Kim, D.-G., 2012. Estimation of net gain of soil carbon in a nitrogen-fixing tree and crop intercropping system in sub-Saharan Africa: results from re-examining a study.

Kim, D.-G., 2012. Estimation of net gain of soil carbon in a nitrogen-fixing tree and crop intercropping system in sub-Saharan Africa: results from re-examining a study. Agroforestry Systems, doi:10.1007/s10457-011-9477-1

 

Abstract

Nitrogen (N)-fixing tree and crop intercropping systems can be a sustainable agricultural practice in sub-Saharan Africa and can also contribute to resolving climate change through enhancing soil carbon (C) sequestration. A study conducted by Makumba et al. (Agric Ecosyst Environ 118:237–243, 2007) on the N-fixing tree gliricidia and maize intercropping system in southern Malawi provides a rare dataset of both sequestered soil C and C loss as soil carbon dioxide (CO₂) emissions. However, no soil C gain and loss estimates were made so the study failed to show the net gain of soil C. Also absent from this study was potential benefit or negative impact related to the other greenhouse gas, nitrous oxide (N₂O) and methane (CH₄) emissions from the intercropping system. Using the data provided in Makumba et al. (Agric Ecosyst Environ 118:237–243, 2007) a C loss as soil CO₂ emissions (51.2 ± 0.4 Mg C ha⁻¹) was estimated, amounting to 67.4% of the sequestered soil C (76 ± 8.6 Mg C ha⁻¹ in 0–2 m soil depth) for the first 7 years in the intercropping system. An annual net gain of soil C of 3.5 Mg C ha⁻¹ year⁻¹ was estimated from soil C sequestered and lost. Inclusion of the potential for N₂O mitigation [0.12–1.97 kg N₂O–N ha⁻¹ year⁻¹, 0.036–0.59 Mg CO₂ equivalents (eq.) ha⁻¹ year⁻¹] within this intercropping system mitigation as CO₂ eq. basis was estimated to be 3.5–4.1 Mg CO₂ eq. ha⁻¹ year⁻¹. These results suggest that reducing N₂O emission can significantly increase the overall mitigation benefit from the intercropping system. However, significant uncertainties are associated with estimating the effect of intercropping on soil N₂O and CH₄ emissions. These results stress the importance of including consideration of quantifying soil CO₂, N₂O and CH₄ emissions when quantifying the C sequestration potential in intercropping system.

Sugihara et al. 2012. Effects of land management on CO2 flux and soil C stock in two Tanzanian croplands with contrasting soil texture

Sugihara, S., Funakawa, S., Kilasara, M., and Kosaki, T.: Effects of land management on CO2 flux and soil C stock in two Tanzanian croplands with contrasting soil texture, Soil Biology and Biochemistry, 46, 1-9, 2012.

Abstract

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.

Saunders et al. 2011. Agricultural encroachment: Implications for carbon sequestration in tropical African wetlands

Saunders, M.J., Kansiime, F., Jones, M.B., 2011. Agricultural encroachment: Implications for carbon sequestration in tropical African wetlands. Global Change Biology, 10.1111/j.1365-2486.2011.02633.x.

Abstract

Tropical wetlands have been shown to exhibit high rates of net primary productivity and may therefore play an important role in global climate change mitigation through carbon assimilation and sequestration. Many permanently flooded areas of tropical East Africa are dominated by the highly productive C₄ emergent macrophyte sedge, Cyperus papyrus L. (papyrus). However, increasing population densities around wetland margins in East Africa are reducing the extent of papyrus coverage due to the planting of subsistence crops such as Colocasia esculenta (cocoyam). In this paper we assess the impact of this land use change on the carbon cycle and in particular the impacts of land conversion on net ecosystem carbon dioxide exchange. Eddy covariance techniques were used, on a campaign basis, to measure fluxes of carbon dioxide over both papyrus and cocoyam dominated wetlands located on the Ugandan shore of Lake Victoria. Peak rates of net photosynthetic CO₂ assimilation, derived from monthly diurnal averages of net ecosystem exchange, of 28-35 μmol CO₂ m⁻² s⁻¹ and 15-20 μmol CO₂ m⁻² s⁻¹ were recorded in the papyrus and cocoyam wetlands respectively, while night time respiratory losses ranged between 10-15 μmol CO₂ m⁻² s⁻¹ at the papyrus wetland and 5-10 μmol CO₂ m⁻² s⁻¹ at the cocoyam site. The integration of the flux data suggest that papyrus wetlands have the potential to act as a sink for significant amounts of carbon, in the region of 10 t C ha⁻¹ yr⁻¹. The cocoyam vegetation assimilated ~7 t C ha⁻¹ yr⁻¹ but when carbon exports from crop biomass removal were accounted for these wetlands represent a significant net loss of carbon of similar magnitude. The development of sustainable wetland management strategies are therefore required, to promote the dual wetland function of crop production and the mitigation of greenhouse gas emissions especially under future climate change scenarios.

Werner et al 2007. A global inventory of N2O emissions from tropical rainforest soils

Werner, C., K. Butterbach-Bahl, E. Haas, T. Hickler, and R. Kiese (2007), A global inventory of N2O emissions from tropical rainforest soils using a detailed biogeochemical model, Global Biogeochem. Cycles, 21, GB3010, doi:10.1029/2006GB002909.

Beside agricultural soils, tropical rainforest soils are the main source of atmospheric N₂O. Current estimates of the global N₂O source strength of tropical rainforest soils are still based on rather simplistic upscaling approaches and do have a large range of uncertainty. In this study, the biogeochemical ForestDNDC-tropica model was recalibrated and intensively tested on the site scale prior to inventory calculations. For this, the model was coupled to a newly developed global GIS database holding relevant information on model initialization and driving parameters in 0.25° × 0.25° resolution. On average, the mean annual N₂O emission source strength of rainforests ecosystems worldwide for the 10-year-period 1991–2000 was calculated to be 1.2 kg N₂O-N ha⁻¹ yr⁻¹. Using a total rainforest area of 10.9 × 10⁶ km², this amounts to a total source strength of 1.34 Tg N yr⁻¹. The result of an initialization parameter uncertainty assessment using Latin Hypercube sampling revealed that the global source strength of N₂O emissions from tropical rainforests may range from 0.88 to 2.37 Tg N yr⁻¹. Our calculations also show that N₂O emissions do vary substantially on spatial and temporal scales. Regional differences were mainly caused by differences in soil properties, whereas the pronounced seasonal and interannual variability was driven by climate variability. Our work shows that detailed biogeochemical models are a valuable tool for assessing biosphere-atmosphere exchange even on a global scale. However, further progress and a narrowing of the uncertainty range do crucially depend on the availability of more detailed field measurements for model testing and an improvement of the quality of spatial data sets on soil and vegetation properties.

Gharahi Ghehi et al 2011. Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda

Gharahi Ghehi, N., Werner, C., Cizungu Ntaboba, L., Mbonigaba Muhinda, J.J., Van Ranst, E., Butterbach-Bahl, K., Kiese, R., Boeckx, P., 2011. Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda. Biogeosciences Discuss. 8, 11631-11660. doi:10.5194/bgd-8-11631-2011, 2011

Abstract
Globally, tropical forest soils represent the second largest source of N₂O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. To investigate how soil properties affect N₂O and NO emission, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90% water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m⁻² h⁻¹, while NO emission varied from 6.6 to 265 μg N m⁻² h⁻¹. Mean N₂O emission at different moisture levels was 46.5 ± 11.1 (50% WFPS), 71.7 ± 11.5 (70% WFPS) and 98.8 ± 16.4 (90% WFPS) μg N m⁻² h⁻¹, while mean NO emission was 69.3 ± 9.3 (50% WFPS), 47.1 ± 5.8 (70% WFPS) and 36.1 ± 4.2 (90% WFPS) μg N m⁻² h⁻¹. The latter suggests that climate (i.e. dry vs. wet season) controls N₂O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N₂O and NO production. But interestingly N₂O and NO emissions also showed a negative correlation (only N₂O) with soil pH and a positive correlation with free iron. The latter suggest that chemo-denitrification might, at least for N₂O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will not only benefit from better spatial explicit trace gas emission and basic soil property monitoring, but also by differentiating between biological and chemical pathways for N trace gas formation.

Frimpong et al. 2011. Does incorporation of cowpea-maize residue mixes influence nitrous oxide emission and mineral nitrogen release in a tropical luvisol?

Frimpong, K., Yawson, D., Baggs, E., Agyarko, K., 2011. Does incorporation of cowpea-maize residue mixes influence nitrous oxide emission and mineral nitrogen release in a tropical luvisol? Nutrient Cycling in Agroecosystems 91, 281-292

Abstract

In the face of climate change, quantification of the emission of nitrous oxide from soils in relation to sufficient N availability for crop uptake has assumed much significance. This study used the ¹⁵N stable isotope technique, under controlled laboratory conditions, to quantify the interactive effect on and relative contributions of the component species to N₂O emission and mineral N dynamics in a tropical luvisol incorporated with different rates of cowpea-maize residue mixtures. The results show that increasing the maize residue proportion in the mixture significantly decreases N₂O emission compared to the sole cowpea incorporation but increases mineral N concentration compared to sole maize residue incorporation. It is concluded that mixing low C:N ratio cowpea residue with high C:N ratio maize residue has potential for N management in tropical legume-cereal intercropping systems with the view to minimizing N₂O emission while making N available for crop uptake. 

 

Keywords  Nitrous oxide emission – Mineral N – Cowpea-maize residue –  ¹⁵N stable isotope

Hickman et al. 2010. Impacts of Increasing Chemical Fertilizer Use On Nitrous Oxide Emissions From a Smallholder Agricultural System in Western Kenya.

Impacts of Increasing Chemical Fertilizer Use On Nitrous Oxide Emissions From a Smallholder Agricultural System in Western Kenya.

Jonathan Hickman, Tropical Agriculture Program, The Earth Institute at Columbia University, Palisades, NY, Cheryl Palm, Tropical Agriculture and Rural Environment Program, Earth Institute, Columbia University, Palisades, NY, Jianwu (Jim) Tang, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA and Jerry Melillo, Marine Biological Laboratory, The Ecosystems Center, Woods Hole, MA

SSSA 2010 International annual meetings, Oct. 31 - Nov. 4, Long Beach, CA, USA

Abstract

Impacts of increasing chemical fertilizer use on N2O emissions from a smallholder agricultural system in western Kenya. Over the last several decades, agricultural soils in many parts of sub-Saharan Africa have become depleted of nitrogen (N) and other nutrients, creating challenges to achieving food security in many countries. At only 8 kg N/ha/yr, average fertilizer application rates in the region are an order of magnitude lower than typical rates in the United States, and well below optimal levels. Increased use of nutrient inputs is a centerpiece of most African Green Revolution strategies, making it important to quantify the impacts of this change in practices. Increased N inputs are invariably accompanied by losses of N to the atmosphere as nitrogen oxides, including the greenhouse gas nitrous oxide (N2O). Several investigations of greenhouse gas emissions from sub-Saharan agricultural systems have been conducted over the last 20 years, but they typically include only two levels of fertilizer additions, and so are unable to identify potentially important thresholds in the response of trace gas emissions to fertilization rate. Here we examine the response function of N2O emissions to 5 different levels of inorganic fertilizer additions in a maize field in Maseno, Kenya during the 2010 long rainy season. We used an RCB design incorporating 5 levels of inorganic fertilizer additions (0, 50, 75, 100, and 200 kg/ha). We measured trace gas fluxes daily for one week starting the day before fertilizer application, followed by weekly measurements until trace gas emissions subsided to control levels. In order to identify thresholds in the N2O response, we use a stepwise backwards regression to identify departures from linearity in the mean chamber flux relative to the level of N fertilizer applied. Preliminary data suggest that N2O emissions may be slow to increase with increasing fertilizer additions, though important threshold effects may emerge. The identification of emission response thresholds combined with information to be collected on crop yield responses can provide insight into how to manage fertilizer use to optimize crop production per unit greenhouse gas emitted.

Source: http://a-c-s.confex.com/crops/2010am/webprogram/Paper61989.html