Root density distribution and biomass allocation of co-occurring woody plants on contrasting soils in a subtropical savanna parkland

Background and aims: Root niche partitioning among trees/shrubs and grasses facilitates their coexistence in savannas, but little is known regarding root distribution patterns of co-occurring woody plants, and how they might differ on contrasting soils. Methods: We quantified root distributions of co-occurring shrubs to 2m on argillic and non-argillic soils. Results: Root biomass in the two shrub communities was 3- to 5- fold greater than that in the grassland community. Prosopis glandulosa, the dominant overstory species was deep-rooted, while the dominant understory shrub, Zanthoxylum fagara, was shallow-rooted (47% vs. 25% of root density at depths >0.4m). Shrubs on argillic soils had less aboveground and greater belowground mass than those on non-argillic soils. Root biomass and density on argillic soils was elevated at shallow (0.4m. Root density decreased exponentially with increasing distance from woody patch perimeters. Conclusions: Belowground biomass (carbon) pools increased markedly with grassland-to-shrubland state change. The presence/absence of a restrictive barrier had substantial effects on root distributions and above- vs. belowground biomass allocation. Differences in root distribution patterns of co-occurring woody species would facilitate their co-existence.NSF [BSR-9109240]; NASA [NAGW-2662]; NSF Doctoral Dissertation Improvement Grant [DEB/DDIG-1600790]; USDA/NIFA Hatch Project [1003961]; Sid Kyle Graduate Merit Assistantship from the Department of Ecosystem Science and Management; Tom Slick Graduate Research Fellowship from the College of Agriculture and Life Sciences, Texas AM University; Office of Graduate and Professional Studies at Texas AM University12 month embargo; first online: 11 March 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

A paleoclimate reconstruction for southwestern Texas using oxalate residue from lichen as a paleoclimate proxy.

Abstract A calcium oxalate rock coating is ubiquitous on limestone surfaces inside dry rock shelters and under rock overhangs within the canyons of the southwestern Edwards Plateau in southwestern Texas. The oxalate was likely produced by epilithic lichens that #ourished in these niches during dry climate regimes. During wet climate conditions the productivity of the lichen would be severely reduced due to physiological response to moisture regime. Thus, lichen productivity and the production of calcium oxalate may have changed through time in response to wet}dry climate #uctuations. Twenty-"ve AMS radiocarbon ages of rock crust samples collected from 14 sites demonstrate that oxalate was produced episodically during the middle and late Holocene. The occurrence of oxalate is correlated with periods of dry climate, whereas gaps in the record of oxalate deposition coincide with more mesic climate intervals. The results of this study demonstrate the potential for obtaining paleoclimate data from biogenic residues on rock surfaces

Nematode community development early in ecological restoration: The role of organic amendments

a b s t r a c t Soil food web structure is an integral component of ecosystem function, but there are few strategies orientated towards managing its development in restoration projects. The objective of this study was to direct nematode community structure and function through the application of organic amendments to the soil of an urban landfill remediation project using native grassland vegetation. We used a 2 Â 3 factorial design in which an organic amendment was added to the soil at different locations (incorporated versus surface-applied) and amounts (none, light, heavy). Nematode and plant community structure were monitored over three growing seasons to determine the rate and direction of change. Surface application of organic amendments supported greater grass and total plant densities compared to incorporated amendment treatments, but plant density did not vary with amendment amount. Total nematode density, family diversity and family richness were not affected by the amendment treatments, but both family richness and seasonal nematode density increased over the duration of the experiment. Other descriptors of nematode community development (Structure, Maturity, and Plant Parasite Indexes) were not influenced by either amendment amount or location, but varied significantly over time. Contrary to expectations, the surface amendment treatments significantly increased bacterivorous, plant parasitic, omnivorous and predator nematode densities, but had no influence on fungi/root-tip feeding nematodes. Also contrary to our hypotheses, the surface treatments had smaller Channel Index and greater Enrichment Index values relative to the incorporated treatments during the first 15 month of the experiment. We hypothesize that the surface amendments are indirectly affecting the structure of the nematode community by promoting greater plant density, thus increasing the concentration of highquality organic matter (such as root exudates) in the soil. This promotes the development of a nematode community dominated by opportunistic groups that respond rapidly to increased resource availability. Future studies should aim to distinguish between the organic amendment's direct function as a potential food source for the soil biota versus their indirect role as an environmental variable, including their capability to alter the availability of plant-derived resources

Interactions of long-term grazing and woody encroachment can shift soil biogeochemistry and microbiomes in savanna ecosystems

Semi-arid grasslands and savannas in the southern Great Plains USA are extensively used for livestock grazing. Over the past century, Juniperus (juniper) and Quercus (oak) species abundance have increased due to intensive grazing and reduced fire frequency. We investigated the interactions between livestock grazing history (none, moderate, heavy) and vegetation cover (grassland, juniper, oak) using a ∼ 70-year grazing experiment in west-central Texas. We explored effects on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), microbial community composition, and function. SOC and TN were 50–150 % higher under juniper and oak compared to grasslands, and 10–30 % lower in grazed vs. ungrazed areas. Vegetation × grazing interaction showed greater SOC and TN loss under oak than juniper or grasslands. Ungrazed controls had higher soil TP than grazed areas, with oak and juniper soils having more TP than grasslands. Bacterial and fungal communities differed between grassland and woody vegetation. Grazing affected only bacterial communities. SOC, TN, TP accounted for differences in community structure. Abundances of genes related to methane, nitrogen, sulfur metabolisms, and dominant fungal trophic modes were linked to soil C, N, P ratios. These findings highlight how long-term livestock grazing and woody plant encroachment influence soil C, N, P cycles, altering soil microbial community structure and function. This study provides insights for savanna ecosystem management and integrating land cover effects into biogeochemical models for global change scenarios

Quality of fresh organic matter affects priming of soil organic matter and substrate utilization patterns of microbes

Changes in biogeochemical cycles and the climate system due to human activities are expected to change the quantity and quality of plant litter inputs to soils. How changing quality of fresh organic matter (FOM) might influence the priming effect (PE) on soil organic matter (SOM) mineralization is still under debate. Here we determined the PE induced by two (13)C-labeled FOMs with contrasting nutritional quality (leaf vs. stalk of Zea mays L.). Soils from two different forest types yielded consistent results: soils amended with leaf tissue switched faster from negative PE to positive PE due to greater microbial growth compared to soils amended with stalks. However, after 16 d of incubation, soils amended with stalks had a higher PE than those amended with leaf. Phospholipid fatty acid (PLFA) results suggested that microbial demand for carbon and other nutrients was one of the major determinants of the PE observed. Therefore, consideration of both microbial demands for nutrients and FOM supply simultaneously is essential to understand the underlying mechanisms of PE. Our study provided evidence that changes in FOM quality could affect microbial utilization of substrate and PE on SOM mineralization, which may exacerbate global warming problems under future climate change

Soil colloidal particles in a subtropical savanna : biogeochemical significance and influence of anthropogenic disturbances

Soil colloids (diameter < 1000 nm) are comprised mainly of clay minerals and organic matter, and play major roles in determining ion exchange capacity and in regulating key biogeochemical processes. Consequently, it is important to understand how soil colloids and their functions are influenced by land cover and anthropogenic disturbances. In grasslands, savannas, and other dryland ecosystems across the globe, woody plants are encroaching due to livestock grazing, fire suppression, elevated CO2 concentrations, and climate change. These major land cover changes could influence soil colloidal properties, with implications for soil C, N, and P cycles. We assessed how woody encroachment, livestock grazing, and fire interact to influence soil colloidal properties in a juniper-oak savanna. Surface soils (0–10 cm) from the southern Great Plains (Texas, USA) were collected from long-term treatments differing in grazing intensity (none, moderate, and heavy) and fire history. Within each treatment, soil samples were taken under grass, juniper, and oak canopies. Water dispersible soil colloids (WDC, d < 500 nm) were isolated and analyzed by asymmetric flow field-flow fractionation and their P species by liquid-state 31P-nuclear magnetic resonance spectroscopy (31P NMR). Soil beneath oak and juniper canopies had smaller WDC and elevated colloidal organic carbon (OC) and P concentrations, especially in nanocolloid (<30 nm) and fine colloid (30–160 nm) size fractions. Woody encroachment enriched Ca, Fe, Al, Si and Mg in WDC in the ungrazed control, but not in any of the other grazed or burned areas. Colloidal soil P mainly occurred as orthophosphate and orthophosphate diesters, and was present as OC-Ca-P complexes in fine and medium colloid fractions (30–500 nm), while P in the nanocolloid fraction (<30 nm) was in direct association with Ca. Moderate grazing did not affect the retention of colloidal P, while heavy grazing potentially increased the loss risk of colloidal P. Fire accelerated soil P loss from colloid fractions only in woody areas. Our findings highlight that woody encroachment strengthens the retention of OC and P by soil colloids, consequently increasing overall C and P pools in savanna soils

Woody Plant Encroachment into Grasslands: Spatial Patterns of Functional Group Distribution and Community Development

Woody plant encroachment into grasslands has been globally widespread. The woody species invading grasslands represent a variety of contrasting plant functional groups and growth forms. Are some woody plant functional types (PFTs) better suited to invade grasslands than others? To what extent do local patterns of distribution and abundance of woody PFTs invading grasslands reflect intrinsic topoedaphic properties versus plant-induced changes in soil properties? We addressed these questions in the Southern Great Plains, United States at a subtropical grassland known to have been encroached upon by woody species over the past 50-100 years. A total of 20 woody species (9 tree-statured; 11 shrub-statured) were encountered along a transect extending from an upland into a playa basin. About half of the encroaching woody plants were potential N(2)-fixers (55% of species), but they contributed only 7% to 16 % of the total basal area. Most species and the PFTs they represent were ubiquitously distributed along the topoedaphic gradient, but with varying abundances. Overstory-understory comparisons suggest that while future species composition of these woody communities is likely to change, PFT composition is not. Canonical correspondence analysis (CCA) ordination and variance partitioning (Partial CCA) indicated that woody species and PFT composition in developing woody communities was primarily influenced by intrinsic landscape location variables (e.g., soil texture) and secondarily by plant-induced changes in soil organic carbon and total nitrogen content. The ubiquitous distribution of species and PFTs suggests that woody plants are generally well-suited to a broad range of grassland topoedaphic settings. However, here we only examined categorical and non-quantitative functional traits. Although intrinsic soil properties exerted more control over the floristics of grassland-to-woodland succession did plant modifications of soil carbon and nitrogen concentrations, the latter are likely to influence productivity and nutrient cycling and may, over longer time-frames, feed back to influence PFT distributions

Soil carbon in tropical savannas mostly derived from grasses

Tropical savannas have been increasingly targeted for carbon sequestration by afforestation, assuming large gains in soil organic carbon (SOC) with increasing tree cover. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation. However, the exact contribution of grasses to SOC and the changes in SOC with increasing tree cover remain poorly understood. Here we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived carbon constitutes more than half of total SOC to a soil depth of 1 m, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (&gt;70% of total SOC). Across the tropics, SOC concentration was not explained by tree cover. Both SOC gain and loss were observed following increasing tree cover, and on average SOC storage within a 1-m profile only increased by 6% (s.e. = 4%, n = 44). These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty in SOC responses to increasing tree cover across tropical savannas

Nitrogen fertilizer driven nitrous and nitric oxide production is decoupled from microbial genetic potential in low carbon, semi-arid soil

IntroductionNitrous oxide (N2O) emission from soil is a major concern due to its contribution to global climate change and its function as a loss mechanism of plant-available nitrogen (N) from the soil. This is especially true in intensive agricultural soils with high rates of N fertilizer application such as those on the semi-arid Southern High Plains, USA.MethodsThis study examined emissions of N2O, pore-space concentrations of N2O and nitric oxide (NO), soil chemical properties, water content, and the genetic potential for N cycling five years after conservation system and N management implementation.ResultsFor these semi-arid soils with low N, carbon, and water contents, large soil N2O emissions (up to 8 mL N2O-N m-2 day-1) are directly related to the application of N fertilizer which overwhelms the N2O reducing capacity of the soil. When this fertilizer N is depleted, N2O flux is either low, non-existent, or net-negative and has been observed as early as mid-season for preplant applied N fertilizer (-0.1 mL N2O-N m-2 day-1). Soil pore-space gas concentrations (N2O and NO) remained relatively constant across the growing season (average N2O: 0.78 µL N2O L-1 soil air; NO: 3.3 µL NO L-1 soil air, indicating a base-level of N-cycle activity, but was not directly related to surface emissions of N2O which decreased across the growing season. In addition, genetic potential for N cycle activities increased across the growing season simultaneously with stagnant/reduced N cycle activity. This reflects the difficulty in relating genetic potential to in-situ activity in field research.ConclusionIt is likely that in a nutrient and carbon-poor soil, such as the semi-arid agricultural soil in this study, the microbial processes associated with N cycling are mostly limited by inorganic-N and less directly related to genetic potential at the time of sampling