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Udgivelsesdato: November

There is presently limited knowledge on the influence of field spatial variability on the carbon (C) sink-source relationships in arable landscapes. This is accompanied by the fact that our understanding of soil profile C dynamics is also limited. This study aimed at investigating how spatial variability along a short catena influences C sinksource relationships and temporal dynamics of CO2 concentrations in soils. In spring 2011, soil samples were collected from topsoil (2-5.5 cm) and subsoil (38-41.5 cm) horizons at upslope and footslope positions in a Danish winter wheat field on a sandy loam soil developed on glacial till. Bulk densities and C concentrations of the soils were characterized. From June 2011, gas samples were collected at least bimonthly from the same slope positions in four spatial replicates using stainless steel needles that were permanently installed at 5, 10, 20 and 30 cm soil depths. Concurrently, gas was sampled from 40, 50, and 80 cm depths using steel rods connected to a sampling port. Concentrations of CO2 in the gas samples were analyzed by gas chromatography. The results show that at the upslope position, soils from the topsoil horizon clearly had higher C pools (5.2 Mg C ha-1) compared to those from the subsoil horizon (1.0 Mg C ha-1). At the footslope position, however, C pools in topsoil (6.9 Mg C ha-1) and subsoil (7.0 C Mg ha-1) horizons were similar but higher than those at the upslope position. The gas monitoring study is still ongoing, but preliminary results show that CO2 concentrations generally increased with depth. At the upslope position, CO2 concentrations ranged between 800 and 24000 ppm and were generally lower than the concentrations observed at the footslope position (3000􀀀42000 ppm) for similar soil depths. The upslope position has been subject to soil erosion while the footslope position has been a depositional site; thus the subsoil at the footslope position was to a large extent a buried topsoil horizon. The topographic relationship between the upslope and footslope position made the latter a sink for soil C transported through processes such as tillage erosion. This led to the presence of higher C pools and CO2 concentrations at the footslope compared to the upslope position. Despite the fact that higher CO2 concentrations with depth may be influenced by the relationship between CO2 production and transport, our results indicated that variability across arable landscapes makes footslope soils both a larger sink of buried soil C and a bigger potential CO2 source than upslope soils.

The growth of deep-rooted crops within agricultural soils has the potential to increase carbon deposition within deep subsoil layers potentially mitigating climate change. The growth of these deep-rooted crops and subsequent availability of both labile C substrates and nutrients such as nitrogen (N), phosphorus (P), and/or sulfur (S) have the potential to influence both C turnover and stability within deep subsoil layers. The present study utilized intact soil samples obtained from 5-6 m to study the effects of nutrient limitations for microbial C turnover when glucose or an artificial root exudate (ARE) mixture, and supplementary nutrients (N-P-S) were introduced as different treatments to deep subsoil samples during a 10-week incubation study. Our results document that C substrates alone are not the only drivers in C turnover, although significant differences between the addition of only C substrates were documented with the addition of only glucose compared to the addition of the ARE mixture. Such differences were interpreted as a partial alleviation of the N limitation due to the N-containing amino acid, L-arginine within the ARE mixture, but differences are also a likely a response to the diversity of compounds within the ARE mixture. Furthermore, we found potential effects of a co-limitation of N and P on C turnover in these deep subsoil samples to depths of 5-6 m, far exceeding depths from previous studies. As such, based on the co-limitation of N and P as observed within this study, it is important to know the N and P status of subsoils to predict the fate of organic C in deep soils as the production of microbial residues is based on the coupling of these nutrients to meet the stoichiometric microbial demand. Hence, managements removing such limitations could facilitate the stability and long-term storage of C in deep subsoil.

Udgivelsesdato: November

Background and AimsDrained peatlands are considered to be insignificant CH4 sources, but the effect of drainage on CH4 dynamics has not been extensively studied. We investigated seasonal dynamics of CH4 in two fen peat soils and one bog peat soil under permanent grassland in Denmark.MethodsSoil CH4 concentrations were measured several times throughout the year in parallel to a one year CH4 flux monitoring campaign with static chambers. Additionally, CH4 production potentials were determined in a laboratory incubation assay for the bog soil.ResultsMethane fluxes were generally negligible, even though soil CH4 concentrations of up to 155 and 1000 μmol CH4 dm−3 were measured in one of the fen peats and in the bog peat, respectively. Significant CH4 concentrations were observed above the water table. Methane production assays confirmed the presence of viable methanogens in the upper parts of the bog peat soil. The aerenchymous plant Juncus effusus L. liberated CH4 from the peat at rates of up to 3.3 mg CH4 m−2 h−1. No CH4 dynamics were observed in the second fen peat which, in contrast to the other two sites, had high sulfate concentrations.ConclusionsPeat type and the distribution of aerenchymous plants should be considered before dismissing grasslands on peat as CH4 sources.

Noter om: (1) Økologisk forædling af hvedesorter; (2) Løsnet pløjesål med blandet effekt på udbytte; (3) Skadetærskler for kvik og tidsler; (4) Møde om økologisk vækst.

Udgivelsesdato: December

Note om kartoflers optag af N fra placeret gylle Økologiske spisekartofler er en højværdi-afgrøde som har et betydeligt potentiale for at kunne give bedre rentabilitet i økologisk planteavl. Et hyppigt problem er dog, at mange økologiske kartofler er umodne, når de nedvisner som følge af skimmelangreb. Derfor er det vigtigt, at kartoflerne kommer tidligt i gang i foråret, og at der er næringsstoffer til rådighed tidligt i vækstperioden. Her viser nye undersøgelser, at placering af gylle direkte under kammen øger N-optagelse i afgrøden. Ved begyndende skimmelangreb blev der som gennemsnit optaget 117 og 124 kg N pr. ha i afgrøden dyrket i Jyndevad, mens der blev optaget 148 og 164 kg N pr. ha i afgrøden dyrket i Foulum, når gyllen var henholdsvis udbragt på normal vis eller placeret.

Ukrudt i rækkeafgrøder kan begrænses med en ny teknik, hvor jorden dampes i smalle striber. Derfor kan dampning være interessant i økologisk jordbrug. Teknikken påvirker dog også jordens mikroliv.

Removal of the plant hormone ethylene (C 2 H 4 ) is often required by horticultural storage facilities, which are operated at temperatures below 10°C. The aim of this study was to demonstrate an efficient, biological C 2 H 4 removal under such low-temperature conditions. Peat-soil, acclimated to degradation of C 2 H 4 , was packed in a biofilter (687 cm 3 ) and subjected to an airflow (∼73 ml min −1 ) with 2 ppm (μl liter −1 ) C 2 H 4 . The C 2 H 4 removal efficiencies achieved at 20, 10, and 5°C, respectively, were 99.0, 98.8, and 98.4%. This corresponded to C 2 H 4 levels of 0.022 to 0.032 ppm in the biofilter outlet air. At 2°C, the average C 2 H 4 removal efficiency dropped to 83%. The detailed temperature response of C 2 H 4 removal was tested under batch conditions by incubation of 1-g soil samples in a temperature gradient ranging from 0 to 29°C with increments of 1°C. The C 2 H 4 removal rate was highest at 26°C (0.85 μg of C 2 H 4 g [dry weight] −1 h −1 ), but remained at levels of 0.14 to 0.28 μg of C 2 H 4 g (dry weight) −1 h −1 at 0 to 10°C. At 35 to 40°C, the C 2 H 4 removal rate was negligible (0.02 to 0.06 μg of C 2 H 4 g [dry weight] −1 h −1 ). The Q 10 (i.e., the ratio of rates 10°C apart) for C 2 H 4 removal was 1.9 for the interval 0 to 10°C. In conclusion, the present results demonstrated microbial C 2 H 4 removal, which proceeded at 0 to 2°C and produced a moderately psychrophilic temperature response.

In environments with temperatures above 60°C, thermophilic prokaryotes are the only metabolically active life-forms. By using the 35 SO 4 2- tracer technique, we studied the activity of sulfate-reducing microorganisms (SRM) in hot sediment from a hydrothermal vent site in the northern part of freshwater Lake Tanganyika (East Africa). Incubation of slurry samples at 8 to 90°C demonstrated meso- and thermophilic sulfate reduction with optimum temperatures of 34 to 45°C and 56 to 65°C, respectively, and with an upper temperature limit of 80°C. Sulfate reduction was stimulated at all temperatures by the addition of short-chain fatty acids and benzoate or complex substrates (yeast extract and peptone). A time course experiment showed that linear thermophilic sulfate consumption occurred after a lag phase (12 h) and indicated the presence of a large population of SRM in the hydrothermal sediment. Thermophilic sulfate reduction had a pH optimum of about 7 and was completely inhibited at pH 8.8 to 9.2. SRM could be enriched from hydrothermal chimney and sediment samples at 60 and 75°C. In lactate-grown enrichments, sulfide production occurred at up to 70 and 75°C, with optima at 63 and 71°C, respectively. Several sporulating thermophilic enrichments were morphologically similar to Desulfotomaculum spp. Dissimilatory sulfate reduction in the studied hydrothermal area of Lake Tanganyika apparently has an upper temperature limit of 80°C.