Soil organic carbon (SOC) dynamics are regulated by the complex interplay of climatic, edaphic and biotic conditions. However, the interrelation of SOC and these drivers and their potential ...connection networks are rarely assessed quantitatively. Using observations of SOC dynamics with detailed soil properties from 90 field trials at 28 sites under different agroecosystems across the Australian cropping regions, we investigated the direct and indirect effects of climate, soil properties, carbon (C) inputs and soil C pools (a total of 17 variables) on SOC change rate (rC, Mg C ha−1 yr−1). Among these variables, we found that the most influential variables on rC were the average C input amount and annual precipitation, and the total SOC stock at the beginning of the trials. Overall, C inputs (including C input amount and pasture frequency in the crop rotation system) accounted for 27% of the relative influence on rC, followed by climate 25% (including precipitation and temperature), soil C pools 24% (including pool size and composition) and soil properties (such as cation exchange capacity, clay content, bulk density) 24%. Path analysis identified a network of intercorrelations of climate, soil properties, C inputs and soil C pools in determining rC. The direct correlation of rC with climate was significantly weakened if removing the effects of soil properties and C pools, and vice versa. These results reveal the relative importance of climate, soil properties, C inputs and C pools and their complex interconnections in regulating SOC dynamics. Ignorance of the impact of changes in soil properties, C pool composition and C input (quantity and quality) on SOC dynamics is likely one of the main sources of uncertainty in SOC predictions from the process‐based SOC models.
We quantitatively assessed the interconnections of a series of climatic, edaphic and biotic factors in controlling soil organic carbon (SOC) dynamics. We found that, overall, soil properties, soil C pools, climate and C inputs were almost equally important in terms of overall effect on SOC dynamics. However, if the effect of soil was controlled, the effect of climate was significantly weakened, and vice versa, while the effects of carbon input and soil carbon pool composition were significantly strengthened if climate and/or soil were controlled. These results force us to revise SOC predictions based on modelling and empirical approaches mainly considering climatic factors, and provide new insights into mechanistic understanding of SOC dynamics under different soil and climate conditions across ecosystems.
Soil organic carbon (SOC) in the subsoil below 0.3 m accounts for the majority of total SOC and may be as sensitive to climate change as topsoil SOC. Here we map global SOC turnover times (τ) in the ...subsoil layer at 1 km resolution using observational databases. Global mean τ is estimated to be Formula: see text yr (mean with 95% confidence interval), and deserts and tundra show the shortest (Formula: see text yr) and longest (Formula: see text yr) τ respectively. Across the globe, mean τ ranges from 9 (the 5% quantile) to 6332 years (the 95% quantile). Temperature is the most important factor negatively affecting τ, but the overall effect of climate (including temperature and precipitation) is secondary compared with the overall effect of assessed soil properties (e.g., soil texture and pH). The high-resolution mapping of τ and the quantification of its controls provide a benchmark for diagnosing subsoil SOC dynamics under climate change.
Our meta-analysis based on data from 69 paired-experiments indicated that: ▶ Cultivation with conventional tillage (CT) and no-tillage (NT) resulted in comparable soil C loss comparing with adjacent ...natural soils. ▶ In most cases, adopting NT did not increase the total C in the soil profile. ▶ Climatic conditions and fertilization did not significantly regulate the response of soil C to the adoption of NT. ▶ However, the type of crops and cropping systems caused variations in soil C change after adopting NT. Compared with CT, NT in double cropping systems significantly increased soil total C, while NT in single cropping systems with more diverse crop types could lead to net reduction in total soil C.
Adopting no-tillage in agro-ecosystems has been widely recommended as a means of enhancing carbon (C) sequestration in soils. However, study results are inconsistent and varying from significant increase to significant decrease. It is unclear whether this variability is caused by environmental, or management factors or by sampling errors and analysis methodology. Using meta-analysis, we assessed the response of soil organic carbon (SOC) to conversion of management practice from conventional tillage (CT) to no-tillage (NT) based on global data from 69 paired-experiments, where soil sampling extended deeper than 40
cm. We found that cultivation of natural soils for more than 5 years, on average, resulted in soil C loss of more than 20
t
ha
−1, with no significant difference between CT and NT. Conversion from CT to NT changed distribution of C in the soil profile significantly, but did not increase the total SOC except in double cropping systems. After adopting NT, soil C increased by 3.15
±
2.42
t
ha
−1 (mean
±
95% confidence interval) in the surface 10
cm of soil, but declined by 3.30
±
1.61
t
ha
−1 in the 20–40
cm soil layer. Overall, adopting NT did not enhance soil total C stock down to 40
cm. Increased number of crop species in rotation resulted in less C accumulation in the surface soil and greater C loss in deeper layer. Increased crop frequency seemed to have the opposite effect and significantly increased soil C by 11% in the 0–60
cm soil. Neither mean annual temperature and mean annual rainfall nor nitrogen fertilization and duration of adopting NT affected the response of soil C stock to the adoption of NT. Our results highlight that the role of adopting NT in sequestrating C is greatly regulated by cropping systems. Increasing cropping frequency might be a more efficient strategy to sequester C in agro-ecosystems. More information on the effects of increasing crop species and frequency on soil C input and decomposition processes is needed to further our understanding on the potential ability of C sequestration in agricultural soils.
Priming of soil organic matter decomposition by fresh carbon inputs is a key ecological process determining soil carbon (C) and nutrient cycling in terrestrial ecosystems. Although this priming ...effect (PE) has been studied under various environmental conditions, the conclusions are inconsistent across space and time and the underlying mechanisms unclear. We used a meta-analysis with extensive datasets of CO2 effluxes from soils with 13C or 14C labelled fresh C inputs and without fresh C inputs under various soil conditions to synthesize and assess the temporal dynamics of the PE. The results indicated that the PE declined in 20 days on average from 67−21+26% (95% confidence interval) immediately following the fresh C inputs to less than 7.6−1.8+2.0% and remained relatively stable thereafter. We also assessed the variability of the temporal dynamics of the PE in the collected datasets and the underlying drivers. The results showed that the magnitude of PE at a specific time (i.e., the instantaneous PE after the fresh C inputs) was significantly and positively correlated with the instantaneous quantity of remaining fresh C. Under the same quantity of remaining fresh C, the PE varied significantly across ecosystems (in the order of grasslands < farmlands < forests < other ecosystems such as lake beds and volcanic soils), but, contrary to our expectation, the PE was independent of the quality of the added fresh C. We found that the PE experienced a faster decrease in soils with higher clay and moisture contents. These results describe the temporal dynamics of PE and the underlying drivers, underpinning the robust predictions of PE dynamics and their impact on soil C and nutrient balances.
•The datasets from 171 experiments were used to assess the temporal PE dynamics.•The average PE declines from 67% to <10% in 20 days after fresh C inputs.•The quantity not the quality of fresh C significantly affects the PE temporal dynamics.•The PE varies significantly across ecosystems under certain fresh C inputs.•The PE decreases faster with time in soils with higher clay and moisture content.
Soil is the largest reservoir of carbon (C) in the terrestrial biosphere and a slight variation in this pool could lead to substantial changes in the atmospheric CO
2 concentration, thus impact ...significantly on the global climate. Cultivation of natural ecosystems has led to marked decline in soil C storage, such that conservation agricultural practices (CAPs) are widely recommended as options to increase soil C storage, thereby mitigating climate change. In this review, we summarise soil C change as a result of cultivation worldwide and in Australia. We then combine the available data to examine the effects of adopting CAPs on soil C dynamics in Australian agro-ecosystems. Finally, we discuss the future research priorities related to soil C dynamics. The available data show that in Australian agro-ecosystems, cultivation has led to C loss for more than 40
years, with a total C loss of approximately 51% in the surface 0.1
m of soil. Adoption of CAPs generally increased soil C. Introducing perennial plants into rotation had the greatest potential to increase soil C by 18% compared with other CAPs. However, the same CAPs could result in different outcomes on soil C under different climate and soil combinations. No consistent trend of increase in soil C was found with the duration of CAP applications, implying that questions remain regarding long-term impact of CAPs. Most of the available data in Australia are limited to the surface 0.1 to 0.3
m of soil. Efforts are needed to investigate soil C change in deeper soil layers in order to understand the impact of crop root growth and various agricultural practices on C distribution in soil profile. Elevated atmospheric CO
2 concentration, global warming and rainfall change could all alter the C balance of agricultural soils. Because of the complexity of soil C response to management and environmental factors, a system modelling approach supported by sound experimental data would provide the most effective means to analyse the impact of different management practices and future climate change on soil C dynamics.
Abstract
Soil organic carbon (SOC) changes under future climate warming are difficult to quantify in situ. Here we apply an innovative approach combining space-for-time substitution with ...meta-analysis to SOC measurements in 113,013 soil profiles across the globe to estimate the effect of future climate warming on steady-state SOC stocks. We find that SOC stock will reduce by 6.0 ± 1.6% (mean±95% confidence interval), 4.8 ± 2.3% and 1.3 ± 4.0% at 0–0.3, 0.3–1 and 1–2 m soil depths, respectively, under 1 °C air warming, with additional 4.2%, 2.2% and 1.4% losses per every additional 1 °C warming, respectively. The largest proportional SOC losses occur in boreal forests. Existing SOC level is the predominant determinant of the spatial variability of SOC changes with higher percentage losses in SOC-rich soils. Our work demonstrates that warming induces more proportional SOC losses in topsoil than in subsoil, particularly from high-latitudinal SOC-rich systems.
Changes in litter and nutrient inputs into soil could have significant consequences on forest carbon (C) dynamics via controls on the structure and microbial utilization of soil organic C (SOC). In ...this study, we assessed changes in physical fractions (250–2000 μm, 53–250 μm, and < 53 μm soil aggregates) and chemical fractions (labile, intermediate and recalcitrant pools) of SOC in the top 20 cm mineral soil layer and their influences on microbial substrate utilization after eight years of experiment in a mixed pine-oak forest. The litter treatments included: control (L
con
), litter removal (L
nil
), fine woody litter addition (L
woody
), leaf litter addition (L
leaf
) and a mix of leaf and fine woody litter (L
mix
). Nitrogen (N) addition (application rates of 0, 5 and 10 g N m
−2
year
−1
, respectively) was also applied. We found that complete removal of forest-floor litter (L
nil
) significantly reduced the pool sizes of all SOC fractions in both the physical and chemical fractions compared with treatments that retained either leaf litter (L
leaf
) or mixture of leaves and fine woody materials (L
mix
). The type of litter was more important in affecting SOC fractions than the quantity of inputs; neither the level of N addition rate nor its interaction with litter treatment had significant effects on both physical and chemical SOC fractions. Microbial respiration differed significantly among the treatments of varying litter types. However, the effectiveness of microbial C utilization inferred by microbial C use efficiency and biomass-specific respiration was not affected by either the litter treatments or N addition. These results suggest that despite significant changes in SOC composition due to long-term treatments of forest-floor litter and N addition in this mixed pine-oak forest of temperate climate, microbial C utilization strategies remain unaffected.
•Microbial community diversity and composition are sensitive to temperature.•Instantaneous microbial community is a poor predictor of SOC mineralization.•Microbial community is a proxy of substrate ...quality and availability.•Substate quality and availability are predominant regulators of SOC mineralization.
Soil microbes drive soil organic carbon (SOC) mineralization. Because microbial groups differ in metabolic efficiency and respond differently to temperature variation, it is reasonable to expect a close association of SOC mineralization and its temperature sensitivity (Q10 which is defined as the factor of the change of soil carbon mineralization induced by 10 °C temperature increase) with microbial community diversity and composition. However, these relations have rarely been tested. Here, we conducted an incubation experiment to assess the temperature responses of microbial α diversity and the relative abundance of microbial r- and K-strategists in soils from a wide range of ecosystems across a climate gradient in the southeast Tibet. The results indicated that the instantaneous α diversity and the relative abundance of r- and K-strategists are significantly (P < 0.05) influenced by temperature, but these microbial variables are poor predictors of SOC mineralization measured at the same time. Rather, microbial community diversity and the relative abundance of r- and K-strategists of fresh soils showed consistent and significant (P < 0.05) effects on both SOC mineralization and Q10 at different incubation stages. Importantly, path analysis indicated that microbial α diversity and r- and K-strategists exerts no independent effects on SOC mineralization and Q10 when variation in climate, SOC chemistry, physical protection, and edaphic properties are accounted for. Together, our results suggest that while soil microbial community diversity and composition are a strong proxy of SOC quality and availability, they are not a fundamental determinant of SOC mineralization and Q10.
Purpose
Regional values for prospective soil organic carbon (SOC) change in Australian cropland were derived via state-of-the-art modelling. This paper evaluates the applicability of the results in ...the context of life cycle assessment (LCA).
Methods
Results of soil carbon modelling need to align with LCA requirements in order to be applicable. The following aspects were investigated in more detail: effect of SOC and variability on product carbon footprint results, data symmetry and consistency, attribution of the SOC change to activity and allocation of the SOC change to crops in rotations.
Results and discussion
Results show that greenhouse gas (GHG) emissions or removals associated with SOC change even in the absence of recent land use change or land management change can potentially change Australian crop carbon footprints considerably. Over a modelling period of 62 years, the SOC continues to change. In attributional LCA, issues with attribution, allocation and data symmetry of the SOC change values are complex. Without a comprehensive understanding of the causal link between individual crops and pasture in a rotation and the change in SOC, and without a SOC change figure for an appropriate reference baseline, application in attributional LCA is limited to certain types of studies only. In consequential LCA, the data symmetry issues as well as the need for allocation and attribution can be avoided.
Conclusions
The SOC change results cannot be allocated to individual crops and are therefore valid only for a full rotation cycle. This means results may be applied in attributional LCA but only in certain contexts. The main applicability is foreseen as a business-as-usual baseline for consequential LCA. A set of SOC change values will be derived for this purpose and made available with accompanying guidance for use and interpretation.
Increases in carbon (C) inputs to soil can replenish soil organic C (SOC) through various mechanisms. However, recent studies have suggested that the increased C input can also stimulate the ...decomposition of old SOC via priming. Whether the loss of old SOC by priming can override C replenishment has not been rigorously examined. Here we show, through data-model synthesis, that the magnitude of replenishment is greater than that of priming, resulting in a net increase in SOC by a mean of 32% of the added new C. The magnitude of the net increase in SOC is positively correlated with the nitrogen-to-C ratio of the added substrates. Additionally, model evaluation indicates that a two-pool interactive model is a parsimonious model to represent the SOC decomposition with priming and replenishment. Our findings suggest that increasing C input to soils likely promote SOC accumulation despite the enhanced decomposition of old C via priming.
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