Acid rain, cation dissolution, and sulphate retention in three tropical soils

Surface and subsurface samples of three tropical soils were examined with respect to their interaction with dilute solutions of sulphuric acid of pH 3. In calcareous clayey samples with a large cation exchange capacity the H⁺ was replaced by an equivalent concentration of metal cations which remained in solution along with SO^2-₄ as counterion. In a coarse-textured neutral soil with small cation exchange capacity, there was less chemical interaction and a major proportion of the H₂SO₄ remained unchanged in the equilibrium solution. Another soil exhibited considerable ability to remove SO^2-₄ from solution and, therefore, the total ionic concentration was greatly reduced. Other samples showed behaviour which was intermediate to these three types.
The ability to adsorb SO^2-₄ is one of the most important factors which determines the nature of the interaction of soil with dilute sulphuric acid. This ability was shown to be affected by the content of hydrous sesquioxides and organic matter in these soils.     

Decomposition in a peaty soil improved for pastoral agriculture

Abstract. The rates of CO₂ production and decomposition of ¹³C-enriched Lolium perenne leaves and roots in soil from the surface five cm of two upland stagnohumic gley soils were measured in laboratory experiments. One of the soils had been limed (pH 6.8) 13 years earlier. The other was unlimed (pH 3.7). Liming increased the rate of CO₂ release from soil to which no L. perenne had been added. About 30% of the ¹³C in L. perenne leaves remained in both limed and unlimed soil after 224 days. By contrast, less ¹³C-remained in the limed soil amended with L. perenne roots (44%) than in the limed soils (55%). Although the daily rate of CO₂ from the plant material-amended soils was initially greater in the improved than in the unimproved soil, it subsequently declined more rapidly.     

Short-term temporal changes of bare soil CO2 fluxes after tillage described by first-order decay models

To further understand the impact of tillage on carbon dioxide (CO₂) emission, we compare the performance of two conceptual models that describe CO₂ emission after tillage as a function of the non-tilled emission plus a correction resulting from the tillage disturbance. The models assume that C in the readily decomposable organic matter follows a first-order reaction kinetics equation as     and that soil C-CO₂ emission is proportional to the C decay rate in soil, where C _soil( t ) is the available labile soil C (g m⁻²) at any time ( t ) and k is the decay constant (time⁻¹). Two possible relationships are derived between non-tilled ( F _NT) and tilled ( F _T) soil fluxes:     (model 1) and     (model 2), where t is time after tillage. The difference between these two models comes from an assumption related to the k factor of labile C in the tilled plot and its similarity to the k factor of labile C in the non-till plot. Statistical fit of experimental data to conceptual models showed good agreement between predicted and observed CO₂ fluxes based on the index of agreement (d-index) and with model efficiency as large as 0.97. Comparisons reveal that model 2, where all C pools are assigned the same k factor, produces a better statistical fit than model 1. The advantage of this modelling approach is that temporal variability of tillage-induced emissions can be described by a simple analytical function that includes the non-tilled emission plus an exponential term, which is dependent upon tillage and environmental conditions.     

Greenhouse gas emissions from farmed organic soils: a review

Abstract. The large boreal peatland ecosystems sequester carbon and nitrogen from the atmosphere due to a low oxygen pressure in waterlogged peat. Consequently they are sinks for CO₂ and strong emitters of CH₄. Drainage and cultivation of peatlands allows oxygen to enter the soil, which initiates decomposition of the stored organic material, and in turn CO₂ and N₂O emissions increase while CH₄ emissions decrease. Compared to undrained peat, draining of organic soils for agricultural purposes increases the emissions of greenhouse gases (CO₂, CH₄, and N₂O) by roughly 1t CO₂ equivalents/ha per year. Although farmed organic soils in most European countries represent a minor part of the total agricultural area, these soils contribute significantly to national greenhouse gas budgets. Consequently, farmed organic soils are potential targets for policy makers in search of socially acceptable and economically cost-efficient measures to mitigate climate gas emissions from agriculture. Despite a scarcity of knowledge about greenhouse gas emissions from these soils, this paper addresses the emissions and possible control of the three greenhouse gases by different managements of organic soils. More precise information is needed regarding the present trace gas fluxes from these soils, as well as predictions of future emissions under alternative management regimes, before any definite policies can be devised.     

Assessing phosphorus 'Change-Points' and leaching potential by isotopic exchange and sequential fractionation

Abstract. There is increasing evidence that phosphorus (P) can be transferred to surface waters by leaching as well as by erosion and surface runoff. Recently it has been suggested that P soluble in 0.01 m CaCl₂ may be a good indicator of the specific Olsen-P concentration (usually termed the 'Change Point') at which the rate of P leaching from soil suddenly increases and poses a greater threat of eutrophication to standing waters. We know that these 'Change Points' vary from soil to soil but, so far, we do not fully understand the mechanism(s) involved. Here, we combine methods for assessing isotopically exchangeable P and P sequential fractionation to gain an insight into the processes which cause this sudden increase in P solubilization. We suggest that Change-Points simply define the asymtote of rapid desorption isotherms relating to that P which is most readily isotopically exchangeable (i.e at 24 h –³³P₂₄) with the soil solution. This involves ligand exchange at hydroxyl sites associated with Fe and Al cations, which is kinetically governed by the concentration of surface complexes on soil minerals. Individual Change-Points reflect the mineralogy and surface chemistry of different soil types. Laboratory and field measurements of the Olsen-P Change-Point reflect these surface phenomena and are similar. Olsen-P extracts the portion of the exchangeable pool that most readily controls solution P, and the Olsen-P/³³P₂₄ ratio is linearly related to Olsen-P Change-Points. This may provide a method for estimating P Change-Points where gradients of soil P are not available.     

Mediterranean natural forest living at elevated carbon dioxide: soil biological properties and plant biomass growth

Abstract. Biomass productivity and soil microbial responses to long-term CO₂ enrichment have been investigated in a Mediterranean natural forest ecosystem. Several biochemical parameters have been measured on soil samples taken from six open top chambers (OTCs), enclosing clumps of natural Mediterranean woody vegetation including: Quercus ilex L., Phillyrea angustifolia L., Pistacia lentiscus L. and Myrtus communis L. The CO₂ concentration of the air inside the OTCs was either ambient or ambient plus 350 μmol mol^–1 ( c . 710 ppm as mean daily value). Microbial C biomass, microbial respiration, dehydrogenase, β-glucosidase, acid phosphatase and protease activities, inorganic N and soluble P, were tested in order to evaluate soil microbial size and activity. Statistically correlated seasonal patterns have been identified in some biochemical parameters in response to climatic conditions, soil nutritional status and the physiology of the vegetative cover. In situ soil respiration and above- and below-ground productivity were also measured. Microbial responses to CO₂ enrichment were observed only at the beginning of the study and a general progressive reduction of the CO₂ effect was recorded as monitoring continued. These results are in agreement with data from literature regarding similar studies on natural complex communities.     

A modelling analysis of the potential for soil carbon sequestration under short rotation coppice willow bioenergy plantations

Abstract. Rising atmospheric CO₂ concentrations and their association with global climate change have led to several major international initiatives to reduce net CO₂ emissions, including the promotion of bioenergy crops such as short rotation coppice (SRC) willow. Although the above-ground harvested bio-fuel is likely to be the major contributor to the CO₂ mitigation potential of bioenergy crops, additional carbon may be sequestered through crop inputs into plantation soils.   Here, we describe a process-based model specifically designed to evaluate the potential for soil carbon sequestration in SRC willow plantations in the UK. According to the model predictions, we conclude that the potential for soil carbon sequestration in these plantations is comparable to, or even greater than, that of naturally regenerating woodland. Our preliminary, site-specific model output suggests that soil carbon sequestration may constitute about 5% of the overall carbon mitigation benefit arising from SRC plantations. Sensitivity analyses identified the following factors as the principal controls on rates and amounts of soil carbon sequestration under SRC: carbon inputs (net primary production), decomposition rates of the major soil carbon pools, initial soil carbon content (an inverse relationship with rates of soil carbon sequestration), crop/plantation management, and depth of soil being influenced by the bioenergy crop. Our results suggest that carbon sequestration potential is greatest in soils whose carbon content has been depleted to relatively low levels due to agricultural land use practices such as annual deep ploughing of agricultural soils.     

Impact of water percolation on nutrient leaching from an irrigated paddy field in Japan

Abstract. We examined the effect on soil nutrient status and sustainability of water percolation through an irrigated paddy field in Japan, to the depth of drainage (40 cm). The difference between amounts of nutrients leached by percolation and those supplied by irrigation indicated that 25–130 kg ha⁻¹ Ca, 8–24 kg ha⁻¹ Mg, from −1 to 9 kg ha⁻¹ K, and 8–17 kg ha⁻¹ Fe, respectively, were lost each year from the 0–40 cm soil layer during rice cultivation, when the supply from fertilization and rainfall and the loss in grain harvest were not accounted for. When the supply of K from rainfall and the loss in grain harvest were taken into account, a total K loss of about 10 kg ha⁻¹ was estimated. The electrical neutrality of inorganic ions in the percolating water was always maintained. From these results we estimate that the amounts of exchangeable Ca and Mg in the soil to a depth of 40 cm would decrease by 50% within 50–260 and 30–100 years, respectively, if similar management were continued without fertilization. The total amount of carbon dioxide (ΣCO₂) leached in percolating water during the period of rice cultivation was 120–325 kg C ha⁻¹, which corresponded to 0.47–0.94% of the soil organic carbon to 40 cm depth.     

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.     

Automated diffusion chambers to monitor diurnal and seasonal dynamics of the soil CO2 concentration profile

To better understand the factors controlling carbon dioxide (CO₂) production and transport in soil, we developed a new method to continuously monitor soil CO₂ concentration at multiple depths, by using diffusion chambers. The soil diffusion chambers are constructed from a high-density polyethylene cylindrical frame enclosed by a micro-polyvinylidene difluoride flat membrane (PVDF). All chambers are linked to an infrared gas analyser positioned above-ground through a multi-port valve system. We set up two experimental sites for long-term measurements of soil CO₂ concentration, soil temperature and soil water content at depths of 0, 10, 20, 40 and 80 cm. The system provides the following advantages : (i) the use of the PVDF combined with the small dimensions of the diffusion chambers allows rapid diffusion of soil gas into the chambers and therefore a short equilibration time of the gas phase with the surrounding soil atmosphere, (ii) the equilibrating closed loop system allows the semi-continuous measurement of soil profile CO₂ concentrations without creating a pressure differential within the chambers, thus reducing gas concentration distortions in the soil, (iii) the small size of the closed diffusion chambers reduces the initial soil disturbance during installation, (iv) it allows sampling in wet, humid soils, including ones that are waterlogged or temporarily saturated, and (v) the chambers do not require removal for maintenance purposes and are inexpensive.     

Mitigation of greenhouse gas emissions from soil under silage production by use of organic manures or slow-release fertilizer

Abstract. Grassland is a major source of nitrous oxide (N₂O) and methane (CH₄) emissions in the UK, resulting from high rates of fertilizer application. We studied the effects of substituting mineral fertilizer by organic manures and a slow-release fertilizer in silage grass production on greenhouse gas emissions and soil mineral N content in a three-year field experiment. The organic manures investigated were sewage sludge pellets and composted sewage sludge (dry materials), and digested sewage sludge and cattle slurry (liquid materials). The organic manures produced N₂O and carbon dioxide (CO₂) consistently from time of application up to harvest. However, they mitigated N₂O emissions by around 90% when aggregate emissions of 15.7 kg N ha⁻¹ from NPK fertilizer were caused by a flux of up to 4.9 kg N ha⁻¹ d⁻¹ during the first 4 days after heavy rainfall subsequent to the NPK fertilizer application. CH₄ was emitted only for 2 or 3 days after application of the liquid manures. CH₄ and CO₂ fluxes were not significantly mitigated. Composting and dried pellets were useful methods of conserving nutrients in organic wastes, enabling slow and sustained release of nitrogen. NPK slow-release fertilizer also maintained grass yields and was the most effective substitute for the conventional NPK fertilizer for mitigation of N₂O fluxes.     

Quantitative Modeling of Photoassimilate Flow in an Intact Plant Using the Positron Emitting Tracer Imaging System (PETIS)

The photoassimilate flow in an intact plant stem was imaged in real-time and its dynamics was quantitatively described using the Positron Emitting Tracer Imaging System (PETIS). Radioactive ¹¹CO₂ was fed to a leaf of an intact broad bean ( Vicia faba L.) plant, together with air containing an ambient concentration of non-radioactive carrier CO₂ gas. Movies of flow of the ¹¹C-labeled photoassimilates in the plant body were captured with PETIS. Here we demonstrate that the average flow speeds and the distribution ratios of photoassimilates in the respective nodes and internodes of the observed stem can be estimated by the transfer function analysis, one of the mathematical modeling methods. We also estimated the changes in the spatial distribution of the average flow speeds in the same stem when the fed leaf was exposed to enriched carrier CO₂ gas.     

Carbon cycling and sequestration opportunities in temperate grasslands

Abstract. Temperate grasslands account for c. 20% of the land area in Europe. Carbon accumulation in grassland ecosystems occurs mostly below ground and changes in soil organic carbon stocks may result from land use changes (e.g. conversion of arable land to grassland) and grassland management. Grasslands also contribute to the biosphere–atmosphere exchange of non-CO₂ radiatively active trace gases, with fluxes intimately linked to management practices. In this article, we discuss the current knowledge on carbon cycling and carbon sequestration opportunities in temperate grasslands. First, from a simple two-parameter exponential model fitted to literature data, we assess soil organic carbon fluxes resulting from land use change (e.g. between arable and grassland) and from grassland management. Second, we discuss carbon fluxes within the context of farming systems, including crop–grass rotations and farm manure applications. Third, using a grassland ecosystem model (PaSim), we provide estimates of the greenhouse gas balance, in CO₂ equivalents, of pastures for a range of stocking rates and of N fertilizer applications. Finally, we consider carbon sequestration opportunities for France resulting from the restoration of grasslands and from the de-intensification of intensive livestock breeding systems. We emphasize major uncertainties concerning the magnitude and non-linearity of soil carbon stock changes in agricultural grasslands as well as the emissions of N₂O from soil and of CH₄ from grazing livestock.     

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.     

The nature and kinetics of organic matter release from soil by salt solutions

Laboratory experiments in soil columns were performed to study the influence of dissolved salts on the amount and composition of organic matter (OM) released from soil. Samples of two surface soils from former wastewater infiltration sites were leached with solutions containing dissolved salts (NaH₂PO₄, NaNO₃, CaCl₂) and by deionized water. The NaH₂PO₄ solution induced strongest release with 0.6% of soil organic carbon (C_org) with 700 ml for 100 g of soil, while CaCl₂ released the least, summing to 0.1–0.2% of C_org. The OM released was characterized by UV absorbance (aromaticity), ultrafiltration (molecular size distribution) and solid-phase extraction (polarity). The results suggest that CaCl₂ preferentially released readily soluble OM. For the other solutions we assume solubilization by enhanced electrostatic repulsion (water), sodium exchange (NaNO₃), and sodium exchange and calcium decomplexation and displacement of sorbed organic anions (NaH₂PO₄) to be the major mechanisms of release. In all experiments a phase of spontaneous desorption was observed, followed by a phase of steady-state desorption. Activation energies for steady-state release were estimated from kinetic investigations and suggest that the release is controlled by diffusion towards the phase boundary. These investigations emphasize the influence of dissolved salts on the nature and quantity of organic matter released from soil. The method presented seems able to characterize soil organic matter with respect to its availability and its mode of association with the soil matrix.     

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.     

Comparison of approaches for estimating carbon sequestration at the regional scale

Abstract. Many former estimates of regional scale C sequestration potential have made use of linear regressions based on long-term experimental data, whilst some have used dynamic soil organic matter (SOM) models linked to spatial databases. Few studies have compared the two methods. We present a case study in which the potential of different land management practices to sequester carbon in soil in arable land is estimated by different methods. Two dynamic SOM models were chosen for this study, RothC (a soil process model) and CENTURY (a whole ecosystem model with a SOM module). RothC and CENTURY are the two most widely used and validated SOM models worldwide. A Geographic Information System (GIS) containing soil, land use and climate layers, was assembled for a case study in central Hungary. GIS interfaces were developed for the RothC and CENTURY models, thus linking them to the spatial datasets at the regional level. This allowed a comparison of estimates of the C sequestration potential of different land management practices obtained using the two models and using regression based approaches. Although estimates obtained by the different approaches were of the same order of magnitude, differences were observed. Some of the land management scenarios studied here showed sufficient C mitigation potential to meet Hungarian CO₂ reduction commitments. For example, afforestation of 12% current arable land could sequester 0.042–0.092 Tg yr^–1 in the soil alone, or 0.285–0.588 Tg C yr^–1 in both soil and biomass; 1990 level CO₂ emissions for the study area were 4.7 Tg C with a corresponding reduction commitment of 0.282 Tg C. It is not, however, suggested that this is the only, or the most favourable way, in which to meet the commitments.     

Causes of soil acidification: a summary

Abstract. A review of recent data shows that (i) dissolved CO₂ has its greatest acidifying effect in soils with pH values above about 6.5, (ii) fertilizers containing NH₋₁⁺ ions or urea will acidify soil whether the ions are taken up directly by plants or are first nitrified, (iii) oxidation of nitrogen and sulphur in soil organic matter causes acidification especially after deforestation, and (iv) the acidifying effect of rainfall and dry deposition is due to sulphuric and nitric acids, SO₂ and NH₋₁⁺ ions. A table is given showing the order of magnitude of each source of acidification.     

The reactions of copper and zinc with calcium carbonate surfaces

The reaction of copper and zinc were studied by adding the metal nitrate to a 4g:50ml suspension of calcite and water which had been equilibrated for two days, and then equilibrating for a further three days. Zinc behaved in a similar manner to cadmium in forming a surface-solid solution of Zn_χCa_1-χCO₃ as a result of adsorption, and the pIAP of the equilibrium solutions were close to those expected from the Thorstenson & Plummer equation. However, the continuity from adsorption to precipitation was broken by the formation of Zn₅(OH)₆(CO₃)₂ which has a higher stability than ZnCO₃. The CaCO₃ surface appears to constrain the adsorbed Zn to conform to a carbonate structure despite its lower stability than the hydroxy carbonate.
The adsorption data for Cu can also be explained in terms of a surface-solid solution of Cu_χCa_1-XCO₃, but CuCO₃ is not found naturally because of much greater stability of Cu(OH)₂, and so there is no independent value for the solubility of a CuCO₃ end-member if a surface-solid solution is formed. Secondary reactions are more likely to occur for Cu, especially close to the adsorption-precipitation boundary. Continuity from adsorption to precipitation again was not found. Theory predicts that Cu is less likely to form a surface-solid solution than Zn.
Upper Chalk (calcite with 4% other minerals, mostly quartz) behaved in a similar way but with increased adsorption due to its higher surface area.     

Soils as carbon sinks: the global context

Abstract. Soil carbon sequestration could meet at most about one-third of the current yearly increase in atmospheric CO₂-carbon, but the duration of the effect would be limited, with significant impacts lasting only 20–50 years. Coupled with this limited duration, increases in population and per-capita energy demand mean that soil carbon sequestration could play only a minor role in closing the difference between predicted and target carbon emissions by 2100. However, if atmospheric CO₂ concentrations are to be stabilized at reasonable levels (450–650 ppm), drastic reductions in carbon emissions will be required over the next 20–30 years. Given this, carbon sequestration should form a central role in any portfolio of measures to reduce atmospheric CO₂ concentrations over this crucial period, while new energy technologies are developed and implemented. International agreements, such as the Kyoto Protocol, encourage soil carbon sequestration and could be used to formulate soil carbon sequestration polices. Such policies need to take account of other environmental impacts as well as political, economic and societal needs, so that they form part of a raft of measures encouraging sustainable development. Of the carbon sequestration options available, those of a 'win–win' nature, that is, those that increase carbon stocks at the same time as improving other aspects of the environment, and those that protect or enhance existing stocks ('no regrets' implementation) show the greatest promise in meeting these goals.