Effects of land management on CO2 flux and soil C stock in two Tanzanian croplands with contrasting soil texture

Evaluation of carbon dynamics is of great concern worldwide in terms of climate change and soil fertility. However, the annual CO₂ flux and the effect of land management on the carbon budget are poorly understood in Sub-Saharan Africa, owing to the relative dearth of data for in situ CO₂ fluxes. Here, we evaluated seasonal variations in CO₂ efflux rate with hourly climate data in two dry tropical croplands in Tanzania at two sites with contrasting soil textures, viz. clayey or sandy, over four consecutive crop-cultivation periods of 40 months. We then: (1) estimated the annual CO₂ flux, and (2) evaluated the effect of land management (control plot, plant residue treatment plot, fertilizer treatment plot, and plant residue and fertilizer treatment plot) on the CO₂ flux and soil carbon stock at both sites. Estimated annual CO₂ fluxes were 1.0–2.2 and 0.9–1.9 Mg C ha^?1 yr^?1 for the clayey and sandy sites, respectively. At the end of the experiment, crop cultivation had decreased the surface soil carbon stocks by 2.4 and 3.0 Mg C ha^?1 (soil depth 0–15 cm) at the clayey and sandy sites, respectively. On the other hand, plant residue application (7.5 Mg C ha^?1 yr^?1) significantly increased the surface soil carbon stocks, i.e., 3.5–3.8 and 1.7–2.1 Mg C ha^?1 (soil depth 0–15 cm) at the clayey and sandy sites, respectively, while it also increased the annual CO₂ fluxes substantially, i.e., 2.5–4.0 and 2.4–3.4 Mg C ha^?1 yr^?1 for the clayey and sandy soils, respectively. Our results indicate that these dry tropical croplands at least may act as a carbon sink, though the efficiency of carbon accumulation was substantially lower in sandy soil (6.8–8.4%) compared to clayey soil (14.0–15.2%), possibly owing to higher carbon loss by leaching and macro-faunal activity.     

Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season

The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ, rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO₂ pulses increased from 0.84 to 2.08 g CO₂–C m^? 2 d^? 1 with the intensity of wetting up to ca. 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH₄–C m^? 2 d^? 1 with the intensity of wetting. Wetting dry forest floor increased N₂O fluxes from 6.2 to 25.9 μg N₂O–N m^? 2 d^? 1, but there was no significant difference between all experimental wetted plots. The rainfall-induced N₂O pulses with increasing wetting intensity were opposite to that of the CO₂ pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO₂ flux and CH₄ uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q₁₀ value of the temperature response of CO₂ fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO₂ pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.     

Estimating heterotrophic and autotrophic soil respiration in a semi-natural forest of Lombardy,Italy

We studied a semi-natural forest in Northern Italy that was set aside more than 50 years ago, in order to better understand the soil carbon cycle and in particular the partitioning of soil respiration between autotrophic and heterotrophic respiration. Here we report on soil organic carbon, root density, and estimates of annual fluxes of soil CO₂ as measured with a mobile chamber system at 16 permanent collars about monthly during the course of a year. We partitioned between autotrophic and heterotrophic respiration by the indirect regression method, which enabled us to obtain the seasonal pattern of single components.The soil pool of organic carbon, with 15.8 (±4.5) kg m^?2, was very high over the entire depth of 45 cm. The annual respiration rates ranged from 0.6 to 6.9 μmol CO₂ m^?2 s^?1 with an average value of 3.4 (±2.3) μmol CO₂ m^?2 s^?1, and a cumulative flux of 1.1 kg C m^?2 yr^?1. The heterotrophic component accounted for 66% of annual CO₂ efflux. Soil temperature largely controlled the heterotrophic respiration (R² = 0.93), while the autotrophic component followed irradiation, pointing to the role of photosynthesis in modulating the annual course of soil respiration.Most studies on soil respiration partitioning indicate autotrophic root respiration as a first control of the spatial variability of the overall respiration, which originates mainly from the uppermost soil layers. Instead, in our forest the spatial variability of soil respiration was mainly linked to soil carbon, and deeper layers seemed to provide a significant contribution to soil respiration, a feature that may be typical for an undisturbed, naturally maturing ecosystem with well developed pedobiological processes and high carbon stocks.     

How do environmental factors and different fertilizer strategies affect soil CO2 emission and carbon sequestration in the upland soils of southern China?

Upland soils have been identified as a major CO₂ source induced by human activities, such as fertilizer applications. The aim of this study is to identify the characteristics of soil CO₂ emission and carbon balance in cropland ecosystems after continuous fertilizer applications over decades. The measurements of soil surface CO₂ fluxes throughout the years of 2009 and 2010 were carried out based on a fertilization experiment (from 1990) in a double cropping system rotated with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) in upland soil in southern China. Four treatments were chosen from the experiment for this study: no-fertilizer application (SR), nitrogen–phosphorus–potassium chemical fertilizers (NPK), NPK plus pig manure (NPKM) and pig manure alone (M). Results showed that the mean value of soil CO₂ fluxes from 08:00 to 10:00 am could represent its daily mean value in summer period (June–August) and that from 09:00 am to 12:00 pm for the rest season of a year. Soil temperature and moisture combined together could explain 70–83% of variations of CO₂ emission. Annual cumulative soil CO₂ fluxes in the treatments with manure applications (8.2 ± 0.8 and 11.0 ± 1.2 t C ha⁻¹ in 2009, and 7.9 ± 0.9 and 11.1 ± 1.2 t C ha⁻¹ in 2010 in NPKM and M, respectively) were significantly higher than those in the treatments with non-manure addition (2.5 ± 0.2 and 3.4 ± 0.2 t C ha⁻¹ in 2009, and 2.1 ± 0.2 and 3.7 ± 0.3 t C ha⁻¹ in 2010 in SR and NPK, respectively). However, the treatments with manure applications represented a carbon sink in the soil (carbon output/input ratio < 1.0), which demonstrated potential for carbon sequestration.     

The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils

Contradictory effects of simultaneous available organic C and N sources on nitrous oxide (N₂O), carbon dioxide (CO₂) and nitric oxide (NO) fluxes are reported in the literature. In order to clarify this controversy, laboratory experiments were conduced on two different soils, a semiarid arable soil from Spain (soil I, pH=7.5, 0.8%C) and a grassland soil from Scotland (soil II, pH=5.5, 4.1%C). Soils were incubated at two different moisture contents, at a water filled pore space (WFPS) of 90% and 40%. Ammonium sulphate, added at rates equivalent to 200 and 50 kg N ha^?1, stimulated N₂O and NO emissions in both soils. Under wet conditions (90% WFPS), at high and low rates of N additions, cumulative N₂O emissions increased by 250.7 and 8.1 ng N₂O–N g^?1 in comparison to the control, respectively, in soil I and by 472.2 and 2.1 ng N₂O–N g^?1, respectively, in soil II. NO emissions only significantly increased in soil I at the high N application rate with and without glucose addition and at both 40% and 90% WFPS. In both soils additions of glucose together with the high N application rate (200 kg N ha^?1) reduced cumulative N₂O and NO emissions by 94% and 55% in soil I, and by 46% and 66% in soil II, respectively. These differences can be explained by differences in soil properties, including pH, soil mineral N and total and dissolved organic carbon content. It is speculated that nitrifier denitrification was the main source of NO and N₂O in the C-poor Spanish soil, and coupled nitrification–denitrification in the C-rich Scottish soil.     

Environmental impact of introducing plant covers in the taluses of terraces: Implications for mitigating agricultural soil erosion and runoff

South-eastern Spain, and in particular the coastal areas of Granada and Malaga, feature a large area under subtropical crops, with orchards established on terraces built along the slopes of the mountainous areas. The climate, characterized by periodically heavy rainfall, variable in space and time, and with the common agricultural practice of leaving the taluses with bare soil, are the main factors encouraging soil erosion, runoff, and subsequent transport of pollutants. Over a two-year period, six plant covers were applied [(Thymus mastichina (Th), Lavandula dentata (La), native spontaneous vegetation (Sv), Anthyllis cytisoides (An), Satureja obovata (Sa), Rosmarinus officinalis (Ro)] in comparison to a control of bare soil (Bs) to determine the effectiveness of the covers in reducing soil erosion, runoff, and potential pollution risk by agricultural nutrients (N, P, and K) and heavy metals. Also, carbon losses were monitored in the transported sediments by runoff and in eroded soils. For this purpose, 16 m² erosion plots (4 m × 4 m) were laid out in the taluses of the terraces. When the yearly data were compared, the control plot (Bs) showed significantly higher soil erosion and runoff rates (26.4 t ha^? 1 yr^? 1 and 55.7 mm yr^? 1, respectively) than the treatments with plant covers. The plant covers studied registered the following results in runoff: Ro > Sa > An > Th ≈ La > Sv (41.7, 38.2, 35.5, 16.9, 16.1, and 12.4 mm yr^? 1, respectively) while annual soil erosion gave the following results: Sa > An > Ro > Th > Sv > La (18.0, 13.5, 13.4, 5.5, 4.4, and 3.2 Mg ha^? 1 yr^? 1, respectively). This means that Sv reduced runoff and soil-erosion rates compared to Bs by not less than 78 and 83%, respectively. Nevertheless, La and Th plots were also very effective plant covers in reducing runoff and soil erosion (71.2 and 87.8; 69.5 and 79.2%, respectively) in comparison with the Bs plot. The heaviest nutrient losses in runoff and eroded soils were found in Bs and the lowest in the La, Th, and Sv plots. Bs and Ro plots registered the highest carbon losses (829.9 and 652.1 kg ha^? 1, respectively), the lowest carbon-loss rates being measured in La, Sv, and Th plots (145.2, 140.3, and 109.3 kg ha^? 1, respectively). The results indicate that heavy metals (Mn, Cr, Co, Ni, Cu, Zn, Mo, Cd, and Pb) in these types of agroecosystems may also be a potential pollutant due to transport by agricultural runoff. There was a major reduction of heavy-metal transport by plant covers in relation to the control of bare soil. The results of this research support the recommendation of using plant covers with multiple purposes (aromatic–medicinal–culinary) on the taluses of subtropical-crop terraces in order to reduce erosion and pollution risk.     

Nitrogen fertilization and cropping systems effects on soil organic carbon and total nitrogen pools under chisel-plow tillage in Illinois

Agricultural soils can be a major sink for atmospheric carbon (C) with adoption of recommended management practices (RMPs). Our objectives were to evaluate the effects of nitrogen (N) fertilization and cropping systems on soil organic carbon (SOC) and total N (TN) concentrations and pools. Replicated soil samples were collected in May 2004 to 90 cm depth from a 23-year-old experiment at the Northwestern Illinois Agricultural Research and Demonstration Center, Monmouth, IL. The SOC and TN concentrations and pools, soil bulk density (ρ_b) and soil C:N ratio were measured for five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)]. Long-term N fertilization and cropping systems significantly influenced SOC concentrations and pools to 30 cm depth. The SOC pool in 0–30 cm depth ranged from 68.4 Mg ha⁻¹ for N₀ to 75.8 Mg ha⁻¹ for N₄. Across all N treatments, the SOC pool in 0–30 cm depth for CC was 4.7 Mg ha⁻¹ greater than for CS. Similarly, TN concentrations and pools were also significantly affected by N rates. The TN pool for 0–30 cm depth ranged from 5.36 Mg ha⁻¹ for N₀ to 6.14 Mg ha⁻¹ for N₄. In relation to cropping systems, the TN pool for 0–20 cm depth for CC was 0.4 Mg ha⁻¹ greater than for CS. The increase in SOC and TN pools with higher N rates is attributed to the increased amount of biomass production in CC and CS systems. Increasing N rates significantly decreased ρ_b for 0–30 cm and decreased the soil C:N ratio for 0–10 cm soil depth. However, none of the measured soil properties were significantly correlated with N rates and cropping systems below 30 cm soil depth. We conclude that in the context of developing productive and environmentally sustainable agricultural systems on a site and soil specific basis, the results from this study is helpful to strengthening the database of management effects on SOC storage in the Mollisols of Midwestern U.S.     

Observed and modelled soil carbon and nitrogen changes after planting a Pinus radiata stand onto former pasture

After reforesting pasture land, it is often observed that soil carbon stocks decrease. The present work reports findings from a site near Canberra, Australia, where a pine forest (Pinus radiata) was planted onto a former unimproved pasture site. We report a number of detailed observations seeking to understand the basis of the decline in soil C stocks. This is supported by simulations using the whole-ecosystem carbon and nitrogen cycling model CenW 3.1. The model indicated that over the first 18 years after forest establishment, the site lost about 5.5 t C ha^?1 and 588 kgN ha^?1 from the soil. The C:N ratio of soil organic matter did not change in a systematic manner over the observational period. Carbon and nitrogen stocks contained in the biomass of the 18-year old pine stand exceeded that of the pasture by 88 t C ha^?1 and 393 kgN ha^?1. An additional 6.1 t C ha^?1 and 110 kgN ha^?1 accumulated in above-ground litter. These changes, together with the vertical distribution of carbon and nitrogen in the soil, agreed well with the observation at the site. It was assumed that over 18 years, there was also a loss of 86 kgN ha^?1 from the ecosystem because of normal gaseous losses during nitrogen turn-over and a small amount of nitrogen leaching. Those losses could not be replenished in the pine system without symbiotic biological nitrogen fixation, and there were no fertiliser additions. A simple mass balance approach indicated that the amount of nitrogen accumulating in plant biomass and the litter layer plus the assumed nitrogen loss from the site matched the amount of nitrogen lost from the soil organic nitrogen pool. This reduction in soil nitrogen, together with an unchanged C:N ratio, provided a simple and internally consistent explanation for the observed reduction of soil carbon after reforestation. It supports the general notion that trends in soil carbon upon land-use change can often be controlled by the possible fates of available soil nitrogen.     

Estimating greenhouse gas emissions from soil following liquid manure applications using a unit response curve method

Quantitative information is critical in policy making related to the roles of agriculture in greenhouse gas (GHG) emissions. A Unit Response (UR) curve method was developed in this study for modeling GHG emissions from soil after liquid manure applications. The emission sources (soils and liquid manures) are conceptualized as a set of linear cascaded chambers with equal storage-release coefficients, or two sets of cascaded chambers in parallel, each set having equal storage-release coefficients. The model is based on a two-parameter gamma distribution. Three parameters in this model denote the number of cascaded chambers, the storage-release coefficient, and the multiplier (referring to the total net emissions) added to the gamma distribution function. These parameters can be expressed as functions of site-specific background fluxes without applications of manure/fertilizer. The method was assessed with emissions data from five fields in Washington State. The results showed that at the WSU and Lynden sites, the average excess CH₄ emissions due to manure applications were 0.39 and 0.17 kg CH₄–C ha^? 1, respectively; the average excess CO₂ emissions were 216.50 and 25.20 kg CO₂–C ha^? 1, respectively; and the average excess N₂O were 0.37 and 0.03 kg N₂O–N ha^? 1, respectively. The UR method may fill the gaps between field measurements, simple emission factor (EF) method, and complex process-oriented models. This method has the potential to be used for estimating additional GHG emissions due to manure/fertilizer applications.     

Spatial and seasonal variations of atmospheric N2O and CO2 fluxes from a subtropical mangrove swamp and their relationships with soil characteristics

Marine ecosystems are a known net source of greenhouse gases emissions but the atmospheric gas fluxes, particularly from the mangrove swamps occupying inter-tidal zones, are characterized poorly. Spatial and seasonal fluxes of nitrous oxide (N₂O) and carbon dioxide (CO₂) from soil in Mai Po mangrove swamp in Hong Kong, South China and their relationships with soil characteristics were investigated. The N₂O fluxes averaged from 32.1 to 533.7 μg m⁻² h⁻¹ and the CO₂ fluxes were between 10.6 and 1374.1 mg m⁻² h⁻¹. Both N₂O and CO₂ fluxes in this swamp showed large spatial and seasonal variations. The fluxes were higher at the landward site than the foreshore bare mudflat, and higher fluxes were recorded in warm, rather than cold, seasons. The landward site had the highest content of soil organic carbon (OC), total Kjeldahl nitrogen (TKN), nitrate (NO₃⁻–N) and total phosphorus (TP), while the bare mudflat had the highest ammonium nitrogen (NH₄⁺–N) concentration and soil denitrification potential activity. The N₂O flux was related, positively, to CO₂ flux. Soil NO₃⁻–N and TP increased N₂O flux, while soil OC and TP concentrations contributed to the CO₂ flux. The results indicated that the Mai Po mangrove swamp emitted significant amounts of greenhouse gases, and the N₂O emission was probably due to soil denitrifcation.     

Modeling organic carbon dynamics under no-tillage and plowed systems in tropical soils of Brazil using CQESTR

CQESTR simulates the effect of management practices on soil organic carbon (SOC) stocks. The beta version of the model had been calibrated and validated for temperate regions. Our objective was to evaluate the CQESTR model performance for simulating carbon dynamics as affected by tillage practices in two tropical soils (Ultisol and Oxisol) in southeastern and northeastern Brazil. In the southeast (20.75 S 42.81 W), tillage systems consisted of no tillage (NT); reduced tillage (RT) (one disc plow and one harrow leveling [RT1] or one heavy disc harrow and one harrow leveling [RT2]); and conventional tillage (CT) (two heavy disc harrows followed by one disc plow and two harrow levelings). In the northeast (7.55 S 45.23 W), tillage systems consisted of NT, RT (one chisel plow and one harrow leveling), and CT (one disk plow, two heavy disk harrowings, and two harrow levelings). CQESTR underestimated SOC at both sites, especially under NT systems, indicating that adjustments (e.g., the inclusion of clay mineralogy factor) are necessary for more accurate simulation of SOC in the tropics. In spite of this, measured and simulated values of SOC in the 0–20 cm depth were well correlated (southeast, R² = 0.94, p < 0.01; northeast, R² = 0.88, p < 0.05). With respect to initial conditions (native forest), CQESTR estimated a decrease in SOC stocks in plowed and no-tillage systems. In 2006, in the southeast, SOC stocks were 28.8, 23.7, 23.2, and 22.0 Mg ha^?1 under NT, RT2, RT1, and CT, respectively; in the northeast, stocks were 36.0, 33.8, and 32.5 Mg ha^?1 under NT, RT, and CT, respectively. The model estimated carbon emissions varying from 0.36 (NT) to 1.05 Mg ha^?1 year^?1 (CT) in the southeast and from 0.30 (NT) to 0.82 (CT) Mg ha^?1 year^?1 in the northeast. CQESTR prediction of SOC dynamics illustrates acceptable performance for the two tropical soils of Brazil.     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

Effects of alternative management practices on the economics,energy and GHG emissions of a wheat–pea cropping system in the Canadian prairies

In recent years alternative farming practices have received considerable attention from Canadian producers as a means to improve their net return from grain and oilseed production. Enhancing the efficiency of nitrogen fertilizer use, including a pulse crop in the rotation, reducing tillage and pesticide use are seen as viable options to reduce reliance on fossil fuel, lower input costs and decrease the risk of soil, air and water degradation. The objective of this study was to determine the effects of 16 alternative management practices for a 2-year spring wheat (Triticum aestivum L.)–field pea (Pisum sativum L.) rotation on economic returns, non-renewable energy use efficiency, and greenhouse gas emissions. The alternative management methods for wheat consisted of a factorial combination of high vs. low soil disturbance one pass seeding, four nitrogen (N) fertilizer rates (20 kg N ha^?1, 40 kg N ha^?1, 60 kg N ha^?1 and 80 kg N ha^?1), and recommended vs. reduced rates of in-crop herbicide application. Alternative management practices for field pea were high vs. low soil disturbance one pass seeding. The resulting 16 cropping systems were evaluated at the whole farm level based on 4 years (two rotation cycles) of data from field experiments conducted on two Orthic Black Chernozem soils (clay loam and loam textures) in Manitoba, Canada. The highest net returns on the clay loam soil were for the high disturbance system with 60 kg N ha^?1 applied to wheat and the recommended rates of in-crop herbicides. The lowest application rate of N, together with low disturbance seeding, provided the highest economic returns on the loam soil. Energy use efficiency was highest for the lowest rate of N application for both tillage systems. The highest rate of N fertilizer and recommended rates of in-crop herbicide produced little additional yield response, lower net returns, and higher GHG emissions. An increase in N fertilizer application from 20 kg ha^?1 to 80 kg ha^?1 increased whole farm energy requirements by about 40%, while reducing herbicide rates had negligible effects on grain yields and total energy input. Overall, as N fertilizer rate increased, the associated GHG emissions were not offset by an increase in carbon retained in the above-ground crop biomass. Moderate to high soil test NO₃-N levels at experimental sites reduced the potential for positive yield responses to N fertilizer in this study, thus minimizing the economic benefits derived from N fertilizer application.     

Factors affecting soil loss at plot scale and sediment yield at catchment scale in a tropical volcanic agroforestry landscape

Tropical deforestation and land use change is often perceived as the major cause of soil loss by water erosion and of sediment load in rivers that has a negative impact on the functioning of hydropower storage reservoirs. The Sumberjaya area in Sumatra, Indonesia is representative for conflicts and evictions arising from this perception. The purpose of this study as part of a Negotiation Support System approach was to assess sediment yield both at plot and catchment scale and to relate it to a variety of possible clarifying factors i.e. land use, geology, soil and topography. Sediment yield at catchment scale per unit area, was found to be 3–10 times higher than soil loss measured in erosion plots. A stepwise regression showed that the dominant factors explaining sediment yield differences at catchment scale in this volcanic landscape were a particular lithology (Old Andesites) and slope angle followed by the silt fraction of the top soil. In lithologically sensitive areas soil loss at the plot scale under monoculture coffee gardens decreases over time from on average 7–11 Mg ha^? 1 yr^? 1 to 4–6 Mg ha^? 1 yr^? 1, mainly because of the development of surface litter layers as filters and top soil compaction in the areas without litter, but remains higher than under shade coffee systems or forest. The runoff coefficient under monoculture coffee remains on average significantly higher (10–15%) than under forest (4%) or under shade coffee systems (4–7%). In lithologically stable areas soil loss remained below 1.8 Mg ha^? 1 yr^? 1 and the runoff coefficient below 2.5% under all land use types, even bare soil plots or monoculture coffee gardens. Less than 20% of the catchment area produces almost 60% of the sediment yield. The reduction of negative off-site effects on e.g. the life time of a storage reservoir would benefit greatly from an improved assessment of the lithologies in volcanic landscapes and the consideration of potential sediment source and sink areas. In lithologically sensitive areas, a shift from sun to shade coffee systems may result in reducing surface runoff and soil loss, although water erosion at the plot scale is not the main contributor to sediment yield at the catchment scale. The quantification of land use effects on dominant erosive processes such as river bank and river bed erosion, landslides and the concentrated flow erosion on footpaths and roads can contribute to more targeted efforts and relevant incentives to reduce (or live with) sediment load of the rivers.     

Nitrogen mineralization in the ruderal sub-alpine communities in Mount Uludağ, Turkey

Nitrogen mineralization and nitrification in the soil of sub-alpine ruderal community of Mount Uludağ, Bursa, Turkey was measured for 1 year, under field conditions with Verbascum olympicum and Rumex olympicus being the dominant pioneer species under dry and wet sites, respectively. Seasonal fluctuations were observed in N mineralization and nitrification. The net N mineralization and nitrification were high in early summer and winter, due to high moisture. The annual net N mineralization rate (for the 0–15 cm soil layer) was higher under R. olympicus (188 kg N ha⁻¹ yr⁻¹) than under V. olympicum (96 kg N ha⁻¹ yr⁻¹). A significant positive correlation between net N mineralization and soil organic C (r² = 0.166), total N (r² = 0.141) and water content (r² = 0.211) was found. Our results indicate that N mineralization rate is high in soils of ruderal communities on disturbed sites and varies with dominant species and, a difference in net N mineralization rate can be attributed to organic C, total N and moisture content of soils.     

Recycling gamma irradiated sewage sludge as fertilizer: A case study using onion (Alium cepa)

Recycling sewage sludge into fertilizer for agricultural purposes may improve soil fertility by influencing the physical, chemical, and biological properties of the land. However, there is concern regarding elevated levels of heavy metals and pathogenic microorganisms, which may result from the use of untreated sewage sludge. Gamma radiation is found to be an efficient tool in the hygienization of municipal sewage sludge. In order to evaluate the agricultural potential of gamma irradiated sewage sludge and to assess the safety of this fertilizer, field experiments were performed in a root crop, onion (Alium cepa), during the 2003–2004 and 2004–2005 winter months. The influence over major nutrients, metallic micronutrients, and heavy metals in soil and crop plant were key factors to be analyzed. Treatments consisted of three source of fertilizers {S₁: farmyard manure (FYM); S₂: gamma irradiated sewage sludge (GISS); and S₃: non-irradiated sewage sludge (NISS)}, each at three separate levels (L₁: 5 t ha^?1; L₂: 10 t ha^?1; and L₃: 15 t ha^?1), which were evaluated and compared. The growth parameters and onion yield were not significantly influenced by the different sources of fertilizer, or the different application levels. Values of pH, organic carbon, organic N, available P and K, metallic micronutrients (Zn, Mn, Fe, Cu) and heavy metals (Ni, Cd, Pb, Co) indicate no negative effects on either soil or plant properties. Concentrations of heavy metals in soil and plant were slightly higher in NISS treatment in compare to GISS; however, the concentrations did remain within the prescribed limit, and no significant increase was consistently noted. The results prove that the gamma irradiated sludge material was of equal quality compared to the conventional FYM.     

Measurements of ozone deposition to a potato canopy

Potatoes are an important staple crop, grown in many parts of the world. Although ozone deposition to many vegetation types has been measured in the field, no data have been reported for potatoes. Such measurements, including the latent-heat flux, were made over a fully grown potato field in central Scotland during the summer of 2006, covering a 4-week period just after rainfall and then dry, sunny weather. The magnitude of the flux was typical of many canopies showing the expected diurnal cycles. Although the bulk-canopy stomatal conductance declined as the field dried out (～300 mmol-O₃ m^?2 s^?1 to ～70 mmol-O₃ m^?2 s^?1), the total ozone flux did not follow the same trend, indicating that non-stomatal deposition was significant. Over a dry surface non-stomatal resistance (R_ns) was 270–450 s m^?1, while over a wet surface R_ns was ～50% smaller and both decreased with increasing surface temperature and friction velocity. From the variation with relative humidity (RH) it is suggested that three processes occur on leaf surfaces: on a very dry surface ozone is removed by thermal decomposition, possibly enhanced by photolytic reactions in the daytime and so R_ns decreases as temperature increases; at 50–70% RH a thin film of liquid blocks the “dry” process and resistance increases; above 60–70% RH sufficient surface water is present for aqueous reactions to remove ozone and resistance decreases.     

Conservation farming strategies in East and Southern Africa: Yields and rain water productivity from on-farm action research

Improved agricultural productivity using conservation farming (CF) systems based on non-inversion tillage methods, have predominantly originated from farming systems in sub-humid to humid regions where water is not a key limiting factor for crop growth. This paper presents evidence of increased yields and improved water productivity using conservation farming in semi-arid and dry sub-humid locations in Ethiopia, Kenya, Tanzania and Zambia. Results are based on on-farm farmer and research managed experiments during the period 1999–2003. Grain yield of maize (Zea mays L.) and tef (Eragrostis Tef (Zucc)) from conventional (inversion) tillage are compared with CF with and without fertilizer. Rain water productivity (WP_rain) is assessed for the locations, treatments and seasons. Results indicate significantly higher yields (p < 0.05) for CF+ fertilizer treatments over conventional treatments in most locations, increasing from 1.2 to 2 t ha^?1 with 20–120% for maize. For tef in Ethiopian locations, the yield gains nearly doubled from 0.5–0.7 to 1.1 t ha^?1 for “best bet” CF+ fertilizer. WP_rain improved for CF+ fertilizer treatments with WP gains of 4500–6500 m³ rainwater per t maize grain yield in the lower yield range from 0 to 2.5 t ha^?1. This is explained by the large current unproductive water losses in the on-farm water balance. There was a tendency of improved WP_rain in drier locations, which can be explained by the water harvesting effect obtained in the CF treatments. The experiences from East and Southern Africa presented in this paper indicate that for smallholder farmers in savannah agro-ecosystems, conservation farming first and foremost constitutes a water harvesting strategy. It is thus a non-inversion tillage strategy for in situ moisture conservation, rather than solely aimed at minimum tillage with mulch cover. Challenges for the future adoption of CF in sub-Saharan Africa include how to improve farmer awareness of CF benefits, and how to efficiently incorporate green manure/cover crops and manage weeds.     

Evapotranspiration and water use efficiency in different-aged Pacific Northwest Douglas-fir stands

We used the eddy-covariance technique to measure evapotranspiration (E) and gross primary production (GPP) in a chronosequence of three coastal Douglas-fir (Pseudotsuga menziesii) stands (7, 19 and 58 years old in 2007, hereafter referred to as HDF00, HDF88 and DF49, respectively) since 1998. Here, we focus on the controls on canopy conductance (g_c), E, GPP and water use efficiency (WUE) and the effect of interannual climate variability at the intermediate-aged stand (DF49) and then analyze the effects of stand age following clearcut harvesting on these characteristics. Daytime dry-foliage Priestley–Taylor α and g_c at DF49 were 0.4–0.8 and 2–6 mm s^?1, respectively, and were linearly correlated (R² = 0.65). Low values of α and g_c at DF49 as well at the other two stands suggested stomatal limitation to transpiration. Monthly E, however, showed strong positive linear correlations to monthly net radiation (R² = 0.94), air temperature (R² = 0.77), and daytime vapour pressure deficit (R² = 0.76). During July–September, monthly E (mm) was linearly correlated to monthly mean soil water content (θ, m³ m^?3) in the 0–60 cm layer (E = 453θ ? 21, R² = 0.69), and GPP was similarly affected. Annual E and GPP of DF49 for the period 1998–2007 varied from 370 to 430 mm and from 1950 to 2390 g C m^?2, respectively. After clearcut harvesting, E dropped to about 70% of that for DF49 while ecosystem evapotranspiration was fully recovered when stand age was ～12 years. This contrasted to GPP, which varied hyperbolically with stand age. Monthly GPP showed a strong positive linear relationship with E irrespective of the stand age. While annual WUE of HDF00 and HDF88 varied with age from 0.5 to 4.1 g C m^?2 kg^?1 and from 2.8 to 4.4 g C m^?2 kg^?1, respectively, it was quite conservative at ～5.3 g C m^?2 kg^?1 for DF49. N-fertilization had little first-year response on E and WUE. This study not only provides important results for a more detailed validation of process-based models but also helps in predicting the influences of climate change and forest management on water vapour and CO₂ fluxes in Douglas-fir forests.     

Turnover of low molecular weight dissolved organic C (DOC) and microbial C exhibit different temperature sensitivities in Arctic tundra soils

Polar ecosystems are currently experiencing some of the fastest rates of climate warming. An increase in soil temperature in High Arctic regions may stimulate soil permafrost melting and microbial activity, thereby accelerating losses of greenhouse gases. It is therefore important to understand the factors regulating the rates of C turnover in polar soils. Consequently, our aims were to: (1) assess the concentration of low molecular weight (MW) dissolved organic carbon (DOC) in soil, (2) to investigate the temperature-dependent turnover of specific low MW compounds, and (3) to analyse the influence of substrate concentration on C cycling. Microbial mineralisation of labile low MW DOC in two High Arctic tundra soils was investigated using soil solutions spiked with either ¹⁴C-labelled glucose or amino acids. Spiked solutions were added to the top- and sub-soil from two ecosystem types (lichen and Carex dominated tundra), maintained at three temperatures (4–20 °C), and their microbial mineralisation kinetics monitored. ¹⁴CO₂ evolution from the tundra soils in response to ¹⁴C-glucose and -amino acid addition could best be described by a double first order exponential kinetic equation with rate constants k₁ and k₂. Both forms of DOC had a short half-life (t_1/2) in the pool of microbial respiratory substrate (t_1/2 = 1.07 ± 0.10 h for glucose and 1.63 ± 0.14 h for amino acids; exponential coefficient k₁ = 0.93 ± 0.07 and 0.64 ± 0.06 h^?1 respectively) whilst the second phase of mineralisation, assumed to be C that had entered the microbial biomass, was much slower (average k₂ = 1.30 × 10^?3 ± 0.49 × 10^?4 h^?1). Temperature had little effect on the rate of mineralisation of ¹⁴C used directly as respiratory substrate. In contrast, the turnover rate of the ¹⁴C immobilized in the microbial biomass prior to mineralisation was temperature sensitive (k₂ values of 0.99 × 10^?3 h^?1 and 1.66 × 10^?3 h^?1 at 4 and 20 °C respectively). Concentration-dependent glucose and amino acid mineralisation kinetics of glucose and amino acids (0–10 mM) were best described using Michaelis–Menten kinetics; there was a low affinity for both C substrates by the microbial community (K_m = 4.07 ± 0.41 mM, V_max = 0.027 ± 0.005 mmol kg^?1 h^?1). In conclusion, our results suggest that in these C limiting environments the flux of labile, low MW DOC through the soil solution is extremely rapid and relatively insensitive to temperature. In contrast, the turnover of C incorporated into higher molecular weight microbial C pools appears to show greater temperature sensitivity.