Use of plant residues for improving soil fertility, pod nutrients, root growth and pod weight of okra (Abelmoschus esculentum L)

The effect of wood ash, sawdust, ground cocoa husk, spent grain and rice bran upon root development, ash content, pod yield and nutrient status and soil fertility for okra (Abelmoschus esculentum L NHAe 47 variety) was studied. The five organic fertilizer treatments were compared to chemical fertilizer (400kg/ha/crop NPK 15-15-15) and unfertilized controls in four field experiments replicated four times in a randomized complete block design. The results showed that the application of 6tha(-1) of plant residues increased (P<0.05) the soil N, P, K, Ca, Mg, pH, and SOM; pod N, P, K, Ca, Mg and ash; root length; and pod yield of okra in all four experiments relative to the control treatment. For instance, spent grain treatment increased the okra pod yield by 99%, 33%, 50%, 49%, 65% and 67% compared to control, NPK, wood ash, cocoa husk, rice bran and sawdust treatments respectively. In the stepwise regression, out of the total R(2) value of 0.83 for the soil nutrients to the pod yield of okra; soil N accounted for 50% of the soil fertility improvement and yield of okra. Spent grain, wood ash and cocoa husk were the most effective in improving okra pod weight, pod nutrients, ash content, root length and soil fertility whereas the rice bran and sawdust were the least effective. This was because the spent grain, wood ash and cocoa husk had lower C/N ratio and higher nutrient composition than rice bran and sawdust, thus, the former enhanced an increase in pod nutrients, composition for better human dietary intake, increased the root length, pod weight of okra and improved soil fertility and plant nutrition crop. The significance of the increases in okra mineral nutrition concentration by plant residues is that consumers will consume more of these minerals in their meals and monetarily spend less for purchasing vitamins and mineral supplement drugs to meet health requirements. In addition, the increase in plant nutrition and soil fertility would help to reduce the high cost of buying synthetic inorganic fertilizers and maintain the long term productivity of soils for sustainable cultivation of okra.     

Wood ash applications to temperate forest ecosystems—potential benefits and drawbacks

The objectives of the present work were (a) to quantify the effects of wood ash on forest ecosystems through a meta-analysis approach associated with a detailed review of the literature (mainly composed of work carried out in Nordic countries) and (b) to extrapolate the effects on forest growth to other contexts (i.e. warm temperate countries) by identifying the cases for which wood ash applications can be beneficial to forest production. Three databases were built regarding the effects of wood ash on soil (151 observations; 33 experimental field trials), on nutrient concentrations of tree foliage (68 observations; 28 trials) and on annual stem biomass growth rate (70 observations; 27 trials). We obtained information on the wide variability of ash properties due to differences in burnt compounds, combustion processes and ash conditioning. Two important properties of wood ash are its high pH value and neutralizing capacity. These properties result in biochemical modifications of forest soils limed with ash. In the short term, soil solution composition was dramatically modified. Intense peaks of the K, Na or SO₄ concentrations were observed, resulting from the dissolution of salts contained in ash. At the same time, Ca and Mg concentrations increased as the carbonate pool of wood ash started to dissolve. The consequence of this dissolution process was an increase in the pH in all the soil phases. These modifications increased the activity of the soil microflora and some isolated peaks in the mineralization of soil organic matter may be observed in mineral soils. In the longer term, that is to say after the first year following ash application, only the effect on the acidity status of the soil remained significant. The effects of ash addition on forest ecosystems usually increased with the dose and were more pronounced with loose ash compared to aggregated ash. The addition of wood ash into forest ecosystems increased the foliar Ca status of trees. Some modifications of other nutrients, like P or K, were also observed but only for a few years after treatment. For most stands growing on mineral soils of Nordic countries, this treatment did not result in an increase in tree growth, probably because of the absence of N in the ash. For stands growing on organic soils of the same area, this input, associated with a long-lasting increase of soil organic matter mineralization, was sufficient to improve tree growth significantly (median = +59% compared to the control). For soils located in warm temperate regions, similar responses are expected for organic soils. For mineral soils, the wood ash application is expected to be suitable for stands showing deficiencies in K, Ca or Mg. Ash may contain high amounts of toxic heavy metals such as Cd. The bioavailability of most of these elements appeared to be very low in a forest context. No contamination of food chains has been observed, except possibly via some species of fungi, and heavy metals remain in the forest litter or in the topsoil. Based on all the reviewed results, several guidelines for wood ash application into forest ecosystems are proposed. Wood ash application should be restricted to acidic soils. Applications should consist of low doses of a stabilized ash form. Wood ash should be applied to adult stands rather than onto seedlings.     

Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the Central Amazon basin: fertilizer,manure and charcoal amendments

Soil fertility and leaching losses of nutrients were compared between a Fimic Anthrosol and a Xanthic Ferralsol from Central Amazônia. The Anthrosol was a relict soil from pre-Columbian settlements with high organic C containing large proportions of black carbon. It was further tested whether charcoal additions among other organic and inorganic applications could produce similarly fertile soils as these archaeological Anthrosols. In the first experiment, cowpea (Vigna unguiculata (L.) Walp.) was planted in pots, while in the second experiment lysimeters were used to quantify water and nutrient leaching from soil cropped to rice (Oryza sativa L.). The Anthrosol showed significantly higher P, Ca, Mn, and Zn availability than the Ferralsol increasing biomass production of both cowpea and rice by 38–45% without fertilization (P<0.05). The soil N contents were also higher in the Anthrosol but the wide C-to-N ratios due to high soil C contents led to immobilization of N. Despite the generally high nutrient availability, nutrient leaching was minimal in the Anthrosol, providing an explanation for their sustainable fertility. However, when inorganic nutrients were applied to the Anthrosol, nutrient leaching exceeded the one found in the fertilized Ferralsol. Charcoal additions significantly increased plant growth and nutrition. While N availability in the Ferralsol decreased similar to the Anthrosol, uptake of P, K, Ca, Zn, and Cu by the plants increased with higher charcoal additions. Leaching of applied fertilizer N was significantly reduced by charcoal, and Ca and Mg leaching was delayed. In both the Ferralsol with added charcoal and the Anthrosol, nutrient availability was elevated with the exception of N while nutrient leaching was comparatively low.     

Forest regrowth as the controlling factor of soil nutrient availability 75 years after fire in a deciduous forest of Southern Quebec

The effects of secondary succession on soil nutrient availability following fire in the 1920s was investigated in a hardwood forest of southern Quebec by correlation analyses between soil and solution chemistry, tree species composition, tree growth, litterfall nutrient fluxes and presence of charcoal monitored in six 300 m² plots between 1994 and 1998. The results suggests that the pioneer stand of largetooth aspen, paper birch and yellow birch that followed fire enriched the forest floor and upper mineral soil with its K-rich litter, but lowered solution NH₄, NO₃ and Mg concentrations through its high nutrient demand. High woody biomass primarily observed in the maple-dominated stands was associated with low exchangeable base cation concentrations in the forest floor, suggesting that nutrient immobilisation in trees is also a significant process leading to spatial variation in base cation availability in the forest floor. Finally, charcoal was positively correlated with exchangeable base cations in the forest floor which leads to believe that charcoal has a high affinity for base cations and that it can increase base cation availability decades after forest fire disturbance.     

Complementary Models of Tree Species–Soil Relationships in Old-Growth Temperate Forests

Ecosystem-level studies identify plant–soil feedbacks as important controls on soil nutrient availability, particularly for nitrogen and phosphorus. Although site- and species-specific studies of tree species–soil relationships are relatively common, comparatively fewer studies consider multiple co-existing species in old-growth forests across a range of sites that vary in underlying soil fertility. We characterized patterns in forest floor and mineral soil nutrients associated with four common tree species across eight undisturbed old-growth forests in Oregon, USA, and used two complementary conceptual models to assess tree species–soil relationships. Plant–soil feedbacks that could reinforce site-level differences in nutrient availability were assessed using the context-dependent relationships model, whereby relative species-based differences in each soil nutrient diverged or converged as nutrient status changed across sites. Tree species–soil relationships that did not reflect strong feedbacks were evaluated using a site-independent relationships model, whereby forest floor and surface mineral soil nutrient pools differed consistently by tree species across sites, without variation in deeper mineral soils. We found that the organically cycled elements carbon, nitrogen, and phosphorus exhibited context-dependent differences among species in both forest floor and mineral soil, and most often followed a divergence model, whereby species differences were greatest at high-nutrient sites. These patterns are consistent with theory emphasizing biotic control of these elements through plant–soil feedback mechanisms. Site-independent species differences were strongest for pools of the weatherable cations calcium, magnesium, potassium, as well as phosphorus, in mineral soils. Site-independent species differences in forest floor nutrients were attributable to one species that displayed significantly greater forest floor mass accumulation. Our findings confirm that site-independent and context-dependent tree species-soil relationships occur simultaneously in old-growth temperate forests, with context-dependent relationships strongest for organically cycled elements, and site-independent relationships strongest for weatherable elements with inorganic cycling phases. These models provide complementary explanations for patterns of nutrient accumulation and cycling in mixed-species old-growth temperate forests.     

The relative importance of the bacterial pathway and soil inorganic nitrogen increase across an extreme wood‐ash application gradient

Ash from combustion of biofuels, for example wood chips, is often deposited as waste, but due to its high content of essential plant nutrients and alkalinity, it has been proposed to recycle ash as a fertilizer and liming agent in biofuel production forest. However, current legislation sets rather strict limitations for wood‐ash application in biofuel production systems. The soil microfood web, that is microorganisms and their microfaunal grazers, protozoa and nematodes, is pivotal for essential ecosystem processes such as decomposition and plant nutrient release. Therefore, a thorough assessment of the impacts on microfood web structure and functioning must precede actions towards raising the currently allowed application rates. In a Danish Norway spruce plantation, we evaluate the impact of wood ash applied at dosages from 0 to the extreme case of 90 t ash ha^?1 on the microfood web, the bacterial community structure, soil content of inorganic nitrogen, organic matter, dissolved organic carbon and nitrogen. Using structural equation modelling (SEM), we disentangled the direct effect of the disturbance imposed by ash per se, the associated pH increase and changes in prey abundance on individual organism groups in the microfood web. The SEM showed that the pH rise was the main driver of increasing abundances of culturable heterotrophic bacteria with increasing ash doses, and via trophical transfer, this also manifested as higher abundances of bacterial grazers. Fungal‐feeding nematodes were unaffected by ash, whereas carnivorous/omnivorous nematodes decreased due to the direct effect of ash. Increasing ash doses enhanced the difference between bacterial communities of control plots and ash‐amended plots. The ash‐induced stimulation of culturable heterotrophic bacteria and bacterial grazers increased inorganic nitrogen availability at ash doses of 9 t ha^?1 and above. Hence, raised limits for ash application may potentially benefit tree growth via enhanced N mineralization activity of the soil food web.     

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.     

矿质养分输入对森林生物固氮的影响

生物固氮是森林生态系统重要的氮素来源,并且在全球氮循环中占有重要的地位。近代以来,因人类活动加剧而导致氮沉降的增加以及其它矿质养分元素(如磷、钼、铁等)输入的改变已成为影响森林生态系统生物固氮的重要因素之一,并引起了学术界的普遍关注。综述了国内外关于森林生物固氮对矿质养分输入的响应及机理。主要内容包括:(1)森林生物固氮的概念及主要的测定方法;(2)矿质养分输入对森林生物固氮的影响。整体上讲,氮素输入抑制了森林生物固氮,磷和其他营养元素输入则表现为促进作用。氮和磷、磷和微量元素同时添加均提高了森林的固氮量;(3)矿质养分改变森林生物固氮的机理。包括生物作用机制(如改变地表层固氮菌的数量或群落丰度、改变结瘤植物的根瘤生物量和附生植物的丰度或盖度)和环境作用机制(如引起土壤酸化、改变碳源物质的含量);(4)探讨了矿质养分输入对森林生物固氮影响研究中所存在的问题,并对未来该领域的研究提出建议。     

Tree mineral nutrition is deteriorating in Europe

The response of forest ecosystems to increased atmospheric CO₂ is constrained by nutrient availability. It is thus crucial to account for nutrient limitation when studying the forest response to climate change. The objectives of this study were to describe the nutritional status of the main European tree species, to identify growth‐limiting nutrients and to assess changes in tree nutrition during the past two decades. We analysed the foliar nutrition data collected during 1992–2009 on the intensive forest monitoring plots of the ICP Forests programme. Of the 22 significant temporal trends that were observed in foliar nutrient concentrations, 20 were decreasing and two were increasing. Some of these trends were alarming, among which the foliar P concentration in F. sylvatica, Q. Petraea and P. sylvestris that significantly deteriorated during 1992–2009. In Q. Petraea and P. sylvestris, the decrease in foliar P concentration was more pronounced on plots with low foliar P status, meaning that trees with latent P deficiency could become deficient in the near future. Increased tree productivity, possibly resulting from high N deposition and from the global increase in atmospheric CO₂, has led to higher nutrient demand by trees. As the soil nutrient supply was not always sufficient to meet the demands of faster growing trees, this could partly explain the deterioration of tree mineral nutrition. The results suggest that when evaluating forest carbon storage capacity and when planning to reduce CO₂ emissions by increasing use of wood biomass for bioenergy, it is crucial that nutrient limitations for forest growth are considered.     

Implications of pre- and post-fertilizing changes in growth and nitrogen pools following multiple applications of nitrogen fertilizer to a Pinus radiata stand over 12 years

Around the world large tracts of forest, previously available for production, have been reserved for nature conservation. This means that wood supply must be met from a reduced land base, including land of low productivity. In addition there are likely to be increasing demands on the use of managed forests for sequestering C as one means of reducing the build up of atmospheric CO2. One way for the forest industry to meet the demands of increased production would be through the use of fertilizers. Substantially increased growth from fertilizer N application has been measured in many cases while, in the northern hemisphere, atmospheric N deposition has been associated with increased growth in some forests.The possibility of using fertilizer N to increase growth, and the effect on the forest and soil, was studied in a research trial area in north-east Tasmania, Australia. Nitrogen was applied for 12 years to a 16 year old P. radiata stand in a low rainfall zone. Growth and foliar nutrient concentrations were measured to age 34 years, to determine change after attainment of steady state growth at age 25 years, and following cessation of fertilizing at age 29 years. Biomass sampling was carried out at ages 25 years, 29 years, and 34 years.Growth at the steady peak rate achieved in the fertilizer plots, of 31 m3 ha-1 periodic annual increment, was accompanied by changes in the N nutrient pools. Surface applied N built up in the surface litter layer while this layer was increasing, from 15 t ha-1 to nearly 50 t ha-1, and fertilizer was being applied between ages 25 and 29 years. Decline of total N in the soil, between ages 25 and 29 years, indicated continued uptake from that source even though fertilizer was being applied. Subsequent increases in soil N, through leaching from the litter layer, followed cessation of fertilizing.Following cessation of annual applications of N fertilizer, growth rates declined by less than 15% and remained substantially (>150%) above the control growth rates for 4 years. Over a 3 year period foliar N concentrations declined steadily, to the same as control plots, although mass of needles was not reduced 5 years after cessation of fertilizer applications and remained significantly above that of the control plots.Without fertilizers, N for new foliage must be supplied from internal recycling and from mineralization of litter and soil organic matter. There were net losses of N from the foliage and the wood, estimated at around 12 kg N ha-1 yr-1, for the fertilized trees for the 5 years following cessation of fertilizing. This indicated retention and recycling within the tree of a considerable proportion of the 104 kg N ha-1 in the foliage, at age 29 years. As rates of mineralization of soil N were likely to be low, this retention of N within the tree was important in maintaining growth rates.In the forest ecosystems investigated, with low initial content of soil organic matter and N, fertilizer N produced improved tree health and substantially increased growth, thus providing the opportunity to manage this forest for increased wood production or C sequestration. Although there was a substantial build up of N and C in the litter on the fertilized treatment, incorporation of N and C into the mineral soil was slow. Over 12 years 1.34 t N ha-1 had been applied, comparable to 40 years of atmospheric deposition at 30 kg N ha-1. There were no signs of detrimental effects from this application.     

酶化学计量揭示5年氮添加加剧毛竹林土壤微生物碳磷限制

土壤胞外酶活性和酶化学计量比能很好地反映土壤养分有效性和微生物对养分的需求变化。然而,氮(N)沉降对亚热带森林土壤微生物养分相对限制情况的影响尚不清楚。通过在亚热带毛竹林进行N添加试验来模拟N沉降,并在试验满5年时进行取样,测定不同处理下土壤养分和与碳(C)、N、磷(P)循环相关的酶活性,利用酶化学计量比及矢量分析探究微生物的养分分配情况。结果表明: N添加显著降低土壤可溶性有机碳、有效磷含量,显著提高有效氮含量。此外,N添加显著降低β-N-乙酰氨基葡糖苷酶(NAG)活性和NAG/微生物生物量碳(MBC),显著提高酸性磷酸酶(ACP)和ACP/MBC。低N和中N处理显著提高酶C/N、矢量长度和矢量角度,但显著降低酶N/P。冗余分析表明,N添加下,土壤有效磷含量的变化是影响土壤酶活性及酶化学计量比变化的主要因子。综上可知,N添加改变了微生物的养分获取策略,即通过减少分配给合成N获取酶的养分来增加合成P获取酶的养分。此外,N添加还加剧了微生物的C、P限制,未来可以施加适量P肥来提高亚热带毛竹林的土壤肥力。     

Short‐term growth response of jack pine and spruce spp. to wood ash amendment across Canada

Wood ash amendment to forest soils contributes to the sustainability of the growing bioenergy industry, not only through decreased wood ash waste disposal in landfills but also by increasing soil/site productivity and tree growth. However, tree growth studies to date have reported variable responses to wood ash, highlighting the need to identify proper application rates under various soil/site conditions to maximize their benefits. We explored the influence of tree species, wood ash nutrient application rates, time since application, stand development stage, and initial (i.e., before wood ash application) soil pH and N on short‐term tree growth response to wood ash amendment across eight unique study sites spanning five Canadian Provinces. Jack pine (Pinus banksiana Lamb) had the most positive response to wood ash amendment compared to white (Picea glauca Moench), hybrid (Picea engelmannii x glauca Parry), and black spruce (Picea mariana Miller), where increasing nutrient application rates increased height growth response. In comparison, black spruce had the most negative response to wood ash amendment, where increasing nutrient application rates slightly decreased height growth response. Site as a random effect explained additional variation, highlighting the importance of other unidentified site characteristics. By examining trends in short‐term growth response across multiple studies with variable site characteristics, we found growth response differed by tree species and nutrient application rates, and that jack pine is a promising candidate for wood ash amendment. These results contribute to our knowledge of optimal wood ash amendment practices and environmentally sustainable bioenergy production.     

佛山地区存留自然林和存留人工林的土壤无机磷组分

为探讨热带亚热带森林,尤其城市及其周边地区残存森林土壤磷的有效性,对佛山地区14个残存林(7个自然林和7个人工林)的0~3 cm和3~23 cm矿质土壤的P有效性进行研究。结果表明,铁结合态无机P和还原剂可溶解无机P是土壤无机P的主要组分。在0~3 cm矿质层中,自然林土壤铝结合态无机P、Bray 1提取无机P和总无机P含量显著高于人工林;而在3~23 cm矿质土层中,自然林土壤钙结合态无机P含量显著高于人工林。其它土壤营养指标在自然林和人工林间差异不显著。相关分析结果表明,土壤有机质含量与钙结合态无机P除外的其它无机P组分含量均成显著正相关。聚类分析结果表明14个残存林土壤P有效性可分成3组,整体上人工林土壤P有效性比自然林低。这有助于认识城市化影响下城市及其周边地区残存森林土壤营养状况及加强养分管理。     

Functional traits determine tree growth and ecosystem productivity of a tropical montane forest: Insights from a long‐term nutrient manipulation experiment

Trait‐response effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long‐term tree growth in experimental nutrient addition plots (N, P, and N + P) varied as a function of morphological traits, tree size, and species identity. We also evaluated how trait‐based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N + P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N + P grew faster irrespective of their initial size relative to trees in control or to trees in other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long‐term experimental evidence that trait‐based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.     

Temporal patterns of soil nutrients in a Panamanian moist forest revealed by ion-exchange resin and experimental irrigation

The effect of seasonal water availability on soil nutrients and soil N transformations was investigated by irrigating two large plots of mature tropical forest on Barro Colorado Island (BCI), Panama, during the dry season for five consecutive years. Methods included (i) nutrient accumulation by ion-exchange resins placed on the surface of the mineral soil for contiguous 21-day periods, (ii) monthly mineral soil (0–10 cm) extractions and incubations for inorganic N and P concentrations, and (iii) leaching loss of nutrients from leaf litter samples. Rates of nutrient accumulation by the resins showed a great deal of variation between sampling dates and among years in control plots; albeit, seasonal patterns were slight, except for the highest Ca values near the end of the wet season and inorganic P (P_i) and SO₄ values that peaked during the dry season. Irrigation had remarkably little effect on nutrient accumulation rates by resins, except for an increase in Mg and Na values, but did affect the timing in the temporal variation in K, Na, N_i and P_i values. In contrast, inorganic N (N_i) and P_i pools and N transformation rates in the mineral soil hardly varied among sampling dates and did not show any response to irrigation. We hypothesize that the timing of leaf litterfall and nutrient leaching from forest floor litter can set up temporal patterns in the levels of soil nutrient at the surface of the mineral soil, but the temporal patterns essentially disappear with depth in the mineral soil.     

Effects of irrigation and balanced fertilization on nutrient cycling in a Douglas fir stand

The effects on growth and nutrient status of an increased availability of all major nutrients was studied in a 40 year old Douglas fir stand. The nutrient amounts were applied by daily sprinkling and were in fixed, optimal proportions to the estimated annual uptake of N. Irrigation was included to avoid drought stress. The nutrient applications were also done in combination with a lower NH₄ load, realized under a roof that was placed above the ground. According to nutritional standards nitrogen supply was optimal and was related to the moderately high atmospheric N inputs. Nutrition of P, K and in some years Ca seemed critical.In all four studied growing seasons water additions resulted in a growth increase, of 30 to 40%. Irrigation decreased needle fall. Nutrient applications increased the needle concentrations of P and K considerably, but this did not result in a growth enhancement. Fertilization also increased K and P in shedded needles. Only minor leaching losses were calculated. The recovery in trees of fertilizer K and P was maximally 15%. Input-output budgets suggested that P was strongly retained in the soil. The decrease in NH₄ load had slightly decreased N availability and soil acidification, and seemed to enhance tree growth. The use of tree growth as a parameter to evaluate the effects of an improved nutritional balance is discussed.     

The Effect of Phosphorus Availability on Decomposition Dynamics in a Seasonal Lowland Amazonian Forest

Once the weathering of parent material ceases to supply significant inputs of phosphorus (P), vegetation depends largely on the decomposition of litter and soil organic matter and the associated mineralization of organic P forms to provide an adequate supply of this essential nutrient. At the same time, the decomposition of litter is often characterized by the immobilization of nutrients, suggesting that nutrient availability is a limiting factor for this process. Immobilization temporally decouples nutrient mineralization from decomposition and may play an important role in nutrient retention in low-nutrient ecosystems. In this study, we used a common substrate to study the effects of native soil P availability as well as artificially elevated P availability on litter decomposition rates in a lowland Amazonian rain forest on highly weathered soils. Although both available and total soil P pools varied almost three fold across treatments, there was no significant difference in decomposition rates among treatments. Decomposition was rapid in all treatments, with approximately 50% of the mass lost over the 11-month study period. Carbon (C) and nitrogen (N) remaining and C:N ratios were the most effective predictors of amount of mass remaining at each time point in all treatments. Fertilized treatments showed significant amounts of P immobilization (P < 0.001). By the final collection point, the remaining litter contained a quantity equivalent to two-thirds of the initial P and N, even though only half of the original mass remained. In these soils, immobilization of nutrients in the microbial biomass, late in the decomposition process, effectively prevents the loss of essential nutrients through leaching or occlusion in the mineral soil.     

Phosphorus conservation during post‐fire regeneration in a Chilean temperate rainforest

Nitrogen and phosphorus are the main elements limiting net primary production in terrestrial ecosystems. When growing in nutrient‐poor soils, plants develop physiological mechanisms to conserve nutrients, such as reabsorbing elements from senescing foliage (i.e. nutrient retranslocation). We investigated the changes in soil N and P in post‐fire succession in temperate rainforests of southern Chile. In this area, forest recovery often leads to spatially scattered, discrete regeneration with patches varying in age, area, species richness and tree cover, representing different degrees of recovery from disturbance. We hypothesized that soil nutrient concentrations should differ among tree regenerating patches depending on the progress of forest regeneration and that nutrient resorption should increase over time as colonizing trees respond to limited soil nutrients. To evaluate these hypotheses, we sampled 40 regeneration patches in an area of 5 ha, spanning a broad range of vegetation complexity, and collected soil, tree foliage and litter samples to determine N and P concentrations. Nutrient concentrations in leaf litter were interpreted as nutrient resorption proficiency. We found that soil P was negatively correlated with all the indicators of successional progress, whereas total soil N was independent of the successional progress. Foliar N and P were unrelated to soil nutrient concentrations; however, litter N was negatively related to soil N, and litter P was positively related with soil P. Finally, foliar N:P ratios ranged from 16 to 25, which suggests that P limitation can hamper post‐fire regeneration. We provide evidence that after human‐induced fires, succession in temperate forests of Chile can become nutrient limited and that high nutrient retranslocation is a key nutrient conservation strategy for regenerating tree communities.     

The influence of tree species on canopy soil nutrient status in a tropical lowland wet forest in Costa Rica

The canopy is host to a large percentage of the flora and fauna in tropical wet forests and is distinct from the forest floor in plant richness, soil type and microclimate. In this study, we examined the influence of tree species and season on soil nutrient cycling processes in canopy soils of four tree species common to Costa Rican wet forests. We also compared the canopy soils to the associated forest floor mineral soils. Both tree species and season had strong effects on canopy soil nutrients and processes. Canopy soils from trees with high litter lignin concentrations had higher net N-mineralization rates and higher dissolved inorganic N concentrations than those with low lignin concentrations. During the dry season, net N-immobilization occurred and dissolved organic and inorganic N and available P concentrations were significantly higher than during the wet season. Overall, canopy soils had higher N levels and higher fungi + bacteria richness than forest floor mineral soils. The differences in canopy soil properties observed among tree species indicates that these species have distinct N cycles that reflect differences in both soil origin and biological controls.     

Long term effects of manure, charcoal and mineral fertilization on crop production and fertility on a highly weathered Central Amazonian upland soil

Application of organic fertilizers and charcoal increase nutrient stocks in the rooting zone of crops, reduce nutrient leaching and thus improve crop production on acid and highly weathered tropical soils. In a field trial near Manaus (Brazil) 15 different amendment combinations based on equal amounts of carbon (C) applied through chicken manure (CM), compost, charcoal, and forest litter were tested during four cropping cycles with rice (Oryza sativa L.) and sorghum (Sorghum bicolor L.) in five replicates. CM amendments resulted in the highest (P < 0.05) cumulative crop yield (12.4 Mg ha⁻¹) over four seasons. Most importantly, surface soil pH, phosphorus (P), calcium (Ca), and magnesium (Mg) were significantly enhanced by CM. A single compost application produced fourfold more grain yield (P < 0.05) than plots mineral fertilized in split applications. Charcoal significantly improved plant growth and doubled grain production if fertilized with NPK in comparison to the NPK-fertilizer without charcoal (P < 0.05). The higher yields caused a significantly greater nutrient export in charcoal-amended fields, but available nutrients did not decrease to the same extent as on just mineral fertilized plots. Exchangeable soil aluminum (Al) was further reduced if mineral fertilizer was applied with charcoal (from 4.7 to 0 mg kg⁻¹). The resilience of soil organic matter (SOM) in charcoal amended plots (8 and 4% soil C loss, mineral fertilized or not fertilized, respectively) indicates the refractory nature of charcoal in comparison to SOM losses over 20 months in CM (27%), compost amended (27%), and control plots (25% loss).