Total Soil Organic C Research Articles

Long-term tillage, straw and N rate effects on quantity and quality of organic C and N in a Gray Luvisol soil

Long-term use of soil, crop and fertilizer management practices alters some soil properties, but the magnitude of change depends on soil type and climatic conditions. A field experiment with a barley (Hordeum vulgare L.)–wheat (Triticum aestivum L.)–canola (Brassica napus L.) rotation was conducted on a Gray Luvisol (Typic Cryoboralf) loam soil at Breton, Alberta, Canada. Effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [SRem] and straw retained [SRet]) and N fertilizer rate (0, 50 and 100 kg N ha−1 in SRet, and 0 kg N ha−1 in SRem plots) were determined on total organic C (TOC) and N (TON), light fraction organic C (LFOC) and N (LFON), macro organic matter C (MOM-C) and N (MOM-N), microbial biomass C (MB-C), and mineralizable C (Cmin) and N (Nmin) in the 0–7.5 and 7.5–15 cm or 0–5, 5–10 and 10–15 cm soil layers. Zero tillage and SRet tended to have higher, and N fertilizer treatment usually had higher mass of TOC, TON, LFOC, LFON, Cmin and Nmin in soil compared to the corresponding CT, SRem and zero-N control treatments, especially in the surface soil layers. Soil MB-C, MOM-C and MOM-N in soil generally tended to be higher with SRet than SRem, and also with N fertilizer than zero-N. There was no additional beneficial effect of ZT in increasing MB-C in soil. There were close and significant correlations among most soil organic C or N fractions, except for MB-C which did not correlate with MOM-N, and Nmin did not correlate with MOM-C. Linear regressions between crop residue C input and soil organic C or N were significant in most cases, except for MB-C and Nmin. Compared to the 1979 data, all treatments that did not receive N fertilizer (CTSRem0, CTSRet0, ZTSRem0 and ZTSRet0) showed a decrease in TOC concentration in the 0–15 cm soil layer over time, with the highest decrease in the CTSRem0 treatment. Straw retention and N fertilizer application at 50 and 100 kg N ha−1 under both ZT (ZTSRet50 and ZTSRet100) and CT (CTSRet50 and CTSRet100) resulted in a strongest increase in TOC during the first 11 years, and since then the TOC decreased under both N rates but 50 kg N ha−1 rate under CT (CTSRet50) showed the strongest negative effect on TOC in soil. In conclusion, elimination of tillage, straw retention and N application all improved organic C and N in soil, and generally differences were more pronounced for light fraction organic C and N, and between the most extreme treatments (CTSRem0 vs. ZTSRet100) for each dynamic organic fraction. This may be better for the long-term sustainability of soil quality and productivity.

Conversion of Wheat–Maize to Vegetable Cropping Systems Changes Soil Organic Matter Characteristics

We investigated the effects of land use conversion from cereal grain fields (GF) to greenhouse vegetable fields (VF) on total soil organic C (SOC) and total N (TN) and the labile soil organic matter pools dissolved organic C (DOC), permanganate‐oxidizable C (POC), dissolved organic N (DON), mineral N, and microbial biomass C (MBC) in the top 30 cm of the soil profile. The soil samples were collected in 20 sets of paired soils (greenhouse vegetable vs. wheat [Triticum aestivum L.]–maize [Zea mays L.] rotation fields) in northeast China. Compared with those in GF, the SOC and TN concentrations in VF increased by 28 and 58%, respectively. The accumulation rate of TN vs. SOC, however, led to a significant decrease in soil C/N ratio in VF compared with GF systems. The greatest difference in C/N ratios between GF and VF was observed in the large macroaggregates (>2000 μm). Conversion to VF led to increased DOC, POC, and DON concentrations and an increase in the percentage of SOC as DOC and POC. The pH, MBC, and percentage of SOC as MBC, however, were lower in the VF than GF systems. Furthermore, a significant positive relationship was found between pH and MBC in the VF systems, while MBC was positively related to total SOC and POC in the GF systems. In conclusion, the results show tradeoffs between increasing SOC sequestration when agricultural land is converted from wheat–maize to vegetable systems and decreased C/N ratios and soil acidification. Improvements in soil and crop management for vegetable production systems in China, particularly the avoidance of excess N fertilizer and manure applications, are recommended.

Predicting the Spatial Variation of the Soil Organic Carbon Pool at a Regional Scale

Estimates of soil organic C (SOC) storage and their variability at various spatial scales are essential to better understand the global C cycle, estimate C sink capacity, identify effective C sequestration strategies, and quantify the amount of SOC sequestered during a specific period of time. This study used a geographically weighted regression (GWR) approach to predict the SOC pool at a regional scale. The GWR considers varying relationships between the SOC pool and environmental variables across the study area. The range of the variogram of SOC observations was used to define a search radius in the GWR. Terrain attributes, climate data, land use data, bedrock geology, and normalized difference vegetation index data were used to predict the SOC pool for seven states in the midwestern United States. The prediction accuracy of this SOC pool map was compared with the multiple linear regression (MLR) and regression kriging (RK) approaches. Higher contrast and wider variability (1.73–39.3 kg m−2) of the SOC pool were predicted with lower global prediction errors (mean estimation error = −0.11 kg m−2, RMSE = 6.40 kg m−2) in GWR compared with the other approaches. A relative improvement of 22% over MLR and 2% over RK was observed in SOC prediction. The total SOC pool to the 0.5‐m depth was estimated to be 6.22 Pg. The results suggest that the GWR approach is a promising tool for regional‐scale SOC prediction.

Composition and Organic Carbon Distribution of Organomineral Complex in Black Soil under Different Land Uses and Management Systems

Organomineral complexes form the basis of soil fertility and have significant effects on the soil environment. In this research, we aimed to study the composition and organic carbon (C) distribution of organomineral complexes in a black soil under different land uses and management by means of ultrasonic dispersion and particle assortment. The results showed that the fine sand–size complex (20–200 μm) was dominant under different land uses and management. Silt-size (2–20 μm) and fine sand-size content increased with nitrogen and phosphorus application (NP) and NPM (NP together with organic manure) treatment, whereas clay-size (0–2 μm) content decreased. The content of <20-μm complex in GL (grassland) was less than in BL (bareland), and >20-μm complex showed the opposite trend. The silt-size content increased with the increase of SOC (soil organic C). A negative relationship was observed between the clay-size complex content and SOC content. Land-use change resulted in different dynamics in C sequestration in soil. The content of <20-μm complex in GL was more than in NP and NPM; GL has potential to sequester more C than tilled soil because of the stability of SOC stored in the <20-μm fraction. Long-term application of organic manure and vegetation restoration increased the OC (organic carbon) content of all sizes of complexes; the OC contents of clay-size complex were in the order GL > NPM > NP > BL > NF (no fertilizer applied) and increased the proportion of OC in >20-μm complexes, indicating that OC content in sand-size fractions increased with total SOC content.

Organic‐carbon and nitrogen stocks and organic‐carbon fractions in soil under mixed pine and oak forest stands of different ages in NE Germany

AbstractThe objective of this work was to evaluate the C and N stocks and organic‐C fractions in soil under mixed forest stands of Scots pine (Pinus sylvestris L.) and Sessile oak (Quercus petraea [Matt.] Liebl.) of different ages in NE Germany. Treatments consisted of pure pine (age 102 y), and pine (age 90–102 y) mixed with 10‐, 35‐, 106‐, and 124‐y‐old oak trees. After sampling O layers, soils in the mineral layer were taken at two different depths (0–10 and 10–20 cm). Oak admixture did not affect total organic‐C (TOC) and N stocks considering the different layers separately. However, when the sum of TOC stocks in the organic and mineral layers was considered, TOC stocks decreased with increasing in oak age (r2 = 0.58, p &lt; 0.10). The microbial C (CMB) was not directly correlated with increase of oak age, however, it was positively related with presence of oak species. There was an increase in the percentage of the CMB‐to‐TOC ratio with increase of oak‐tree ages. On average, light‐fraction C (CLF) comprised 68% of the soil TOC in upper layer corresponding to the highest C pool in the upper layer. CLF and heavy‐fraction C (CHF) were not directly affected by the admixture of oak trees in both layers. The CHF accounted on average for 30% and 59% of the TOC at 0–10 and 10–20 cm depths, respectively. Despite low clay contents in the studied soils, the differences in the DCB‐extractable Fe and Al affected the concentrations of the CHF and TOC in the 10–20 cm layers (p &lt; 0.05). Admixture of oak in pine stands contributed to reduce topsoil C stocks, probably due to higher soil organic matter turnover promoted by higher quality of oak litter.

Carbon Storage in Soil Size Fractions Under Two Cacao Agroforestry Systems in Bahia, Brazil

Shaded perennial agroforestry systems contain relatively high quantities of soil carbon (C) resulting from continuous deposition of plant residues; however, the extent to which the C is sequestered in soil will depend on the extent of physical protection of soil organic C (SOC). The main objective of this study was to characterize SOC storage in relation to soil fraction-size classes in cacao (Theobroma cacao L.) agroforestry systems (AFSs). Two shaded cacao systems and an adjacent natural forest in reddish-yellow Oxisols in Bahia, Brazil were selected. Soil samples were collected from four depth classes to 1 m depth and separated by wet-sieving into three fraction-size classes (>250 microm, 250-53 microm, and <53 microm)-corresponding to macroaggregate, microaggregate, and silt-and-clay size fractions-and analyzed for C content. The total SOC stock did not vary among systems (mean: 302 Mg/ha). On average, 72% of SOC was in macroaggregate-size, 20% in microaggregate-size, and 8% in silt-and-clay size fractions in soil. Sonication of aggregates showed that occlusion of C in soil aggregates could be a major mechanism of C protection in these soils. Considering the low level of soil disturbances in cacao AFSs, the C contained in the macroaggregate fraction might become stabilized in the soil. The study shows the role of cacao AFSs in mitigating greenhouse gas (GHG) emission through accumulation and retention of high amounts of organic C in the soils and suggests the potential benefit of this environmental service to the nearly 6 million cacao farmers worldwide.

Carbohydrates in cyanobacterial soil crusts as a source of carbon in the southwest Kalahari, Botswana

Semiarid ecosystems, also known as drylands, contain small amounts of carbon (C). To date only few studies have evaluated soil C dynamics in these ecosystems. Cyanobacterial soil crusts are considered a major source of organic C in semiarid ecosystems through photosynthesis, increasing soil organic C (SOC) pool as carbohydrates. This study considers cyanobacterial soil crusts as a source of C in the southwest Kalahari. Cyanobacterial soil crusts are well adapted to drylands because extracellular polysaccharide (EPS) secretions provide stabilization of soil and resistance to desiccation. The carbohydrate and chlorophyll a content were evaluated in the upper soil profile on Kalahari Sands and pan soils. Topsoil carbohydrate concentration decreased exponentially with depth. The carbohydrate content produced in cyanobacterial soil crusts in the southwest Kalahari represents up to 75% of the total SOC and is thus an essential component for the fertility of Kalahari Sand soils.

Long-term fertilization and manuring effects on physically-separated soil organic matter pools under a wheat–wheat–maize cropping system in an arid region of China

With increasing food demand worldwide, agriculture in semiarid and arid regions becomes increasingly important, though knowledge about organic matter (OM) conserving management systems is scarce. This study aimed at examining organic C (OC) and nitrogen (N) concentrations in various soil OM pools affected by 26-years application of chemical fertilizer and farmyard manure at an arid site of Gansu Province, China. Macro OM (>0.05 mm) was extracted by wet sieving and then separated into light macro OM (<1.8 g cm −3) and heavy macro OM (>1.8 g cm −3) sub-fractions; bulk soil was differentiated into free particulate OM (FPOM, <1.6 g cm −3), occluded particulate OM (OPOM, 1.6–1.8 g cm −3) and mineral-associated OM (>1.8 g cm −3). OC and N concentrations of heavy macro OM and FPOM were slightly affected by long-term N fertilization alone and its combination with P and K, but their magnitudes of change had not significantly contributed to total soil OC and N concentrations. Farmyard manure increased light macro OC and N by 58 and 70%, heavy macro OC and N by 86 and 117%, free particulate OC and N by 29 and 55%, occluded particulate OC and N by 29 and 55%, and mineral-associated OC and N by 44 and 48%, respectively, compared to nil-manure. Mineral fertilization improved soil OM quality by decreasing C/N ratio in the light macro OM and FPOM fractions where farmyard manure was absent. Organic manure led to a decline of the C/N ratio in all physically-separated OM fractions possibly due to the increased input of processed organic materials. We found about two thirds of macro OM was actually located within 2–0.05 mm organo-mineral associations or/and aggregates. In conclusion, this study stresses the vital importance to apply organic manure to the wheat-corn production system characterized by straw removal and conventional tillage in the region.

Effects of reduced tillage, crop residue management and manure application practices on crop yields and soil carbon sequestration on an Andisol in northern Japan

Soil carbon sequestration in agricultural lands has been deemed a sustainable option to mitigate rising atmospheric CO2 levels. In this context, the effects of different tillage and C input management (residue management and manure application) practices on crop yields, residue C and annual changes in total soil organic C (SOC) (0–30 cm depth) were investigated over one cycle of a 4-year crop rotation (2003–2006) on a cropped Andisol in northern Japan. For tillage practices, the effects of reduced tillage (no deep plowing, a single shallow harrowing for seedbed preparation [RT]) and conventional deep moldboard plow tillage (CT) were compared. The combination of RT, residue return and manure application (20 Mg ha−1 in each year) increased spring wheat and potato yields significantly; however, soybean and sugar beet yields were not influenced by tillage practices. For all crops studied, manure application enhanced the production of above-ground residue C. Thus, manure application served not only as a direct input of C to the soil, but the greater crop biomass production engendered enhanced subsequent C inputs to the soil from residues. The SOC contents in both the 0–5 cm and 5–10 cm layers of the soil profile were greater under RT than under CT treatments because the crop residue and manure were densely incorporated into the shallow soil layers. Comparatively, neither tillage nor C input management practices had significant effects on annual changes in SOC content in either the 10–20 cm or 20–30 cm layers of the soil profile. When soil C sequestration rates, as represented by annual changes in total SOC (0–30 cm), were assessed on a total soil mass basis, an anova showed that tillage practices had no significant effect on total C sequestration, but C input management practices had significant positive effects (P ≤ 0.05). These results indicate that continuous C input to the soil through crop residue return and manure application is a crucial practice for enhancing crop yields and soil C sequestration in the Andisol region of northern Japan.

Can Apparent Electrical Conductivity Improve the Spatial Characterization of Soil Organic Carbon?

Ancillary information, such as apparent electrical conductivity (ECa), can improve the spatial and temporal estimation of soil properties. The purpose of this study was to determine if ECacould be used for the spatial characterization of soil organic C (SOC) within a long‐term tillage experiment. Apparent electrical conductivity was measured using an electromagnetic induction sensor, the EM38DD, and its predictive potential for mapping SOC was evaluated. The ECamaps showed clear differences between the conventional tillage and direct drilling plots, with higher ECaand SOC in the direct drilling plots. A normalized ECadifference (ΔECa), calculated as the difference between the normalized vertical and horizontal dipole ECavalues (ECaVand ECaH, respectively) successfully classified the SOC observations according to their corresponding management systems. Maps of ΔECa(FKM1) and ECaVand ECaH(FKM2) classified by fuzzyk‐means accounted for 30% of the total SOC variability, whereas the individual plots and management strategy explained 44 and 41%, respectively. Simple kriging with local varying means using either FKM2 or plot‐average SOC as secondary information reduced the RMSE by 8% and increased the efficiency index by about 70% compared with ordinary kriging. Despite the low point‐to‐point correlation between ECaand SOC, ECawas shown to be useful for the spatial estimation of SOC.

Fertilization effects on yield sustainability and soil properties under irrigated wheat–soybean rotation of an Indian Himalayan upper valley

To date, the sustainability of wheat (Triticumaestivum)–soybean (Glycinemax) cropping systems has not been well assessed, especially under Indian Himalayas. Research was conducted in 1995–1996 to 2004 at Hawalbagh, India to study the effects of fertilization on yield sustainability of irrigated wheat–soybean system and on selected soil properties. The mean wheat yield under NPK + FYM (farmyard manure) treated plots was ~27% higher than NPK (2.4 Mg ha−1). The residual effect of NPK + FYM caused ~14% increase in soybean yield over NPK (2.18 Mg ha−1). Sustainable yield index values of wheat and the wheat–soybean system were greater with annual fertilizer N or NPK plots 10 Mg ha−1 FYM than NPK alone. However, benefit:cost ratio of fertilization, agronomic efficiency and partial factor productivity of applied nutrients were higher with NPK + FYM than NPK, if FYM nutrients were not considered. Soils under NPK + FYM contained higher soil organic C (SOC), total soil N, total P and Olsen-P by ~10, 42, 52 and 71%, respectively, in the 0–30 cm soil layers, compared with NPK. Non-exchangeable K decreased with time under all treatments except NPK. Total SOC in the 0–30 cm soil layer increased in all fertilized plots. Application of NPK + FYM also improved selected soil physical properties over NPK. The NPK + FYM application had better soil productivity than NPK but was not as economical as NPK if farmers had to purchase manure.

Soil organic carbon contents in long‐term experimental grassland plots in the UK (Palace Leas and Park Grass) have not changed consistently in recent decades

AbstractA recent report of widespread declines in soil organic C (SOC) in the UK over the 10–25 years until the early 2000s has focussed attention on the importance of resampling previously characterized sites to assess long‐term trends in SOC contents and the importance of soils as a potentially volatile and globally significant reservoir of terrestrial C. We have used two sets of long‐term experimental plots which have been under constant and known management for over a century and for which historical data exist that allow comparison over recent decades to determine what, if any, changes in SOC content have occurred. The plots used are the Palace Leas (PL) Meadow Hay Plots in north‐east England (UK) established in 1897, and from the Park Grass (PG) Continuous Hay experiment established in 1856 at Rothamsted in south‐east England. Collectively, these plots represent the only grassland sites in the UK under long‐term management where changes in SOC over several decades can be assessed, and are probably unique in the world. The plots have received different manure and fertilizer treatment and have been under known management for at least 100 years. In 1982, total SOC contents were determined for the 0–27 cm layer of six of the PL plots using measurements of SOC concentrations, bulk density and soil depth. In 2006, the same six PL plots were resampled and SOC contents determined again. Four of the plots showed no net change in SOC content, but two plots showed net loss of SOC of 15% and 17% (amounting to decreases of 18 and 15 t C ha−1) since 1982. However, these differences in total SOC content were in a similar range to the variations in bulk density (6–31%) with changing soil water content. In 1959, the soil masses and SOC concentrations to 23 cm depth were measured on six PG plots with fertilizer and manure treatments corresponding closely with those measured on PL. In 2002, the SOC concentrations on the same plots were measured again. On three of the PG plots, SOC concentrations had declined by 2–10%, but in the other three it had increased by 4–8% between 1959 and 2002. If it is assumed that the soil bulk density had not changed over this period, the losses of SOC from the top soils ranged range from 10 to 3 t C ha−1, while the gains ranged from 4 to 7 t C ha−1. When the differences with time in SOC contents for the six PL and the six PG plots were examined using paired t‐tests, that is, regarding the plots as two sets of six replicate permanent grasslands, there were no significant differences between 1982 and 2006 for the PL plots or between 1959 and 2002 for the PG plots. Thus, these independent observations on similar plots at PL and PG indicate there has been no consistent decrease in SOC stocks in surface soils under old, permanent grassland in England in recent decades, even though meteorological records for both sites indicate significant warming of the soil and air between 1980 and 2000. Because the potential influences of changes in management or land use have been definitively excluded, and measured rather than derived bulk densities have been used to convert from SOC concentrations to SOC amounts, our observations question whether for permanent grassland in England, losses in SOC in recent decades reported elsewhere can be attributed to widespread environmental change.

Soil Organic Carbon Stocks of Rio Grande do Sul, Brazil

Robust and accurate regional estimates of C storage in soils are currently an important research topic because of ongoing debate about human‐induced changes in the terrestrial C cycle. Widely available geoprocessing tools were applied to estimate native soil organic C (SOC) stocks of Rio Grande do Sul state in southern Brazil to a depth of 30 cm from previously sampled soil pedons under undisturbed vegetation. The study used a statewide comprehensive soil survey comprising a small‐scale soil map, a climate map, and a soil pedon database. Soil organic C stocks under native vegetation were calculated with two different approaches: the Tier 1 method of the Intergovernmental Panel on Climate Change (IPCC) and a refined method based on actual field measurements derived from soil profile data. Highest SOC stocks occurred in Neossolos Quartzarênico hidromórfico (Aquents), Organossolos Tiomórficos (Hemists), Latossolos Brunos (Udox), and Vertissolos Ebânicos (Uderts) soil classes. Before human use of soils, most C was stored in the Latossolos Vermelhos (Udox) and Neossolos Regolíticos (Orthents), which occupy a large area of Rio Grande do Sul. Generally, IPCC default reference SOC stocks compared well with SOC stocks calculated from soil pedons. The total SOC stock of Rio Grande do Sul was estimated at 1510.3 Tg C (5.8 kg C m−2) by the IPPC method and 1597.5 ± 363.9 Tg C (7.4 ± 1.9 kg C m−2) calculated from soil pedons. The SOC digital map and SOC database developed in this study provide crucial background information for state‐level contemporary assessment of C stocks and soil C sequestration programs and initiatives.

Changes in soil organic carbon, nutrients and aggregation after conversion of native desert soil into irrigated arable land

This study aimed at investigating the effects of agricultural exploitation on desert soil organic C, N and P, and soil aggregation. Four land uses were assessed: (1) 5-year wheat ( Triticum aestivum L.)/barley ( Hordeum vulgare L.) + 5-year maize ( Zea mays L.); (2) 5-year wheat/barley + 5-year alfalfa ( Medicago sativa L.); (3) 6-year wheat/barley + 4-year acacia ( Robinia pseudoacacia L.) and (4) uncultivated desert soil. The desert soil contained total organic C (TOC) of 3.1, 3.7 and 4.2 g kg −1 and particulate organic C (POC) of 0.6, 0.7 and 0.8 g kg −1 at 0–10, 10–20 and 20–30 cm depths, respectively. The soil TOC concentration was increased by 32–68% under wheat–maize rotation and by 27–136% under wheat–acacia at 0–20 cm depth, and by 48% under wheat–alfalfa only at 0–10 cm depth. This contrasted with an increase in the soil POC concentration by 143–167% at depth 0–20 cm under wheat–maize and by 217%, 550% at depth 0–10 cm under wheat–alfalfa and wheat–acacia, respectively. The desert soil had 13 Mg ha −1 TOC stock and 2 Mg ha −1 POC stock at depth 0–30 cm, whereas crop rotations increased the soil TOC stock by 30–65% and POC stock by 200–350%. Over the 10-year period, the rates of TOC accumulation were 0.6, 0.3, 0.8 Mg ha −1 year −1 and the rates of POC accumulation were 0.4, 0.4 and 0.7 Mg ha −1 year −1 under wheat–maize, wheat–alfalfa and wheat–acacia rotations, respectively. At 0–30 cm depth, total soil N was increased by 61–64% under wheat–maize and wheat–acacia, but total soil P was reduced by 38% under wheat–alfalfa. A significant improvement in clay stability but not in aggregate water-stability was observed in cultivated soils. The results showed a significant increase in soil organic C pool but unimproved macro-aggregation of the desert soil after 10 years of cultivation.

Regional Study of No‐Till Effects on Carbon Sequestration in the Midwestern United States

No‐till (NT) agriculture has been promoted as one of the optimal management practices that preserves soil and water, and increases soil organic C (SOC) compared with conventional tillage (CT) practices. Information on SOC sequestration in NT systems, however, has been based on measurements from the surface soil (&lt;30 cm) and little is known about the extent of SOC sequestration in NT across the entire 0‐ to 60‐cm soil profile. We conducted a regional study of NT farming to assess the extent of SOC sequestration in the whole soil profile across 12 contrasting but representative soils in the Midwestern United States, each within a Major Land Resource Area (MLRA: 98, 111C, 114B, 122 in Indiana; 111A, 111B, 111D, 124, and 126 in Ohio; and 127 and 147 in Pennsylvania). Soils on gentle terrain were sampled in paired NT and CT fields as well as in an adjacent woodlot in each MLRA. The SOC and N concentrations were greater in the surface 0‐ to 5‐cm soil in NT than CT in MLRA 124. The SOC concentration in CT soil was greater than in NT soil at 10 to 30 cm in MLRAs 98 and 126. The total SOC pool for the whole soil profile did not differ between NT and CT in eight of the 12 MLRAs and the total profile SOC was actually greater under CT in MLRAs 98, 127, and 126, resulting in negative C sequestration rates on conversion from CT to NT in these three MLRAs. This regional study suggests that the entire soil profile must be examined and ecosystem C budget assessed when elucidating SOC sequestration in NT vs. CT fields.

Modeling of Soil Organic Carbon Fractions Using Visible–Near‐Infrared Spectroscopy

There is a pressing need for rapid and cost‐effective tools to estimate soil C across larger landscapes. Visible–near‐infrared diffuse reflectance spectroscopy (VNIRS) offers comparable levels of accuracy to conventional laboratory methods for estimating various soil properties. We used VNIRS to estimate soil total organic C (TC) and four organic C fractions in 141 samples collected in the Santa Fe River watershed of Florida. The C fractions measured were (in order of decreasing potential residence time in soils): recalcitrant C (RC), hydrolyzable C (HC), hot‐water‐soluble C (SC), and mineralizable C (MC). Soil samples were scanned in the visible–near‐infrared spectral range. Six preprocessing transformations were applied to the soil reflectance, and five multivariate techniques were tested to model soil TC and the organic C fractions: stepwise multiple linear regression (SMLR), principal components regression, partial least squares regression (PLSR), regression tree, and committee trees. Total organic C was estimated with the highest accuracy, obtaining a coefficient of determination using a validation set (Rv2) of 0.86, followed by RC (Rv2 = 0.82), both using PLSR. The SC fraction was modeled best by SMLR (Rv2 = 0.70), while PLSR produced the best models of MC (Rv2 = 0.65) and HC (Rv2 = 0.40). The addition of TC as a predictor improved the VNIRS models of the soil organic C fractions. Our study indicates the suitability of VNIRS to quantify soil organic C pools with widely varying turnover times in soils, which are important in the context of C sequestration and climate change.

Microaggregation and organic matter storage at the silt-size scale

The relationship between soil aggregation at the silt-size scale, and soil organic C dynamics has scarcely been studied. In this work, we developed a fractionation protocol comprising complete dispersion of the fraction > 50 μm of the original soil sample, and subsequent size and density fraction of the < 50 μm fraction to isolate stable aggregates from the silt-size fraction of a silty cultivated temperate soil. This fraction also contained non-aggregated silt-size mineral particles and small amounts of non-occluded light organic particles. Slaking-resistant silt-size aggregates stored almost half of the total soil organic C (TOC) as organic matter bound to silt (8% TOC) and clay minerals (28% TOC) and as occluded organic particles (8% TOC). Clay mineralogy was related to aggregation at this scale, since aggregates were selectively formed from 2:1 clays, while quartz and kaolinite-like clays were preferentially found in the non-aggregated clay fraction. We conclude that stable microaggregates are formed within the silt-size fraction and store organic C both by adsorption and by entrapment of small organic particles. The turnover rate of organic carbon in such microstructures, their sensitivity to land use and cultivation practices, and their importance in different soil types remain to be determined.

Indications for Soil Carbon Saturation in a Temperate Agroecosystem

The soil C saturation concept postulates that there is an upper limit to the equilibrium soil C level of mineral soils even when soil C input is increased. To test this concept, we analyzed the relationship between steady‐state soil C input and soil organic C (SOC) concentration in a temperate corn (Zea mays L.) agroecosystem experiment located in Lexington, KY. In this experiment, a gradient in plant productivity and consequent C input to the soil was produced with four N application rates (0, 84, 168, and 336 kg N ha−1 yr−1) under two disturbance regimes, no‐till (NT) and moldboard plowing (MP). We examined whether physical protection of organic matter by soil aggregation and chemical protection by association with silt and clay particles led to SOC saturation. We used wet sieving to physically separate SOC pools that differ in C stabilization potential, and determined the C sequestration in each fraction. Total SOC was positively related to C input, and this was primarily due to C stabilization in small macroaggregates. In both tillage systems, however, microaggregate C and silt‐plus‐clay C did not increase with greater C input. Within the small macroaggregates, coarse particulate organic matter C and microaggregate C increased with C input, but there was no increase in silt‐plus‐clay C. Our results indicate that soil fractions with low C stabilization potential exhibit C saturation behavior. Apparent C saturation of some of the fractions indicates that SOC pools have a limited capacity to stabilize added C and that such a limit to C stabilization will constrain the ecosystem services provided by these SOC pools.

Short-term and long-term effects of tillage and crop rotation on soil physical properties, organic C and N in a Black Chernozem in northeastern Saskatchewan

A field experiment was conducted for 8 yr (from 1994 to 2001) in a Black Chernozem (Udic Haploboroll) high in organic C (5.5%) at Melfort, Saskatchewan, Canada, to assess the environmental impact of tillage and crop rotation. Short-term effects on residue cover, soil temperature, moisture, aggregation, bulk density and hydraulic conductivity were measured, and long-term effects on total organic C (TOC) and N (TON), and light fraction organic C (LFOC) and N (LFON) were determined. There were two tillage systems [no tillage (NT) and conventional tillage (CT)] and three 4-yr annual crop rotations (canola-wheat-barley-barley,canola-barley-pea-wheat,and canola-pea-flax-barley). Straw was returned to soil every year in all treatments with the exception of flax residue. Soil samples were taken for analysis of organic C and N after the harvest of canola at the end of second 4-yr rotation cycle. Residue cover on soil was higher under NT than CT. Soil temperature was lower and soil moisture was higher in spring, under NT than CT. The proportion of fine aggregates (&lt; 1.3 mm) in soil was lower and that of medium (2.0–12.7 mm) and large aggregates (&gt; 12.7 mm) was generally higher under NT compared with CT. The mean weight diameter (MWD) of aggregates was also larger under NT compared with CT in most cases. The proportion of fine aggregates was higher after canola compared with pea in some cases, but the proportion of medium and large aggregates was higher after pea compared with canola, and in rotations with 50% than 75% broadleaf crops in 2yr. Soil hydraulic conductivity and bulk density were lower under NT than CT in some instances. Compared with CT, NT had greater mass of TOC, TON, LFOC and LFON in soil. However, the differences were significant only for LFOC and LFON, in dicating that light fraction organic C and N was more responsive to NT than CT compared with total organic C and N. In conclusion, the findings suggest that there is no interaction between tillage and rotation, and that light fraction C and N accumulate when tillage is eliminated despite high levels of soil organic C in the soil. Key words: Aggregation, bulk density, crop rotation, hydraulic conductivity, organic C and N, moisture, residue cover soil, temperature, tillage

Tillage Effects on Soil Organic Carbon Fractions in Mediterranean Dryland Agroecosystems

Under semiarid conditions, soil quality and productivity can be improved by enhancing soil organic matter content by means of alternative management practices. In this study, we evaluated the feasibility of no‐till (NT) and cropping intensification as alternative soil practices to increase soil organic C (SOC). At the same time, we studied the influence of these management practices on two SOC fractions (particulate organic matter C, POM‐C, and the mineral‐associated C, Min‐C), in semiarid agroecosystems of the Ebro River valley. Soil samples were collected from five soil layers (0–5‐, 5–10‐, 10–20‐, 20–30‐, 30–40‐cm depth) during July 2005 at three long‐term tillage experiments located at different sites in the Ebro River valley (northeast Spain). Soil bulk density, SOC concentration and content, SOC stratification ratio, POM‐C, and Min‐C were measured. Higher soil bulk density was observed under NT than under reduced tillage (RT), subsoil tillage (ST), or conventional tillage (CT). At the soil surface (0–5‐cm depth), the highest total SOC concentration, POM‐C, and Min‐C were measured under NT, followed by RT, ST, and CT, respectively. In the whole soil profile (0–40 cm), similarly, slightly greater SOC content was measured under NT than under CT with the exception of the Selvanera site, where deep subsoil tillage combined with moldboard plowing accumulated more SOC than NT. In semiarid Mediterranean agroecosystems where CT consists in moldboard plowing, NT is a viable management practice to increase SOC.