Leif Klemedtsson

Energy exchange and water budget partitioning in a boreal minerogenic mire

This study investigated patterns and controls of the seasonal and inter-annual variations in energy fluxes (i.e., sensible heat, H, and latent heat, lambda E) and partitioning of the water budget (i.e., precipitation, P; evapotranspiration, ET; discharge, Q; and soil water storage, Delta S) over five years (2001-2005) in a boreal oligotrophic fen in northern Sweden based on continuous eddy covariance, water table level (WTL), and weir measurements. For the growing season (May 1 to September 31), the 5 year averages (+/- standard deviation) of the midday (10:00 to 14:00 h) Bowen ratio (beta, i.e., H/lambda E) was 0.86 +/- 0.08. Seasonal and inter-annual variability of beta was mainly driven by lambda E which itself was strongly controlled by both weather (i.e., vapor pressure deficit, D, and net radiation, R-n) and physiological parameters (i.e., surface resistance). During the growing season, surface resistance largely exceeded aerodynamic resistance, which together with low mean values of the actual ET to potential ET ratio (0.55 +/- 0.05) and Priestley-Taylor alpha (0.89) suggests significant physiological constrains on ET in this well-watered fen. Among the water budget components, the inter-annual variability of ET was lower (199 to 298 mm) compared to Q (225 to 752 mm), with each accounting on average for 34 and 65% of the ecosystem water loss, respectively. The fraction of P expended into ET was negatively correlated to P and positively to R-n. Although a decrease in WTL caused a reduction of the surface conductance, the overall effect of WTL on ET was limited. Non-growing season (October 1 to April 30) fluxes of H, lambda E, and Q were significant representing on average -67%, 13%, and 61%, respectively, of their growing season sums (negative sign indicates opposite flux direction between the two seasons). Overall, our findings suggest that plant functional type composition, P and R-n dynamics (i.e., amount and timing) were the major controls on the partitioning of the mire energy and water budgets. This has important implications for the regional climate as well as for ecosystem development, nutrient, and carbon dynamics. Citation: Peichl, M., J. Sagerfors, A. Lindroth, I. Buffam, A. Grelle, L. Klemedtsson, H. Laudon, and M. B. Nilsson (2013), Energy exchange and water budget partitioning in a boreal minerogenic mire, J. Geophys. Res. Biogeosci., 118, 1-13, doi:10.1029/2012JG002073.

Inhibitory effect of low partial pressures of acetylene on nitrification

Abstract The inhibiting effect of C 2 H 2 on nitrification was investigated in two agricultural soils. Nitrification was totally inhibited at 10 Pa partial pressure of C 2 H 2 which is lower than previously reported for soils. There were no differences in rates of nitrate production between flasks without C 2 H 2 and flasks with C 2 H 2 at 0.01 Pa, while there was an effect on nitrification at 0.1 PaC 2 H 2 . At 0.1 Pa no inhibition was noted during the first 3 days; after this period nitrification was partially inhibited. The inhibiting effect did not cease until 7 days after removal of C 2 H 2 . The sensitivity of nitrification to low concentrations of C 2 H 2 should be noted when denitrification rates are determined by the use of the acetylene inhibition method (usually C 2 H 2 at 10kPa).

Nitrous oxide emission from Swedish forest soils in relation to liming and simulated increased N-deposition

Abstract Fluxes of N2O were studied in a Norway spruce forest in the southwest of Sweden. Three differently treated catchments were compared: Limed (6 t dolomite ha–1), Nitrex (additional N-deposition corresponding to 35 kg ha–1 year–1, in small doses) and Control (used as control site). The N-retention was still high (95%) after 2years of N-addition at the Nitrex site when the flux measurements were performed. Each catchment contained both well-drained and poorly drained soils (covered with Sphagnum sp.). The emissions of N2O were in general low with both a high spatial and temporal variation for all three sites. The measured emissions were 25, 71 and 96 (gN2O-N ha–1 year–1) for the well-drained Limed, Control and Nitrex sites, respectively. The average emissions of N2O from the wet areas were significantly higher than the well-drained areas within the catchments. For the wet areas the measured emissions were larger: 90, 118 and 254 (g N2O-N ha–1 year–1) for the Limed, Control and Nitrex sites, respectively. Comparison between treatments showed the wet Nitrex site to have a significantly higher emission than all other sites. The increased N-deposition at the Nitrex site increased the N2O emissions by 0.2% of the added N for the well-drained soils and about 1% for the wet areas, compared with the control site. Since the wet areas represented only a small part of the forest, their larger emissions did not contribute significantly to the overall emission of the forest. Neither temperature nor water content of the soil was well correlated with the N2O emissions. Soil gas samples showed that most of the N2O was produced below a 0.3-m depth in the soil.

A method of selective inhibition to distinguish between nitrification and denitrification as sources of nitrous oxide in soil

SummaryNitrapyrin and C2H2 were evaluated as nitrification inhibitors in soil to determine the relative contributions of denitrification and nitrification to total N2O production. In laboratory experiments nitrapyrin, or its solvent xylene, stimulated denitrification directly or indirectly and was therefore considered unsuitable. Low partial pressures of C2H2 (2.5–5.0 Pa) inhibited nitrification and had only a small effect on denitrification, which made it possible to estimate the contribution of denitrification. The contribution of nitrification was estimated by subtracting the denitrification value from total N2O production (samples without C2H2). The critical C2H2 concentrations needed to achieve inhibition of nitrification, without affecting the N2O reductase in denitrifiers, must be individually determined for each set of experimental conditions.

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

SummaryThe effect of soil water content [60%–100% water-holding capacity (WHC)] on N2O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C2H2 concentration (2.1 Pa). Nitrifiers usually produced more N2O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N2O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N2O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O2 remaining in the soil. The use of 10 kPa C2H2 to inhibit reduction of N2O to N2 stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.

Nitrous oxide production in a forest soil at low temperatures – processes and environmental controls

Recent investigations have highlighted the relative importance of the winter season for emissions of N(2)O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N(2)O formation at temperatures below 0 degrees C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N(2)O formation and subsequent emission. Net N(2)O production rates at -4 degrees C equaled those observed at +10 to +15 degrees C at moisture contents >60% of the soils water-holding capacity. The source of this N(2)O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 degrees C. Furthermore, both net nitrogen-mineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N(2)O formation in soils at low temperatures.

AMMONIA AND NITROUS OXIDE EMISSIONS FROM GRASS AND ALFALFA MULCHES

Ammonia (NH3) and nitrous oxide (N-2O) emissions were measured in the field for three months from three different herbage mulches and from bare soil, used as a control. The mulches were grass with a low N-content (1.15% N in DM), grass with a high N-content (2.12% N in DM) and alfalfa with a high N-content (4.33% N in DM). NH3 volatilization was measured using a micrometeorological technique. N-2O emissions were measured using closed chambers. NH3 and N-2O emissions were found to be much higher from the N-rich mulches than from the low-N grass and bare soil, which did not differ significantly. Volatilization losses of NH3 and N-2O occurred mainly during the first month after applying the herbage and were highest from wet material shortly after a rain. The extent of NH3-N losses was difficult to estimate, due to the low frequency of measurements and some problems with the denuder technique, used on the first occasions of measurements. Nevertheless, the results indicate that NH3-N losses from herbage mulch rich in N can be substantial. Estimated losses of NH3-N ranged from the equivalent of 17% of the applied N for alfalfa to 39% for high-N grass. These losses not only represent a reduction in the fertilizer value of the mulch, but also contribute appreciably to atmospheric pollution. The estimated loss of N-2O-N during the measurement period amounted to 1% of the applied N in the N-rich materials, which is equivalent to at least 13 kg N-2O-N ha-1 lost from alfalfa and 6 kg ha-1 lost from high-N grass. These emission values greatly exceed the 0.2 kg N-2O-N ha-1 released from bare soil, and thus contribute to greenhouse gas emissions.

Dinitrogen and nitrous oxide produced by denitrification and nitrification in soil with and without barley plants

SummaryTo examine the effect of barley roots on denitrification, a pot experiment was designed to compare N2O production and denitrification in soils with and without barley plants. Denitrification, N2O resulting from denitrification and nitrification, and respiration were estimated by incubating pots with soil with and without intact plants in plastic bags at high moisture levels. C2H2-inhibition of nitrous oxide reductase (partial pressure of 10 kPa C2H2) was used to determine total denitrification rates while incubations with ambient air and with C2H2 at partial pressures of 2.5–5 Pa were used to estimate the amounts of N2O released from autotrophic nitrification and from denitrification processes. Other sources of N2O were presumed to be negligible. Potential denitrification, nitrification and root biomass were measured in subsamples collected from four soil depths.A positive correlation was found between denitrification rates and root biomass. N2 was the predominant denitrification product found close to roots; N2O formed by non autotrophic nitrifiers, assumed to be denitrifiers originated in soil not affected by growing roots. Apparently, roots promote denitrification because they consumed oxygen, thereby increasing the anaerobic volume of the soil. The ratio of actual to potential denitrification rates increased over time, especially in the presence of roots.

Microbial nitrogen transformations in the root environment of barley

Abstract To determine the influence of barley roots on microorganisms and N-transfonning processes in soil, numbers of nitrifiers and potential nitrification and denitrification rates were measured every week for 5 wks. The barley plants were grown in growth chambers in which the root-containing soil layer (A) was separated from three outer soil layers (B, C, D). The numbers and biomass of bacteria, numbers of flagellates and amoebae, total and FDA-active hyphal lengths, microbial biomass carbon and respiration were also determined. The numbers of ammonium oxidizers were positively correlated with root biomass but did not differ significantly between soil layers. Potential ammonium oxidation was stimulated in the root-layer, while potential nitrite oxidation was stimulated in the B- and C-layers. The denitrification activity (measured anaerobically in the presence of excess No − 3 ) was positively correlated with root biomass in the A-layer. Denitrification activity in the B-layer was positively correlated with the water content of the soil. When roots grew near the nets separating the root layer from the other layers, denitrification activity was stimulated in the next layer (B). We propose that nitrite oxidation in the root zone partly depends on the reduction of nitrate. This would explain why nitrite-oxidizer numbers were usually several orders of magnitude higher than ammonium-oxidizer numbers. Bacterial numbers decreased between wks 1 and 5. Increases in bacteria, naked amoebae and flagellates in all layers between wks 2 and 3 indicated that bacteria were produced until wk 3. There were no signs of bacterial production after wk 3. The total length of hyphae and the length of FDA-active hyphae were not significantly different between layers. However, both of these parameters, as well as total microbial biomass carbon and respiration, were consistently highest in the A-layer.

Methane uptake in Swedish forest soil in relation to liming and extra N-deposition

Abstract Methane uptake to soil was examined in individual chambers at three small forest catchments with different treatments, Control, Limed and Nitrex sites, where N-deposition was experimentally increased. The catchments consisted of both well-drained forest and wet sphagnum areas, and showed uptake of CH4 from the ambient air. The lowest CH4 uptakes were observed in the wet areas, where the different treatments did not influence the uptake rate. In the well-drained areas the CH4 uptakes were 1.6, 1.4 and 0.6 kg ha–1 year–1 for the Limed, Control and Nitrex sites, respectively. The uptake of methane at the well-drained Nitrex site was statistically smaller than at the other well-drained catchments. Both acidification and increase in nitrogen in the soil, caused by the air-borne deposition, are the probable cause for the reduction in the methane uptake potential. Uptake of methane was correlated to soil water content or temperature for individual chambers at the well-drained sites. The uptake rate of methane in soil cores was largest in the 0- to 10-cm upper soil layer. The concentration of CH4 in the soil was lower than the atmospheric concentration up to 30 cm depth, where methane production occurred. Besides acting as a sink for atmospheric methane, the oxidizing process in soil prevents the release of produced methane from deeper soil layers reaching the atmosphere.