Abdalla et al. 2016. Long-term annual burning of grassland increases CO2 emissions from soils

Abdalla, K., Chivenge, P., Everson, C., Mathieu, O., Thevenot, M., Chaplot, V., 2016. Long-term annual burning of grassland increases CO2 emissions from soils. Geoderma 282, 80-86.

Abstract

Grasslands have potential to mitigate against climate change because of their large capacity to store soil organic carbon (SOC). However, the long-term impact of grassland management such as burning, which is still common in many areas of the world, on SOC is still a matter of debate. The objective of this study was to quantify the long-term effects of annual burning on CO₂ output from soils and SOC stocks. The study was performed on a 62 years old field trial comparing annual burning (AB) to no burning associated with tree encroachment (NB), and to annual mowing (AM) with all treatments laid out in randomized block design with three replicates per treatment. CO₂ emissions from soil were continuously measured over two years and were correlated to soil chemical and physical properties. AB and AM produced 30 and 34% greater CO₂ emissions from soil than NB (1.80 ± 0.13 vs. 2.34 ± 0.18 and 2.41 ± 0.17 g C-CO₂ m^− 2 d^− 1 for NB, AB and AM respectively). AB and AM also produced greater CO₂ emissions from soil and per gram of soil carbon (1.32 ± 0.1 and 1.35 ± 0.1 mg C-CO₂ g C^− 1 d^− 1, respectively) than NB (1.05 ± 0.07 mg C-CO₂ g C^− 1 d^− 1), which corresponded to significant differences of respectively 26% and 29%. Overall, CO₂ emissions from soil (per m²) significantly increased with soil water content (r = 0.72) followed by SOC stocks (r = 0.59), SOC content (r = 0.50), soil bulk density (r = 0.49), soil temperature (r = 0.47), C:N ratio (r = 0.46) and mean weight diameter (r = 0.38). These findings suggest that long-term annual burning increases CO₂ output from soils. Additional greenhouse gases emissions from burning itself and alternative grassland management techniques were finally discussed.

Chaplot et al. 2015. Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils.

Chaplot, V., Abdalla, K., Alexis, M., Bourennane, H., Darboux, F., Dlamini, P., Everson, C., McHunu, C., Muller-Nedebock, D., Mutema, M., Quenea, K., Thenga, H., Chivenge, P., 2015. Surface organic carbon enrichment to explain greater CO2 emissions from short-term no-tilled soils. Agriculture, Ecosystems & Environment 203, 110-118.

Abstract

The impact of agricultural practices on CO₂ emissions from soils needs to be understood and quantified to enhance ecosystem functions, especially the ability of soils to sequester atmospheric carbon (C), while enhancing food and biomass production. The objective of this study was to assess CO₂ emissions in the soil surface following tillage abandonment and to investigate some of the underlying soil physical, chemical and biological controls. Maize (Zea mays) was planted under conventional tillage (T) and no-tillage (NT), both without crop residues under smallholder farming conditions in Potshini, South Africa. Intact top-soil (0–0.05 m) core samples (N = 54) from three 5 × 15 m² plots per treatment were collected two years after conversion of T to NT to evaluate the short-term CO₂ emissions. Depending on the treatment, cores were left intact, compacted by 5 and 10%, or had surface crusts removed. They were incubated for 20 days with measurements of CO₂ fluxes twice a day during the first three days and once a day thereafter. Soil organic C (SOC) content, soil bulk density (ρ_b), aggregate stability, soil organic matter quality, and microbial biomass and its activity were evaluated at the onset of the incubation. CO₂ emissions were 22% lower under NT compared with T with CO₂ emissions of 0.9 ± 0.10 vs 1.1 ± 0.10 mg C–CO₂ gC⁻¹ day⁻¹ under NT and T, respectively, suggesting greater SOC protection under NT. However, there were greater total CO₂ emissions per unit of surface by 9% under NT compared to T (1.15 ± 0.03 vs 1.05 ± 0.04 g C–CO₂ m⁻² day⁻¹). SOC protection significantly increased with the increase in soil bulk density (r = 0.89) and aggregate stability (from 1.7 ± 0.25 mm to 2.3 ± 0.31, r = 0.50), and to the decrease in microbial biomass and its activity (r = −0.59 and −0.57, respectively). In contrast, the greater NT CO₂ emissions per m² were explained by top-soil enrichment in SOC by 48% (from 12.4 ± 0.2 to 19.1 ± 0.4 g kg⁻¹, r = 0.59). These results on the soil controls of tillage impact on CO₂ emissions are expected to inform on the required shifts in agricultural practices for enhancing C sequestration in soils. In the context of the study, any mechanism favoring aggregate stability and promoting SOC allocation deep in the soil profile rather than in the top-soil would greatly diminish soil CO₂ outputs and thus stimulate C sequestration.

Roland et al. 2016. Anaerobic methane oxidation in an East African great lake (Lake Kivu)

Roland, F. A. E., Darchambeau, F., Morana, C., Crowe, S. A., Thamdrup, B., and Borges, A. V.: Anaerobic methane oxidation in an East African great lake (Lake Kivu), Biogeosciences Discuss., doi:10.5194/bg-2016-300, in review, 2016. 

 Abstract
 This study investigates methane (CH₄) oxidation in the water column of Lake Kivu, a deep meromictic tropical lake containing large quantities of CH₄ in the anoxic deep waters. Depth profiles of dissolved gases (CH₄ and nitrous oxide (N₂O)) and of the different potential electron acceptors for anaerobic methane oxidation (AOM) (nitrate, sulfate, iron and manganese) were determined during six field campaigns between June 2011 and August 2014. Bacterial abundance all along the vertical profiles was also determined by flow cytometry during three field campaigns, and denitrification measurements based on stable isotopes were performed twice. Incubation experiments were performed to quantify CH₄ oxidation and nitrate consumption rates, with a focus on AOM, without and with an inhibitor of sulfate-reducing bacteria activity (molybdate). Nitrate consumption rates were measured in these incubations. Substantial CH₄ oxidation activity was observed in oxic and anoxic waters, and in the upper anoxic waters of Lake Kivu, CH₄ is a major electron donor to sustain anaerobic metabolic processes coupled to AOM. The maximum aerobic and anaerobic CH₄ oxidation rates were estimated to 27 ± 2 and 16 ± 8 µmol L⁻¹ d⁻¹, respectively. We observed a decrease of AOM rates when molybdate was added for half of the measurements, strongly suggesting the occurrence of AOM linked to sulfate reduction, but an increase of AOM rates was observed for the other half. Nitrate reduction rates and dissolved manganese production rates tended to be higher with the addition of molybdate, but the maximum rates of 0.6 ± 0.02 and 11 ± 2 µmol L⁻¹ d⁻¹, respectively, were never high enough to explain AOM rates observed at the same depths. We also put in evidence a difference in relative importance of aerobic and anaerobic CH₄ oxidation between the seasons, with a higher importance of aerobic oxidation when the oxygenated layer was thicker (in dry season).

Kimaro et al. 2016. Is conservation agriculture 'climate-smart' for maize farmers in the highlands of Tanzania?

Kimaro, A.A., Mpanda, M., Rioux, J., Aynekulu, E., Shaba, S., Thiong?셭, M., Mutuo, P., Abwanda, S., Shepherd, K., Neufeldt, H., Rosenstock, T.S., 2016. Is conservation agriculture 'climate-smart' for maize farmers in the highlands of Tanzania? Nutrient cycling in agroecosystems 105, 217-228.

Abstract

Conservation agriculture (CA) is promoted extensively to increase the productivity and environmental sustainability of maize production systems across sub-Saharan Africa and is often listed as a climate-smart agriculture (CSA) practice. However, the impacts of CA on food security, resilience/adaptive capacity and climate change mitigation are location-dependent and it is unknown whether CA can simultaneously address CSA’s multiple objectives. Here we evaluate four variations of CA: reduced tillage plus mulch (mulch), reduced tillage plus mulch and leguminous cover crop (Lablab), reduced tillage plus mulch and leguminous trees (CAWT), and reduced tillage plus mulch and nitrogen fertilizer (CA + F)—for their effect on CSA-relevant outcomes in highland Tanzania maize production. By comparison to conventional practice in the region, intensification of maize production by Lablab, CAWT, and CA + F significantly increases yields by 40, 89 and 77 %, respectively. Likewise, rainfall use efficiency was highest in these three treatments and significantly greater than conventional practices in 7 of 12 comparisons. Seasonal and annual greenhouse gas fluxes were similar across all treatments; however, yield-scaled global warming potential (Mg CO₂ eq Mg grain⁻¹) was lower in CAWT (2.1–3.1) and CA + F (1.9–2.3) than conventional practice (1.9–8.3), averaging 62 and 68 % of the emission intensity of conventional practice, respectively. The findings demonstrate that CA can deliver benefits consistent with the objectives of CSA for farmers in this region, particularly when soil nitrogen limitation is alleviated, providing other constraints to adoption are removed.

Rosenstock et al. 2016. Greenhouse gas fluxes from agricultural soils of Kenya and Tanzania

Rosenstock, T.S., Mathew, M., Pelster, D.E., Butterbach-Bahl, K., Rufino, M.C., Thiong'o, M., Mutuo, P., Abwanda, S., Rioux, J., Kimaro, A.A., Neufeldt, H.C.J.G., 2016. Greenhouse gas fluxes from agricultural soils of Kenya and Tanzania. Journal of Geophysical Research: Biogeosciences, DOI: 10.1002/2016JG003341.

Abstract

Knowledge of greenhouse gas (GHG) fluxes in soils is a prerequisite to constrain national, continental, and global GHG budgets. However, data characterizing fluxes from agricultural soils of Africa are markedly limited. We measured carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) fluxes at ten farmer-managed sites of six crop types for one year in Kenya and Tanzania using static chambers and gas chromatography. Cumulative emissions ranged between 3.5 – 15.9 Mg CO₂-C ha^-1 yr^-1, 0.4 – 3.9 kg N₂O-N ha^-1 yr^-1, and -1.2 – 10.1 kg CH₄-C ha^-1 yr^-1, depending on crop type, environmental conditions, and management. Manure inputs increased CO₂ (p = 0.03), but not N₂O or CH₄, emissions. Soil cultivation had no discernable effect on emissions of any of the three gases. Fluxes of CO₂ and N₂O were 54 – 208% greater (p  < 0.05) during the wet versus the dry seasons for some, but not all, crop types. The heterogeneity and seasonality of fluxes suggest that the available data describing soil fluxes in Africa, based on measurements of limited duration of only a few crop types and agroecological zones, are inadequate to use as a basis for estimating the impact of agricultural soils on GHG budgets. A targeted effort to understand the magnitude and mechanisms underlying African agricultural soil fluxes is necessary to accurately estimate the influence of this source on the global climate system and for determining mitigation strategies.

Pelster et al. 2016. Methane and Nitrous Oxide Emissions from Cattle Excreta on an East African Grassland

Pelster, D.E., B. Gisore, J. Goopy, D. Korir, J.K. Koske, M.C. Rufino, and K. Butterbach-Bahl. 2016. Methane and Nitrous Oxide Emissions from Cattle Excreta on an East African Grassland. J. Environ. Qual. 0. doi:10.2134/jeq2016.02.0050

Abstract

Greenhouse gas (GHG) emission measurements from livestock excreta in Africa are limited. We measured CH₄ and N₂O emissions from excreta of six Boran (Bos indicus) and six Friesian (Bos taurus) steers near Nairobi, Kenya. The steers were fed one of three diets (T1 [chaffed wheat straw], T2 [T1 + Calliandra calothyrsus Meissner – 0.2% live weight per day], and T3 [T1 + calliandra – 0.4% live weight every 2 d]). The T1 diet is similar in quality to typical diets in the region. Calliandra is a leguminous fodder tree promoted as a feed supplement. Fresh feces and urine were applied to grasslands and emissions measured using static chambers. Cumulative 28-d fecal emissions were 302 ± 52.4 and 95 ± 13.8 mg CH₄–C kg⁻¹ dry matter for Friesen and Boran steers, respectively, and 11.5 ± 4.26 and 24.7 ± 8.32 mg N₂O–N kg⁻¹ dry matter for Friesian and Boran steers, respectively. For urine from Friesian steers, the N₂O emissions were 2.8 ± 0.64 mg N₂O–N 100 mL urine⁻¹. The CH₄ emission factors (EFs) (246 ± 49.5 and 87 ± 12.7 g CH₄–C yr⁻¹ animal⁻¹ for Friesan and Boran, respectively) were lower than the International Panel on Climate Change EFs (750 g CH₄–C animal⁻¹ yr⁻¹), whereas the N₂O EFs (0.1 and 0.2% for the Friesian and Boran feces, respectively, and 1.2% for urine) were also lower than International Panel on Climate Change estimates. The low N content of the excreta likely caused the low emissions and indicates that current models probably overestimate CH₄ and N₂O emissions from African livestock manure.