Understanding soil organic carbon (SOC) sequestration is important to develop strategies to increase the SOC stock and, thereby, offset some of the increases in atmospheric carbon dioxide. Although ...the capacity of soils to store SOC in a stable form is commonly attributed to the fine (clay + fine silt) fraction, the properties of the fine fraction that determine the SOC stabilization capacity are poorly known. The aim of this study was to develop an improved model to estimate the SOC stabilization capacity of Allophanic (Andisols) and non‐Allophanic topsoils (0–15 cm) and, as a case study, to apply the model to predict the sequestration potential of pastoral soils across New Zealand. A quantile (90th) regression model, based on the specific surface area and extractable aluminium (pyrophosphate) content of soils, provided the best prediction of the upper limit of fine fraction carbon (FFC) (i.e. the stabilization capacity), but with different coefficients for Allophanic and non‐Allophanic soils. The carbon (C) saturation deficit was estimated as the difference between the stabilization capacity of individual soils and their current C concentration. For long‐term pastures, the mean saturation deficit of Allophanic soils (20.3 mg C g−1) was greater than that of non‐Allophanic soils (16.3 mg C g−1). The saturation deficit of cropped soils was 1.14–1.89 times that of pasture soils. The sequestration potential of pasture soils ranged from 10 t C ha−1 (Ultic soils) to 42 t C ha−1 (Melanic soils). Although meeting the estimated national soil C sequestration potential (124 Mt C) is unrealistic, improved management practices targeted to those soils with the greatest sequestration potential could contribute significantly to off‐setting New Zealand's greenhouse gas emissions. As the first national‐scale estimate of SOC sequestration potential that encompasses both Allophanic and non‐Allophanic soils, this serves as an informative case study for the international community.
A quantile regression model was developed and used to estimate the soil C stabilization capacity and saturation deficit for a wide range of New Zealand pasture and cropping soils. The overall mean saturation deficit for these soils was 15.1 ± 0.8 mg C g−1 soil. Accounting for differences in bulk density and land area, the mean saturation deficit of each major soil Order was used to estimate a national C sequestration potential of 124 ± 37 Mt C.
As soils under permanent pasture and grasslands have large topsoil carbon (C) stocks, the scope to sequester additional C may be limited. However, because C in pasture/grassland soils declines with ...depth, there may be potential to sequester additional C in the subsoil. Data from 247 continuous pasture sites in New Zealand (representing five major soil Orders and ~80% of the grassland area) showed that, on average, the 0.15–0.30 m layer contained 25–34 t ha−1 less C than the top 0.15 m. High‐production grazed pastures require periodic renewal (re‐seeding) every 7–14 years to maintain productivity. Our objective was to assess whether a one‐time pasture renewal, involving full inversion tillage (FIT) to a depth of 0.30 m, has potential to increase C storage by burying C‐rich topsoil and bringing low‐C subsoil to the surface where C inputs from pasture production are greatest. Data from the 247 pasture sites were used to model changes in C stocks following FIT pasture renewal by predicting (1) the C accumulation in the new 0–0.15 m layer and (2) the decomposition of buried‐C in the new 0.15–0.30 m layer. In the 20 years following FIT pasture renewal, soil C was predicted to increase by an average of 7.3–10.3 (Sedimentary soils) and 9.6–12.7 t C ha−1 (Allophanic soils), depending on the assumptions applied. Adoption of FIT for pasture renewal across all suitable soils (2.0–2.6 M ha) in New Zealand was predicted to sequester ~20–36 Mt C, sufficient to offset 9.6–17.5% of the country's cumulative greenhouse gas emissions from agriculture over 20 years at the current rate of emissions. Given that grasslands account for ~70% of global agricultural land, FIT renewal of pastures or grassland could offer a significant opportunity to sequester soil C and offset greenhouse gas emissions.
Soil organic C (SOC) stocks in permanent pastures and grasslands tend to be strongly stratified, being high near the surface and declining with depth. A one‐time use of full inversion tillage during pasture renewal (FIT‐PR) could increase SOC storage by burying C‐rich topsoil and bringing low‐C subsoil to the surface where C inputs from pasture are greatest. Data from New Zealand permanent pasture soils indicate the potential for FIT‐PR to increase average SOC stocks (over 20 years) in allophanic (9.6–12.7 t C ha−1) and sedimentary (7.3–10.3 t C ha−1) soils, with the amount sequestered related to pre‐renewal SOC stratification ratio.
•We review farm management options to increase grassland soil carbon stocks.•Carbon saturation deficit defines the potential to increase soil carbon stocks.•Increasing carbon stock is dependent on ...carbon inputs and stabilisation processes.•Models highlight trade-offs between increasing soil carbon and milk production.•We recommend assessment criteria and priorities for further research.
Even small increases in the large pool of soil organic carbon could result in large reductions in atmospheric CO2 concentrations sufficient to limit global warming below the threshold of 2 °C required for climate stability. Globally, grasslands occupy 70% of the world’s agricultural area, so interventions to farm management practices to reduce losses or increase soil carbon stocks in grassland are highly relevant. Here, we review the literature with particular emphasis on New Zealand and report on the effects of management practices on changes in soil carbon stocks for temperate grazed grasslands. We include findings from models that explore the trade-offs between multiple desirable outcomes, such as increasing soil carbon stocks and milk production.
Farm management practices can affect soil carbon stocks through changes in net primary production, the proportions of biomass removed, the degree of stabilisation of carbon in the soil and changes to the rate of soil carbon decomposition. The carbon saturation deficit defines the potential for a soil to stabilise additional carbon. Earlier reviews have concluded that, while labile carbon is the dominant substrate for soil carbon decomposition, a fraction of soil carbon stocks is stabilised and protected from decomposition by the formation of organo-mineral complexes. Recent evidence shows that the rate of organic carbon decomposition is determined primarily by the extent of soil organic carbon protection and, therefore, the availability of substrates to microbial activity.
New Zealand grassland systems have moderate to high soil carbon stocks in the surface layers (i.e., upper 0.15 m) where most roots are located, so the carbon saturation deficit is relatively low and the scope to increase soil carbon stocks by carbon inputs from primary production may be limited. International studies have shown that the addition of fertilisers, feed imports, and applications of manure and effluent can increase soil carbon stocks, especially for degraded soils, but the responses in New Zealand soils are uncertain because of the limited number of studies. However, recent evidence shows that irrigation can reduce soil carbon stocks in New Zealand, but neither the processes nor the long-term trends are known. Studies of sward renewal have shown that short-term losses of carbon losses resulting from the disturbance can be mitigated using rapid replacement of the new sward, minimum tillage and avoidance of times when the soil water content is high. Swards comprising multiple species have also shown that soil carbon stocks may be increased after periods of several years. Model simulations have shown that the goal of increasing both soil carbon and milk production could be achieved best by increasing carbon inputs from supplementary animal feed. However, losses of carbon at feed export sites need to be minimised to achieve overall net gains in soil carbon. Grazing intensity can have a big influence on soil carbon stocks but the magnitude and direction of the effects are not consistent between studies.
Biochar addition could possibly increase soil carbon stocks but it is not yet an economical option for large-scale application in New Zealand. There is some evidence that the introduction of earthworms and dung beetles could potentially increase soil carbon stabilisation, but the greenhouse gas benefits are confounded by possible increases in nitrous oxide emissions. The new practice of full inversion tillage during grassland renewal has the potential to increase soil carbon stocks under suitable conditions but full life-cycle analysis including the effects of the disruptive operations has yet to be completed.
We conclude with a list of criteria that determine the success and suitability of management options to increase soil carbon stocks and identify priority research questions that need to be addressed using experimental and modelling approaches to optimise management options to increase soil carbon stocks.
Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost‐effectively from water adsorption by air‐dry soil ...samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water‐adsorption by SOC. Here, we refer to “apparent specific surface area” for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area.
We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r2 = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m2 mgC−1. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area.
Highlights
Soil surface area can be estimated from the H2O content of air‐dry soil but SOC also adsorbs H2O.
We developed a mathematical approach to estimate water adsorption by SOC.
We estimated the contribution of SOC to apparent specific surface area as 0.43 ± 0.02 m2 mgC−1.
Mineral specific surface area can be inferred by subtracting SOC‐based H2O adsorption.
Subsoil carbon is generally older and decomposes more slowly than topsoil carbon. It has, therefore, been suggested that carbon stocks could be increased by burying carbon-rich topsoil at depth to ...slow its decomposition. This has been supported by recent experiments that showed that buried topsoil carbon indeed decomposed more slowly, but the mechanisms causing the reduction have not yet been identified. We investigated three theoretical mechanisms that may explain reduced decomposition rates at depth: (1) lower soil-temperature variability, (2) lower oxygen concentrations/redox potential and (3) less priming (biological synergy). Temperature variability decreases with soil depth. As decomposition rates vary non-linearly with temperature, reduced temperature variability should, therefore, reduce annual decomposition rates. However, detailed simulations showed that it changed annual decomposition rates by only a few percent. Maximal decomposition rates also require adequate oxygen, but our simulations showed that oxygen diffusion rates would need to be reduced 1000 to 10 000-fold compared to the topsoil for it to protect buried soil carbon. Oxygen limitation is, therefore, likely to be confined to soils that are water-logged for extended periods. Priming (or biological synergy) is assumed to be the stimulation of decomposition rates by the availability of labile organic carbon. Our simulations showed that lower labile carbon inputs could reduce priming and potentially preserve up to half of buried carbon for centuries. If experimental work can further substantiate the role of this mechanism, carbon burial at depth could become a practical and useful climate-change mitigation option.
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•We theoretically explored why soil carbon at depth might be more stable.•We tested temperature variability, O2 requirements and priming (biological synergy).•More even temperatures at depth reduce decomposition rates but by only a few percent.•Reduced O2 access by extreme diffusion limits (water-logged) can reduce decomposition.•Cessation of biological synergy (priming) can explain persistence of C at depth.
Soil organic carbon (SOC) sequestration in pastoral soils could offset net greenhouse gas emissions by reducing global atmospheric carbon dioxide (CO2) concentrations and, thereby, slow climate ...warming. Long-term pastures typically have topsoils (0–10 cm) rich in SOC, with subsoils (10–30 cm) storing less than half as much SOC. In New Zealand, lowland farmers are advised to renew (reseed) their pastures every 7–10 years to improve pasture production. Renewal typically involves desiccating the old pasture followed by shallow tillage (or direct drilling) to establish a short season forage crop as a weed break, then direct drilling new pasture species. Minimum till at renewal maintains the vertical stratification of SOC, limiting the scope to increase SOC stocks under new pasture. During the spring of 2016 and 2017, two independent trials were established, on an Alfisol (trial 1) and on an Andisol (trial 2), to assess the effects of establishing a summer Brassica crop with either full inversion tillage (FIT; 30 cm furrow depth, as a one-off or infrequent (every 25–30 years) management), shallow tillage or no-till (direct drill) on SOC stocks and crop/pasture agronomic performance. In autumn, new pasture species were direct drilled into the stubble of the summer forage crop. Changes in SOC vertical distribution, plant growth, herbage quality (at both trials) and nitrogen (N) leaching (trial 1 only) were monitored. At both trials, FIT effectively buried SOC below 0–10 cm depth and increased crop yields compared to no-till treatment. After re-grassing pasture production was similar among all treatments. In trial 1, N leaching losses were 42% lower under FIT than under no-till. These results highlight the potential agronomic and environmental benefits of pasture renewal including FIT. Economic and N cycling benefits depend on the timely inclusion of a crop phase.
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•Full inversion tillage (FIT) prior to spring pasture renewal was field-tested.•FIT increased crop yield, without yield penalty for subsequent pasture.•FIT created a topsoil C gap, avoiding short-term SOC and mineral N losses.•FIT economic and N cycling benefits depend on the inclusion of a summer crop phase.
In principle, greenhouse gas emissions can be offset by increasing soil carbon stocks. Full utilisation of that potential, however, requires a good understanding of the controls on carbon stocks to ...identify factors that can be modified through management changes and distinguish those from factors that are inherent soil properties that cannot be modified. Here, we present a conceptual model of protected (or stabilised) carbon stocks in soils based on observations from two farms in New Zealand, and from a combined soils data set from observations from throughout New Zealand. These data showed that1)When other factors, such as climate, plant cover and pasture management, were identical, soil carbon stocks were highly, and linearly correlated with the soil's specific surface area estimated from soil water adsorption.2)The slopes of these relationships decreased with soil depth.3)Extrapolation of the relationships to zero specific surface area resulted in relatively small intercepts on the soil carbon axis. These intercepts decreased with soil depth.4)The intercepts were considered to correspond to unprotected labile carbon, with highest contents near the soils surface where most carbon inputs are received by soils.5)Together, these observations implied that virtually all protected carbon in the analysed soils was protected by the soil matrix rather than biochemically, and that mineral surface area was the functionally relevant key attribute that defined the soils' protective capacity.6)It implied that protected organic carbon, Cp, in a soil can be described as: Cp=kCinAm/f(T,W,pH,Al,…), where k is a simple constant, Cin is the total carbon inflow rate into the soil, Am is specific surface area, and f(T, W, pH, Al, …) is a specific turn-over rate of protected carbon as a function of temperature, soil water, pH, aluminium concentration, or any other factors apart from soil texture that may affect soil-carbon turn-over rates.
These observations improve our understanding of the important carbon-protection mechanisms in the soil, with significant implications for the optimal manipulation of carbon input rates into different soils to maximise overall soil carbon storage. They imply that overall carbon storage of soils could be enhanced by physically transferring any available carbon from soils with low to soils with high specific surface areas.
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•We found a strong linear correlation between SOC and specific surface area (Am).•Slopes and intercepts of these regression lines decreased with soil depth.•Hence, protected SOC, Cp, is protected by the mineral matrix, not biochemically.•Model: Cp = CinAm/ft, with Cin: carbon input and ft: specific SOC turn-over.•To mitigate climate change: better to deposit C on soils with high surface area.
Objective: Despite recent advances, there are still no interventions that have been developed for the specific treatment of young children who have anxiety disorders. This study examined the impact ...of a new, cognitive-behaviorally based parenting intervention on anxiety symptoms. Method: Families of 74 anxious children (aged 9 years or less) took part in a randomized controlled trial, which compared the new 10-session, group-format intervention with a wait-list control condition. Outcome measures included blinded diagnostic interview and self-reports from parents and children. Results: Intention-to-treat analyses indicated that children whose parent(s) received the intervention were significantly less anxious at the end of the study than those in the control condition. Specifically, 57% of those receiving the new intervention were free of their primary disorder, compared with 15% in the control condition. Moreover, 32% of treated children were free of any anxiety diagnosis at the end of the treatment period, compared with 6% of those in the control group. Treatment gains were maintained at 12-month follow-up. Conclusions: This new parenting-based intervention may represent an advance in the treatment of this previously neglected group. Clinical trial registration information: Anxiety in Young Children: A Randomized Controlled Trial of a New Cognitive-Behaviourally Based Parenting Intervention; http://www.isrctn.org/; ISRCTN12166762. (Contains 5 tables and 1 figure.)
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