C. P. Webster

Soluble organic nitrogen in agricultural soils

Abstract The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20–30 kg SON-N ha–1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding.

Methane oxidation in soil as affected by land use, soil pH and N fertilization☆

Net uptake of CH4 was measured in intact soil cores (6.4 cm dia, 12 cm deep) collected from an arable wheat field, from three sites left uncultivated for more than 110 years following arable cropping and from a permanent grassland with different mineral N treatments subdivided into four pH levels. Soil cores were incubated in sealed 1 litre jars at 25°C for 48 h with a CH4 -amended atmosphere of 10 μl 1−1 at the start of incubation. The decrease in CH4 concentration followed first-order-kinetics and by log-transformation individual uptake rates could be calculated for each treatment. Soil from a calcareous site (pH 7.4) under deciduous woodland (Broadbalk Wilderness wooded section) oxidized CH4 6 times faster than the arable plot (pH 7.8) with the highest activity in the adjacent Broadbalk Wheat Experiment (with uptake rates of −80 and −13 nl CH4 1−1 h−1, respectively). The CH4 uptake rate was only 20% of that in the woodland in an adjacent area that had been uncultivated for the same period but kept as rough grassland by the annual removal of trees and shrubs and, since 1960, grazed during the summer by sheep. It is suggested that the continuous input of urea through animal excreta was mainly responsible for this difference. Another undisturbed woodland area with an acidic soil reaction (pH 4.1) did not oxidize any CH4. On a permanent grassland site (Park Grass Continuous Hay Experiment), the plot without N fertilization showed a distinct pH effect: CH4 consumption decreased from −67 to − 35nl CH4 1−1 h−1 with decreasing pH in the range 6.3–5.6 and declined to zero between pH 5.6 and 5.1. Mineral N applied annually as (NH4)2SO4, at either 96 or 144 kg N ha −1 for 130 years, completely inhibited CH4 oxidation, even where lime was applied to maintain a soil pH of about 6. By contrast, the long-term application of N as NaNO3 (96 kg N ha−1 a−1) caused no decline in CH4 oxidation compared to unfertilized grassland at the same pH and, in some cases, caused a small increase. Withholding NH4-N for 3 years caused no significant recovery of CH4 -oxidizing activity; withholding NO3-N caused a slight decline. Thus, land use (arable, cut grassland, grazed grassland or woodland), soil pH, N fertilizer inputs and form of N (NH4 or NO3) all have marked and interacting effects on the extent to which aerobic soil acts as a sink for CH4. The mechanisms through which the factors operate are not known but some possibilities are discussed. The results have important implications for the planning of land use and agricultural practices that will maximize the extent to which aerobic soils can act as a sink for CH4.

Denitrification in riparian buffer zones: the role of floodplain hydrology

The broad purpose of the study described here was to assess the role of denitrification in riparian zones in ameliorating groundwater pollution through nitrate loss, and as a potential source of nitrous oxide to the atmosphere. A suitable riparian zone was identified at Cuddesdon Mill on the River Thame floodplain near Oxford, England. Measurements were made of water and nitrate moving from arable land through the riparian zone and into the river. Techniques to measure denitrification were tested and applied, and the factors controlling denitrification measured. While there was considerable potential for denitrification at the site, this was not realized because much of the water moving off the farmland bypassed the riparian zone, entering the river directly via springs or through gravel lenses beneath the floodplain soil. Management of this site would not reduce nitrate leaching unless the floodplain hydrology could be substantially modified, and the main conclusion is that nitrate buffer zones will only operate efficiently where the hydrology of the site is appropriate. Copyright

Long-term effects of nitrogen fertilization on methane oxidation in soil of the broadbalk wheat experiment

Abstract Methane uptake by a temperate arable soil was investigated in incubation experiments with intact soil cores. The measurements were carried out with a soil moisture content of 16–17% (w/w) and at 25°C. The decrease of the CH 4 concentrations in an amended atmosphere (10 μl CH 4 1 −1 ) was measured during a 212 h period. There was no decrease of CH 4 if the soil was autoclaved showing that the disappearance of methane was entirely mediated by microbial activity. The long-term application (140 yr) of mineral nitrogen fertilizer caused significant differences in the ability of the soil to oxidize CH 4 : the larger the amount of fertilizer applied the lower the rate of CH 4 oxidation. No significant short-term effect of mineral-N fertilization could be observed whether applied as (NH 4 ) 2 SO 4 or KNO 3 . An organic manure treatment, which has received nearly 240 kg N ha −1 each year as farmyard manure, showed almost the same ability to oxidize CH 4 as an unfertilized plot and had a significantly higher CH 4 oxidation rate after an application of 144kg N ha −1 as nitrate fertilizer. For the mineral-N treatments the inhibition of the CH 4 oxidation increased with increasing N turnover rate but was independent of the mineral nitrogen content of the soil at the time of measurement. Therefore, the continued application of mineral-N fertilizer for an extended period (at least 7 yr) caused a depletion of the bacterial methane sink in soil and may have contributed to the continuous increase in atmospheric CH 4 over the past decades.

N2O, NO and NO2 fluxes from a grassland: Effect of soil pH

Abstract Fluxes of N 2 O, NO and NO 2 between grassland and the atmosphere were measured over 1 y using three plots which have been maintained at a constant pH of 3.9, 5.9 and 7.6 over many years. Net fluxes of N 2 O and NO were always from the soil to the atmosphere, whilst those of NO 2 were invariably from atmosphere to soil. Mean fluxes of N 2 O decreased appreciably with increasing acidity, whilst NO fluxes showed little dependence on pH, with the highest mean flux from the plot at pH 5.9. Sterilization of soil cores by autoclaving reduced N 2 O emissions almost to zero at all pH values, but residual production of NO was found, even at low pH. Increasing the pH of unsterilized soil cores from pH 3.9–6.5±0.5 led to a reduction in NO and especially N 2 O fluxes. It was concluded that the microbial community of the soil had adjusted to the low pH and was responsible for the entire production of N 2 O and much of the NO release. Chemodenitrification is also responsible for some NO production, especially at low pH.

Methane oxidation in temperate soils: Effects of land use and the chemical form of nitrogen fertilizer

Results are presented from long-term experimental sites showing that land use and agricultural management practices play an important role in mediating the sink strength of aerobic soils for methane. At sites located within 1 km2 at Rothamsted Experimental Station, U.K. the methane sink strength of soil follows the order woodland > grassland > arable. Comparison of grassland plots receiving nitrate-N fertilizer compared to ammonium-N fertilizer shows that the long-term (138 years) application of ammonium-N fertilizer caused a significant decrease in the soil sink strength for methane but that the application of nitrate-N for the same length of time did not.These results are discussed in relation to land use and microbial ecology.

Studies on no and N20 fluxes from a wheat field

The importance of soil parameters and the other environmental conditions that affect emission rates of NO and N20 were studied over a fertilised wheat field. Open-chamber and closed-chamber techniques were used for the flux measurement of NO and N20, respectively. Both gases showed variation in the emission rates which followed the seasonal variation in the available NH4+ and N03− and the moisture content of the soil. Whilst N20 emission rates increased with the moisture content of the soil,NO emissions decreased with increasing soil moisture and rainfall. The results suggested that most soil variables and atmospheric parameters had similar effects on both NO and N2O emission rates but that the overriding influence upon the NOJN2O emission ratio is the soil moisture content. The NO flux showed a clear diurnal variation which followed the surface soil temperature with an activation energy of 108 kJ mol− . The annual NO flux estimated from this study (0.79 kg N ha-−) was approximately half the corresponding N20 (1.42 kg N ha−1).

The effect of agriculture on methane oxidation in soil

Aerobic soils are an important sink for methane (CH4) contributing up to 15% of annual global CH4 destruction. However, the sink strength is significantly affected by land management, nitrogen (N) fertilizers and acidity. We tested these effects on samples taken from the Broadbalk Continuous Wheat, Park Grass permanent grassland and Broadbalk and Geescroft Wilderness experiments at Rothamsted. The rates of uptake from the atmosphere of both enhanced (10 ppmv) and ambient (2 ppmv) concentrations of CH4 were measured in laboratory incubations of soil cores under controlled conditions.The most rapid rates of uptake were measured in soil from deciduous woodland at pH 7 (measured in water); acidic (pH 4) woodland soil showed no net CH4 oxidation. While disturbance of the cores used in the experiments did not affect the rate of CH4 uptake, extended (150 years) cultivation of land for arable crops reduced uptake rate by 85% compared to that in the soil under calcareous woodland. The long-term application of ammonium- (NH4) based fertilizer, but not nitrate- (NO3) based fertilizer, completely inhibited CH4 uptake, but the application for the same period of farmyard manure that contained more N than the fertilizer had no inhibitory effect.Although the effects of agricultural practice on the oxidation of CH4 in soil are significant, the differences in oxidation rates between land use types are even greater. The likely effects of forest clearance, agricultural intensification and anthropogenic emissions of CH4 over the last 2500 years have been estimated for the United Kingdom. The calculations indicate that 54% of the current CH4 uptake by UK soils is the result of increased CH4 mixing ratio. They also indicate that land use change has decreased the potential sink strength by 62% or 37 kt CH4 g-1. In countries with much larger land areas than the UK, such as China, aerobic soil is likely to be a more significant factor in calculating net fluxes of CH4. It is important that the impacts of different agricultural managements and land use systems are understood and quantified so that the best possible estimate of CH4 sinks is calculated for comparison with sources.

Effect of fertilizer application on NO and N2O fluxes from agricultural fields

Losses of fertilizer as NO and N2O were studied at Broadbalk field, Rothamsted Experimental Station in England, on which subplots have been subject to differing constant levels of fertilizer application for many years. Fluxes of NO and N2O were measured using open- and closed-chamber techniques, respectively. Fluxes from unfertilized soil ranged from 0.3 to 4.8 ng N m(-2) s(-1) for NO and 0.23 to 3.0 ng N m(-2) s(-1) for N2O. The corresponding fluxes from the plot with the highest fertilizer application (92 kg N ha(-1) yr(-1) as NH4NO3) ranged from 0.5 to 64 ng N m(-2) s(-1) for NO and 0.4 to 240 ng N m(-2) s(-1) for N2O. Application of increasing amounts of fertilizer substantially enhanced emission rates of both NO and N2O. However, the amount of increase was controlled by competition between the crop and the microorganisms for the available soil nutrients, and loss of N2O to the atmosphere increased sharply at superoptimal levels of fertilizer application. The fertilizer-derived NO and N2O emissions represented approximately 90% of the total emission of these gases during the 25-day sampling period after fertilizer application. The results suggest that while increasing the amount of fertilizer increases both NO and N2O fluxes simultaneously, the NO/N2O emission ratio decreases. Results from laboratory experiments showed that the magnitude of the fertilizer loss as N2O was strongly affected by the form of the applied fertilizer.

Effects of waterlogging and drought on winter wheat and winter barley grown on a clay and a sandy loam soil. I. Crop growth and yield

SummaryThe effects of winter waterlogging and a subsequent drought on the growth of winter barley and winter wheat have been examined. We used lysimeters containing soil monoliths with facilities to control the water table and a mobile shelter to control rainfall. Winter wheat was grown on a clay and on a sandy loam, but winter barley only on the clay soil. Lysimeters were either freely-drained during the winter or waterlogged with the water table 10 cm below the soil surface from 2 December until 31 March (that could occur by rainfall with a return period of 2 to 3 years). The lysimeters then were either irrigated so that the soil moisture deficit did not exceed 84 mm, or subjected to drought by limiting rainfall (equivalent to a 1 in 10 dry year in the driest area of England) so that the deficits reached maximum values of 150 mm in the clay and 159 mm in the sandy loam by harvest.Winter waterlogging restricted tillering and restricted the number of ears for all crops; grain yield of the winter barley was decreased by 219 g/m2 (30%), and that of winter wheat by 170 g/m2 (24%) and 153 g/m2 (21% on the clay and sandy loam respectively.The drought treatment reduced the straw weight of winter barley by 75 g/m2 (12%) but did not significantly depress the grain yield. For winter wheat on the clay, where the soil was freely-drained during the winter, drought depressed total shoot weight by 344 g/m2 (17%) and grain weight by 137 g/m2 (17%), but after winter waterlogging, drought did not further depress total or grain weight. In contrast, the winter wheat on the sandy loam was not significantly affected by drought.From these results, which are discussed in relation to other experiments in the United Kingdom, it seems that winter waterlogging is likely to cause more variation in the yield of winter barley and winter wheat than drought.