from plant litter of Norway spruce forest in the Bohemian Forest

The role of litter composition and quality on the nutrient release was studied in three month laboratory experiment. Spruce needles and leaves of four species dominant in understorey vegetation of the Norway spruce forest were collected in early autumn and incubated at 5°C, 10°C and 15°C. C mineralization was measured every two weeks, concentration of NH₄, NO₃, dissolved organic N, dissolved organic C and oxalate extractable P at the beginning and end of incubation and decay rate and nutrient release was calculated. Freshly senescent leaves contained less N and P indicating nutrient reallocation. Effect of temperature on a decay rate and nutrient transformation was not significant while the effect of litter quality expressed by C/N ratio at the end of incubation was. The decay rate was the fastest for the fern (Athyrium alpestre) and decreased in order: Callamagrostis villosa > Vaccinium myrtillus > Avenella flexuosa > spruce needles. The critical C/N ratio bellow which mineral N was released in high amount was around a value of 32. The results indicte that an increase of coverage of understorey vegetation can increase a risk of nutrient release.     

Effects of harvesting on spatial and temporal diversity of carbon stocks in a boreal forest landscape

Carbon stocks in managed forests of Ontario, Canada, and in harvested wood products originated from these forests were estimated for 2010–2100. Simulations included four future forest harvesting scenarios based on historical harvesting levels (low, average, high, and maximum available) and a no‐harvest scenario. In four harvesting scenarios, forest carbon stocks in Ontario's managed forest were estimated to range from 6202 to 6227 Mt C (millions of tons of carbon) in 2010, and from 6121 to 6428 Mt C by 2100. Inclusion of carbon stored in harvested wood products in use and in landfills changed the projected range in 2100 to 6710–6742 Mt C. For the no‐harvest scenario, forest carbon stocks were projected to change from 6246 Mt C in 2010 to 6680 Mt C in 2100. Spatial variation in projected forest carbon stocks was strongly related to changes in forest age (r = 0.603), but had weak correlation with harvesting rates. For all managed forests in Ontario combined, projected carbon stocks in combined forest and harvested wood products converged to within 2% difference by 2100. The results suggest that harvesting in the boreal forest, if applied within limits of sustainable forest management, will eventually have a relatively small effect on long‐term combined forest and wood products carbon stocks. However, there was a large time lag to approach carbon equality, with more than 90 years with a net reduction in stored carbon in harvested forests plus wood products compared to nonharvested boreal forest which also has low rates of natural disturbance. The eventual near equivalency of carbon stocks in nonharvested forest and forest that is harvested and protected from natural disturbance reflects both the accumulation of carbon in harvested wood products and the relatively young age at which boreal forest stands undergo natural succession in the absence of disturbance.     

不同施氮处理玉米根茬在土壤中矿化分解特性

以黄土高原南部地区7a定位试验不同氮肥处理玉米根茬为研究对象,通过室内培养试验研究了施氮量分别为0、120和240 kg N/hm2处理玉米根茬(分别用R0、R120、R240表示)在15-20 cm和45-50 cm土层土壤中有机碳矿化及其对土壤微生物量碳、可溶性有机碳和矿质态氮含量的影响.结果表明,不同处理玉米根茬C/N为R0＞R240 ＞R120.培养条件下,R120和R240根茬的碳矿化速率高于R0根茬,R120与R240根茬之间差异不显著.不同处理根茬C/N与其培养过程中碳素累积表观矿化量呈极显著负相关关系.3种施氮量处理的玉米根茬在培养过程中有机碳矿化率、潜在碳矿化量、土壤微生物量碳、可溶性有机碳含量均为添加R120根茬的处理最高,R240次之,R0最低.添加R120和R240根茬显著提高了培养起始时土壤矿质态氮含量.R0、R120和R240根茬在15-20 cm土层土壤中的碳矿化率分别比其在45-50 cm土层土壤中高51.70％、26.41％和27.84％.在评价根茬还田对农田生态系统碳、氮等养分循环的作用时,应同时考虑施肥对根茬分解和转化的影响.     

Effects of irradiance on diel and seasonal patterns of nutrient uptake by stream periphyton

1. Irradiance strongly affects the abundance of stream periphyton communities that in turn influence patterns of instream nutrient uptake. We examined relationships between irradiance and periphyton nutrient uptake taking into account diel and seasonal variation in ambient irradiance. 2. Uptake of dissolved N, P and C by periphyton as areal uptake (U) and demand (V_f) was determined under 11 irradiance levels (0–100% of ambient conditions) using shallow stream‐side experimental channels. Experiments were conducted once per season over one annual cycle with both day and night uptake rates assessed, together with periphyton biomass and autotrophic production rates. 3. No consistent diel variation in areal uptake or demand was detected for the predominant inorganic or total dissolved nutrients even at the highest irradiances. Lack of variation may indicate nutrient limitation, with photosynthetic sequestration and storage of C during the day for subsequent utilisation at night. Alternatively, oxygen consumption by photoautotrophs at night may stimulate compensatory heterotrophic uptake (e.g. denitrification). 4. In all seasons, release of dissolved organic N was detected during the day but to a lesser extent at night. This was not directly related to irradiance levels, indicating that heterotrophic metabolism (e.g. microbial decomposition) contributes to this phenomenon. 5. Areal uptake and demand for the predominant inorganic and total dissolved nutrients increased in response to increasing irradiance in some or all seasons, but rates were typically higher during the spring and summer. Saturation of areal uptake and demand at elevated irradiances was evident during the spring. demand was also saturated at higher irradiances in the summer and autumn. Maximum demand was comparable during spring and summer, but saturation occurred at lower irradiance in summer (24 h average 135–145 μmol m^?2 s^?1) relative to spring (312–424 μmol m^?2 s^?1), indicating more efficient nutrient uptake in summer. Higher total periphyton biomass in summer, but comparable autotrophic biomass (chlorophyll a), implies that heterotrophic metabolism may contribute to this greater efficiency. In spring, autotrophic biomass peaked at an irradiance level of 225 μmol m^?2 s^?1, also suggesting a role for heterotrophic metabolism in demand at higher irradiances. 6. The results of this study show that irradiance levels exert a strong influence on the nature and quantity of instream nutrient uptake with N demand saturated at elevated irradiance levels during the spring, summer and autumn. Our results also suggest that heterotrophic metabolism makes a measurable contribution to instream nutrient uptake even under higher irradiances that favour autotrophic activity.     

Soil nutrient dynamics in response to irrigation of a Panamanian tropical moist forest

We measured concentrations of soil nutrients (0–15 and 30–35 cm depths) before and after the dry season in control and dry-season irrigated plots of mature tropical moist forest on Barro Colorado Island (BCI) in central Panama to determine how soil moisture affects availability of plant nutrients. Dry-season irrigation (January through April in 1986, 1987, and 1988) enhanced gravimetric soil water contents to wet-season levels (ca. 400 g kg^–1 but did not cause leaching beyond 0.8 m depth in the soil. Irrigation increased concentrations of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), but it had little effect on concentrations of inorganic N (NH₄ ^+C, NO₃ ^– and S (SO₄ ^2–). These BCI soils had particularly low concentrations of extractable P especially at the end of the dry season in April, and concentrations increased in response to irrigation and the onset of the rainy season. We also measured the response of soil processes (nitrification and S mineralization) to irrigation and found that they responded positively to increased soil moisture in laboratory incubations, but irrigation had little effect on rates in the field. Other processes (plant uptake, soil organic matter dynamics) must compensate in the field and keep soil nutrient concentrations at relatively low levels.