Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

Abstract Conversion of tropical forests is among the primary causes of global environmental change. The loss of their important environmental services has prompted calls to integrate ecosystem services (ES) in addition to socio-economic objectives in decision-making. To test the effect of accounting for both ES and socio-economic objectives in land-use decisions, we develop a new dynamic approach to model deforestation scenarios for tropical mountain forests. We integrate multi-objective optimization of land allocation with an innovative approach to consider uncertainty spaces for each objective. These uncertainty spaces account for potential variability among decision-makers, who may have different expectations about the future. When optimizing only socio-economic objectives, the model continues the past trend in deforestation (1975–2015) in the projected land-use allocation (2015–2070). Based on indicators for biomass production, carbon storage, climate and water regulation, and soil quality, we show that considering multiple ES in addition to the socio-economic objectives has heterogeneous effects on land-use allocation. It saves some natural forest if the natural forest share is below 38%, and can stop deforestation once the natural forest share drops below 10%. For landscapes with high shares of forest (38%–80% in our study), accounting for multiple ES under high uncertainty of their indicators may, however, accelerate deforestation. For such multifunctional landscapes, two main effects prevail: (a) accelerated expansion of diversified non-natural areas to elevate the levels of the indicators and (b) increased landscape diversification to maintain multiple ES, reducing the proportion of natural forest. Only when accounting for vascular plant species richness as an explicit objective in the optimization, deforestation was consistently reduced. Aiming for multifunctional landscapes may therefore conflict with the aim of reducing deforestation, which we can quantify here for the first time. Our findings are relevant for identifying types of landscapes where this conflict may arise and to better align respective policies.

Abstract:

Bio-economic modelling has become a useful tool for anticipating the outcomes of policies and technologies before their implementation. Advances in mathematical programming have made it possible to build more comprehensive models. In an overview of recent studies about bio-economic models applied to land-use problems in agriculture and forestry,we evaluated howaspects such as uncertainty,multiple objective functions, system dynamics and time have been incorporated into models. We found that single objective models were more frequently applied at the farm level, while multiple objective modelling has been applied to meet concerns at the landscape level. Among the objectives, social aspects are seldom represented in allmodels, when being compared to economic and environmental aspects. The integration of uncertainty is occasionally a topic, while stochastic approaches are more frequently applied than non-stochastic robust methods. Mostmultiple-objectivemodels do not integrate uncertainty or sequential decision making. Static approaches continue to be more recurrent than truly dynamic models. Even though integrating multiple aspects may enhance our understanding of a system; it involves a tradeoff between complexity and robustness of the results obtained. Land-use models have to address this balance between complexity and robustness in order to evolve towards robust multiple-objective spatial optimization as a prerequisite to achieve sustainability goals.

Abstract:

Forest site classification adapted to the respective site conditions is one prerequisite for sustainable silviculture. This work aims to initiate the forest site classification for pine plantations in the southern Andean region of Ecuador. Forest productivity, estimated by the dominant height of 20-year-old trees (DH20), was related to data from climate, topography, and soil using 23 plots installed in pine plantations in the province of Loja. Forest site productivity was classified as: low (class C: 13.4 m), middle (class B: 16.6 m), and high (Class A: 22.3 m). Strong determinants to differentiate the forest site classes were: the short to medium term available Ca and K stocks (organic layer + mineral soil standardized to a depth of 60 cm), soil acidity, the C:N ratio, clay and sand content, forest floor thickness, altitude, and slope. The lowest forest productivity (Class C) is mainly associated with the lowest short to medium term available K and Ca stocks. Whereas, in site classes with the highest forest productivity, pines could benefit from a more active microbial community releasing N and P, since the soil pH was about 1 unit less acidic. This is supported by the lowest forest floor thickness and the narrowest C:N ratio.

Abstract:

High landscape diversity is assumed to increase the number and level of ecosystem services. However, the interactions between ecosystem service provision, disturbance and landscape composition are poorly understood. Here we present a novel approach to include uncertainty in the optimization of land allocation for improving the provision of multiple ecosystem services. We refer to the rehabilitation of abandoned agricultural lands in Ecuador including two types of both afforestation and pasture rehabilitation, together with a succession option. Our results show that high compositional landscape diversity supports multiple ecosystem services (multifunction effect). This implicitly provides a buffer against uncertainty. Our work shows that active integration of uncertainty is only important when optimizing single or highly correlated ecosystem services and that the multifunction effect on landscape diversity is stronger than the uncertainty effect. This is an important insight to support a land-use planning based on ecosystem services.

Abstract:

Increasing demands for livelihood resources in tropical rural areas have led to progressive clearing of biodiverse natural forests. Restoration of abandoned farmlands could counter this process. However, as aims and modes of restoration differ in their ecological and socio-economic value, the assessment of achievable ecosystem functions and bene?ts requires holistic investigation. Here we combine the results from multidisciplinary research for a unique assessment based on a normalization of 23 ecological, economic and social indicators for four restoration options in the tropical Andes of Ecuador. A comparison of the outcomes among afforestation with native alder or exotic pine, pasture restoration with either low-input or intense management and the abandoned status quo shows that both variants of afforestation and intense pasture use improve the ecological value, but low-input pasture does not. Economic indicators favour either afforestation or intense pasturing. Both Mestizo and indigenous Saraguro settlers are more inclined to opt for afforestation.

Abstract:

Linking nutrient balances and flows to soil nutrient stocks creates a valuable indicator for sustainability assessment in agricultural land-use systems. Therefore, we investigated the impact of management on soil fertility at farm/field scale using the Nutmon approach. A detailed methodology for the adaptation of the difficult-to-quantify flows to the local conditions is described. Research was carried out in the three farming systems of Yantzaza (low-external-input), El Tambo (irrigated cash crops) and San Lucas (integrated nutrient management) in southern Ecuador. For each land-use within a farm (annual and perennial crops, pasture, forest), soil nutrient balances and flows were modeled with Nutmon and soil nutrient stocks were calculated for NPK. Soil nutrient balances were evaluated using potential socio-economic and soil fertility explanatory variables. Balances for the different land-uses in the three research areas varied between -151 to 66 kg ha-1 a-1 for N, -4 to 33 kg ha-1 a-1 for P and -346 to 39 kg ha-1 a-1 for K and were mainly negative. Up to 70 % of the balances’ variability was explained by soil fertility variables and financial flows. Highest external inputs existed in land-uses with a strong market orientation. Land-uses benefiting from a surplus of within-farm flows had the highest soil nutrient stocks. The focus on N fertilization induced highly negative PKbalances in annual crops of El Tambo. In contrast, the application of organic fertilizers and nutrient recycling in San Lucas resulted in positive NP balances particularly for perennial crops. NP balances in annual crops of Yantzaza were most negative due to nonexistent fertilization, leaching and burning of crop residues. A non-sustainable landuse of annual crops in Yantzaza was illustrated by total N stock decreases of 4.9 % a-1 and decreased soil organic carbon stocks to 85 % of adjacent forest sites. Results indicated a potential risk regarding sustainable management of soils in the research area and provide a basis for policy and decision makers to develop appropriate management strategies.

Abstract:

Quantification of nutrient stocks and their temporal changes are considered of prime importance in farm-ing systems of the humid tropics to answer the question of sustainable management. The research areain the Southern Ecuadorian Andes included forest, annual (0–5 years old) and different aged perennial(0–5, 6–10, 11–20, 21–30) and pasture (0–5, 6–10, 11–20, 21–30, >30) sites. Soil organic carbon (SOC),total and plant available soil nutrient stocks and nutrient balances were investigated to assess temporalsoil nutrient dynamics in relation to management activities. Forest conversion by slash-and-burn caused a decrease in SOC stocks in all three land-uses amounting between 14% and 19%. This was mainly due tothe absence of an organic layer and losses in the upper five cm of the mineral soil. Stocks of exchangeablebases and pH values increased in annuals which however, had the most negative nutrient balance of allland-uses amounting to ?128, ?25 and ?226 kg ha?1 a?1 for N, P and K, respectively. The abandonmentof annual sites after five years was linked to a shortage of available N and P due to low-external-input management which caused SOC stock decreases. Major soil nutrient changes in perennials and pasturesdid not occur directly but 6–20 years after forest conversion with increases in stocks for total N, P, S and exchangeable bases above forest level. SOC stocks of medium aged perennials and pastures increased above forest level in the mineral soil. Easily available inorganic N and P stocks remained low throughout the chronosequence in perennials and pastures, indicating a quick uptake by plant roots and microor-ganisms. Compared to medium aged sites, oldest perennials and pastures showed a strong decrease in SOC (?28% and ?16%) and soil nutrient stocks. Stocks for total N, P, S and exchangeable bases were up to50% below those of medium aged sites in oldest perennials and pastures.

Abstract:

A tropical smallholder cash crop farming system in a semi-arid Inter-Andean valley of Ecuador was investigated. Intensive irrigated agriculture with up to 400 kg of urea-N fertilization ha?1·a?1 prevails in colluvial sites whereas urea application in the steep eroded sites is lower. Farmers did recently introduce manure to partly replace urea fertilization. Knowledge about the effects of mineral and organic fertilizers on nitrogen and carbon cycling needs to be improved to maintain sustainable agriculture in the research area. Therefore, a laboratory incubation experiment was conducted to investigate effects of urea and guinea pig manure on microbial activity of colluvial foot slope and eroded upper slope soils. Soil samples were treated with 200 kg·N·ha?1 either applied as urea, guinea pig manure or a combination of both and compared to a control. Urea fertilizer was labeled with 15N to trace its fate. Microbial biomass and basal respiration were determined in different time intervals. Urea fertilization induced an acceleration of soil organic matter (SOM) mineralization (positive priming effect) in the first seven days of incubation. The combined fertilization served as a positive alternative to the present urea management since it increased the amount of microbial biomass and provided mineral nitrogen for immediate plant uptake. Moreover, low recovery rates suggested an immobilization of fertilized nitrogen into the microbial biomass inducing an efficient and sustained nutrient release throughout the entire growing season. SOM stocks in colluvial soils were 40% below those of eroded soils which was partly due to the positive priming effect after urea fertilization. Microbial activity and efficiency were not reduced by soil erosion.

Abstract:

In the present thesis, implications of pasture establishment, fertilization and abandonment on soil C and nutrient dynamics were investigated for the mountain rainforest region of southern Ecuador. Over the past decades the natural forest of the study area has been threatened by conversion to cattle pastures. However, the soil fertility of these extensively grazed pastures (active pastures) declines continuously during pasture use. The invasion of bracken fern (Pteridium arachnoideum) leads to pasture abandonment when bracken becomes dominant. In order to reveal the mechanisms behind the deterioration of soil fertility, biotic and abiotic soil properties and their interaction were analyzed along a land?use gradient (natural forest – active pasture – abandoned pasture). The ecosystem disturbance of the mountain rainforest through pasture use changed the microbial function and structure, and affected soil CO2?C fluxes. Annually, 2 Mg soil CO2?C ha?1 were additionally emitted from the pasture land. This acceleration in soil respiration rates was related to accelerated rates of microbial C mineralization and fine?root respiration. The high?quality, N?rich above? and belowground residues of the pasture grass (S. sphacelata, C4?plant), especially the huge fine?root biomass, provided a high C and N availability for soil microbes. Compared to the forest, increased soil pH and accelerated base saturation were further factors beneficial for soil microbial growth and metabolism of the upper mineral soil at active pastures. Three times higher amounts of microbial biomass C and a significant shift in the microbial community structure towards a higher relative abundance of Gram(?)? bacteria and fungi were observed. Long?term pasture use and the invasion of bracken (C3?plant) diminished beneficial effects for microbes, causing a significant decrease in the C, net, and gross N mineralization rates as well as a two?third reduction in the microbial biomass. A preferential substrate utilization of grass?derived C4 by the soil microbes resulted in a rapid decline of the C4?pool. As a consequence, the less available C3?pool from bracken and former forest increased its dominance in the SOC?pool, further decreasing pasture productivity and finally causing pasture abandonment. The lower quality and quantity of above? and belowground residues of the bracken (high lignin content, C/N) resulted in resource?limited conditions that influenced the microbial function to greater extent than their structure. The microbial structure seemed to be sensitive mainly to soil pH along the land?use gradient. Thus, a disconnection between microbial structure and function was identified. Fertilization experiments were conducted both in the lab and in the field to evaluate the impact of urea and/or rock phosphate amendment on SOM dynamics and on pasture productivity of active pastures. After combined fertilization the pasture yield was most efficiently increased by 2 Mg ha?1 a?1, indicating a NP?limitation of grass growth. Furthermore, the fodder quality was improved by a higher content of P and Ca in the grass biomass. The microorganisms of the active pasture soil responded with an adaptation of their structure to the increased substrate availability in the short term, but did not change their initial functions in the long term. After urea/ rock phosphate addition a significant increase in the relative fungal abundance was detected, but neither a microbial limitation of energy nor of N or P was observed. However, urea addition accelerated gaseous losses of soil CO2?C in the short term. In the study area, pronounced alterations in ecosystem functioning due to land?use changes were detected, especially in soil C and N cycling rates. For a sustainable land?use in this region it is crucial to prevent pasture degradation and to rehabilitate degraded pastures in order to protect the prevailing mountain rainforest ecosystem. It is of crucial importance for active pasture soils to maintain or even increase resource availability, being one indicator of soil fertility. In this context, the soil organic matter has to be retained in the long?term to maintain high microbial activity and biomass, and thus pasture productivity. A moderate fertilization with urea and rock phosphate can be a first step to provide continuous nutrient supply for grass growth and to strengthen livestock health through increased fodder quality. However, the risk of further additional emissions of soil CO2?C due to increased loads of urea fertilizer application has to be kept in mind. Overall, for the establishment of a sustainable land?use management the control of bracken invasion and an adjusted nutrient management are needed. Further investigations on the reduction of soil nutrient losses and increased nutrient use efficiencies of plants, such as combined planting with legumes or the usage of cultivars with special nutrient acquisition strategies, should be in the focus of future work.

Abstract:

In the mountain rainforest region of southern Ecuador, soils of active pastures, established after slashand- burn of the forest, are characterized by improved quantity and quality of soil organic matter favoring microbial conditions. However, these beneficial conditions decrease with increasing pasture age and burning frequency. As a consequence, rates of soil nutrient cycling decrease, supporting the infestation of bracken fern and, in turn, causing further decreases in pasture productivity. Finally, farmers are forced to abandon the degraded pastures and to establish new ones by continuous deforestation. To investigate whether an application of N and/or P nutrients to an extensively grazed pasture (active pasture) can improve grass productivity and maintain soil fertility, a pasture fertilization experiment was conducted. On an active pasture site, planted with Setaria sphacelata, moderate rates of urea (50 kg N ha−1 a−1), rock phosphate (10 kg P ha−1 a−1), and a combination of both were applied. It was examined whether soil mineralization (gross and net N mineralization, SOC mineralization) and microbial community structure (PLFA-analysis), as well as quantity and quality of the grass biomass, were affected by fertilization. Furthermore, the impact of fertilization on in situ soil respiration rates was studied. The combined application of urea and rock phosphate increased the pasture yield by 2 Mg ha−1 a−1 most efficiently, indicating a co-limitation of growth. Additionally, the fodder quality was improved by a 67% higher content of P and by a 7% higher content of Ca in the grass biomass compared to the control. While carbon, nitrogen, and phosphorus in the microbial biomass remained unaffected and the microbial activity increased only temporarily, the relative abundance of fungi (18:2n6,9) increased significantly due to fertilizer addition. Urea addition induced a short-lived increase in the in situ soil CO2-C effluxes, assuming a positive priming effect due to an activation of soil microbes. In total, plots amended with urea emitted 0.8 Mg CO2- C ha−1 a−1 more than the control. Results reveal that already moderate fertilization significantly improved pasture productivity and maintained soil quality. However, it is expected that higher loads of NP fertilizer will increase pasture productivity at the expense of soil organic carbon sequestration due to enhanced soil CO2-C losses. Hence, to establish a sustainable pasture management in the study region, the soil C management must also be carefully considered.

Abstract:

Die von ROOS et al. (2010a) durchgeführten Bekämpfungsmaßnahmen auf der Llashipa Fläche, führten zu einer deutlichen Verringerung von P. arachnoideum. Im Anschluss erfolgte eine Bepflanzung der Flächen mit S. sphacelata, um zu untersuchen ob hierdurch aufgegebene Weideflächen in eine erneute Nutzung überführt werden können. Aufgrund von Untersuchungsergebnissen (HAMER et al. (2009), HAMER et al. (2012), POTTHAST et al. (2010), POTTHAST et al. (2011)) von verlassenen und aktiven Weideflächen, welche Unterschiede in verschiedenen chemischen und biologischen Bodenindikatoren nachgewiesen haben, erfolgte eine Untersuchung nach einem vergleichbaren Schema für die Llashipa Fläche. Hierbei dienten weiterhin mit Farn bewachsene Flächen als Referenz, um mögliche Auswirkungen der Anpflanzung zu identifizieren. Die Messergebnisse zeigten zwischen den mit S. sphacelata bepflanzten Flächen und den Kontrollflächen keine signifikanten Unterschiede, ließen für verschiedene Indikatoren jedoch Tendenzen erkennen. So war der pH-Wert, die Basensättigung, der mikrobiell gebundene Phosphor und die Kohlenstoffmineralisierung auf der von P. arachnoideum dominierten Kontrollflache geringer als auf den mit S. sphacelata bepflanzten Flachen. Auffällig ist, dass die Kontrollflache den höchsten Gehalt an mikrobiell gebundenen Stickstoff bei vergleichbarem mikrobiell gebundenem Kohlenstoffgehalt aufweist. Weiterhin lag im Vergleich die höchste Stickstoffmineralisierung auf der Kontrollfläche vor. Allgemein weisen etliche der aufgenommenen Indikatoren eine hohe Variabilität auf, was durch den Stichprobenumfang nicht ausreichend abgedeckt wurde. Eine hohe Variabilität führt dazu, dass mögliche Unterschiede in den Indikatoren sehr groß sein müssen, damit eine Teststatistik Signifikanz anzeigt. Da jedoch ein relativ geringer Zeitraum zwischen Bepflanzung (Anfang 2008) und Probennahme (Oktober 2010) lag, ist es wahrscheinlich, dass mögliche Veränderungen in den verschiedenen Indikatoren nicht stark genug ausgeprägt waren. Damit mögliche Veränderungen deutlich werden, sollten Folgebeprobungen mit einem erhöhten Stichprobenumfang auf der Fläche durchgeführt werden. Hierbei sollte nicht zwischen verschiedenen Bekämpfungsmaßnahmen unterschiedenen werden, sondern ausschließlich zwischen S. sphacelata und P. arachnoideum dominierten Flächen. Eine weiterführende Untersuchung sollte dazu dienen, dass vorhandene Prozessverständnis von aktiven und verlassenen Weideflachen zu verbessern. Die Ergebnisse würden dazu beitragen, effektive Weidemanagementpläne zu erstellen, damit eine Degradation vermieden wird und bereits degradierte Weideflachen erneut in die Produktion überführt werden könnten.

Abstract:

Forest-to-pasture conversion has been reported to increase soil organic matter (SOM) in mineral topsoils in the tropical mountain rainforest region of south Ecuador, with subsequent decreases following pasture abandonment. Until now the mechanisms behind these changes have not been fully understood. To elucidate their varied preservation patterns, we analysed root- and shoot-derived organic matter and assessed their contribution to the formation of SOM in topsoils (0?5 cm) on a chronosequence of pastures (Setaria sphacelata (Schumach.); C4) established after slash and burn of the natural forest (diverse C3 plant species) and an abandoned pasture site invaded by bracken fern (Pteridium arachnoideum (Kaulf.) Maxon.; C3). Cutin and suberin biomarkers of the two plant species (grass and bracken) and of forest litter were identified after saponification and their contribution to SOM was studied by compound-specific stable carbon isotope analyses. Our results showed specific root and shoot biomarkers for the two plant species and for forest litter, which often did not correspond to the classification of root-versus shoot-specific monomers reported in the literature. This illustrates the importance of direct biomarker determination rather than using results from studies with different plants. Shoot- as well as root-derived OM of forest and grass origin contributed to the stable SOM pool with decadal turnover times. Forest-derived monomers contributed more to the stable SOM pool compared with grass-derived monomers. ω-hydroxy carboxylic acids and α,ω-alkanedioic acids of forest origin may have been stabilized in these tropical soils by bonding to soil minerals. Rapid degradation of grass-derived lipids from the same compound classes suggests a saturation of the mineral binding capacity. In pasture soils, the accumulation of SOM was mainly driven by large inputs of root OM. The accumulated SOM during pasture use is, however, lost rapidly after abandonment.

Abstract:

Understanding pasture degradation processes is the key for sustainable land management in the tropical mountain rainforest region of the South Ecuadorian Andes. We estimated the stocks of total carbon and nutrients, microbial biomass and different P fractions along a gradient of land-uses that is typical of the eastern escarpment of the Cordillera Real i.e., old-growth evergreen lower montane forest, active pastures (17 and 50 years-old), abandoned pastures 10 and 20 years old with bracken fern or successional vegetation. Conversion of forest to pasture by slashand- burn increased the stocks of SOC, TN, P and S in mineral topsoil of active pasture sites. Microbial growth in pasture soils was enhanced by improved availability of nutrients, C:N ratio, and increased soil pH. Up to 39 %of the total P in mineral soil was stored in the microbial biomass indicating its importance as a dynamic, easily available P reservoir at all sites. At a 17 years-old pasture the stock of NH4F extractable organic P, which is considered to be mineralisable in the short-term, was twice as high as in all other soils. The importance of the NaOH extractable organic Ppool increased with pasture age. Pasture degradation was accelerated by a decline of this P stock, which is essential for the long-term P supply. Stocks of microbial biomass, total N and S had returned to forest levels 10 years after pasture abandonment; soil pH and total P 20 years after growth of successional bush vegetation. Only the C:N ratio increased above forest level indicating an ongoing loss of N after 20 years. Soil nutrient depletion and microbial biomass decline enforced the degradation of pastures on the investigated Cambisol sites.

Abstract:

Over the past decades, the tropical mountain rainforest of southern Ecuador has been threatened by conversion to cattle pastures. Frequently, these pastures are invaded by bracken fern and abandoned when bracken becomes dominant. Changes in land-use (forest–pasture–abandoned pasture) can affect soil microorganisms and their physiological responses with respect to soil carbon and nutrient cycling. In situ investigations on litter decomposition and soil respiration as well as biogeochemical characterization of the soil were carried out to identify the driving factors behind. The conversion of forest to pasture induced a pronounced increase in CO2–C effluxes to 12.2 Mg ha-1 a-1 which did not decrease after abandonment. Soil microbial activity and biomass showed a different pattern with lowest values at forest and abandoned pasture sites. With 3445 mg kg-1 (0–5 cm) microbial biomass carbon (MBC by CFE-method), the active pasture had a more than three times higher value than forest and abandoned pasture, which was among the highest in tropical pasture soils. A shift in the microbial community structure (phospholipid fatty acid, PLFA) was also induced by the establishment of pasture land; the relative abundance of fungi and Gram-negative bacteria increased. PLFA fingerprints of the forest organic layer were more similar to pasture than to forest mineral soil. Chemical properties (pH value, exchangeable cations) were the main factors influencing the respective microbial structure. Bracken-invasion resulted in a decrease in the quantity and quality of aboveand belowground biomass. The lower organic substance and nutrient availability induced a significant decline in microbial biomass and activity. After pasture abandonment, these differences in soil microbial function were not accompanied by pronounced shifts in the community structure and in soil pH as was shown for the conversion to pasture. A disconnection between microbial structure and function was identified. Similar soil CO2–C effluxes between active and abandoned pasture sites might be explained by an underestimation of the effluxes from the active pasture site. All measurements were carried out between grass tussocks where fine-root density was about 2.6 times lower than below tussocks. Thus, lower proportions of root respiration were expected than below tussocks. Overall, soil microorganisms responded differently to changes in land-use from forest to pasture and from pasture to abandoned pasture resulting in pronounced changes of carbon and nutrient cycling and hence of ecosystem functioning.

Abstract:

Tropical regions are currently undergoing remarkable rates of land use change accompanied by altered litter inputs to soil. In vast areas of Southern Ecuador forests are clear cut and converted for use as cattle pastures. Frequently these pasture sites are invaded by bracken fern, when bracken becomes dominant pasture productivity decreases and the sites are abandoned. In the present study implications of invasive bracken on soil biogeochemical properties were investigated. Soil samples (0–5 cm) were taken from an active pasture with Setaria sphacelata as predominant grass and from an abandoned pasture overgrown by bracken. Grass (C4 plant) and bracken (C3 plant) litter, differing in C:N ratio (33 and 77, respectively) and lignin content (Klason-lignin: 18% and 45%, respectively), were incubated in soils of their corresponding sites and vice versa for 28 days at 22 C. Unamended microcosms containing only the respective soil or litter were taken as controls. During incubation the amount of CO2 and its d13C-signature were determined at different time intervals. Additionally, the soil microbial community structure (PLFA-analysis) aswell as the concentrations of KCl-extractable C and N were monitored. The comparison between the control soils of active and abandoned pasture sites showed that the massive displacement of Setaria-grass by bracken after pasture abandonment was characterized by decreased pH values accompanied by decreased amounts of readily available organic carbon and nitrogen, a lower microbial biomass and decreased activity as well as a higher relative abundance of actinomycetes. The d13C-signature of CO2 indicated a preferential mineralization of grass-derived organic carbon in pasture control soils. In soils amended with grass litter the mineralization of soil organic matter was retarded (negative priming effect) and also a preferential utilization of easily available organic substances derived from the grass litter was evident. Compared to the other treatments, the pasture soil amended with grass litter showed an opposite shift in the microbial community structure towards a lower relative abundance of fungi. After addition of bracken litter to the abandoned pasture soil a positive priming effect seemed to be supported by an N limitation at the end of incubation. This was accompanied by an increase in the ratio of Gram-positive to Gram-negative bacterial PLFA marker. The differences in litter quality between grass and bracken are important triggers of changes in soil biogeochemical and soil microbial properties after land use conversion.

Abstract:

The Andes of Ecuador are known for their outstanding biodiversity but also as the region with the highest deforestation rate in South America. This process is accompanied by accelerating degradation and loss of environmental services. Despite an extraordinary richness in native tree diversity, more than 90% of all forest plantations established in Ecuador consist of exotic species, primarily Eucalyptus spp. and Pinus spp. This is mainly due to the lack of information about the autecological and synecological requirements of the native species. The present study aims at providing basic knowledge on the early height development of native species in comparison to exotics. 12,000 seedlings of exotic and native species were planted in experimental trials at three sites of different successional stages: recently abandoned pastures (Setaria sphacelata), bracken (Pteridium arachnoideum) and shrubs. The results presented in this study refer to the status of the seedlings 3 years after planting. Soil data were revealed from soil core analysis from a total of 1008 soil samples distributed systematically over all plots. Soil chemical data were derived from a subsample of 125 randomly selected soil core sites. Soil properties in the study area emerged to be extremely heterogeneous. More than 60% of all plots presented two or more soil clusters. Soils in general were very poor in plant available N. Soil heterogeneity affected extractable Mn and Mg, dominating vegetation cover in turn affected Mn and P. Differences in soil properties had a strong effect for Eucalyptus saligna and Alnus acuminata. Manual above ground weeding showed species-specific effects: Tabebuia chrysantha and Heliocarpus americanus showed improved height growth, while that of Cedrela montana was reduced. There is evidence that A. acuminata can compete in growth with exotic species. Early successional species and exotics performed best on pasture dominated sites. Height growth of the mid-successional species C. montana was facilitated by bracken fern under certain soil conditions, and shrubs facilitated growth of T. chrysantha. The results indicate that reforestation with native species in Ecuador is possible but requires intensive consideration of interactions with soil properties and accompanying vegetation. Macroscopic soil core analysis can be a suitable instrument for detecting small-scale variation of soil properties. Nevertheless, a characterisation of both small-scale variation as well as variation on higher spatial scales, for instance by aerial photographs, is essential for effective planning of reforestation measures in the Andes.