Woody plant encroachment and proliferation in grasslands is occurring worldwide and has significant but variable effects on ecosystem processes, including primary production, litter decomposition, ...and N cycling. In mesic systems, recent reports suggest aboveground net primary production (ANPP) is stimulated by grass-to-woodland conversion; however, significant losses of soil C may accompany this land cover change in these environments. This study aimed to quantify how changes in plant production inputs (both above- and belowground), litter decomposition, and N cycling may feed back to reduce soil C pools in a mesic tallgrass prairie in central Texas, USA, that has undergone grass-to-woodland conversion over the past 60–70 y. Belowground net primary production (BNPP) in woodland was half that of adjacent grassland, and contrary to recent reports, no significant differences in ANPP between the two vegetation types were observed. Decomposition rates of aboveground woody and grass material were surprisingly similar. However, higher N concentrations in woodland leaves and stems meant woodland received more than 3× the amount of N via litterfall than adjacent grassland, and woody litter mineralized N when decomposing, while grassland litter tended to retain it. Losses of soil C accompanying grass-to-woodland conversion at this site (woodland soil C was ∼20% less than that of adjacent grassland) may be the result of both reduced BNPP inputs to the soil C pool and increased rates of N cycling stimulating soil organic matter decomposition. Given that the processes controlling whether grasslands accrue or lose C and N when they are replaced by woody species vary across time and depend on the species involved and the biotic and edaphic conditions and management history of the site, more complex models that incorporate these parameters may be required to understand and predict when gains and losses of C will accompany vegetation change.
The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a ...function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C${\rm m}^{-2}\ {\rm y}^{-1}$) to tallgrass prairie (180, 1221, and 208 g C${\rm m}^{-2}\ {\rm y}^{-1}$for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.
Soil nitrogen mineralisation (N
), the conversion of organic into inorganic N, is important for productivity and nutrient cycling. The balance between mineralisation and immobilisation (net N
) ...varies with soil properties and climate. However, because most global-scale assessments of net N
are laboratory-based, its regulation under field-conditions and implications for real-world soil functioning remain uncertain. Here, we explore the drivers of realised (field) and potential (laboratory) soil net N
across 30 grasslands worldwide. We find that realised N
is largely explained by temperature of the wettest quarter, microbial biomass, clay content and bulk density. Potential N
only weakly correlates with realised N
, but contributes to explain realised net N
when combined with soil and climatic variables. We provide novel insights of global realised soil net N
and show that potential soil net N
data available in the literature could be parameterised with soil and climate data to better predict realised N
.
Rates of nutrient cycling vary across landscape and regional scales. This biogeochemical variability can be partially attributed to patterns in plant community characteristics and abiotic and edaphic ...conditions across topographic gradients at the landscape-scale or across regional climatic gradients. However, it is also possible that concomitant changes in the microbial communities performing these biogeochemical processes occur across the same spatial scales and may therefore contribute to the observed biogeochemical trends. To assess patterns of microbial community composition across regional and landscape scales, we sampled upland and lowland topographic positions at three grassland communities spanning a 500-mm regional precipitation gradient across the central Great Plains. Soil microbial community composition and biomass were determined using phospholipid fatty acid (PLFA) analysis. Microbial biomass increased across the regional gradient, and different inicrobial communities were associated with the different grassland community types. The relative abundance of fungi decreased while gram-negative anaerobic bacteria increased from shortgrass steppe to tallgrass prairie. There were no differences in microbial biomass at the landscape-scale, and the only alteration in microbial community composition between upland and lowland landscape positions was a shift toward more nonspecific bacteria in lowlands. The fact that the trends in microbial biomass and community composition at the landscape-scale were less pronounced suggests that variability in microbial community composition is larger regionally across the Great Plains than landscape variability associated with topographical features at any particular site. Alterations in the inicrobial community may play a role in determining the biogeochemical patterns of grasslands in the Great Plains region. PUBLICATION ABSTRACT
Tall fescue (Schedonurus arundinaceus), an exotic invasive grass in North America, can associate with a fungal endophyte that causes livestock toxicity. Native prairies are frequently managed with ...interactive fire and grazing, yet little is known regarding tall fescue's endophytic and toxicological responses. From 2012 to 2014, we applied patch-burn grazing (PBG—burning a different third annually) or graze and burn (GAB—burning completely in 2012 but no fire in 2013 or 2014) treatments to tall fescue–invaded grasslands. Burning happened in March/April, and cattle grazing occurred during the growing season. Tall fescue tillers were analyzed for Epichloë endophyte presence and alkaloid concentrations (ergovaline, ergovalinine, N-acetylnorloline, N-formylloline, N-acetylloline). Cattle toxicosis was assessed via fecal ergovaline levels. With PBG, tiller defoliation was greater in burned patches versus unburned and was greater than any years in GAB. In GAB, tiller defoliation was no different the year of the burn than the years without fire. Cattle did not discriminate between endophyte-infected or endophyte-free tillers in either treatment. Endophyte infection levels were inversely related to years since fire (YSF), and various alkaloids displayed asynchronous responses to YSF. Cattle had no detectable fecal ergovaline when managed with patchy or complete pasture fires. Only two herds had detectable fecal ergovaline (> 100 ppb), which were in pastures managed without fire and only in 2013. Thus, patch burning tall fescue–invaded grasslands alters alkaloids and tiller defoliation with implications for cattle toxicosis. Future research should incorporate greater intra-annual resolution of plant phenology relative to focal grazing and alkaloid expression.
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 (Epichloe/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. PUBLICATION ABSTRACT
Litter decomposition is a key ecosystem process that determines rates of carbon and nutrient cycling. Photodegradation and soil-litter mixing have emerged as important drivers of dryland litter ...decomposition, but how these processes interact with decomposing microorganisms has received less attention. In this study, we examined the effects of ultraviolet-B radiation (UV-B; 280–315 nm) and soil-litter mixing on the decomposition of litter and its associated microbial community in an arid shrubland. We performed a full factorial litter decomposition experiment using leaf litter from a dominant shrub (
Prosopis velutina
) and a dominant grass (
Eragrostis lehmanniana
) that were exposed to solar radiation with near-ambient or attenuated UV-B, and were either soil-free or soil-covered; we then quantified litter decomposition and microbial community composition over a 12 month period. In general, shrub litter decomposed more rapidly than grass litter regardless of soil coverage, likely due to its lower C:N. Attenuation of UV-B had modest effects on decomposition but UV-B exposure did increase fungal biomass, perhaps reflecting facilitative aspects of photodegradation. Both bacteria and fungi emerged as important regulators of decomposition, and microbial decomposition was indirectly mediated by litter C:N, soil coverage, and UV-B effects on the microbial community. Bacterial colonization was inhibited in soil-free treatments, but was facilitated when litter was soil-covered. These findings suggest that UV-B may play an important role in facilitating fungal decomposition of litter, while soil-litter mixing is fundamental for promoting bacterial decomposition of litter.
Highlights
Soil-litter mixing enhanced bacterial colonization of litter.
Fungi are present on dryland litter regardless of soil and UV-B conditions.
Soil-litter mixing initially accelerated litter decomposition but subsequently reduced it.
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