Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg⁻¹ day⁻¹, and net nitrification rates ranged from 2.2 to 11.6 mg N kg⁻¹ day⁻¹. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg⁻¹ day⁻¹ and net nitrification rates ranged from 0.02 to 2.8 mg N kg⁻¹ day⁻¹. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha⁻¹. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.     

Soil organic matter and related physical properties in a Mediterranean wheat-based rotation trial

Under semi-arid Mediterranean conditions, limited moisture is the main constraint to rainfed cropping with wheat (Triticum aestivum), barley (Hordeum vulgare), and food and forage legumes. With increasing land-use pressure, moisture-conserving fallowing is being replaced by continuous cropping, which is considered an unsustainable practice. Thus, a long-term trial with durum wheat (T. turgidum var. durum) was established in 1983 at Tel Hadya, Aleppo, Syria (mean annual rainfall 330 mm) to assess alternative rotation options to fallow and continuous cropping. Nitrogen (N) and grazing/residue management were secondary factors. Soil aggregation, infiltration, hydraulic conductivity, and total soil organic matter and component fractions (fulvic and humic acids and polysaccharides) were determined at the end of 12 years. Some rotations, e.g., medic (Medicago sativa) and vetch (Vicia faba), significantly increased soil organic matter (12.5–13.8 g kg⁻¹ versus 10.9–11 g kg⁻¹ for continuous wheat and wheat/fallow). All measurements, or indices, indicated parallel trends with increasing organic matter, e.g., coefficients of macro-structure, micro-aggregation, and water-stable aggregates, and decreasing dispersion. Similarly, legume rotations had higher infiltration rates (16.2–21.8 cm h⁻¹ versus 13.9–14.4 cm h⁻¹ with continuous wheat and wheat/fallow) and hydraulic conductivity rates (8.7–12.4 cm h⁻¹ versus 6.2–7.4 cm h⁻¹ with continuous wheat and wheat/fallow). We conclude that cereal/legume rotations, in addition to being biologically and economically attractive, also enhance soil quality and thus promote soil use sustainability in fragile semi-arid areas as in the Mediterranean zone.     

Monitoring of water erosion on arable farms in England and Wales. 1990–94

Abstract. The incidence of soil water erosion was monitored in 12 erosion-susceptible arable catchments ( c . 80 fields) in England and Wales between 1990 and 1994. Factors associated with the initiation of erosion were recorded, and the extent of rills and gullies measured. Approximately 80% of the erosion events were on land cropped to winter cereals. In 30% of cases, the initiation of erosion was linked to valley floor features, which concentrated runoff. Poor crop cover, wheelings and tramlines were also assessed as contributory factors in 22%, 19% and 14% of cases, respectively. In c . 95% of cases rainfall events causing erosion were ≥10 mm day⁻¹ and c . 80% were >15 mm day⁻¹. Erosion was also associated with maximum rainfall intensities of >4 mm h⁻¹ for c . 90% of cases and >10 mm h⁻¹ for c . 20%. Mean net soil erosion rates were approximately 4 t ha⁻¹ per annum (median value 0.41 t ha⁻¹ per annum) and associated mean P losses 3.4 kg ha⁻¹.     

Investigating preferential flow in a large intact soil block under pasture

Abstract. A large soil block was constructed to determine the importance of preferential flow routes compared with matric flow pathways at a pasture site in mid-Devon. The sandy loam soil was well structured and uniform. The soil block measured 5 m×3 m×1 m and was instrumented with an array of 54 tensiometers, TDR wave guides and suction samplers connected to an in situ chloride analysis system. Four steady state irrigation experiments were conducted with a range of rainfall intensities. During each experiment chloride and nitrate tracers were applied and the patterns of movement were observed. Although the application of tracer was uniform and the soil was relatively homogeneous, there was large variability across the block in terms of time taken to reach the peak concentration (TPC) and the peak concentration itself. About 44 samplers operated at the greatest intensities (10–2 mm h⁻¹) and only 35 at the smallest (1 mm h⁻¹). No relationship was found between TPC and depth. The fastest TPC and largest concentrations were associated with the greatest rainfall intensities. Relative importance of the individual water pathways was a function of soil heterogeneity: parts of the soil block were highly active with several pathways having short TPCs and conductivities in excess of 4 m day⁻¹ whereas other areas had longer TPCs and conductivities of 1–2 m day⁻¹. The pattern was also dynamic, with conductivities of the pathways changing through time, though most of the faster pathways maintained their greater conductivities for more than one year.     

Long-term farmyard manure application effects on properties of a silty clay loam soil under irrigated wheat–soybean rotation

Increasing importance has been placed on the use of agricultural soils for the mitigation of atmospheric CO₂ through sequestration of soil C. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, little information is available on C sequestration, C content in different aggregate size fractions and soil water transmission properties (infiltration and saturated hydraulic conductivity) as affected by long-term manure addition. We analyzed results of an 8-year experiment, initiated in 1995–1996 on a silty clay loam soil, to determine the influence of fertilizer and fertilizer + farmyard manure (FYM) application on those important soil properties. The overall increase in soil organic C (SOC) content in the 0–45 cm soil depth in NPK + FYM treatment as compared to NPK and control treatments was 11.0 and 13.9 Mg C ha⁻¹ at the end of 8 years, respectively. Application of FYM significantly reduced soil bulk density and increased mean weight diameter (MWD) and SOC contents in different aggregate size fractions. Soil organic C content in macroaggregates was greater than in microaggregates. The response of SOC content to FYM application was dependent upon inorganic fertilization and more upon balanced application of NPK than N only. Steady state infiltration rate under NPK + FYM (1.98 cm h⁻¹) was higher than under unfertilized (0.72 cm h⁻¹) and NPK (1.2 cm h⁻¹). Soil water sorptivity (calculated from Philip's equation) under NPK + FYM (1.06 cm min^−0.5) was higher than under NPK (0.61 cm min^−0.5). We conclude that hill farmers in northern India should be encouraged to use FYM along with chemical fertilizers to increase SOC content and improve soil physical properties.     

Hydraulic conductivity, residue cover and soil surface roughness under different tillage systems in semiarid conditions

The objective of this study was to investigate the effect of tillage and cropping system on near-saturated hydraulic conductivity, residue cover and surface roughness to improve soil management for moisture conservation under semiarid Mediterranean conditions. Three tillage systems were compared (subsoil tillage, minimum tillage and no-tillage) under three field situations (continuous crop, fallow and crop after fallow) on two soils (Fluventic Xerochrept and Lithic Xeric Torriorthent). Soil under no-tillage had lower hydraulic conductivity (5.0 cm day⁻¹) than under subsoil tillage (15.5 cm day⁻¹) or minimum tillage (14.3 cm day⁻¹) during 1 of 2 years in continuous crop due to a reduction of soil porosity. Residue cover at sowing was greater under no-tillage (60%) than under subsoil or minimum tillage (<10%) in continuous crop. Under fallow, residue cover was low (10%) at sowing of the following crop for all tillage systems in both soils. Surface roughness increased with tillage, with a high value of 16% and decreasing following rainfall. Under no-tillage, surface roughness was relatively low (3–4%). Greater surface residue cover under no-tillage helped conserve water, despite indications of lower hydraulic conductivity. To overcome the condition of low infiltration and high evaporation when no-till fallow is expected in a cropping sequence, either greater residue production should be planed prior to fallow (e.g. no residue harvest) or surface tillage may be needed during fallow.     

Root–soil adhesion as affected by crop species in a volcanic sandy soil of Mexico

Field observations have shown that root residues maintain root-adhering soil for several months after harvest. The aim of this work was to compare post-harvest effect of Amaranthus hypochondriacus (amaranth), Phaseolus vulgaris (common bean) and Zea mays (maize) roots on root-adhering soil, aggregation and organic carbon content. The experimental site was located on a volcanic sandy soil (Typic Ustifluvent) in the Valley of Mexico. In 1999 and 2000, maize had the highest root mass (92 and 94 g m⁻²) and the highest root-adhering soil (9051 and 5876 g m⁻²) when a root–soil monolith of 0.20 m × 0.20 m × 0.30 m was excavated after harvest. In contrast, bean roots (2 and 5 g m⁻²) had only 347 and 23 g m⁻² of adhering soil per monolith in each year. Amaranth had intermediate values between maize and bean. Dry soil aggregate classes (<0.25, 0.5, 1, 2, 5 and >5 mm) were similarly distributed among the three species. The sum of the three soil macro-aggregates classes >1 mm was 0.1 g g⁻¹ in both years. Neither water stability of the 2–5 mm aggregates (0.05–0.09 g g⁻¹) nor soil organic C (SOC) in three aggregate classes (<0.25, 1–2 and >5 mm; mean 14.6 mg g⁻¹) was affected by species (P < 0.05) in either year. Observations of thin sections (10× and 40×) revealed absence of macro-aggregates under maize. Soil compaction was attributed to high mass of maize roots in the sampled soil volume. Root systems sampled after harvest had the capacity to maintain a well structured soil mass, which was proportional to root mass. Root-adhering soil measured in the field could be used to select species promoting soil adhesion by roots.     

Amelioration of saline–sodic soil with mildly saline water in the Songnen Plain, northeast China

Abstract. The saline–sodic soils of the dryland Songnen Plain in northeast China are only slowly permeable to fresh water because of their large content of montmorillinite clay and sodium bicarbonate. Use of slightly saline groundwater containing adequate dissolved calcium and magnesium for leaching and reclamation can potentially prevent dispersion of the clay soil particles during treatment. Amelioration was evaluated using shallow, mildly saline groundwater to irrigate sorghum–corn rotations in a two-year field experiment. After two growing seasons during which a total of 400 mm of leaching water was applied, in addition to some supplemental irrigation water, the average electrical conductivity (EC_e) of the top 1.2 m of the soil profile decreased from 14.5±3.5 to 2.7±0.2 dS m⁻¹, and the sodium absorption ratio (SAR_e) decreased from 35.3±4.1 to 10.1±2.5 (meq L⁻¹)^0.5. The soil physical properties were improved: infiltration rate with mildly saline groundwater increased from 12.1 to 42 mm h⁻¹. Salinity changes in the top 1.2 m of soil layers after 700 mm of leaching produced no further improvement. Crop yields produced on plots undergoing amelioration increased by 64–562% compared with the rainfed control. The improved soil conditions after leaching resulted in 59–548% greater crop yields.     

An experimental and numerical study on flow and transport in a field soil using zero-tension lysimeters and suction plates

Zero-tension lysimeters are widely applied to study the fate of chemicals in the subsurface environment. However, conditions in lysimeters differ from the field situation, because local saturation is required at the lower boundary to collect leachate. The objective was to characterize the influence of the lower boundary on the flow and transport behaviour of bromide observed in six 1.2-m-long lysimeters and in the field by 30 suction plates installed at 1.2-m depth, which were operated with a time-variable suction equal to the ambient soil water potential. A bromide pulse was applied at the bare surface of a silty soil in autumn 1997 and monitored for 2.5 years. The mean leachate flux was 0.98 mm day⁻¹ for the lysimeters versus 0.66 mm day⁻¹ for the suction plates. The lysimeters had a slightly slower effective mean pore-water velocity, expressed as transport distance per unit of leaching depth, and exhibited more solute spreading than the suction plates. Numerical simulations revealed that the amount of water collected with the suction plates was sensitive to the hydraulic conductivity of the plates. The spatial variability in hydraulic properties in the model explained the observed variability in cumulative leachate, at least qualitatively. The arrival time and spreading of the breakthrough curves (BTCs) were well described by the simulations in the lysimeters, but were underestimated in the suction plates. Preferential flow through macropores, which is not an effective carrier for bromide, might be the reason for this discrepancy. Molecular diffusion contributed significantly to solute spreading and enhanced lateral mixing. Both the experiments and the simulations revealed that the dispersivity derived from BTCs is significantly influenced by the observation method and experimental conditions.     

Pollution of carbonate soils in a Mediterranean climate due to a tailings spill

The retention walls in a pond containing the residues from the pyrite mine of Aznalcóllar (southern Spain) broke open on 25 April 1998, spilling approximately 6 × 10⁶ m³ of polluted water and toxic tailings, which affected some 55 km². Drying and aeration of the tailings resulted in oxidation, forming an acidic solution with high pollutant contents, the effects of which were studied in a calcareous soil. The infiltration of this solution markedly affected only the first 12 mm of the soil, where strong acidification caused the weathering of the carbonates, and where the fine mineral particles were hydrolysed. The SO₄²⁻ ions in the acidic solution precipitated almost entirely at this depth, forming gypsum, hydroxysulphates and complex sulphates. The Fe³⁺ ions also precipitated there, mainly in amorphous or poorly crystallized forms, adsorbing to As, Sb, Tl and Pb dissolved in the acidic solution. The Al³⁺ ions, though partly precipitating in the acidic layer, accumulated mostly where the soil pH exceeded 5.5 (12–14 mm in depth). They did so primarily as amorphous or poorly crystallized forms, adsorbing to Cu dissolved in the acidic solution. The Zn²⁺ and Cd²⁺ ions accumulated mainly at pH > 7.0 (19–21 mm in depth), being adsorbed chiefly by clay mineral. After 15 months, only the first 20 mm of the soil were acidified by the oxidation of the tailings and most of the pollutants did not penetrate deeper than 100 mm. Consequently, the speed of the cleanup of the toxic spill is not as important as a thorough removal of tailings together with the upper 10 cm of the soil.     

Alternative tillage and crop residue management in wheat after rice in sandy loam soils of Indo-Gangetic plains

A 3-year field study was conducted to evaluate the effect of three tillage practices (conventional, zero and reduced/strip) with two nitrogen levels (120 and 150 kg N ha⁻¹) applied in primary strips and three crop residue management practices (removal, burning and incorporation) in secondary strips in wheat after rice. Reduced tillage resulted in significantly higher overall mean wheat yield (5.10 Mg ha⁻¹) compared to conventional (4.60 Mg ha⁻¹) and zero tillage (4.75 Mg ha⁻¹). Residue incorporation resulted in highest mean yield (5.86 Mg ha⁻¹) during third year. Maximum mean yield (6.1 Mg ha⁻¹) was obtained in reduced tillage followed by conventional tillage (5.8 Mg ha⁻¹) under residue incorporation in third year. The weed dry weight recorded at 30 days after sowing was highest (0.3 Mg ha⁻¹) under zero tillage and lowest under conventional tillage (0.16 Mg ha⁻¹). Among crop residue management practices, the highest dry weight of weeds (0.22 Mg ha⁻¹) was recorded under residue incorporation. The highest infiltration rate (1.50 cm h⁻¹) was recorded in residue incorporation followed by residue burning (1.44 cm h⁻¹) whereas; the lowest (0.75 cm h⁻¹) in zero tillage. Soil bulk density was the highest (1.69 Mg m⁻³) under zero tillage and the lowest in residue incorporation (1.59 Mg m⁻³). There were no changes in soil available P and K after each crop sequence in relation to tillage practices during first 2 years. Higher organic carbon (5.1–5.4 g kg⁻¹) was measured under zero tillage compared to other treatments. Residue incorporation increased soil organic carbon and available P while higher available K was monitored in burning treatment during the third year. These results suggest that reduced tillage and in situ incorporation of crop residues at 5 Mg ha⁻¹ along with 150 kg N ha⁻¹ were optimum to achieve higher yield of wheat after rice in sandy loam soils of Indo-Gangetic plains of India.     

Total selenium content of agricultural soils in Japan

To evaluate the selenium (Se) level in agricultural soils in Japan and to investigate its determining factors, 180 soil samples were collected from the surface layer of paddy or upland fields in Japan and their total Se contents were determined. Finely ground soil (50 mg) was wet-digested with HNO₃ and HClO₄ solution and the released Se was reduced to Se(IV). The concentration of Se(IV) was then determined by high-performance liquid chromatography with a fluorescence detector after treatment with 2,3-diaminonaphthalene and extraction with cyclohexane. The total Se content ranged from 0.05 to 2.80 mg kg⁻¹ with geometric and arithmetic means of 0.43 and 0.51 mg kg⁻¹, respectively. The overall data showed a log-normal distribution. In terms of soil type, volcanic soils and peat soils had relatively high Se content and regosols and gray lowland soils had relatively low Se content. In terms of land use, upland soils had significantly higher Se content than paddy soils. Among regions, soils in the Kanto, Tohoku, Hokkaido and Kyushu regions had relatively high content. The total Se content had a significant positive correlation with the organic carbon content ( P  < 0.01) and the equation for the estimation of total Se content with organic carbon suggested that on average approximately 48% (0.24 mg kg⁻¹) of the total Se was in inorganic forms and approximately 52% (0.25 mg kg⁻¹) was in organic forms. Soil pH, on the contrary, did not show a significant relationship with the total Se content. In conclusion, the organic matter content, in combination with volcanic materials, was the main determining factor of the total Se content of agricultural soils in Japan.     

Effect of Gypsum and Polyacrylamide Application on Erodibility of an Acid Kunigami Mahji Soil

Calcium carbonates and gypsum are often used to improve the chemical status of acid soils. This study discusses the effects of gypsum and polyacrylamide (PAM) application on infiltration and erodibility of a Japanese acid soil. Acid Kunigami mahji soil (sedimentary rock derived, Typic Hapludult) from Okinawa was packed into an acrylic plastic box, and simulated rainfall of 40 mm h⁻¹ was applied. Prior to the rainfall, 2.5 t ha⁻¹ of gypsum and/or 15 kg ha⁻¹ of non-ionic or anionic PAM were applied onto soil surface. During a rainfall, surface runoff was collected periodically, and sediment concentration, pH, and electric conductivity of the runoff were measured. Gypsum application enhanced surface runoff. During the rainfall, EC of the runoff was greater than the critical coagulation concentration of the clays of the mahji soil, however the soil became dispersive with gypsum application. PAM application could improve infiltration of gypsum amended mahji soil and reduce sediment loss.     

Increased predicted losses of phosphorus to surface waters from soils with high Olsen-P concentrations

Abstract. Diffuse soluble reactive P (SRP) & total P (TP) loads from over 50 major river catchments in Northern Ireland were predicted using an export coefficient modelling approach. Phosphorus export coefficients for each CORINE land cover class, derived from satellite imagery, allowed the prediction of P loads from a breakdown of the CORINE land cover classes by catchment using a GIS. This approach was validated using observed P loads calculated from flow and concentration data. Mean measured Olsen-P concentrations in the soil A-horizon were also determined on a catchment basis. Plots of P loads to the watercourse versus Olsen-P concentrations in the soil showed a breakpoint around 22 mg Olsen- P l⁻¹ for both SRP & TP data. Below Olsen-P concentrations of 22 mg l⁻¹, SRP & TP losses were essentially independent of Olsen-P at 0.28 and 0.63 kg P ha⁻¹ yr⁻¹, respectively. Above Olsen-P concentrations of 22 mg l⁻¹, there was considerable spread in the P loss data. Nevertheless, significant upward trends in SRP and TP losses to watercourses were detected with increasing Olsen-P at a rate of approximately 0.5 and 1.0 kg P ha⁻¹ yr⁻¹, for SRP and TP respectively, for each 10 mg l⁻¹ increase in Olsen-P.     

Effects of vegetation cover on the tendency of soil to crust in South Africa

Abstract. Tendency to crust is a potentially useful index for assessing soil degradation and for assisting land use planning in South Africa. In this study, the influence of land use, geology and vegetation cover on the tendency of soil to form a surface crust was investigated in six vegetation types. Crusting at all sites was greater in exposed soils than soils under vegetation, as determined by infiltration rate, water dispersible clay and modulus of rupture. In Renosterveld, crusting was markedly greater in exposed soil than vegetation covered soil (mean infiltration 16 vs 44 mm h⁻¹; dispersible clay 2.6 vs 2.2%; modulus of rupture 121 vs 64 kPa). Greater crusting in exposed soil was attributed to lower soluble salt and labile carbon (C) contents and an associated increase in the dispersion of clay. In Karoo, crusting of exposed, shale-derived soils was greater than that of exposed, dolerite-derived soils (infiltration 40 vs 83 mm h⁻¹; dispersible clay 2 vs 1.2%), and a similar pattern was evident in Tall Grassveld (infiltration 18 vs 36 mm h⁻¹; dispersible clay 1.2 vs 0.9%; modulus of rupture 31 vs 21 kPa). In Upland Grassland, cultivation of maize and rye enhanced crusting. In Thicket, crusting was greater in soils from open, degraded vegetation than intact, densely wooded sites (infiltration 19 vs 51 mm h⁻¹; modulus of rupture 16 vs 34 kPa), probably due to lower content of soil C. In Bushveld, crusting was greater in annually burnt plots than unburnt plots (infiltration 109 vs 163 mm h⁻¹; dispersible clay 0.9 vs 0.6% on granite-derived soils; and infiltration 56 vs 72 mm h⁻¹; dispersible clay 1.5 vs 1.3% on basalt-derived soils). Greater crusting of soil from burnt plots was ascribed to a reduction in soil C and soluble salts as well as a greater exchangeable sodium percentage.     

Long-term changes in soil organic matter under conventional tillage and no-tillage systems in semiarid Morocco

Abstract. A no-tillage (NT) system was developed in semiarid Morocco to improve the soil fertility and stabilize yield through conservation of water. Results in two long-term trials (4 and 11 years) were able to show the effects of a no-tillage system in increasing total soil organic matter and total nitrogen. Over time, the quality of the NT soil surface was improved compared with that under conventional tillage (CT) with disc harrows. This effect was the result of an increase in soil organic carbon (SOC) and a slight decline in pH. However, over time, nitrogen decreased in both tillage practices, especially in the 0–25 mm layer (from 0.59 to 0.57 t ha⁻¹ and from 0.44 to 0.42 t ha⁻¹ under NT and CT, respectively). After 4 years of NT an extra 5.62 t ha⁻¹ of SOC was sequestered in the 0–25 mm layer, and after 11 years the SOC increased further to 7.21 t ha⁻¹.     

Long-term effects of lantana residue additions on water retention and transmission properties of a medium-textured soil under rice–wheat cropping in northwest India

Abstract. Hydraulic properties of soils after rice cropping are generally unfavourable for wheat cultivation. Poor drainage, delayed planting and oxygen stress in the root zone may adversely affect the wheat crop after lowland rice cultivation. We studied long-term effects of lantana ( Lantana spp. L.) residue additions at 10, 20 and 30 t ha⁻¹ yr⁻¹ (fresh biomass) on physical properties of a silty clay loam soil under rice–wheat cropping in northwest India. At the end of ten cropping cycles, soil water retention, infiltration rate, saturated hydraulic conductivity and drying rate of soil increased significantly with lantana additions. The available water capacity (AWC), on volume basis, declined at rice harvest (from 22.0 to 18.8–20.9%), but increased at wheat harvest (from 12.9 to 13.4–15.0%) after lantana treatment. The volumes of water transmission (>50 μm) and storage pores (0.5–50 μm) were greater, while the volume of residual pores (<0.5 μm) was smaller in lantana-treated plots than in controls at both rice and wheat harvest. Infiltration rate in the lantana-treated soil was 1.6–7.9 times that of the control (61 mm d⁻¹) at rice harvest, and 2–4.1 times that of the control (1879 mm d⁻¹) at wheat harvest. Thus lantana addition improved soil hydraulic properties to the benefit of the wheat crop in a rice–wheat cropping sequence.     

Nitrous oxide and carbon dioxide emissions from grassland amended with sewage sludge

Abstract. Land disposal of sewage sludge in the UK is set to increase markedly in the next few years and much of this will be applied to grassland. Here we applied high rates of digested sludge cake (1–1.5×10³ kg total N ha⁻¹) to grassland and incorporated it prior to reseeding. Using automated chambers, nitrous oxide (N₂O) and carbon dioxide (CO₂) fluxes from the soil were monitored 2–4 times per day, for 6 months after sludge incorporation. Peaks of N₂O emission were up to 1.4 kg N ha⁻¹ d⁻¹ soon after incorporation, and thereafter were regularly detected following significant rainfalls. Gas emissions reflected diurnal temperature variations, though N₂O emissions were also strongly affected by rainfall. Although emissions decreased in the winter, temperatures below 4 °C stimulated short, sharp fluxes of both CO₂ and N₂O as temperature increased. The aggregate loss of nitrogen and carbon over the measurement period was up to 23 kg N ha⁻¹ and 5.1 t C ha⁻¹. Losses of N₂O in the sludge-amended soil were associated with good microbial conditions for N mineralization, and with high carbon and water contents. Since grassland is an important source of greenhouse gases, application of sewage sludge can be at least as significant as fertilizer in enhancing these emissions.     

Methane and nitrous oxide fluxes from a farmed Swedish Histosol

Fluxes of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) from histosolic soils (which account for approximately 10% of Swedish agricultural soils) supporting grassley and barley production in Sweden were measured over 3 years using static chambers. Emissions varied both over area and time. Methane was both produced and oxidized in the soil: fluxes were small, with an average emission of 0.12 g CH₄ m⁻² year⁻¹ at the grassley site and net uptake of −0.01 g CH₄ m⁻² year⁻¹ at the barley field. Methane emission was related to soil water, with more emission when wet. Nitrous oxide emissions varied, with peaks of emission after soil cultivation, ploughing and harrowing. On average, the grassley and barley field had emissions of 0.20 and 1.51 g N₂O m⁻² year⁻¹, respectively. We found no correlation between N₂O and soil factors, but the greatest N₂O emission was associated with the driest areas, with < 60% average water-filled pore space. We suggest that the best management option to mitigate emissions is to keep the soil moderately wet with permanent grass production, which restricts N₂O emissions whilst minimizing those of CH₄.     

Nitrogen fertilizer requirements of cereals following grass

Abstract. The effect of increasing rates of nitrogen (N) fertilizer on the yield response of 3 or 4 consecutive winter cereal crops after ploughing out grass was investigated at six field sites on commercial farms in England and Wales. Amounts of N required for an economically optimum yield (>3 kg of grain for each kg of fertilizer N applied) ranged from 0 to 265 kg ha⁻¹ and were dependent on soil N supply, but not on crop yield. Optimum N rates were large (mean 197 kg N ha⁻¹) at three sites: two sites where cereals followed 2-year grass leys receiving low N inputs (<200 kg N ha⁻¹), and at one site where a cut and grazed 4-year ley had received c . 315 kg N ha⁻¹ of fertilizer N annually. At the other three sites where 4 and 5-year grass leys had received large regular amounts of organic manures (20–30 t or m³ ha⁻¹) plus fertilizer N ( c . 300 kg ha⁻¹ each year), optimum N rates were low (mean 93 kg N ha⁻¹) and consistently over-estimated by the farmer by an average of 107 kg N ha⁻¹. Optimum N rates generally increased in successive years after ploughing as the N supply from the soil declined. Determination of soil C:N ratio and mineral N (NO₃N+NH₄N) to 90 cm depth in autumn were helpful in assessing fertilizer N need. The results suggest there is scope to improve current fertilizer recommendations for cereals after grass by removing crop yield as a determinant and including an assessment of soil mineralizable N during the growing season.