Urban ecosystems are expanding globally, and assessing the ecological consequences of urbanization is critical to understanding the biology of local and global change related to land use. We measured ...carbon (C) fluxes, nitrogen (N) cycling, and soil microbial community structure in a replicated (n=3) field experiment comparing urban lawns to corn, wheat–fallow, and unmanaged shortgrass steppe ecosystems in northern Colorado. The urban and corn sites were irrigated and fertilized. Wheat and shortgrass steppe sites were not fertilized or irrigated. Aboveground net primary productivity (ANPP) in urban ecosystems (383±11 C m−2 yr−1) was four to five times greater than wheat or shortgrass steppe but significantly less than corn (537±44 C m−2 yr−1). Soil respiration (2777±273 g C m−2 yr−1) and total belowground C allocation (2602±269 g C m−2 yr−1) in urban ecosystems were both 2.5 to five times greater than any other land‐use type. We estimate that for a large (1578 km2) portion of Larimer County, Colorado, urban lawns occupying 6.4% of the land area account for up to 30% of regional ANPP and 24% of regional soil respiration from land‐use types that we sampled. The rate of N cycling from urban lawn mower clippings to the soil surface was comparable with the rate of N export in harvested corn (both ∼12–15 g N m−2 yr−1). A one‐time measurement of microbial community structure via phospholipid fatty acid analysis suggested that land‐use type had a large impact on microbial biomass and a small impact on the relative abundance of broad taxonomic groups of microorganisms. Our data are consistent with several other studies suggesting that urbanization of arid and semiarid ecosystems leads to enhanced C cycling rates that alter regional C budgets.
Soil stores approximately twice as much carbon as the atmosphere and fluctuations in the size of the soil carbon pool directly influence climate conditions. We used the Nutrient Network global change ...experiment to examine how anthropogenic nutrient enrichment might influence grassland soil carbon storage at a global scale. In isolation, enrichment of nitrogen and phosphorous had minimal impacts on soil carbon storage. However, when these nutrients were added in combination with potassium and micronutrients, soil carbon stocks changed considerably, with an average increase of 0.04 KgCm−2 year−1 (standard deviation 0.18 KgCm−2 year−1). These effects did not correlate with changes in primary productivity, suggesting that soil carbon decomposition may have been restricted. Although nutrient enrichment caused soil carbon gains most dry, sandy regions, considerable absolute losses of soil carbon may occur in high‐latitude regions that store the majority of the world's soil carbon. These mechanistic insights into the sensitivity of grassland carbon stocks to nutrient enrichment can facilitate biochemical modelling efforts to project carbon cycling under future climate scenarios.
The dynamics of biologic Si cycling in grassland ecosystems are largely unknown and likely to impact mineral weathering rates regionally and diatom productivity globally; key regulatory processes in ...the global Si cycle are closely tied to the global carbon cycle. Across a bioclimatic sequence spanning major grassland ecosystems in the Great Plains, soil biogenic silica depth distributions are similar to that of soil organic carbon; however, unlike soil organic carbon, quantities of soil biogenic silica decrease with increasing precipitation, despite an increase in annual biogenic inputs through litterfall across the same gradient. Though comprising only 1–3% of the total Si pool, faster turnover of biogenic Si and annual cycling by grasses should positively impact mineral dissolution. Our results suggest that the largest reservoir of biogenic Si in terrestrial ecosystems resides in soils, and emphasize the potential significance of grasslands in the global biogeochemical cycle of Si.
Grasslands and savannas worldwide are experiencing increases in woody plant abundance. In the subtropical Rio Grande Plains of southern Texas and northern Mexico, this change in physiognomy typically ...results in soil C and N accumulation. The extent to which this accumulation is the result of increased C and N inputs vs. decreased losses is not known. To address this issue, we compared soil C and N pools, soil respiration, soil microbial biomass, and potential C and N mineralization and nitrification rates in remnant grassland communities and adjacent woody plant communities known to have developed on grassland within the past 100 years. Mean soil organic C (SOC) and total N pools in the upper 20 cm of the profile were 2× larger in wooded communities (3382 and 273 g/m2for C and N, respectively) than in remnant grasslands (1737 and 150 g/m2). The larger pool sizes in the wooded communities supported higher annual soil respiration (SR; 745 vs. 611 g C· m-2· yr-1for woodlands and grasslands, respectively) and greater soil microbial biomass C (444 vs. 311 mg C/kg soil), potential rates of N mineralization (0.9 vs. 0.6 mg N· Kg-1· d-1) and nitrification (0.9 vs. 0.4 mg N· Kg-1· d-1). However, despite higher SR rates, mean residence time of near-surface SOC in wooded communities (11 years) exceeded that of remnant grassland communities (6 years). The fact that increased fluxes of soil C and N were accompanied by increases in SOC and N pools and total SOC mean residence time suggests that shifts from grass to woody plant dominance have increased both labile and recalcitrant pools of SOC and total N, the latter to a greater extent than the former. Given the widespread increase in woody plant abundance in drylands in recent history, the observed net increase in soil C storage that potentially accompanies this change could have global implications for C and N cycling and the climate system.
Tall fescue Lolium arundinaceum (Schreb.) Darbysh. syn. Festuca arundinacea Schreb. and perennial ryegrass (Lolium perenne L.) are important perennial forage grasses utilized throughout the moderate- ...to high-rainfall temperate zones of the world. These grasses have coevolved with symbiotic fungal endophytes (Epichloë/Neotyphodium spp.) that can impart bioactive properties and environmental stress tolerance to the grass compared with endophyte-free individuals. These endophytes have proven to be very important in pastoral agriculture in the United States, New Zealand, and Australia, where forage grasses are the principal feed for grazing ruminants. In this review, we describe the biology of these grass-endophyte associations and implications for the livestock industries that are dependent on these forages. Endophyte alkaloid production is put in context with endophyte diversity, and we illustrate how this has facilitated utilization of grasses infected with different endophyte strains that reduce livestock toxicity issues. Utilization of tall fescue and use of perennial ryegrass in the United States, New Zealand, and Australia are compared, and management strategies focused predominantly on the success of endophyte-infected perennial ryegrass in New Zealand and Australia are discussed. In addition, we consider the impact of grass-endophyte associations on the sustainability of pasture ecosystems and their likely response to future changes in climate.
Explanations for the occurrence of deep-rooted plants in arid and semi-arid ecosystems have traditionally emphasized the uptake of relatively deep soil water. However, recent hydrologic data from ...arid systems show that soil water potentials at depth fluctuate little over long time periods, suggesting this water may be rarely utilized or replenished. In this study, we examine the distributions of root biomass, soil moisture and nutrient contents to 10-m depths at five semi-arid and arid sites across south-western USA. We couple these depth distributions with strontium (Sr) isotope data that show deep (>1 m) nutrient uptake is prevalent at four of the five sites. At all of the sites, the highest abundance of one or more of the measured nutrients occurred deep within the soil profile, particularly for P, Ca²ɺ and Mg²ɺ. Phosphate contents were greater at depth than in the top meter of soil at three of five sites. At Jornada, for example, the 2-3 m depth increment had twice the extractable P as the top meter of soil, despite the highest concentrations of P occurring at the surface. The prevalence of such deep resource pools, and our evidence for cation uptake from them, suggest nutrient uptake as a complementary explanation for the occurrence of deep-rooted plants in arid and semi-arid systems. We propose that hydraulic redistribution of shallow surface water to deep soil layers by roots may be the mechanism through which deep soil nutrients are mobilized and taken up by plants.
Aim
The microbial metabolic quotient (MMQ; mg CO2‐C/mg MBC/h), defined as the amount of microbial CO2 respired (MR; mg CO2‐C/kg soil/h) per unit of microbial biomass C (MBC; mg C/kg soil), is a key ...parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Here, we experimentally tested hypotheses about the individual and interactive effects of multiple nutrient addition (nitrogen + phosphorus + potassium + micronutrients) and herbivore exclusion on MR, MBC and MMQ across 23 sites (five continents). Our sites encompassed a wide range of edaphoclimatic conditions; thus, we assessed which edaphoclimatic variables affected MMQ the most and how they interacted with our treatments.
Location
Australia, Asia, Europe, North/South America.
Time period
2015–2016.
Major taxa
Soil microbes.
Methods
Soils were collected from plots with established experimental treatments. MR was assessed in a 5‐week laboratory incubation without glucose addition, MBC via substrate‐induced respiration. MMQ was calculated as MR/MBC and corrected for soil temperatures (MMQsoil). Using linear mixed effects models (LMMs) and structural equation models (SEMs), we analysed how edaphoclimatic characteristics and treatments interactively affected MMQsoil.
Results
MMQsoil was higher in locations with higher mean annual temperature, lower water holding capacity and lower soil organic C concentration, but did not respond to our treatments across sites as neither MR nor MBC changed. We attributed this relative homeostasis to our treatments to the modulating influence of edaphoclimatic variables. For example, herbivore exclusion, regardless of fertilization, led to greater MMQsoil only at sites with lower soil organic C (< 1.7%).
Main conclusions
Our results pinpoint the main variables related to MMQsoil across grasslands and emphasize the importance of the local edaphoclimatic conditions in controlling the response of the C cycle to anthropogenic stressors. By testing hypotheses about MMQsoil across global edaphoclimatic gradients, this work also helps to align the conflicting results of prior studies.
Conservation agriculture has been marketed to producers as a sustainable way to increase soil productivity and buffer the effects of anticipated fluctuations in both climate and the price of fuel and ...fertilizer. Despite US Farm Bill financial incentives promoting the use of conservation practices such as no-tillage and cover crops among producers, widescale adoption of cover crops in the US remains low. Implementing no-tillage and cover crop use may take years of consistent management to show measurable soil quality improvement. Conversely, costs associated with cover crop planting and management must be incurred immediately and before the accrual of any benefits. Current research has largely focused on the soil quality benefits of cover crops without considering the short-term implementation costs for producers comparing their current production systems to conservation systems they may adopt. To help fill this knowledge gap, soil properties, cash crop yield, and annual production costs were evaluated in a three-year maize /soybean study comparing a conservation system to a conventional production system in Mississippi from 2016 to 2018. The conventional system included frequent tillage and control of winter weeds with a residual soil herbicide (conv. till + bare). We compared five different levels of conservation systems, including a conservation system with no-tillage and cover crops (no-till + cover). We found a lack of consistent improvement in soil properties such as soil organic matter increase or crop yield in the conservation system over the three-year period. The economic benefits of substituting no-tillage and cover crops for expensive conventional inputs were much greater than the modest soil property improvements observed from 2016 to 2018 crops. Though crop yields varied, the cost of cash crop production in the conservation system (no-till + cover) was 43% less than the cost of the conventional system ($29.67 per Mg of crop yield in the conv. till + bare to $17.04 per Mg of crop yield in the no-till + cover system), highlighting a short-term economic incentive for producers to switch to conservation agriculture. The greatest resource benefit from the conservation system was a reduction in estimated soil loss, as mean soil loss in no-till and cover crop decreased by 86% compared to the conventional system (3.5–25.1 t ha-1). These results demonstrate that conservation agriculture has the potential to decrease overall production cost while decreasing the risk of accelerated soil erosion during expected weather extremes associated with climate change, potentially making both farming operations and agroecosystems more resilient.
Display omitted
•Cover crops and no-tillage did not significantly increase soil organic matter in the short term (3 years).•No-till with cover crops cost less than conventional till left bare.•No-till reduced soil loss more than cover crops.•Cover crops did not fully mitigate soil loss in tilled systems.
Soil P transformations and distribution studies under water limited conditions that characterize many grasslands may provide further insight into the importance of abiotic and biotic P controls ...within grass-dominated ecosystems. We assessed transformations between P pools across four sites spanning the shortgrass steppe, mixed grass prairie, and tallgrass prairie along a 400-mm precipitation gradient across the central Great Plains. Pedon total elemental and constituent mass balance analyses reflected a pattern of increased chemical weathering from the more arid shortgrass steppe to the more mesic tallgrass prairie. Soil surface A horizon P accumulation was likely related to increased biocycling and biological mining. Soluble P, a small fraction of total P in surface A horizons, was greatest at the mixed grass sites. The distribution of secondary soil P fractions across the gradient suggested decreasing Ca-bound P and increasing amounts of occluded P with increasing precipitation. Surface A horizons contained evidence of Ca-bound P in the absence of CaCO
3, while in subsurface horizons the Ca-bound P was associated with increasing CaCO
3 content. Calcium-bound P, which dominates in water-limited systems, forms under different sets of soil chemical conditions in different climatic regimes, demonstrating the importance of carbonate regulation of P in semi-arid ecosystems.
Tall fescue (Lolium arundinaceum (Schreb.)) is a cool-season perennial grass within which can live a fungal endophyte (Epichloë coenophiala) thought to provide enhanced edaphic and climactic stress ...tolerance to the host compared to non-infected individuals. Our prior research demonstrated that a variety of root exudate compounds released from tall fescue were differentially affected by tall fescue cultivar, endophyte genotype and their interaction. Changes in root exudates and associated microbial communities could influence soil processes, including carbon and nitrogen cycling, but these effects may differ depending on fescue and endophyte genetics. To test this, we collected rhizosphere and bulk soil samples from six year old field plots located in Lexington, KY planted with two different tall fescue cultivars (PDF and 97TF1), each containing four endophyte treatments endophyte-free (E−) or infected with one of three strains of E. coenophiala (common toxic, novel AR542E+ and novel AR584E+). The influence of fescue cultivar, endophyte strain, and soil sample location (rhizosphere vs. bulk) were assessed for soil organic carbon, soil organic nitrogen, particulate and non-particulate organic matter (POM and n-POM, respectively) - C and – N, and dissolved organic carbon and nitrogen (DOC, DON) pools. Soil functional aspects were evaluated by measuring soil respiration and the activity of seven different soil enzymes related to C, N and P cycling. We found that rhizosphere soils had greater microbial biomass, potential enzyme activity, and oxygen utilization, but lesser POM-N concentrations than bulk soils. In rhizosphere soils, tall fescue cultivar had the greatest influence on soil microbial community structure, while endophyte genotype had a stronger influence on soil C fractions (notably POM-C). Changes in root system architecture, biomass, and tissue composition, together with root exudate chemistry, which we have shown in previous studies to be affected by endophyte infection and fescue cultivar, likely explain these findings. There was greater POM-N in bulk soils which was influenced by the interaction of endophyte and cultivar potentially due to differences in the chemical composition of the tissues brought about by this interaction. Our results support current observations that tall fescue cultivar and fungal endophyte strain influence soil C and N cycling and, by analyzing bulk and rhizosphere soils separately, go further to show the level of influence cultivar and endophyte have within each of these compartments.
Display omitted
•Tall fescue cultivar and fungal endophyte strain influence pasture soil C and N cycling.•Fescue cultivar had greatest influence on rhizosphere microbial community structure.•Endophyte strain significantly effects POM-C concentrations in rhizosphere soils.•Greater POM-N in bulk soils influenced by endophyte × cultivar interaction.•Reveals the influence cultivar and endophyte have within bulk and rhizosphere soils.
PDF
