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Selected Papers of the Workshop on Development of Models and Forest Soil Surveys for Monitoring of Soil Carbon.

Countries need to assess changes in the carbon stocks of forest soils as a part of national greenhouse gas (GHG) inventories under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol (KP). Since measuring these changes is expensive, it is likely that many countries will use alternative methods to prepare these estimates. We reviewed seven well-known soil carbon models from the point of view of preparing country-scale soil C change estimates. We first introduced the models and explained how they incorporated the most important input variables. Second, we evaluated their applicability at regional scale considering commonly available data sources. Third, we compiled references to data that exist for evaluation of model performance in forest soils. A range of process-based soil carbon models differing in input data requirements exist, allowing some flexibility to forest soil C accounting. Simple models may be the only reasonable option to estimate soil C changes if available resources are limited. More complex models may be used as integral parts of sophisticated inventories assimilating several data sources. Currently, measurement data for model evaluation are common for agricultural soils, but less data have been collected in forest soils. Definitions of model and measured soil pools often differ, ancillary model inputs require scaling of data, and soil C measurements are uncertain. These issues complicate the preparation of model estimates and their evaluation with empirical data, at large scale. Assessment of uncertainties that accounts for the effect of model choice is important part of inventories estimating large-scale soil C changes. Joint development of models and large-scale soil measurement campaigns could reduce the inconsistencies between models and empirical data, and eventually also the uncertainties of model predictions.

We integrate a carbon net sink and stand-level wood production to analyze their simultaneous optimization as an economic problem. Carbon is included in living trees, wood products, and forest soil. Forestry is specified by a size-structured model for optimizing thinning timing and intensity, rotation period, and the optimal choice of rotation versus continuous cover forestry. The optimal inclusion of a carbon net sink increases the carbon pool mainly in living trees and forest soil, while the effect on the product carbon pool remains minor. With a 3% interest rate, increasing the C[O.sub.2] price to 40 [euro] per tC[O.sub.2] increases the total steady-state carbon pool by 131% and the soil carbon accounts for ca. 60% of the increased carbon storage. Omitting soil carbon, as in previous studies, leads to underestimates of the carbon sink, significantly decreasing the optimal total C[O.sub.2] net sink and achievable economic net gain from joint wood production and carbon management. The inclusion of soil carbon suggests that, in contrast to previous results, a higher C[O.sub.2] price does not necessarily favor continuous cover forestry. Key words: carbon sequestration, economic profitability, soil carbon, continuous cover forestry, rotation forestry, optimal rotation, 1. Introduction Climate change and the aim of limiting global warming have strongly increased the attention given to forests as carbon sinks (Shukla et al. 2019). Compared with the tropics, [...]

Spatial variation in the density of soil organic carbon (kg/m) and the thickness of soil horizons (F/H, E) were investigated in a 6 m x 8 m area in Scots pine ( L.) stand in Southern Finland for designing an effective sampling for the C density and studying the effect of trees on the variation. The horizon thickness of the podzolized soil were measured on a total of 126 soil cores (50 cm deep) and the C density of the organic F/H and 0â10 cm, 10â20 cm and 20â40 cm mineral soil layers was analysed.2Pinus sylvestris

Organic soil amendments are used to improve soil quality and mitigate climate change. However, their effects on soil structure, nutrient and water retention as well as greenhouse gas (GHG) emissions are still poorly understood. The purpose of this study was to determine the residual effects of a single field application of four ligneous soil amendments on soil structure and GHG emissions. We conducted a laboratory incubation experiment using soil samples collected from an ongoing soil-amendment field experiment at Qvidja Farm in south-west Finland, two years after a single application of four ligneous biomasses. Specifically, two biochars (willow and spruce) produced via slow pyrolysis, and two mixed pulp sludges from paper industry side-streams were applied at a rate of 9–22 Mg ha-1 mixed in the top 0.1 m soil layer. An unamended fertilized soil was used as a control. The laboratory incubation lasted for 33 days, during which the samples were kept at room temperature (21°C) and at 20%, 40%, 70% or 100% water holding capacity. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured periodically after 1, 5, 12, 20 and 33 days of incubation. The application of ligneous soil amendments increased the pH of the sampled soils by 0.4–0.8 units, whereas the effects on soil organic carbon content and soil structure varied between treatments. The GHG exchange was dominated by CO2 emissions, which were mainly unaffected by the soil amendment treatments. The contribution of soil CH4 exchange was negligible (nearly no emissions) compared to soil CO2 and N2O emissions. The soil N2O emissions exhibited a positive exponential relationship with soil moisture. Overall, the soil amendments reduced N2O emissions on average by 13%, 64%, 28%, and 37%, at the four soil moisture levels, respectively. Furthermore, the variation in N2O emissions between the amendments correlated positively with their liming effect. More specifically, the potential for the pulp sludge treatments to modulate N2O emissions was evident only in response to high water contents. This tendency to modulate N2O emissions was attributed to their capacity to increase soil pH and influence soil processes by persisting in the soil long after their application. [ABSTRACT FROM AUTHOR]

Soil organic carbon (C), accumulated over millennia, comprise more than half of the C stored in boreal and temperate forest landscapes. We used the Norwegian national forest inventory and soil survey network (n = 719, no deep organic soils) to explore the validity of a deterministic model representation of this pool (Yasso07). We statistically compared simulated and measured soil C stocks and related differences (measured - simulated) to site factors (drainage, topography, climate, vegetation, C-to-N ratio, and soil classification). Median C stocks were 5.0 kg C x [m.sup.-2] (model) and 14.5 kg C x [m.sup.-2] (measurements). Soil C differences related to site factors ([r.sup.2] of 0.16 to 0.37). For Brunisols, Gleysols, and wet Organic soils, differences related primarily to topographic wetness. For Regosols, Podzols, and Dystric Eluviated Brunisols, they related to climate, profile depth, and, in some cases, drainage class and site index. We argue that soil moisture regimes in our study area overrule tree productivity effects in the determination of soil C stocks and present conditions for soil formation that the model cannot (and does not explicitly) account for. These are processes such as humification and podsolization that involve eluviation and illuviation of dissolved organic C (DOC) with sesquioxides to form spodic B horizons and carbon enrichment due to hampered decomposition in frequently anoxic conditions. Key words: drainage, national forest inventory (NFI), SOC simulation, soil classification, soil-forming factors, soil moisture regime, soil survey, podsolization, Yasso07. Le carbone (C) organique du sol accumule au fil des millenaires correspond a plus de la moitie du C stocke dans les paysages forestiers boreaux et temperes. Nous avons utilise l'inventaire forestier national norvegien et le reseau d'inventaire des sols (n = 719, aucun sol organique epais) pour etudier la validite d'une representation de ce reservoir (Yasso07) a l'aide d'un modele deterministe. Nous avons statistiquement compare les stocks de C du sol simules et mesures, et relie leurs differences (mesures - simules) a des caracteristiques de la station (drainage, topographie, climat, vegetation, rapport de C a N et classification du sol). Les stocks medians de C atteignaient 5,0 (simule) et 14,5 (mesure) kg C x [m.sup.-2]. Les differences de C du sol etaient reliees aux caracteristiques des stations ([r.sup.2] de 0,16 a 0,37). Pour les brunisols, les gleysols et les sols organiques humides, les differences etaient surtout reliees a l'humidite topographique. Pour les regosols, les podzols et les brunisols dystriques eluvies, les differences etaient reliees au climat, a la profondeur du profil et, dans certains cas, a la classe de drainage et a l'indice de qualite de station. Nous avancons que le regime d'humidite du sol de notre aire d'etude est un facteur plus important que les effets de la productivite ligneuse pour determiner les stocks de C, et qu'il engendre des conditions de formation des sols dont le modele ne peut tenir compte et ne tient pas compte explicitement. Ces conditions sont des processus comme l'humification et la podzolisation qui impliquent l'eluviation et l'illuviation du carbone organique dissout avec des sesquioxydes pour former des horizons B spodiques et de l'enrichissement en carbone cause par l'absence de decomposition sous des conditions frequentes d'anoxie. [Traduit par la Redaction] Mots-cles : drainage, inventaire forestier national, simulation du carbone organique du sol, classification des sols, facteurs de pedogenese, regime d'humidite des sols, inventaire des sols, podzolisation, Yasso07., Introduction Of the total carbon (C) stock in the world's forests, 32% (272 Pg) is stored in the boreal region and only about 20% of this is stored in the [...]

Soils account for the largest share of carbon found in terrestrial ecosystems, and their status is of considerable interest for the global carbon cycle budget and atmospheric carbon concentration. The decomposition of soil organic matter depends on environmental conditions and human activities, which raises the question of how permanent are these carbon storages under changing climate. One way to get insight into carbon decomposition processes is to analyse different carbon isotope concentrations in soil organic matter. In this paper we introduce a carbon-13-isotope-specific soil organic matter decomposition add-on into the Yasso soil carbon model and assess its functionality. The new 13 C-dedicated decomposition is straightforward to implement and depends linearly on the default Yasso model parameters and the relative carbon isotope (13 C /12 C) concentration. The model modifications are based on the assumption that the heavier 13 C atoms are not as reactive as 12 C. The new formulations were calibrated using fractionated C, 13 C and δ13 measurements from litterbags containing pine needles and woody material, which were left to decompose in natural environment for 4 years. The introduced model modifications considerably improve the model behaviour in a 100-year-long simulation, where modelled δ13 is compared against fractionated peat column carbon content. The work presented here is a proof of concept and enables 13 C to be used as a natural tracer to detect changes in the underlying soil organic matter decomposition. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) models are important tools for assessing global SOC distributions and how carbon stocks are affected by climate change. Their performances, however, are affected by data and methods used to calibrate them. Here we study how a new version of the Yasso SOC model, here named Yasso20, performs if calibrated individually or with multiple datasets and how the chosen calibration method affects the parameter estimation. We also compare Yasso20 to the previous version of the Yasso model. We found that when calibrated with multiple datasets, the model showed a better global performance compared to a single-dataset calibration. Furthermore, our results show that more advanced calibration algorithms should be used for SOC models due to multiple local maxima in the likelihood space. The comparison showed that the resulting model performed better with the validation data than the previous version of Yasso. [ABSTRACT FROM AUTHOR]

Ecosystems have distinct soil carbon dynamics, including litter decomposition, depending on whether they are dominated by plants featuring ectomycorrhizae (EM) or arbuscular mycorrhizae (AM). However, current soil carbon models treat mycorrhizal impacts on the processes of soil carbon transformation as a black box. We re-formulated the soil carbon model Yasso15 and incorporated impacts of mycorrhizal vegetation on topsoil carbon pools of different recalcitrance. We examined alternative conceptualizations of mycorrhizal impacts on transformations of labile and stable carbon and quantitatively assessed the performance of the selected optimal model in terms of the long-term fate of plant litter 10 years following litter input. We found that mycorrhizal impacts on labile carbon pools are distinct from those on recalcitrant pools. Plant litter of the same chemical composition decomposes slower when exposed to EM-dominated ecosystems compared to AM-dominated ones, and across time, EM-dominated ecosystems accumulate more recalcitrant residues of non-decomposed litter. Overall, adding our mycorrhizal module into the Yasso model improved the accuracy of the temporal dynamics of carbon sequestration predictions. Our results suggest that mycorrhizal impacts on litter decomposition are underpinned by distinct decomposition pathways in AM- and EM-dominated ecosystems. A sensitivity analysis of litter decomposition to climate and mycorrhizal factors indicated that ignoring the mycorrhizal impact on decomposition leads to an overestimation of climate impacts on decomposition dynamics. Our new model provides a benchmark for quantitative modelling of microbial impacts on soil carbon dynamics. It helps to determine the relative importance of mycorrhizal associations and climate on litter decomposition rate and reduces the uncertainties in estimating soil carbon sequestration. [ABSTRACT FROM AUTHOR]

Soil carbon sequestration has gained traction as a mean to mitigate rising atmospheric carbon dioxide concentrations. Verification of different methods' efficiency to increase soil carbon sink requires, in addition to good quality measurements, reliable models capable of simulating the effect of the sequestration practises. One way to get insight of the methods' effects on carbon cycling processes is to analyse different carbon isotope concentrations in soil organic matter. In this paper we introduce a carbon-13 isotope specific soil organic matter decomposition add-on into the Yasso soil carbon model and assess its functionality. The new 13C-dedicated decomposition is straightforward to implement and depends linearly on the default Yasso model parameters and the relative carbon isotope (13C/12C) concentration. Despite of their simplicity, the modifications considerably improve the model behaviour in a 50-year long simulation. [ABSTRACT FROM AUTHOR]