Karin Potthast

Land-use and soil depth affect resource and microbial stoichiometry in a tropical mountain rainforest region of southern Ecuador

Abstract Global change phenomena, such as forest disturbance and land-use change, significantly affect elemental balances as well as the structure and function of terrestrial ecosystems. However, the importance of shifts in soil nutrient stoichiometry for the regulation of belowground biota and soil food webs have not been intensively studied for tropical ecosystems. In the present account, we examine the effects of land-use change and soil depth on soil and microbial stoichiometry along a land-use sequence (natural forest, pastures of different ages, secondary succession) in the tropical mountain rainforest region of southern Ecuador. Furthermore, we analyzed (PLFA-method) whether shifts in the microbial community structure were related to alterations in soil and microbial stoichiometry. Soil and microbial stoichiometry were affected by both land-use change and soil depth. After forest disturbance, significant decreases of soil C:N:P ratios at the pastures were followed by increases during secondary succession. Microbial C:N ratios varied slightly in response to land-use change, whereas no fixed microbial C:P and N:P ratios were observed. Shifts in microbial community composition were associated with soil and microbial stoichiometry. Strong positive relationships between PLFA-markers 18:2n6,9c (saprotrophic fungi) and 20:4 (animals) and negative associations between 20:4 and microbial N:P point to land-use change affecting the structure of soil food webs. Significant deviations from global soil and microbial C:N:P ratios indicated a major force of land-use change to alter stoichiometric relationships and to structure biological systems. Our results support the idea that soil biotic communities are stoichiometrically flexible in order to adapt to alterations in resource stoichiometry.

Fire enhances phosphorus availability in topsoils depending on binding properties

Fire can have large effects on ecosystems, with phosphorus being highly important in this regard, especially when considering productivity in burned or adjacent ecosystems after fire. Unfortunately, existing data pose contradictions and methodological challenges to assessing P availability. Here, the impact of fire on topsoil available P was estimated using three different types of organic soil layers (raw humus from spruce, humus from beech, and peat) and two fire regimes (obtaining black carbon and ash). Our findings suggest a strong fire impact on P availability, by enhancing available P during burning to black carbon (~10-fold), and to ash (~2- to 7.5-fold) dependent on calcite content. Fire impact on P availability is on a similar order of magnitude as mineral weathering and annual P cycling/uptake in/by ecosystems. Furthermore, the proportion of available P in relation to total P depends on the origin of the organic soil layers. When related to the remaining mass after burning, as is commonly done, ...

Current Regulating and Supporting Services: Nutrient Cycles

The study forest regulates nutrient cycles as a supporting ecosystem service mainly via retention in the biosphere and the soil organic layer. How tight the nutrient cycles are depends on environmental conditions. In this chapter, we focus on the roles of (1) deposition from the atmosphere, (2) soil moisture regime, and (3) conversion to pasture in the nutrient cycle. Between 1998 and 2010, there were a seasonal deposition of salpetric acid, an episodic deposition of Ca and Mg from Sahara dusts, and a continuous increase in reactive N inputs related to Amazonian forest fires, the El Nino Southern Oscillation cycle, and the economic development, respectively. Simultaneously, soils became increasingly drier enhancing nutrient release by mineralization. An increasing number of rain storms could considerably increase the export of N and base metals (K, Ca, Mg) via fast surface-near lateral transport in soil. Land-use change from forest to pasture introduces alkaline ashes and grass-derived organic matter. The resulting increases in soil pH and nutrient and substrate supply increase nutrient cycling rates because of enhanced microbial activity.

Land-use and fire drive temporal patterns of soil solution chemistry and nutrient fluxes

Land-use type and ecosystem disturbances are important drivers for element cycling and bear the potential to modulate soil processes and hence ecosystem functions. To better understand the effect of such drivers on the magnitude and temporal patterns of organic matter (OM) and associated nutrient fluxes in soils, continuous flux monitoring is indispensable but insufficiently studied yet. We conducted a field study to elucidate the impact of land-use and surface fires on OM and nutrient fluxes with soil solution regarding seasonal and temporal patterns analyzing short (<3months) and medium-term (3-12months) effects. Control and prescribed fire-treated topsoil horizons in beech forests and pastures were monitored biweekly for dissolved and particulate OM (DOM, POM) and solution chemistry (pH value, elements: Ca, Mg, Na, K, Al, Fe, Mn, P, S, Si) over one post-fire year. Linear mixed model analyses exhibited that mean annual DOM and POM fluxes did not differ between the two land-use types, but were subjected to strong seasonal patterns. Fire disturbance significantly lowered the annual soil solution pH in both land-uses and increased water fluxes, while DOC fluxes remained unaffected. A positive response of POC and S to fire was limited to short-term effects, while amplified particulate and dissolved nitrogen fluxes were observed in the longer run and co-ocurred with accelerated Ca and Mg fluxes. In summary, surface fires generated stronger effects on element fluxes than the land-use. Fire-induced increases in POM fluxes suggest that the particulate fraction represent a major pathway of OM translocation into the subsoil and beyond. With regard to ecosystem functions, pasture ecosystems were less prone to the risk of nutrient losses following fire events than the forest. In pastures, fire-induced base cation export may accelerate soil acidification, consequently exhausting soil buffer systems and thus may reduce the resilience to acidic depositions and disturbances.

Current Provisioning Services : Pasture Development and Use, Weeds (Bracken) and Management

This chapter reports on the historical expansion and current state of the pastures in the Rio San Francisco valley. Its major part is inhabited by the Mestizos, who do not have a long-standing pasture tradition. Three types of pastures were identified by the dominant grass species: the “pastos azules” (Holcus lanatus), the Yaragua pastures (Melinis minutiflora) and the dominating “pastos mieles” (Setaria sphacelata). The peculiarities, species composition, soil dynamics and agricultural values of these pastures are discussed. Except for the pastos azules on small flattenings in the otherwise steep slopes of the valley, pastures in the area suffer from invasion by aggressive weeds, mainly the tropical bracken fern. Abandonment of pastures is fostered by the use of fire to combat weeds and stimulate grass growth. This type of low-yield pasture farming is not sustainable. The earnings of livestock farming are not sufficient for subsistence. Diversification of the income portfolio is necessary.

Nutrient Additions Affecting Matter Turnover in Forest and Pasture Ecosystems

Nutrient inputs into ecosystems of the tropical mountain rainforest region are projected to further increase in the next decades. To investigate whether important ecosystem services such as nutrient cycling and matter turnover in native forests and pasture ecosystems show different patterns of response, two nutrient addition experiments have been established: NUMEX in the forest and FERPAST at the pasture. Both ecosystems already responded 1.5 years after the start of nutrient application (N, P, NP, Ca). Interestingly, most nutrients remained in the respective systems. While the pasture grass was co-limited by N and P, most tree species responded to P addition. Soil microbial biomass in the forest litter layer increased after NP fertilization pointing to nutrient co-limitation. In pasture soils, microorganisms were neither limited by N nor P. The results support the hypothesis that multiple and temporally variable nutrient limitations can coexist in tropical ecosystems.

Tracing the fate and transport of secondary plant metabolites in a laboratory mesocosm experiment by employing mass spectrometric imaging

AbstractMass spectrometric imaging (MSI) has received considerable attention in recent years, since it allows the molecular mapping of various compound classes, such as proteins, peptides, glycans, secondary metabolites, lipids, and drugs in animal, human, or plant tissue sections. In the present study, the application of laser-based MSI analysis of secondary plant metabolites to monitor their transport from the grass leaves of Dactylis glomerata, over the crop of the grasshopper Chorthippus dorsatus to its excrements, and finally in the soil solution is described. This plant-herbivore-soil pathway was investigated under controlled conditions by using laboratory mesocosms. From six targeted secondary plant metabolites (dehydroquinic acid, quinic acid, apigenin, luteolin, tricin, and rosmarinic acid), only quinic acid, and dehydroquinic acid, an in-source-decay (ISD) product of quinic acid, could be traced in nearly all compartments. The tentative identification of secondary plant metabolites was performed by MS/MS analysis of methanol extracts prepared from the investigated compartments, in both the positive and negative ion mode, and subsequently compared with the results generated from the reference standards. Except for tricin, all secondary metabolites could be tentatively identified by this approach. Additional liquid-chromatography mass spectrometry (LC-MS) experiments were carried out to verify the MSI results and revealed the presence of quinic acid only in grass and chewed grass, whereas apigenin-hexoside-pentoside and luteolin-hexoisde-pentoside could be traced in the grasshopper body and excrement extracts. In summary, the MSI technique shows a trade-off between sensitivity and spatial resolution. Graphical abstractMonitoring quinic acid in a mesocosm experiment by mass spectrometric imaging (MSI).

Future Provisioning Services: Repasturisation of Abandoned Pastures, Problems, and Pasture Management

More and more pastures in the Rio San Francisco valley were and still are abandoned as a result of ecologically unbalanced pasture management, which promotes the invasion of weeds like bracken. Under the common pasture management, using fire as an agricultural tool, bracken by virtue of several ecophysiological traits can outcompete the grass. Competition of both species was investigated by the growth model SoBraCoMo. Vegetation development after burning of a bracken-infected pasture was followed by automated monitoring, using a balloon. To rehabilitate abandoned pastures, a three-step experiment was performed. Bracken control was followed by planting of the pasture grass Setaria sphacelata. Subsequently, different strategies for pasture management were examined. Fertilisation was crucial for the achievement of reasonable yields as well as for bracken suppression. Additionally, the prevention of negative nutrient balances of active pastures was investigated in an extended pasture fertilisation experiment (FERPAST). A specific combination of N and P is necessary to maintain soil productivity and to increase fodder quality.

Spatiotemporal variation of aluminium and micro- and macronutrients in the soil solution of a coniferous forest after low-intensity prescribed surface fires

Even though the functioning of nutrient-poor forest ecosystems strongly depends on the cycling of various elements, rather little is known about the effects of fires on the fluxes of Al, Ca, Fe, K, Mg, Mn, Na, P and S. Solution fluxes at three different soil depths (organic (O) layer, upper mineral soil (A) and lower mineral soil (B) horizon) were measured every 2 weeks with free-draining lysimeters before and after low-intensity prescribed surface fires in a Scots pine forest in Germany. Measurements of element content in pre-fire litterfall and soil were also conducted. Linear mixed-effect modelling revealed that low-intensity fires caused a short-term (<3 months) increase of element fluxes from the O layer and a medium-term (3–8 months) increase from the A horizon. This solute flush was followed by retention processes in the B horizon, except for S, Ca and Mg, which were removed from the soil system, probably because anion exchange sites favoured dissolved organic carbon over SO42−, and because Ca2+ and Mg2+ partially maintained the charge balance. Our findings indicated that fires affected nutrient-poor soil systems by causing a short-and medium-term element translocation from the O layer into the B horizon, which functioned as a retaining soil horizon by reducing the losses of important elements.

Proteome data on the microbial microbiome of grasshopper feces

We present proteome data from the microbiota (feces) after a diet shift from a natural diverse to a monocultural meadow with Dactylis glomerata. The abundant grasshopper species, Chorthippus dorsatus, was taken from the wild and kept in captivity and were fed with Dactylis glomerata for five days. For phytophagous insects, the efficiency of utilization of hemicellulose and cellulose depends on the gut microbiota. Shifts in environmental and management conditions alter the presence and abundance of plant species which may induce adaptations in the diversity of gut microbiota. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD005126.