Plant community effects on soil moisture and nitrogen cycling in a semi-arid ecosystem

Biogeochemistry - Salinization of freshwater ecosystems impacts carbon cycling, a particular concern for coastal wetlands, which are important agents of carbon sequestration. Previous experimental...     

Biological soil crusts (biocrusts) as a model system in community,landscape and ecosystem ecology

Model systems have had a profound influence on the development of ecological theory and general principles. Compared to alternatives, the most effective models share some combination of the following characteristics: simpler, smaller, faster, general, idiosyncratic or manipulable. We argue that biological soil crusts (biocrusts) have unique combinations of these features that should be more widely exploited in community, landscape and ecosystem ecology. In community ecology, biocrusts are elucidating the importance of biodiversity and spatial pattern for maintaining ecosystem multifunctionality due to their manipulability in experiments. Due to idiosyncrasies in their modes of facilitation and competition, biocrusts have led to new models on the interplay between environmental stress and biotic interactions and on the maintenance of biodiversity by competitive processes. Biocrusts are perhaps one of the best examples of micro-landscapes—real landscapes that are small in size. Although they exhibit varying patch heterogeneity, aggregation, connectivity and fragmentation, like macro-landscapes, they are also compatible with well-replicated experiments (unlike macro-landscapes). In ecosystem ecology, a number of studies are imposing small-scale, low cost manipulations of global change or state factors in biocrust micro-landscapes. The versatility of biocrusts to inform such disparate lines of inquiry suggests that they are an especially useful model system that can enable researchers to see ecological principles more clearly and quickly.     

Effects of nitrogen addition on vegetation and ecosystem carbon in a semi-arid grassland

To clarify responses of plant and soil carbon (C) and nitrogen (N) pools in grassland ecosystem to N addition, a field experiment was performed in a grassland in Keerqin Sandy Lands, Northeast China. We investigated vegetation composition and C and N pools of plant and soil (0–30 cm) after five consecutive years of N addition at a rate of 20 g N m^?2 y^?1. Vegetation composition and species diversity responded dramatically to N addition, as dominance by C₄ perennials was replaced with C₃ annuals. Carbon in aboveground pool increased significantly (over two-fold), mainly due to the increase of the C in aboveground living plants and surface litter, which increased by 98 and 134%, respectively. Although soil C did not change significantly, the root C pool decreased in response to 5 years of N addition. The total ecosystem C pool was not significantly impacted by N addition because the large soil pool did not respond to N addition, and the increase in aboveground C was offset by the decrease in root C pool. Moreover, N addition significantly increased the aboveground N pool, but had no significant effects on belowground and total ecosystem N pools. Our results suggest that in the mid-term N addition alters the C and N partitioning in above- and belowground pools, but has no significant effects on total ecosystem C and N pools in these N-limited grasslands.     

Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance

Effects of annual additions of mineral N and P (100 kg ha^–1) on plant species composition and annual aboveground net primary production (ANPP) were investigated during the first three years following disturbance in a semi-arid ecosystem. Additions of N reduced richness of perennial plant species during years 2 and 3, while P reduced the number of perennial species only in year 3. From year 1 to year 2, annual and biennial species richness declined in all treatments while ANPP of annual species increased greatly. Added N increased ANPP of annual species while it decreased ANPP of most perennial species relative to the unfertilized control treatment. Community similarities were higher for the control and native vegetation than for other pairs of treatments using both species presence and plant production data. Nitrogen additions have retarded but not completely arrested secondary succession in this system.     

Does nitrogen deposition affect soil microfungal diversity and soil N and P dynamics in a high Arctic ecosystem?

In a high Arctic polar semidesert ecosystem (ambient N deposition c. 0.1 g N m⁻² a⁻¹), the effects of N enrichment on the diversity of soil microfungi and on N content and availability in organic and mineral soils were determined. Three N (total: 0, 0.5, 5 g N m⁻² a⁻¹) and two P (total 0, 1 g m⁻² a⁻¹) treatments were applied, since P may limit response to N in this ecosystem. Organic and mineral soils were sampled in June and August in the second year of treatment for microfungi, pH, moisture content, and total N and P. In the third year, soils were resampled for extractable and total N and P. The fungi isolated were typical of high pH soils in the High Arctic and Antarctic. The species richness and diversity of soil microfungi were very low, with ranges as follows: Shannon diversity, 0.56–1.5; richness, 2–6; evenness, 0.79–0.9. There was no significant effect of treatment on the frequency of occurrence of different taxa of soil microfungi. Time of sampling also had no significant impact on fungal assemblages, although different, more diverse communities were isolated from organic, rather than mineral, soils. Nitrate-N in organic soil decreased significantly when P was added alone, but not when P and N were added together. Addition of 0.5 g N m⁻² a⁻¹, a rate deposition already occurring in Greenland and Iceland, appeared to exceed N demand even when P limitation was relieved. There was no apparent soil acidification as a result of the N treatments.     

Temporal dynamics of ultraviolet radiation impacts on litter decomposition in a semi-arid ecosystem

Background and aims

The emerging consensus posits that ultraviolet (UV) radiation accelerates litter decomposition in xeric environments mainly by preconditioning litter for subsequent microbial decomposition. However, how UV radiation affects the interactions among litter chemistry, microbes, and eventually litter mass during different decomposition stages is still poorly understood.

Methods

Here, we conducted a 29-month in situ decomposition experiment with litter exposed to ambient and reduced UV in a semi-arid grassland.

Results

The decomposition rate for Cleistogenes squarrosa and Stipa krylovii under ambient UV was 82 and 111% greater than that under reduced UV, respectively. UV’s positive effect showed three-stage temporal dynamics. During the early stage, UV had no impact on either litter chemistry or mass loss. During the intermediate stage, UV decreased litter carbon concentration and increased dissolved organic carbon concentration, but still had no effect on litter mass. During the late stage, UV exposure increased microbial population size in the surface soil and significantly increased litter mass loss.

Conclusions

Overall, our study suggested that UV exposure accelerated litter decomposition first by improving litter biodegradability during the intermediate stage and then by enhancing microbial decomposition during the late stage. More long-term photodegradation experiments are needed to explore the biotic and abiotic interactions during different decomposition stages.

     

Denitrification in a semi-arid grazing ecosystem

The effect of large herbivores on gaseous N loss from grasslands, particularly via denitrification, is poorly understood. In this study, we examined the influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of artificial urine application on denitrification, and (2) comparing rates inside and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after application at hilltop, mid-slope, and slope-bottom sites. Likewise, grazers had no effect on community-level denitrification at dry exclosure sites, where rates were low. At mesic sites, however, ungulates enhanced denitrification by as much as 4 kg N ha⁻¹ year⁻¹, which was double atmospheric N inputs to this ecosystem. Denitrification enzyme activity (DEA, a measure of denitrification potential) was positively associated with soil moisture at exclosure sites, and herbivores stimulated DEA when accounting for the soil moisture effect. Glucose additons to soils increased denitrification and nitrate additions had no influence, suggesting that denitrification was limited by the amount of labile soil carbon, which previously has been shown to be enhanced by ungulates in Yellowstone. These results indicate that denitrification can be an ecologically important flux in portions of semi-arid landscapes, and that there is a previously unsuspected regulation of this process by herbivores. Received: 6 March 1998 / Accepted: 28 August 1998     

Soil properties and C dynamics in abandoned and cultivated farmlands in a semi-arid ecosystem

Background and Aims

Land abandonment might be an alternative management for restoring soil conditions and C from prolonged cultivation and agricultural practices. In the present study, the influence of 18–22?years of land abandonment on soil properties, C dynamics and microbial biomass was evaluated in closely situated wheat and alfalfa farmlands, and abandoned lands on calcareous soils, Central Iran.

Methods

Soil properties of the 0–15 and 15–30?cm depths from abandoned lands were compared to those from conventionally cultivated lands (i.e., continuous wheat–fallow and alfalfa–wheat rotation) common in calcareous soils of Central Zagros Mountains.

Results

Soil bulk density in the 0–15 and 15–30?cm layers decreased significantly while total porosity increased significantly in abandoned lands. Generally, soil aggregate stability tended to increase within the abandoned fields owing to increased water-stable macro-aggregates. Soil organic C (OC) contents (g kg^?1) and pools (Mg ha^?1) in the 0–15?cm soil layer increased significantly in abandoned lands compared with cultivated lands, with no effect in the 15–30?cm soil layer after 18–22?years of land abandonment, suggesting the restoration of C is pronounced in the upper 0–15?cm soil depth . The total C accumulation in abandoned lands was 7.0?Mg?C?ha^?1 for the entire sampling depth (0–30?cm) over the 18–22?years of land abandonment, which was 26% greater relative to cultivated lands. Carbon mineralization (Cmin) followed a trend similar to organic C, whereas C turnover (Cmin/OC ratio) was slightly greater in wheat fields. However, soil microbial biomass C (MBC) did not vary considerably among the three land uses.

Conclusions

In brief, improvements, albeit slowly, in soil properties of the top layer with the cessation of cultivation indicated that land abandonment may result in enhanced soil C sequestration, and would maintain fertility and productivity of the farmlands of semi-arid climates.     

Combined climate factors alleviate changes in gross soil nitrogen dynamics in heathlands

The ongoing climate change affects biogeochemical cycling in terrestrial ecosystems, but the magnitude and direction of this impact is yet unclear. To shed further light on the climate change impact, we investigated alterations in the soil nitrogen (N) cycling in a Danish heathland after 5 years of exposure to three climate change factors, i.e. warming, elevated CO₂ (eCO₂) and summer drought, applied both in isolation and in combination. By conducting laboratory ¹⁵N tracing experiments we show that warming increased both gross N mineralization and nitrification rates. In contrast, gross nitrification was decreased by eCO₂, an effect that was more pronounced when eCO₂ was combined with warming and drought. Moreover, there was an interactive effect between the warming and CO₂ treatment, especially for N mineralization: rates increased at warming alone but decreased at warming combined with eCO₂. In the full treatment combination, simulating the predicted climate for the year 2075, gross N transformations were only moderately affected compared to control, suggesting a minor alteration of the N cycle due to climate change. Overall, our study confirms the importance of multifactorial field experiments for a better understanding of N cycling in a changing climate, which is a prerequisite for more reliable model predictions of ecosystems responses to climate change.     

Upscaling understanding of nitrogen dynamics associated with overland flow in a semi-arid environment

An experiment was designed to further the empirical understanding of the effects of scale on fluxes of water and dissolved nitrogen from hillslopes in semi-arid shrubland. It was hypothesised that the behaviour of dissolved nitrogen is related to the scale of the contributing hillslope/catchment area and dynamics of the overland flow as has been demonstrated to be the case for soil erosion (Parsons et al. 2006). Data from four hillslope scales (ca. 21–300 m²) and one subcatchment (ca. 1,500 m²), collected over two monsoon seasons, support this hypothesis and demonstrate that the key controls of average dissolved nitrogen yields are flow discharge and plot scale. The slope of the best-fit line describing the relationship between flow discharge and total dissolved nitrogen (TDN) yields descreases with increasing scale, from 0.0183 at 21.01 m², 0.0092 at 56.84 m², 0.0059 at 115.94 m², 0.0024 at 302.19 m² to 0.0004 at 1,500 m². An implication of these findings is that care must be taken when upscaling results describing nutrient behaviour from small, plot experiments, as this behaviour appears to be scale dependent. For example, average yields of TDN in overland flow increase to a maximum with increasing plot area until an area of 50 m² is reached, and decline with increasing plot size thereafter. Thus, studies that rely upon fixed plot scales may misrepresent catchment- or landscape-scale fluxes as they do not describe the changing relationship between overland flow and nutrient fluxes with increasing spatial scale. Further investigations into intra-event behaviour illustrate that nitrogen losses from natural rainfall/runoff events are supply limited as over the course of the events monitored, decreasing concentrations illustrate a pattern of nutrient exhaustion. When events are compared at the same sites through the monsoon season, however, the anticipated seasonal exhaustion effect is not present. This work provides an empirical basis to upscale the understanding of dissolved nitrogen behaviour from small hillslope plots to catchment scales in degraded semi-arid environments.     

Co-limitation by nitrogen and water constrains allocation response to drought in deciduous and evergreen shrubs in a semi-arid ecosystem

Plant Ecology - Semi-arid regions worldwide are expected to experience reduced precipitation with future climate change, in addition to increased intensity of precipitation events, heightening the...     

Digital mapping of soil organic matter stocks using Random Forest modeling in a semi-arid steppe ecosystem

Spatial prediction of soil organic matter is a global challenge and of particular importance for regions with intensive land use and where availability of soil data is limited. This study evaluated a Digital Soil Mapping (DSM) approach to model the spatial distribution of stocks of soil organic carbon (SOC), total carbon (C_tot), total nitrogen (N_tot) and total sulphur (S_tot) for a data-sparse, semi-arid catchment in Inner Mongolia, Northern China. Random Forest (RF) was used as a new modeling tool for soil properties and Classification and Regression Trees (CART) as an additional method for the analysis of variable importance. At 120 locations soil profiles to 1 m depth were analyzed for soil texture, SOC, C_tot, N_tot, S_tot, bulk density (BD) and pH. On the basis of a digital elevation model, the catchment was divided into pixels of 90 m?×?90 m and for each cell, predictor variables were determined: land use unit, Reference Soil Group (RSG), geological unit and 12 topography-related variables. Prediction maps showed that the highest amounts of SOC, C_tot, N_tot and S_tot stocks are stored under marshland, steppes and mountain meadows. River-like structures of very high elemental stocks in valleys within the steppes are partly responsible for the high amounts of SOC for grasslands (81?C84% of total catchment stocks). Analysis of variable importance showed that land use, RSG and geology are the most important variables influencing SOC storage. Prediction accuracy of the RF modeling and the generated maps was acceptable and explained variances of 42 to 62% and 66 to 75%, respectively. A decline of up to 70% in elemental stocks was calculated after conversion of steppe to arable land confirming the risk of rapid soil degradation if steppes are cultivated. Thus their suitability for agricultural use is limited.     

Biological invasions increase the richness of arbuscular mycorrhizal fungi from a Hawaiian subtropical ecosystem

Biological invasions can have various impacts on the diversity of important microbial mutualists such as mycorrhizal fungi, but few studies have tested whether the effects of invasions on mycorrhizal diversity are consistent across spatial gradients. Furthermore, few of these studies have taken place in tropical ecosystems that experience an inordinate rate of invasions into native habitats. Here, we examined the effects of plant invasions dominated by non-native tree species on the diversity of arbuscular mycorrhizal (AM) fungi in Hawaii. To test the hypothesis that invasions result in consistent changes in AM fungal diversity across spatial gradients relative to native forest habitats, we sampled soil in paired native and invaded sites from three watersheds and used amplicon sequencing to characterize AM fungal communities. Whether our analyses considered phylogenetic relatedness or not, we found that invasions consistently increased the richness of AM fungi. However, AM fungal species composition was not related to invasion status of the vegetation nor local environment, but stratified by watershed. Our results suggest that while invasions can lead to an overall increase in the diversity of microbial mutualists, the effects of plant host identity or geographic structuring potentially outweigh those of invasive species in determining the community membership of AM fungi. Thus, host specificity and spatial factors such as dispersal need to be taken into consideration when examining the effects of biological invasions on symbiotic microbes.     

Non-parallel changes in soil microbial carbon and nitrogen dynamics due to reindeer grazing in northern boreal forests

Reindeer grazing has a considerable influence on mineralization processes in northern Fennoscandian boreal forests, but the mechanisms underlying the observed differences between grazed and ungrazed areas are not well understood. We studied the below-ground impacts of reindeer grazing by comparing the carbon and nitrogen mineralization rates inside and outside long-term fenced reindeer exclosure areas in five oligotrophic, lichen-dominated and five mesotrophic, dwarf-shrub dominated forests. The soil C mineralization rates and microbial metabolic activity (qCO₂) were significantly lower in the grazed than the ungrazed areas in both oligotrophic and mesotrophic forests. The reductions occurred irrespective of the impact on soil moisture. We conclude that reindeer grazing causes a reduction in the supply of labile C substrates to microbes, resulting in reduced organic matter decomposition rates through changes in the activity of the microbial biomass. Simultaneously, grazing had no consistent effect on the microbial N dynamics, but the impact ranged from no change to increased or decreased in N mineralization rates at the different study sites. The impact of grazing on the N mineralization potential thus seems to be site-specific and uncoupled from the impact of grazing on soil C mineralization. Reciprocal transplant incubations showed no interactions between N mineralization rates and the reindeer-mediated impact on the soil microclimate. We suggest that plant root damage due to trampling by reindeer may be an important mechanism for the deceleration of soil C cycling. In some cases, however, the impact of grazing on the soil active N pool may be strong enough to outweigh the reduction in soil organic matter decomposition, and by these means uncouple soil N dynamics from soil C quality.     

Temporal dynamics of arbuscular mycorrhizal fungi colonizing roots of representative shrub species in a semi-arid Mediterranean ecosystem

Arbuscular mycorrhizal (AM) symbiosis plays an important role in improving plant fitness and soil quality, particularly in fragile and stressed environments, as those in certain areas of Mediterranean ecosystems. AM fungal communities are usually affected by dynamic factors such as the plant community structure and composition, which in turn are imposed by seasonality. For this reason, a one-year-round time-course trial was performed by sampling the root system of two representative shrubland species (Rosmarinus officinalis and Thymus zygis) within a typical Mediterranean ecosystem from the Southeast of Spain. The 18S rDNA gene, of the AM fungal community in roots, was subjected to PCR-SSCP, sequencing, and phylogenetic analysis. Forty-three different AM fungal sequence types were found which clustered in 16 phylotypes: 14 belonged to the Glomeraceae and two to the Diversisporaceae. Surprisingly, only two of these phylotypes were related with sequences of morphologically defined species: Glomus intraradices and Glomus constrictum. Significant differences were detected for the relative abundance of some phylotypes while no effects were found for the calculated diversity indices. These results may help to design efficient mycorrhizal-based revegetation programs for this type of ecosystems.     

Spatial heterogeneity in the herbaceous layer of a semi-arid savanna ecosystem

Despite increasing recognition of the role spatial pattern can play in ecosystem function, few studies have quantified spatial heterogeneity in savanna ecosystems. The spatial distribution of herbaceous biomass and species composition was measured across three scales in a semi-arid savanna in central Kenya, and patterns were related to environmental variables at different scales. Herbaceous biomass declined across a rainfall gradient and from upper to lower topographic positions, but variation within a site (across 5–50 m) was similar in magnitude to among-site variation associated with rainfall and topography. Geostatistical analyses showed that patchiness at scales of 5–25 m explained 20% of total variation in herbaceous biomass. This pattern arose from the presence of both 5–10-m diameter patches containing high herbaceous biomass (> 170 g m^–2) and 5–10-m diameter patches characterized by nearly bare soil surfaces (< 40 g m^–2). Patch structure was contingent on topography, with larger bare patches at ridgeline and upper hillslope positions. Grass species distributions showed the greatest degree of patch structure and species turnover across distances of 5–45 m. Additional community variation was associated with topography, with minimal variation in species composition across the rainfall gradient. Pattern diversity significantly exceeded levels reported for four other grassland ecosystems, suggesting fundamental differences in local processes generating spatial pattern. It is hypothesized that heterogeneously distributed grazing pressure, interacting with the distribution of shrub canopies, is an important factor generating such high levels of small-scale patch structure in this savanna.