Abstract. Agricultural soils, having been depleted of much of their native carbon stocks, have a significant CO2 sink capacity. Global estimates of this sink capacity are in the order of 20‐30 Pg C over the next 50‐100 years. Management practices to build up soil C must increase the input of organic matter to soil and/or decrease soil organic matter decomposition rates. The most appropriate management practices to increase soil C vary regionally, dependent on both environmental and socioeconomic factors. In temperate regions, key strategies involve increasing cropping frequency and reducing bare fallow, increasing the use of perennial forages (including N‐fixing species) in crop rotations, retaining crop residues and reducing or eliminating tillage (i.e. no‐till). In North America and Europe, conversion of marginal arable land to permanent perennial vegetation, to protect fragile soils and landscapes and/or reduce agricultural surpluses, provides additional opportunities for C sequestration. In the tropics, increasing C inputs to soil through improving the fertility and productivity of cropland and pastures is essential. In extensive systems with vegetated fallow periods (e.g. shifting cultivation), planted fallows and cover crops can increase C levels over the cropping cycle. Use of no‐till, green manures and agroforestry are other beneficial practices. Overall, improving the productivity and sustainability of existing agricultural lands is crucial to help reduce the rate of new land clearing, from which large amounts of CO2 from biomass and soil are emitted to the atmosphere. Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized countries. More are needed, especially for the tropics, to capture region‐specific interactions between climate, soil and management resources that are lost in global level assessments. By itself, C sequestration in agricultural soils can make only modest contributions (e.g. 3‐6% of total fossil C emissions) to mitigating greenhouse gas emissions. However, effective mitigation policies will not be based on any single ‘magic bullet’ solutions, but rather on many modest reductions which are economically efficient and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option. Additional advantages of pursuing strategies to increase soil C are the added benefits of improved soil quality for improving agricultural productivity and sustainability.

(2011) Roots contribute more to refractory soil organic matter than aboveground crop residues, as revealed by a long-term field experiment.Agriculture Corresponding author: Thomas.Katterer@slu.se AbstractWe revisited the well documented and ongoing long-term 'Ultuna continuous soil organic matter field experiment' which started in 1956 at the Swedish University of Agricultural Sciences. The objective of the experiment is to quantify effects of six organic amendments and mineral N fertilizers on the crop and soil. We used the 'equivalent soil mass' concept for estimating changes in the topsoil C stocks in all 15 treatments. C inputs from amendments were measured and those from crops were calculated using allometric functions and crop yields. Clustering C inputs into seven categories by quality allowed us to calculate a 'humification' coefficient for each category. Here, these coefficients simply were based on the fraction of total C input that still remains in the topsoil after about 50 years. As indicated by previous studies, this coefficient was highest for peat, followed by sewage sludge, manure, sawdust and aboveground crop residues. The most interesting result from the current investigation is that the optimized coefficient for rootderived C was about 2.3 times higher than that for aboveground plant residues. The calculated results were found to be robust in a sensitivity analysis. Our findings strongly support the hypothesis that root-derived C contributes more to relatively stable soil C pools than the same amount of aboveground crop residue-derived C.

The literature was reviewed regarding laboratory incubation studies where C mineralization was measured. Experiments were select-ed in which the same substrate was incubated at least at two different temperatures and where time-series were available with at least four measurements for each substrate and temperature. A first-order one-component model and a parallel first-order two-component model were fitted to the CO2-C evolution data in each experiment using a least-squares procedure. After normalising for a reference temperature, the temperature coefficient (Q(10)) function and three other temperature response functions were fitted to the estimated rate constants. The two-component model could describe the dynamics of the 25 experiments much more adequately than the one-component model (higher R-2, adjusted for the number of parameters), even when the rate constants for both were assumed to be equally affected by temperature. The goodness-of-fit did not differ between the temperature response models, but was affected by the choice of the reference temperature. For the whole data set, a Q(10) of 2 was found to be adequate for describing the temperature dependence of decomposition in the intermediate temperature range (about 5-35 degrees C). However, for individual experiments, Q(10) values deviated greatly from 2. At least at temperatures below 5 degrees C, functions not based on Q(10) are probably more adequate. However, due to the paucity of data from low-temperature incubations, this conclusion is only tentative, and more experimental work is called for

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Models were applied to results from a field experiment on the decomposition of barley straw, incubated 10—15 cm below the soil surface in a barley field. Litter bags were sampled 14 times during a 2—yr period to follow the dynamics of total mass and chemical components, e.g., water—solubles and total N. Zero— and first—order regression models were fitted to total mass, with and without adjustment for ambient temperature. R2, adjusted for the number of model parameters, was used for comparisons of model fits. The first—order model showed a good fit (R2 = 0.9875) if days with mean soil temperatures @<0°C were excluded. An improved fit was obtained using a temperature correction with Q10 = 1.21 (R2 = 0.9913). A one—compartment simulation model, using temperature and moisture as driving variables, showed a further improved fit (R2 = 0.9952) and a best Q10 = 1.78. Parallel and consecutive first—order models with two components did not improve the overall fit (R2 = 0.9896), but the initial loss of water—solubles coincided well with the predicted initial loss from the labile fraction. To describe the dynamics of selected chemical components, a four—compartment simulation model, including decomposition product formation, was fitted to total mass, water—solubles and total nitrogen. The observed dynamics of these components were well reproduced by the model. Influences of experimental and statistical techniques on interpretations of model results are discussed.

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A two‐component model was devised, comprising young and old soil C, two decay constants, and parameters for litter input, “humification,” and external influences. Due to the model’s simplicity, the differential equations were solved analytically, and parameter optimizations can be made using generally available nonlinear regression programs. The calibration parameter values were derived from a 35‐yr experiment with arable crops on a clay soil in central Sweden. We show how the model can be used for medium‐term (30 yr) predictions of the effects of changed inputs, climate, initial pools, litter quality, etc., on soil carbon pools. Equations are provided for calculating steady‐state pool sizes as well as model parameters from litter bag or 14C‐labeled litter decomposition data. Strategies for model parameterization to different inputs, climatic regions, and soils, as well as the model’s relations to other model families, are briefly discussed.

Yearly, per-area carbon sequestration rates are used to estimate mitigation potentials by comparing types and areas of land management in 1990 and 2000 and projected to 2010, for the European Union (EU)-15 and for four country-level case studies for which data are available: UK, Sweden, Belgium and Finland. Because cropland area is decreasing in these countries (except for Belgium), and in most European countries there are no incentives in place to encourage soil carbon sequestration, carbon sequestration between 1990 and 2000 was small or negative in the EU-15 and all case study countries. Belgium has a slightly higher estimate for carbon sequestration than the other countries examined. This is at odds with previous reports of decreasing soil organic carbon stocks in Flanders. For all countries except Belgium, carbon sequestration is predicted to be negligible or negative by 2010, based on extrapolated trends, and is small even in Belgium. The only trend in agriculture that may be enhancing carbon stocks on croplands at present is organic farming, and the magnitude of this effect is highly uncertain.Previous studies have focused on the potential for carbon sequestration and have shown quite significant potential. This study, which examines the sequestration likely to occur by 2010, suggests that the potential will not be realized. Without incentives for carbon sequestration in the future, cropland carbon sequestration under Article 3.4 of the Kyoto Protocol will not be an option in EU-15.