Rengen Grassland Experiment Research Articles

Monitoring Forage Mass with Low-Cost UAV Data: Case Study at the Rengen Grassland Experiment

Monitoring and predicting above ground biomass yield of grasslands are of key importance for grassland management. Established manual methods such as clipping or rising plate meter measurements provide accurate estimates of forage yield, but are time consuming and labor intensive, and do not provide spatially continuous data as required for precision agriculture applications. Therefore, the main objective of this study is to investigate the potential of sward height metrics derived from low-cost unmanned aerial vehicle-based image data to predict forage yield. The study was conducted over a period of 3 consecutive years (2014–2016) at the Rengen Grassland Experiment (RGE) in Germany. The RGE was established in 1941 and is since then under the same management regime of five treatments in a random block design and two harvest cuts per year. For UAV-based image acquisition, a DJI Phantom 2 with a mounted Canon Powershot S110 was used as a low-cost aerial imaging system. The data were investigated at different levels (e.g., harvest date-specific, year-specific, and plant community-specific). A pooled data model resulted in an R2 of 0.65 with a RMSE of 956.57 kg ha−1, although cut-specific or date-specific models yielded better results. In general, the UAV-based metrics outperformed the traditional rising plate meter measurements, but was affected by the timing of the harvest cut and plant community.

Response of grassland vegetation composition to different fertilizer treatments recorded over ten years following 64 years of fertilizer applications in the Rengen Grassland Experiment

AbstractQuestionsFertilizer application is a key driver affecting the diversity and conservation value grassland vegetation. Using a long‐term fertilization experiment in mountain grassland, we addressed the following questions: (a) what is the effect of long‐term fertilizer applications on species richness and plant species composition, and (b) is there any detectable trend in plant species composition during ten years of continuous observation?LocationThe Rengen Grassland Experiment, Eifel Mountains, Germany (established in 1941).MethodsFive treatments including different fertilizers applied annually and one unfertilized control were analysed: Ca (718 kg Ca ha−1); CaN (752 Ca and 100 N kg ha−1); CaNP (752 Ca, 100 N and 35 P kg ha−1); CaNP‐KCl (752 Ca, 100 N, 35 P and 133 K kg ha−1); CaNP‐K2SO4 (752 Ca, 100 N, 35 P and 133 K kg ha−1). The experiment included five replicates per treatment in a fully randomized block design. All treatments were cut twice a year in late June or early July, and in mid‐October. Percentage cover of individual plant species was estimated by visual observation in each plot in late June in the years 2005–2014.ResultsDespite inter‐annual variability in the cover of the individual vascular plant species, the multivariate data analyses revealed a relatively similar response of the plant community to the different fertilizer applications throughout the ten years. With phosphorus application, no differences in botanical composition among treatments were found; however, they did differ from other treatments without phosphorus application. In the unfertilized control, there was a certain directed trend in plant species composition in response to ongoing nutrient impoverishment.ConclusionSpecies‐rich grasslands of high nature conservation value were only maintained under P limitation in the control without fertilizer application (Violion caninae) and in the liming treatment (Polygono‐Trisetion), but also in the treatment with liming and pure N addition. It seems that after 74 years, some stage of equilibrium of the grassland community was achieved in all treatments receiving any type of fertilizer application, but less so in the unfertilized control treatment.

POTENTIAL OF NON-CALIBRATED UAV-BASED RGB IMAGERY FOR FORAGE MONITORING: CASE STUDY AT THE RENGEN LONG-TERM GRASSLAND EXPERIMENT (RGE), GERMANY

Abstract. Forage monitoring in grassland is an important task to support management decisions. Spatial data on (i) yield,(ii) quality, and (iii) floristic composition are of interest. The spatio-temporal variability in grasslands is significant and requires fast and low-cost methods for data delivery. Therefore, the overarching aim of this contribution is the investigation of low-cost and non-calibrated UAV-derived RGB imagery for forage monitoring. Study area is the Rengen Grassland Experiment (RGE) in Germany which is a long-term field experiment since 1941. Due to the experiment layout, destructive biomass sampling during the growing period was not possible. Hence, non-destructive Rising Plate Meter (RPM) measurements, which are a common method to estimate biomass in grasslands, were carried out. UAV campaigns with a Canon Powershot 110 mounted on a DJI Phantom 2 were conducted in the first growing season in 2014. From the RGB imagery, the RGB vegetation index (RGBVI) and the Grassland Index (GrassI) introduced by Bendig et al. (2015) and Bareth et al. (2015), respectively, were computed. The RGBVI and the GrassI perform very well against the RPM measurements resulting in R2 of 0.84 and 0.9, respectively. These results indicate the potential of low-cost UAV methods for grassland monitoring and correspond well to the studies of Viljanen et al. (2018) and Näsi et al. (2018).

USING CALIBRATED RGB IMAGERY FROM LOW-COST UAVS FOR GRASSLAND MONITORING: CASE STUDY AT THE RENGEN GRASSLAND EXPERIMENT (RGE), GERMANY

Abstract. Monitoring the spectral response of intensively managed grassland throughout the growing season allows optimizing fertilizer inputs by monitoring plant growth. For example, site-specific fertilizer application as part of precision agriculture (PA) management requires information within short time. But, this requires field-based measurements with hyper- or multispectral sensors, which may not be feasible on a day to day farming practice. Exploiting the information of RGB images from consumer grade cameras mounted on unmanned aerial vehicles (UAV) can offer cost-efficient as well as near-real time analysis of grasslands with high temporal and spatial resolution. The potential of RGB imagery-based vegetation indices (VI) from consumer grade cameras mounted on UAVs has been explored recently in several. However, for multitemporal analyses it is desirable to calibrate the digital numbers (DN) of RGB-images to physical units. In this study, we explored the comparability of the RGBVI from a consumer grade camera mounted on a low-cost UAV to well established vegetation indices from hyperspectral field measurements for applications in grassland. The study was conducted in 2014 on the Rengen Grassland Experiment (RGE) in Germany. Image DN values were calibrated into reflectance by using the Empirical Line Method (Smith & Milton 1999). Depending on sampling date and VI the correlation between the UAV-based RGBVI and VIs such as the NDVI resulted in varying R2 values from no correlation to up to 0.9. These results indicate, that calibrated RGB-based VIs have the potential to support or substitute hyperspectral field measurements to facilitate management decisions on grasslands.

Long-term agricultural management maximizing hay production can significantly reduce belowground C storage

Liming and fertilization of grasslands have been used for centuries to sustain hay production. Besides improving hay yields, these practices induce compositional shifts in plant and soil microbial communities, including symbiotic arbuscular mycorrhizal (AM) fungi. However, in spite of increasing interest in soil carbon (C) sequestration to offset anthropogenic CO2 emissions, little is known about the long-term effects of these agronomic interventions on soil C stocks. We examined how plants, AM fungi, and soil C respond to more than seven decades of annual applications of lime, mineral nitrogen (N), and mineral phosphorus (P) to test the hypotheses that (1) management practices increasing aboveground plant production decrease C allocation to roots, AM fungi and the soil; and (2) the relative availability of N and P predicts belowground C allocation in a consistent manner. Our study was conducted at the Rengen Grassland Experiment, established in 1941. Lime combined with N increased hay yields and promoted development of AM fungal hyphae in soil, while reducing relative C allocation to roots. Simultaneous enrichment of soil with lime, N, and P further boosted hay production, promoted grasses and suppressed other plant functional groups. This treatment also decreased soil organic C and strongly suppressed AM fungi in the soil, although the response to P varied among different AM fungal taxa. Our results indicate that agricultural management practices aimed at maximization of hay production may, in the long run, significantly (−20%) reduce belowground C storage. This is a great concern with respect to the intended use of grasslands as anthropogenic CO2 sinks because the fertilization-induced decrease in soil C stocks can partly or fully negate the C sequestration potential of the grassland ecosystems as a whole.

Soil phosphorus fractions after seven decades of fertilizer application in the Rengen Grassland Experiment

AbstractDeclining global P reserves require a better understanding of P cycling in soil and related plant uptake. On managed grasslands, application of lime and fertilizer affects not only soil nutrient status, but also plant‐species composition of the sward. We examined the P fractionation in the Rengen Grassland Experiment (RGE) on a naturally acid Stagnic Cambisol in the Eifel Mts. (Germany) 69 y after the setup of the experiment. A modified sequential Hedley fractionation was carried out for samples from 30 plots at 0–10 cm depth. Application of inorganic phosphorus fertilizer had diverse effects on inorganic (Pi) and organic P (Po) fractions. Resin‐Pi, NaHCO3‐Pi, NaHCO3‐Po, NaOH‐Pi, HCldil‐Pi, HClconc‐Pi, and HClconc‐Po contents increased, while NaOH‐Po significantly decreased and residual‐P remained unaffected. Strongest enrichment occurred in the HCldil‐Pi fraction, probably due to the chemical nature of the basic Thomas slag applied as P fertilizer. Without P fertilization, all fractions except residual‐P were more or less depleted. Strong P limitation of the vegetation in the limed treatments without P led to lowered contents also for NaOH‐Pi and NaOH‐Po. However, NaOH‐Po was largest in the Control and even exceeded the respective content in the treatments with P. It remained unclear why species adapted to a low soil P status did not access this P fraction though being P‐limited. Published theory on the availability of Hedley P fractions does neither match P exploitation nor P nutritional status of the vegetation in the RGE. Regarding NaOH‐Po as stable and HCldil‐Pi as moderately labile led to a more realistic evaluation of plant P uptake. Evaluation of P availability on the basis of chemical extractions alone is questionable for conditions like in the RGE. On long‐term grassland, plant‐species composition has to be taken into account to estimate access of plants to soil P.

Competitive ability of Rhinanthus minor L. in relation to productivityin the Rengen Grassland Experiment

Rhinanthus minor (yellow-rattle) can be used for restoration of species-rich grasslands but is vulnerable to competitive exclusion from high total aboveground biomass production of vascular plants. We asked (1) whether there is a threshold limit for total annual aboveground biomass production of vascular plants above which R. minor cannot establish viable population in grasslands and (2) how is cover of R. minor in grassland related to standing biomass of bryophytes. Data were collected in the Rengen Grassland Experiment (RGE) established in Germany in 1941 with following fertilizer treatments: unfertilized control, application of Ca, CaN, CaNP, CaNPKCl and CaNPK<sub>2</sub>SO<sub>4</sub>. Cover of R. minor and total annual aboveground biomass production of vascular plants were determined from 2005 to 2009. Further relationship between standing biomass of bryophytes and cover of R. minor was analyzed in 2006. Mean cover of R. minor over five years ranged from 0.7% to 12.3% in CaNPK<sub>2</sub>SO<sub>4</sub> and control treatments, respectively. Cover of R. minor was significantly negatively related to total annual aboveground biomass production of vascular plants and cover of R. minor was below 3% in all plots with total annual aboveground dry matter biomass production of vascular plants higher than 5 t/ha. Although cover of R. minor was markedly reduced in highly productive plots in the RGE, high standing biomass of bryophytes (1.8 t/ha) in low productive control was not an obstacle for establishment of its viable population. We concluded, that viable population of R. minor can be established in grasslands only if total annual aboveground dry matter biomass production of vascular plants is below 5 t/ha regardless on standing biomass of bryophytes.

Dactylorhiza maculata, Platanthera bifolia and Listera ovata survive N application under P limitation

Increased nutrient availability is generally believed to be harmful for terrestrial orchids. However, no information is available on whether grassland orchids survive decades-long calcium (Ca), nitrogen (N), phosphorus (P) and potassium (K) application. We took advantage of sampling in one of the oldest long-term experiments in the world, the Rengen Grassland Experiment (RGE), which was established in Germany in 1941 on low-productive Violion caninae grassland, and we monitored orchids from 2005 to 2009. Dactylorhiza maculata, Platanthera bifolia and Listera ovata survived decades-long application of Ca and CaN fertilizer, but not CaN together with P. Cover of D. maculata was highest in areas of Ca treatment. The number of flowers per plant of L. ovata was significantly higher in the CaN treatment compared with the control indicating that enhanced N availability stimulated its growth. Long-term N application (NH 4NO 3 at an annual rate of 100 kg N ha −1) was not fatal to the studied orchids if not accompanied by P application. It is more likely that enhanced N availability can cause loss of studied orchids only where the availability of P in soil is high enough to enable the growth of productive species that can better compete for light on eutrophic soils.

The Rengen Grassland Experiment: Effect of Soil Chemical Properties on Biomass Production, Plant Species Composition and Species Richness

The Rengen Grassland Experiment (RGE), set up on a Nardus grassland in 1941, consists of a control and five fertilizer treatments (Ca, CaN, CaNP, CaNP-KCl and CaNP-K2SO4). In 2005, soil chemical properties were analyzed to investigate the effect of soil variables on biomass production, plant species composition and species richness of vascular plants. Further, the effect of sampling scale (from 0.02 to 5.76 m2) on species richness was investigated. Soil properties (plant-available contents of K, P, C:N ratio, and pH) and biomass production were found to be strictly dependent on the fertilizers applied. Diversification of soil P content between treatments with and without P application is still in progress. Biomass production was most positively affected by P and K soil contents under N application. Furthermore, pH had a small positive effect on biomass production, and C:N ratio a moderately negative one. Two types of nutrient limitation were recognized: (1) limitation of total biomass production and (2) limitation of individual plant species. Long-term addition of a limiting nutrient affected the grassland ecosystem in three ways: (1) causing a change in plant species composition without significant increase in total biomass production, (2) causing no change in species composition but with significant increase in total biomass production, and (3) causing substantial change in plant species composition accompanied by significant increase in total biomass production. The explanatory power of all measured soil properties on plant species composition was almost the same as the power of the treatment effect (61.7% versus 62% of explained variability in RDA). The most powerful predictors of plant species composition were soil P, K and Mg contents, pH, and biomass production. The soil P content and biomass production were the only variables leading to a significant negative effect on species richness. An almost parallel increase in species richness with increasing sampling area was detected in all treatments. Constant differences among treatments were independent of sampling area.

The Rengen Grassland Experiment: relationship between soil and biomass chemical properties, amount of elements applied, and their uptake

The Rengen Grassland Experiment (RGE) was established in the Eifel Mountains (Germany) on a low productive Nardetum in 1941. Since then, the following fertilizer treatments have been applied with a late two-cut system: unfertilized control, Ca, CaN, CaNP, CaNPKCl and CaNPK2SO4. We aimed to understand how concentrations of macro (N, P, K, Ca and Mg), micro (Cu, Fe, Mn and Zn) and trace (As, Cd, Cr, Ni and Pb) elements in the plant biomass were affected by long-term fertilizer application, soil chemical properties and biomass production. In 2008, biomass samples from the first cut (early July) and the second cut (mid-October) were collected and analyzed. The simultaneous application of N, P and K decreased nitrogen concentration in the aboveground biomass, but substantially increased biomass production. Late cutting management decreased forage quality in highly productive more than in low productive plant communities. The concentrations of P and K in the plant biomass were positively related to P and K application and, therefore, to plant available P and K concentrations in the soil. The concentrations of some micro (Fe, Mn and Zn) and trace (As, Cd, Cr, Ni and Pb) elements in the plant biomass were negatively correlated with the amount of elements supplied by fertilizers and biomass production, probably because of the dilution effect. Long-term fertilizer application resulted in the accumulation of macro (P, Ca and Mg), micro (Fe and Mn) and trace (As and Cr) elements in the soil, but in many cases this accumulation was not connected with an increase in the concentrations of these elements in the plant biomass. Nutritional status, as indicated by the biomass N:P ratio, was consistent with N or P limitation as indicated by the nitrogen and phosphorus nutrition indices. Furthermore, additional K (co-)limitation was indicated by the N:K and K:P ratios in the biomass from the NP treatment. The results from the RGE indicate that there is no simple positive relationship between the applied elements and their concentrations in the plant biomass.

The Rengen Grassland experiment: bryophytes biomass and element concentrations after 65 years of fertilizer application

The Rengen Grassland Experiment in Germany, established in 1941, consists of the following fertilizer treatments applied under a two cut management: control, Ca, CaN, CaNP, CaNP-KCl, and CaNP-K(2)SO(4). The aim of this study was (1) to identify effects of fertilizer application on biomass and species composition of bryophytes and (2) to investigate the impact of fertilizer application on macro- (N, P, K, Ca, Mg), micro- (Cu, Fe, Mn, Zn), and toxic (As, Cd, Cr, Pb, Ni) element concentrations in bryophyte biomass. In June 2006, Rhytidiadelphus squarrosus was the only bryophyte species recorded in the control. In treatment Ca, R. squarrosus was the dominant bryophyte species whereas Brachythecium rutabulum occurred sporadically only in a single plot of that treatment. The latter was the only bryophyte species collected in CaN, CaNP, CaNP-KCl, and CaNP-K(2)SO(4) treatments. Dry matter accumulation of bryophytes was highest in the control (180 g m(-2)) followed by Ca (46 g m(-2)), CaNP (25 g m(-2)), CaNP-KCl (15 g m(-2)), CaNP-K(2)SO(4) (9 g m(-2)), and CaN (2 g m(-2)) treatments. A negative correlation between biomass production of bryophytes and dry matter production of vascular plants was revealed up to a threshold value of 400 g m(-2). Above this limit, biomass production of bryophytes remained obviously unaffected by further increase in biomass production of vascular plants. A significant effect of treatment on As, Cd, Cr, Fe, Mn, Ni, Pb, P, Ca, Mg, K, and N concentrations was revealed. Concentrations of these elements were a function of amount of elements supplied with fertilizers. Bryophytes seem to be promising bio-indicators not only for airborne deposition of toxic element but also for fertilizer introduced as well.

Changes in vegetation types and Ellenberg indicator values after 65 years of fertilizer application in the Rengen Grassland Experiment, Germany

AbstractQuestion: How does semi‐natural grassland diversify after 65 years of differential application of Ca, N, P, and K fertilizers? Is fertilizer application adequately reflected by the Ellenberg indicator values (EIVs)?Location: Eifel Mountains, West Germany.Methods: The Rengen Grassland Experiment (RGE) was established in an oligotrophic grassland in 1941. Six fertilizer treatments (Ca, CaN, CaNP, CaNP‐KCl, CaNP‐K2SO4, and unfertilized control) were applied annually in five complete randomized blocks. Species composition of experimental plots was sampled in 2006 and compared with constancy tables representing grassland types in a phytosociological monograph of a wider area. Each plot was matched to the most similar community type using the Associa method. Mean EIVs were calculated for each treatment.Results: The control plots supported oligotrophic Nardus grassland of the Polygalo‐Nardetum association (Violion caninae alliance). Vegetation in the Ca and CaN treatments mostly resembled montane meadow of Geranio‐Trisetetum (Polygono‐Trisetion). Transitional types between Poo‐Trisetetum and Arrhenatheretum (both from the Arrhenatherion alliance) developed in the CaNP treatment. In the CaNP‐KCl and CaNP‐K2SO4 treatments, vegetation corresponded to the mesotrophic Arrhenatheretum meadow. Major discontinuity in species composition was found between control, Ca, and CaN treatments, and all treatments with P application. EIVs for both nutrients and soil reaction were considerably higher in P treatments than in Ca and CaN treatments. Surprisingly, the control plots had the lowest EIVs for continentality and moisture, although these factors had not been manipulated in the experiment.Conclusions: Long‐term fertilizer application can create different plant communities belonging to different phytosociological alliances and classes, even within a distance of a few meters. Due to their correlated nature, EIVs can erroneously indicate changes in factors that actually did not change, but co‐varied with factors that did change. In P‐limited ecosystems, EIVs for nutrients may indicate availability of P rather than N.

The Rengen Grassland Experiment: soil contamination by trace elements after 65 years of Ca, N, P and K fertiliser application

The Rengen Grassland Experiment (RGE) was established in the Eifel Mts. (Germany) on a low productive Nardetum in 1941. Since then, the following fertiliser treatments have been applied along with a two cut system: unfertilised control, Ca, CaN, CaNP, CaNP–KCl and CaNP–K2SO4 with basic slag (syn. Thomas phosphate) as the only P fertiliser. The effect of long-term fertilisation on plant-available (extracted with 0.01 mol l−1 CaCl2), easily-mobilisable (extracted with 0.05 mol l−1 EDTA), potentially-mobilisable (extracted with 2 mol l−1 HNO3) and total concentrations of trace elements (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in the top 0–10 and 10–20 cm of soil were investigated in 2006. According to redundancy analysis (RDA), the effect of treatment on the concentrations of risk elements was significant and explained 82.3 and 90.6% of the variability in the data in the 0–10 and 10–20 cm soil layers, respectively. Basic slag supplied the soil with considerable amounts of As, Cr, Cu, Fe, Mn and Zn. Following 65 years of fertiliser application the concentrations of risk elements in the soil profile had increased substantially, especially with basic slag. However, threshold limits for total trace element concentration in soil permitted by Czech national legislation were exceeded only in the case of As. The increase in plant-available As concentrations was most critical as it increased the potential uptake of As by plants in plots fertilised with P. Although P treatments received more than 300 g of Cr ha−1 annually, no effect on plant-available Cr soil content was detected. This contrasted with the accumulation of total Cr in the 0–10 and 10–20 cm soil layers. Furthermore, plant availability of Cd, Fe, Mn and Zn was affected by soil pH and generally decreased with the application of quick lime. Plant availability of these elements was not correlated with amounts supplied by fertilisers.

The Rengen Grassland Experiment: Plant species composition after 64 years of fertilizer application

A long-term fertilizer experiment (the Rengen Grassland Experiment, RGE) was established in 1941 in the Eifel Mountains of Germany on low productive grassland naturally dominated by Calluna vulgaris and Nardus stricta. Six treatments combinations of Ca, N, P, and K fertilizer were applied annually: an unfertilized control, Ca, CaN, CaNP, CaNP–KCl, and CaNP–K 2SO 4. In mid-June 2005, plant cover was visually estimated and sward height was measured aiming to detect changes in floristic composition caused by long-term fertilization. Calculated by redundancy analysis (RDA), the effect of treatment was found to be a significant predictor of sward structure in the experimental area and explained 62% of cover data variability. The largest difference in vegetation structure and composition was between the treatments without and with P application. Briza media was the dominant short grass in the control, Ca, and CaN treatments. Lathyrus linifolius was the dominant legume in the control and Carex panicea was dominant in the CaN treatment. Among treatments with P application, plant species composition was similar with tall grasses such as Alopecurus pratensis, Arrhenatherum elatius, and Trisetum flavescens dominating in the sward. Sward heights were lowest in treatments without P addition in contrast to tall canopies in the P fertilized treatments. Cover of the moss layer was continuously developed in treatments without P application only and was negatively correlated with sward height as was species richness of vascular plants. Long-term fertilization caused significant diversification of plant communities. Species indicative of low productivity grasslands (short grasses, orchids, and sedges) survived in the CaN treatment but not in plots with P or K fertilizer. It was often thought that N enrichment is detrimental to vascular plant richness. However, the RGE shows that this is not necessarily so if N application is not accompanied by another limiting nutrient like P.