Higher temperature sensitivity for stable than for labile soil organic carbon – Evidence from incubations of long‐term bare fallow soils

The impact of climate change on the stability of soil organic carbon (SOC) remains a major source of uncertainty in predicting future changes in atmospheric CO₂ levels. One unsettled issue is whether the mineralization response to temperature depends on SOC mineralization rate. Long‐term (>25 years) bare fallow experiments (LTBF) in which the soil is kept free of any vegetation and organic inputs, and their associated archives of soil samples represent a unique research platform to examine this issue as with increasing duration of fallow, the lability of remaining total SOC decreases. We retrieved soils from LTBF experiments situated at Askov (Denmark), Grignon (France), Ultuna (Sweden), and Versailles (France) and sampled at the start of the experiments and after 25, 50, 52, and 79 years of bare fallow, respectively. Soils were incubated at 4, 12, 20, and 35 °C and the evolved CO₂ monitored. The apparent activation energy (E_a) of SOC was then calculated for similar loss of CO₂ at the different temperatures. The E_a was always higher for samples taken at the end of the bare‐fallow period, implying a higher temperature sensitivity of stable C than of labile C. Our results provide strong evidence for a general relationship between temperature sensitivity and SOC stability upon which significant improvements in predictive models could be based.     

Temporal dynamics of soil organic carbon after land‐use change in the temperate zone – carbon response functions as a model approach

Land‐use change (LUC) is a major driving factor for the balance of soil organic carbon (SOC) stocks and the global carbon cycle. The temporal dynamic of SOC after LUC is especially important in temperate systems with a long reaction time. On the basis of 95 compiled studies covering 322 sites in the temperate zone, carbon response functions (CRFs) were derived to model the temporal dynamic of SOC after five different LUC types (mean soil depth of 30±6 cm). Grassland establishment caused a long lasting carbon sink with a relative stock change of 128±23% and afforestation on former cropland a sink of 116±54%, 100 years after LUC (mean±95% confidence interval). No new equilibrium was reached within 120 years. In contrast, there was no SOC sink following afforestation of grasslands and 75% of all observations showed SOC losses, even after 100 years. Only in the forest floor, there was carbon accumulation of 0.38±0.04 Mg ha^?1 yr^?1 in afforestations adding up to 38±4 Mg ha^?1 labile carbon after 100 years. Carbon loss after deforestation (?32±20%) and grassland conversion to cropland (?36±5%), was rapid with a new SOC equilibrium being reached after 23 and 17 years, respectively. The change rate of SOC increased with temperature and precipitation but decreased with soil depth and clay content. Subsoil SOC changes followed the trend of the topsoil SOC changes but were smaller (25±5% of the total SOC changes) and with a high uncertainty due to a limited number of datasets. As a simple and robust model approach, the developed CRFs provide an easily applicable tool to estimate SOC stock changes after LUC to improve greenhouse gas reporting in the framework of UNFCCC.     

Modeling and empirical validation of long‐term carbon sequestration in forests (France, 1850–2015)

The development of appropriate tools to quantify long‐term carbon (C) budgets following forest transitions, that is, shifts from deforestation to afforestation, and to identify their drivers are key issues for forging sustainable land‐based climate‐change mitigation strategies. Here, we develop a new modeling approach, CRAFT (CaRbon Accumulation in ForesTs) based on widely available input data to study the C dynamics in French forests at the regional scale from 1850 to 2015. The model is composed of two interconnected modules which integrate biomass stocks and flows (Module 1) with litter and soil organic C (Module 2) and build upon previously established coupled climate‐vegetation models. Our model allows to develop a comprehensive understanding of forest C dynamics by systematically depicting the integrated impact of environmental changes and land use. Model outputs were compared to empirical data of C stocks in forest biomass and soils, available for recent decades from inventories, and to a long‐term simulation using a bookkeeping model. The CRAFT model reliably simulates the C dynamics during France's forest transition and reproduces C‐fluxes and stocks reported in the forest and soil inventories, in contrast to a widely used bookkeeping model which strictly only depicts C‐fluxes due to wood extraction. Model results show that like in several other industrialized countries, a sharp increase in forest biomass and SOC stocks resulted from forest area expansion and, especially after 1960, from tree growth resulting in vegetation thickening (on average 7.8 Mt C/year over the whole period). The difference between the bookkeeping model, 0.3 Mt C/year in 1850 and 21 Mt C/year in 2015, can be attributed to environmental and land management changes. The CRAFT model opens new grounds for better quantifying long‐term forest C dynamics and investigating the relative effects of land use, land management, and environmental change.     

Soil carbon sequestration due to post‐Soviet cropland abandonment: estimates from a large‐scale soil organic carbon field inventory

The break‐up of the Soviet Union in 1991 triggered cropland abandonment on a continental scale, which in turn led to carbon accumulation on abandoned land across Eurasia. Previous studies have estimated carbon accumulation rates across Russia based on large‐scale modelling. Studies that assess carbon sequestration on abandoned land based on robust field sampling are rare. We investigated soil organic carbon (SOC) stocks using a randomized sampling design along a climatic gradient from forest steppe to Sub‐Taiga in Western Siberia (Tyumen Province). In total, SOC contents were sampled on 470 plots across different soil and land‐use types. The effect of land use on changes in SOC stock was evaluated, and carbon sequestration rates were calculated for different age stages of abandoned cropland. While land‐use type had an effect on carbon accumulation in the topsoil (0–5 cm), no independent land‐use effects were found for deeper SOC stocks. Topsoil carbon stocks of grasslands and forests were significantly higher than those of soils managed for crops and under abandoned cropland. SOC increased significantly with time since abandonment. The average carbon sequestration rate for soils of abandoned cropland was 0.66 Mg C ha^?1 yr^?1 (1–20 years old, 0–5 cm soil depth), which is at the lower end of published estimates for Russia and Siberia. There was a tendency towards SOC saturation on abandoned land as sequestration rates were much higher for recently abandoned (1–10 years old, 1.04 Mg C ha^?1 yr^?1) compared to earlier abandoned crop fields (11–20 years old, 0.26 Mg C ha^?1 yr^?1). Our study confirms the global significance of abandoned cropland in Russia for carbon sequestration. Our findings also suggest that robust regional surveys based on a large number of samples advance model‐based continent‐wide SOC prediction.     

Major limitations to achieving “4 per 1000” increases in soil organic carbon stock in temperate regions: Evidence from long‐term experiments at Rothamsted Research,United Kingdom

We evaluated the “4 per 1000” initiative for increasing soil organic carbon (SOC) by analysing rates of SOC increase in treatments in 16 long‐term experiments in southeast United Kingdom. The initiative sets a goal for SOC stock to increase by 4‰ per year in the 0–40 cm soil depth, continued over 20 years. Our experiments, on three soil types, provided 114 treatment comparisons over 7–157 years. Treatments included organic additions (incorporated by inversion ploughing), N fertilizers, introducing pasture leys into continuous arable systems, and converting arable land to woodland. In 65% of cases, SOC increases occurred at >7‰ per year in the 0–23 cm depth, approximately equivalent to 4‰ per year in the 0–40 cm depth. In the two longest running experiments (>150 years), annual farmyard manure (FYM) applications at 35 t fresh material per hectare (equivalent to approx. 3.2 t organic C/ha/year) gave SOC increases of 18‰ and 43‰ per year in the 23 cm depth during the first 20 years. Increases exceeding 7‰ per year continued for 40–60 years. In other experiments, with FYM applied at lower rates or not every year, there were increases of 3‰–8‰ per year over several decades. Other treatments gave increases between zero and 19‰ per year over various periods. We conclude that there are severe limitations to achieving the “4 per 1000” goal in practical agriculture over large areas. The reasons include (1) farmers not having the necessary resources (e.g. insufficient manure); (2) some, though not all, practices favouring SOC already widely adopted; (3) practices uneconomic for farmers—potentially overcome by changes in regulations or subsidies; (4) practices undesirable for global food security. We suggest it is more realistic to promote practices for increasing SOC based on improving soil quality and functioning as small increases can have disproportionately large beneficial impacts, though not necessarily translating into increased crop yield.     

Soil carbon changes under Miscanthus driven by C4 accumulation and C3 decompostion – toward a default sequestration function

Bioenergy has to meet increasing sustainability criteria in the EU putting conventional bioenergy crops under pressure. Alternatively, perennial bioenergy crops, such as Miscanthus, show higher greenhouse gas savings with similarly high energy yields. In addition, Miscanthus plantations may sequester additional soil organic carbon (SOC) to mitigate climate change. As the land‐use change in cropland to Miscanthus involves a C₃‐C₄ vegetation change (VC), it is possible to determine the dynamic of Miscanthus‐derived SOC (C₄ carbon) and of the old SOC (C₃ carbon) by the isotopic ratio of ¹³C to ¹²C. We sampled six croplands and adjacent Miscanthus plantations exceeding the age of 10 years across Europe. We found a mean C₄ carbon sequestration rate of 0.78 ± 0.19 Mg ha^?1 yr^?1, which increased with mean annual temperature. At three of six sites, we found a significant increase in C₃ carbon due to the application of organic fertilizers or difference in baseline SOC, which we define as non‐VC‐induced SOC changes. The Rothamsted Carbon Model was used to disentangle the decomposition of old C₃ carbon and the non‐VC‐induced C₃ carbon changes. Subsequently, this method was applied to eight more sites from the literature, resulting in a climate‐dependent VC‐induced SOC sequestration rate (0.40 ± 0.20 Mg ha^?1 yr^?1), as a step toward a default SOC change function for Miscanthus plantations on former croplands in Europe. Furthermore, we conducted a SOC fractionation to assess qualitative SOC changes and the incorporation of C₄ carbon into the soil. Sixteen years after Miscanthus establishment, 68% of the particulate organic matter (POM) was Miscanthus‐derived in 0–10 cm depth. POM was thus the fastest cycling SOC fraction with a C₄ carbon accumulation rate of 0.33 ± 0.05 Mg ha^?1 yr^?1. Miscanthus‐derived SOC also entered the NaOCl‐resistant fraction, comprising 12% in 0–10 cm, which indicates that this fraction was not an inert SOC pool.     

Impact of tropical land‐use change on soil organic carbon stocks – a meta‐analysis

Land‐use changes are the second largest source of human‐induced greenhouse gas emission, mainly due to deforestation in the tropics and subtropics. CO₂ emissions result from biomass and soil organic carbon (SOC) losses and may be offset with afforestation programs. However, the effect of land‐use changes on SOC is poorly quantified due to insufficient data quality (only SOC concentrations and no SOC stocks, shallow sampling depth) and representativeness. In a global meta‐analysis, 385 studies on land‐use change in the tropics were explored to estimate the SOC stock changes for all major land‐use change types. The highest SOC losses were caused by conversion of primary forest into cropland (?25%) and perennial crops (?30%) but forest conversion into grassland also reduced SOC stocks by 12%. Secondary forests stored less SOC than primary forests (?9%) underlining the importance of primary forests for C stores. SOC losses are partly reversible if agricultural land is afforested (+29%) or under cropland fallow (+32%) and with cropland conversion into grassland (+26%). Data on soil bulk density are critical in order to estimate SOC stock changes because (i) the bulk density changes with land‐use and needs to be accounted for when calculating SOC stocks and (ii) soil sample mass has to be corrected for bulk density changes in order to compare land‐use types on the same basis of soil mass. Without soil mass correction, land‐use change effects would have been underestimated by 28%. Land‐use change impact on SOC was not restricted to the surface soil, but relative changes were equally high in the subsoil, stressing the importance of sufficiently deep sampling.     

Microbial necromass in cropland soils: A global meta-analysis of management effects

Microbial necromass is a large and persistent component of soil organic carbon (SOC), especially under croplands. The effects of cropland management on microbial necromass accumulation and its contribution to SOC have been measured in individual studies but have not yet been summarized on the global scale. We conducted a meta-analysis of 481-paired measurements from cropland soils to examine the management effects on microbial necromass and identify the optimal conditions for its accumulation. Nitrogen fertilization increased total microbial necromass C by 12%, cover crops by 14%, no or reduced tillage (NT/RT) by 20%, manure by 21%, and straw amendment by 21%. Microbial necromass accumulation was independent of biochar addition. NT/RT and straw amendment increased fungal necromass and its contribution to SOC more than bacterial necromass. Manure increased bacterial necromass higher than fungal, leading to decreased ratio of fungal-to-bacterial necromass. Greater microbial necromass increases after straw amendments were common under semi-arid and in cool climates in soils with pH <8, and were proportional to the amount of straw input. In contrast, NT/RT increased microbial necromass mainly under warm and humid climates. Manure application increased microbial necromass irrespective of soil properties and climate. Management effects were especially strong when applied during medium (3–10 years) to long (10+ years) periods to soils with larger initial SOC contents, but were absent in sandy soils. Close positive links between microbial biomass, necromass and SOC indicate the important role of stabilized microbial products for C accrual. Microbial necromass contribution to SOC increment (accumulation efficiency) under NT/RT, cover crops, manure and straw amendment ranged from 45% to 52%, which was 9%–16% larger than under N fertilization. In summary, long-term cropland management increases SOC by enhancing microbial necromass accumulation, and optimizing microbial necromass accumulation and its contribution to SOC sequestration requires site-specific management.     

Accumulation of soil organic carbon after cropland conversion to short‐rotation willow and poplar

The demand for bioenergy has increased the interest in short‐rotation woody crops (SRWCs) in temperate zones. With increased litter input and ceased annual soil cultivation, SRWC plantations may become soil carbon sinks for climate change mitigation. A chronosequence of 26 paired plots was used to study the potential for increasing soil organic carbon (SOC) under SRWC willow and poplar after conversion from cropland (CR) on well‐drained soils. We estimated SOC stocks in SRWC stands and adjacent CR and related the difference to time since conversion, energy crop species, SOC stock of the adjacent CR (proxy for initial SOC of SRWC) and the fine soil percentage (<63 μm) (FS). Soil cores to 40 cm depth were sampled and separated by layers of fixed depths (0–5, 5–10, 10–15, 15–25 and 25–40 cm). Additionally, soils were sampled from soil pits by genetic horizons to 100 cm depth. Comparisons of SOC stocks by equivalent soil masses showed that mean SOC stocks in SRWC were 1.7 times higher than those of CR in the top 5 cm of the soil (P < 0.001). The differences between SRWC and CR remained significant for the plough layer (0–25 cm) by a factor of 1.2 (P = 0.003), while no changes were detectable for the 0–40 cm (P = 0.32), or for the entire 0–100 cm soil layer (P = 0.29). The SOC stock ratio, that is the ratio of SOC stock in SRWC relative to CR, did not change significantly with time since conversion, although there was a tendency to an increase over time for the top 40 cm (P = 0.09). The SOC stock ratio was negatively correlated to SOC in CR and FS percentage, but there was no significant difference between willow and poplar at any depth. Our results suggest that SOC stocks in the plough layer increase after conversion to SRWC.     

Carbon accumulation in agricultural soils after afforestation: a meta-analysis

Deforestation usually results in significant losses of soil organic carbon (SOC). The rate and factors determining the recovery of this C pool with afforestation are still poorly understood. This paper provides a review of the influence of afforestation on SOC stocks based on a meta-analysis of 33 recent publications (totaling 120 sites and 189 observations), with the aim of determining the factors responsible for the restoration of SOC following afforestation. Based on a mixed linear model, the meta-analysis indicates that the main factors that contribute to restoring SOC stocks after afforestation are: previous land use, tree species planted, soil clay content, preplanting disturbance and, to a lesser extent, climatic zone. Specifically, this meta-analysis (1) indicates that the positive impact of afforestation on SOC stocks is more pronounced in cropland soils than in pastures or natural grasslands; (2) suggests that broadleaf tree species have a greater capacity to accumulate SOC than coniferous species; (3) underscores that afforestation using pine species does not result in a net loss of the whole soil-profile carbon stocks compared with initial values (agricultural soil) when the surface organic layer is included in the accounting; (4) demonstrates that clay-rich soils (> 33%) have a greater capacity to accumulate SOC than soils with a lower clay content (< 33%); (5) indicates that minimizing preplanting disturbances may increase the rate at which SOC stocks are replenished; and (6) suggests that afforestation carried out in the boreal climate zone results in small SOC losses compared with other climate zones, probably because trees grow more slowly under these conditions, although this does not rule out gains over time after the conversion. This study also highlights the importance of the methodological approach used when developing the sampling design, especially the inclusion of the organic layer in the accounting.     

No detectable upper limit of mineral-associated organic carbon in temperate agricultural soils

Soil organic carbon (SOC) sequestration is a promising climate change mitigation option. In this context, the formation of the relatively long-lived mineral-associated organic carbon (MAOC) is key. To date, soils are considered to be limited in their ability to accumulate MAOC, mainly by the amount of clay and silt particles present. Using the comprehensive German Agricultural Soil Inventory, we selected 189 samples with a wide range of SOC (5–118 g kg⁻¹) and clay contents (30–770 g kg⁻¹) to test whether there is a detectable upper limit of MAOC content. We found that the proportion of MAOC was surprisingly stable for soils under cropland and grassland use across the whole range of bulk SOC contents. Soil texture influenced the slope of the relationship between bulk SOC and MAOC, but no upper limit was observed in any texture class. Also, C content in the fine fraction (g C kg⁻¹ fraction) was negatively correlated to fine fraction content (g kg⁻¹ bulk soil). Both findings challenge the notion that MAOC accumulation is limited by soil fine fraction content per se.     

Effects of straw carbon input on carbon dynamics in agricultural soils: a meta‐analysis

Straw return has been widely recommended as an environmentally friendly practice to manage carbon (C) sequestration in agricultural ecosystems. However, the overall trend and magnitude of changes in soil C in response to straw return remain uncertain. In this meta‐analysis, we calculated the response ratios of soil organic C (SOC) concentrations, greenhouse gases (GHGs) emission, nutrient contents and other important soil properties to straw addition in 176 published field studies. Our results indicated that straw return significantly increased SOC concentration by 12.8 ± 0.4% on average, with a 27.4 ± 1.4% to 56.6 ± 1.8% increase in soil active C fraction. CO₂ emission increased in both upland (27.8 ± 2.0%) and paddy systems (51.0 ± 2.0%), while CH₄ emission increased by 110.7 ± 1.2% only in rice paddies. N₂O emission has declined by 15.2 ± 1.1% in paddy soils but increased by 8.3 ± 2.5% in upland soils. Responses of macro‐aggregates and crop yield to straw return showed positively linear with increasing SOC concentration. Straw‐C input rate and clay content significantly affected the response of SOC. A significant positive relationship was found between annual SOC sequestered and duration, suggesting that soil C saturation would occur after 12 years under straw return. Overall, straw return was an effective means to improve SOC accumulation, soil quality, and crop yield. Straw return‐induced improvement of soil nutrient availability may favor crop growth, which can in turn increase ecosystem C input. Meanwhile, the analysis on net global warming potential (GWP) balance suggested that straw return increased C sink in upland soils but increased C source in paddy soils due to enhanced CH₄ emission. Our meta‐analysis suggested that future agro‐ecosystem models and cropland management should differentiate the effects of straw return on ecosystem C budget in upland and paddy soils.     

Temporal response of soil organic carbon after grassland‐related land‐use change

The net flux of CO₂ exchanged with the atmosphere following grassland‐related land‐use change (LUC) depends on the subsequent temporal dynamics of soil organic carbon (SOC). Yet, the magnitude and timing of these dynamics are still unclear. We compiled a global data set of 836 paired‐sites to quantify temporal SOC changes after grassland‐related LUC. In order to discriminate between SOC losses from the initial ecosystem and gains from the secondary one, the post‐LUC time series of SOC data was combined with satellite‐based net primary production observations as a proxy of carbon input to the soil. Globally, land conversion from either cropland or forest into grassland leads to SOC accumulation; the reverse shows net SOC loss. The SOC response curves vary between different regions. Conversion of cropland to managed grassland results in more SOC accumulation than natural grassland recovery from abandoned cropland. We did not consider the biophysical variables (e.g., climate conditions and soil properties) when fitting the SOC turnover rate into the observation data but analyzed the relationships between the fitted turnover rate and these variables. The SOC turnover rate is significantly correlated with temperature and precipitation (p < 0.05), but not with the clay fraction of soils (p > 0.05). Comparing our results with predictions from bookkeeping models, we found that bookkeeping models overestimate by 56% of the long‐term (100 years horizon) cumulative SOC emissions for grassland‐related LUC types in tropical and temperate regions since 2000. We also tested the spatial representativeness of our data set and calculated SOC response curves using the representative subset of sites in each region. Our study provides new insight into the impact grassland‐related LUC on the global carbon budget and sheds light on the potential of grassland conservation for climate mitigation.     

Long‐term carbon storage through retention of dissolved aromatic acids by reactive particles in soil

Soils retain large quantities of carbon, thereby slowing its return to the atmosphere. The mechanisms governing organic carbon sequestration in soil remain poorly understood, yet are integral to understanding soil‐climate feedbacks. We evaluated the biochemistry of dissolved and solid organic carbon in potential source and sink horizons across a chronosequence of volcanic soils in Hawai'i. The soils are derived from similar basaltic parent material on gently sloping volcanic shield surfaces, support the same vegetation assemblage, and yet exhibit strong shifts in soil mineralogy and soil carbon content as a function of volcanic substrate age. Solid‐state¹³carbon nuclear magnetic resonance spectra indicate that the most persistent mineral‐bound carbon is comprised of partially oxidized aromatic compounds with strong chemical resemblance to dissolved organic matter derived from plant litter. A molecular mixing model indicates that protein, lipid, carbohydrate, and char content decreased whereas oxidized lignin and carboxyl/carbonyl content increased with increasing short‐range order mineral content. When solutions rich in dissolved organic matter were passed through Bw‐horizon mineral cores, aromatic compounds were preferentially sorbed with the greatest retention occurring in horizons containing the greatest amount of short‐range ordered minerals. These minerals are reactive metastable nanocrystals that are most common in volcanic soils, but exist in smaller amounts in nearly all major soil classes. Our results indicate that long‐term carbon storage in short‐range ordered minerals occurs via chemical retention with dissolved aromatic acids derived from plant litter and carried along preferential flow‐paths to deeper B horizons.     

Surficial gains and subsoil losses of soil carbon and nitrogen during secondary forest development

Reforestation of formerly cultivated land is widely understood to accumulate above‐ and belowground detrital organic matter pools, including soil organic matter. However, during 40 years of study of reforestation in the subtropical southeastern USA, repeated observations of above‐ and belowground carbon documented that significant gains in soil organic matter (SOM) in surface soils (0–7.5 cm) were offset by significant SOM losses in subsoils (35–60 cm). Here, we extended the observation period in this long‐term experiment by an additional decade, and used soil fractionation and stable isotopes and radioisotopes to explore changes in soil organic carbon and soil nitrogen that accompanied nearly 50 years of loblolly pine secondary forest development. We observed that accumulations of mineral soil C and N from 0 to 7.5 cm were almost entirely due to accumulations of light‐fraction SOM. Meanwhile, losses of soil C and N from mineral soils at 35 to 60 cm were from SOM associated with silt and clay‐sized particles. Isotopic signatures showed relatively large accumulations of forest‐derived carbon in surface soils, and little to no accumulation of forest‐derived carbon in subsoils. We argue that the land use change from old field to secondary forest drove biogeochemical and hydrological changes throughout the soil profile that enhanced microbial activity and SOM decomposition in subsoils. However, when the pine stands aged and began to transition to mixed pines and hardwoods, demands on soil organic matter for nutrients to support aboveground growth eased due to pine mortality, and subsoil organic matter levels stabilized. This study emphasizes the importance of long‐term experiments and deep measurements when characterizing soil C and N responses to land use change and the remarkable paucity of such long‐term soil data deeper than 30 cm.     

土地利用对崇明岛围垦区土壤有机碳库和土壤呼吸的影响

土地利用方式是影响农业土壤碳固持和温室气体减排的关键因子之一,而准确地评价土地利用变化的影响往往因土壤本底的不均一和土地利用历史多变而复杂化。为此,在崇明东滩湿地围垦区选取了本底均匀、利用历史简单的几种土地利用类型(水-旱轮作农田、人工林、鱼塘撂荒地),研究其土壤有机碳库和土壤呼吸的变化及其与土壤环境间的关系,以期评价其各自的固碳和温室气体减排潜力。农田土壤的表层(20cm)有机碳和微生物生物量碳含量最高,分别为12.62g/kg和225.34mg/kg,包括苗圃栾树林、水杉林带以及桔园在内的人工林地次之,鱼塘撂荒地最低;但撂荒地深层土壤(40—100cm)的有机碳含量高于其它类型,反映了围垦前湿地土壤有机碳累积的残留影响。土壤呼吸强度的顺序则为鱼塘撂荒地农田桔园苗圃栾树林水杉林带。农耕地在前作小麦收割种植水稻后,土壤CO2通量显著下降,不及旱作时的10%。除农田和撂荒地以外,土壤表层5 cm深处温度可以很好地解释土壤呼吸速率的变化,但在高温高湿季节呼吸速率较为离散。研究表明:在有机质含量较低的土壤中,水-旱轮作可增加土壤有机碳的储量;受人类活动干扰较小的林地土壤,有机碳含量反而有可能低于农田土壤。在中国南方湿润亚热带地区,水旱轮作可较好地协调农业土壤的碳固持和释放过程的矛盾,可能具有相当大的农业减排潜力。     

Changes in organic carbon distribution with depth in agricultural soils in northern Belgium, 1960–2006

In most studies concerning the carbon (C) exchange between soil and atmosphere only the topsoil (0–0.3 m) is taken into account. However, it has been shown that important amounts of stable soil organic carbon (SOC) are also stored at greater depth. Here, we developed a quantitative model to estimate the evolution of the distribution of SOC with depth between 1960 (database 'Aardewerk') and 2006 in northern Belgium. This temporal analysis was conducted under different land use, texture and drainage conditions. The results indicate that intensified land management practices seriously affect the SOC status of the soil. The increase in plough depth and a change in crop rotation result in a significant decrease of C near the surface for dry silt loam cropland soils, (i.e. 1.02 ± 0.23 kg C m⁻² in the top 0.3 m between 1960 and 2006). In wet to extremely wet grasslands, topsoil SOC decreased significantly, indicating a negative influence of intensive soil drainage on SOC stock. This resulted in a decline of SOC between 1960 and 2006 in the top 1 m, ranging from 3.99 ± 2.57 kg C m⁻² in extremely wet silt loam soils to 2.04 ± 2.08 kg C m⁻² in wet sandy soils. A slight increase of SOC stock is observed under dry to moderately wet grasslands at greater depths corresponding to increased livestock densities in the region. The increase of SOC in the top 1 m under grassland ranges from 0.65 ± 1.39 kg C m⁻² in well drained silt loam soils to 2.59 ± 6.49 kg C m⁻² in moderately drained silt loam soils over entire period.     

Conventional intensive logging promotes loss of organic carbon from the mineral soil

There are few data, but diametrically opposed opinions, about the impacts of forest logging on soil organic carbon (SOC). Reviews and research articles conclude either that there is no effect, or show contradictory effects. Given that SOC is a substantial store of potential greenhouse gasses and forest logging and harvesting is routine, resolution is important. We review forest logging SOC studies and provide an overarching conceptual explanation for their findings. The literature can be separated into short‐term empirical studies, longer‐term empirical studies and long‐term modelling. All modelling that includes major aboveground and belowground biomass pools shows a long‐term (i.e. ≥300 years) decrease in SOC when a primary forest is logged and then subjected to harvesting cycles. The empirical longer‐term studies indicate likewise. With successive harvests the net emission accumulates but is only statistically perceptible after centuries. Short‐term SOC flux varies around zero. The long‐term drop in SOC in the mineral soil is driven by the biomass drop from the primary forest level but takes time to adjust to the new temporal average biomass. We show agreement between secondary forest SOC stocks derived purely from biomass information and stocks derived from complex forest harvest modelling. Thus, conclusions that conventional harvests do not deplete SOC in the mineral soil have been a function of their short time frames. Forest managers, climate change modellers and environmental policymakers need to assume a long‐term net transfer of SOC from the mineral soil to the atmosphere when primary forests are logged and then undergo harvest cycles. However, from a greenhouse accounting perspective, forest SOC is not the entire story. Forest wood products that ultimately reach landfill, and some portion of which produces some soil‐like material there rather than in the forest, could possibly help attenuate the forest SOC emission by adding to a carbon pool in landfill.     

Net annual global warming potential and greenhouse gas intensity in Chinese double rice‐cropping systems: a 3‐year field measurement in long‐term fertilizer experiments

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long‐term fertilizer experiment in Chinese double rice‐cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH₄) and nitrous oxide (N₂O) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0–20 cm) organic carbon (SOC) density over the 10‐year period 1999–2009. Long‐term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha^?1 yr^?1 for the control and 1.01–1.43 t C ha^?1 yr^?1 for the fertilizer plots. Long‐term inorganic fertilizer application tended to increase CH₄ emissions during the flooded rice season and significantly increased N₂O emissions from drained soils during the nonrice season. Annual mean CH₄ emissions ranged from 621 kg CH₄ ha^?1 for the control to 1175 kg CH₄ ha^?1 for the FOM plots, 63–83% of which derived from the late‐rice season. Annual N₂O emission averaged 1.15–4.11 kg N₂O–N ha^?1 in the double rice‐cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application.     

Evolution of soil carbon stocks under Miscanthus × giganteus and Miscanthus sinensis across contrasting environmental conditions

Miscanthus is a C4 bioenergy perennial crop characterized by its high potential yield. Our study aimed to compare the carbon storage capacities of Miscanthus sinensis (M. sinensis) with that of Miscanthus × giganteus (M. × giganteus) in field conditions in different types of soils in France. We set up a multi‐environment experimental network. On each trial, we tested two treatments: M. × giganteus established from rhizomes (Gr) and M. sinensis transplanted seedlings (Sp). We quantified the soil organic carbon (SOC) stock at equivalent soil mass for both genotypes in 2014 and 2019 and for two sampling depths: L1 (ca. 0–5 cm) and L1‐2 (ca. 0–30 cm). We also calculated the total and annual variation of the SOC stock and investigated factors that could explain the variation and the initial state of the SOC stock. ANOVAs were performed to compare the SOC stock, as well as the SOC stock variation rates across treatments and soil layers. Results showed that the soil bulk density did not vary significantly between 2014 and 2019 for both treatments (Gr and Sp). The SOC concentration (i.e. SOC expressed in g/kg) increased significantly between 2014 and 2019 in L1, whereas no significant evolution was found in L2 (ca. 5–30 cm). The SOC stock (i.e. SOC expressed in t/ha) increased significantly in the superficial layer L1 for M. × giganteus and M. sinensis, by 0.48 ± 0.41 and 0.54 ± 0.25 t ha^?1 year^?1 on average, respectively, although no significant change was detected in the layer L1‐2 for both genotypes. Moreover, SOC stocks in 2019 did not differ significantly between M. × giganteus and M. sinensis in the soil layers L1 and L1‐2. Lastly, our results showed that the initial SOC stock was significantly higher when miscanthus was grown after set‐aside than after annual crops.