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.     

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.     

Use of the RothC model to estimate the carbon sequestration potential of organic matter application in Japanese arable soils

We estimated the carbon (C) sequestration potential of organic matter application in Japanese arable soils at a country scale by applying the Rothamsted carbon (RothC) model at a 1-km resolution. After establishing the baseline soil organic carbon (SOC) content for 1990, a 25-year simulation was run for four management scenarios: A (minimum organic matter application), B (farmyard manure application), C (double cropping for paddy fields) and D (both B and C). The total SOC decreased during the simulation in all four scenarios because the C input in all four scenarios was lower than that required to maintain the baseline 1990 SOC level. Scenario A resulted in the greatest depletion, reflecting the effects of increased organic matter application in the other scenarios. The 25-year difference in SOC accumulation between scenario A and scenarios B, C and D was 32.3, 11.1 and 43.4 Mt C, respectively. The annual SOC accumulation per unit area was similar to a previous estimate, and the 25-year averages were 0.30, 0.10 and 0.41 t C ha⁻¹ year⁻¹ for scenarios B, C and D, respectively. The system we developed in the present study, that is, linking the RothC model and soil spatial data, can be useful for estimating the potential C sequestration resulting from an increase in organic matter input to Japanese arable soils, although more feasible scenarios need to be developed to enable more realistic estimation.     

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.     

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.     

Wind tunnel test and Cs tracing study on wind erosion of several soils in Tibet

The soils of alpine meadows and alpine grassland steppes, aeolian soils, coarse-grained soils, and farm soils cultivated from alpine grasslands in Tibet are typical soils that are suffering from different degrees of soil erosion by wind. Based on field investigations, wind tunnel experiments, and a ¹³⁷Cs trace study, this work tested the erodibility of these soils by wind, simulated the protective functions of natural vegetation and the accelerative effects of damage by livestock, woodcutting, and cultivation on erosion, and estimated erosion rates from 1963 to 2001. The results indicated that alpine meadows have the strongest resistance to wind erosion, and that undamaged alpine meadow soils generally sustain only weak or no wind erosion. Alpine grassland steppes with good vegetation cover and little damage by humans exhibit good resistance to wind erosion and suffered from only slight erosion. However, soil erodibility increased remarkably in response to serious disturbance by livestock and woodcutting; wind erosion reached 33.03 t ha⁻¹ year⁻¹. The erodibility of semi-stabilized aeolian soil and mobile aeolian soil was highest, at 52.17 and 56.4 t ha⁻¹ year⁻¹, respectively. The mean erosion rates of coarse-grained soil with various levels of vegetation coverage and of farm soil were intermediate, at 45.85 and 51.33 t ha⁻¹ year⁻¹, respectively. Restricting livestock, woodcutting, and excessive grassland cultivation are the keys to controlling wind erosion in Tibet. In agricultural regions, taking protective cultivation and management to enhance surface roughness is a useful way to control wind erosion.     

Is burial of wheat straw in ditches a way to reduce CH4 emissions from rice cultivation?

Burial of wheat straw in ditches and incorporation of wheat straw are the two main ways of returning wheat straw prior to rice cultivation in China. To examine the effect of burying wheat straw in ditches on CH₄ emissions from rice cultivation, a field experiment was conducted at Yixing, Jiangsu, China in 2004. CH₄ flux was measured using a closed-chamber technique in three treatments (CK, no wheat straw application; WI, evenly incorporating 3.75 t ha⁻¹ wheat straw into the 0.1 m topsoil; WD, burying 3.75 t ha⁻¹ wheat straw in 0.14-m deep by 0.25-m wide ditches). Seasonal CH₄ emissions ranged from 49.7 to 218.4 kg CH₄ ha⁻¹. The application of wheat straw in these two ways significantly increased CH₄ emissions by 4.0-fold and 4.4-fold, respectively ( P  < 0.05). Although CH₄ flux from the non-ditch area in the WD treatment was as low as that in the CK treatment, it was counter-balanced by extremely high CH₄ flux from the ditch, which was approximately 6.0-fold as much as that from WI, leading to comparability between treatments WI and WD in total CH₄ emissions ( P  > 0.05). No significant difference was observed between the three treatments in grain yield ( P  > 0.05). The results indicated that burial of wheat straw in ditches is not a way to reduce CH₄ emission from rice cultivation.     

Water quality implications of dairy slurry applied to cut pastures in the northeast USA

Abstract. Nitrate nitrogen (NO₃-N) leaching from animal production systems in the northeast USA is a major non-point source of pollution in the Chesapeake Bay. We conducted a study to measure NO₃-N leaching from dairy slurry applied to orchardgrass ( Dactylis glomerata L., cv. Pennlate) using large drainage lysimeters to measure the direct impact of four rates of slurry (urine and faeces) N application (0, 168, 336, 672 kg N ha⁻¹ yr⁻¹) on NO₃-N leaching on three soil types. We then used experimentally-based relationships developed earlier between stocking density and NO₃-N leaching loss and leachate NO₃-N concentration to estimate the added impact of animal grazing. Nitrate N leaching losses from only dairy slurry applied at the 0, 158, 336, and 672 kg N ha⁻¹ yr⁻¹ rates were 5.85, 8.26, 8.83, and 12.1 kg N ha⁻¹ yr⁻¹, respectively with corresponding NO₃-N concentrations of 1.60, 2.30, 2.46, and 3.48 mg l⁻¹. These NO₃-N concentrations met the 10 mg l⁻¹ US EPA drinking water standard. However, when a scenario was constructed to include the effect of NO₃-N leaching caused by animal grazing, the NO₃-N drinking water standard was calculated to be exceeded.     

Potassium retention and leaching in an organic crop rotation on loamy sand as affected by contrasting potassium budgets

Abstract. In organic farming, potassium (K) deficiency may become a significant problem due to nutrient import restrictions. Knowledge about potential K leaching in systems with different K budgets is therefore important for effective agricultural management. We investigated the effect of four organic farming systems (two livestock densities in combination with two types of organic manure) on crop yields, K leaching and K balances in a six course crop rotation from 1993/94 to 1997/98. Average K concentrations in soil water extracted by means of ceramic suction cups at 1 m depth were 0.6 mg K l⁻¹ corresponding to a K leaching loss of 1.5 kg ha⁻¹ yr⁻¹ which was less than expected from values reported in the literature. Variation in K budgets from −12 to +30 kg ha⁻¹ yr⁻¹ did not affect K leaching. In an additional experiment with application of 988 kg K ha⁻¹ as KCl, K leaching accounted for only 0.2% of the applied K although 40% of the accompanying Cl was leached. The main part of the applied K was retained in the topsoil. It was concluded that K leaching was a result of the fertilizer history rather than of the current K budget.     

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.     

Soil phosphorus status and turnover in central-European beech forest ecosystems with differing tree species diversity

Problems in phosphorus (P) nutrition of forest trees raise questions concerning the soil P concentrations, pools and turnover in forests. In addition, it is not clear if, and to what extent, tree species diversity has an influence on the soil P status and turnover. The aim of this study was to investigate the P status and turnover in beech ( Fagus sylvatica L.) -dominated forest ecosystems on loess over limestone and to elucidate what role heterogeneities in tree species diversity would play. The soils of mixed species stands contained more organically bound P (710–772 kg ha⁻¹) than those of pure beech stands (378 kg ha⁻¹), whereas the inorganic P content differed little between the stand types. A large proportion (44–55%) of the total soil P was organically bound. This fraction was mainly dependent on the clay content of the soils and not on the tree diversity. The P input with leaf litter (1.4–2.1 kg ha⁻¹ year⁻¹) showed a tendency to increase with increasing diversity. The apparent P turnover times in the organic surface layers differed, with shorter turnover times in mixed species stands (2–3 years) than in pure beech stands (10 years). Possible explanations for the different turnover times were differences in the litter quality, interactions in mixed species litters and the soil pH and base saturation. Hence, the tree species mainly influence the apparent P turnover time in the organic surface layer, whereas the P concentrations and pools in the mineral soil are determined by the soil properties, particularly the clay content.     

Airborne nitrogen load in Japanese and Chinese agroecosystems

The objective of this review is to make current knowledge on the nitrogen (N) load throughout the atmosphere (airborne N load) available to readers, with special emphasis on Japanese and Chinese agroecosystems. Key species of airborne N are ammonia, nitrogen dioxide, nitrogen oxide, nitric acid, nitrous acid and particulate ammonium and nitrate. Organic N also exists in the atmosphere. The main processes in terms of the airborne N load involve emission, atmospheric transportation and transformation, atmospheric deposition and environmental impacts. Agricultural activities are the largest emitters of ammonia through emissions mainly from livestock waste and field-applied N fertilizers. The ammonia emissions in China in 1995 from chemical fertilizers and organic fertilizers, such as animal excreta, were estimated to be 3.56 and 2.04 Tg N year⁻¹, respectively, and the emissions in Japan were 0.059 and 0.069 Tg N year⁻¹, respectively. The most fundamental causes of the airborne N load in relation to Japanese and Chinese agriculture were intensive livestock farming in Japan and over use of N fertilizers in China. However, agroecosystems are also a sink for airborne N. Atmospheric N deposition was up to 20 and 60 kg N ha⁻¹ year⁻¹ in Japan and China, respectively. The unrelenting load of airborne N continues in Japan and China. The development of a simple, but accurate method to determine the dry deposition flux that is applicable to simultaneous and multipoint observations would be valuable. The establishment of cross relationships among in situ observations, remote sensing and numerical modeling is also needed to cope with the issue by assessing the actual status, predicting the future status and working out effective measures.     

Phytoextraction of Cd and Zn with Thlaspi caerulescens in field trials

Abstract. Phytoextraction is the remediation of heavy metal contaminated soils using plants that take up metals. Hyperaccumulating plants such as Thlaspi caerulescens are often studied for their possible use for decontamination of Cd and Zn rich soils, but few field trials have been reported, although they are necessary to validate the results of hydroponic and pot studies. This article reports field data for T . caerulescens grown on a calcareous and an acidic soil, both contaminated 20 years ago by either atmospheric depositions or septic-tank wastes. Accelerated cropping using transplants grown three times in eight months was compared to Thlaspi sown twice during the same period. Both were followed by one crop of sown Thlaspi . High Cd and Zn concentrations in the plant shoots compensated for the low biomass production. Annual metal exports with transplanted Thlaspi were 130 g Cd ha⁻¹ and 3.7 kg Zn ha⁻¹ on the calcareous soil and 540 g Cd ha⁻¹ and 20 kg Zn ha⁻¹ on the acidic soil. We concluded that within the framework of the Swiss legislation, remediation of Cd-contaminated soils could be achieved within less than 10 years with one crop of Thlaspi per year, but differences in soil properties could affect the rate of phytoextraction significantly. Total Zn content in both soils was too high to be remediated by T . caerulescens in a realistic time span. Thlaspi did not decrease the NaNO₃-extractable fraction of Cd or Zn in either of the soils.     

Effects of Plant Species on CH4 and N2O Fluxes from a Volcanic Grassland Soil in Nasu, Japan

To investigate the effects of plant species in grassland on methane (CH₄) and nitrous oxide (N₂O) fluxes from soil, fluxes from an orchardgrass ( Dactylis glomerata L.) grassland, white clover ( Trifolium repens L.) grassland and orchardgrass/white clover mixed grassland were measured weekly from April 2001 to March 2002 using a vented closed chamber method. Related environmental parameters (soil inorganic N content, soil pH (H₂O) value, soil moisture content, soil temperature, grass yield, and the number of soil microorganisms) were also regularly monitored. On an annual basis, CH₄ consumption in the soil of the orchardgrass grassland, white clover grassland and orchardgrass/white clover mixed grassland was 1.8, 2.4, and 1.8 kg C ha⁻¹ year⁻¹, respectively. The soil bulk density of the white clover grassland was lower than that of the other grasslands. Fluxes of CH₄ were positively correlated with the soil moisture content. White clover increased the CH₄ consumption by improving soil aeration. Nitrogen supply to the soil by white clover did not decrease the CH₄ consumption in the soil of our grasslands. On the other hand, annual N₂O emissions from the orchardgrass grassland, white clover grassland, and orchardgrass/white clover mixed grassland were 0.39, 1.59, and 0.67 kg N ha⁻¹ year⁻¹, respectively. Fluxes of N₂O were correlated with the NO₃⁻ content in soil and soil temperature. White clover increased the N₂O emissions by increasing the inorganic N content derived from degrading white clover in soil in summer.     

A short-term investigation of trace gas emissions following tillage and no-tillage of agroforestry residues in western Kenya

Improved-fallow agroforestry systems are increasingly being adopted in the humid tropics for soil fertility management. However, there is little information on trace gas emissions after residue application in these systems, or on the effect of tillage practice on emissions from tropical agricultural systems. Here, we report a short-term experiment in which the effects of tillage practice (no-tillage versus tillage to 15 cm depth) and residue quality on emissions of N₂O, CO₂ and CH₄ were determined in an improved-fallow agroforestry system in western Kenya. Emissions were increased following tillage of Tephrosia candida (2.1 g N₂O-N ha⁻¹ kg N applied⁻¹; 759 kg CO₂-C ha⁻¹ t C applied⁻¹; 30 g CH₄-C ha⁻¹ t C applied⁻¹) and Crotalaria paulina residues (2.8 g N₂O-N ha⁻¹ kg N applied⁻¹; 967 kg CO₂-C ha⁻¹ t C applied⁻¹; 146 g CH₄-C ha⁻¹ t C applied⁻¹) and were higher than from tillage of natural-fallow residues (1.0 g N₂O-N ha⁻¹ kg N applied⁻¹; 432 kg CO₂-C ha⁻¹ t C applied⁻¹; 14.7 g CH₄-C ha⁻¹ t C applied⁻¹) or from continuous maize cropping systems. Emissions from these fallow treatments were positively correlated with residue N content (r = 0.62–0.97; P < 0.05) and negatively correlated with residue lignin content (r = −0.56, N₂O; r = −0.92, CH₄; P < 0.05). No-tillage of surface applied Tephrosia residues lowered the total N₂O and CO₂ emitted over 99 days by 0.33 g N₂O-N ha⁻¹ kg N applied⁻¹ and 124 kg CO₂-C ha⁻¹ t C applied⁻¹, respectively; estimated to provide a reduction in global warming potential of 41 g CO₂ equivalents. However, emissions were increased from this treatment over the first 2 weeks. The responses to tillage practice and residue quality reported here need to be verified in longer term experiments before they can be used to suggest mitigation strategies appropriate for all three greenhouse gases.     

Methane fluxes on agricultural and forested boreal organic soils

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH₄. Surprisingly, the agricultural soils emitted CH₄ during a warm dry summer. The CH₄ oxidation capacity and CH₄ uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH₄ during a dry summer. Despite periods of CH₄ emission, the agricultural soils were annual sinks for CH₄, with uptake rate of CH₄-C varying from 0.1 to 3.7 kg ha⁻¹ yr⁻¹. The forested soil had a methane uptake rate of 3.9 kg CH₄-C ha⁻¹ yr⁻¹. All the soils acted as sinks for CH₄ during winter, which contributed up to half of the annual CH₄ uptake. The capacity of soils to transport gases did not explain the larger CH₄ uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH₄ uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH₄ uptake. The results suggest that CH₄ oxidation in agricultural organic soil is more sensitive to soil drying than CH₄ oxidation in forested organic soil.     

Leaching and balances of phosphorus and other nutrients in lysimeters after application of organic manures or fertilizers

Abstract. A long-term lysimeter experiment with undisturbed monoliths studied leaching behaviour and balances of phosphorus (P), potassium (K) and nitrogen (N) during a seven year crop rotation on four types of soil receiving inorganic fertilizers, manure and grass compost respectively. It was shown that application of manure did not lead to any direct change in nutrient leaching, unlike the application of fertilizers to soils of normal fertility. However, soil type considerably affected the nutrient concentrations in the drainage water.
Manure applied in amounts equal to the maximum animal density allowed by Swedish legislation slightly oversupplied P and N (0.5–3.5 and 18–38 kg ha⁻¹ y⁻¹ respectively) compared to the crop requirement and leaching losses for most of the soils. The relationship between lactate-soluble P in the topsoil and the concentrations of dissolved P in the drainage water was very strong. However the strength of this relationship was dependent on just one or two soils. P losses from a fertile sandy soil were large (1–11 kg ha⁻¹ y⁻¹) throughout the crop rotation and average crop removal (13 kg ha⁻¹ y⁻¹) plus the leaching losses were not balanced (average deficit 3–6 kg ha⁻¹ y⁻¹) by the addition of fertilizer, manure or grass compost. No decreasing trend was found in the P losses during seven years. However, the K deficit (average 26 kg ha⁻¹ y⁻¹) led to a significant reduction in the leaching trend from this soil. The other soils that had a smaller K deficit showed no significant reduction in the leaching of K.     

Carbon and nitrogen mineralization of sewage sludge compost in soils with a different initial pH

Soil properties may affect the decomposition of added organic materials and inorganic nitrogen (N) production in agricultural soils. Three soils, Potu (Pu), Sankengtzu (Sk) and Erhlin (Eh) soils, mixed with sewage sludge compost (SSC) at application rates of 0 (control), 25, 75 and 150 Mg ha⁻¹ were selected from Taiwan for incubation for 112 days. The aim of the present study was to examine the effects of SSC application rates on the carbon decomposition rate, N transformation and pH changes in three soils with different initial soil pH values (4.8–7.7). The results indicated that the highest peaks of the CO₂ evolution rate occurred after 3 days of incubation, for all treatments. The Pu soil (pH 4.8) had a relatively low rate of CO₂ evolution, total amounts of CO₂ evolution and percentage of added organic C loss, all of which resulted from inhibition of microbial activity under low pH. For the Pu and Sk soils, the concentration of NH₄⁺-N reached its peak after 7–14 days of incubation, which indicated that ammonification might have occurred in the two soils with low initial pH values. NO₃⁻-N rapidly accumulated in the first 7 days of incubation in the Eh soil (pH 7.7). The direction and extent of the soil pH changes were influenced by the N in the SSC and the initial soil pH. Ammonification of organic N in the SSC caused the soil pH to increase, whereas nitrification of mineralized N caused the soil pH to decline. Consequently, the initial soil pH greatly affected the rate of carbon decomposition, ammonification and nitrification of SSC.     

Stock, turnover time and accumulation of organic matter in bulk and density fractions of a Podzol soil

Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m⁻² year⁻¹ whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.     

Soil physical fertility and crop performance as affected by long term application of FYM and inorganic fertilizers in rice–wheat system

Soil fertility, one of the important determinants of agricultural productivity, is generally thought to be supplemented through the application of nutrients mainly through inorganic fertilizers. The physical fertility of the soil, which creates suitable environment for the availability and uptake of these nutrients, is generally ignored. The present study aims to characterize the soil physical environment in relation to the long term application of farm yard manure (FYM) and inorganic fertilizers in rice–wheat. The treatments during both rice and wheat crops were (i) farm yard manure @ 20 t ha⁻¹ (FYM); (ii) nitrogen @ 120 kg ha⁻¹ (N₁₂₀); (iii) nitrogen and phosphorus @ 120 and 30 kg ha⁻¹ (N₁₂₀P₃₀) and (iv) nitrogen, phosphorus and potassium @ 120, 30 and 30 kg ha⁻¹ (N₁₂₀P₃₀K₃₀) in addition to (iv) control treatment, i.e. without any fertilizer and/or FYM addition. The treatments were replicated four times in randomized block design in a sandy loam (typic Ustipsament, non-saline, slightly alkaline). Bulk density, structural stability of soil aggregates and water holding capacity of 0–60 cm soil layer were measured.

The average mean weight diameter (MWD) was highest in FYM-plots both in rice (0.237 mm) and wheat (0.249 mm) closely followed by that in N₁₂₀P₃₀K₃₀ plots. The effect of FYM in increasing the MWD decreased with soil depth. The addition of both FYM and N₁₂₀P₃₀K₃₀ increased the organic carbon by 44 and 37%, respectively in rice. The total porosity of soil increased with the application of both FYM and N₁₂₀P₃₀K₃₀ from that in control plots. In 0–15 cm soil layer, the total porosity increased by 25% with FYM from that in control plots. This difference decreased to 13% in 15–30 cm soil layer. The average water holding capacity (WHC) was 16 and 11% higher with FYM and N₁₂₀P₃₀K₃₀ application from that in control plots. The MWD, total porosity and WHC improved with the application of balanced application of fertilizers. The grain yield and uptake of N, P and K by both rice and wheat were higher with the application of FYM and inorganic fertilizers than in control plots. The carbon sequestration rate after 32 years was maximum (0.31 t ha⁻¹ year⁻¹) in FYM-plots, followed by 0.26 t ha⁻¹ year⁻¹ in N₁₂₀P₃₀K₃₀-plots, 0.19 t ha⁻¹ year⁻¹ in N₁₂₀P₃₀ and minimum (0.13 t ha⁻¹ year⁻¹) in N₁₂₀-plots.