Importance of resilient pastures for New Zealand’s agricultural soil carbon stocks

New Zealand’s agricultural pastures contain significant soil carbon (C) stocks that are susceptible to change when impacted by management and natural processes (e.g., climate). Inputs of C to these pastoral soils is through photosynthetic uptake of atmospheric CO2 either on-site or elsewhere. Changes in soil C stocks are in response to the management of the system that alters the input-output balance. Increasing the resilience of pastures to climatic events such as hot and dry summers or cool and wet winters can increase inputs of C to the soil while sustaining above-ground production and so provide an opportunity for C sequestration. Furthermore, increased pasture for grazing can reduce the need for management practices identified as detrimental for soil C stocks such as irrigation or the production of cropped supplemental feed. A reduction in the need for renewal and its associated soil C losses, and the establishment of a more diverse sward, especially if deeper-rooting species are included, has the potential for increasing soil C stocks provided the diversity can be maintained. From a soil C perspective, a resilient pasture maximises CO2 uptake to ensure adequate above- and below-ground inputs to maintain or increase soil C stocks and minimise the need for management activities detrimental to soil C.

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Integrated management of a Swiss cropland is not sufficient to preserve its soil carbon pool in the long-term

10.5194/bg-2018-205 ◽

2018 ◽

Cited By ~ 1

Author(s):

Carmen Emmel ◽

Annina Winkler ◽

Lukas Hörtnagl ◽

Andrew Revill ◽

Christof Ammann ◽

...

Keyword(s):

Soil Carbon ◽

Cover Crops ◽

Management Practices ◽

Integrated Management ◽

Co2 Uptake ◽

Soil C ◽

C Stocks ◽

Soil Measurements ◽

Carbon Dioxide Co2

Abstract. Croplands are involved in the exchange of carbon dioxide (CO2) between the atmosphere and the biosphere. Further- more, soil carbon (C) stocks play an important role in soil fertility. It is, thus, of great interest to know whether croplands act as a net source or sink of atmospheric CO2, and if soil C stocks are preserved over long timescales. The FLUXNET site CH-Oe2 in Oensingen, Switzerland has been operational since the end of 2003. This cropland is managed under the Swiss framework of the Proof of Ecological Performance (PEP, a variant of integrated management) with a crop rotation centred on winter wheat, which also includes winter barley, winter rapeseed, peas, potato and intermediate cover crops. In addition to eddy covariance measurements, meteorological and soil measurements were available along with information on C imports and exports from organic fertilisation, sowing and harvesting. This study investigates cropland C budgets over 13 years and assesses whether the PEP regulations resulted in a balanced C budget. Strongest CO2 uptake was observed during cereal seasons. C export through harvest, however, offset the strong uptake of the cereal crops. The largest net CO2 emissions to the atmosphere were observed during pea and cover crop seasons. Net biome production, representing the overall C budget, typically ranged between close to C neutral to C losses of up to 407 g C m−2 per season, with peas being the largest source. Overall, the field lost 1674 g C m−2 over thirteen years (129 g C m−2 yr−1), which was confirmed by soil C stock measurements at the beginning and the end of the study period. Although managing the field under the regulations of PEP did not result in an overall C sink, model simulations showed that the use of cover crops reduced the C losses compared to leaving the field bare. The use of solid manure improved the C budget by importing substantial amounts of C into the soil while liquid manure had only a small effect. We thus conclude that additional efforts are needed to bring Swiss management practices closer to the goal of preserving soil C in the long-term.

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Integrated management of a Swiss cropland is not sufficient to preserve its soil carbon pool in the long term

Biogeosciences ◽

10.5194/bg-15-5377-2018 ◽

2018 ◽

Vol 15 (17) ◽

pp. 5377-5393 ◽

Cited By ~ 9

Author(s):

Carmen Emmel ◽

Annina Winkler ◽

Lukas Hörtnagl ◽

Andrew Revill ◽

Christof Ammann ◽

...

Keyword(s):

Soil Carbon ◽

Cover Crops ◽

Management Practices ◽

Integrated Management ◽

Co2 Uptake ◽

Soil C ◽

C Stocks ◽

Intensively Managed ◽

Soil Measurements

Abstract. Croplands are involved in the exchange of carbon dioxide (CO2) between the atmosphere and the biosphere. Furthermore, soil carbon (C) stocks play an important role in soil fertility. It is thus of great interest to know whether intensively managed croplands act as a net source or sink of atmospheric CO2 and if soil C stocks are preserved over long timescales. The FluxNet site CH-Oe2 in Oensingen, Switzerland, has been operational since the end of 2003. This cropland is managed under the Swiss framework of the Proof of Ecological Performance (PEP, a variant of integrated management) with a crop rotation centred on winter wheat, which also includes winter barley, winter rapeseed, peas, potato and intermediate cover crops. In addition to eddy covariance measurements, meteorological and soil measurements were available along with information on C imports and exports from organic fertilisation, sowing and harvesting. This study investigates cropland C budgets over 13 years and assesses whether the PEP regulations resulted in a balanced C budget. The strongest CO2 uptake was observed during cereal seasons. C export through harvest, however, offset the strong uptake of the cereal crops. The largest net CO2 emissions to the atmosphere were observed during pea and cover crop seasons. Net biome production, representing the overall C budget (assuming carbon leaching to groundwater to be negligible), typically ranged between close to C neutral to C losses of up to 407 g C m−2 per season, with peas being the largest source. Overall, the field lost 1674 g C m−2 over 13 years (129 g C m−2 yr−1), which was confirmed by soil C stock measurements at the beginning and the end of the study period. Although managing the field under the regulations of PEP did not result in an overall C sink, model simulations showed that the use of cover crops reduced the C losses compared to leaving the field bare. The use of solid manure improved the C budget by importing substantial amounts of C into the soil, while liquid manure had only a small effect. We thus conclude that additional efforts are needed to bring Swiss management practices closer to the goal of preserving soil C in the long term.

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Carbon stocks in Tasmanian soils

Soil Research ◽

10.1071/sr11211 ◽

2012 ◽

Vol 50 (2) ◽

pp. 83 ◽

Cited By ~ 14

Author(s):

W. E. Cotching

Keyword(s):

Management Practices ◽

Agricultural Soils ◽

C Sequestration ◽

Mean Annual Temperature ◽

Soil C ◽

C Stocks ◽

Initial Soil ◽

If Current ◽

Intensive Cropping ◽

The Difference

Soil carbon (C) stocks were calculated for Tasmanian soil orders to 0.3 and 1.0 m depth from existing datasets. Tasmanian soils have C stocks of 49–117 Mg C/ha in the upper 0.3 m, with Ferrosols having the largest soil C stocks. Mean soil C stocks in agricultural soils were significantly lower under intensive cropping than under irrigated pasture. The range in soil C within soil orders indicates that it is critical to determine initial soil C stocks at individual sites and farms for C accounting and trading purposes, because the initial soil C content will determine if current or changed management practices are likely to result in soil C sequestration or emission. The distribution of C within the profile was significantly different between agricultural and forested land, with agricultural soils having two-thirds of their soil C in the upper 0.3 m, compared with half for forested soils. The difference in this proportion between agricultural and forested land was largest in Dermosols (0.72 v. 0.47). The total amount of soil C in a soil to 1.0 m depth may not change with a change in land use, but the distribution can and any change in soil C deeper in the profile might affect how soil C can be managed for sequestration. Tasmanian soil C stocks are significantly greater than those in mainland states of Australia, reflecting the lower mean annual temperature and higher precipitation in Tasmania, which result in less oxidation of soil organic matter.

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Biochar from sugarcane residues: An overview of its sequestration potential in Sao Paulo, Brazil

10.5194/egusphere-egu2020-2880 ◽

2020 ◽

Author(s):

David Lefebvre ◽

Jeroen Meersmans ◽

Guy Kirk ◽

Adrian Williams

Keyword(s):

Soil Carbon ◽

C Sequestration ◽

Sao Paulo ◽

São Paulo ◽

Soil C ◽

Paulo State ◽

Sequestration Potential ◽

C Stocks ◽

C Stock ◽

Soil C Stock

Harvesting sugarcane (Saccharum officinarum) produces large quantities of biomass residues. We investigated the potential for converting these residues into biochar (recalcitrant carbon rich material) for soil carbon (C) sequestration. We modified a version of the RothC soil carbon model to follow changes in soil C stocks considering different amounts of fresh sugarcane residues and biochar (including recalcitrant and labile biochar fractions). We used Sao Paulo State (Brazil) as a case study due to its large sugarcane production and associated soil C sequestration potential.Mechanical harvesting of sugarcane fields leaves behind > 10 t dry matter of trash (leaves) ha-1 year-1. Although trash blanketing increases soil fertility, an excessive amount is detrimental and reduces the subsequent crop yield. After the optimal trash blanketing amount, sugarcane cultivation still produces 5.9 t C ha-1 year-1 of excess trash and bagasse (processing residues) which are available for subsequent use.The available residues could produce 2.5 t of slow-pyrolysis (550&#176;C) biochar C ha-1 year-1. The model predicts this could increase sugarcane field soil C stock on average by 2.4 &#177; 0.4 t C ha&#8209;1 year&#8209;1, after accounting for the climate and soil type variability across the State. Comparing different scenarios, we found that applying fresh residues into the field results in a smaller increase in soil C stock compared to the biochar because the soil C approaches a new equilibrium. For instance, adding 1.2 t of biochar C ha&#8209;1 year&#8209;1 along with 3.2 t of fresh residue C ha&#8209;1 year&#8209;1 increased the soil C stock by 1.8 t C ha&#8209;1 year&#8209;1 after 10 years of repeated applications. In contrast, adding 0.62 t of biochar C ha&#8209;1 year&#8209;1 with 4.5 t of fresh sugarcane residues C ha&#8209;1 year&#8209;1 increased the soil carbon soil stock by 1.4 t C ha&#8209;1 year&#8209;1 after 10 years of application. These are reductions 25% and 40% of the potential soil C accumulation rates compared with applying available residues as biochar.&#160; &#160;We also tested the sensitivity of the model to biochar-induced positive priming (i.e. increased mineralization of soil organic C) using published values. This showed that the C sequestration balance remains positive over the long term, even considering an extremely high positive-priming factor. Upscaling our results to the total 5 Mha of sugarcane in Sao Paulo State, biochar application could sequester up to 50 Mt of CO2 equivalent per year, representing 31% of the emissions attributed to the State in 2016.This study provides first insights into the sequestration potential of biochar application on sugarcane fields. Measurements of changes in soil C stocks in sugarcane field experiments are needed to further validate the model, and the emissions to implement the practice at large scale need to be taken into account. As the climate crisis grows, the need for greenhouse gas removal technologies becomes crucial. Assessing the net effectiveness of readily available technologies is essential to guide policy makers.&#160;&#160;

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Vinasse application and cessation of burning in sugarcane management can have positive impact on soil carbon stocks

PeerJ ◽

10.7717/peerj.5398 ◽

2018 ◽

Vol 6 ◽

pp. e5398 ◽

Cited By ~ 1

Author(s):

Caio F. Zani ◽

Arlete S. Barneze ◽

Andy D. Robertson ◽

Aidan M. Keith ◽

Carlos E.P. Cerri ◽

...

Keyword(s):

Soil Carbon ◽

Greenhouse Gas ◽

Management Practices ◽

Positive Impact ◽

Clay Fraction ◽

Bioenergy Crops ◽

Soil C ◽

Sequestration Potential ◽

C Stocks ◽

Management Changes

Bioenergy crops, such as sugarcane, have the potential to mitigate greenhouse gas emissions through fossil fuel substitution. However, increased sugarcane propagation and recent management changes have raised concerns that these practices may deplete soil carbon (C) stocks, thereby limiting the net greenhouse gas benefit. In this study, we use both a measured and modelled approach to evaluate the impacts of two common sugarcane management practices on soil C sequestration potential in Brazil. We explore how transitions from conventional (mineral fertiliser/burning) to improved (vinasse application/unburned) practices influence soil C stocks in total and in physically fractionated soil down to one metre. Results suggest that vinasse application leads to an accumulation of soil C of 0.55 Mg ha−1yr−1 at 0–30 cm depth and applying unburned management led to gains of ∼0.7 Mg ha−1yr−1 at 30–60 cm depth. Soil C concentration in the Silt+Clay fraction of topsoil (0–20 cm) showed higher C content in unburned management but it did not differ under vinasse application. The CENTURY model was used to simulate the consequences of management changes beyond the temporal extent of the measurements. Simulations indicated that vinasse was not the key factor driving increases in soil C stocks but its application may be the most readily available practice to prevent the soil C losses under burned management. Furthermore, cessation of burning may increase topsoil C by 40% after ∼50 years. These are the first data comparing different sugarcane management transitions within a single area. Our findings indicate that both vinasse application and the cessation of burning can play an important role in reducing the time required for sugarcane ethanol production to reach a net C benefit (payback time).

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CARBON STOCKS, LITTERFALL AND PRUNING RESIDUES IN MONOCULTURE AND AGROFORESTRY CACAO PRODUCTION SYSTEMS

Experimental Agriculture ◽

10.1017/s001447971800011x ◽

2018 ◽

Vol 55 (3) ◽

pp. 452-470 ◽

Cited By ~ 4

Author(s):

ULF SCHNEIDEWIND ◽

WIEBKE NIETHER ◽

LAURA ARMENGOT ◽

MONIKA SCHNEIDER ◽

DANIELA SAUER ◽

...

Keyword(s):

Management Practices ◽

Stand Structure ◽

Production Systems ◽

Plant Biomass ◽

Farming Systems ◽

C Sequestration ◽

Agroforestry Systems ◽

Soil C ◽

C Stocks ◽

Pruning Residues

SUMMARYAgroforestry systems (AFS) can serve to decrease ecosystem carbon (C) losses caused by deforestation and inadequate soil management. Because of their shade tolerance, cacao plants are suitable to be grown in AFS, since they can be combined with other kinds of trees and shrubs. The potential for C sequestration in cacao farming systems depends on various factors, such as management practices, stand structure and plantation age. We compared conventionally and organically managed cacao monoculture systems (MCS) and AFS in Sara Ana (Bolivia) with respect to C stocks in plant biomass and to amounts of litterfall and pruning residues. The total aboveground C stocks of the AFS (26 Mg C ha−1) considerably exceeded those of the MCS (~7 Mg C ha−1), although the biomass of cacao trees was greater in the MCS compared to the AFS. Due to higher tree density, annual litterfall in the AFS (2.2 Mg C ha−1 year−1) substantially exceeded that in the MCS (1.2 Mg C ha−1 year−1). The amounts of C in pruning residues (2.6 Mg C ha−1 year−1 in MCS to 4.3 Mg C ha−1 year−1 in AFS) was more than twice those in the litterfall. Annual nitrogen (N) inputs to the soil through pruning residues of cacao and N-fixing trees were up to 10 times higher than the N inputs through external fertiliser application. We conclude that appropriate management of cacao AFS, involving the pruning of leguminous trees, will lead to increases in biomass, litter quantity and quality as well as soil C and N stocks. Thus, we recommend stimulating the expansion of well-managed AFS to improve soil fertility and enhance C sequestration in soils.

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The potential for full inversion tillage to increase soil carbon storage following pasture renewal in New Zealand

10.5194/egusphere-egu2020-13082 ◽

2020 ◽

Author(s):

Mike Beare ◽

Erin Lawrence-Smith ◽

Denis Curtin ◽

Sam McNally ◽

Frank Kelliher ◽

...

Keyword(s):

New Zealand ◽

Management Practices ◽

Management Practice ◽

Mineral Soil ◽

C Sequestration ◽

Atmospheric Concentration ◽

Soil C ◽

C Stocks ◽

Soil C Sequestration ◽

Accumulation Efficiency

The global atmospheric concentration of CO2 and other greenhouse gases (GHG) is steadily increasing. It is estimated that, worldwide, soil C sequestration could offset GHG emissions by 400&#8211;1200 Mt C per year. Relative to 1990, New Zealand&#8217;s CH4 and N2O emissions in 2013 had increased by 7% and 23% respectively, which translates to an annual emission increase of 1.09 Mt C that could be offset by a similar annual increase in soil C stock. Recent research has shown that some New Zealand pastoral soils are under-saturated in SOC. Subsurface soils (15&#8211;30 cm depth) typically have a greater soil C saturation deficit than topsoil (0-30 cm) because plant C inputs (roots) are lower. Using management practices that expose more of the under-saturated soil to higher C inputs could result in increased soil C storage and stabilisation.Pasture renewal (destruction and re-establishment of pasture) is promoted to livestock farmers to improve pasture performance. This typically involves shallow cultivation or direct drilling to establish new grass. Whereas shallow cultivation of soil typically results in a loss of SOC, deeper full inversion tillage (FIT) of soil would result in the burial of C-rich topsoil in closer proximity to mineral material that has a higher stabilisation capacity.&#160; Buried SOC is expected to have a slower decomposition rate owing to less variable temperatures and more anoxic conditions. Deep FIT would also bring under-saturated mineral soil to the surface, where the deposition of SOC from high producing pastures could increase the stabilisation of SOC.&#160; Both the slower turnover of buried SOM and greater stabilisation of new carbon on under-saturated minerals at the soil surface are expected to result in increased SOC sequestration. There is a lack of experimental data to directly address the effect of FIT on soil C stocks in pastoral soils. We applied a simple empirical model to predicting changes in soil C stocks following a one-off application of FIT (30 cm) during pasture renewal. The model accounts for the decomposition of SOC in buried topsoil and the accumulation of C in the new topsoil (inverted subsoil). The model was used to derive national estimates of soil C sequestration under different scenarios of C accumulation efficiency, farmer adoption of FIT and pasture renewal rates.Our modelled estimates suggest that 32 Mt C could be sequestered over 20 years following a one-time application of FIT (0-30 cm) to 2 M ha of High Producing Grasslands on suitable New Zealand soils. This estimate is based on 100% accumulation efficiency (i.e. topsoil C stocks are returned to pre-inversion levels within 20 years) and a 10% annual rate of pasture renewal. In the absence of direct experimental evidence, a more conservative estimate is warranted, where topsoil C stocks are projected to return to 80% of pre-inversion levels, thus sequestering 20 Mt C. This paper will present our modelled estimates of SOC sequestration during FIT pasture renewal and discuss the potential benefits and adverse effects of deploying this management practice.

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Linking Vegetation, Soil Carbon Stocks, and Earthworms in Upland Coniferous–Broadleaf Forests

Forests ◽

10.3390/f12091179 ◽

2021 ◽

Vol 12 (9) ◽

pp. 1179

Author(s):

Anastasiia I. Kuznetsova ◽

Anna P. Geraskina ◽

Natalia V. Lukina ◽

Vadim E. Smirnov ◽

Elena V. Tikhonova ◽

...

Keyword(s):

Soil Carbon ◽

Carbon Stocks ◽

Mineral Layer ◽

Forest Zone ◽

Soil C ◽

C Stocks ◽

European Part Of Russia ◽

Organic Horizons ◽

Soil Carbon Stocks ◽

The Impact

Linking vegetation, soil biota, and soil carbon stocks in forests has a high predictive value. The specific aim of this study was to identify the relationships between vegetation, earthworms, and soil carbon stocks in nine types of forests dominating autonomous landscape positions in a coniferous–broadleaf forest zone of the European part of Russia. Mountain forests were selected in the Northwest Caucasus, while plain forests were selected in Bryansk Polesie and on the Moskva-Oka plain. One-way analysis of variance (ANOVA) and v-tests were used to assess the impact of different factors on soil C stocks. To assess the contribution of vegetation, litter quality, and earthworms to variation of carbon stocks in organic (FH-layer) and mineral layer (0–50 cm), the method of hierarchical partitioning was performed. The highest C stocks in the organic horizons were associated with the low-quality litter, i.e., with a low base saturation, high acidity, and wide C/N ratio. The highest soil C stocks in the mineral layers were found in mixed forests with the highest richness of plant species, producing litterfall of different quality. The С stock in the organic horizon was negatively related to the biomass of worms that process the litter, while the carbon stock in the mineral layers was positively related to the biomass of worms whose life activity is related to the mineral layers. These findings demonstrated the substantial influence of plants producing a litter of different quality, and of earthworms, belonging to different functional groups, on soil С stocks in coniferous–broadleaf forests.

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Potential contribution by cotton roots to soil carbon stocks in irrigated Vertosols

Soil Research ◽

10.1071/sr08180 ◽

2009 ◽

Vol 47 (3) ◽

pp. 243 ◽

Cited By ~ 11

Author(s):

N. R. Hulugalle ◽

T. B. Weaver ◽

L. A. Finlay ◽

N. W. Luelf ◽

D. K. Y. Tan

Keyword(s):

Root Growth ◽

Soil Carbon ◽

Crop Residues ◽

Climatic Factors ◽

Carbon Stocks ◽

Potential Contribution ◽

Soil C ◽

C Stocks ◽

Root Death ◽

Soil Carbon Stocks

The well-documented decline in soil organic carbon (SOC) stocks in Australian cotton (Gossypium hirsutum L.) growing Vertosols has been primarily analysed in terms of inputs from above-ground crop residues, with addition to soil C by root materials being little studied. Potential contribution by cotton roots to soil carbon stocks was evaluated between 2002 and 2008 in 2 ongoing long-term experiments near Narrabri, north-western New South Wales. Experiment 1 consisted of cotton monoculture sown either after conventional tillage or on permanent beds, and a cotton–wheat (Triticum aestivum L.) rotation on permanent beds; Experiment 2 consisted of 4 cotton-based rotation systems sown on permanent beds: cotton monoculture, cotton–vetch (Vicia villosa Roth.), cotton–wheat, and cotton–wheat–vetch. Roundup-Ready™ (genetically modified) cotton varieties were sown until 2005, and Bollgard™ II-Roundup Ready™-Flex™ varieties thereafter. Root growth in the surface 0.10 m was measured with the core-break method using 0.10-m-diameter cores. A subsample of these cores was used to evaluate relative root length and root C concentrations. Root growth in the 0.10–1.0 m depth was measured at 0.10-m depth intervals with a ‘Bartz’ BTC-2 minirhizotron video microscope and I-CAP image capture system (‘minirhizotron’). The video camera was inserted into clear, plastic acrylic minirhizotron tubes (50-mm-diameter) installed within each plot, 30° from the vertical. Root images were captured 4–5 times each season in 2 orientations, left and right side of each tube, adjacent to a furrow, at each time of measurement and the images analysed to estimate selected root growth indices. The indices evaluated were the length and number of live roots at each time of measurement, number of roots which changed length, number and length of roots which died (i.e. disappeared between times of measurement), new roots initiated between times of measurement, and net change in root numbers and length. These measurements were used to derive root C turnover between times of measurements, root C added to soil through intra-seasonal root death, C in roots remaining at end of season, and the sum of the last 2 indices: root C potentially available for addition to soil C stocks. Total seasonal cotton root C potentially available for addition to soil C stocks ranged between ~50 and 400 g/m2 (0.5 and 4 t/ha), with intra-seasonal root death contributing 25–70%. These values are ~10–60% of that contributed by above-ground crop residues. As soil organic carbon in irrigated Vertosols can range between 40 and 60 t/ha, it is unlikely that cotton roots will contribute significantly to soil carbon stocks in irrigated cotton farming systems. Seasonal root C was reduced by cotton monoculture, stress caused by high insect numbers, and sowing Bollgard II varieties; and increased by sowing non-Bollgard II varieties and wheat rotation crops. Permanent beds increased root C but leguminous rotation crops did not. Climatic factors such as cumulative day-degrees and seasonal rainfall were positively related to seasonal root C. Root C turnover was, in general, highest during later vegetative/early reproductive growth. Large variations in root C turnover and seasonal C indices occurred due to a combination of environmental, management and climatic factors.

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Nitrogen deposition accelerates soil carbon sequestration in tropical forests

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2020790118 ◽

2021 ◽

Vol 118 (16) ◽

pp. e2020790118

Author(s):

Xiankai Lu ◽

Peter M. Vitousek ◽

Qinggong Mao ◽

Frank S. Gilliam ◽

Yiqi Luo ◽

...

Keyword(s):

Tropical Forests ◽

C Sequestration ◽

N Deposition ◽

Sink Strength ◽

Soil C ◽

C Cycle ◽

C Stocks ◽

Below Ground ◽

Soil C Sequestration ◽

N Additions

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

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