Miko U. F. Kirschbaum

The temperature dependence of soil organic matter decomposition, and the effect of global warming on soil organic C storage

One of the key questions in climate change research relates to the future dynamics of the large amount of C that is currently stored in soil organic matter. Will the amount of C in this pool increase or decrease with global warming? The future trend in amounts of soil organic C will depend on the relative temperature sensitivities of net primary productivity and soil organic matter decomposition rate. Equations for the temperature dependence of net primary productivity have been widely used, but the temperature dependence of decomposition rate is less clear. The literature was surveyed to obtain the temperature dependencies of soil respiration and N dynamics reported in different studies. Only laboratory-based measurements were used to avoid confounding effects with differences in litter input rates, litter quality, soil moisture or other environmental factors. A considerable range of values has been reported, with the greatest relative sensitivity of decomposition processes to temperature having been observed at low temperatures. A relationship fitted to the literature data indicated that the rate of decomposition increases with temperature at 0°C with a Q10 of almost 8. The temperature sensitivity of organic matter decomposition decreases with increasing temperature, indicated by the Q10 decreasing with temperature to be about 4.5 at 10°C and 2.5 at 20°C. At low temperatures, the temperature sensitivity of decomposition was consequently much greater than the temperature sensitivity of net primary productivity, whereas the temperature sensitivities became more similar at higher temperatures. The much higher temperature sensitivity of decomposition than for net primary productivity has important implications for the store of soil organic C in the soil. The data suggest that a 1°C increase in temperature could ultimately lead to a loss of over 10% of soil organic C in regions of the world with an annual mean temperature of 5°C, whereas the same temperature increase would lead to a loss of only 3% of soil organic C for a soil at 30°C. These differences are even greater in absolute amounts as cooler soils contain greater amounts of soil organic C. This analysis supports the conclusion of previous studies which indicated that soil organic C contents may decrease greatly with global warming and thereby provide a positive feed-back in the global C cycle.

Will changes in soil organic carbon act as a positive or negative feedback on global warming

The worlds soils contain about 1500 Gt of organic carbon to a depth of 1m and a further 900 Gt from 1--2m. A change of total soil organic carbon by just 10% would thus be equivalent to all the anthropogenic CO2 emitted over 30 years. Warming is likely to increase both the rate of decomposition and net primary production (NPP), with a fraction of NPP forming new organic carbon. Evidence from various sources can be used to assess whether NPP or the rate of decomposition has the greater temperature sensitivity, and, hence, whether warming is likely to lead to an increase or decrease in soil organic carbon.Evidence is reviewed from laboratory-based incubations, field measurements of organic carbon storage, carbon isotope ratios and soil respiration with either naturally varying temperatures or after experimentally increasing soil temperatures. Estimates of terrestrial carbon stored at the Last Glacial Maximum are also reviewed. The review concludes that the temperature dependence of organic matter decomposition can be best described as: d(T) = exp[3.36 (T − 40)/(T + 31.79)] where d(T) is the normalised decomposition rate at temperature T (in °C). In this equation, decomposition rate is normalised to ‘1’ at 40 °C.The review concludes by simulating the likely changes in soil organic carbon with warming. In summary, it appears likely that warming will have the effect of reducing soil organic carbon by stimulating decomposition rates more than NPP. However, increasing CO2 is likely to simultaneously have the effect of increasing soil organic carbon through increases in NPP. Any changes are also likely to be very slow. The net effect of changes in soil organic carbon on atmospheric CO2 loading over the next decades to centuries is, therefore, likely to be small.

Organic nutrient uptake by mycorrhizal fungi enhances ecosystem carbon storage: a model‐based assessment

Understanding the factors that drive soil carbon (C) accumulation is of fundamental importance given their potential to mitigate climate change. Much research has focused on the relationship between plant traits and C sequestration, but no studies to date have quantitatively considered traits of their mycorrhizal symbionts. Here, we use a modelling approach to assess the contribution of an important mycorrhizal fungal trait, organic nutrient uptake, to soil C accumulation. We show that organic nutrient uptake can significantly increase soil C storage, and that it has a greater effect under nutrient-limited conditions. The main mechanism behind this was an increase in plant C fixation and subsequent increased C inputs to soil through mycorrhizal fungi. Reduced decomposition due to increased nutrient limitation of saprotrophs also played a role. Our results indicate that direct uptake of nutrients from organic pools by mycorrhizal fungi could have a significant effect on ecosystem C cycling and storage.

CenW, a forest growth model with linked carbon, energy, nutrient and water cycles

A comprehensive forest growth model (CenW) is described here. The model links the flows of carbon, energy, nutrients and water in trees and soil organic matter. Modelled tree growth depends on physiological plant factors, the size of plant pools, such as foliage mass, environmental factors, such as temperature and rainfall, and the total amount and turn-over rates of soil organic matter, which drives mineralisation of soil organic nitrogen. The model was validated against experimental data obtained for Pinus radiata from the Biology of Forest Growth (BFG) site near Canberra, Australia. The BFG experiment was conducted over 5 years, and included controls and treatments with irrigation and/ or fertiliser addition. Growth rates across treatments ranged about 2.5-fold. The model realistically simulated water use, foliage production and turn-over, foliar nitrogen dynamics, wood production and stand architecture across a wide range of responses under variable water and nitrogen supply due to experimental treatments and natural variations in rainfall. The model can be used to simulate the effects of silvicultural treatments, such as stand thinning, or to assess the sustainability of long-term site productivity based on simulation of the nutrient budget of forest stands. A sensitivity analysis was carried out to identify the parameters and external drivers to which overall growth was most sensitive. Sensitivity differed depending on growth conditions, but growth was always highly sensitive to biomass allocation factors and photosynthetic parameters. Growth was also highly sensitive to parameters describing crown nutritional relationship when nutrients were limiting and to ambient CO2 concentration and stomatal parameters when water was limiting.

Key issues and options in accounting for carbon sequestration and temporary storage in life cycle assessment and carbon footprinting

PurposeBiological sequestration can increase the carbon stocks of non-atmospheric reservoirs (e.g. land and land-based products). Since this contained carbon is sequestered from, and retained outside, the atmosphere for a period of time, the concentration of CO2 in the atmosphere is temporarily reduced and some radiative forcing is avoided. Carbon removal from the atmosphere and storage in the biosphere or anthroposphere, therefore, has the potential to mitigate climate change, even if the carbon storage and associated benefits might be temporary. Life cycle assessment (LCA) and carbon footprinting (CF) are increasingly popular tools for the environmental assessment of products, that take into account their entire life cycle. There have been significant efforts to develop robust methods to account for the benefits, if any, of sequestration and temporary storage and release of biogenic carbon. However, there is still no overall consensus on the most appropriate ways of considering and quantifying it.MethodThis paper reviews and discusses six available methods for accounting for the potential climate impacts of carbon sequestration and temporary storage or release of biogenic carbon in LCA and CF. Several viewpoints and approaches are presented in a structured manner to help decision-makers in their selection of an option from competing approaches for dealing with timing issues, including delayed emissions of fossil carbon.ResultsKey issues identified are that the benefits of temporary carbon removals depend on the time horizon adopted when assessing climate change impacts and are therefore not purely science-based but include value judgments. We therefore did not recommend a preferred option out of the six alternatives presented here.ConclusionsFurther work is needed to combine aspects of scientific and socio-economic understanding with value judgements and ethical considerations.

Modelling C and N dynamics in forest soils with a modified version of the CENTURY model

Abstract The CENTURY model of soil organic matter turn-over developed by Parton and co-workers has been used successfully for grasslands to predict dynamics of C, N and other nutrients. It was tested here for decomposition of a range of forest litters and for N mineralisation in forest soils. Modifications to the CENTURY model were necessary to match model output to empirical findings. These modifications included: (1) incorporation of additional woody litter pools (i.e. fine-wood and coarse-wood) (2) allowing the N content of soil organic matter (SOM) pools to vary (3) constraining N mineralisation and immobilisation to the active SOM pool (4) incorporation of a small flux of mineral N to the resistant SOM pool (5) allowance of mycorrhizal uptake of N, and (6) re-formulation of temperature and moisture effects on decomposition. Other possible changes, such as giving greater flexibility to the critical N concentration for mineralisation, were tested but found not to improve model performance. The modified model largely accounted for the effects of initial lignin and N concentration on subsequent litter decomposition rate, litter N concentrations and critical N concentrations for the commencement of mineralisation. Consequently, the model could successfully simulate realistic time courses of N immobilisation and subsequent mineralisation for a range of litter types. The present model also successfully predicted the critical N concentration for N mineralisation across a wide range of litter samples from forests and other vegetation sources. Net N mineralisation was successfully simulated in forest soils, which were either untreated, irrigated or fertilised.

Does Enhanced Photosynthesis Enhance Growth? Lessons Learned from CO2 Enrichment Studies

Plants typically convert only 2-4% of the available energy in radiation into new plant growth. This low efficiency has provided an impetus for trying to genetically manipulate plants in order to achieve greater efficiencies. But to what extent can increased photosynthesis be expected to increase plant growth? This question is addressed by treating plant responses to elevated CO2 as an analogue to increasing photosynthesis through plant breeding or genetic manipulations. For plants grown optimal growth conditions and elevated CO2, photosynthetic rates can be more than 50% higher than for plants grown under normal CO2 concentrations. This reduces to 40% higher for plants grown under the average of optimal and sub-optimal conditions, and over the course of a full day, average photosynthetic enhancements under elevated CO2 are estimated to be about 30%. The 30% enhancement in photosynthesis is reported to increase relative growth rate by only about 10%. This discrepancy is probably due to enhanced carbohydrate availability exceeding many plant9s ability to fully utilise it due to nutrient or inherent internal growth limitations. Consequently, growth responses to elevated CO2 increase with plant9s sink capacity and nutrient status. However, even a 10% enhancement in relative growth rate can translate into absolute growth enhancements of up to 50% during the exponential growth phase of plants. When space constraints and self-shading force an end to exponential growth, on-going growth enhancements are likely to be closer to the enhancement of relative growth rate. The growth response to elevated CO2 suggests that increases in photosynthesis almost invariably increase growth, but that growth response is numerically much smaller than the initial photosynthetic enhancement. This lends partial support to the usefulness of breeding plants with greater photosynthetic capacity, but dramatic growth stimulation should not be expected. The usefulness of increasing photosynthetic capacity can be maximised through changes in management practices and manipulation of other genetic traits to optimise the conditions under which increased photosynthesis can lead to maximal growth increases.

A comment on the quantitative significance of aerobic methane release by plants

A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH4 emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH4 emissions as 10-60 Mt CH4 year-1. This estimate is much smaller than the 62-236 Mt CH4 year-1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH4 budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH4 emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH4 emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH4 emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH4 emission, how CH4 emissions change with light, temperature and the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.

To sink or burn? A discussion of the potential contributions of forests to greenhouse gas balances through storing carbon or providing biofuels

Abstract Forests can affect net CO 2 emissions by increasing or decreasing the amount of stored carbon, or by supplying biofuels for power generation to substitute for fossil fuels. However, forests store the most carbon when they remain undisturbed and are allowed to grow to maturity, whereas using wood for bioenergy requires wood removal from forests, which reduces on-site carbon storage. Hence, it is difficult to manage a forest simultaneously for maximum carbon storage and supplying fuelwood. For developing optimal strategies for the use of vegetation sinks, it is necessary to consider the feedbacks via the inherent natural adjustments in the global carbon cycle. Increased atmospheric CO 2 currently provides a driving force for carbon uptake by natural carbon reservoirs, such as the worlds oceans. When carbon is removed from the atmosphere and stored in biomass, it lowers the concentration gradient between the atmosphere and these other reservoirs. This reduces the subsequent inherent rate of CO 2 removal from the atmosphere. This means that transferring a quantity of CO 2 from the atmosphere to a biomass pool lowers the atmospheric concentration the most immediately after the initial removal, but subsequently, the atmospheric concentration trends back towards the values without biospheric removal. The optimal timing for the use of vegetation sinks therefore depends on a number of factors: the length of time over which forest growth can be maintained, whether biomass is used for energy generation and on the nature of the most detrimental aspects of climate-change impacts. Climate-change impacts related to the instantaneous effect of temperature are mitigated less by vegetation sinks than impacts that act via the cumulative effect of increased temperature. It also means that short-term carbon storage in temporary sinks is not generally beneficial in mitigating climate change.

Can Trees Buy Time? An Assessment of the Role of Vegetation Sinks as Part of the Global Carbon Cycle

Atmospheric CO2 concentrations can be reduced by storing carbon in vegetation. However, this lowers the concentration gradient between the atmosphere and other potential carbon reservoirs, such as the oceans, and thereby reduces the subsequent inherent rate of removal of CO2 from the atmosphere. Hence, storage of carbon in temporary reservoirs can reduce atmospheric CO2 concentrations in the short term, but if the carbon is released again, it will increase concentrations in the long term. It must, therefore, be considered when, or, indeed whether, to store carbon in vegetation sinks.To determine an optimal strategy, the exact nature of climate-change impacts needs to be considered first. Impacts can be mediated by:1. the direct and instantaneous effect of CO2 and its associated temperature;2. the rate of change in CO2 and its associated temperature;3. the cumulative effect of CO2 and its associated temperature.Carbon stored in permanently maintained vegetation sinks can lower atmospheric CO2 concentrations, but this can be done most effectively if sequestration occurs close to the time when atmospheric concentrations are to be lowered. Similarly, maximal rates of change can be most effectively reduced by carbon sequestration close to the time of anticipated maximal rates of change. For reducing impacts via cumulative forcing, however, early sink activity would be more effective than delayed activity.Temporary carbon stores would only be beneficial for climate change impacts related to the cumulative impact of CO2, but it could even worsen impacts mediated via the instantaneous effect of temperature or those related to the rate of change. Hence, the planting of trees is only beneficial in reducing climate-change impacts if the most serious impacts are those related to the cumulative effect of increased temperature. If other impacts are more serious, then the planting of trees would bring greater benefits if it is delayed until closer to the time when the most severe impacts are to be expected. However, if serious land degradation would result from deforestation, or from a failure to plant trees in the near future, then trees should still be planted in order to maximise the amount of carbon stored on land.