CO2 Mitigation by Agriculture: An Overview

Agriculture currently contributes significantly to the increase of CO2 in the atmosphere, primarily through the conversion of native ecosystems to agricultural uses in the tropics. Yet there are major opportunities for mitigation of CO2 and other greenhouse gas emissions through changes in the use and management of agricultural lands. Agricultural mitigation options can be broadly divided into two categories: (I) strategies to maintain and increase stocks of organic C in soils (and biomass), and (ii) reductions in fossil C consumption, including reduced emissions by the agricultural sector itself and through agricultural production of biofuels to substitute for fossil fuels.Reducing the conversion of new land to agriculture in the tropics could substantially reduce CO2 emissions, but this option faces several difficult issues including population increase, land tenure and other socio-political factors in developing countries. The most significant opportunities for reducing tropical land conversions are in the humid tropics and in tropical wetlands. An important linkage is to improve the productivity and sustainability of existing agricultural lands in these regions.Globally, we estimate potential agricultural CO2 mitigation through soil C sequestration to be 0.4-0.9 Pg C y-1, through better management of existing agricultural soils, restoration of degraded lands, permanent "set-asides" of surplus agricultural lands in temperate developed countries and restoration of 10-20% of former wetlands now being used for agriculture. However, soils have a finite capacity to store additional C and therefore any increases in C stocks following changes in management would be largely realized within 50-100 years.Mitigation potential through reducing direct agricultural emissions is modest, 0.01-0.05 Pg C y-1. However, the potential to offset fossil C consumption through the use of biofuels produced by agriculture is substantial, 0.5-1.6 Pg C y-1, mainly through the production of dedicated biofuel crops with a smaller contribution (0.2-0.3 Pg C y-1) from crop residues.Many agricultural mitigation options represent "win-win" situations, in that there are important side benefits, in addition to CO2 mitigation, that could be achieved, e.g. improved soil fertility with higher soil organic matter, protection of lands poorly suited for permanent agriculture, cost saving for fossil fuel inputs and diversification of agricultural production (e.g. biofuels). However, the needs for global food production and farmer/societal acceptability suggest that mitigation technologies should conform to: (I) the enhancement of agricultural production levels in parts of the world where food production and population demand are in delicate balance and (ii) the accrual of additional benefits to the farmer (e.g., reduced labor, reduced or more efficient use of inputs) and society at large.     

iSPITM— the improved source parameter imaging method

Interpretation of an anomalous magnetic response involves determining the parameters that characterize the source of the anomaly. The depth to the top of the structure is a parameter that is commonly sought, and the Source Parameter Imaging^TM (SPI^TM) method is one way of determining this depth estimate. One advantage of the SPI method is that the depths can be displayed on an image. Typically there can be one image for an assumed contact (fault) model and another image for an assumed dipping thin sheet (dike) model. The depth estimate obtained will depend on the model assumed. An improvement to the source parameter imaging method extends the method to horizontal cylinders and at the same time allows the most appropriate model to be determined automatically. This model can be displayed on an image and the correct depth estimate for each anomaly can also be determined. The depth estimates can therefore be summarized on one map independent of an assumed model. The images generated from synthetic and field data show that the improved SPI method makes the task of interpreting magnetic data significantly easier.     

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997     

Carbon-Nitrogen-Phosphorus Cyclingin Tomales Bay, California

Tomales Bay, California, has been the site oflong-term study of carbon-nitrogen-phospho-rus fluxes in the coastal zone. It has also one of severalcoastal sites being used for comparison of C-N-Pbiogeochemical fluxes. The site releases phosphorusand consumes dissolved nitrogen. It also producesdissolved inorganic carbon, mostly as elevatedalkalinity. The overall interpretation placed on thebiogeochemical fluxes is as follows. The system is netheterotrophic; that is, it consumes more organicmatter than it produces. A pathway of consumption thatis of particular importance to the nitrogen cycle isdenitrification. The combination of net heterotrophyand denitrification is the simultaneous release ofdissolved inorganic phosphorus and the uptake ofdissolved inorganic nitrogen. Much of the dissolvedinorganic carbon released during the net heterotrophyis bound in alkalinity, apparently the result ofsulfate reduction.Because this work can be traced by to the trainingthat one of the authors (SVS) received from KeithChave, it is appropriate to present a summary of theTomales Bay research in memory of Keith.     

Electrogeochemical sampling with

 Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1996 distributed at least 36×10⁶ m³ of sulphur(S)-rich tephra over the central and eastern North Island of New Zealand. The tephras added between 30–1500 kg ha^–1 S to at least 25 000 km² of land in primary production. Smaller but beneficial amounts of selenium (Se) and in some areas potassium and magnesium were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a drop in soil pH and an increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in S and Se, but following the eruptions S was not required in fertilizer applications in many areas. The strongest and longest lasting effects of S and Se deposition were in high anion-retention soils particularly Hapludands (moist, moderately weathered soils, derived from volcanic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to starvation when tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements. Received: 17 January 1997 · Accepted: 31 March 1997     

Agronomic impact of tephra fallout from the 1995 and 1996 Ruapehu Volcano eruptions, New Zealand

 Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1996 distributed at least 36×10⁶ m³ of sulphur(S)-rich tephra over the central and eastern North Island of New Zealand. The tephras added between 30–1500 kg ha^–1 S to at least 25 000 km² of land in primary production. Smaller but beneficial amounts of selenium (Se) and in some areas potassium and magnesium were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a drop in soil pH and an increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in S and Se, but following the eruptions S was not required in fertilizer applications in many areas. The strongest and longest lasting effects of S and Se deposition were in high anion-retention soils particularly Hapludands (moist, moderately weathered soils, derived from volcanic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to starvation when tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements. Received: 17 January 1997 · Accepted: 31 March 1997     

Methane distribution and cycling in Tomales Bay, California

Cycling of methane (CH₄) in Tomales Bay, a 28-km² temperature estuary in northern California with relatively low inputs of organic carbon, was studied over a 1-yr period. Water column CH₄ concentrations showed spatial and temporal variability (range=8–100 nM), and were supersaturated with respect to the atmosphere by a factor of 2–37. Rates of net water column CH₄ production-oxidation were determined by in situ experiments, and were not found to be significantly different from zero. Fluxes across the sediment-water interface, determined by direct measurement using benthic chambers, varied from ?0.1 μmol m^?2 d^?1 to +16 μmol m^?2 d^?1 (positive fluxes into water). Methane concentrations in the two perennial creeks feeding the bay varied annually (140–950 nM); these creeks were a significant CH₄ source to the bay during winter. In addition, mass-balance calculations indicate a significant additional inter CH₄ source, which is hypothesized to result from storm-related runoff from dairy farms adjacent to the bay. Systemwide CH₄ budgets of the 16-km² inner bay indicate benthic production (110 mol d^?1) and atmospheric evasion (110 mol d^?1) dominated during summer, while atmospheric evasion (160 mol d^?1) and runoff from dairy farms (90 mol d^?1) dominated during winter.