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The conversion of tropical rainforest to agricultural systems such as oil palm alters biodiversity across a large range of interacting taxa and trophic levels. Yet, it remains unclear how direct and cascading effects of land-use change simultaneously drive ecological shifts. Combining data from a multi-taxon research initiative in Sumatra, Indonesia, we show that direct and cascading land-use effects alter biomass and species richness of taxa across trophic levels ranging from microorganisms to birds. Tropical land use resulted in increases in biomass and species richness via bottom-up cascading effects, but reductions via direct effects. When considering direct and cascading effects together, land use was found to reduce biomass and species richness, with increasing magnitude at higher trophic levels. Our analyses disentangle the multifaceted effects of land-use change on tropical ecosystems, revealing that biotic interactions on broad taxonomic scales influence the ecological outcome of anthropogenic perturbations to natural ecosystems.

Agroforestry, which is the integration of trees into monoculture cropland, can alter soil properties and nutrient cycling. Temperate agroforestry practices have been shown to affect soil microbial communities as indicated by changes in enzyme activities, substrate-induced respiration, and microbial biomass. Research exploring soil microbial communities in temperate agroforestry with the help of molecular tools which allow for the quantification of microbial taxa and selected genes is scarce. Here, we quantified 13 taxonomic groups of microorganisms and nine genes involved in N cycling (N2 fixation, nitrification, and denitrification) in soils of three paired temperate agroforestry and conventional monoculture croplands using real-time PCR. The agroforestry croplands were poplar-based alley-cropping systems in which samples were collected in the tree rows as well as within the crop rows at three distances from the tree rows. The abundance of Acidobacteria, Actinobacteria, Alpha- and Gammaproteobacteria, Firmicutes, and Verrucomicrobia increased in the vicinity of poplar trees, which may be accounted for by the presence of persistent poplar roots as well as by the input of tree litter. The strongest population increase was observed for Basidiomycota, which was likely related to high soil moisture, the accumulation of tree litter, and the absence of tillage in the tree rows. Soil microorganisms carrying denitrification genes were more abundant in the tree rows than in the crop rows and monoculture systems, suggesting a greater potential for nitrate removal through denitrification, which may reduce nitrate leaching. Since microbial communities are involved in critical soil processes, we expect that the combination of real-time PCR with soil process measurements will greatly enhance insights into the microbial control of important soil functions in agroforestry systems. Open-Access-Publikationsfonds 2020 peerReviewed

Aunque el clima ejerce un control importante sobre la meteorizacion de los minerales y los procesos de formacion del suelo, el efecto combinado de la vegetacion y la topografia puede influir en la velocidad y el alcance de la meteorizacion quimica a escala de ladera. En este trabajo, examinamos los patrones espaciales en la tension volumetrica y la extension de la meteorizacion del suelo asociados con los gradientes topograficos y los patrones de vegetacion. En unEn la cuenca alta andina, seleccionamos 10 secuencias de suelo para flujos andesiticos: 5 bajo pastos de mata, 3 bajo plantas formadoras de almohadillas y 2 bajo bosque nativo. A lo largo de cada secuencia de propulsion, se excavo un pozo en el hombro, la pendiente posterior y la pendiente de los pies, lo que resulto en 30 perfiles de suelo. La porosidad total del suelo ponderada en profundidad de los 30 perfiles del suelo promedio 64 ± 6%. La asociacion entre la tension volumetrica y la C organica del suelo indica que los agentes bioticos pueden ser efectivos para dilatar el regolito durante la meteorizacion. Los suelos volcanicos jovenes postglaciales se agotaron en cationes mono-divalentes y divalentes, con perdidas de masa total que oscilan entre 793 y 1610 kg / m 2.. La acumulacion de complejos de Al-humus en la matriz del suelo juega un papel esencial en la transformacion quimica de los suelos no alofanicos. Mas alla del control topografico marginalmente significativo sobre la extension de la meteorizacion quimica, nuestros datos muestran diferencias altamente significativas en la extension de la meteorizacion quimica entre las comunidades de vegetacion, con perdidas totales de masa en los suelos forestales, respectivamente, 19% y 22% mas altas que en los pastizales y las plantas formadoras de almohadillas. Por lo tanto, el mosaico de vegetacion en los ecosistemas alpinos podria proporcionar pistas esenciales para comprender los patrones de meteorizacion quimica del suelo causados ​​por las particulas de suelo y los tiempos de residencia del agua que varian espacialmente.

Forest conversion to agriculture can decrease soil nutrient stocks overtime. However, inherent spatial variability in soil biochemical properties in converted landscapes could be high, and may supersede effects of land-use change on soil nutrient changes. Our aims were to assess changes in soil nutrient stocks with land-use change, and to quantify the proportions of spatial variability and land-use change effects on the overall variance of soil nutrient stocks. This study was conducted in Jambi Province, Sumatra, Indonesia in two distinct landscapes defined by the dominant soil texture and type: loam and clay Acrisol soils. In each landscape, four land-use types were examined: lowland forest and rubber interspersed in naturally regenerating forest (referred here as “jungle rubber”) as reference land uses and smallholder plantations of rubber and oil palm. In the 0–0.5 m soil depth of the reference land uses, the clay Acrisol had higher total N (660.1 ± 63.8–1074.2 ± 158.8 g N m − 2 ; P ≤ 0.05), exchangeable Ca (24.1 ± 5.7–80.6 ± 22.8 g Ca m − 2 ; P ≤ 0.06), Mg (4.3 ± 0.6–39.2 ± 16.3 g Mg m − 2 ; P ≤ 0.02), K (11.7 ± 0.7–34.7 ± 12.1 g K m − 2 ; P ≤ 0.06), extractable P (1.1 ± 0.1–2.6 ± 0.1 g P m − 2 ; P ≤ 0.001) and effective cation exchange capacity (ECEC; 11.4 ± 3.1–40.6 ± 11.0 cmol c kg − 1 ; P = 0.02), illustrating that clay content influenced soil fertility in these highly weathered soils. Compared to the reference land uses, the oil palm plantations had higher soil pH (4.2 ± 0.0–4.6 ± 0.1; P ≤ 0.04), base saturation (8.9 ± 1.6–6.5 ± 1.3%; P ≤ 0.07) and extractable P (0.8 ± 0.1–6.1 ± 3.2 g P m − 2 ; P ≤ 0.01) in the top 0.5 m depth, which was probably due to the legacy effect of biomass burning and fertilization. We were unable to detect significant effects of land-use change on other soil biochemical characteristics (i.e., ECEC, stocks of exchangeable bases, soil organic carbon (SOC), total N). Based on variance components analysis, a large proportion of the variance of these parameters was accounted by the variation among replicate plots (26–91%) rather than by land-use types (only 0–6%). Power analysis showed that the optimum number of replicate plots to detect land-use change effects on these parameters ranged from 5 to 7. Our results suggest that spatial variability must be represented in the experimental design in order to detect land-use change effects on soil nutrient changes through stratifying the area of inference (i.e., landscape or region) based on known drivers of soil fertility and determining the optimal number of experimental units.

Canopy soils can significantly contribute to aboveground labile biomass, especially in tropical montane forests. Whether they also contribute to the exchange of greenhouse gases is unknown. To examine the importance of canopy soils to tropical forest-soil greenhouse gas exchange, we quantified gas fluxes from canopy soil cores along an elevation gradient with 4 yr of nutrient addition to the forest floor. Canopy soil contributed 5–12 percent of combined (canopy + forest floor) soil CO2 emissions but CH4 and N2O fluxes were low. At 2000 m, phosphorus decreased CO2 emissions (>40%) and nitrogen slightly increased CH4 uptake and N2O emissions. Our results show that canopy soils may contribute significantly to combined soil greenhouse gas fluxes in montane regions with high accumulations of canopy soil. We also show that changes in fluxes could occur with chronic nutrient deposition.

Oil palm plantations have expanded rapidly in recent decades. This large-scale land-use change has had great ecological, economic, and social impacts on both the areas converted to oil palm and their surroundings. However, research on the impacts of oil palm cultivation is scattered and patchy, and no clear overview exists. We address this gap through a systematic and comprehensive literature review of all ecosystem functions in oil palm plantations, including several (genetic, medicinal and ornamental resources, information functions) not included in previous systematic reviews. We compare ecosystem functions in oil palm plantations to those in forests, as the conversion of forest to oil palm is prevalent in the tropics. We find that oil palm plantations generally have reduced ecosystem functioning compared to forests: 11 out of 14 ecosystem functions show a net decrease in level of function. Some functions show decreases with potentially irreversible global impacts (e.g. reductions in gas and climate regulation, habitat and nursery functions, genetic resources, medicinal resources, and information functions). The most serious impacts occur when forest is cleared to establish new plantations, and immediately afterwards, especially on peat soils. To variable degrees, specific plantation management measures can prevent or reduce losses of some ecosystem functions (e.g. avoid illegal land clearing via fire, avoid draining of peat, use of integrated pest management, use of cover crops, mulch, and compost) and we highlight synergistic mitigation measures that can improve multiple ecosystem functions simultaneously. The only ecosystem function which increases in oil palm plantations is, unsurprisingly, the production of marketable goods. Our review highlights numerous research gaps. In particular, there are significant gaps with respect to socio-cultural information functions. Further, there is a need for more empirical data on the importance of spatial and temporal scales, such as differences among plantations in different environments, of different sizes, and of different ages, as our review has identified examples where ecosystem functions vary spatially and temporally. Finally, more research is needed on developing management practices that can offset the losses of ecosystem functions. Our findings should stimulate research to address the identified gaps, and provide a foundation for more systematic research and discussion on ways to minimize the negative impacts and maximize the positive impacts of oil palm cultivation.

Our goals were (1) to determine whether tree species diversity affects nutrient (N, P and K) cycling, and (2) to assess whether there is competition for these nutrients between microbial biomass and trees. We measured nutrient resorption efficiency by trees, nutrient contents in leaf litterfall, decomposition rates of leaf litter, nutrient turnover in decomposing leaf litter, and plant-available nutrients in the soil in mono-species stands of beech, oak, hornbeam and lime and in mixed-species stands, each consisting of three of these species. Cycling of nutrients through leaf litter input and decomposition were influenced by the types of tree species and not simply by tree species diversity. Trees and microbial biomass were competing strongly for P, less for K and only marginally for N. Such competition was most pronounced in mono-species stands of beech and oak, which had low nutrient turnover in their slow decomposing leaf litter, and less in mono-species stands of hornbeam and lime, which had high nutrient turnover in their fast decomposing leaf litter. The low soil P and K availability in beech stands, which limit the growth of beech at Hainich, Germany, were alleviated by mixing beech with hornbeam and lime. These species-specific effects on nutrient cycling and soil nutrient availability can aid forest management in improving productivity and soil fertility.

Land-use change has a strong impact on carbon, energy and water fluxes and its effect is particularly pronounced in tropical regions. Uncertainties exist in the prediction of future land-use change impacts on these fluxes by land surface models due to scarcity of suitable measured data for parametrization and poor representation of key biogeochemical processes associated with tropical vegetation types. Rubber plantations (Havea brasilliensis) are a crucial land-use type across tropical landscapes that has greatly expanded in recent decades. Here, we first synthesize the relevant data for describing the biogeochemical processes of rubber from our past measurement campaigns in Jambi province, Indonesia. We then use these data-sets to develop a rubber plant functional type (PFT) for the Community Land Model (CLM4.5). Field measured data from small-holder plantations on leaf litterfall, soil respiration, latex harvest, leaf area index, transpiration, net primary productivity, and above-ground and fine root biomass were used to develop and calibrate a new PFT-based model (CLM4.5-rubber). CLM-rubber predictions adequately captured the annual net primary productivity and above-ground biomass as well as the seasonal dynamics of leaf litterfall, soil respiration, soil moisture and leaf area index. All of the predicted water fluxes of CLM-rubber were very similar to a site-specific calibrated soil water model. Including temporal variations in leaf life span enabled CLM-rubber to better capture the seasonality of leaf litterfall. Increased sensitivity of stomata to soil water stress and the enhancement of growth and maintenance respiration of fine roots in response to soil nutrient limitation enabled CLM-rubber to capture the magnitude of transpiration and leaf area index. Since CLM-rubber predicted reasonably well the carbon and water use, we think that the current model can be used for larger-scale simulations within Jambi province because more than 99 % of the rubber plantations are smallholder owned in Jambi province and have low soil fertility.

Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We studied the response of soil carbon dioxide (CO2) efflux to long-term experimental N addition (125 kg N ha−1 yr−1) in mature lowland and montane forests in Panama. In the lowland forest, on soils with high nutrient-supplying and buffering capacity, fine litterfall and stem-growth were neither N- nor phosphorus-limited. In the montane forest, on soils with low nutrient supplying capacity and an organic layer, fine litterfall and stem-growth were N-limited. Our objectives were to 1) explore the influence of soil temperature and moisture on the dynamics of soil CO2 efflux and 2) determine the responses of soil CO2 efflux from an N-rich and N-limited forest to elevated N input. Annual soil CO2-C efflux was larger in the lowland (15.44 ± 1.02 Mg C ha−1) than in the montane forest (9.37 ± 0.28 Mg C ha−1). In the lowland forest, soil moisture explained the largest fraction of the variance in soil CO2 efflux while soil temperature was the main explanatory variable in the montane forest. Soil CO2 efflux in the lowland forest did not differ between the control and 9–11 yr N-addition plots, suggesting that chronic N input to nutrient-rich tropical lowland forests on well-buffered soils may not change their C balance on a decadal time scale. In the montane forest, first year N addition did not affect soil CO2 efflux but annual CO2 efflux was reduced by 14% and 8% in the 2nd and 3rd year N-addition plots, respectively, compared to the control. This reduction was caused by a decrease in soil CO2 efflux during the high stem-growth period of the year, suggesting a shift in carbon partitioning from below- to aboveground in the N-addition plots in which stem diameter growth was promoted.

Tropical lowland forest soils are significant sources and sinks of trace gases. In order to model soil trace gas flux for future climate scenarios, it is necessary to be able to predict changes in soil trace gas fluxes along natural gradients of soil fertility and climatic characteristics. We quantified trace gas fluxes in lowland forest soils at five locations in Panama, which encompassed orthogonal precipitation and soil fertility gradients. Soil trace gas fluxes were measured monthly for 1 (NO) or 2 (CO2, CH4, N2O) years (2010–2012) using vented dynamic (for NO only) or static chambers with permanent bases. Across the five sites, annual fluxes ranged from 8.0 to 10.2 Mg CO2-C, −2.0 to −0.3 kg CH4-C, 0.4 to 1.3 kg N2O-N and −0.82 to −0.03 kg NO-N ha−1 yr−1. Soil CO2 emissions did not differ across sites, but they did exhibit clear seasonal differences and a parabolic pattern with soil moisture across sites. All sites were CH4 sinks; within-site fluxes were largely controlled by soil moisture, whereas fluxes across sites were positively correlated with an integrated index of soil fertility. Soil N2O fluxes were low throughout the measurement years, but the highest emissions occurred at a mid-precipitation site with high soil N availability. Net negative NO fluxes at the soil surface occurred at all sites, with the most negative fluxes at the low-precipitation site closest to Panama City; this was likely due to high ambient NO concentrations from anthropogenic sources. Our study highlights the importance of both short-term (climatic) and long-term (soil and site characteristics) factors in predicting soil trace gas fluxes.

Conversion of forest to rubber and oil palm plantations is widespread in Sumatra, Indonesia, and it is largely unknown how such land-use conversion affects nutrient leaching losses. Our study aimed to quantify nutrient leaching and nutrient retention efficiency in the soil after land-use conversion to smallholder rubber and oil palm plantations. In Jambi province, Indonesia, we selected two landscapes on highly weathered Acrisol soils that mainly differed in texture: loam and clay. Within each soil type, we compared two reference land uses, lowland forest and jungle rubber (defined as rubber trees interspersed in secondary forest), with two converted land uses: smallholder rubber and oil palm plantations. Within each soil type, the first three land uses were represented by 4 replicate sites and the oil palm by three sites, totaling 30 sites. We measured leaching losses using suction cup lysimeters sampled biweekly to monthly from February to December 2013. Forests and jungle rubber had low solute concentrations in drainage water, suggesting low internal inputs of rock-derived nutrients and efficient internal cycling of nutrients. These reference land uses on the clay Acrisol soils had lower leaching of dissolved N and base cations (P= 0.01–0.06) and higher N and base cation retention efficiency (P P P P=0.08) and organic C (P&thinsp

Fertilization experiments in tropical forests have shown that litterfall increases in response to the addition of one or more soil nutrients. However, the relationship between soil nutrient availability and litterfall is poorly defined along natural soil fertility gradients, especially in tropical montane forests. Here, we measured litterfall for two years in five lower montane 1-ha plots spanning a soil fertility and precipitation gradient in lower montane forest at Fortuna, Panama. Litterfall was also measured in a concurrent nitrogen fertilization experiment at one site. Repeated-measures ANOVA was used to test for site (or treatment), year, and season effects on vegetative, reproductive and total litterfall. We predicted that total litterfall, and the ratio of reproductive to leaf litterfall, would increase with nutrient availability along the fertility gradient, and in response to nitrogen addition. We found that total annual litterfall varied substantially among 1-ha plots (4.78 Mg/ha/yr to 7.96 Mg/ha/yr), and all but the most aseasonal plot showed significant seasonality in litterfall. However, litterfall accumulation did not track soil nutrient availability; instead forest growing on relatively infertile soil, but dominated by an ectomycorrhizal tree species, had the highest total litterfall accumulation. In the fertilization plots, significantly more total litter fell in nitrogen addition relative to control plots, but this increase in response to nitrogen (13%) was small compared to variation observed among 1-ha plots. These results suggest that while litterfall at Fortuna is nutrient-limited, compositional and functional turnover along the fertility gradient obscure any direct relationship between soil resource availability and canopy productivity. Resumen Se demostro con experimentos de fertilizacion en bosques tropicales incrementos en la caida de hojarasca asociados con la adicion de nutrientes; empero, se desconoce la relacion entre la disponibilidad de nutrientes del suelo con la produccion de hojarasca en gradientes naturales de fertilidad del suelo, especialmente en bosques montanos tropicales. Medimos la produccion de hojarasca durante dos anos en cinco parcelas (1-ha) en bosques montanos bajos, sobre gradientes de fertilidad y precipitacion y en un experimento paralelo de fertilizacion de nitrogeno en la Reserva Forestal Fortuna, Panama. Predijimos incrementos de hojarasca total y reproductiva con la disponibilidad de nutrientes en el gradiente de fertilidad del suelo y en respuesta a la adicion de nitrogeno (13%). Encontramos que la hojarasca total varia sustancialmente entre parcelas (4.78 Mg/ha/yr hasta 7.96 Mg/ha/yr) y que en todas las parcelas, excepto la menos estacional, hay estacionalidad significativa en la cantidad de hojarasca; pero, la acumulacion de hojarasca no coincide con la disponibilidad de nutrientes en el suelo, porque el bosque sobre suelo poco fertil con dominio de una especie ectomicorricica, presento mayor acumulacion de hojarasca. Las parcelas con fertilizacion tenian significativamente mas hojarasca que las control; pero, el incremento en hojarasca en respuesta a la adicion de nitrogeno fue menor respecto a la variacion entre parcelas. Los resultados sugieren que mientras la produccion de hojarasca en Fortuna es limitada por nutrientes, la variacion en la composicion de especies a traves de gradientes de fertilidad ocultan cualquier relacion directa entre la disponibilidad de nutrientes del suelo y la productividad del dosel.

Large areas in the tropics receive elevated atmospheric nutrient inputs. Presently, little is known on how nitrogen (N) cycling in tropical montane forest soils will respond to such increased nutrient inputs. We assessed how gross rates of mineral N production (N mineralization and nitrification) and microbial N retention (NH4+ and NO3- immobilization and dissimilatory NO3- reduction to NH4+ [DNRA]) change with elevated N and phosphorus (P) inputs in montane forest soils at 1000-, 2000-, and 3000-m elevations in south Ecuador. At each elevation, four replicate plots (20 x 20 m each) of control, N (added at 50 kg N x ha(-1) x yr(-1)), P (added at 10 kg P x ha(-1) x yr(-1)), and combined N+P additions have been established since 2008. We measured gross N cycling rates in 2010 and 2011, using 15N pool dilution techniques with in situ incubation of intact soil cores taken from the top 5 cm of soil. In control plots, gross soil-N cycling rates decreased.with increase in elevation, and microbial N retention was tightly coupled with mineral N production. At 1000 m and 2000 m, four-year N and combined N + P additions increased gross mineral N production but decreased NH4+ and NO3- immobilization and DNRA compared to the control. At 3000 m, four-year N and combined N + P additions increased gross N mineralization rates and decreased DNRA compared to the control; although NH4+ and NO3- immobilization in the N and N + P plots were not different' from the control, these were lower than their respective mineral N production. At all elevations, decreased microbial N retention was accompanied by decreased microbial biomass C and C:N ratio. P addition did not affect any of the soil-N cycling processes. Our results signified that four years of N addition, at a rate expected to occur at these sites, uncoupled the soil-N cycling processes, as indicated by decreased microbial N retention. This fast response of soil-N cycling processes across elevations implies that greater attention should be paid to the biological implications on montane forests of such uncoupled soil-N cycling.

There are contrasting reports whether and how tree diversity influences stand productivity in temperate deciduous forests. Tree species diversity may increase stand productivity in temperate forests through complementary resource use and/or facilitation if the resource considered limits productivity. In unpolluted temperate forests, net primary production is typically limited by nitrogen (N). However, in many parts of Europe high N deposition has alleviated N limitation and there is some evidence that phosphorus (P) and/or potassium (K) limitation has become more widespread. Here, we report on a study where we investigated whether complementarity and/or facilitation increase productivity in a typical German deciduous forest with tree species of beech (Fagus sylvatica), oak (Quercus petraea and Quercus robur), hornbeam (Carpinus betulus) and lime (Tilia cordata and Tilia platyphyllus). We measured biomass production and availability of soil N, P, K, calcium (Ca) and magnesium (Mg) in stands of single species (mono-species stands) and in stands with different combinations of three of the tree species above (mix-species stands). We used nutrient response efficiency (NRE) to evaluate whether a specific nutrient limits tree growth. At a stand level, above-ground net primary productivity did not differ between mono- and mix-species stands. At a tree level, using a neighborhood approach, relative growth rates of beech trees in mono-species stands were smaller than when they were in mix with lime and hornbeam whereas growth of lime trees in mono-species stands was larger than in mix with beech and oak. The NRE curve for beech showed that beech trees in mix-species stands had optimal P and K response efficiencies whereas beech trees in mono-species stands showed P and K limitations. The NRE curve for oak with exchangeable soil K showed that K levels were beyond the optimum NRE and thus K was not limiting oak growth. NRE curves for hornbeam and lime showed no significant relationships with any of the soil nutrients. Hence, nutrient limitation was species-dependent. Our results showed that using both NRE and a neighborhood approach are useful tools in quantifying the effects of individual tree species on a species’ productivity between mono- and mix-species stands. Such tools provide important basis for improving management of typical mix-species, temperate forests.

Forest conversion to agricultural land may affect nutrient budget due to different soil management intensity. Our study aimed to assess partial nutrient budget from forest converted to rubber and oil palm plantations in Jambi Province, Sumatra, Indonesia. The research investigated four land uses: lowland forest and jungle rubber (as reference) and the converted land uses of rubber and oil palm plantations in loam and clay Acrisol soil landscapes, with four replicates in each site, except in oil palm plantation with three replicates. Partial nutrient budgets were quantified from nutrient input (bulk precipitation and fertilizers) and output (leaching and harvest) for N, P, K, Ca, Mg and Na. Annual leaching fluxes were the main output pathways for N, P and base cations in the reference land uses, except for P in jungle rubber sites where the major output pathway was harvest export. The high nutrient output through leaching losses and harvest export resulted in the lowest annual partial budgets of Ca and Mg in oil palm plantations than in the other land uses in both landscapes. Proper soil management is needed to minimize the net negatives balances and their effect on sustainability of oil palm plantations.

Although often overlooked in forest research, the canopy can play an important role in forest nutrient cycling. Since the canopy is spatially isolated from the forest floor, nutrient cycling in the two areas may differ as terrestrial nutrients accumulate. We measured rates of free-living N2 fixation along an elevation gradient (1,000, 2,000 and 3,000 m) of tropical montane canopy soils, compared these to rates measured in the top 5 cm of forest floor soils (excluding fresh litter), and assessed the effects of elevated nutrient inputs to the forest floor. N2 fixation was measured using the acetylene reduction assay. Measurements occurred in the field, in the wet and dry seasons, using intact cores of soil. The forest floor had been fertilized biannually with moderate amounts of nitrogen (N) and phosphorus (P) for 4 years; treatments included control, N, P and N + P. N2 fixation rates exhibited little variation with elevation but were higher in the dry season than the wet season. Fixation was inhibited in forest floor N plots compared to control and P plots, and stimulated in canopy P plots compared to control. At 2,000 m, the canopy contributed 12 % of measured canopy and forest floor N2 fixation (1.2 kg N ha−1 year−1). Results suggest that N2 fixation is an active process in canopy soils, which is variable across seasons and sensitive to changes in terrestrial nutrient availability. Long-term terrestrial accumulation of N and/or P has the potential to significantly change the dynamics of soil N cycling in these canopies.

Maintaining nitrogen retention efficiency (NRE) is crucial in minimizing N losses when intensifying management of temperate grasslands. Our aim was to evaluate how grassland management practices and sward compositions affect NRE (1 − N losses/soil available N), defined as the efficiency with which soil available N is retained in an ecosystem. A three-factorial grassland management experiment was established with two fertilization treatments (without and combined N, phosphorus and potassium fertilization), two mowing frequencies (cut once and thrice per year) and three sward compositions (control, monocot- and dicot-enhanced swards). We measured N losses as leaching and nitrous oxide emissions, and soil available N as gross N mineralization rates. Fertilization increased N losses due to increased nitrification and decreased microbial N immobilization, and consequently decreased NRE. Intensive mowing partly dampened high N losses following fertilization. Sward compositions influenced NRE but not N losses: control swards that developed for decades under extensive management had the highest NRE, whereas monocot-enhanced sward had the lowest NRE. NRE was highly correlated with microbial NH 4 + immobilization and microbial biomass and only marginally correlated with plant N uptake, underlining the importance of microbial N retention in the soil-plant system. Microbial N retention is reflected in NRE but not in indices commonly used to reflect plant response. NRE was able to capture the effects of sward composition and fertilization whereas N losses were only sensitive to fertilization; thus, NRE is a better index when evaluating environmental sustainability of sward compositions and management practices of grasslands.

Estimates of asymbiotic biological N fixation (BNF) in temperate grasslands are few with large variations. In the past six decades, European grasslands have been subjected to intensive management practices and presently it is not known how asymbiotic BNF is influenced by these practices. Our objective was to assess the impact of fertilizer application and mowing frequency on asymbiotic BNF in a Central European grassland. In 2008, we established a three-factorial experiment with two fertilizer treatments (no fertilizer application and combined nitrogen (N), phosphorus (P) and potassium (K) fertilization at 180–30–100 kg ha −1 yr −1 ), two mowing frequencies (cut once and thrice per year) and three sward compositions through the application of herbicides (control, monocot- and dicot-enhanced swards). Three years after the initial sward manipulation, there was no more difference in functional group composition. Between June 2011 and May 2012, we measured in-situ asymbiotic BNF using the acetylene reduction assay, calibrated with 15 N 2 -fixation method. Across treatments, asymbiotic BNF rates in the 0–5-cm soil depth ranged from 1.7 (±0.2 SE) kg ha −1 yr −1 for fertilized plots cut once a year to 5.7 (±2.3 SE) kg ha −1 yr −1 for unfertilized plots cut thrice a year. Fertilization decreased asymbiotic BNF, suggesting that the potential positive effect of increased soil P levels might have been overruled by the negative effect of increased soil mineral N levels. Intensive mowing stimulated asymbiotic BNF, which was probably due to an increase in rhizodeposition. Our calibration of the acetylene reduction assay with the 15 N 2 -fixation method resulted in a conversion factor of 0.61, which largely deviates from the theoretical conversion factor of 3. Furthermore, laboratory incubations under increased soil moisture and temperature conditions overestimated BNF rates compared to in-situ measurements. Thus, laboratory measurements with altered soil moisture, temperature or disturbed soil may lead to strong biases in estimates of asymbiotic BNF. Our results suggest that input of N through BNF may be considerable in temperate grasslands. We conclude that BNF studies should be conducted in-situ and that the acetylene reduction assay should be calibrated against 15 N 2 -fixation calibration for reliable estimates.

Secondary forest-to-rubber ( Hevea brasiliensis ) plantation conversion is an important recent land-use change in the montane regions of mainland Southeast Asia. This land-use conversion caused a reduction of soil organic carbon (SOC) stocks by on average 19% down to 1.2 m over 46 years. Due to the mountainous topography of the region, most rubber plantations include narrow terraces parallel to contours. Manual terrace construction involves cutting of the soil from the upper slope and piling up the removed soil on the soil surface downslope. Soil redistribution by terrace construction may affect SOC dynamics through exposure of the subsurface soil at the terrace inner sides (cut section) and soil burial at the terrace outer edges (fill section). Our study, conducted in southern Yunnan province of China, aimed to quantify SOC stock changes induced by terrace construction. In three rubber plantations aged 5, 29 and 44 years, we systematically sampled the terraces according to soil redistribution zones, and the original sloping areas in between the terraces were used as reference. At the cut section of the terrace, topsoil removal caused a depletion of SOC stocks in the youngest plantation followed by SOC stock recovery in the two oldest plantations. The recovery of SOC stocks at the cut section in the two oldest plantations was attributed to the capacity of the exposed subsurface soil to store new organic carbon inputs from roots and litter, and to sedimentation of eroded topsoil materials from the upper slope. At the fill section of the terrace, soil deposition resulted in higher total SOC stocks compared to the reference position in all plantations. This was due to the deposition of redistributed soil material on top of the original soil surface combined with the partial preservation of carbon in the buried soil. Overall, the increase of SOC in the exposed subsurface soil at the cut sections, and the partial preservation of SOC in the buried soil at the fill sections resulted in higher SOC stocks down to 1.2 m at the terraces compared to the reference positions in the two oldest plantations. Our results imply that terracing may alleviate SOC losses caused by the conversion of secondary forest to terraced rubber plantation.

Although disturbances associated with selective logging can cause pronounced changes in soil characteristics and nutrient stocks, such information is very limited for highly weathered soils in Africa. We assessed the effects of reduced impact logging (RIL, with a 30-year rotation management plan) and conventional logging (CL, without a management plan) on physical and biochemical characteristics of Ferralsol soils that developed on pre-Cambrian rocks in rainforests of Cameroon. Five to seven months after the logging operations were completed, we mapped the CL and RIL sites and quantified the disturbed areas: felling gaps, skidding trails, logging decks and roads. We selected four replicate plots at each site that encompassed these four disturbed strata and an adjacent undisturbed area as the reference. At each disturbed stratum and reference area per plot, we took soil samples down to 50 cm, and quantified soil physical and biochemical characteristics. Nutrient exports with timber harvest were also quantified. The logging intensity was very low with removals of 0.2 and 0.3 tree per hectare, and the ground area disturbed accounted only 5.2% and 4.0% of the total area in CL and RIL, respectively. In terms of area disturbance for each harvested tree, CL had 753 m2 tree−1 more affected ground area than RIL. Roads and logging decks were the most affected by logging operations, where effective cation exchange capacity, soil organic carbon (SOC), total nitrogen (N), Bray-extractable phosphorus (P) and exchangeable aluminum decreased whereas pH, 15N natural abundance and exchangeable manganese increased compared to the undisturbed reference area (P

In tropical forests, multi-year studies on the impact of chronic nitrogen (N) enrichment on soil N-oxide fluxes are lacking. Our objectives were to: (1) assess the changes in soil N-oxide (NO+N2O) fluxes from a montane forest in response to 3–4 years of N addition and compare these to the response of a lowland forest with 11–12 years of N addition, (2) quantify the contributions of nitrification and denitrification to nitrous oxide (N2O) emissions, and (3) assess how deep in the soil profile the N2O concentrations are affected by N input. These measurements were conducted in montane and lowland forests in Panama in 2008–2009, which covered the last 2 years of a 4-year investigation that started in 2006. Each forest had a control and N addition treatment (125 kg urea N ha−1 year−1) with four replicate plots (1,600 m2 each) per treatment. N-oxide emissions from the montane forest started to increase within the first 2 years of treatment and continued to increase in the 3rd and 4th years of N addition, during which the emissions were already equivalent to those from the lowland forest with 11–12 years of N addition. The large N-oxide response of the montane forest to N addition were due to the large increases in gross nitrification rates in the organic layer and the high moisture contents of the mineral soil due to the high rainfall (5.5 ± 0.2 m year−1). In the lowland forest (2.7 ± 0.1 m rain year−1), N-oxide response to N addition was more pronounced in wet years (i.e. 2006–2007 with 5–17 % higher rainfall than average) than in dry years (i.e. 2008–2009 with 5–26 % lower rainfall than average). Denitrification was the dominant source of N2O not only for the organic layer and the top 5 cm of mineral soil but possibly also for the entire 2 m depth. Soil–air N2O concentrations were elevated by N addition down to at least 2 m. Our results suggest that the best indicators of these responses were the presence of an organic layer and rainfall quantity and seasonality.

Abstract. Presently, the lack of data on soil organic carbon (SOC) in relation to land-use types and biophysical characteristics prevents reliable estimates of carbon stocks in montane landscapes of mainland SE Asia. Our study, conducted in a 10,000-hectare landscape in Xishuangbanna, SW China, aimed at assessing the spatial variability in SOC and its relationships with land-use cover and key biophysical characteristics at multiple spatial scales. We sampled 27 one-hectare plots including 10 plots in mature forests, 11 plots in regenerating or highly disturbed forests, and six plots in open land including tea plantations or grasslands. We used a sampling design with a hierarchical structure. The landscape was first classified according to land-use types. Within each land-use type, sampling plots of 100 m × 100 m each were randomly selected, and within each plot we sampled nine subplots. This hierarchical sampling design allowed partitioning of the overall variance in SOC, vegetation, soil properties and topography that was accounted for by the variability among land-use types, among plots nested within land-use types, and within plots. SOC concentrations and stocks did not differ significantly across land-use types. The SOC stocks to a depth of 0.9 m were 177.6 ± 19.6 Mg C ha−1 in tea plantations, 199.5 ± 14.8 Mg C ha−1 in regenerating or highly disturbed forests, 228.6 ± 19.7 (SE) Mg C ha−1 in mature forests, and 236.2 ± 13.7 Mg C ha−1 in grasslands. In this montane landscape, variability within plots accounted for more than 50 % of the overall variance in SOC. The relationships between SOC, biophysical characteristics and land-use types varied across spatial scales. Variability in SOC within plots was determined by tree basal area, litter layer carbon stocks and slope. Variability in SOC among plots in open land was influenced by land-use type – SOC concentrations and stocks in grasslands were higher than in tea plantations. In forests, the variability in SOC among plots was related to elevation. The scale-dependent relationships between SOC and its controlling factors demonstrate that studies which aim to investigate the land-use effects on SOC need an appropriate sampling design reflecting the controlling factors of SOC so that land-use effects will not be masked by the variability between and within sampling plots.

Oil palm and rubber plantations cover large areas of former rainforest in Sumatra, Indonesia, supplying the global demand for these crops. Although forest conversion is known to influence soil nitrous oxide (N2O) and nitric oxide (NO) fluxes, measurements from oil palm and rubber plantations are scarce (for N2O) or nonexistent (for NO). Our study aimed to (1) quantify changes in soil-atmosphere fluxes of N oxides with forest conversion to rubber and oil palm plantations, and (2) determine their controlling factors. In Jambi, Sumatra, we selected two landscapes that mainly differed in texture but both on heavily weathered soils: loam and clay Acrisol soils. Within each landscape, we investigated lowland forest, rubber trees interspersed in secondary forest (termed as jungle rubber), both as reference land uses, and smallholder rubber and oil palm plantations, as converted land uses. Each land use had four replicate plots within each landscape. Soil N2O fluxes were measured monthly from December 2012 to December 2013, and soil NO fluxes were measured four times between March and September 2013. In the loam Acrisol landscape, we also conducted weekly to bi-weekly soil N2O flux measurements from July 2014 to July 2015 in a large-scale oil palm plantation with four replicate plots for comparison with smallholder oil palm plantations. Land-use conversion to smallholder plantations had no effect on soil N-oxide fluxes (P = 0.58 to 0.76) due to the generally low soil N availability in the reference land uses that further decreased with land-use conversion. Over one-year measurements, the temporal patterns of soil N-oxide fluxes were influenced by soil mineral N and water contents. Across landscapes, annual soil N2O emissions were controlled by gross nitrification and sand content, which also suggest the influence of soil N and water availability. Soil N2O fluxes (µg N m−2 h−1) were: 7 ± 2 to 14 ± 7 (reference land uses), 6 ± 3 to 9 ± 2 (rubber), 12 ± 3 to 12 ± 6 (smallholder oil palm), and 42 ± 24 (large-scale oil palm). Soil NO fluxes (µg N m−2 h−1) were: −0.6 ± 0.7 to 5.7 ± 5.8 (reference land uses), −1.2 ± 0.5 to −1.0 ± 0.2 (rubber) and −0.2 ± 1.2 to 0.7 ± 0.7 (smallholder oil palm). The low N fertilizer application in smallholder oil palm plantations (commonly 48 to 88 kg N ha−1 yr−1) resulted in N-oxide losses of only 0.2–0.7 % of the applied N. To improve estimate of soil N-oxide fluxes from oil palm plantations in this region, studies should focus on large-scale plantations (which usually have two to four times higher N fertilization rates than smallholders) with frequent measurements following fertilizer application.

Oil palm plantations have expanded rapidly in recent decades. This large-scale land-use change has had great ecological, economic, and social impacts on both the areas converted to oil palm and their surroundings. However, research on the impacts of oil palm cultivation is scattered and patchy, and no clear overview exists. We address this gap through a systematic and comprehensive literature review of all ecosystem functions in oil palm plantations, including several (genetic, medicinal and ornamental resources, information functions) not included in previous systematic reviews. We compare ecosystem functions in oil palm plantations to those in forests, as the conversion of forest to oil palm is prevalent in the tropics. We find that oil palm plantations generally have reduced ecosystem functioning compared to forests: 11 out of 14 ecosystem functions show a net decrease in level of function. Some functions show decreases with potentially irreversible global impacts (e.g. reductions in gas and climate regulation, habitat and nursery functions, genetic resources, medicinal resources, and information functions). The most serious impacts occur when forest is cleared to establish new plantations, and immediately afterwards, especially on peat soils. To variable degrees, specific plantation management measures can prevent or reduce losses of some ecosystem functions (e.g. avoid illegal land clearing via fire, avoid draining of peat, use of integrated pest management, use of cover crops, mulch, and compost) and we highlight synergistic mitigation measures that can improve multiple ecosystem functions simultaneously. The only ecosystem function which increases in oil palm plantations is, unsurprisingly, the production of marketable goods. Our review highlights numerous research gaps. In particular, there are significant gaps with respect to socio-cultural information functions. Further, there is a need for more empirical data on the importance of spatial and temporal scales, such as differences among plantations in different environments, of different sizes, and of different ages, as our review has identified examples where ecosystem functions vary spatially and temporally. Finally, more research is needed on developing management practices that can offset the losses of ecosystem functions. Our findings should stimulate research to address the identified gaps, and provide a foundation for more systematic research and discussion on ways to minimize the negative impacts and maximize the positive impacts of oil palm cultivation. peerReviewed

Smallholder-dominated agricultural mosaic landscapes are highlighted as model production systems that deliver both economic and ecological goods in tropical agricultural landscapes, but trade-offs underlying current land-use dynamics are poorly known. Here, using the most comprehensive quantification of land-use change and associated bundles of ecosystem functions, services and economic benefits to date, we show that Indonesian smallholders predominantly choose farm portfolios with high economic productivity but low ecological value. The more profitable oil palm and rubber monocultures replace forests and agroforests critical for maintaining above- and below-ground ecological functions and the diversity of most taxa. Between the monocultures, the higher economic performance of oil palm over rubber comes with the reliance on fertilizer inputs and with increased nutrient leaching losses. Strategies to achieve an ecological-economic balance and a sustainable management of tropical smallholder landscapes must be prioritized to avoid further environmental degradation., Small-scale farmers in Southeast Asia are increasingly turning to monocultures of oil palm and rubber to maximize income. Clough and colleagues demonstrate that this land-use change in Indonesia comes at a cost to a wide array of ecosystem functions and biodiversity.

Accurately quantifying changes in soil carbon (C) stocks with land-use change is important for estimating the anthropogenic fluxes of greenhouse gases to the atmosphere and for implementing policies such as REDD (Reducing Emissions from Deforestation and Degradation) that provide financial incentives to reduce carbon dioxide fluxes from deforestation and land degradation. Despite hundreds of field studies and at least a dozen literature reviews, there is still considerable disagreement on the direction and magnitude of changes in soil C stocks with land-use change. We conducted a meta-analysis of studies that quantified changes in soil C stocks with land use in the tropics. Conversion from one land use to another caused significant increases or decreases in soil C stocks for 8 of the 14 transitions examined. For the three land-use transitions with sufficient observations, both the direction and magnitude of the change in soil C pools depended strongly on biophysical factors of mean annual precipitation and dominant soil clay mineralogy. When we compared the distribution of biophysical conditions of the field observations to the area-weighted distribution of those factors in the tropics as a whole or the tropical lands that have undergone conversion, we found that field observations are highly unrepresentative of most tropical landscapes. Because of this geographic bias we strongly caution against extrapolating average values of land-cover change effects on soil C stocks, such as those generated through meta-analysis and literature reviews, to regions that differ in biophysical conditions.

Degraded lands are common in human-influenced tropical semiarid areas, and the potential for C sequestration through rehabilitation of these areas is substantial. In this study, we investigated changes in ecosystem C stocks (ECS) after establishing exclosures on degraded communal grazing lands, and identified easily measurable biophysical and management-related factors that can be used to predict ECS restoration in the highlands of Tigray, Ethiopia. We selected replicated (n = 3) 5-, 10-, 15-, and 20-yr-old exclosures and paired each exclosure with an adjacent communal grazing land. All exclosures displayed higher ECS than the communal grazing lands. Differences in ECS between exclosures and grazing lands varied between 29 (±4.9) and 61 (±6.7) Mg C ha-1 and increased with exclosure duration. In exclosures, much of the variability in ECS was explained by a combination of the following variables: precipitation, clay content, vegetation canopy cover, woody biomass, and exclosure duration (R2 = 0.77–0.90). Precipitation and vegetation canopy cover also explained much of the variability of ECS in communal grazing lands (R2 = 0.48–0.55). Our results help to establish baseline information for C sequestration projects and to predict the expected ecosystem C sequestration under exclosures. Expansion of exclosures would increase grazing pressure on the remaining communal grazing area. Therefore, the decision to establish additional exclosures should also include an economic analysis and an evaluation of the social consequences.

Methane concentrations above tropical forests in the neotropics are high, according to space-borne observations. Flux measurements in the field suggest that tank bromeliads, herbaceous plants common throughout tropical forests, emit methane and may contribute to the tropical source.

Soil respiration is the second largest flux in the global carbon cycle, yet the underlying below-ground process, carbon dioxide (CO2) production, is not well understood because it can not be measured in the field. CO2 production has frequently been calculated from the vertical CO2 diffusive flux divergence, known as "soil-CO2 profile method". This relatively simple model requires knowledge of soil CO2 concentration profiles and soil diffusive properties. Application of the method for a tropical lowland forest soil in Panama gave inconsistent results when using diffusion coefficients (D) calculated based on relationships with soil porosity and moisture ("physically modeled" D). Our objective was to investigate whether these inconsistencies were related to (1) the applied interpolation and solution methods and/or (2) uncertainties in the physically modeled profile of D. First, we show that the calculated CO2 production strongly depends on the function used to interpolate between measured CO2 concentrations. Secondly, using an inverse analysis of the soil-CO2 profile method, we deduce which D would be required to explain the observed CO2 concentrations, assuming the model perception is valid. In the top soil, this inversely modeled D closely resembled the physically modeled D. In the deep soil, however, the inversely modeled D increased sharply while the physically modeled D did not. When imposing a constraint during the fit parameter optimization, a solution could be found where this deviation between the physically and inversely modeled D disappeared. A radon (Rn) mass balance model, in which diffusion was calculated based on the physically modeled or constrained inversely modeled D, simulated observed Rn profiles reasonably well. However, the CO2 concentrations which corresponded to the constrained inversely modeled D were too small compared to the measurements. We suggest that, in well-structured soils, a missing description of steady state CO2 exchange fluxes across water-filled pores causes the soil-CO2 profile method to fail. These fluxes are driven by the different diffusivities in inter- vs. intra-aggregate pores which create permanent CO2 gradients if separated by a "diffusive water barrier". These results corroborate other studies which have shown that the theory to treat gas diffusion as homogeneous process, a precondition for use of the soil-CO2 profile method, is inaccurate for pore networks which exhibit spatial separation between CO2 production and diffusion out of the soil.

Summary 1. Soil nutrients influence plant productivity and community composition in tropical forests. In lower montane tropical forests in western Panama, the distribution of understory palm species over a scale of 1–20 km correlates with differences in soil nitrogen (N). We hypothesized that soil N determines seedling performance in the forest understory, and, may therefore influence species distributions along the soil N gradient. 2. We explored the potential for N availability to generate species-habitat associations through species-specific differences in biomass allocation, photosynthetic capacity, N use-efficiency, and susceptibility to herbivory. Seedlings of nine palm species from two sub-families and four habitat types were transplanted into N-addition and control plots at a low N site. Growth, mortality, biomass allocation, photosynthesis, foliar N content and herbivory were measured over 21 months. 3. Foliar N increased for all species (15–68%) following N addition. Most species showed strong (20–200%) increases in photosynthetic rates with N addition except two species with marginal decreases in photosynthetic rates (5–15%). However, shifts in physiological traits did not increase relative growth rate or change in biomass allocation for any species or N treatment combination. Rather, increased leaf quality contributed to greater levels of herbivory in species associated with soils of intermediate and high inorganic N availability. 4. Thus, potential increases in overall growth with N addition were masked by herbivory, resulting in no apparent growth response with increased N. We suggest that for understory palms, and potentially other montane forest plants, distribution patterns are driven by a combination of physiological and herbivore-mediated responses to soil nutrient availability.

Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but changes in soil-N-cycling processes in tropical ecosystems under elevated N input are less well understood. We used N-addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. Our objectives were to (1) assess changes in soil mineral N production (gross rates of N mineralization and nitrification) and retention (microbial immobilization and rapid reactions to organic N) during 1- and 9-yr N additions in the lowland forest and during 1-yr N addition in the montane forest and (2) relate these changes to N leaching and N-oxide emissions. In the old-growth lowland forest located on an Inceptisol, with high base saturation and net primary production not limited by N, there was no immediate effect of first-year N addition on gross rates of mineral-N production and N-oxide emissions. Changes in soil-N processes were only apparent in chronic (9 yr) N-addition plots: gross N mineralization and nitrification rates, NO3- leaching, and N-oxide emissions increased, while microbial biomass and NH4+ immobilization rates decreased compared to the control. Increased mineral-N production under chronic N addition was paralleled by increased substrate quality (e.g., reduced C:N ratios of litterfall), while the decrease in microbial biomass was possibly due to an increase in soil acidity. An increase in N losses was reflected in the increase in 15N signatures of litterfall under chronic N addition. In contrast, the old-growth montane forest located on an Andisol, with low base saturation and aboveground net primary production limited by N, reacted to first-year N addition with increases in gross rates of mineral-N production, microbial biomass, NO3- leaching, and N-oxide emissions compared to the control. The increased N-oxide emissions were attributed to increased nitrification activity in the organic layer, and the high NO3- availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO3-leaching. These results suggest that soil type, presence of an organic layer, changes in soil-N cycling, and hydrological properties are more important indicators than vegetation as an N sink on how tropical forests respond to elevated N input.

To evaluate N limitation on above-ground net primary production in a tropical lower montane rain forest, an N fertilization experiment was conducted for 2 y. The study site is located at 1200–1300 m asl in the Fortuna forest reserve in western Panama and has a mature, mixed-species stand growing on an Andisol soil. Control and N-fertilized (125 kg urea-N ha−1 y−1) treatments were represented by four replicate plots (each 40 × 40 m, separated by at least 40 m). Stem diameter growth was analysed by diameter at breast height classes and also for the three most abundant species. The three species did not respond to N addition. The response of stem growth and above-ground woody biomass production to N fertilization varied among dbh classes. Stem growth of trees of 10–30 cm dbh increased only in the first year of N addition while trees of 30–50 cm dbh responded in the second year of N addition, which may be due to differences in light conditions between years. Trees >50 cm dbh did not respond during 2 years of N addition. As a result, the overall stem growth and above-ground woody biomass production were not affected by N fertilization. Annual total fine litterfall increased in the first year of N fertilization, while annual leaf litterfall increased in both years of N addition. Above-ground net primary production, of which total fine litterfall constituted 68%, also increased only in the first year of N addition. The magnitude and timing of response of stem diameter growth and litterfall suggest that these aspects of above-ground productivity are not uniformly limited by N availability.

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate-relevant trace gases NO and N2O are expected, but few studies address this possibility. We used N addition experiments to achieve N-enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N-oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long-term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first-time N additions caused rapid increases in soil N-oxide emissions. During the first 2 years of N addition, annual N-oxide emissions were five times (transitory effect) and two times (long-term effect) larger than controls. This contradicts the current assumption that N-limited tropical montane forests will respond to N additions with only small and delayed increases in soil N-oxide emissions. We attribute this fast and large response of soil N-oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first-time N additions caused only gradual and minimal increases in soil N-oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9- and 10-year N-addition plots displayed instantaneous and large soil N-oxide emissions. Annual N-oxide emissions under chronic N addition were seven times (transitory effect) and four times (long-term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N-oxide emissions from the 9- and 10-year N-addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N2O emissions from tropical forests exposed to elevated N deposition.

In a Panamanian lower montane rain forest we: (1) analyzed the vertical and horizontal distribution of fine roots; and (2) assessed the relationship of fine root mass to thickness of the soil organic layer, soil pH, and soil-extractable nitrogen. The soil in the study area has developed on volcanic ash deposits and was classified as Hapludand. In randomly distributed samples, the median fine root mass (biomass and necromass, diam ≤ 2 mm) to a depth of 100 cm mineral soil was 544 g/m2, 41 percent of which was found in the organic layer. Fine root mass was approximately twice as high in the vicinity of stems of the tree species Oreomunnea mexicana (1069 g/m2) and the palm species Colpothrinax aphanopetala (1169 g/m2) and was associated with thick organic layers. The median thickness of the soil organic layer in a larger random sample (N= 64) was 8 cm with a considerable variation (interquartile range: 7 cm). In these samples, the density of fine root biomass was correlated with the concentration of extractable nitrogen (r= 0.33, P= 0.011), and on an areal basis, fine root biomass in the organic layer increased with increasing thickness of the organic layer (r= 0.63, P < 0.001) and decreasing pHKCl (r=−0.33, P < 0.01). Fine root biomass in the upper mineral soil did not show significant correlations with any of the studied parameters. RESUMEN En un bosque panameno bajo montano tropical (1) analizamos la distribucion horizontal y vertical de las raices finas, y (2) evaluamos la relacion de la masa de las raices finas con el espesor de la capa de suelo organico, pH del suelo, y nitrogeno extraible del suelo. El suelo del area de estudio se ha desarrollado en depositos de ceniza volcanica y fue clasificado como Hapludand. En muestras distribuidas aleatoriamente, la media de la masa de raices finas (biomasa y masa necrosada, diametro ≤ 2 mm) a una profundidad de 100 cm del suelo mineral fue 544 g/m2, 41 por ciento de las cuales fueron encontradas en suelo organico. La masa de raices finas fue aproximadamente el doble en la vecindad entre los pies de especies de arboles Oreomunnea mexicana (1069 g/m2) y especies de palmera Colpothrinax aphanopetala (1169 g/m2) y fue asociada con el espesor de capa organica. El espesor mediano de la capa de suelo organico en una amplia muestra aleatoria (N= 64) fue 8 cm con una considerable variacion (intervalo entre cuartilas: 7 cm). En estas muestras, la densidad de raices finas fue correlacionada con la concentracion de nitrogeno extraible (r= 0.33, P= 0.011), y en base al area, la biomasa de raices finas en la capa organica aumento con el incremento del espesor de la capa organica (r= 0.63, P < 0.001) y decrecimiento del pHKCl (r=−0.33, P < 0.01). La biomasa de raices finas en el suelo mineral superior no mostro ninguna significante correlacion con los parametros estudiados.

Measurements of N transformation rates in tropical forest soils are commonly conducted in the laboratory from disturbed or intact soil cores. On four sites with Andisol soils under old-growth forests of Panama and Ecuador, we compared N transformation rates measured from laboratory incubation (at soil temperatures of the sites) of intact soil cores after a period of cold storage (at 5 °C) with measurements conducted in situ. Laboratory measurements from stored soil cores showed lower gross N mineralization and NH 4 + consumption rates and higher gross nitrification and NO 3 − immobilization rates than the in-situ measurements. We conclude that cold storage and laboratory incubation change the soils to such an extent that N cycling rates do not reflect field conditions. The only reliable way to measure N transformation rates of tropical forest soils is in-situ incubation and mineral N extraction in the field.

Indirect evidence of the nitrogen (N) status of tropical forests strongly suggests that in heavily weathered soils under old-growth lowland tropical forests nitrogen is in relative excess. However, within the lowland forests of the Amazon basin, there is substantial evidence that soil texture influences soil NH4+ and NO3− concentrations and hence possibly N availability and retention in the soil. Here, we evaluate the soil N status of two heavily weathered soils which contrast in texture (sandy versus clay Oxisol). Using 15N pool dilution, we quantified gross rates of soil N cycling and retention. We also measured the δ15N signatures from the litter layer down to 50-cm depth mineral soil and calculated the overall 15N enrichment factor (e) for each soil type. The clay soil showed high gross N mineralization and nitrification rates and a high overall 15N enrichment factor, signifying high N losses. The sandy soil had low gross rates of N cycling and 15N enrichment factor, manifesting a conservative soil N cycling. Faster turnover rates of NH4+ compared to NO3− indicated that NH4+ cycles faster through microorganisms than NO3−, possibly contributing to better retention of NH4+ than NO3−. However this was opposite to abiotic retention processes, which showed higher conversion of NO3− to the organic N pool than NH4+. Our combined results suggest that clay Oxisol in Amazonian forest have higher N availability than sandy Oxisol, which will have important consequences for changes in soil N cycling and losses when projected increase in anthropogenic N deposition will occur.

5 Páginas Tropical deforestation for the establishment of tree cash crop plantations causes significant alterations to soil organic carbon (SOC) dynamics. Despite this recognition, the current Intergovernmental Panel on Climate Change (IPCC) tier 1 method has a SOC change factor of 1 (no SOC loss) for conversion of forests to perennial tree crops, because of scarcity of SOC data. In this pantropic study, conducted in active deforestation regions of Indonesia, Cameroon, and Peru, we quantified the impact of forest conversion to oil palm (Elaeis guineensis), rubber (Hevea brasiliensis), and cacao (Theobroma cacao) agroforestry plantations on SOC stocks within 3-m depth in deeply weathered mineral soils. We also investigated the underlying biophysical controls regulating SOC stock changes. Using a space-for-time substitution approach, we compared SOC stocks from paired forests (n = 32) and adjacent plantations (n = 54). Our study showed that deforestation for tree plantations decreased SOC stocks by up to 50%. The key variable that predicted SOC changes across plantations was the amount of SOC present in the forest before conversion—the higher the initial SOC, the higher the loss. Decreases in SOC stocks were most pronounced in the topsoil, although older plantations showed considerable SOC losses below 1-m depth. Our results suggest that (i) the IPCC tier 1 method should be revised from its current SOC change factor of 1 to 0.6 ± 0.1 for oil palm and cacao agroforestry plantations and 0.8 ± 0.3 for rubber plantations in the humid tropics; and (ii) land use management policies should protect natural forests on carbon-rich mineral soils to minimize SOC losses. Study Description. Results. Discussion. Methods. References

Expansion of palm oil and rubber production, for which global demand is increasing, causes rapid deforestation in Sumatra, Indonesia, and is expected to continue in the next decades. Our study aimed to (1) quantify changes in soil CO2 and CH4 fluxes with land-use change and (2) determine their controlling factors. In Jambi Province, Sumatra, we selected two landscapes on heavily weathered soils that differ mainly in texture: loam and clay Acrisol soils. In each landscape, we investigated the reference land-use types (forest and secondary forest with regenerating rubber) and the converted land-use types (rubber, 7–17 years old, and oil palm plantations, 9–16 years old). We measured soil CO2 and CH4 fluxes monthly from December 2012 to December 2013. Annual soil CO2 fluxes from the reference land-use types were correlated with soil fertility: low extractable phosphorus (P) coincided with high annual CO2 fluxes from the loam Acrisol soil that had lower fertility than the clay Acrisol soil (P < 0.05). Soil CO2 fluxes from the oil palm (107.2 to 115.7 mg C m−2 h−1) decreased compared to the other land-use types (between 178.7 and 195.9 mg C m−2 h−1; P < 0.01). Across land-use types, annual CO2 fluxes were positively correlated with soil organic carbon (C) and negatively correlated with 15N signatures, extractable P and base saturation. This suggests that the reduced soil CO2 fluxes from oil palm were the result of strongly decomposed soil organic matter and reduced soil C stocks due to reduced litter input as well as being due to a possible reduction in C allocation to roots due to improved soil fertility from liming and P fertilization in these plantations. Soil CH4 uptake in the reference land-use types was negatively correlated with net nitrogen (N) mineralization and soil mineral N, suggesting N limitation of CH4 uptake, and positively correlated with exchangeable aluminum (Al), indicating a decrease in methanotrophic activity at high Al saturation. Reduction in soil CH4 uptake in the converted land-use types (ranging from −3.0 to −14.9 μg C m−2 h−1) compared to the reference land-use types (ranging from −20.8 to −40.3 μg C m−2 h−1; P < 0.01) was due to a decrease in soil N availability in the converted land-use types. Our study shows for the first time that differences in soil fertility control the soil–atmosphere exchange of CO2 and CH4 in a tropical landscape, a mechanism that we were able to detect by conducting this study on the landscape scale.

The lower montane forest zone of Indonesia is undergoing rapid conversion of indigenous forests to agriculture. In this tropical region, however, the effects of forest conversion on soil N processes have not been investigated. Corn (Zea mays L.) and cacao (Theobroma cacao L.)–coffee (Coffea canephora Pierre ex Froehner) agroforestry are the main land use types in cleared lower montane forests in Central Sulawesi, Indonesia. Our main objective was to compare the soil N dynamics under agroforest systems and corn cultivation with indigenous forest. We measured the gross rates of N transformation processes using 15 N pool dilution. The agroforest systems and indigenous forests had higher gross N mineralization rates and faster turnover rates of NH4 1 and microbial N pools than the long-term cultivated corn sites. Faster soil N turnover rates in agroforest systems suggest a more dynamic soil N cycling. Leguminous shade trees, which are important components of these agroforest systems, may have influenced the fast microbial N cycling through release of N-rich root exudates and plant residues. Our results show that compared with corn, agroforestry is a better option in terms of sustainability in the N-supplying capacity of the soil. In addition, we measured higher 15 NH4 1 recoveries than 15 NO3 2 recoveries after 15 min of 15 N addition in all our sites. Our measured rates of gross nitrification were very low to negligible, due to rapid disappearance of added 15 NO3 2 . Such fast reaction of NO3 2 warrants further investigation, especially in tropical areas where 15 N studies are very few.

An ongoing roof experiment, where N and acid inputs were reduced to the recommended critical load levels, has been conducted since 1991 in an N-saturated spruce stand in Solling, Germany. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling under ambient and reduced N conditions, and (2) relating the soil N dynamics to the changes in N leaching and N status of trees. Two roofs were used, one to achieve “ambient” and the other reduced (“clean rain”) inputs, with a roofless plot as a control for possible roof effects. In 2001, the ambient roof and ambient no-roof plots showed an apparent decrease in gross N mineralization rates and significantly lower microbial NH4+ immobilization rates and turnover rates of NH4+ and microbial N pools. The microbial NO3− immobilization rates and NO3− pool turnover rates were lower than the microbial NH4+ immobilization rates and NH4+ pool turnover rates, showing that less NO3− cycled through microorganisms than NH4+. There was also low abiotic NO3− immobilization. High NO3− input from throughfall and low microbial turnover rates of the NO3− pool, combined with low abiotic NO3− retention, may have contributed to the high NO3− leaching losses in these ambient plots. The clean rain plot showed a slight increase in gross N mineralization rates and significantly higher microbial NH4+ immobilization rates and turnover rates of NH4+ and microbial N pools. Neither nitrification nor soil NO3− was detectable. There was an increase in abiotic NO3− immobilization. Foliar N concentration had decreased but was still adequate. An efficient cycling of NH4+ through microorganisms, combined with the high abiotic NO3− immobilization, indicated efficient mineral N retention in the clean rain plot. These results indicated that long-term reduction of throughfall N and acid inputs had induced high but tightly coupled microbial NH4+ cycling and an increase in abiotic NO3− retention, which contributed to the reversal of N saturation.

This study focuses on the microbial N cycle in the acid soil of a beech forest that falls in the upper range of the N saturation continuum. Our objectives were: (1) to quantify microbial N cycling under long-term N-saturated and limed conditions and (2) to determine the factors controlling the differences in microbial N cycling. Our study site has a long history of high N deposition: ?25 kg N-ha- Iyr-' since measurements began in 1971. This was further enhanced by 11 yr (1983-1993) of fertilization (140 kg ammonium sulfate- N-ha-I-yr-') to create an N saturation plot. Another plot was limed with 30 Mg/ha dolomitic limestone in 1982. In 1999-2000, gross rates of microbial N cycling were measured using '5N pool dilution techniques. Despite the chronic high N deposition, the control plot showed a tightly coupled microbial N cycle; NH4 and NO3- immobilization rates were comparable to gross N mineralization and nitrification rates, respectively. These were supported by low levels of NH4+, NO3-, and dissolved organic N (DON) in percolate. Liming increased gross N mineralization and nitrification rates but did not cause similar increases in microbial biomass or NH4 +, and NO~- immobilization rates. In addition, NO3- immobilization rates were somewhat less than gross nitrification rates; relatively high levels of NO3- and DON in percolate were also observed. The N-saturated plot suggested an uncoupled microbial N cycle; NH4+ immobilization rates were lower than gross N mineralization rates, and NO3- immobilization rates were somewhat less than gross nitrification rates. These were corrob- orated by high levels of NH4+, NO3-, and DON in percolate. The reduced NH4+ and N03- immobilization rates in the N-saturated plot could be attributed to the measured decreases in microbial biomass, and the low microbial biomass was likely due to decreases in the supply of labile C. Our study demonstrates that while hydrological N input/output budgets can indicate whether or not a forest ecosystem is in a state of N saturation, the microbial N cycle can provide quantitative information on key processes that govern N losses.

Measured data from two experimental sites in Canada were used to test the ability of the DeNitrification and DeComposition model (DNDC) to predict N2O emissions from agricultural soils. The two sites, one from eastern Canada, and one from western Canada, provided a variety of crops, management practices, soils, and climates for testing the model. At the site in eastern Canada, the magnitude of total seasonal N2O flux from the seven treatments was accurately predicted with a slight average over-prediction (ARE) of 3% and a coefficient of variation of 41%. Nitrous oxide emissions based on International Panel for Climate Change (IPCC) methodology had a relative error of 62% for the seven treatments. The DNDC estimates of total yearly emissions of N2O from the field site in western Canada showed an underestimation of 8% for the footslope landscape position and an overestimation of 46% for the shoulder position. The data input for the DNDC model were not of sufficient detail to characterize the moisture difference between the landscape positions. The estimates from IPCC guidelines showed an underestimation of 54% for the footslope and an overestimation of 161% for the shoulder. The results indicate that the DNDC model was more accurate than IPCC methodology at estimating N2O emissions at both sites. Key words: Nitrous oxide, DNDC, soil model, greenhouse gas, testing

Understanding how N-cycling processes in unmanaged grassland vary spatially and seasonally would aid the development of management strategies which capitalize on the behavior of its N cycle, in order to manage N losses when such system is altered. Our objectives were: (1) to quantify gross rates of internal N-cycling processes (i.e. mineralization, nitrification, and immobilization) from an unmanaged grassland, and (2) to investigate the role of topography and climatic factors on the spatial and seasonal variation of these processes. We delineated our study site into three topographic units based on soil and drainage types: upper, lower, and drained lower slopes. The dynamics of NH 4 + in the internal N cycle was influenced by the spatial and seasonal variation of soil microbial biomass, in which the spatial variation resulted from long-term topographic influence and the seasonal variation from seasonal flushes of available organic matter. The drained lower slope had the highest microbial biomass (647 mg C kg −1 and 90 mg N kg −1 ), gross N mineralization (8 mg N kg −1 d −1 ), NH 4 + immobilization (6 mg N kg −1 d −1 ), and fastest NH 4 + turnover (0.5 d), indicating that the drainage favored microbial growth and activity. The seasonal pattern of NH 4 + transformations and microbial biomass showed that the increases in microbial N (in fall and late spring towards summer) were paralleled by high NH 4 + immobilization and a decrease in the microbial C–N ratio. Spatial variation of NO 3 − transformations showed the effect of topography through water redistribution; higher gross nitrification was observed in the upper (1.7 mg N kg −1 d −1 ) than lower slopes (1.1 mg N kg −1 d −1 ), with drainage also favoring gross nitrification (1.4 mg N kg −1 d −1 ) through improved soil aeration. The seasonal pattern of gross nitrification was related to soil moisture ( r =−0.79, P ≤0.01) and temperature ( r =0.55, P ≤0.05). Gross nitrification accounted 32% of the NH 4 + produced. NO 3 − immobilization was about 50% of NH 4 + immobilization. NO 3 − immobilization was highest when microbial immobilization was most favorable and available NH 4 + was insufficient to meet microbial demand. NO 3 − was rapidly produced and consumed (0.7 d average turnover time), and hence its usually small pool size in unmanaged grassland cannot be used as basis to dismiss its importance in the internal N cycle. Our study supported the concept of considering both the spatial and seasonal variation of soil biochemical processes in refining nutrient management strategies in an ecosystem.

Although the canopy can play an important role in forest nutrient cycles, canopy-based processes are often overlooked in studies on nutrient deposition. In areas of nitrogen (N) and phosphorus (P) deposition, canopy soils may retain a significant proportion of atmospheric inputs, and also receive indirect enrichment through root uptake followed by throughfall or recycling of plant litter in the canopy. We measured net and gross rates of N cycling in canopy soils of tropical montane forests along an elevation gradient and assessed indirect effects of elevated nutrient inputs to the forest floor. Net N cycling rates were measured using the buried bag method. Gross N cycling rates were measured using (15) N pool dilution techniques. Measurements took place in the field, in the wet and dry season, using intact cores of canopy soil from three elevations (1000, 2000 and 3000 m). The forest floor had been fertilized biannually with moderate amounts of N and P for 4 years; treatments included control, N, P, and N + P. In control plots, gross rates of NH4 (+) transformations decreased with increasing elevation; gross rates of NO3 (-) transformations did not exhibit a clear elevation trend, but were significantly affected by season. Nutrient-addition effects were different at each elevation, but combined N + P generally increased N cycling rates at all elevations. Results showed that canopy soils could be a significant N source for epiphytes as well as contributing up to 23% of total (canopy + forest floor) mineral N production in our forests. In contrast to theories that canopy soils are decoupled from nutrient cycling in forest floor soil, N cycling in our canopy soils was sensitive to slight changes in forest floor nutrient availability. Long-term atmospheric N and P deposition may lead to increased N cycling, but also increased mineral N losses from the canopy soil system.

Landscape-scale approaches to research in soil science are explicitly focused on transfers of components within and between landscapes. Despite wide-spread recognition of the importance of these transfers, the application of landscape-scale approaches has been hindered by the lack of clear, reproducible research designs. Landform segmentation is used to divide natural and human-influenced landscapes into functionally distinct units. A specific type of landform segmentation, landform element classification, was used in a comparative mensurative design to compare the effects of cultivation on soil distribution and soil organic carbon (SOC) storage and in a manipulative design to determine the relationship between N2O emissions and fertilizer rate in a hummocky till geomorphic surface in southern Saskatchewan. Significant transfers of SOC and surface soil from convex shoulder units to lower slope positions occurred over the past 90 years, resulting in a change in the type of soils that occupy these positions at two research sites. The observed pattern is consistent with a tillage translocation dominated surface. The dominant control on N2O emissions in the landscape are spatial differences in water-filled pore space (WFPS) that are strongly controlled by water redistribution. Emissions from drier, shoulder landform element complexes are consistently low throughout the year, whereas a strong positive relationship between N fertilizer rate and N2O emissions occur in the wettest, level depressional elements.