Data are needed to identify optimum response to potassium (K) and phosphorus (P) amendment and associated mycorrhizal colonization for native warm-season grasses (NWSGs; big bluestem [BB; Andropogon gerardii Vitman] and switchgrass [SG; Panicum virgatum L.]). To evaluate these responses, experiments were conducted in Knoxville and Springfield, Tennessee, from 2013 to 2019. In twice-annual harvests, we assessed BB and SG dry matter (DM) yield, crude protein (CP), total digestible nutrients (TDNs), P and K removed by grasses (removal), and soil test P and K in response to P (29 to 88 kg ha−1) and K (70 to 257 kg ha−1) elemental rates, and rates of root colonization by mycorrhizal fungi in response to P. Amendments had no effect (p > 0.05) on DM yield, CP, or TDN for either species. Yield, CP, and TDN fluctuated among years (p < 0.001) for both species, but no consistent temporal trends were observed. Although removal exceeded inputs at the control (no input) for P and K, and at 70 kg K ha−1, there was not an associated reduction in soil test K and P values. Phosphorus rate affected (p = 0.02) total mycorrhizal colonization, with an average of 62% colonization across both species and 70% at the highest P rates. Given the lack of response for yield, CP, TDN, or associated soil nutrient test levels, NWSGs appear to offer a low-input option for forage production.

Reduced floral resources and habitat fragmentation have led to pollinator decline. Increased diversity of native plants in pastures could support cattle and pollinators. However, the relationship between grazing and plant diversity needs to be investigated. We explored how grazing rest periods impacted persistence and forage characteristics of Andropogon gerardii (BB)/Sorghastrum nutans (IG; BBIG) and Panicum virgatum (SG) pastures interseeded with forbs and grazed over five years. ANOVA analysis was conducted using R with significance set at p ≤ 0.05. Forb species exhibited different establishment and flowering characteristics. Coreopsis tinctoria, Rudbeckia hirta (BESU), and Coreopsis lanceolata (LCOR) established early, while Helianthus maximiliani, Heliopsis helianthoides (OSUN), and Echinacea purpurea (PURC) established the second season. Rudbeckia hirta, LCOR, OSUN, and PURC flowered most frequently, and the grazing regimen did not influence the flowering frequency of any species. Desmodium tortuosum (TTFL) was one of the most selected by cattle. Total forage mass declined in 2022, but forb mass interacted with treatment and year where mass declined each year but varied among treatments annually. Based on persistence and forage characteristics, BESU, LCOR, OSUN, PURC, and TTFL could successfully provide forage in native pastures under a variety of grazing regimens.

Insect pollinators are in population decline due to environmental and chemical stressors. Including native forbs in pastures could benefit grazers and pollinators; however, their forage and flowering characteristics are not fully documented. The objectives of our research were to evaluate 12 native forbs for persistence, forage mass, nutrient composition, and flowering patterns under repeated defoliation. Twelve species were planted in a small-plot experiment in 2018. Response variables were measured from 2020 to 2022. Annual (partridge pea, PPEA, Chamaecrista fasciculata) and biennial (black-eyed Susan, BESU, Rudbeckia hirta) species established high (p < 0.05) plant populations during the first season; however, the PPEA declined (p < 0.05) in forage mass during 2021. Tall species (Maximilian sunflower, MSUN, Helianthus maximiliani; cup plant, CUPP, Silphium perfoliatum) increased in forage mass, produced high-quality forage, and flowered during early fall. Lanceleaf coreopsis (LCOR, Coreopsis lanceolata) produced consistent (p > 0.05) forage mass and flowered in spring. The purple coneflower (PURC, Echinacea purpurea), Illinois bundleflower (ILBF, Desmanthus illinoensis), and oxeye sunflower (OSUN, Helopsis helianthoides) produced high-quality, consistent (p > 0.05) forage mass and flowered mid-season. Interseeding the BESU, ILBF, PPEA, LCOR, PURC, OSUN, and MSUN or CUPP would produce high-quality forage and floral resources throughout summer.

Incorporating native forbs within native warm-season grass (NWSG) pastures has the potential to benefit cattle, pollinators, and wildlife beyond that of NWSG monocultures. However, when grazing NWSG pastures, rotational stocking is recommended as opposed to continuous stocking. Therefore, to evaluate whether within-season rest is needed for native pasture sustainability, two NWSG grazing experiments were conducted near Greeneville, TN, 2017-2020, to assess the persistence of native forbs when an 11-species native forb blend was interseeded into established switchgrass (SG; Panicum virgatum L.) and big bluestem/indiangrass (BBIG; Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash) pastures. Each experiment was a completely randomized design with four replicates of each within-season rest grazing treatment (no rest, early rest, middle rest, late rest, and no graze). Within-season rest was not influential for NWSG tiller density or total forb plant density thus indicating persistence of forbs may not require rotational stocking. Purple prairie clover (Dalea purpurea Vent.) did not establish while Illinois bundleflower (Desmanthus illinoensis (Michx.) MacMill. ex B.L. Rob. & Fernald) was only observed flowering once despite having the greatest seeding rate among forbs. Based on establishment and flowering of the 11 species in the current mixture, interseeding a 6-species polyculture of black-eyed Susan (Rudbeckia hirta L.), dixie ticktrefoil (Desmodium tortuosum (Sw.) DC.), eastern purple coneflower (Echinacea purpurea (L.) Moench), lanceleaf coreopsis (Coreopsis lanceolata L.), Maximilian sunflower (Helianthus maximiliani Schrad.), and oxeye sunflower (Heliopsis helianthoides (L.) Sweet) could allow for plant biodiversity while offering floral resources for pollinators during the NWSG grazing season.

The identification of appropriate nitrogen (N) rates for native warm-season grasses (NWSG) is needed to inform forage management in the southeastern United States. Experiments were conducted in Knoxville and Springfield, TN, from 2015 to 2019, to evaluate dry matter (DM) yield, forage nutritive value (FNV), the influence of temperature and precipitation on yield, and partial factor productivity (PFP) responses. Three NWSG species (big bluestem [BB; Andropogon gerardii Vitman], switchgrass [SG; Panicum virgatum L.], and eastern gamagrass [EG; Tripsacum dactyloides L.]) were grown at each location and harvested twice annually. Five N rates in the form of urea were applied annually in split applications. The yields for all species responded positively to nitrogen (p < 0.001) and the time of harvest (p < 0.001) at both sites, except for BB yield at Springfield; no consistent N effects were observed over years. Nitrogen affected the FNV (p < 0.001) of all species, increasing CP by three to five percentage points (p < 0.001). Yields across all species and locations responded positively to precipitation (p < 0.001) and temperature (p < 0.001). A moderate N amendment (<135 kg N ha−1 yr−1, based on PFP) can enhance the productivity of NWSG, but responses were site-dependent and influenced by temperature and precipitation.

Pollinator declines and expectations for more sustainable agriculture, including pasture-based enterprises, bring attention to strategies to enhance the habitat value of grazing lands. We evaluated native warm-season grass (NWSG) pastures with (FORB) and without (CONT) interseeded native forbs in 2021–2023. An analysis was conducted using R with the significance set at p ≤ 0.05. The grass appeared to be weakened predominantly by grazing management practices. Forb density and mass had an inverse relationship in seasons two and three. Total forage mass declined in response to increased grazing days and weakened stands. The forage nutritive compositions differed, with more stable, season-long crude protein and lower fiber concentrations in late-season FORB, which supported higher bodyweight gains and season-long average daily gain. Black-eyed Susan (Rudbeckia hirta; BESU), lanceleaf coreopsis (Coreopsis lanceolata; LCOR), and showy ticktrefoil (Desmodium canadensis; STTF) were the most abundant forbs, and BESU, LCOR, and purple coneflower (Echinacea purpurea; PURC) produced long flowering windows. Cattle grazed STTF, cup plant (Silphium perfoliatum; CUPP), and oxeye sunflower (Helopsis helianthoides) the most. Under continuous stocking, a blend of BESU, LCOR, PURC, STTF, and CUPP produced acceptable cattle gains and provided pollinator resources, suggesting that this model may be a viable means to enhance the sustainability of pastures.

Soil microbial transformation of nitrogen (N) in nutrient-limited native C 4 grasslands can be affected by N fertilization rate and C 4 grass species. Here, we report in situ dynamics of the population size (gene copy abundances) and activity (transcript copy abundances) of five functional genes involved in soil N cycling ( nifH , bacterial amoA , nirK , nirS , and nosZ ) in a field experiment with two C 4 grass species (switchgrass ( Panicum virgatum ) and big bluestem ( Andropogon gerardii )) under three N fertilization rates (0, 67, and 202 kg N ha −1 ). Diazotroph ( nifH ) abundance and activity were not affected by N fertilization rate nor grass species. However, moderate and high N fertilization promoted population size and activity of ammonia oxidizing bacteria (AOB, quantified via amoA genes and transcripts) and nitrification potential. Moderate N fertilization increased abundances of nitrite-reducing bacterial genes ( nirK and nirS ) under switchgrass but decreased these genes under big bluestem. The activity of nitrous oxide reducing bacteria ( nosZ transcripts) was also promoted by moderate N fertilization. In general, high N fertilization had a negative effect on N-cycling populations compared to moderate N addition. Compared to big bluestem, the soils planted with switchgrass had a greater population size of AOB and nitrite reducers. The significant interaction effects of sampling season, grass species, and N fertilization rate on N-cycling microbial community at genetic-level rather than transcriptional-level suggested the activity of N-cycling microbial communities may be driven by more complex environmental factors in native C 4 grass systems, such as climatic and edaphic factors.

Soil microbial transformation of nitrogen (N) in nutrient-limited native C 4 grasslands can be affected by N fertilization rate and C 4 grass species.Here, we report in situ dynamics of the population size (gene copy abundances) and activity (transcript copy abundances) of five functional genes involved in soil N cycling (nifH, bacterial amoA, nirK, nirS, and nosZ) in a field experiment with two C 4 grass species (switchgrass [Panicum virgatum] and big bluestem [Andropogon gerardii]) under three N fertilization rates (0, 67, and 202 kg N ha -1). Diazotroph (nifH) abundance and activity were not affected by N fertilization rate nor grass species.However, moderate and high N fertilization promoted population size and activity of ammonia oxidizing bacteria (AOB, quantified via amoA genes and transcripts) and nitrification potential.Moderate N fertilization increased abundances of nitritereducing bacterial genes (nirK and nirS) under switchgrass but decreased these genes under big bluestem.The activity of nitrous oxide reducing bacteria (nosZ transcripts) was also promoted by moderate N fertilization.In general, high N fertilization had a negative effect on N-cycling populations compared to moderate N addition.Compared to big bluestem, the soils planted with switchgrass had a greater population size of AOB and nitrite reducers.The significant interaction effects of sampling season, grass species, and N fertilization rate on N-cycling microbial community at genetic-level rather than transcriptional-level suggested the activity of N-cycling microbial communities may be driven by more complex environmental factors in native C4 grass systems, such as climatic and edaphic factors.

Native C4 grasses have become the preferred species for native perennial pastures and bioenergy production due to their high productivity under low soil nitrogen (N) status. One reason for their low N requirement is that C4 grasses may benefit from soil diazotrophs and promote biological N fixation. Our objective was to evaluate the impact of N fertilization rates (0, 67, and 202 kg N ha-1) and grass species (switchgrass [Panicum virgatum] and big bluestem [Andropogon gerardii]) on the abundance, activity, diversity, and community composition of soil diazotrophs over three agricultural seasons (grass green-up, initial harvest, and second harvest) in a field experiment in East Tennessee, United States. Nitrogen fertilization rate had a stronger influence on diazotroph population size and activity (determined by nifH gene and transcript abundances) and community composition (determined by nifH gene amplicon sequencing) than agricultural season or grass species. Excessive fertilization (202 kg N ha-1) resulted in fewer nifH transcripts compared to moderate fertilization (67 kg N ha-1) and decreased both richness and evenness of diazotrophic community, reflecting an inhibitory effect of high N application rates on soil diazotrophic community. Overall, cluster I and cluster III diazotrophs were dominant in this native C4 grass system. Diazotroph population size and activity were directly related to soil water content (SWC) based on structural equation modeling. Soil pH, SWC, and C and N availability were related to the variability of diazotrophic community composition. Our results revealed relationships between soil diazotrophic community and associated soil properties, adding to our understanding of the response of soil diazotrophs to N fertilization and grass species in native C4 grass systems.

The lack of forage production during the seedling year is a barrier to wide-scale adoption of native warm-season grasses (NWSG). To address this, two NWSG establishment experiments were conducted in Knoxville, TN, 2016-2018, to determine the efficacy of big bluestem (BB; Andropogon gerardii Vitman) and switchgrass (SG; Panicum virgatum L.) establishment with browntop millet [BTM; Urochloa ramosa (L.) Nguyen] as a companion crop. Each experiment was a randomized complete block arranged as a 2 x 3 factorial. Two defoliation strategies [(1) harvests based on BTM maturity (boot to heading stage) for hay (HAY) or (2) clipping to control BTM competition by maintaining >50% sunlight reaching BB and SG seedlings (CLIP)] were coupled with three BTM seeding rates [0 (control), 11.2 (half-recommended rate), and 22.4 (full-recommended rate) kg pure live seed (PLS) ha(-1)]. Only BTM seeding rate affected BB and SG plant density at dormancy. In all cases, the control had greater BB and SG plant density than the full-recommended rate, indicating that BTM impeded BB and SG establishment. All BTM seeding rates resulted in acceptable stands (>= 5.4 plants m(-2)) of BB (both years) and SG (2017 only). Only the control allowed for acceptable stands of SG in 2016 (8.5 plants m(-2)). Managing BTM for HAY produced a mean cumulative dry matter (DM) yield of 3.15 and 2.68 Mg ha(-1) in 2016 and 2017, respectively. These findings show that BTM can be a companion crop that helps offset production losses during BB and SG establishment.

Background Fertilizer addition can contribute to nitrogen (N) losses from soil by affecting microbial populations responsible for nitrification. However, the effects of N fertilization on ammonia oxidizing bacteria under C 4 perennial grasses in nutrient-poor grasslands are not well studied. Methods In this study, a field experiment was used to assess the effects of N fertilization rate (0, 67, and 202 kg N ha −1 ) and grass species (switchgrass ( Panicum virgatum ) and big bluestem ( Andropogon gerardii )) on ammonia-oxidizing bacterial (AOB) communities in C 4 grassland soils using quantitative PCR, quantitative reverse transcription-PCR, and high-throughput amplicon sequencing of amoA genes. Results Nitrosospira were dominant AOB in the C 4 grassland soil throughout the growing season. N fertilization rate had a stronger influence on AOB community composition than C 4 grass species. Elevated N fertilizer application increased the abundance, activity, and alpha-diversity of AOB communities as well as nitrification potential, nitrous oxide (N 2 O) emission and soil acidity. The abundance and species richness of AOB were higher under switchgrass compared to big bluestem. Soil pH, nitrate, nitrification potential, and N 2 O emission were significantly related to the variability in AOB community structures ( p < 0.05).

Current knowledge of yield potential and best agronomic management practices for perennial bioenergy grasses is primarily derived from small‐scale and short‐term studies, yet these studies inform policy at the national scale. In an effort to learn more about how bioenergy grasses perform across multiple locations and years, the U.S. Department of Energy ( US DOE )/Sun Grant Initiative Regional Feedstock Partnership was initiated in 2008. The objectives of the Feedstock Partnership were to (1) provide a wide range of information for feedstock selection (species choice) and management practice options for a variety of regions and (2) develop national maps of potential feedstock yield for each of the herbaceous species evaluated. The Feedstock Partnership expands our previous understanding of the bioenergy potential of switchgrass, Miscanthus, sorghum, energycane, and prairie mixtures on Conservation Reserve Program land by conducting long‐term, replicated trials of each species at diverse environments in the U.S. Trials were initiated between 2008 and 2010 and completed between 2012 and 2015 depending on species. Field‐scale plots were utilized for switchgrass and Conservation Reserve Program trials to use traditional agricultural machinery. This is important as we know that the smaller scale studies often overestimated yield potential of some of these species. Insufficient vegetative propagules of energycane and Miscanthus prohibited farm‐scale trials of these species. The Feedstock Partnership studies also confirmed that environmental differences across years and across sites had a large impact on biomass production. Nitrogen application had variable effects across feedstocks, but some nitrogen fertilizer generally had a positive effect. National yield potential maps were developed using PRISM ‐ ELM for each species in the Feedstock Partnership. This manuscript, with the accompanying supplemental data, will be useful in making decisions about feedstock selection as well as agronomic practices across a wide region of the country.

Background.Fertilizer addition can contribute to nitrogen (N) losses from soil by affecting microbial populations responsible for nitrification.However, the effects of N fertilization on ammonia oxidizing bacteria under C 4 perennial grasses in nutrient-poor grasslands are not well studied.Methods.In this study, a field experiment was used to assess the effects of N fertilization rate (0, 67, and 202 kg N ha -1) and grass species (switchgrass [Panicum virgatum] and big bluestem [Andropogon gerardii]) on ammonia-oxidizing bacterial (AOB) communities in C 4 grassland soils using quantitative PCR, quantitative reverse transcription-PCR, and high-throughput amplicon sequencing of amoA genes.Results.Nitrosospira were dominant AOB in the C 4 grassland soil throughout the growing season.N fertilization rate had a stronger influence on AOB community composition than C 4 grass species.Elevated N fertilizer application increased the abundance, activity, and alpha-diversity of AOB communities as well as nitrification potential, nitrous oxide (N 2 O) emission and soil acidity.The abundance and species richness of AOB were higher under switchgrass compared to big bluestem.Soil pH, nitrate, nitrification potential, and N 2 O emission were significantly related to the variability in AOB community structures (p < 0.05).