Intact Pasture Research Articles

Complex dissolved organic matter (DOM) on the roof of the world – Tibetan DOM molecular characteristics indicate sources, land use effects, and processing along the fluvial–limnic continuum

Abstract. The Tibetan Plateau (TP) is the world's largest and highest plateau, comprising the earth's biggest alpine pasture system. It is sensitive to the impacts of climate change and anthropogenic pressure. Carbon cycling on the TP is influenced by glaciation and degradation of the pasture ecosystem. Dissolved organic matter (DOM) connects carbon reservoirs, following the hydrological continuum from glaciers and headwaters to lakes. Due to its complexity, DOM cycling along the aquatic continuum and the impact of land use and climate change on DOM characteristics are still not well understood. Here, we study solid phase extracted (SPE) DOM molecular characteristics using ultrahigh-resolution mass spectrometry (FT-ICR-MS) along the TP hydrological continuum from glaciers, groundwater springs, and wetlands, including pastures and alpine steppes, to the endorheic Lake Nam Co. Our study revealed that the SPE-DOM composition was largely influenced by local sources of glaciers, wetlands, and groundwater springs as well as pasture degradation. Glacial meltwater SPE-DOM contained more saturated compounds suggesting microbial sources together with aromatic compounds probably derived from aeolian deposition. In comparison, wetland and stream SPE-DOM were characterised by a higher percentage of highly unsaturated and aromatic molecular formulae. These were likely derived from inputs of vascular plants and soils. Groundwater spring SPE-DOM from degraded pastures differed from intact pasture samples. In degraded systems a strongly oxidised signature with the lowest counts of P heteroatoms, a lower O/C ratio, and a higher aromaticity of SPE-DOM together with a high degradation index suggested a strong transformation of SPE-DOM. SPE-DOM of the endorheic lake was richer in unsaturated molecular formulae compared to the tributaries. This suggests algae and microbial sources and production in the lake. The SPE-DOM rich in aromatic and highly unsaturated formulae visible in the brackish zone of the lake shore contrasted sharply with that of the lake. Aromatic molecular formulae were strongly depleted in the lake deep water suggesting photooxidation of riverine SPE-DOM. This indicates that alpine SPE-DOM signatures are shaped by small-scale catchment properties, land degradation, and the influence of glaciers and wetlands. The close link of alpine SPE-DOM composition to landscape properties is indicative of a strong susceptibility of DOM characteristics to climatic and land use changes in High Asia.

Soil‐Gas Diffusivity and Soil‐Moisture effects on N2O Emissions from Intact Pasture Soils

Grazed pastures are recognized as a dominant source of nitrous oxide (N2O), a highly potent greenhouse gas. Studies have examined soil physical controls on N2O emissions, including soil moisture status. Limited attempts to link N2O emissions with soil‐diffusivity (Dp/Do), using repacked soil cores, have shown peak N2O emissions to align with a relatively narrow window ofDp/Do, despite a relatively wide range in water‐filled pore space (WFPS), across a range of soil bulk densities. Such detailed studies have not been performed withintactsoil cores. We investigated the effects of soil‐water characteristic (SWC) andDp/Doon N2O emissions from intact soil samples, retrieved at three depths (0–5, 5–10, 10–15 cm) from three perennial pasture sites that received a KNO3solution (1800 mg, N mL−1). We observed distinct fingerprints of SWC andDp/Do, which showed clear effects of soil structure on diffusion‐controlled gas emissions. Depth‐wise variation in soil moisture diminished as the soil was subjected to higher matric potential (> ∼ ‐100 kPa). Variation inDp/Do, was more pronounced in the dry soil (> ∼ ‐1000 kPa), being largely constrained by soil moisture in wet soil (∼ ‐100 kPa) with little depth‐wise variation. Measured N2O fluxes peaked within narrow ranges of WFPS andDp/Do, 0.90–0.95 and 0.005–0.01, respectively. The value ofDp/Docan be determined using parametric models and presents a pasture management (e.g., irrigation, soil physical disturbance such as pasture renovation and animal treading)) tool to minimize N2O emissions: soilDp/Doshould be maintained above a range of 0.005–0.01 to minimize N2O emissions.Core IdeasPeak N2O fluxes fromintactsoil cores occurred when diffusivity ranged from 0.005–0.01This peak N2O flux diffusivity range was 0.005–0.01 regardless of soil or depth (0–15 cm)The diffusivity range for peak N2O flux equalled that observed inrepackedsoil coresDiffusivity values are readily determined and add to the suite of soil management tools

Sustained functional composition of pollinators in restored pastures despite slow functional restoration of plants.

Habitat restoration is a key measure to counteract negative impacts on biodiversity from habitat loss and fragmentation. To assess success in restoring not only biodiversity, but also functionality of communities, we should take into account the re‐assembly of species trait composition across taxa. Attaining such functional restoration would depend on the landscape context, vegetation structure, and time since restoration. We assessed how trait composition of plant and pollinator (bee and hoverfly) communities differ between abandoned, restored (formerly abandoned) or continuously grazed (intact) semi‐natural pastures. In restored pastures, we also explored trait composition in relation to landscape context, vegetation structure, and pasture management history. Abandoned pastures differed from intact and restored pastures in trait composition of plant communities, and as expected, had lower abundances of species with traits associated with grazing adaptations. Further, plant trait composition in restored pastures became increasingly similar to that in intact pastures with increasing time since restoration. On the contrary, the trait composition of pollinator communities in both abandoned and restored pastures remained similar to intact pastures. The trait composition for both bees and hoverflies was influenced by flower abundance and, for bees, by connectivity to other intact grasslands in the landscape. The divergent responses across organism groups appeared to be mainly related to the limited dispersal ability and long individual life span in plants, the high mobility of pollinators, and the dependency of semi‐natural habitat for bees. Our results, encompassing restoration effects on trait composition for multiple taxa along a gradient in both time (time since restoration) and space (connectivity), reveal how interacting communities of plants and pollinators are shaped by different trait–environmental relationships. Complete functional restoration of pastures needs for more detailed assessments of both plants dispersal in time and of resources available within pollinator dispersal range.

Grass competition surpasses the effect of defoliation on a woody plant invader

Woody encroachment in grasslands represents a global phenomenon with strong consequences on ecosystem functioning. While the causes triggering this process can be multiple, there is consensus on the fact that anthropogenic activities play a central role in woody expansion. In particular, the loss of grass cover increases the chances of woody invasion, whereas the role of defoliation is less known. In this study our objective was to assess the simultaneous effect of competition generated by resident vegetation and woody seedling defoliation on the growth and survival of Gleditsia triacanthos seedlings, a woody invader in Argentina. We established a factorial pot experiment with two main factors: Gleditsia defoliation (2 levels: with and without defoliation) and pasture competition (3 levels: without pasture, clipped pasture and intact pasture). Our results showed that pasture competition reduced Gleditsia survival and tree growth, but that the effect of tree defoliation on tree growth depended on the magnitude of pasture competition. More widely, our results stress the existence of a hierarchy order of factors controlling Gleditsia establishment (survival + growth): grass competition was the main control and tree defoliation became important only in the absence of competition. This evidence suggests that maintaining a competitive grass cover along with a frequency of tree defoliation could diminish tree establishment in herbaceous communities.

Recovery of plant diversity in restored semi‐natural pastures depends on adjacent land use

AbstractQuestionsDoes restoration success of formerly abandoned semi‐natural pastures depend on adjacent land use? Is species richness higher in restored pastures adjacent to an intact semi‐natural pasture than in restored pastures adjacent to arable land? Does community similarity between a restored and an adjacent intact pasture decrease with distance from the border between the two pastures? Do differences in species richness and community similarity decrease over time?LocationAgricultural landscapes in south‐central Sweden.MethodsThe plant community in previously abandoned but now restored semi‐natural pastures was surveyed along a distance gradient from the border between the restored pastures and adjacent fields towards the centre of the pastures. The restored pastures were located adjacent to either a crop field (N = 8) or a continuously grazed pasture (N = 6), and differed in time since restoration (1–13 yr).ResultsThe total species richness was higher in pastures adjoining continuously grazed pastures compared to crop fields. Richness of both total and specialist species increased with time since restoration. Irrespective of adjacent land use, richness of specialist species decreased with increasing distance from the edge, an effect that became weaker with increasing time since restoration. The similarity in species composition compared to that in adjacent continuously grazed pasture also decreased towards the centre of the restored pasture.ConclusionsOur results suggest that restoration of biodiversity in semi‐natural pastures benefits from adjacent pastures that can act as source habitats. The most likely mechanism is step‐wise short‐distance dispersal, but also other processes, such as more long‐distance dispersal, seed bank dynamics and historical legacies are probably involved. To best succeed in habitat restoration in fragmented landscapes, the spatial location of source populations must be considered.

Population growth of Nassella trichotoma in grasslands in New Zealand slower today than in the past

Nassella trichotoma established in modified tussock-grasslands in New Zealand from about 1860. Management programmes since 1946 have reduced populations to levels no longer impacting pastoral production. Optimising future management requires knowledge of the trajectory of population growth and its regulating demographic processes. To that end, four long-term field experiments were conducted. Net reproductive rate varied from 1.021 to 1.237 year −1 and growth in plant basal diameter from 8.1 to 16.6 mm year −1. The probability of flowering increased with basal diameter and was essentially unity above 50 mm diameter. Populations grubbed annually declined abruptly but recruitment was unaffected and extinction did not occur. Of seeds sown into disturbed and intact pastures, 0–51% produced seedlings and more arose on sunny slopes and from disturbed than intact pasture. Death rates were high; 7 years after sowing, surviving plants represented only 0–9% of the seed sown. Seeds buried 25 mm deep in the pasture litter on two occasions declined in viability at rates of 74 and 89% in the first year and first three months respectively and 26 and 0% year −1 thereafter. Seed production plant −1 (square root scale) increased linearly with plant basal diameter; for example, plants of 11 and 100 mm diameter are predicted to produce 0 and 11,092 spikelets (each with one seed) respectively. We estimate that a N. trichotoma population today will, in the absence of management, take 210 years to increase to 90% of its carrying capacity supporting the hypothesis that population growth in this species is slower than occurred historically. We show that the rates of some demographic processes may be much lower than in the past and suggest this is due to more competitive vegetation resulting from improved management. The size-dependence of many processes supports the need for a size-structured model to explain population growth in this weed.

Respiration from soil and litter in a sown perennial grass pasture

The severe nitrogen (N) deficiency which occurs in many sown grass pastures in Queensland is believed to be exacerbated by large and continuous inputs of carbon (C) from decomposing plant residues. In this study we attempted to quantify the importance of surface litter, roots and soil organic matter as sources of respiration in an established green panic (Panicum maximum var. trichoglume) pasture in south-east and Queensland. Intact pasture cores were taken from the field and the surface litter was removed before applying the following treatments: (i) surface litter added, (ii) roots pruned to kill approximately 60% of roots but not kill the plant, and (iii) plant shoots removed. Cores from bare soil between green panic plants were also included. The cores were kept in a glasshouse and CO2 evolution measured continuously for 117 days using an alkali absorption method. Respiration from the various components of the system was estimated. Evolution of CO2 from the cores was increased by litter addition and decreased by shoot removal. Root pruning stimulated CO2 evolution in litter-removed treatments but had no effect in litter-added treatments. Root respiration and microbial respiration of root-derived C accounted for an average of 53% of the total evolved CO2. Surface litter, soil organic matter and dead roots accounted for an average of 40%, 4% and 3% respectively. The importance of a particular C source to microorganisms varied depending on the availability of other C sources. Cores were destructively sampled on five occasions and the soils incubated at 25�C for 10 days to measure CO2 evolution with surface litter and roots removed. Evolution of CO2 in incubated soils was increased by litter and, to a lesser extent, by live roots, demonstrating that some of the labile C from these plant components was soluble or well incorporated into the soil.

Effects of modifying plant carbon inputs on nitrogen distribution in intact cores of a perennial grass pasture

Perennial pastures can accumulate large quantities of roots and surface litter of high C:N ratio, which may reduce N availability to the plant by stimulating microbial immobilization. We studied the effects of modifying carbon inputs from roots and litter on the distribution of nitrogen (N) in plant and soil fractions of an old N-deficient green panic (Panicum maximum var. trichoglume) pasture. Intact pasture cores were taken from the field to a glasshouse, and the surface litter was removed before applying the following treatments: (i) surface litter added, (ii) roots pruned to kill approximately 60% of roots, and (iii) plant shoots removed. A small pulse of 15N as ammonium sulfate was added to the soil surface, and the cores were destructively sampled on several occasions over the following 4 months. Litter addition had little effect on N uptake by uncut plants. When plant shoots were removed, litter markedly reduced plant N uptake. Litter increased N and 15N in microbial biomass and N and 15N stabilized in non-biomass soil organic matter, and reduced loss of N from the cores. Root pruning had little effect on N distribution, except for an initial reduction in plant uptake. Removal of pasture shoots markedly increased soil nitrate and loss of 15N, and decreased non-biomass organic N and 15N. Recovery of 15N in non-biomass organic matter was around three times greater than 15N in microbial biomass, and was closely associated with microbial CO2 production. There was evidence that 15N entered the non-biomass organic matter by both abiotic and microbially mediated processes. In these pastures, the non-biomass soil organic matter may be a more important sink for N than the microbial biomass.

The influence of sward mass, defoliation and watering on ammonia volatilization losses from an Italian ryegrass sward topdressed with urea

Using intact pasture sods with 0.15m2 surface area and sealed in volatilization chambers, the influence of the mass of herbage on the ammonia volatilization losses following a surface application of urea was determined. Ammonia volatilization loss increased as sward mass decreased. This effect was still evident in poorly established pastures.

A radiotracer technique for studying the nutrient release from different fertilizer materials and its uptake by plants

Abstract In the technique presented, 35S was used to label the soil available sulfur pool in intact pasture cores, which were subsequently treated with different sulfur fertilizers. Dilution of the 35S in the soil by sulfur released from the fertilizers was monitored by measuring the changes in specific radioactivity of sulfur in plants growing in the cores. The results showed the effects of fertilizer form, particle size and application rate on the supply of sulfur to plants. This technique will be useful for making direct comparisons of the ability of commercially available fertilizers to maintain the supply of sulfur to plants.