Increase Soil Carbon Stocks Research Articles

The location- and scale- specific correlation between temperature and soil carbon sequestration across the globe

Much research has been conducted to understand the spatial distribution of soil carbon stock and its temporal dynamics. However, an agreement has not been reached on whether increasing global temperature has a positive or negative feedback on soil carbon stocks. By analysing global maps of soil organic carbon (SOC) using a spherical wavelet analysis, it was found that the correlation between SOC and soil temperature at the regional scale was negative between 52° N and 40° S parallels and positive beyond this region. This was consistent with a few previous studies and it was assumed that the effect was most likely due to the temperature-dependent SOC formation (photosynthesis) and decomposition (microbial activities and substrate decomposability) processes. The results also suggested that the large SOC stocks distributed in the low-temperature areas might increase under global warming while the small SOC stocks found in the high-temperature areas might decrease accordingly. Although it remains unknown whether the potential increasing soil carbon stocks in the low-temperature areas can offset the loss of carbon stocks in the high-temperature areas, the location- and scale- specific correlations between SOC and temperature should be taken into account for modeling SOC dynamics and SOC sequestration management.

Simulation on the Future Change of Soil Organic Carbon from Phaeozems under Different Management Practices in Northeast China

The Phaeozem region is an important grain production base in China and soil fertility has declined under long-term cultivation. Studying soil organic carbon (SOC) change in the upland phaeozem of Northeast China under different tillage modes has great theoretical and practical significance for reducing greenhouse gases emissions, increasing soil carbon stocks, and ensuring food security. This study applied the DAYCENT model to simulate six long-term experimental sites in the phaeozem region, and through calibration and validation analysis, it concluded that the DAYCENT model could effectively simulate the dynamic change of SOC in the upland phaeozems; hence, the relevant parameters of each site were determined. Under future climate scenarios (Representative Concentration Pathway 4.5, RCP 4.5), SOC change in agricultural soils (0–20 cm topsoil) of four different management scenarios (fertilizer application, manure amendment, straw return, no-tillage) was simulated. The overall simulated trend was no-tillage > straw return > manure amendment > fertilizer application. Fertilizer had no evident effect on increasing SOC, but the effect would be better if combined with manure; both straw return and no-tillage had the general effect of improving SOC and the simulated values rose rapidly with a large increasing rate within a short period; however, the increasing rate became gentle after 2050, reached a relatively stable level after 2080, then gradually reached carbon saturation. Untill 2100, the SOC content of each site reached a relatively higher level by adopting the no-tillage treatment, where the SOC contents of Harbin, Hailun, Nehe, Dehui, Gongzhuling and Lishu were 2.36 g/100 g, 2.81 g/100 g, 2.22 g/100 g, 2.38 g/100 g, 1.97 g/100 g and 2.01 g/100 g, respectively. The SOC contents increased by 52.47% when compared with the initial value, and the simulated average annual increase of SOC for 84 years was 0.0082 g/100 g.

Low carbon resource conservation techniques for energy savings, carbon gain and lowering GHGs emission in lowland transplanted rice

Six resource conservation techniques (RCTs) including control were imposed in field during 2013, 2014 on lowland rice (dry season) – rice (wet season) system to assess their impacts on rice production, energy savings, carbon storage, soil carbon fractions, functional microbial diversities and greenhouse gases (methane and nitrous oxide) emission. The resource conservation techniques broadly included i) residue incorporation, ii) in-situ green manuring, iii) zero tillage, iv) mechanical transplanting and v) mechanical transplanting with real time nitrogen management. The control refers to conventional practices (manual transplanting, weeding and harvesting). The energy savings was found highest under zero tillage technique which was 14.4, 17.9% and 13.3, 16.4% higher than conventional practices in dry and wet cropping seasons in 2013 and 2014, respectively. However, in 2013, energy output-input ratio was found maximum under mechanical transplanting with real time nitrogen management (7.64) in dry seasons and under residue incorporation technique (10.78) in wet season. But, in the year 2014 the energy ratio was found maximum 7.62 (dry season) and 10.0 (wet season) under in-situ green manuring. Soil organic carbon (SOC) stock change was significantly higher under zero tillage transplanted rice by 5.3, 6.8% and 8.3, 9.7% as compared to control in the dry and wet seasons of 2013 and 2014, respectively. Methane (CH4) emission was significantly less in zero tillage than control (11.7%). However, nitrous oxide (N2O) emission was significantly low (18.3%) in real time nitrogen management technique as compared to other treatments. In general, soil labile carbon fractions and enzymatic activities were significantly higher under RCTs over control, signified improvement of soil quality. Resource conservation technique having zero tillage and real time nitrogen management in transplanted rice could offer a low carbon technology in long run in one hand as these minimize greenhouse gases (GHGs) emission and increase soil carbon stock and also sustain yield in tropical lowland rice.

PLANT SHOOTS AND ROOTS BIOMASS OF BRACHIARIA GRASSES AND THEIR EFFECTS ON SOIL CARBON IN THE SEMI-ARID TROPICS OF KENYA

<p>Grassland management practices that improve carbon uptake by increasing productivity or reducing carbon losses can lead to net accumulation of carbon in soils. A study was conducted to quantify the amounts of shoots and roots biomass of Brachiaria grass cultivars and their effects on soil carbon in two sites, Ithookwe and Katumani in semi-arid tropical Kenya. The grass cultivars were<em> Brachiaria decumbens </em>cv. Basilisk, <em>B. brizantha</em> cvs. Marandu, MG4, Piatã and Xaraes, <em>B. humidicola</em> cv. Llanero and <em>B. hybrid</em> cv. Mulato II. These were compared with two locally cultivated grasses (<em>Chloris gayana</em> cv<em>. </em>KAT R3 and <em>Pennisetum purpureum </em>cv<em>. </em>Kakamega 1). The grass treatments were evaluated with fertilizer application (40 kg P applied at sowing and 50 kg N ha<sup>-1</sup> in each wet season) and with no fertilizer application. Shoots biomass of the Brachiaria cultivars ranged from 3.0 to 11.3 t ha<sup>-1</sup> and 5.5 to 8.3 t ha<sup>-1</sup> at Ithookwe and Katumani sites respectively in year 1. The highest shoots biomass recorded at Ithookwe was from cv. Piata while cv. MG4 gave the highest biomass at Katumani. Similar trends were recorded in year 2 of growth though the shoots biomass was lower at Katumani. However, the yields were significantly lower than those recorded from control, Napier grass in both years. The cv. Marandu, Xaraes, Basilisk and Piata had higher roots biomass than the controls (Rhodes grass and Napier grass) indicating greater potential for the Brachiaria grasses to sequester more carbon in the soil. The results of this study indicate that introduction of Brachiaria grasses in the semi-arid tropics of Kenya and in other similar environments can help increase soil carbon stocks that would mitigate the adverse effects of climate change and have greater economic returns. </p>

Shade trees have limited benefits for soil fertility in cocoa agroforests

Agroforestry is often promoted as a sustainable agricultural practice that can ameliorate causes of declining yields, such as soil degradation. However, despite the often-stated potential of agroforestry, quantitative data on the benefits of shade trees are limited to relatively few cropping systems, particularly maize and coffee. Furthermore, agroforests are not cost-free and the benefits of agroforests might not be sufficient to outweigh these costs in all cropping systems or environments. Here we quantify costs and benefits of agroforests for cocoa production in Ghana, West Africa. Specifically, we quantified the ability of shade trees to increase soil carbon stocks and soil fertility (i.e. total soil carbon, nitrogen and phosphorus, available phosphorus and potassium, cation exchange capacity, soil aggregation, pH, and foliar nitrogen and phosphorus concentrations), and investigate if these benefits are sufficient to outweigh the negative effects of shade trees on cocoa growth and yields. We measured cocoa yields, soil fertility and carbon-sequestration under individual shade trees, and in 30 × 30 m plots that were distributed along a gradient of shade-tree cover (plot-scale). We found localized positive effects of individual shade trees on soil carbon and nitrogen content, as well as soil aggregation. However, we found no evidence for positive effects of agroforests via improved soil fertility or carbon-sequestration with increasing shade-tree cover at the plot scale, a scale that more closely matches the scale at which agroforests are managed. Cocoa growth was lower under individual shade trees and decreased with increasing shade-tree cover in plots, and cocoa yields also decreased with increasing shade-tree cover. Our results indicate that the benefits of agroforestry for soil fertility and carbon sequestration in cocoa cultivation systems might not be as extensive as believed, and may not be sufficient to compensate for short-term costs to production.

Grassland management impacts on soil carbon stocks: a new synthesis.

Grassland ecosystems cover a large portion of Earths' surface and contain substantial amounts of soil organic carbon. Previous work has established that these soil carbon stocks are sensitive to management and land use changes: grazing, species composition, and mineral nutrient availability can lead to losses or gains of soil carbon. Because of the large annual carbon fluxes into and out of grassland systems, there has been growing interest in how changes in management might shift the net balance of these flows, stemming losses from degrading grasslands or managing systems to increase soil carbon stocks (i.e., carbon sequestration). A synthesis published in 2001 assembled data from hundreds of studies to document soil carbon responses to changes in management. Here we present a new synthesis that has integrated data from the hundreds of studies published after our previous work. These new data largely confirm our earlier conclusions: improved grazing management, fertilization, sowing legumes and improved grass species, irrigation, and conversion from cultivation all tend to lead to increased soil C, at rates ranging from 0.105 to more than 1Mg C·ha-1 ·yr-1 . The new data include assessment of three new management practices: fire, silvopastoralism, and reclamation, although these studies are limited in number. The main area in which the new data are contrary to our previous synthesis is in conversion from native vegetation to grassland, where we find that across the studies the average rate of soil carbon stock change is low and not significant. The data in this synthesis confirm that improving grassland management practices and conversion from cropland to grassland improve soil carbon stocks.

Changes in Soil Carbon Stocks under Integrated Crop-Livestock-Forest System in the Brazilian Amazon Region

Several studies indicate that the use of integrated production systems, such as integrated crop-livestock-forest systems (ICLF), improves the quality of the soil and consequently the sequestration of organic carbon in the soil. In this way, this work aims to evaluate the carbon stocks in soil under different management systems in the Cerrado/Amazonia transition zone, namely: ICLF, no-tillage, pasture and eucalyptus plantation. For this, two soil samplings were done in 2011 and 2014, in the 0 - 5, 5 - 10 and 10 - 30 cm layers. Soil carbon and nitrogen stocks were analyzed. ICLF system was the treatment that obtained the highest percentage of carbon gain (7.8%) after three years of establishment which represents to an increase of 5.5 Mg·ha-1. Management systems, such as ICLF, with minimal soil disturbance combined with crop rotations that contribute to the quantity and quality of residues input, increase soil organic matter content. Carbon stock data show the potential of ICLF systems to increase soil carbon stocks.

Assessment of Carbon Stock in Chronosequence Rehabilitated Tropical Forest Stands in Malaysia

The loss and degradation in tropical forest region are some of the current global concern. Hence, these issues elevated the role of rehabilitated forests in providing ecological products and services. The information on the carbon stock is important in relation to global carbon and biomass use, but lacking from the tropical region. This paper reports the assessment of tree and soil carbon stock in a chronosequence rehabilitated tropical forest stands in Malaysia. The study site was at the UPM-Mitsubishi Forest Rehabilitation Project, UPMKB. <TEX>$20{\times}20m$</TEX> plot was established each and assessed in 2009 at 1-, 10- and 19-year-old sites while an adjacent <TEX>${\pm}23-year-old$</TEX> natural regenerating secondary forest plot was established for comparison. The overall total carbon stock was in the order of 19-year-old><TEX>${\pm}23-year-old$</TEX>>10-year-old>1-year-old. When forest carbon stock is low, the soil component plays an important role in the carbon storage. The forest carbon recovery is crucial to increase soil carbon stock. The variations in the carbon stock showed the different stages of the forest recovery. Species survived after 19-years of planting are potential species for carbon sequestration activities in rehabilitated forest. Human intervention in rehabilitating degraded forest areas through tree planting initiatives is crucial towards recovering the forest ecological role especially in forest carbon stock capacity.

Soil carbon stock changes due to edge effects in central Amazon forest fragments

Amazon forest stocks large quantities of carbon both in plant biomass and in soil. Deforestation has accelerated the process of forest fragmentation in the Brazilian Amazon, resulting in changes in carbon stocks in both biomass and soil. Logging, including that under legal forest management, can create edge-like conditions inside the forest. We investigated the relationship between changes in carbon stocks in the soil and the distance to the nearest edge in forest remnants after about 30years of isolation. We assessed the effect of edges using geographically weighted regression (GWR), which considers the non-stationary character of soil carbon stocks and assigns relative weights to the observations according to the distance between them. Data from 265 georeferenced plots distributed over 28ha of forest fragments in the Manaus region were included in these analyses. Soil-carbon stocks were estimated for areas before (1984–1986) and after (2012–2013) isolation of the fragments. The GWR model indicated an apparent relationship between change in carbon stocks and distance from the edge (R2=0.79). The largest changes occurred in plots located closest to the edges. In 202 plots ⩽100m from an edge, soil-carbon stock increased significantly (p=0.01) by a mean of 1.34Mgha−1 over the ∼30-year period. Such changes in soil carbon stocks appear to be associated with higher rates of tree mortality caused by microclimatic changes in these areas. Increased necromass inputs combined with changes in composition and structure of vegetation may result in increased rates of decomposition of organic matter, transferring carbon to the soil compartment and increasing soil carbon stocks. Considering both “hard” edges adjacent to deforestation and “soft” edges in logging areas, the soil-carbon increase we measured implies an absorption of 6×106MgC in Brazilian Amazonia. In hard edges maintained for ∼30years, the soil-carbon increase offsets 8.3% of the carbon losses from “biomass collapse” in the first 100m from a clearing. Soil carbon did not change significantly in 63 forest-interior plots, suggesting that global climate change has not yet had a detectible effect on this forest carbon compartment.

Total soil organic carbon and carbon sequestration potential in Nigeria

This study aimed to quantify SOC stocks and potential C sequestration for Nigeria using legacy soil data. Mass preserving splines were fitted to legacy SOC and bulk density (BD) pedon data based on GlobalSoilmap soil depths. SOC concentrations (g kg−1) were predicted using Random Forest model (RFM), Cubist and Boosted regression tree (BRT). Thereafter, the soil carbon density (MgCha−1) was calculated from the SOC concentration and BD (Mgm−3). The information was combined with land use/land cover (LULC) map and agro-ecological zone (AEZ) digital maps to estimate SOC sequestration. The mean SOC concentration ranged between 4.2 and 23.7gkg−1 in the top 30cm and between 2.6 and 9.2gkg−1 at the lower soil depth. Total SOC stock in the top 1m was 6.5Pg with an average density of 71.60MgCha−1. Almost half of the SOC stock was found in the 0–30cm layer. SOC stocks decreased from the southwest to the northeast of Nigeria, and increased from Sahel to Humid forest agro-ecological zones. Restoration of the various land use types has the potential to sequester about 0.2 to 30.8MgCha−1 depending on the AEZ. The Derived Guinea Savannah presents a potential hotspot for targeted carbon sequestration projects in Nigeria. Knowledge of SOC stock and sequestration is vital for framing appropriate management regimes to increase soil carbon stocks, for C accounting and environmental monitoring purposes.

Windthrows increase soil carbon stocks in a central Amazon forest

Abstract. Windthrows change forest structure and species composition in central Amazon forests. However, the effects of widespread tree mortality associated with wind disturbances on soil properties have not yet been described in this vast region. We investigated short-term effects (7 years after disturbance) of widespread tree mortality caused by a squall line event from mid-January of 2005 on soil carbon stocks and concentrations in a central Amazon terra firme forest. The soil carbon stock (averaged over a 0–30 cm depth profile) in disturbed plots (61.4 ± 8.2 Mg ha−1, mean ±95 % confidence interval) was marginally higher (p = 0.09) than that from undisturbed plots (47.7 ± 13.6 Mg ha−1). The soil organic carbon concentration in disturbed plots (2.0 ± 0.17 %) was significantly higher (p &lt; 0.001) than that from undisturbed plots (1.36 ± 0.24 %). Moreover, soil carbon stocks were positively correlated with soil clay content (r2 = 0.332, r = 0.575 and p = 0.019) and with tree mortality intensity (r2 = 0.257, r = 0.506 and p = 0.045). Our results indicate that large inputs of plant litter associated with large windthrow events cause a short-term increase in soil carbon content, and the degree of increase is related to soil clay content and tree mortality intensity. The higher carbon content and potentially higher nutrient availability in soils from areas recovering from windthrows may favor forest regrowth and increase vegetation resilience.

Bacterial Mobilization of Nutrients From Biochar-Amended Soils.

Soil amendments with biochar to improve soil fertility and increase soil carbon stocks have received some high-level attention. Physical and chemical analyses of amended soils and biochars from various feedstocks are reported, alongside some evaluations of plant growth promotion capabilities. Fewer studies investigated the soil microbiota and their potential to increase cycling and mobilization of nutrients in biochar-amended soils. This review is discussing the latest findings in the bacterial contribution to cycling and mobilizing nitrogen, phosphorus, and sulfur in biochar-amended soils and potential contributions to plant growth promotion. Depending on feedstock, pyrolysis, soil type, and plant cover, changes in the bacterial community structure were observed for a majority of the studies using amplicon sequencing or genetic fingerprinting methods. Prokaryotic nitrification largely depends on the availability of ammonium and can vary considerably under soil biochar amendment. However, denitrification to di-nitrogen and in particular, nitrous oxide reductase activity is commonly enhanced, resulting in reduced nitrous oxide emissions. Likewise, bacterial fixation of di-nitrogen appears to be regularly enhanced. A paucity of studies suggests that bacterial mobilization of phosphorus and sulfur is enhanced as well. However, most studies only tested for extracellular sulfatase and phosphatase activity. Further research is needed to reveal details of the bacterial nutrient mobilizing capabilities and this is in particular the case for the mobilization of phosphorus and sulfur.

Radiocarbon-Based Assessment of Heterotrophic Soil Respiration in Two Mediterranean Forests

The amount of soil organic carbon (SOC) released into the atmosphere as carbon dioxide (CO2), which is referred to as heterotrophic respiration (Rh), is technically difficult to measure despite its necessity to the understanding of how to protect and increase soil carbon stocks. Within this context, the aim of this study is to determine Rh in two Mediterranean forests dominated by pine and oak using radiocarbon measurements of the bulk SOC from different soil layers. The annual Rh was 3.22 Mg C ha−1 y−1 under pine and 3.13 Mg C ha−1 y−1 under oak, corresponding to 38 and 31% of the annual soil respiration, respectively. The accuracy of the Rh values was evaluated by determining the net primary production (NPP), as the sum of the Rh and the net ecosystem production measured by eddy covariance, then comparing it with the NPP obtained through independent biometric measurements. No significant differences were observed, which suggested the suitability of our methodology to infer Rh. Assuming the C inputs to soil to consist exclusively of the aboveground and belowground litter and the C output exclusively of the Rh, both soils were C sinks, which is consistent with a previous modeling study that was performed in the same stands. In conclusion, radiocarbon analysis of bulk SOC provided a reliable estimate of the average annual amount of soil carbon released to the atmosphere; hence, its application is convenient for calculating Rh because it utilizes only a single soil sampling and no time-consuming monitoring activities.

Erratum to: Dwarf pine invasion in an alpine tundra of discontinuous permafrost area: effects on fine root and soil carbon dynamics

Recent invasion of Pinus pumila , a highly productive shrub-like tree species, into alpine tundra does not significantly modify the dynamics of fine root and soil carbon in the tundra. Climate warming may directly and indirectly affect the large carbon stock in discontinuous permafrost soil at high latitudes. In recent decades, Siberian dwarf pine [Pinus pumila (Pall.) Regel] has been invading dry heath alpine tundra in the northern Amur region of Far East Russia. Siberian dwarf pine is known to have high aboveground productivity, comparable to that of tall coniferous trees. We hypothesised that the invasion of Siberian dwarf pine into alpine tundra could increase soil carbon stocks via an increase in fine roots. Contrary to our expectations, the invasion of dwarf pine did not significantly increase the fine root biomass and productivity of the tundra, probably due to the belowground competitive exclusion between the dwarf pine and alpine tundra plants. Furthermore, the invasion of the dwarf pine did not affect soil carbon in the alpine tundra ecosystem. These results show that the recent invasion of Siberian dwarf pine into tundra did not influence the fine root dynamics or the soil carbon stock in the study site. Together, these results implied that (1) it takes a long time for pine invasion to change the belowground ecosystem properties of tundra vegetation to that of pine thickets and therefore (2) the lack of an increase in soil carbon from recent tree invasion should be taken into account when modelling future carbon dynamics in alpine tundra.

Sensitivity of soil carbon to management and environmental factors within Australian perennial pasture systems

Environmental factors such as parent material and climate can have a large effect on total carbon concentration and soil carbon stocks, yet unlike vegetation type, fertiliser use and grazing pressure, these cannot be changed by management. The relative effects of these environmental and land management factors were compared in the Monaro and Boorowa regions of New South Wales (NSW), Australia. Parent material, geographic region, soil depth and soil fertility had a significant influence on soil carbon stocks to 0.70m while pasture type (introduced vs native pastures) did not. Parent material and soil depth significantly (P<0.05) influenced the mean soil carbon stock (Mg C/ha) in the Monaro region; 159 (11 se) in basalt-derived soils, 77 (11 se) in deep granite-derived soils and 43 (3 se) in shallow granite-derived soils. Climate also significantly (P<0.05) influenced the mean carbon stock, with deep granite-derived soils in the Monaro region having 76.5 (11 se) compared with 51.8 (3 se) Mg C/ha in the Boorowa region. A considerable proportion of the total carbon stock to 0.70m for all sites was measured in the subsoil (0.30 to 0.70m). In the Monaro region, basalt-derived soil contained 43% of the total carbon stock in the subsoil, compared with 28% in deep granite and shallow granite-derived soil. In the Boorowa region, deep granite-derived soil contained 33% of the total carbon stock in the subsoil. Restricting soil carbon measurements to the surface 0.30m of soil may result in erroneous conclusions with respect to the influence of land management on the accumulation of carbon in soil. Total carbon concentration was positively correlated with labile carbon, total nitrogen, cation exchange capacity and extractable sulfur, suggesting that for a given parent material and climate, maintaining adequate pasture nutrition may substantially increase soil carbon stocks.

Simulation of Long-Term Carbon and Nitrogen Dynamics in Grassland-Based Dairy Farming Systems to Evaluate Mitigation Strategies for Nutrient Losses

Many measures have been proposed to mitigate gaseous emissions and other nutrient losses from agroecosystems, which can have large detrimental effects for the quality of soils, water and air, and contribute to eutrophication and global warming. Due to complexities in farm management, biological interactions and emission measurements, most experiments focus on analysis of short-term effects of isolated mitigation practices. Here we present a model that allows simulating long-term effects at the whole-farm level of combined measures related to grassland management, animal housing and manure handling after excretion, during storage and after field application. The model describes the dynamics of pools of organic carbon and nitrogen (N), and of inorganic N, as affected by farm management in grassland-based dairy systems. We assessed the long-term effects of delayed grass mowing, housing type (cubicle and sloping floor barns, resulting in production of slurry and solid cattle manure, respectively), manure additives, contrasting manure storage methods and irrigation after application of covered manure. Simulations demonstrated that individually applied practices often result in compensatory loss pathways. For instance, methods to reduce ammonia emissions during storage like roofing or covering of manure led to larger losses through ammonia volatilization, nitrate leaching or denitrification after application, unless extra measures like irrigation were used. A strategy of combined management practices of delayed mowing and fertilization with solid cattle manure that is treated with zeolite, stored under an impermeable sheet and irrigated after application was effective to increase soil carbon stocks, increase feed self-sufficiency and reduce losses by ammonia volatilization and soil N losses. Although long-term datasets (>25 years) of farm nutrient dynamics and loss flows are not available to validate the model, the model is firmly based on knowledge of processes and measured effects of individual practices, and allows the integrated exploration of effective emission mitigation strategies.

Relationship between environmental and land-use variables on soil carbon levels at the regional scale in central New South Wales, Australia

The potential to change agricultural land use to increase soil carbon stocks has been proposed as a mechanism to offset greenhouse gas emissions. To estimate the potential carbon storage in the soil from regional surveys it is important to understand the influence of environmental variables (climate, soil type, and landscape) before land management can be assessed. A survey was done of 354 sites to determine soil organic carbon stock (SOC stock; Mg C/ha) across the Lachlan and Macquarie catchments of New South Wales, Australia. The influences of climate, soil physical and chemical properties, landscape position, and 10 years of land management information were assessed. The environmental variables described most of the regional variation compared with management. The strongest influence on SOC stock at 0–10 cm was from climatic variables, particularly 30-year average annual rainfall. At a soil depth of 20–30 cm, the proportion of silica (SiO2) determined by mid-infrared spectra (SiMIR) had a negative relationship with SOC stock, and sand and clay measured by particle size analysis also showed strong relationships at sites where measured. Of the difference in SOC stock explained by land use, cropping had lower soil carbon than pasture in rotation or permanent pasture at 0–10 cm. This relationship was consistent across a rainfall gradient, but once soil carbon was standardised per mm of average annual rainfall, there was a greater difference between cropping and permanent pasture with increasing SiMIR in soils. Land use is also regulated by climate, topography, and soil type, and the effect on SOC stock is better assessed in smaller land-management units to remove some variability due to climate and soil.

Modelling the long-term response to positive and negative priming of soil organic carbon by black carbon

Observed increases in the mineralization rate of labile organic carbon (LOC) in the presence of black carbon (BC) have led to speculation that corresponding decreases in non-pyrogenic (i.e. non-BC) soil organic carbon (npSOC) could significantly reduce or negate the C sequestration benefit of BC in soils. Here we show that the potential effect of an increased LOC decomposition rate on long-term npSOC stocks is negligible, even when using assumptions that would favour large losses, potentially causing no more than 3–4 % loss of npSOC over 100 years if 50 % of above-ground crop residues were converted to BC annually. Conversely, if the BC-stimulated enhanced stabilization of npSOC that has been observed in laboratory trials is extrapolated to the long-term, it would greatly increase soil carbon stocks by 30–60 %. Annual additions of BC derived from crop residues would increase total SOC (including both BC and npSOC) by an amount five times greater than the potential increase from enhanced stabilization and an order of magnitude greater than losses of npSOC caused by annual removals of biomass to provide BC feedstock.

The potential to increase soil carbon stocks through reduced tillage or organic material additions in England and Wales: A case study

Results from the UK were reviewed to quantify the impact on climate change mitigation of soil organic carbon (SOC) stocks as a result of (1) a change from conventional to less intensive tillage and (2) addition of organic materials including farm manures, digested biosolids, cereal straw, green manure and paper crumble. The average annual increase in SOC deriving from reduced tillage was 310 kg C ± 180 kg C ha −1 yr −1. Even this accumulation of C is unlikely to be achieved in the UK and northwest Europe because farmers practice rotational tillage. N 2O emissions may increase under reduced tillage, counteracting increases in SOC. Addition of biosolids increased SOC (in kg C ha −1 yr −1 t −1 dry solids added) by on average 60 ± 20 (farm manures), 180 ± 24 (digested biosolids), 50 ± 15 (cereal straw), 60 ± 10 (green compost) and an estimated 60 (paper crumble). SOC accumulation declines in long-term experiments (>50 yr) with farm manure applications as a new equilibrium is approached. Biosolids are typically already applied to soil, so increases in SOC cannot be regarded as mitigation. Large increases in SOC were deduced for paper crumble (>6 t C ha −1 yr −1) but outweighed by N 2O emissions deriving from additional fertiliser. Compost offers genuine potential for mitigation because application replaces disposal to landfill; it also decreases N 2O emission.

Impact of pre-harvest burning versus trash conservation on soil carbon and nitrogen stocks on a sugarcane plantation in the Brazilian Atlantic forest region

Owing to the increased demand for ethanol biofuel from sugar cane, the area planted to this crop in Brazil has increased from 4.8 to 9.5 Mha since 2000. At the same time there has been pressure from environmental groups and others to cease the pre-harvest burning of cane, and today over 40% of the crop is harvested without burning, thus conserving the trash on the soil surface. While most trash decomposes during the year, it is generally assumed that this transition from burning to trash conservation will have benefits for cane productivity and increase soil carbon stocks. To investigate the possible benefits of this change of practice an experiment was carried out in the state of Espírito Santo, south-eastern Brazil, to investigate the long-term effects of the practice of pre-harvested burning compared to trash conservation on soil fertility and soil C and N stocks. The results showed that over a 14-year period, trash conservation marginally decreased soil acidity and significantly increased soil C and N stocks in 0–10 cm depth interval. Although the trash conservation treatment accumulated 13 Mg C ha−1 more than the burned treatment, this difference was not statistically different. However, the stocks of N to 100 cm depth were 900 kg ha−1 higher under the trash conservation treatment and this difference was statistically significant. The 13C abundance data suggested that where trash was conserved, more soil C was derived from the sugar cane than from the original native vegetation.