Are patterns in nutrient limitation belowground consistent with those aboveground: results from a 4 million year chronosequence

Accurately predicting the effects of global change on net carbon (C) exchange between terrestrial ecosystems and the atmosphere requires a more complete understanding of how nutrient availability regulates both plant growth and heterotrophic soil respiration. Models of soil development suggest that the nature of nutrient limitation changes over the course of ecosystem development, transitioning from nitrogen (N) limitation in ‘young’ sites to phosphorus (P) limitation in ‘old’ sites. However, previous research has focused primarily on plant responses to added nutrients, and the applicability of nutrient limitation-soil development models to belowground processes has not been thoroughly investigated. Here, we assessed the effects of nutrients on soil C cycling in three different forests that occupy a 4 million year substrate age chronosequence where tree growth is N limited at the youngest site, co-limited by N and P at the intermediate-aged site, and P limited at the oldest site. Our goal was to use short-term laboratory soil C manipulations (using ¹⁴C-labeled substrates) and longer-term intact soil core incubations to compare belowground responses to fertilization with aboveground patterns. When nutrients were applied with labile C (sucrose), patterns of microbial nutrient limitation were similar to plant patterns: microbial activity was limited more by N than by P in the young site, and P was more limiting than N in the old site. However, in the absence of C additions, increased respiration of native soil organic matter only occurred with simultaneous additions of N and P. Taken together, these data suggest that altered nutrient inputs into ecosystems could have dissimilar effects on C cycling above- and belowground, that nutrients may differentially affect of the fate of different soil C pools, and that future changes to the net C balance of terrestrial ecosystems will be partially regulated by soil nutrient status.     

Phosphorus limitation of forest leaf area and net primary production on a highly weathered soil

We tested the hypothesis that P was the nutrient limiting net primary production of a nativeMetrosideros polymorpha forest on a highly weathered montane tropical soil in Hawaii. A factorial experiment used all combinations of three fertilizer treatments: nitrogen (N), phosphorus (P) and a mix of other essential nutrients (OE), consisting primarily of mineral derived cations and excluding N and P. P addition, but not N or OE, increased leaf area index within 12 months, foliar P concentration measured at 18 months, and stem diameter increment within 18 months. Stem growth at 18 months was even greater when trees fertilized with P also received the OE treatment. N and P additions increased leaf litterfall and N and P in combination further increased litterfall. The sequence of responses suggests that increased available P promoted an increase in photosynthetic area which led to increased wood production. P was the essential element most limiting to primary production on old volcanic soil in contrast to the N limitation found on young volcanic soils.     

Determination of the Limiting Plant Nutrient in the Water of Lake Balaton by Algal Assay Procedure

Algal assays were performed on water samples taken from different sites in Lake Balaton. Selenastrum capricornutum was used in the test to determine the primarily limiting plant nutrient. The results of supplementary nutrient additions to the bottle tests were evaluated by cell counts. The tests have indicated phosphorus as growth rate limiting, and in two cases as biomass limiting plant nutrient, but periodic occurrences of nitrogen limitation cannot be excluded.     

Soil microbial properties and plant growth responses to carbon and water addition in a temperate steppe: the importance of nutrient availability

Background

Global climatic change is generally expected to stimulate net primary production, and consequently increase soil carbon (C) input. The enhanced C input together with potentially increased precipitation may affect soil microbial processes and plant growth.

Methodology/Principal Findings

To examine the effects of C and water additions on soil microbial properties and plant growth, we conducted an experiment lasting two years in a temperate steppe of northeastern China. We found that soil C and water additions significantly affected microbial properties and stimulated plant growth. Carbon addition significantly increased soil microbial biomass and activity but had a limited effect on microbial community structure. Water addition significantly increased soil microbial activity in the first year but the response to water decreased in the second year. The water-induced changes of microbial activity could be ascribed to decreased soil nitrogen (N) availability and to the shift in soil microbial community structure. However, no water effect on soil microbial activity was visible under C addition during the two years, likely because C addition alleviated nutrient limitation of soil microbes. In addition, C and water additions interacted to affect plant functional group composition. Water addition significantly increased the ratio of grass to forb biomass in C addition plots but showed only minor effects under ambient C levels. Our results suggest that soil microbial activity and plant growth are limited by nutrient (C and N) and water availability, and highlight the importance of nutrient availability in modulating the responses of soil microbes and plants to potentially increased precipitation in the temperate steppe.

Conclusions/Significance

Increased soil C input and precipitation would show significant effects on soil microbial properties and plant growth in the temperate steppe. These findings will improve our understanding of the responses of soil microbes and plants to the indirect and direct climate change effects.     

Nutrient requirements of ephemeral plant species from wet,mesotrophic soils

Abstract. Nanocyperion plant communities occur on wet, more or less nutrient‐poor and sparsely vegetated soils in temperate climates and are characterized by tiny, very shortlived plant species. Most of these have become locally extinct. It is generally assumed that drainage and eutrophication were the most important reasons for this decrease. However, chemical analysis of soil pore water from plots on growth sites of these ephemerals showed that phosphorus availability was relatively high. In a greenhouse experiment, the growth of ephemeral species was strongly limited by the amount of available phosphorus, whereas there was little or no limitation to the growth of other plant species from this habitat. At low phosphorus concentrations, the ephemeral species reached their reproductive phase within the same period, but showed a strong reduction in the amount of flowers that were produced. We concluded that ephemeral species in particular require a minimum amount of phosphorus for reproduction. Other species on nutrient‐poor, wet soils have a longer life span and can postpone flowering in nutrient‐poor soils. In contrast to other short‐lived plant species from the same habitat, the growth of ephemeral species was barely stimulated by enhanced nitrogen availability. Apparently, the ephemerals are adapted to low nitrogen concentrations. The occurrence on nitrogen‐poor and relatively phosphorus‐rich soils suggests that this community may be very sensitive to nitrogen deposition. Reduced phosphorus availability below the minimum requirements of ephemerals, for example after acidification or the exclusion of human activities, has possibly contributed to the decrease of ephemeral plant species.     

Nitrogen and phosphorus additions impact arbuscular mycorrhizal abundance and molecular diversity in a tropical montane forest

Increased nitrogen (N) depositions expected in the future endanger the diversity and stability of ecosystems primarily limited by N, but also often co‐limited by other nutrients like phosphorus (P). In this context a nutrient manipulation experiment (NUMEX) was set up in a tropical montane rainforest in southern Ecuador, an area identified as biodiversity hotspot. We examined impacts of elevated N and P availability on arbuscular mycorrhizal fungi (AMF), a group of obligate biotrophic plant symbionts with an important role in soil nutrient cycles. We tested the hypothesis that increased nutrient availability will reduce AMF abundance, reduce species richness and shift the AMF community toward lineages previously shown to be favored by fertilized conditions. NUMEX was designed as a full factorial randomized block design. Soil cores were taken after 2 years of nutrient additions in plots located at 2000 m above sea level. Roots were extracted and intraradical AMF abundance determined microscopically; the AMF community was analyzed by 454‐pyrosequencing targeting the large subunit rDNA. We identified 74 operational taxonomic units (OTUs) with a large proportion of Diversisporales. N additions provoked a significant decrease in intraradical abundance, whereas AMF richness was reduced significantly by N and P additions, with the strongest effect in the combined treatment (39% fewer OTUs), mainly influencing rare species. We identified a differential effect on phylogenetic groups, with Diversisporales richness mainly reduced by N additions in contrast to Glomerales highly significantly affected solely by P. Regarding AMF community structure, we observed a compositional shift when analyzing presence/absence data following P additions. In conclusion, N and P additions in this ecosystem affect AMF abundance, but especially AMF species richness; these changes might influence plant community composition and productivity and by that various ecosystem processes.     

Both nitrogen and phosphorus limit plant production on young Hawaiian lava flows

We applied fertilizers in a 2³complete factorial design to determine the effects of nutrient amendments on plant growth in Hawaiian montane forests growing on two different volcanic substrates: a and phoehoe lava. Both sites were about 140 years old and their overstories were nearly monospecific stands of Metrosideros polymorpha. Fertilizer applications included N, P, a mixture of essential macro- and micronutrients excepting P and N, and all combinations thereof in each of four blocks. Additions of nutrients other than N or P had no significant effects on measured plant-growth variables. In contrast, additions of either N or P significantly increased tree height growth, diameter increments, biomass growth, and height growth of the understory fern Dicranopteris linearis in both sites. The effect of N was greater than that of P. Greatest growth rates occurred in plots receiving both N and P, and signficant N^*P interactions occurred in several cases, suggesting a synergistic effect between these two elements. Plant growth on these young, poorly weathered, basaltic lavas is colimited by N and P availability. Growth in a similar-aged stand growing on a mixture of volcanic ash and cinders is N but not P limited, indicating that the texture of the parent material influences nutrient-availability patterns during early primary succession.     

Changes in microbial nutrient status during secondary succession and its modification by earthworms

Summary Microbial biomass, nutrient (N and P) status, and carbon and nutrient limitation of the microflora were investigated in soils from five different sites (field, 5-, 12-, and about 50-year-old fallow, beechwood), which represent different stages of a secondary succession from a wheat field to the climax ecosystem of a beechwood on limestone. In addition, the effect of faeces production by the substrate feeding earthworm species Octolasion lacteum (Örley) on the nutrient status of the soil microflora of these sites was studied. Humus had accumulated in the soil of the third fallow site, with an enhanced biomass of microflora. However, in the beechwood soil, which had the highest humus content, microbial biomass was lower than in the soil of the third fallow site and similar to that of the field and the two younger fallow sites. In general, soil microbial biomass was little affected by the passage of soil through the gut of O. lacteum. The soil microflora of the field, the 5-, 12-, and about 50-year-old fallow was limited by carbon, whereas in the beechwood soil phosphorus limited microbial growth. NItrogen availability to the soil microflora was low in the two younger fallow sites and high in the field and the third fallow. In the beechwood soil nitrogen supply did not affect microbial carbon utilization. Application of phosphorus stimulated glucose mineralization in the soil of the field, the third fallow, and the beechwood, but not in the two younger fallow sites. Therefor, the nutrient status of the soil microflora seems to have changed during secondary succession: presumably, during the first phase the availability of nitrogen decreased, whereas during the second phase microbial phosphorus supply became more important, which resulted in phosphorus limitation of the soil microflora in the climax ecosystem. The passage of soil through the gut of O. lacteum caused an alteration in the microbial nutrient status. Generally, microbial growth in earthworm casts was limited by carbon. The relative effect of the gut passage of the soils on microbial carbon utilization seems to increase during succession. Therefore, the effect of decomposer invertebrates on microbial nutrient supply seems to increase during secondary succession. In general, nitrogen did not limit microbial carbon utilization in earthworm casts. Phosphorus requirements of the soil microflora were lowered by the gut passage of the soil of the third fallow site and the beechwood, which indicates an increased phosphorus supply in earthworm casts. Howerver, this additional supply was not sufficient to enable optimal carbon utilization by the soil microflora. The results indicate that the effect of decomposer invertebrates on the soil microflora depends on the nutrient status of the ecosystem.     

Phosphatase activity and nitrogen fixation reflect species differences,not nutrient trading or nutrient balance,across tropical rainforest trees

A fundamental biogeochemical paradox is that nitrogen‐rich tropical forests contain abundant nitrogen‐fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen‐fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen‐for‐phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen‐for‐phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen‐fixing trees cannot provide a positive nitrogen‐phosphorus‐carbon feedback to alleviate nutrient limitation of the tropical carbon sink.     

Effects of Nutrient Limitation on Aboveground Carbon Dynamics during Tropical Dry Forest Regeneration in Yucatán,Mexico

Tree growth (as diameter increment), litterfall production, and litter biomass were studied in two secondary tropical dry forests of the Yucatán Peninsula under four treatments of nutrient addition. The studys objective was to assess how variations in the nutrient supply affect aboveground net primary production and carbon (C) accumulation on the floor of two forests in different stages of regeneration. The study included an area of young forest (10 years old) with phosphorus (P)-poor soils and an area of old forest (around 60 years old) where soil P was comparatively less limiting. Four replicate plots (12 × 12 m) at each forest were either left intact (controls) or fertilized with nitrogen (N), P, or N plus P during 3 consecutive years. After 3 years of fertilization, relaxation of the constraints on nutrient limitation resulted in increased trunk growth rates at both the young and old forests. This effect was more pronounced with the addition of P or N plus P (trunk growth doubled with respect to controls), whereas N addition increased tree growth by 60% in comparison to trees in plots without nutrient supplements. In both forests, there were no significant differences in litterfall production among treatments during the first 2 years after fertilization. In the 3rd year of nutrient addition, litterfall production was significantly higher in plots fertilized with N plus P compared to control plots at both forest sites; however, changes in litterfall were not accompanied by litter accumulation in the floor of the two forests. The results of this study support the hypothesis that there is nutrient limitation during tropical dry forest regeneration. They further show that it may be maintained in the long term during secondary succession.     

Comparative analyses of physiological assays and chlorophyll <Emphasis Type="Italic">a</Emphasis> variable fluorescence parameters: investigating the importance of phosphorus availability in oligotrophic and eutrophic freshwater systems

Diverse measurements of nutrient status indicators were used to test the severity of physiological phosphorus (P) limitation of phytoplankton among lake systems ranging from oligotrophic to eutrophic, based on P and chlorophyll a (Chl a) concentrations. Metabolic assays and particulate nutrient ratios were used to estimate nutrient status at sites located in Lake Erie, Lake Ontario and Lake Huron. Variable fluorescence ratios (F _v/F _m), relative electron transport rates and their response to irradiance were measured by the pulse-amplitude-modulated fluorometer. Under summer stratified conditions, P deficiency was strongest in the oligotrophic sites and nitrogen (N) status indicators and Chl a variable parameters revealed no severe N deficiency. Nutrient amendment assays showed positive associations with P additions and Chl a fluorescence parameters at P-deficient sites. In the most oligotrophic sites, N additions revealed a modest increase only detected by the Chl a fluorescence parameters. Phytoplankton communities were also associated with nutrient status, where chrysophytes and cryptophytes were important in P-deficient sites and cyanobacteria, phyrrophyta, and diatoms were prevalent in nutrient-rich sites. The results confirmed that Chl a fluorescence parameters can reveal P deficiency and indicate its severity among the range of trophic status in aquatic systems.     

Nutrient Limitation to Primary Productivity in a Secondary Savanna in Venezuela1

We examined nutrient limitation to primary productivity in a secondary savanna in the interior branch of the Coastal Range of Venezuela, which was converted from forest to savanna more than 100 years ago. We manipulated soil nutrients by adding nitrogen (+N), phosphorus and potassium (+PK), and nitrogen, phosphorus, and potassium (+NPK) to intact savanna. Eleven months after fertilization, we measured aboveground biomass and belowground biomass as live fine roots in the top 20 cm of soil, and species and functional group composition in response to nutrient additions. Aboveground biomass was highest in the NPK treatment ([mean g/m2]; control = 402, +N = 718, +PK = 490, +NPK = 949). Aboveground production, however, appeared to be limited primarily by N. Aboveground biomass increased 78 percent when N was added alone but did not significantly respond to PK additions when compared to controls. In contrast to aboveground biomass, belowground biomass increased with PK additions but showed no significant increase with N (depth 0–20 cm; [mean g/m2]; control = 685, +N = 443, +PK = 827, +NPK = 832). There was also a 36 percent increase in root length with PK additions when compared to controls. Whole savanna shoot:root ratios were similar for control and +PK (0.6), while those for +N or +NPK fertilization were significantly higher (1.7 and 1.2, respectively). Total biomass response (above + belowground) to nutrient additions showed a strong N and PK co‐limitation ([mean g/m2]; control = 1073, +N = 1111, +PK = 1258, +NPK = 1713). Aboveground biomass of all monocots increased with N additions, whereas dicots showed no response to nutrient additions. Trachypogon spp. (T. plumosus+T. vestitus) and Axonopus canescens, the two dominant grasses, made up more than 89 percent of the total aboveground biomass in these sites. Trachypogon spp. responded to NPK, whereas A. canescens, sedges, and the remaining monocots only responded to N. Even though nutrient additions resulted in higher aboveground biomass in N and NPK fertilized plots, this had little effect on plant community composition. With the exception of sedges, which responded positively to N additions and increased from 4 to 8 percent of die plant community, no changes were observed in plant community composition after 11 months.     

Productive phenology of tropical montane forests: Fertilization experiments along a moisture gradient

Phenological responses of leaves and roots were studied in the tropical montane forests of Mount Kinabalu, Borneo. Soil nutrient supply, in addition to the supply of light and water, is a potential abiotic factor influencing plant phenology in the tropics. The main objective of this study was to evaluate the contribution of soil nutrient supply to plant productive phenology. Fertilization experiments, including controls, nitrogen fertilized and nitrogen and phosphorus fertilized treatments, were conducted on three vegetation types in different moisture environments. Responses of leaves and roots were compared among treatments and among vegetation types. Leaf flushing was induced by nitrogen fertilization in the upper montane forest, where extremely wet moisture conditions are associated with cloud cover. This induction of leaf flushing by fertilization was not observed in the other forests. Root growth was suppressed by fertilization when leaf flushing was not induced by fertilization. These results indicate that soil pulsed nutrient release could be a cue for leaf flushing in a tropical wet environment, and that leaf phenology could be regulated by external abiotic factors and root phenology could be regulated by internal plant demands.     

Nutrient Addition Differentially Affects Soil Carbon Sequestration in Secondary Tropical Dry Forests: Early‐ versus Late‐Succession Stages

There is considerable interest in the potential use of soils to sequester carbon for climate change mitigation. As such, there is a need to evaluate the potential for carbon accumulation in tropical regions. We compared the effects of three annual additions of nitrogen and/or phosphorus on soil carbon and nitrogen contents and pools (bulk soil, macro‐, meso‐, and microaggregates) of two regenerating secondary tropical dry forest differing in nutrient status and succession stage (10‐year‐old early‐succession stage and approximately 60‐year‐old late‐succession stage). The selected forest sites were located on a shallow calcareous soil in the Yucatán Peninsula (Mexico). The primary production is limited by nitrogen and phosphorus in early‐succession stage and by phosphorus in late‐succession stage. In each forest site, four independent plots (12 × 12 m²) were established, the treatments being: controls and plots fertilized during three consecutive years with nitrogen, phosphorus, or nitrogen plus phosphorus. In both forests, soil carbon and nitrogen contents were consistently high, with soil carbon:nitrogen ratios generally greater than 10. Results indicate that usually there are no significant increases of soil carbon stock associated to late succession but can be increased to 3.7 Mg·ha^?1·yr^?1 with adoption of fertilizer practices. The potential soil carbon sequestration in early‐succession forest was estimated to be 2.7 Mg·ha^?1·yr^?1, and there is no indication that fertilization improves carbon sequestration. In short, results suggest that the soil potential for carbon sequestration in these ecosystems is high and depends on the specific nutrient status of the site.     

Nutrient limitation of phytoplankton and periphyton growth in upland lakes

SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth‐rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient‐diffusing substrata. 2. Over the whole season the percentage frequency of P, N and co‐limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation. 3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co‐limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively. 4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co‐limitation became progressively more common as the season progressed. 5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small. 6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was <6.5 mmol m^?3 and the ratio of DIN : TDP was <53. Co‐limitation was the most likely outcome at a DIN concentration <13 mmol m^?3 and at a DIN : TDP molar ratio <250. Above these values phosphorus limitation was most likely. 7. The relatively high frequency of nitrogen limitation and co‐limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen‐fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.     

Reversibility of Soil Productivity Decline with Organic Matter of Differing Quality Along a Degradation Gradient

In the highlands of Western Kenya, we investigated the reversibility of soil productivity decline with increasing length of continuous maize cultivation over 100 years (corresponding to decreasing soil organic carbon (SOC) and nutrient contents) using organic matter additions of differing quality and stability as a function of soil texture and inorganic nitrogen (N) additions. The ability of additions of labile organic matter (green and animal manure) to improve productivity primarily by enhanced nutrient availability was contrasted with the ability of stable organic matter (biochar and sawdust) to improve productivity by enhancing SOC. Maize productivity declined by 66% during the first 35 years of continuous cropping after forest clearing. Productivity remained at a low level of 3.0 t grain ha^-1 across the chronosequence stretching up to 105 years of continuous cultivation despite full N–phosphorus (P)–potassium (K) fertilization (120–100–100 kg ha⁻¹). Application of organic resources reversed the productivity decline by increasing yields by 57–167%, whereby responses to nutrient-rich green manure were 110% greater than those from nutrient-poor sawdust. Productivity at the most degraded sites (80–105 years since forest clearing) increased in response to green manure to a greater extent than the yields at the least degraded sites (5 years since forest clearing), both with full N–P–K fertilization. Biochar additions at the most degraded sites doubled maize yield (equaling responses to green manure additions in some instances) that were not fully explained by nutrient availability, suggesting improvement of factors other than plant nutrition. There was no detectable influence of texture (soils with either 11–14 or 45–49% clay) when low quality organic matter was applied (sawdust, biochar), whereas productivity was 8, 15, and 39% greater (P < 0.05) on sandier than heavier textured soils with high quality organic matter (green and animal manure) or only inorganic nutrient additions, respectively. Across the entire degradation range, organic matter additions decreased the need for additional inorganic fertilizer N irrespective of the quality of the organic matter. For low quality organic resources (biochar and sawdust), crop yields were increasingly responsive to inorganic N fertilization with increasing soil degradation. On the other hand, fertilizer N additions did not improve soil productivity when high quality organic inputs were applied. Even with the tested full N–P–K fertilization, adding organic matter to soil was required for restoring soil productivity and most effective in the most degraded sites through both nutrient delivery (with green manure) and improvement of SOC (with biochar).     

Micronutrient and phosphorus limitation of phytoplankton abundance in Gem Lake,Sierra Nevada,California

Nutrient addition experiments conducted during the ice-free seasons of 1983 and 1984 in Gem Lake, an alpine lake in the Sierra Nevada mountains of California, indicate that algal biomass is limited by phosphorus, in combination with iron or copper. Phosphorus additions were always required to stimulate growth, but did not do so when phosphorus was the only nutrient added. Simultaneous additions of phosphorus and iron resulted in increased levels of chlorophyll, particulate carbon, particulate nitrogen and particulate phosphorus. Simultaneous additions of phosphorus and copper resulted in increases in chlorophyll, particulate nitrogen and particulate phosphorus, not in particulate carbon. Neither iron nor copper by itself stimulated growth.Particulate N : P ratios from all seasons in Gem Lake suggest that simultaneous micronutrient and phosphorus limitation exists throughout the summer, when nutrient and biomass levels remain low; limitation by phosphorus alone may appear in the fall and spring, when biomass and major ion concentrations increase dramatically.     

中亚热带丘陵红壤区森林演替典型阶段土壤氮磷有效性

在中亚热带典型丘陵红壤区选取裸露地、马尾松(Pinus massoniana)林地、针阔混交林地、常绿阔叶林地为研究对象,开展土壤氮(N)、磷(P)供应、有效性及其耦合过程的研究.结果表明,土壤有机C、全N、净矿化速率、中性磷酸酶活性表现为随森林演替进展呈现逐步提高的变化趋势;而土壤全P、C/N、C/P、氨化速率、硝化速率、树脂P、NaHCO3-P、NaOH-P、声波P、酸性P、总有效P、酸性磷酸酶活性未表现出此趋势;但反映N、P有效供应的指标,除氨化速率、树脂P和酸性磷酸酶外,在常绿阔叶林中均为最高.相关分析表明大部分N、P供应指标之间存在显著相关性(P<0.05).丘陵红壤区森林演替初级阶段P的限制性明显强于N,土壤N、P供应在森林演替进展过程中可以逐步得到优化而实现协调供应.以常绿阔叶林为中亚热带丘陵红壤区植被恢复的最终目标是可行和理想的.     

Periphyton nutrient limitation and other potential growth-controlling factors in Lake Okeechobee,U.S.A.

Periphyton nutrient limitation was assessed in Lake Okeechobee, a large, shallow, eutrophic lake in the southeastern U.S.A. Nutrient assays were performed to determine if the same nutrients that limit phytoplankton also limit periphyton growth in the lake. Nutrient diffusing clay substrates containing agar spiked with nitrogen, phosphorus, or both, along with nutrient-free controls, were incubated at four sites in the lake. Three sites were located in a pelagic–littoral interface (ecotone) and one site was located in the interior littoral region. Incubations lasted for 20–26 days, and were repeated on a quarterly basis between 1996 and 1997, to incorporate seasonal variability into the experimental design. The physical and chemical conditions at each site also were measured. Periphyton biomass (chlorophyll a and ash-free dry mass) was highest at the littoral and northern ecotone sites. At the littoral site, nitrogen limited biomass in four of five incubations, although the largest biomass differences between the treatments and controls (3 g cm^–2 as chl) were probably not ecologically significant. Periphyton biomass at the western and southern ecotone sites was low compared to the other two sites. Increases in water column depth and associated declines in light penetration strongly correlated with periphyton growth and suggested that they may have limited growth most often at all three ecotone sites. Nitrogen also was found to limit periphyton growth approximately 20% of the time at the ecotone sites and phosphorus was found to limit growth once at the west site.     

黄土丘陵区香梨园土壤水分、养分分布特征及其与产量的关系

本研究以晋西黄土丘陵区玉露香梨种植下的3种不同坡位坡耕地土壤为对象,对不同坡位、不同生育期、不同深度的土壤水分、养分和产量进行观测分析。结果表明: 不同坡位中,香梨产量为高坡位>中坡位>低坡位,高坡位香梨产量与土壤含水量(SWC)、有机质(SOM)、速效钾(AK)显著相关,其中AK对产量的影响最显著,中、低坡位香梨产量与SWC、SOM、全氮(TN)显著相关,其中SWC对产量的影响最显著;高坡位SWC、SOM、速效磷(AP)和TN含量高于中、低坡位,而中坡位AK含量最高。不同土层深度中,土壤养分含量在0～20 cm最高,20～40 cm最低,而SWC在0～20 cm显著低于其他土层,在20～40 cm最高。不同生育期中,开花期SOM、AP和TN含量最高,结果期SWC最高,成熟期AK含量最高。建议该区域在今后香梨水肥管理中,开花期高坡位应加强对K肥的施用,结果期以N、P复合肥施用为主。在中、低坡位应增加灌溉量,灌溉量在300 m³·hm^-2可有效降低该区域水分对产量的限制。本研究可为黄土丘陵区香梨种植精准灌溉和科学施肥提供理论支持和数据参考。