Paludiculture, and in particular the cultivation of perennial grasses as biomass feedstock for green biorefineries, may be an economic and environmentally sustainable option for agricultural peatlands in temperate regions. However, the optimal biomass quality for protein extraction from flood-tolerant grasses is largely unknown. The aim of this study was to define the combined effect of harvest and fertilization frequency, with one to five annual cuts, on protein yield and extractability for the grasses tall fescue (TF) and reed canary grass (RCG), cultivated on an agricultural fen peatland in Denmark.The content of protein fractions was determined according to the Cornell Net Carbohydrate and Protein System (CNCPS). We assessed protein extractability by lab-scale biorefinery techniques using a screw-press followed by acid precipitation of true protein. The two methods were compared to correlate potential extractable protein yields with actual biorefinery outputs. We found the highest annual biomass and crude protein (CP) yields in the two cut treatments, with 13.4 and 15.6 t dry matter (DM) ha−1 year−1, containing 2.9–3.4 t CP ha−1 year−1 for TF and RCG, respectively. The highest neutral-extractable (fractions B1 and B2) true protein yields of 1.1 and 1.5 t ha−1 year−1 were found in the two cut treatments, representing 39% (TF) - 45% (RCG) of total CP. Using biorefining techniques, we were able to precipitate up to 2.2 t DM ha−1 year−1 of protein concentrate, containing up to 39% CP. Significant correlations between methods were found, with a distinct relationship between CNCPS fractions B1 + B2 and CP yield of the protein concentrate, indicating the suitability of the CNCPS as an indicator for extractable protein yields. Biomass and CP yields were not significantly improved beyond two annual cuts. However, timing and harvest frequencies significantly affected plant maturity and consequently extractable CP contents and protein concentrate yields. We conclude that TF and RCG are promising feedstocks for green biorefineries due to high biomass, extractable CP, and protein concentrate yields, and highlight the potential of flood-tolerant grasses, cultivated on wet agricultural peatlands, for an enhanced valorisation beyond the common utilisation for bioenergy.

Time domain reflectometry (TDR) measurements of the volumetric water content (θ) of soils are based on the dielectric permittivity (ε), relating ε to θ, using an empirical calibration function. Accurate determination of θ for peaty soils is vital but complicated by the complexity of organic soils and the lack of a general calibration model. Site-specific calibration models were developed to determine θ from TDR measurements for a heterogenous peatland across gradients of peat decomposition and organic carbon (OC) content; derived by soil organic matter conversion. The possibility of predicting OC contents based on the corrected θ (θcor); ε; electrical impedance (Ζ); and a categorical predictor variable was explored. The application of plot-specific and local area calibration models resulted in similar results. Compared to common calibrations, the threshold for accurate determination of θ was at ε = 5; with higher ε underestimating θ by up to 25%. Including the von Post degree of peat humification as a bioindicator, the OC content could be modelled across the area and the full range of θ with an accuracy of ±1.2% for 496 measurements. In conclusion, a strong indication was found for determining OC in peatlands in situ using TDR and a site-specific calibration model for θ together with indices of peat decomposition.

Quantifying soil organic carbon stocks (SOC) is a critical task in decision support related to climate and land management. Carbon inputs in soils are affected by development of belowground (BGB) and aboveground (AGB) biomass. However, uncertain fixed values of root:shoot ratios (R/S) are widely used for calculating SOC inputs in agroecosystems. In this study, we 1) assessed the effect of harvest frequency (zero, one, two, and five times annually) on the root and shoot development of the perennial grasses Phalaris arundinacea (RCG), Festuca arundinacea (TF), and Festulolium (FL); 2) determined the effect of management on the carbon and nitrogen content in AGB and BGB; and 3) assessed the implications of R/S for SOC quantification. We found the highest yields of BGB in zero-cut treatments with 59% (FL)–70% (RCG) of total biomass. AGB yield was highest in the five-cut treatments with 54% (RCG)–60% (FL), resulting in a decreasing R/S with frequent management, ranging from 1.6–2.3 (zero cut) to 0.6–0.8 (five cuts). No differences in R/S between species were observed. Total carbon yield ranged between 5.5 (FL, one cut) and 18.9 t ha−1 year−1 (FL, zero cut), with a higher carbon content in AGB (45%) than BGB (40%). We showed that the input of total organic carbon into soil was highest in the zero-cut treatments, ranging between 6.6 and 7.6 t C ha−1 year−1, although, in the context of agricultural management the two-cut treatments showed the highest potential for carbon input (3.4–5.4 t C ha−1 year−1). Our results highlighted that using default values for R/S resulted in inaccurate modeling estimations of the soil carbon input, as compared to a management-specific application of R/S. We conclude that an increasing number of annual cuts significantly lowered the R/S for all grasses. Given the critical role of BGB carbon input, our study highlights the need for comprehensive long-term experiments regarding the development of perennial grass root systems under AGB manipulation by harvest. In conclusion, we indicated the importance of using more accurate R/S for perennial grasses depending on management to avoid over- and underestimation of the carbon sink functioning of grassland ecosystems.

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<p>Drainage of peatlands for agriculture causes substantial degradation and finally loss, including associated ecosystem functions, but also creates emission hotspots of carbon dioxide (CO<sub>2</sub>). Mean CO<sub>2</sub> emission from drained temperate grassland on peat was reported by IPCC as 22.4 (18.3-26.7) Mg<br>CO<sub>2</sub>-eq ha<sup>-1</sup> y<sup>-1</sup> (95% CI) while methane (CH<sub>4</sub>) emissions were close to zero. Rewetting of peatlands reduces CO<sub>2</sub> emissions while at the same time favouring CH<sub>4 </sub>emissions. From wet or rewetted nutrient-rich grassland, emissions of CO<sub>2</sub> and CH<sub>4</sub> were reported by IPCC as 1.8 (-2.8-2.8) and<br>9.8 (0-39) Mg CO<sub>2</sub>-eq ha<sup>-1</sup> y<sup>-1</sup>, respectively (GWP CH<sub>4 </sub>= 34). The uncertainties of the estimates reflect the large variation among the reported studies, which could be caused by different climate conditions, vegetation, groundwater table (GWT), peat composition and biogeochemistry. A mesocosm experiment was established to assess biogeochemical causes of variation in CO<sub>2</sub> and CH<sub>4</sub> flux dynamics under controlled GWT for peatsoils derived from five different Danish bogs and fens. A total number of 75 mesocosms were grouped into three treatments: GWT -40 cm, bare; GWT -5 cm, bare; and GWT -5 cm, cultivated with reed canary grass (RCG). GHG fluxes were measured using opaque chambers at biweekly intervals from July 2019 to 2020 and extrapolated to annual values. Preliminary results indicate significant differences regarding CO<sub>2</sub> and CH<sub>4</sub> fluxes across all sites and depending on soil biogeochemical and physical properties. Rewetting raised the contribution of CH<sub>4</sub> most on soils from Store Vildmose and Vejrumbro with 1.9 to 12.9 t CO<sub>2</sub>eq ha<sup>-1</sup> yr<sup>-1</sup> and 0.1 to 5.7 t CO<sub>2</sub>eq ha<sup>-1</sup> yr<sup>-1</sup>, respectively. On an annual average, these high emissions were with 69 % and 48 % mitigated by the cultivation of RCG in a paludiculture scenario. Further, the results show that CH<sub>4</sub> spikes of up to 37.5 mg m<sup>-2</sup> h<sup>-1</sup> at elevated GWT during warmer summer months may be mitigated by cultivation with RCG, with maximum peaks of 2.1 mg m<sup>-2</sup> h<sup>-1</sup>. Soil analyses highlighted distinct differences in the soil mineralogical composition across sites and soil depths.</p>