Urbanization associated changes in biogeochemical cycles.

Abstract. This paper summarizes currently available data on greenhouse gas (GHG) emissions from African natural ecosystems and agricultural lands. The available data are used to synthesize current understanding of the drivers of change in GHG emissions, outline the knowledge gaps, and suggest future directions and strategies for GHG emission research. GHG emission data were collected from 75 studies conducted in 22 countries (n = 244) in sub-Saharan Africa (SSA). Carbon dioxide (CO2) emissions were by far the largest contributor to GHG emissions and global warming potential (GWP) in SSA natural terrestrial systems. CO2 emissions ranged from 3.3 to 57.0 Mg CO2 ha−1 yr−1, methane (CH4) emissions ranged from −4.8 to 3.5 kg ha−1 yr−1 (−0.16 to 0.12 Mg CO2 equivalent (eq.) ha−1 yr−1), and nitrous oxide (N2O) emissions ranged from −0.1 to 13.7 kg ha−1 yr−1 (−0.03 to 4.1 Mg CO2 eq. ha−1 yr−1). Soil physical and chemical properties, rewetting, vegetation type, forest management, and land-use changes were all found to be important factors affecting soil GHG emissions from natural terrestrial systems. In aquatic systems, CO2 was the largest contributor to total GHG emissions, ranging from 5.7 to 232.0 Mg CO2 ha−1 yr−1, followed by −26.3 to 2741.9 kg CH4 ha−1 yr−1 (−0.89 to 93.2 Mg CO2 eq. ha−1 yr−1) and 0.2 to 3.5 kg N2O ha−1 yr−1 (0.06 to 1.0 Mg CO2 eq. ha−1 yr−1). Rates of all GHG emissions from aquatic systems were affected by type, location, hydrological characteristics, and water quality. In croplands, soil GHG emissions were also dominated by CO2, ranging from 1.7 to 141.2 Mg CO2 ha−1 yr−1, with −1.3 to 66.7 kg CH4 ha−1 yr−1 (−0.04 to 2.3 Mg CO2 eq. ha−1 yr−1) and 0.05 to 112.0 kg N2O ha−1 yr−1 (0.015 to 33.4 Mg CO2 eq. ha−1 yr−1). N2O emission factors (EFs) ranged from 0.01 to 4.1 %. Incorporation of crop residues or manure with inorganic fertilizers invariably resulted in significant changes in GHG emissions, but results were inconsistent as the magnitude and direction of changes were differed by gas. Soil GHG emissions from vegetable gardens ranged from 73.3 to 132.0 Mg CO2 ha−1 yr−1 and 53.4 to 177.6 kg N2O ha−1 yr−1 (15.9 to 52.9 Mg CO2 eq. ha−1 yr−1) and N2O EFs ranged from 3 to 4 %. Soil CO2 and N2O emissions from agroforestry were 38.6 Mg CO2 ha−1 yr−1 and 0.2 to 26.7 kg N2O ha−1 yr−1 (0.06 to 8.0 Mg CO2 eq. ha−1 yr−1), respectively. Improving fallow with nitrogen (N)-fixing trees led to increased CO2 and N2O emissions compared to conventional croplands. The type and quality of plant residue in the fallow is an important control on how CO2 and N2O emissions are affected. Throughout agricultural lands, N2O emissions slowly increased with N inputs below 150 kg N ha−1 yr−1 and increased exponentially with N application rates up to 300 kg N ha−1 yr−1. The lowest yield-scaled N2O emissions were reported with N application rates ranging between 100 and 150 kg N ha−1. Overall, total CO2 eq. emissions from SSA natural ecosystems and agricultural lands were 56.9 ± 12.7 × 109 Mg CO2 eq. yr−1 with natural ecosystems and agricultural lands contributing 76.3 and 23.7 %, respectively. Additional GHG emission measurements are urgently required to reduce uncertainty on annual GHG emissions from the different land uses and identify major control factors and mitigation options for low-emission development. A common strategy for addressing this data gap may include identifying priorities for data acquisition, utilizing appropriate technologies, and involving international networks and collaboration.

Incorporating organic matter alters soil greenhouse gas emissions and increases grain yield in a semi-arid climate

Nitrous oxide (N 2 O) emissions comprise the major share of agriculture's contribution to greenhouse gases; however, our understanding of what is actually happening in the field remains incomplete, especially concerning the multiple interactions between agricultural practices and N 2 O emissions. Soil compaction induces major changes in the soil structure and the key variables controlling N 2 O emissions. Our objective was to analyse the ability of a process‐based model (Nitrous Oxide Emissions (NOE)) to simulate the impact of soil compaction on N 2 O emission kinetics obtained from field experiments. We used automatic chambers to continuously monitor N 2 O and CO 2 emissions on uncompacted and compacted areas in sugar beet fields during 2 years. Soil compaction led to smaller CO 2 emissions and larger N 2 O emissions by inducing anoxic conditions favourable for denitrification. Cumulative N 2 O emissions during the crop cycles were 944 and 977 g N ha −1 in uncompacted plots and 1448 and 1382 g N ha −1 in compacted plots in 2007 and 2008, respectively. The NOE model () simulated 106 and 138 g N 2 O‐N ha −1 in uncompacted plots and 1550 and 650 g N 2 O‐N ha −1 in compacted plots in 2007 and 2008, respectively, markedly under‐estimating the nitrification rates and associated N 2 O emissions. We modified the model on the basis of published results in order to better simulate nitrification and account for varying N 2 O fractions of total end‐products in response to varying soil water and nitrate contents. The modified model (NOE2) better predicted nitrification rates and N 2 O emissions following fertilizer addition. Using a fine vertical separation of soil layers of configurable, but constant, thickness (1 cm) also improved the simulations. NOE2 predicted 428 and 416 g N‐N 2 O ha −1 in uncompacted plots and 1559 and 1032 g N‐ N 2 O ha −1 in compacted plots in 2007 and 2008, respectively. These results show that a simple process‐based model can be used to predict successfully the post‐fertilizer addition kinetics of N 2 O emissions and the impact of soil compaction on these emissions. However, large emissions later on during the cropping cycle were not captured by the model, emphasizing the need for further research.

The extent of anthropogenic land use in watersheds determines the amount of pollutants discharged to streams. This indirectly and directly affects stream water quality and biological health. Most studies have therefore focused on ways to reduce non-point pollution sources to streams from the surrounding land use in watersheds. However, the mechanistic pathways between land use and the deterioration of stream water quality and biological assemblages remain unclear. This study estimated a structural equation model (SEM) representing the impact of agricultural and urban land use on water quality and the benthic macroinvertebrate index (BMI) using IBM AMOS in the Nam-Han river systems, South Korea. The estimated SEM showed that the percent of urban and agricultural land in the watersheds significantly affected both the water quality and the BMI of the streams. Specifically, a higher percent of urban land use had directly increased the biochemical oxygen demand (BOD) and total phosphorus (TP), and deteriorated the BMI of streams. Similarly, higher proportions of agricultural land use had also directly increased the BOD, total nitrogen (TN), and total phosphorus (TP) concentrations, and lowered the BMI of streams. In addition, it was observed that the percent of urban and agricultural land use had indirectly deteriorated the BMI through increased BOD. However, we were not able to observe any significant indirect effect of the percent of urban and agricultural land use through increased nutrients including TN and TP. These results indicate that increased urban and agricultural land use in the watersheds had directly and indirectly affected the physicochemical characteristics and benthic macroinvertebrate communities in streams. Our findings emphasize the need to develop more elaborate environmental management and restoration strategies to improve the water quality and biological status of streams.

Biochar is the product of pyrolysis produced from feedstock of biological origin. Due to its aromatic structure and long residence time, biochar may enable long-term carbon sequestration. At the same time, biochar has the potential to improve soil fertility and reduce greenhouse gas (GHG) emissions from soils. However, the effect of biochar application on GHG fluxes from soil must be investigated before recommendations for field-scale biochar application can be made. A laboratory experiment was designed to measure carbon dioxide (CO) and nitrous oxide (NO) emissions from two Irish soils with the addition of two different biochars, along with endogeic (soil-feeding) earthworms and ammonium sulfate, to assist in the overall evaluation of biochar as a GHG-mitigation tool. A significant reduction in NO emissions was observed from both low and high organic matter soils when biochars were applied at rates of 4% (w/w). Earthworms significantly increased NO fluxes in low and high organic matter soils more than 12.6-fold and 7.8-fold, respectively. The large increase in soil NO emissions in the presence of earthworms was significantly reduced by the addition of both biochars. biochar reduced the large earthworm emissions by 91 and 95% in the low organic matter soil and by 56 and 61% in the high organic matter soil (with and without N fertilization), respectively. With peanut hull biochar, the earthworm emissions reduction was 80 and 70% in the low organic matter soil, and only 20 and 10% in the high organic matter soil (with and without N fertilization), respectively. In high organic matter soil, both biochars reduced CO efflux in the absence of earthworms. However, soil CO efflux increased when peanut hull biochar was applied in the presence of earthworms. This study demonstrated that biochar can potentially reduce earthworm-enhanced soil NO and CO emissions. Hence, biochar application combined with endogeic earthworm activity did not reveal unknown risks for GHG emissions at the pot scale, but field-scale experiments are required to confirm this.

Dairy production systems represent a significant source of air pollutants such as greenhouse gases (GHG), that increase global warming, and ammonia (NH3), that leads to eutrophication and acidification of natural ecosystems. Greenhouse gases and ammonia are emitted both by conventional and organic dairy systems. Several studies have already been conducted to design practices that reduce greenhouse gas and ammonia emissions from dairy systems. However, those studies did not consider options specifically applied to organic farming, as well as the multiple trade-offs occurring between these air pollutants. This article reviews agricultural practices that mitigate greenhouse gas and ammonia emissions. Those practices can be applied to the most common organic dairy systems in northern Europe such as organic mixed crop-dairy systems. The following major points of mitigation options for animal production, crop production and grasslands are discussed. Animal production: the most promising options for reducing greenhouse gas emissions at the livestock management level involve either the improvement of animal production through dietary changes and genetic improvement or the reduction of the replacement rate. The control of the protein intake of animals is an effective means to reduce gaseous emissions of nitrogen, but it is difficult to implement in organic dairy farming systems. Considering the manure handling chain, mitigation options involve housing, storage and application. For housing, an increase in the amounts of straw used for bedding reduces NH3 emissions, while the limitation of CH4 emissions from deep litter is achieved by avoiding anaerobic conditions. During the storage of solid manure, composting could be an efficient mitigation option, depending on its management. Addition of straw to solid manure was shown to reduce CH4 and N2O emissions from the manure heaps. During the storage of liquid manure, emptying the slurry store before late spring is an efficient mitigation option to limit both CH4 and NH3 emissions. Addition of a wooden cover also reduces these emissions more efficiently than a natural surface crust alone, but may increase N2O emissions. Anaerobic digestion is the most promising way to reduce the overall greenhouse gas emissions from storage and land spreading, without increasing NH3 emissions. At the application stage, NH3 emissions may be reduced by spreading manure during the coolest part of the day, incorporating it quickly and in narrow bands. Crop production: the mitigation options for crop production focus on limiting CO2 and N2O emissions. The introduction of perennial crops or temporary leys of longer duration are promising options to limit CO2 emissions by storing carbon in plants or soils. Reduced tillage or no tillage as well as the incorporation of crop residues also favour carbon sequestration in soils, but these practices may enhance N2O emissions. Besides, the improvement of crop N-use efficiency through effective management of manure and slurry, by growing catch crops or by delaying the ploughing of leys, is of prime importance to reduce N2O emissions. Grassland: concerning grassland and grazing management, permanent conversion from arable to grassland provides high soil carbon sequestration while increasing or decreasing the livestock density seems not to be an appropriate mitigation option. From the study of the multiple interrelations between gases and between farm compartments, the following mitigation options are advised for organic mixed crop-dairy systems: (1) actions for increasing energy efficiency or fuel savings because they are beneficial in any case, (2) techniques improving efficiency of N management at field and farm levels because they affect not only N2O and NH3 emissions, but also nitrate leaching, and (3) biogas production through anaerobic digestion of manure because it is a promising efficient method to mitigate greenhouse gas emissions, even if the profitability of this expensive investment needs to be carefully studied. Finally, the way the farmer implements the mitigation options, i.e. his practices, will be a determining factor in the reduction of greenhouse gas and NH3 emissions.

Greenhouse gases (GHGs) emission from arable field is a hot topic recently, adopting appropriate cropping systems is an effective way to reduce GHGs emission. This paper reviewed the impacts and mechanisms of intercropping on soil CO2 and N2O emissions in upland field. Rational intercropping systems could increase soil organic carbon (SOC), promote the transformation of straw to SOC, slow down mineralization rate of SOC, and hence reduce soil CO2 emissions. The Poaceae intercropping with legume could maintain the stability of yield while reducing synthetic N inputs, formation of inorganic N by residue decomposition and soil mineral N, and further reducing soil N2O emission. In addition, crop interactions in intercropping system and filed microclimate were important factors on GHGs emission as well. It is necessary to extent the period of researches in field GHGs emission in order to fully understand the underlying mechanisms of GHGs emission in farm land, especially the function of soil microorganisms at molecular level. It would provide theoretical knowledge in building environment-friendly agricultural system in the future.

Whole farm modelling the effect of grass silage harvest date and nitrogen fertiliser rate on nitrous oxide emissions from grass-based suckler to beef farming systems

We explored land use, fish assemblage structure, and stream habitat associations in 20 catchments in Opequon Creek watershed, West Virginia. The purpose was to determine the relative importance of urban and agriculture land use on stream biotic integrity, and to evaluate the spatial scale (i.e., whole-catchment vs riparian buffer) at which land use effects were most pronounced. We found that index of biological integrity (IBI) scores were strongly associated with extent of urban land use in individual catchments. Sites that received ratings of poor or very poor based on IBI scores had > 7% of urban land use in their respective catchments. Habitat correlations suggested that urban land use disrupted flow regime, reduced water quality, and altered stream channels. In contrast, we found no meaningful relationship between agricultural land use and IBI at either whole-catchment or riparian scales despite strong correlations between percent agriculture and several important stream habitat measures, including nitrate concentrations, proportion of fine sediments in riffles, and the abundance of fish cover. We also found that variation in gradient (channel slope) influenced responses of fish assemblages to land use. Urban land use was more disruptive to biological integrity in catchments with steeper channel slopes. Based on comparisons of our results in the topographically diverse Opequon Creek watershed with results from watersheds in flatter terrains, we hypothesize that the potential for riparian forests to mitigate effects of deleterious land uses in upland portions of the watershed is inversely related to gradient.

The accessibility provided by the transportation system plays an essential role in driving urban growth and urban functional land use changes. Conventional studies on land use simulation usually simplified the accessibility as proximities and adopted the grid-based simulation strategy, leading to the insufficiencies of characterizing spatial geometry of land parcels and simulating subtle land use changes among urban functional types. To overcome these limitations, an Accessibility-interacted Vector-based Cellular Automata (A-VCA) model was proposed for the better simulation of realistic land use change among different urban functional types. The accessibility at both local and zonal scales derived from actual travel time data was considered as a key driver of fine-scale urban land use changes and was integrated into the vector-based CA simulation process. The proposed A-VCA model was tested through the simulation of urban land use changes in the City of Toronto, Canada, during 2012–2016. A vector-based CA without considering the driving factor of accessibility (VCA) and a popular grid-based CA model (Future Land Use Simulation, FLUS) were also implemented for comparisons. The simulation results reveal that the proposed A-VCA model is capable of simulating fine-scale urban land use changes with satisfactory accuracy and good morphological feature (kappa = 0.907, figure of merit = 0.283, and cumulative producer’s accuracy = 72.83% ± 1.535%). The comparison also shows significant outperformance of the A-VCA model against the VCA and FLUS models, suggesting the effectiveness of the accessibility-interactive mechanism and vector-based simulation strategy. The proposed model provides new tools for a better simulation of fine-scale land use changes and can be used in assisting the formulation of urban and transportation planning.

Given the contribution from the livestock sector to greenhouse gas emissions (GHGe) and soil degradation via compaction and defecations it is imperative that best grazing management approaches are identified. Compacted soil from animal treading can lead to conditions that activate denitrifying bacteria and result in heightened nitrous oxide (N2O) emissions. In addition, nitrogen input from cattle urination events amplify these emissions, however more data is required in this area. In this study we aimed to quantify soil N2O emissions in parallel with soil compaction following a simulated grazing event using three stocking densities (control, low (LSD) and high (HSD) with 0, 10 and 100 cows/ha/day respectively) on a temperate Southwest UK pasture. In addition, data was collected to assess how cattle urinations affect soil GHGe under the different stocking densities. Following the grazing event measurements were taken for 12 weeks, soil GHGe were sampled via the use of static chambers and compaction was assessed via penetration resistance. Immediately after the simulated grazing the penetration resistance was 1379 kPa (SD ±368) for the control, 1544 kPa (SD ±429) for the LSD, and 1767 kPa (SD ±490) for the HSD. Differences in penetration resistance were found between the HSD and LSD (p=0.03), and HSD and control (p=0.0001), with a tendency between the control and LSD (p=0.07). Cumulative N2O emissions were 493 g N2O - N/ha for the control +urine, 804 g N2O - N/ha for the LSD +urine and 1237 g N2O - N/ha for the HSD +urine, with differences found between the control +urine and HSD +urine (p=0.0342). The N2O emissions from the stocking densities without urine were however similar. This suggests that denitrification in soils is enhanced under high stocking densities from animal treading when there is a source of nitrogen present, such as cattle urine. The developing results are informative for improving pasture management under cattle grazing and provide data for models simulating soil mechanisms in these systems.

Agriculture is the second-largest contributor of greenhouse gasses (GHGs) globally, after fossil fuels combustion. The excessive application of mineral fertilizers and the inadequate disposal of large amounts of livestock waste in agricultural soils result in elevated N2O and CO2 emissions, which surpass 17% of the global GHG emissions.Approximately 1.4 billion tons of cow manure (CM) are produced every year in the EU and current EU policies promote CM incorporation into the soil, as a cost-efficient and sustainable agronomic practice. The European Green Deal urges a 20% reduction in chemical fertilization by 2030 and reuse of organic fertilizers, i.e. cow manure The beneficial use of CM is linked to enhanced soil fertility, soil organic matter content and carbon sequestration. However, soil organic amendments may fuel soil nutrient transformations and potentially increase nutrient losses i.e. GHG emissions.To test the short-term effects of combined organic and inorganic fertilization on GHG emissions, we conducted a mesocosm experiment using two soil types (Sandy-loam (SL) & Loamy (L)) and including five treatments: Control (C: No fertilization), Urea as Chemical Fertilization in two rates (100U:200 kg/ha & 80U:160 kg/ha), Cow Manure (CM:50 Mg/ha) and the combination of 80U and CM (80U-CM:160 kg/ha Urea & 50 Mg/ha Cow Manure). During a 90-days incubation period, CO2 and N2O flux rates and soil NO-3, NO-2 and NH+4 were measured regularly. Soil type was the only significant factor (p≤0.05) driving cum. CO2 emissions. A 20% increase of cum. CO2 was found for L soil treatments than SL. The combined treatment 80U-CM had similar emissions to conventional fertilization (100U) that were on average 762 mg/kg C-CO2, approx. 28.5% greater than C (591 mg/kg). CM incorporation led to 19% increase in cum. CO2 emissions than C.Contrary to CO2, soil (p<0.001), fertilization (p<0.001) and their interaction (p=0.002) were significant factors explaining cum. N2O emissions. The SL soil had 60% higher cum. N2O emissions compared to L. The use of CM in L soil decreased (39%), while in SL soil increased (5%) cum N2O emissions, relative to C. A 20% reduction in urea application resulted in 90% and 19% reduction for SL and L soil, respectively when compared to 100U. The combined application 80U-CM increased cum. N2O emissions than CM and 80U and had lower cum. N2O emissions than 100U, for both soils. Soil, fertilization, and their interaction were accounted for statistically significant (p≤0.05) differences in soil NO3- , NO2- and NH4+ availability (AUC) .According to our study, the combined application of 80U-CM cannot be an effective alternative to conventional fertilization (100U), as it generates similar levels of GHG emissions and has lower nutrient (N) supply potential. Furthermore, our preliminary research indicates the need to further quantify the effects of different organic amendments on GHG soil emissions and soil microbial communities.Funding: The BSc and MSc research work by George Kourtidis and Elpida Pasvadoglou, respectively, was supported in part by the Hellenic Foundation for Research and Innovation (HFRI) Post-Doctoral Grant #1053 awarded to Principal Investigator Dr. Georgios Giannopoulos.

Biochar is widely used in agriculture to enhance crop yield, improve soil fertility, and regulate greenhouse gas (GHG) emissions. Its effectiveness, however, depends not only on its properties but also on soil moisture conditions, making integrated water management essential for maximizing its benefits. The study reports the results of a laboratory incubation experiment using three biochar application rates (0, 20, and 40 t ha−1) and two irrigation regimes—flooded irrigation and alternate wetting and drying (AWD)—to investigate the effects of biochar amendment and water management on soil greenhouse gas (GHG) emissions. The results indicated that there was no significant interaction between biochar and water regulation on GHG emissions, and changes in soil moisture and biochar application levels had no significant impact on carbon dioxide (CO2) emissions. Compared to flooded irrigation, AWD effectively enhanced soil microbial activity, increasing nitrous oxide (N2O) emissions by 62.50% to 88.35%, but significantly reducing methane (CH4) emissions by 44.30% to 68.55%, thereby lowering the soil’s global warming potential (GWP). Additionally, biochar amendment significantly increased soil SOC and TN contents, enhanced soil enzyme activities, and significantly improved microbial carbon use efficiency (CUE), the C/N ratio, and the net nitrification rate (NNR). However, it had no significant effect on soil N2O and CO2 emissions, while significantly suppressed CH4 emissions. Throughout the entire growth period, biochar application increased soil GWP overall. However, during the first water cycle, GWP increased with higher biochar application rates, whereas in the second water cycle, biochar application exhibited a suppressive effect on GWP. In conclusion, integrating biochar application with AWD irrigation can optimize soil CUE, enhance soil nutrient supply, and mitigate, to some extent, the potential increase in GHG emissions induced by biochar. This provides valuable insights for carbon management and sustainable agricultural development.

Release of dissolved organic carbon (DOC) and nitrogen (DON) from subtropical urban soils has had little investigation, even though the growth of metropolitan areas is unprecedented in southern states. The time of initial disturbance from native or agricultural to urban land use and irrigation with sodic water may compromise urban soils' ability to retain DOC and DON. We examined 21 soils from beneath remnant native land use and urban land use in south-central Texas. Soils were collected from five classes of urban land use and four classes of remnant native land use and analyzed using initial mass isotherms. We found that release of DOC was significantly higher from soils under urban parks (range, 433–782 mg kg−1) relative to soils from residential lawns (range, 89–228 mg kg−1) and remnant native soils (range, 43–264 mg kg−1). Release of DON and DON reactive soil pools were also significantly increased in soils under urban relative to remnant land use (P < 0.05). Older urban soils with minimal irrigation tended to have DOC and DON reactive soil pools similar to remnant soils, perhaps illustrating that equilibrium is reached over time.

Revealing the spatial–temporal pattern and convergence characteristics of urban land use efficiency has important guiding significance for adjusting and optimizing the regional urban land use structure. Taking the provincial units in China as the research object, the urban land use efficiency evaluation system considering the unexpected output was constructed, and the slack-based measure (SBA) model was used to quantitatively measure the provincial urban land use efficiency from 2000 to 2020. The exploratory spatial data analysis (ESDA) model and spatial convergence index were combined to reveal the spatial–temporal pattern and convergence characteristics of provincial urban land use efficiency. The results showed that the provincial urban land use efficiency has been continuously improving, with regional differences as shown in eastern region > northeast region > central region > western region. Moran’s I of provincial urban land use efficiency was greater than 0, there was a positive spatial correlation, and the clustering feature became increasingly significant. The spatial form of LISA was characterized by “small agglomeration and large dispersion”; the H(High)-H(High) type was clustered in the Yangtze River Delta and Beijing–Tianjin–Hebei, while the L(Low)-L(Low) type was clustered in Xizang, Xinjiang and Qinghai. There was a σ convergence in provincial urban land use efficiency, and there was significant absolute β convergence and conditional β convergence of provincial urban land use efficiency. The results showed that the differences in provincial urban land use efficiency were shrinking, showing a “catch-up effect”, and converging to their respective stable states over time. Based on the analysis of the spatial–temporal pattern and convergence characteristics of provincial urban land use efficiency in China, we could provide a direction for the optimization of the urban land use structure and efficiency improvement in China, in order to narrow the differences in urban land use efficiency in China’s four major regions.