Erosional redistribution of topsoil controls soil nitrogen dynamics

In recent years, the role of soil erosion on terrestrial carbon sequestration had been the focus of a growing number of studies. However, relatively little attention has been paid so far to the role of erosion on the lateral distribution of soil nitrogen (N) and the role of geomorphic processes on soil N dynamics. Here, we present primary data on the stock of nitrogen in soil and its rate of erosion at a relatively undisturbed, zero-order watershed in northern California. Erosion transports 0.26–0.47 g N m^?2 year^?1 from eroding slope positions (Summit and Slope), and about two-thirds of the eroded N enters depositional landform positions (Hollow and Plain). Our results show that depositional-position soil profiles contain up to 3 times more N than soil profiles in the eroding positions. More than 92% of all soil nitrogen was chemically bound to soil minerals in all the landform positions, compared to 2–4% each found in the free light and occluded light fractions. Nitrogen associated with the free light fraction in topsoil is particularly susceptible to loss by soil erosion. By comparison, soil N associated with the aggregate-protected occluded light fractions and the mineral-associated dense fractions is likely to be protected from gaseous and dissolved losses. On average, we found that soil N has mean residence time of 694 years in eroding landform positions, compared to 2951 years in depositional landform positions. Our results also show that microbial processing of organic matter exerts strong control on overall soil N storage and N stabilized through sorptive interactions with soil minerals only in poorly drained depositional landform positions. Soil erosion exerts important control on stock, distribution, and long-term fate of soil N in dynamic landscapes.     

Soil C erosion and burial in cropland

Erosion influences the lateral and vertical distribution of soil in agricultural landscapes. A better understanding of the effects of erosion and redistribution on soil organic carbon (C) within croplands would improve our knowledge of how management practices may affect global C dynamics. In this study, the vertical and lateral distribution of soil organic C was characterized to evaluate the amounts and timescales of soil organic C movement, deposition and burial over the last 50 years in different agroecosystems across Canada. There was strong evidence that a substantial portion of eroded sediment and soil organic C was deposited as colluvium close to its source area, thereby burying the original topsoil. The deepest aggraded profile was in a potato field and contained over 70 cm of deposited soil indicating an accumulation rate of 152 Mg ha yr^?1; aggraded profiles in other sites had soil deposition rates of 40–90 Mg ha^?1 yr^?1. The largest stock of soil organic C was 463 Mg ha^?1 (to 60 cm depth) and soil C deposition ranged from about 2 to 4 Mg ha^?1 yr^?1 across all sites. A distinct feature observed in the aggraded profiles at every site was the presence of a large increase in soil organic C concentration near the bottom of the A horizon; the concentration of this C was greater than that at the soil surface. Compared to aggraded profiles, the SOC concentration in eroded profiles did not differ with depth, suggesting that dynamic replacement of soil organic C had occurred in eroded soils. A large amount of soil organic C is buried in depositional areas of Canadian croplands; mineralization of this stock of C appears to have been constrained since burial, but it may be vulnerable to future loss by management practices, land use change and a warming climate.     

Stocks and dynamics of SOC in relation to soil redistribution by water and tillage erosion

Soil organic carbon (SOC) displaced by soil erosion is the subject of much current research and the fundamental question, whether accelerated soil erosion is a source or sink of atmospheric CO₂, remains unresolved. A toposequence of terraced fields as well as a long slope was selected from hilly areas of the Sichuan Basin, China to determine effects of soil redistribution rates and processes on SOC stocks and dynamics. Soil samples for the determination of caesium‐137 (¹³⁷Cs), SOC, total N and soil particle size fractions were collected at 5 m intervals along a transect down the two toposequences. ¹³⁷Cs data showed that along the long slope transect soil erosion occurred in upper and middle slope positions and soil deposition appeared in the lower part of the slope. Along the terraced transect, soil was lost over the upper parts of the slopes and deposition occurred towards the downslope boundary on each terrace, resulting in very abrupt changes in soil redistribution over short distances either side of terrace boundaries that run parallel with the contour on the steep slopes. These data reflect a difference in erosion process; along the long slope transect, water erosion is the dominant process, while in the terraced landscape soil distribution is mainly the result of tillage erosion. SOC inventories (mass per unit area) show a similar pattern to the ¹³⁷Cs inventory, with relatively low SOC content in the erosional sites and high SOC content in depositional areas. However, in the terraced field landscape C/N ratios were highest in the depositional areas, while along the long slope transect, C/N ratios were highest in the erosional areas. When the samples are subdivided based on ¹³⁷Cs‐derived erosion and deposition data, it is found that the erosional areas have similar C/N ratios for both toposequences, while the C/N ratios in depositional areas are significantly different from each other. These differences are attributed to the difference in soil erosion processes; tillage erosion is mainly responsible for high‐SOC inventories at depositional positions on terraced fields, whereas water erosion plays a primary role in SOC storage at depositional positions on the long slope. These data support the theory that water erosion may cause a loss of SOC due to selective removal of the most labile fraction of SOC, while on the other hand tillage erosion only transports the soil over short distances with less effect on the total SOC stock.     

Influence of soil aggregation on SOC sequestration: A preliminary model of SOC protection by aggregate dynamics

Aggregation dynamics and soil organic carbon (SOC) fractions collected from long-term tillage trials at two sites in Illinois were used to develop a model to simulate the aggregate dynamics and physical protection of SOC. We used two litter pools which are surface litter and root litter and three SOC pools which are directly measurable from the fractionation: loose particulate organic matter (LPOM), aggregate-occluded particulate organic matter (OPOM), and humified fractions (HF). Decay rates of all of five pools were modified by soil temperature and moisture. In the model, the decay rate of LPOM was not influenced by any type of physical protection and the OPOM decay rate was influenced by dry aggregate mean weight diameter (DMWD) size. The effect of DMWD on OPOM decay rate was expressed as logistic equation based on the threshold value beyond which OPOM decay rate was influenced by the reactive mass concept which is that it is primarily outer layer of aggregates that participates in chemical and biological reactions. The decay of HF was influenced by clay contents. The relative aggregate turnover modified the humification coefficients. The faster aggregate turnover speeded the carbon transfer from LPOM to OPOM by providing more chances for organic matter to be incorporated with macroaggregates and retarded carbon transfer from OPOM to HF due to the fact that there is not enough time for organic mater to be associated with microaggregates and clay particles. Simulated results were compared against actual SOC fraction contents obtained from two long-term tillage trials located in Illinois, DeKalb (silty clay loam) and Monmouth (silt loam). Both actual and simulated data showed that after 10 and 17 years of no tillage (NT) practice adoption, OPOM content was increased at the surface in Monmouth and HF content was increased at the surface in DeKalb. Agreement between the output of aggregate dynamics-based model and actual data suggested that DMWD size, relative aggregate turnover, and their interaction with soil moisture and clay contents can be used to predict the inconsistent effects of tillage practices on SOC sequestration.     

Net changes of soil C stocks in two grassland soils 26 months after simulated pasture renovation including biochar addition

The use of deep‐rooting pasture species as a management practice can increase the allocation of plant carbon (C) below ground and enhance C storage. A 2‐year lysimeter trial was set up to compare changes in C stocks of soils under either deep‐ or shallow‐rooting pastures and investigate whether biochar addition below the top 10 cm could promote root growth at depth. For this i) soil ploughing at cultivation was simulated in a silt loam soil and in a sandy soil by inverting the 0 to 10 and 10‐ to 20‐cm‐depth soil layers, and a distinctive biochar (selected for each soil to overcome soil‐specific plant growth limitations) was mixed at 10 Mg ha^?1 in the buried layer, where appropriate and ii) three pasture types with contrasting root systems were grown. In the silt loam, soil inversion resulted in a general loss of C (2.0–8.1 Mg ha^?1), particularly in the buried horizon, under shallow‐rooting pastures only. The addition of a C‐rich biochar (equivalent to 7.6 Mg C ha^?1) to this soil resulted in a net C gain (21–40% over the non‐biochar treatment, P < 0.10) in the buried layer under all pastures; this overcame the loss of C in this horizon under shallow‐rooting pastures. In the sandy soil, all pastures were able to maintain soil C stocks at 10–20 cm depth over time, with minor gains of C (1.6–5.1 Mg ha^?1) for the profile. In this soil, the exposure of a skeletal‐ and nutrient‐depleted soil layer at the surface may have fostered root growth at depth. The addition of a nutrient‐rich biochar (equivalent to 3.6 Mg C ha^?1) to this soil had no apparent effect on C stocks. More research is needed to understand the mechanisms through which soil C stocks at depth are preserved.     

Hierarchical saturation of soil carbon pools near a natural CO2 spring

Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO₂ concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO₂ levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO₂ spring. Total and aggregate‐associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO₂ concentration. An asymptotic function showed a better fit of SOC and aggregation with CO₂ level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO₂ concentration. Litter inputs appeared to be positively related to CO₂ concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate‐size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO₂ levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.     

Decomposition of organic substrates at eroding vs. depositional landform positions

Introduction

Proper understanding of how rate of OM decomposition varies across a given watershed is important to determine the potential of soil erosion to induce terrestrial carbon (C) sequestration. However, as of yet, our understanding of the spatial variability of rate of organic matter (OM) decomposition (k) across a watershed is incomplete, at best.

Aim

The objective of this study is to determine how rates of organic substrate decomposition vary on the surface and in soil profiles of eroding vs. depositional landform positions.

Methods

To determine rate of organic substrate decomposition in eroding vs. depositional landform positions, a field litterbag decomposition study was conducted in Tennessee Valley, Northern California using in situ foliage (from grasses and a shrub) and two standard substrates (filter paper and birch tongue depressors, that served as proxies for OM that is relatively easier vs. harder to breakdown during microbial decomposition). We conducted the experiment at 3–4 depths at each landform position.

Results

The effect of erosional transport (surface to surface transfer of topsoil and associated SOM from eroding to depositional landform positions) and burial (after deposition of eroded SOM by successive erosional events) on decomposition rate of eroded SOM was different depending on the nature of eroding and depositional landform positions considered. The k of organic substrates at 25?cm soil depth in the depositional positions was up to 2 orders of magnitude higher than on the surface of the eroding positions. Results of this litterbag decomposition study suggest that transport of SOM from topsoil of eroding positions to the surface of depositional positions can reduce its k; but burial of eroded SOM in soil profiles at the depositional positions can lead to increasing k.

Conclusion

Because erosion-induced C sequestration is a function of changes in rate of OM decomposition and input post-compared to pre-erosion, our findings suggest that higher rates of plant productivity in eroding watersheds is needed to create and maintain a C sink in such eroding watersheds.     

Spatial scaling of ecosystem C and N in a subtropical savanna landscape

Widely occurred woody encroachment in grass‐dominated ecosystems has the potential to influence soil organic carbon (SOC) and total nitrogen (TN) pools at local, regional, and global scales. Evaluation of this potential requires assessment of both pool sizes and their spatial patterns. We quantified SOC and TN, their relationships with soil and vegetation attributes, and their spatial scaling along a catena (hill‐slope) gradient in the southern Great Plains, USA where woody cover has increased substantially over the past 100 years. Quadrat variance analysis revealed spatial variation in SOC and TN at two scales. The larger scale variation (40–45 m) was approximately the distance between centers of woody plant communities and their adjoining herbaceous patches. The smaller scale variation (10 m) appeared to reflect the local influence of shrubs on SOC and TN. Litter, root biomass, shrub, and tree basal area (a proxy for plant age) exhibited not only similar spatial scales, but also strong correlations with SOC and TN, suggesting invasive woody plants alter both the storage and spatial scaling of SOC and TN through ecological processes related primarily to root turnover and, to a lesser extent litter production, as mediated by time of occupancy. Forb and grass biomass were not significantly correlated with SOC and TN suggesting that changes in herbaceous vegetation have not been the driving force for the observed changes in SOC and TN. Because SOC and TN varied at two scales, it would be inappropriate to estimate SOC and TN pools at broad scales by extrapolating from point sampling at fine scales. Sampling designs that capture variation at multiple scales are required to estimate SOC and TN pools at broader scales. Knowledge of spatial scaling and correlations will be necessary to design field sampling protocols to quantify the biogeochemical consequences of woody plant encroachment at broad scales.     

Soil organic carbon dynamics jointly controlled by climate,carbon inputs,soil properties and soil carbon fractions

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 (r_C, Mg C ha^?1 yr^?1). Among these variables, we found that the most influential variables on r_C 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 r_C, 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 r_C. The direct correlation of r_C 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.     

Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation

Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global CO₂ mitigation. However, the potential of soils to sequester soil organic carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of 516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long‐term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse‐textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO₂‐equivalents could theoretically be stored in A horizons of cultivated soils – four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to CO₂ mitigation. Moreover, increasing SOC stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.     

Faster accumulation and greater contribution of glomalin to the soil organic carbon pool than amino sugars do under tropical coastal forest restoration

Microbial metabolic products play a vital role in maintaining ecosystem multifunctionality, such as soil physical structure and soil organic carbon (SOC) preservation. Afforestation is an effective strategy to restore degraded land. Glomalin-related soil proteins (GRSP) and amino sugars are regarded as stable microbial-derived C, and their distribution within soil aggregates affects soil structure stability and SOC sequestration. However, the information about how afforestation affects the microbial contribution to SOC pools within aggregates is poorly understood. We assessed the accumulation and contribution of GRSP and amino sugars within soil aggregates along a restoration chronosequence (Bare land, Eucalyptus exserta plantation, native species mixed forest, and native forest) in tropical coastal terraces. Amino sugars and GRSP concentrations increased, whereas their contributions to the SOC pool decreased along the restoration chronosequence. Although microaggregates harbored greater microbial abundances, amino sugars and GRSP concentrations were not significantly affected by aggregate sizes. Interestingly, the contributions of amino sugars and GRSP to SOC pools decreased with decreasing aggregate size which might be associated with increased accumulation of plant-derived C. However, the relative change rate of GRSP was consistently greater in all restoration chronosequences than that of amino sugars. The accumulation of GRSP and amino sugars in SOC pools was closely associated with the dynamics of soil fertility and the microbial community. Our findings suggest that GRSP accumulates faster and contributes more to SOC pools during restoration than amino sugars did which was greatly affected by aggregate sizes. Afforestation substantially enhanced soil quality with native forest comprising species sequestering more SOC than the monoculture plantation did. Such information is invaluable for improving our mechanistic understanding of microbial control over SOC preservation during degraded ecosystem restoration. Our findings also show that plantations using arbuscular mycorrhizal plants can be an effective practice to sequester more soil carbon during restoration.     

Ecosystem carbon pools,fluxes, and balances within mature tallgrass prairie restorations

Tallgrass prairie restorations can quickly accrue organic C in soil and biomass, but the rate of C accumulation diminishes through time and is highly variable among more mature prairies. Long‐term soil organic carbon (SOC) accumulation in prairies has been linked to edaphic factors such as soil texture, soil moisture, and SOC content, but it is unclear how these factors affect the ecosystem processes that are responsible for observed differences in C accumulation rates in older prairies. We measured belowground plant and SOC pools and fluxes within 27–36‐year‐old restored tallgrass prairies in order to quantify total C storage, determine the net ecosystem production of C (NEP‐C), and explore which edaphic factors influence the ecosystem processes responsible for divergent NEP‐C. We found that 11% of organic C was stored in biomass, and we estimate that one‐third of post‐restoration C sequestration has occurred in biomass, thereby highlighting biomass as a large but often overlooked C pool. Belowground biomass and soil C pools were notably smaller than those reported for remnant prairie, suggesting that future belowground C accumulation could still occur. During this study, the prairies appeared to be a net source of C, although the range of NEP‐C values encompassed zero. Sand content positively affected NEP‐C via increased belowground biomass production‐C inputs, and SOC negatively affected NEP‐C due to increased soil respiration C outputs. However, soil moisture had a smaller negative effect on soil respiration, indicating that both SOC and soil moisture play important roles in determining prairie C balance.     

Topographic and ungulate regulation of soil C turnover in a temperate grassland ecosystem

A major obstacle for predicting the effects of climate and land use changes on global soil carbon (C) stores is the very limited knowledge about the long timescale dynamics of the relatively stable fraction of soil C, which represents the bulk of soil C and the primary determinant of the long‐term C balance of terrestrial ecosystems. In this study, we examined how variable topo‐edaphic conditions and herds of native migratory ungulates influenced turnover of the stable pool (total minus active fraction) of soil C in grasslands of Yellowstone National Park (YNP). Soil C properties were determined for grasslands located inside and outside long‐term ungulate exclosures established 1958–1962 at seven variable topographic positions. Active C pool sizes, estimated with soil laboratory incubations, and soil radiocarbon measures were used to parameterize a process‐based model to determine turnover of the stable C pool at the sites. Stable C turnover ranged 37–653 and 89–869 years for 0–10 and 0–20 cm soils, respectively. Among ungrazed communities, there was a trend for stable soil C turnover to slow along topographic gradients of increasing soil moisture, soil C content, and shoot biomass from hilltop to slope‐bottom positions. This was likely a result of an increasing amount of support tissue resulting in greater concentrations of lignin and cellulose as shoot biomass increased down slope. In contrast, across the grazed landscape, stable C turnover sped up from hilltop to slope‐bottom positions, which was likely a consequence of grazer effects on plant species composition along the topographic gradient. These findings indicated that despite topography playing the primary role in controlling such important site characteristics as soil moisture, soil C content, and plant production in YNP grassland, the long‐term turnover of the stable C pool was determined by herbivores. The results demonstrate the important regulatory role of herbivores in controlling the C balance of this semiarid grassland ecosystem.     

Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.     

Carbon sequestration potential in perennial bioenergy crops: the importance of organic matter inputs and its physical protection

To date, only few studies have compared the soil organic carbon (SOC) sequestration potential between perennial woody and herbaceous crops. The main objective of this study was to assess the effect of perennial woody (poplar, black locust, willow) and herbaceous (giant reed, miscanthus, switchgrass) crops on SOC stock and its stabilization level after 6 years from plantation on an arable field. Seven SOC fractions related to different soil stabilization mechanisms were isolated by a combination of physical and chemical fractionation methods: unprotected (cPOM and fPOM), physically protected (iPOM), physically and chemically protected (HC‐μs + c), chemically protected (HC‐ds + c), and biochemically protected (NHC‐ds + c and NHC‐μs + c). The continuous C input to the soil and the minimal soil disturbance increased SOC stocks in the top 10 cm of soil, but not in deeper soil layers (10–30; 30–60; and 60–100 cm). In the top soil layer, greater SOC accumulation rates were observed under woody species (105 g m^?2 yr^‐1) than under herbaceous ones (71 g m^?2 yr^‐1) presumably due to a higher C input from leaf‐litter. The conversion from an arable maize monoculture to perennial bioenergy crops increased the organic C associated to the most labile organic matter (POM) fractions, which accounted for 38% of the total SOC stock across bioenergy crops, while no significant increments were observed in more recalcitrant (silt‐ and clay‐sized) fractions, highlighting that the POM fractions were the most prone to land‐use change. The iPOM fraction increased under all perennial bioenergy species compared to the arable field. In addition, the iPOM was higher under woody crops than under herbaceous ones because of the additional C inputs from leaf‐litter that occurred in the former. Conversion from arable cropping systems to perennial bioenergy crops can effectively increase the SOC stock and enlarge the SOC fraction that is physically protected within soil microaggregates.     

Soil carbon sequestration potential of permanent pasture and continuous cropping soils in New Zealand

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.     

Moisture and vegetation controls on decadal‐scale accrual of soil organic carbon and total nitrogen in restored grasslands

Revitalization of degraded landscapes may provide sinks for rising atmospheric CO₂, especially in reconstructed prairies where substantial belowground productivity is coupled with large soil organic carbon (SOC) deficits after many decades of cultivation. The restoration process also provides opportunities to study the often‐elusive factors that regulate soil processes. Although the precise mechanisms that govern the rate of SOC accrual are unclear, factors such as soil moisture or vegetation type may influence the net accrual rate by affecting the balance between organic matter inputs and decomposition. A resampling approach was used to assess the control that soil moisture and plant community type each exert on SOC and total nitrogen (TN) accumulation in restored grasslands. Five plots that varied in drainage were sampled at least four times over two decades to assess SOC, TN, and C₄‐ and C₃‐derived C. We found that higher long‐term soil moisture, characterized by low soil magnetic susceptibility, promoted SOC and TN accrual, with twice the SOC and three times the TN gain in seasonally saturated prairies compared with mesic prairies. Vegetation also influenced SOC and TN recovery, as accrual was faster in the prairies compared with C₃‐only grassland, and C₄‐derived C accrual correlated strongly to total SOC accrual but C₃‐C did not. High SOC accumulation at the surface (0–10 cm) combined with losses at depth (10–20 cm) suggested these soils are recovering the highly stratified profiles typical of remnant prairies. Our results suggest that local hydrology and plant community are critical drivers of SOC and TN recovery in restored grasslands. Because these factors and the way they affect SOC are susceptible to modification by climate change, we contend that predictions of the C‐sequestration performance of restored grasslands must account for projected climatic changes on both soil moisture and the seasonal productivity of C₄ and C₃ plants.     

Shifts in priming partly explain impacts of long‐term nitrogen input in different chemical forms on soil organic carbon storage

Input of labile organic carbon can enhance decomposition of extant soil organic carbon (SOC) through priming. We hypothesized that long‐term nitrogen (N) input in different chemical forms alters SOC pools by altering priming effects associated with N‐mediated changes in plants and soil microbes. The hypothesis was tested by integrating field experimental data of plants, soil microbes and two incubation experiments with soils that had experienced 10 years of N enrichment with three chemical forms (ammonium, nitrate and both ammonium and nitrate) in an alpine meadow on the Tibetan Plateau. Incubations with glucose–¹³C addition at three rates were used to quantify effects of exogenous organic carbon input on the priming of SOC. Incubations with microbial inocula extracted from soils that had experienced different long‐term N treatments were conducted to detect effects of N‐mediated changes in soil microbes on priming effects. We found strong evidence and a mechanistic explanation for alteration of SOC pools following 10 years of N enrichment with different chemical forms. We detected significant negative priming effects both in soils collected from ammonium‐addition plots and in sterilized soils inoculated with soil microbes extracted from ammonium‐addition plots. In contrast, significant positive priming effects were found both in soils collected from nitrate‐addition plots and in sterilized soils inoculated with soil microbes extracted from nitrate‐addition plots. Meanwhile, the abundance and richness of graminoids were higher and the abundance of soil microbes was lower in ammonium‐addition than in nitrate‐addition plots. Our findings provide evidence that shifts toward higher graminoid abundance and changes in soil microbial abundance mediated by N chemical forms are key drivers for priming effects and SOC pool changes, thereby linking human interference with the N cycle to climate change.     

Responses of soil C pools to combined warming and altered precipitation regimes: A meta-analysis

Aim

Global warming and altered precipitation substantially affect soil carbon (C) pools and can, in turn, feed back into climate change. However, how soil C pools respond to the combined effects of warming and altered precipitation remains unclear.

Location

Global.

Time period

1996–2021.

Major taxa studied

Soil organic C pools.

Method

A meta-analysis was performed using 657 observations obtained from 34 published articles that focused on both individual and combined effects of warming and altered precipitation on soil organic C (SOC), dissolved organic C (DOC) and microbial biomass C (MBC) to quantify the responses of soil C pools.

Results

Across all combined warming and increased precipitation experiments, SOC and MBC increased by an average of 4.0% and 15.4%, respectively. In contrast, warming combined with decreased precipitation led to a substantial decline in SOC and MBC by an average of 8.2% and 12.3%, respectively. The responses of DOC to combined warming and altered precipitation were marginal. The direction and magnitude of the responses to the combined treatment were more similar to those in the individual altered precipitation treatment than to those in the individual warming treatment. Furthermore, these combined effects were substantially influenced by altered precipitation magnitudes. Combined warming and altered precipitation had greater impacts on soil C pools than their individual treatments but were not substantially different from the sum of their respective individual effects, showing overall additive effects. The responses of soil C pools to combined warming and altered precipitation were observed to be more pronounced in grasslands than in forests.

Main conclusion

The results demonstrated that altered precipitation regimes often dominated over warming in regulating soil C pools under combined warming and altered precipitation and improved our understanding of soil C cycles under climate change scenarios.     

Microbial models with data‐driven parameters predict stronger soil carbon responses to climate change

Long‐term carbon (C) cycle feedbacks to climate depend on the future dynamics of soil organic carbon (SOC). Current models show low predictive accuracy at simulating contemporary SOC pools, which can be improved through parameter estimation. However, major uncertainty remains in global soil responses to climate change, particularly uncertainty in how the activity of soil microbial communities will respond. To date, the role of microbes in SOC dynamics has been implicitly described by decay rate constants in most conventional global carbon cycle models. Explicitly including microbial biomass dynamics into C cycle model formulations has shown potential to improve model predictive performance when assessed against global SOC databases. This study aimed to data‐constrained parameters of two soil microbial models, evaluate the improvements in performance of those calibrated models in predicting contemporary carbon stocks, and compare the SOC responses to climate change and their uncertainties between microbial and conventional models. Microbial models with calibrated parameters explained 51% of variability in the observed total SOC, whereas a calibrated conventional model explained 41%. The microbial models, when forced with climate and soil carbon input predictions from the 5th Coupled Model Intercomparison Project (CMIP5), produced stronger soil C responses to 95 years of climate change than any of the 11 CMIP5 models. The calibrated microbial models predicted between 8% (2‐pool model) and 11% (4‐pool model) soil C losses compared with CMIP5 model projections which ranged from a 7% loss to a 22.6% gain. Lastly, we observed unrealistic oscillatory SOC dynamics in the 2‐pool microbial model. The 4‐pool model also produced oscillations, but they were less prominent and could be avoided, depending on the parameter values.