Soil Organic Carbon Pool Sizes Research Articles

Soil organic carbon under decadal elevated CO2: Pool size unchanged but stability reduced

Soil organic carbon (SOC) dynamics under elevated atmospheric CO2 concentration has been widely reported, however, in which the behaviors of active and passive fractions remain inadequately explored. Here we studied this issue using three pairs of active and passive fractions of SOC under a 10-year free-air CO2 enrichment experiment (550 ​± ​17 ​ppm) in a cropland in the North China Plain. We found that decadal elevated CO2 increased the root biomass, root exudation rate and microbial biomass, but had little effects on SOC pool size. Elevated CO2 increased the readily oxidizable organic carbon (ROOC) and particulate organic carbon (POC) due to the increments of root C input, but decreased their paired passive fractions possibly because of the carbon input-induced positive priming effect. Our results indicate the reduced stability of SOC pool under elevated CO2. This is significant for better predicting SOC feedback to future climate change.

Benchmarking carbon sequestration potentials in arable soils by on-farm research on innovative pioneer farms

PurposeTackling the global carbon deficit through soil organic carbon (SOC) sequestration in agricultural systems has been a focal point in recent years. However, we still lack a comprehensive understanding of actual on-farm SOC sequestration potentials in order to derive effective strategies.MethodsTherefore, we chose 21 study sites in North-Eastern Austria covering a wide range of relevant arable soil types and determined SOC pool sizes (0–35 cm soil depth) in pioneer versus conventional management systems in relation to permanently covered reference soils. We evaluated physico-chemical predictors of SOC stocks and SOC quality differences between systems using Fourier-transform infrared (FTIR) spectroscopy.ResultsCompared to conventional farming systems, SOC stocks were 14.3 Mg ha− 1 or 15.7% higher in pioneer farming systems, equaling a SOC sequestration rate of 0.56 Mg ha− 1 yr− 1. Reference soils however showed approximately 30 and 50% higher SOC stocks than pioneer and conventional farming systems, respectively. Nitrogen and dissolved organic carbon stocks showed similar patterns. While pioneer systems could close the SOC storage deficit in coarse-textured soils, SOC stocks in medium- and fine-textured soils were still 30–40% lower compared to the reference soils. SOC quality, as inferred by FTIR spectra, differed between land-use systems, yet to a lesser extent between cropping systems.ConclusionsInnovative pioneer management alleviates SOC storage. Actual realized on-farm storage potentials are rather similar to estimated SOC sequestration potentials derived from field experiments and models. The SOC sequestration potential is governed by soil physico-chemical parameters. More on-farm approaches are necessary to evaluate close-to-reality SOC sequestration potentials in pioneer agroecosystems.

A robust initialization method for accurate soil organic carbon simulations

Abstract. Changes in soil organic carbon (SOC) stocks are a major source of uncertainty for the evolution of atmospheric CO2 concentration during the 21st century. They are usually simulated by models dividing SOC into conceptual pools with contrasted turnover times. The lack of reliable methods to initialize these models, by correctly distributing soil carbon amongst their kinetic pools, strongly limits the accuracy of their simulations. Here, we demonstrate that PARTYSOC, a machine-learning model based on Rock-Eval® thermal analysis, optimally partitions the active- and stable-SOC pools of AMG, a simple and well-validated SOC dynamics model, accounting for effects of soil management history. Furthermore, we found that initializing the SOC pool sizes of AMG using machine learning strongly improves its accuracy when reproducing the observed SOC dynamics in nine independent French long-term agricultural experiments. Our results indicate that multi-compartmental models of SOC dynamics combined with a robust initialization can simulate observed SOC stock changes with excellent precision. We recommend exploring their potential before a new generation of models of greater complexity becomes operational. The approach proposed here can be easily implemented on soil monitoring networks, paving the way towards precise predictions of SOC stock changes over the next decades.

Evaluating storage and pool size of soil organic carbon in degraded soils: Tillage effects when crop residue is returned

Conventional tillage in Northeast China involves complete removal of crop residue and deep plowing, both of which cause significant loss of soil organic carbon (SOC). Our study aimed to evaluate SOC storage and pool sizes in degraded soils when post-harvest residue is returned to degraded soil and different tillage systems are used. We measured SOC under no-tillage (NT), ridge-tillage (RT) and moldboard plow (MP) and compared storage across treatments. We used sulfuric acid hydrolysis to separate and evaluate the size of two labile pools and one recalcitrant pool. There was no difference in crop yields (and hence C inputs) across tillage treatments over 12 years. SOC storage in the plow layer (0–20 cm) increased in all tillage systems compared to soils in the degraded state but the rate of increase was greater under NT and RT than MP. There was increased variability when we assessed deep SOC storage (0–30 cm) and SOC storage at this depth was the same across tillage systems, but still higher than in the initially degraded state. Changes observed in the size of labile and recalcitrant pools indicated these occurred mainly in the surface 0–5 cm and that RT induced slight changes in the chemical composition of SOC. Our results indicate that returning residues to degraded soils: 1) replenished SOC levels but the rate of replenishment was highest under conservation tillage, 2) had no effect on yields (C inputs) and deep SOC storage across tillage treatments, and 3) induced subtle differences in SOC pool size.

Nematodes and microbial community affect the sizes and turnover rates of organic carbon pools in soil aggregates

Soil aggregates provide microhabitats for nematodes and microorganisms, but how nematodes and microbial community interactively drive the dynamics of soil organic carbon (SOC) pool remains unclear. Here, we examined the relationships between bacterivorous nematodes, microbial community, and the sizes and turnover rates of three SOC pools in a red soil under four fertilization regimes. The abundance and community composition of nematode and bacterial communities were examined within aggregate fractions, including large macroaggregates (LMA), small macroaggregates (SMA), and microaggregates (MA). The sizes of SOC pools in soil aggregates increased with decreasing aggregate size, while the turnover rates of SOC pools followed the opposite trend. The ratios of bacteria to fungi (B/F) and Gram-positive to Gram-negative bacteria (GP/GN) were higher in the MA fraction than in the SMA and LMA fractions. The assemblages of bacterivorous nematodes in the LMA fraction were significantly different from those in the MA and SMA fractions, primarily because of the higher abundance of the dominant genus Protorhabditis. Results of structural equation modelling indicated that the ratios of B/F and GP/GN showed stronger positive correlations with the sizes and turnover rates of SOC pools in the MA fraction compared with the LMA fraction. Conversely, bacterivores exhibited indirect relationships with the sizes and turnover rates of SOC pools through the B/F ratio in the SMA and LMA fractions. Taken together, these results highlight the functional role of nematodes and microbial community in controlling SOC pool dynamics at the aggregate scale.

Soil organic carbon pools in ploughed and no‐till Alfisols of central Ohio

AbstractNo‐till (NT) farming can restore the soil organic carbon (SOC) pool of agricultural soils, but the SOC pool size and retention rate can vary with soil type and duration of NT. Therefore, the objectives of this study were to determine the effects of NT and soil drainage characteristics on SOC accumulation across a series of NT fields on Alfisols in Ohio, USA. Sites under NT for 9 (NT9), 13 (NT13), 36 (NT36), 48 (NT48) and 49 (NT49) years were selected for the study. Soil was somewhat poorly drained at the NT48 site but moderately well drained at the other sites. The NT48 and NT49 on‐station sites were under continuous corn (Zea mays), while the other sites were farmers' fields in a corn–soybean (Glycine max) rotation. At each location, the SOC pool (0–30 cm) in the NT field was compared to that of an adjacent plough‐till (PT) and woodlot (WL). At the NT36, NT48 and NT49 sites, the retention rate of corn‐derived C was determined using stable C isotope (13C) techniques. In the 0‐ to 10‐cm soil layer, SOC concentration was significantly larger under NT than PT, but a tillage effect was rarely detected below that depth. Across sites, the SOC pool in that layer averaged 36.4, 20 and 40.8 Mg C/ha at the NT, PT and WL sites, respectively. For the 0‐ to 30‐cm layer, the SOC pool for NT (83.4 Mg C/ha) was still 57% greater than under PT. However, there was no consistent trend in the SOC pool with NT duration probably due to the legacy of past management practices and SOC content differences that may have existed among the study sites prior to their conversion to NT. The retention rate of corn‐derived C was 524, 263 and 203 kg C/ha/yr at the NT36, NT48 and NT49 sites. In contrast, the retention rate of corn‐C under PT averaged 25 and 153 kg C/ha/yr at the NT49 (moderately well‐drained) and NT48 (somewhat poorly drained) sites, respectively. The conversion from PT to NT resulted in greater retention of corn‐derived C. Thus, adoption of NT would be beneficial to SOC sequestration in agricultural soils of the region.

Soil Organic Carbon Mineralization as Affected by Cyclical Temperature Fluctuations in a Karst Region of Southwestern China

The diurnal fluctuation in soil temperature may influence soil organic carbon (SOC) mineralization, but there is no consensus on SOC mineralization response to the cyclical fluctuation in soil temperature. A 56-d incubation experiment was conducted to investigate the effects of constant and variable temperatures on SOC mineralization. Three soils were collected from the karst region in western Guizhou Province, southwestern China, including a limestone soil under forest, a limestone soil under crops and a yellow soil under crops. According to the World Reference Base (WRB) classification, the two limestone soils were classified as Haplic Luvisols and the yellow soil as a Dystric Luvisol. These soils were incubated at three constant temperatures (15, 20 and 25 0C) and cyclically fluctuating temperatures (diurnal cycle between 15 and 25 0C). The results showed that the 56-d cumulative SOC mineralized (C56) at the fluctuating temperatures was between those at constant 15 and 25 0C, suggesting that the cumulative SOC mineralization was restricted by temperature range. The SOC mineralization responses to the fluctuating temperatures were different among the three soils, especially in contrast to those at constant 20 0C. Compared with constant 20 0C, significant (P < 0.05) decreases and increases in C56 value were found in the limestone soil under forest and yellow soil under crops at the fluctuating temperatures, respectively. At the fluctuating temperatures, the forest soil with lower temperature coefficient Q10 (the relative change in SOC mineralization rate as a result of increasing the temperature by 10 0C) had a significantly (P < 0.05) lower SOC mineralization intensity than the two cropland soils. These indicated that differences in temperature pattern (constant or fluctuating) could significantly influence SOC mineralization, and SOC mineralization responses to the fluctuating temperatures might be affected by soil characteristics. Moreover, the warmer temperatures might improve the ability of soil microbes to decompose the recalcitrant SOC fraction, and cyclical fluctuations in temperature could influence SOC mineralization through changing the labile SOC pool size and the mineralization rate of the recalcitrant SOC in soils.

The implications of microbial and substrate limitation for the fates of carbon in different organic soil horizon types of boreal forest ecosystems: a mechanistically based model analysis

Abstract. The large amount of soil carbon in boreal forest ecosystems has the potential to influence the climate system if released in large quantities in response to warming. Thus, there is a need to better understand and represent the environmental sensitivity of soil carbon decomposition. Most soil carbon decomposition models rely on empirical relationships omitting key biogeochemical mechanisms and their response to climate change is highly uncertain. In this study, we developed a multi-layer microbial explicit soil decomposition model framework for boreal forest ecosystems. A thorough sensitivity analysis was conducted to identify dominating biogeochemical processes and to highlight structural limitations. Our results indicate that substrate availability (limited by soil water diffusion and substrate quality) is likely to be a major constraint on soil decomposition in the fibrous horizon (40–60% of soil organic carbon (SOC) pool size variation), while energy limited microbial activity in the amorphous horizon exerts a predominant control on soil decomposition (&gt;70% of SOC pool size variation). Elevated temperature alleviated the energy constraint of microbial activity most notably in amorphous soils, whereas moisture only exhibited a marginal effect on dissolved substrate supply and microbial activity. Our study highlights the different decomposition properties and underlying mechanisms of soil dynamics between fibrous and amorphous soil horizons. Soil decomposition models should consider explicitly representing different boreal soil horizons and soil–microbial interactions to better characterize biogeochemical processes in boreal forest ecosystems. A more comprehensive representation of critical biogeochemical mechanisms of soil moisture effects may be required to improve the performance of the soil model we analyzed in this study.

Soil organic carbon mineralization rates in aggregates under contrasting land uses

Measuring soil organic carbon (SOC) mineralization in macro-aggregates (250–2000μm), micro-aggregates (250–53μm) and the <53μm fraction helps to understand how spatial separation of SOC inside soil aggregates regulates its dynamics. We hypothesized that (i) compared with macro-aggregates SOC mineralization rate of micro-aggregates would be slower, (ii) adsorption of SOC on <53μm fraction decreases the SOC mineralization rate, and (iii) land use has a significant influence on SOC decomposition rate. To test these hypotheses we collected topsoil from Dermosol (Acrisols in FAO Soil Classification) sites under three contrasting land uses namely native pasture (NP), crop–pasture rotation (CP) and woodland (WL). Macro-aggregates, micro-aggregates and the <53μm fraction were separated from bulk soil by wet sieving. The three aggregate size ranges were then incubated for six months and CO2 evolution was measured at different time intervals. The chemically stable SOC of <53μm fraction of macro-aggregates, micro-aggregates and the <53μm fraction (separated by wet sieving) was measured by oxidation of SOC with 10% H2O2. On average, cumulative mineralization, Cmin (gCO2–Ckg−1 aggregate) of the <53μm fraction, was 28% lower than that of macro-aggregates and micro-aggregates. However, SOC mineralized (SOCmin) was similar in all size fractions. The size of slow SOC pool (percent of SOC concentration in aggregates) was also significantly higher in the <53μm fraction and ranged from 58 to 96%, across aggregate sizes. However, the chemically stable SOC (percent of SOC concentration in aggregates) was significantly higher in macro-aggregates and micro-aggregates than that of the <53μm fraction. Mean residence time (MRT) of slow SOC pool (MRTs) was higher in the <53μm fraction than for either macro-aggregates or micro-aggregates. Among the land uses NP had higher SOCmin compared with CP and WL. In conclusion, the insignificant difference in SOCmin, slow SOC pool sizes and MRTs between macro-aggregates and micro-aggregates indicated that SOC mineralization rate and thus the protection of SOC was similar in both macro-aggregates and micro-aggregates.

The relative impact of land use and soil properties on sizes and turnover rates of soil organic carbon pools in Subtropical China

AbstractThe dynamics of soil organic carbon (SOC) pools determine potential carbon sequestration and soil nutrient improvement. This study investigated the characteristics of SOC pools in five types of cultivated topsoils (0–15 cm) in subtropical China using laboratory incubation experiments under aerobic conditions. The sizes and turnover rates of the active, slow and resistant C pools were simulated using a first‐order kinetic model. The relative influence of soil environmental properties on the dynamics of different SOC pools was evaluated by applying principal component analysis (PCA) and aggregated boosted trees (ABTs) analysis. The results show that there were significantly greater sizes of different SOC pools and lower turnover rates of slow C pool in two types of paddy soils than in upland soils. Land use exerted the most significant influence on the sizes of all SOC pools, followed by clay content and soil pH. The soil C/N ratio and pH were the major determinants for turnover rates of the active and slow C pools, followed by clay content which had more impact on the turnover rates of the active C pool than the slow C pool. It is concluded that soil type exerts a significant impact on the dynamics of SOC.

Restoring ecosystem carbon sequestration through afforestation: A sub-tropic restoration case study

The long-term Forest Restoration Experimental Project (FREP) was established in 1991 on a sub-tropical, barren, degraded, red soil with a rolling terrain located in Taihe County, Jianxi province, China. The objective of the FREP was to evaluate the effects of restoration, through afforestation of various local climax species, on ecological functions to provide guidance for future restoration projects on severely deteriorated landscapes, which are very common in southern China. In this study, we selected five restoration forests: Chinese sweetgum (Liquidamber formosana), schima (Schima superb), masson’s pine (Pinus massoniana), slash pine (Pinus elliottii), Chinese sweetgum×slash pine mixtures, and one experimental control (natural recovery) to evaluate the differences in carbon storage and pool structures, including above- and below-ground carbon pools, forest floor litter, woody debris, and soil organic carbon (SOC). A similar assessment was also conducted on the species functional groups (coniferous forest, broad-leaved forest, and mixed-species forest – broad-leaved×coniferous species mixture) based on groupings of studied species. We also evaluated the recovery trajectory of FREP’s evergreen broad-leaved forest by comparing it with local ecosystems.Over the 19-year study period, the Total Ecosystem Carbon (TEC) stocks in the five restoration types were significantly higher than that in the control sites, but there were no significant differences in the TEC stock among the restoration types. The TEC was 119MgCha−1 for Chinese sweetgum, 118MgCha−1 for schima, 105MgCha−1 for masson’s pine, 104MgCha−1 for slash pine, and 124MgCha−1 for the mixture. However, there was a significant variation in the carbon pool structure among restoration functional groups. The SOC pool sizes of broad-leaved and mixed-species forest were 25% and 16% significantly higher than the coniferous forest, respectively. This difference may be explained by the recovery trajectory, which suggests that the evergreen forest (schima) in FREP is still in the early developmental stage, and its projected rate of growth is much slower than the average growth rate in the region. This study clearly demonstrated that active restoration can enhance ecosystem carbon sequestration. Clearly, a long-term monitoring program is critical for obtaining more information that will enable us to extrapolate our findings for broader restoration plans and to increase the carbon sequestration strength of restored forests.

Spatially Explicit Parameterization of a Terrestrial Ecosystem Model and Its Application to the Quantification of Carbon Dynamics of Forest Ecosystems in the Conterminous United States

Abstract The authors use a spatially explicit parameterization method and the Terrestrial Ecosystem Model (TEM) to quantify the carbon dynamics of forest ecosystems in the conterminous United States. Six key parameters that govern the rates of carbon and nitrogen dynamics in TEM are selected for calibration. Spatially explicit data for carbon and nitrogen pools and fluxes are used to calibrate the six key parameters to more adequately account for the spatial heterogeneity of ecosystems in estimating regional carbon dynamics. The authors find that a spatially explicit parameterization results in vastly different carbon exchange rates relative to a parameterization conducted for representative ecosystem sites. The new parameterization method estimates that the net ecosystem production (NEP), the annual gross primary production (GPP), and the net primary production (NPP) of the regional forest ecosystems are 61% (0.02 Pg C; 1 Pg = 1015 g) higher and 2% (0.11 Pg C) and 19% (0.45 Pg C) lower, respectively, than the values obtained using the traditional parameterization method for the period 1948–2000. The estimated vegetation carbon and soil organic carbon pool sizes are 51% (18.73 Pg C) lower and 29% (7.40 Pg C) higher. This study suggests that, to more adequately quantify regional carbon dynamics, spatial data for carbon and nitrogen pools and fluxes should be developed and used with the spatially explicit parameterization method.

Spatially explicit parameterization of a terrestrial ecosystem model and its application to the quantification of carbon dynamics of forest ecosystems in the conterminous United States

The authors use a spatially explicit parameterization method and the Terrestrial Ecosystem Model (TEM) to quantify the carbon dynamics of forest ecosystems in the conterminous United States. Six key parameters that govern the rates of carbon and nitrogen dynamics in TEM are selected for calibration. Spatially explicit data for carbon and nitrogen pools and fluxes are used to calibrate the six key parameters to more adequately account for the spatial heterogeneity of ecosystems in estimating regional carbon dynamics. The authors find that a spatially explicit parameterization results in vastly different carbon exchange rates relative to a parameterization conducted for representative ecosystem sites. The new parameterization method estimates that the net ecosystem production (NEP), the annual gross primary production (GPP), and the net primary production (NPP) of the regional forest ecosystems are 61% (0.02 Pg C; 1 Pg = 1015 g) higher and 2% (0.11 Pg C) and 19% (0.45 Pg C) lower, respectively, than the values obtained using the traditional parameterization method for the period 1948–2000. The estimated vegetation carbon and soil organic carbon pool sizes are 51% (18.73 Pg C) lower and 29% (7.40 Pg C) higher. This study suggests that, to more adequately quantify regional carbon dynamics, spatial data for carbon and nitrogen pools and fluxes should be developed and used with the spatially explicit parameterization method.

Kinetic Responses of Soil Carbon Dioxide Emission to Increasing Urea Application Rate

BACKGROUND: Application of urea may increase CO2 emission from soils due both to CO2 generation from urea hydrolysis and fertilizer-induced decomposition of soil organic carbon (SOC). The objective of this study was to investigate the effects of increasing urea application on CO2 emission from soil and mineralization kinetics of indigenous SOC. METHODS AND RESULTS: Emission of CO2 from a soil amended with four different rates (0, 175, 350, and 700 mg N/kg soil) of urea was investigated in a laboratory incubation experiment for 110 days. Cumulative CO2 emission (Ccum) was linearly increased with urea application rate due primarily to the contribution of urea-C through hydrolysis to total CO2 emission. First-order kinetics parameters (C0, mineralizable SOC pool size; k, mineralization rate) became greater with increasing urea application rate; C0 increased from 665.1 to 780.3 mg C/kg and k from 0.024 to 0.069 day -1 , determinately showing fertilizer-induced SOC mineralization. The relationship of C0 (non-linear) and k (linear) with urea-N application rate revealed different responses of C0 and k to increasing rate of fertilizer N. CONCLUSION(s): The relationship of mineralizable SOC pool size and mineralization rate with urea-N application rate suggested that increasing N fertilization may accelerate decomposition of readily decomposable SOC; however, it may not always stimulate decomposition of non-readily decomposable SOC that is protected from microbial

Quantifying soil organic carbon in complex landscapes: an example of grassland undergoing encroachment of woody plants

AbstractThe invasion of woody plants into grass‐dominated ecosystems has occurred worldwide during the past century with potentially significant impacts on soil organic carbon (SOC) storage, ecosystem carbon sequestration and global climate warming. To date, most studies of tree and shrub encroachment impacts on SOC have been conducted at small scales and results are equivocal. To quantify the effects of woody plant proliferation on SOC at broad spatial scales and to potentially resolve inconsistencies reported from studies conducted at fine spatial scales, information regarding spatial variability and uncertainty of SOC is essential. We used sequential indicator simulation (SIS) to quantify spatial uncertainty of SOC in a grassland undergoing shrub encroachment in the Southern Great Plains, USA. Results showed that both SOC pool size and its spatial uncertainty increased with the development of woody communities in grasslands. Higher uncertainty of SOC in new shrub‐dominated communities may be the result of their relatively recent development, their more complex above‐ and belowground architecture, stronger within‐community gradients, and a greater degree of faunal disturbance. Simulations of alternative sampling designs demonstrated the effects of spatial uncertainty on the accuracy of SOC estimates and enabled us to evaluate the efficiency of sampling strategies aimed at quantifying landscape‐scale SOC pools. An approach combining stratified random sampling with unequal point densities and transect sampling of landscape elements exhibiting strong internal gradients yielded the best estimates. Complete random sampling was less effective and required much higher sampling densities. Results provide novel insights into spatial uncertainty of SOC and its effects on estimates of carbon sequestration in terrestrial ecosystem and suggest effective protocol for the estimating of soil attributes in landscapes with complex vegetation patterns.

SOIL CARBON SEQUESTRATION TO MITIGATE CLIMATE CHANGE AND ADVANCE FOOD SECURITY

World soils have been a source of atmospheric carbon dioxide since the dawn of settled agriculture, which began about 10 millennia ago. Most agricultural soils have lost 30% to 75% of their antecedent soil organic carbon (SOC) pool or 30 to 40 t C ha−1. The magnitude of loss is often more in soils prone to accelerated erosion and other degradative processes. On a global scale, CO2-C emissions since 1850 are estimated at 270 ± 30 giga ton (billion ton or Gt) from fossil fuel combustion compared with 78 ± 12 Gt from soils. Consequently, the SOC pool in agricultural soils is much lower than their potential capacity. Furthermore, depletion of the SOC pool also leads to degradation in soil quality and declining agronomic/biomass productivity. Therefore, conversion to restorative land uses (e.g., afforestation, improved pastures) and adoption of recommended management practices (RMP) can enhance SOC and improve soil quality. Important RMP for enhancing SOC include conservation tillage, mulch farming, cover crops, integrated nutrient management including use of manure and compost, and agroforestry. Restoration of degraded/desertified soils and ecosystems is an important strategy. The rate of SOC sequestration, ranging from 100 to 1000 kg ha−1 year−1, depends on climate, soil type, and site-specific management. Total potential of SOC sequestration in the United States of 144 to 432 Mt year−1 (288 Mt year−1) comprises 45 to 98 Mt in cropland, 13 to 70 Mt in grazing land, and 25 to 102 Mt in forestland. The global potential of SOC sequestration is estimated at 0.6 to 1.2 Gt C year−1, comprising 0.4 to 0.8 Gt C year−1 through adoption of RMP on cropland (1350 Mha), and 0.01 to 0.03 Gt C year−1 on irrigated soils (275 Mha), and 0.01 to 0.3 Gt C year−1 through improvements of rangelands and grasslands (3700 Mha). In addition, there is a large potential of C sequestration in biomass in forest plantations, short rotation woody perennials, and so on. The attendant improvement in soil quality with increase in SOC pool size has a strong positive impact on agronomic productivity and world food security. An increase in the SOC pool within the root zone by 1 t C ha−1 year−1 can enhance food production in developing countries by 30 to 50 Mt year−1 including 24 to 40 Mt year−1 of cereal and legumes, and 6 to 10 Mt year−1 of roots and tubers. Despite the enormous challenge of SOC sequestration, especially in regions of warm and arid climates and predominantly resource-poor farmers, it is a truly a win-win strategy. While improving ecosystem services and ensuring sustainable use of soil resources, SOC sequestration also mitigates global warming by offsetting fossil fuel emissions and improving water quality by reducing nonpoint source pollution.

CLIMATIC/EDAPHIC CONTROLS ON SOIL CARBON/NITROGEN RESPONSE TO SHRUB ENCROACHMENT IN DESERT GRASSLAND

The proliferation of woody plants in grasslands over the past 100+ years can alter carbon, nitrogen, and water cycles and influence land surface-atmosphere interactions. Although the majority of organic carbon in these ecosystems resides belowground, there is no consensus on how this change in land cover has affected soil organic carbon (SOC) and total nitrogen (TN) pools. The degree to which duration of woody plant occupation, climate, and edaphic conditions have mediated SOC and TN responses to changes in life-form composition are poorly understood. We addressed these issues at a desert grassland site in Arizona, USA, where the leguminous shrub velvet mesquite (Prosopis velutina) has proliferated along an elevation/precipitation/temperature gradient and on contrasting soil morphologic surfaces. On sandy loam complexes of mid-Holocene origin, mean SOC and TN of soils in the grassland matrix increased approximately 68% and approximately 45%, respectively, with increasing elevation. Soil organic carbon pools were comparable and TN pools were approximately 23% higher in Pleistocene-aged clay loam complexes co-occurring with Holocene-aged soils at the upper elevation/climatic zone. Across the site, belowground resources associated with large Prosopis plants were 21-154% (SOC) and 18-127% (TN) higher than those in the grassy matrix. The variance in SOC and TN pools accounted for by Prosopis stem size (a rough surrogate for time of site occupation) was highest at the low- and mid-elevation sites (69-74%) and lowest at the upper elevation site (32-38%). Soil delta15N values ranged from 5.5 per thousand to 6.7 per thousand across the soil/elevation zones but were comparable in herbaceous and shrub-impacted soils and exhibited a weak relationship with Prosopis basal stem diameter (r2 < 0.1) and TN (r2 < 0.08). The SOC delta13C values decreased linearly with increasing Prosopis basal diameter, suggesting that size and isotopic composition of the SOC pool is a function of time of Prosopis site occupation. Isotopic mixture models indicate that encroachment of C3 woody plants has also promoted SOC additions from C4 plant sources, indicative of long-term herbaceous facilitation. Grassy sites in contrasting soil/elevation combinations, initially highly distinctive in their SOC pool size and delta13C, appear to be converging on similar values following approximately 100 years of woody plant proliferation.

Soil carbon pools and fluxes in long-term corn belt agroecosystems

The dynamics of soil organic carbon (SOC) play an important role in long-term ecosystem productivity and the global C cycle. We used extended laboratory incubation and acid hydrolysis to analytically determine SOC pool sizes and fluxes in US Corn Belt soils derived from both forest and prairie vegetation. Measurement of the natural abundance of 13C made it possible to follow the influence of continuous corn on SOC accumulation. The active pools (C a) comprised 3 to 8% of the SOC with an average field mean residence time (MRT) of 100 d. The slow pools (C s) comprised 50% of SOC in the surface and up to 65% in subsoils. They had field MRTs from 12–28 y for C 4-C and 40–80 y for C 3-derived C depending on soil type and location. No-till management increased the MRT of the C 3-C by 10–15 y above conventional tillage. The resistant pool (C r) decreased from an average of 50% at the surface to 30% at depth. The isotopic composition of SOC mineralized during the early stages of incubation reflected accumulations of labile C from the incorporation of corn residues. The CO 2 released later reflected 13C characteristic of the C s pool. The 13C of the CO 2 did not equal that of the whole soil until after 1000 d of incubation. The SOC dynamics determined by acid hydrolysis, incubation and 13C content were dependent on soil heritage (prairie vs. forest), texture, cultivation and parent material, depositional characteristics. Two independent methods of determining C 3 pool sizes derived from C 3 vegetation gave highly correlated values.