ALBERT

Description: SUMMARYIn autumn 1991, sugarbeet tops (Beta vulgaris L.) and different components of oilseed rape residues (Brassica napus L.), both labelled with 15N, were incorporated into the soil under field conditions at Askov Experimental Station, Denmark, using stainless steel cylinders to contain the treatments. The availability of this labelled N to a subsequent crop was measured, using as test crops autumn-sown rye (Secale cereale L.), wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.). In spring 1992, cylinders with 15 N-residues received NH4NO3 and those without 15NH415NO3. In a parallel experiment, 15N-labelled beet tops were incorporated in lysimeters. A four-course rotation of sugarbeet, spring barley (undersown with perennial ryegrass Lolium perenne L.), perennial ryegrass and winter wheat at two rates of calcium ammonium nitrate (CAN) or animal slurry was grown in these lysimeters. Leaching and the availability of beet top N to successive crops were followed for 2 years. The soil in the cylinders and lysimeters was a light sandy loam (˜ 10% clay).Of the 7·10 g N/m2 added in beet tops, 10–15% was harvested in two subsequent crops of barley and ryegrass and 13–19% was lost by nitrate leaching. Beet top N accounted for 3–7% of the total N offtake in 1992. In 1993 〈 1·5% of the total N offtake in ryegrass was from the beet tops applied in 1991. Combining results from mineral fertilized treatments, it was found that 9% of the beet top N was removed in the first cereal crop, 9% was lost by nitrate leaching and 68% remained in the 0–20 cm soil layer (including roots), suggesting that the denitrification loss was 〈 15%.Incorporation of oilseed rape stubble (1·35 g N/m2), two rates of pods (6·25 and 18·75 g N/m2) or mixed residues (12·25 g N/m2) contributed 0·5, 2·3, 7·4 and 4·6%, respectively, to the total N harvested in the following crop of winter wheat. The percentage of the added labelled N taken up by the wheat ranged from 4·9 to 6·1%, with 60–79% remaining in the 0–20 cm layer after harvest.For beet tops it was calculated that 100 kg N/ha in residues incorporated in the autumn could replace 18 kg N/ha given in the following spring as mineral fertilizer. For oilseed residues, the corresponding average value was 9 kg N/ha.In fertilized cropping systems, oilseed rape residues had minor effects on the subsequent crop, so that an uneven return of residues, as often occurs with combined crops, would do little harm. A considerable proportion of the N applied in sugarbeet tops was lost by leaching and the residual value of the sugarbeet tops to subsequent crops was low.

Description: The current study evaluated the effect of sowing date (early, mid-August or timely, mid-September) on two winter wheat (Triticum aestivum L.) cultivars (Hereford, Mariboss) with different rates of nitrogen (N) (0–225 kg total N/ha) applied as animal manure (AM; cattle slurry) or mineral fertilizers (N: phosphorus: potassium; NPK). Overwinter plant N uptake and soil mineral N content were determined during 2014/15, while harvest yields (grain, straw, N content) were determined during 2014/15 and 2015/16. Overwinter uptake of N was 14 kg N/ha higher in early than in timely-sown wheat. Despite very different yield levels in 2015 and 2016 harvests, the advantage of early sowing on grain yields was similar (1.1 and 0.9 t/ha); straw yield benefits were greater in 2015 (1.7 t/ha more) than in 2016 (0.4 t/ha more). In 2015 and 2016, N offtake was 35 and 17 kg N/ha higher in early than in timely-sown wheat, respectively. The mineral N fertilizer value of cattle slurry averaged 50%. Early sowing increased the apparent N recovery (ANR) for wheat regardless of nutrient source. However, ANR was substantially higher for NPK (82% in 2015; 52% in 2016) than for AM (39% in 2015; 27% in 2016). Performance of the two cultivars did not differ consistently with respect to the effect of early sowing on crop yield, N concentration and offtake, or ANR. Within the north-west European climatic region, moving the sowing time of winter wheat from mid-September to mid-August provides a significant yield and N offtake benefit.

Description: SUMMARYAmmonia losses from surface-applied cattle slurry were measured under field conditions using a wind tunnel system that allows variables affecting ammonia loss to be examined under controlled conditions. The experiments were carried out on a sandy soil with seven different surface covers. This report considers the effect of wind speed, temperature and water vapour deficit on the ammonia loss over a series of 6-day periods. During October 1986 to November 1989 42 treatments were examined, using slurries taken from the same slurry tank to provide slurries of similar chemical composition.When temperatures were near zero, the rate of ammonia loss was generally low. The accumulated loss over 6 days was high, however, because the rate of loss was constant throughout the period. In these experiments the soil was saturated with water and partly frozen, and the infiltration of slurry into the soil reduced. At 19 °C initial loss rates were high but, after 12 h, almost no further loss occurred. Apart from these extremes, the ammonia loss rates within the initial 24 h were significantly affected by temperature and wind speed.Ammonia volatilization after 6 h was exponentially related to temperature (r2 = 0·841) but the correlation weakened with time after slurry application. An increase in ammonia volatilization with increasing water vapour pressure deficit was considered to be an effect of temperature.The ammonia loss rate increased when wind speeds increased up to 2·5 m/s. No consistent increase in ammonia volatilization was found when the wind speed increased from 2·5 to 4 m/s. Ammonia loss after 24 h increased with increasing initial pH of the slurry.A two-stage pattern for ammonia volatilization from slurry is proposed. During the first stage (the initial 24 h) ammonia loss rate is high due to an elevated pH at the slurry surface followingv application, and temperature significantly affects the loss rate. In the next stage, pH declines and the rate of ammonia volatilization decreases. During this stage other factors, including the dry matter content of the slurry, control the rate of ammonia loss.

Description: SUMMARYGaseous NH3 losses from pig and cattle slurry stored in eight storage tanks were measured simultaneously using wind-tunnels. The slurry was either stirred weekly (uncovered), or was allowed to develop a natural surface crust. Oil, peat, chopped cereal straw, PVC foil, leca® (pebbles of burned montmorillonitic clay) and a lid were tested as additional covers. Convective transport of ammonium to the surface layers caused NH3 volatilization losses of 3–5 g NH3-N/m2 per day from the stirred, uncovered tanks. The loss of NH3 from the stirred slurry was related to air temperature. The development of a natural surface crust reduced NH3 losses to 20% of those from stirred slurry. NH3 losses from slurry not developing a natural surface crust layer and left undisturbed were similar to the losses from stirred slurry. A 15 cm layer of straw was as effective as a surface crust layer in reducing NH3 losses. In one experiment, cracks developed in the oil cover and losses were therefore only reduced to 50% of those of uncovered slurry. Apart from this experiment, NH3 losses from slurry covered with oil, leca®, peat and foil were small.

Description: The stability of soil carbon is a major source of uncertainty for the prediction of atmospheric CO2 concentration during the 21st century. Isolating experimentally the stable soil carbon from other, more vulnerable, pools is of prime importance for calibrating soil C models, and gaining insights on the mechanisms leading to soil organic carbon (SOC) stability. Long-term bare fallow experiments, in which the decay of SOC is monitored for decades after inputs from plant material have stopped, represent a unique opportunity to assess the stable organic carbon. We synthesized data from 6 bare fallow experiments of long-duration, covering a range of soil types and climate conditions, at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). The conceptual model of SOC being divided into three pools with increasing turnover times, a labile pool (~ years), an intermediate pool (~ decades) and a stable pool (~ several centuries or more) fits well with the long term SOC decays observed in bare fallow soils. The modeled stable pool estimates ranged from 2.7 gC kg−1 at Rothamsted to 6.8 gC kg−1 at Grignon. The uncertainty over the identification of the stable pool is large due to the short length of the fallow records relative to the time scales involved in the decay of soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for future experimentation addressing the characteristics of stable SOC and its vulnerability to global change.

Description: The stability of soil organic matter (SOM) is a major source of uncertainty in predicting atmospheric CO2 concentration during the 21st century. Isolating the stable soil carbon (C) from other, more labile, C fractions in soil is of prime importance for calibrating soil C simulation models, and gaining insights into the mechanisms that lead to soil C stability. Long-term experiments with continuous bare fallow (vegetation-free) treatments in which the decay of soil C is monitored for decades after all inputs of C have stopped, provide a unique opportunity to assess the quantity of stable soil C. We analyzed data from six bare fallow experiments of long-duration (〉30 yrs), covering a range of soil types and climate conditions, and sited at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). A conceptual three pool model dividing soil C into a labile pool (turnover time of a several years), an intermediate pool (turnover time of a several decades) and a stable pool (turnover time of a several centuries or more) fits well with the long term C decline observed in the bare fallow soils. The estimate of stable C ranged from 2.7 g C kg−1 at Rothamsted to 6.8 g C kg−1 at Grignon. The uncertainty associated with estimates of the stable pool was large due to the short duration of the fallow treatments relative to the turnover time of stable soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for characterization of the nature of stable SOM and its vulnerability to global change.

Description: An understanding of the dynamics of soil organic matter (SOM) is important for our ability to develop management practices that preserve soil quality and sequester carbon. Most SOM decomposition models represent the heterogeneity of organic matter by a few discrete compartments with different turnover rates, while other models employ a continuous quality distribution. To make the multi-compartment models more mechanistic in nature, it has been argued that the compartments should be related to soil fractions actually occurring and having a functional role in the soil. In this paper, we make the case that fractionation methods that can measure continuous quality distributions should be developed, and that the temporal development of these distributions should be incorporated into SOM models. The measured continuous SOM quality distributions should hold valuable information not only for model development, but also for direct interpretation. Measuring continuous distributions requires that the measurements along the quality variable are so frequent that the distribution is approaching the underlying continuum. Continuous distributions leads to possible simplifications of the model formulations, which considerably reduce the number of parameters needed to describe SOM turnover. A general framework for SOM models representing SOM across measurable quality distributions is presented and simplifications for specific situations are discussed. Finally, methods that have been used or have the potential to be used to measure continuous quality SOM distributions are reviewed. Generally, existing fractionation methods have to be modified to allow measurement of distributions or new fractionation techniques will have to be developed. Developing the distributional models in concert with the fractionation methods to measure the distributions will be a major task. We hope the current paper will help spawning the interest needed to accommodate this.

Description: An understanding of the dynamics of soil organic matter (SOM) is important for our ability to develop management practices that preserve soil quality and sequester carbon. Most SOM decomposition models represent the heterogeneity of organic matter by a few discrete compartments with different turnover rates, while other models employ a continuous quality distribution. To make the multi-compartment models more mechanistic in nature, it has been argued that the compartments should be related to soil fractions actually occurring and having a functional role in the soil. In this paper, we make the case that fractionation methods that can measure continuous quality distributions should be developed, and that the temporal development of these distributions should be incorporated into SOM models. The measured continuous SOM quality distributions should hold valuable information not only for model development, but also for direct interpretation. Measuring continuous distributions requires that the measurements along the quality variable are so frequent that the distribution approaches the underlying continuum. Continuous distributions lead to possible simplifications of the model formulations, which considerably reduce the number of parameters needed to describe SOM turnover. A general framework for SOM models representing SOM across measurable quality distributions is presented and simplifications for specific situations are discussed. Finally, methods that have been used or have the potential to be used to measure continuous quality SOM distributions are reviewed. Generally, existing fractionation methods will have to be modified to allow measurement of distributions or new fractionation techniques will have to be developed. Developing the distributional models in concert with the fractionation methods to measure the distributions will be a major task. We hope the current paper will help generate the interest needed to accommodate this.