Soil Organic C Content Research Articles

The impact of long-term liming on soil organic carbon and aggregate stability in low-input acid soils

This study used two field trials with 5 and 34 years of liming histories, respectively, and aimed to elucidate the long-term effect of liming on soil organic C (SOC) in acid soils. It was hypothesized that long-term liming would increase SOC concentration, macro-aggregate stability and SOC concentration within aggregates. Surface soils (0–10 cm) were sampled and separated into four aggregate-size classes: large macro-aggregates (>2 mm), small macro-aggregates (0.25–2 mm), micro-aggregates (0.053–0.25 mm) and silt and clay fraction (<0.053 mm) by wet sieving, and the SOC concentration of each aggregate-size was quantified. Liming decreased SOC in the bulk soil and in aggregates as well as macro-aggregate stability in the low-input and cultivated 34-year-old trial. In contrast, liming did not significantly change the concentration of SOC in the bulk soil or in aggregates but improved macro-aggregate stability in the 5-year-old trial under undisturbed unimproved pastures. Furthermore, the single application of lime to the surface soil increased pH in both topsoil (0–10 cm) and subsurface soil (10–20 cm) and increased K2SO4-extractable C, microbial biomass C (Cmic) and basal respiration (CO2) in both soil layers of both lime trials. Liming increased the percentage of SOC present as microbial biomass C (Cmic/Corg) and decreased the respiration rate per unit biomass (qCO2). The study concludes that despite long-term liming decreased total SOC in the low-input systems, it increased labile C pools and the percentage of SOC present as microbial biomass C.

Repeated monitoring of organic carbon stocks after eight years reveals carbon losses from intensively managed agricultural soils in Western Germany

AbstractPast land‐use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne‐Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability.We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg−1 in 2005 to 9.8 g kg−1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg−1 in 2005 to 3.5 g kg−­1 in 2013). Intriguingly, these changes were due to changes in mineral‐bound SOC rather than to changes in sand‐sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)−1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils [−0.73 t SOC (ha · y)−1], and less pronounced for loamy soils [−0.64 t SOC (ha · y)−1]; silty soils revealed the smallest reduction in SOC [−0.3 t SOC (ha · y)−1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)−1 (2003–2005) to about 133 kg C (ha · y)−1 (2005–2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics.

Eucalyptus and Pinus stand density effects on soil carbon sequestration

Although afforestation of pasture and agricultural systems can increase C sequestration in soils, studies have shown wide variation in the magnitude, timing and direction of soil organic C (SOC) dynamics, depending on site conditions, management practices, and previous land use. Effects of stocking density on SOC have yet to be elucidated, however. The objectives proposed for this work were to quantify: (i) SOC content and distribution for the top 30cm of the A horizon under native pasture compared with that following 8years of afforestation (Eucalyptus grandis and Pinus taeda planted at different densities); (ii) SOC accumulation in the AB and the top of Bt horizons in afforested soils; and (iii) the contribution of the new vegetation (Eucalyptus and Pinus) to SOC. A field trial with three stand-densities of E. grandis and P. taeda and corresponding native pasture was established and soil samples were collected and assayed for five layers of the A horizon (0–5, 5–10, 10–15, 15–20, 20–30cm), and the AB and Bt1 horizons. Soil organic C concentration and δ13C were determined, and the total SOC stock and C derived from the Eucalyptus and Pinus vegetation (young C) were calculated. Our results suggest that there was likely no significant change in SOC stocks in response to 8years of afforestation with either Eucalyptus or Pinus. No significant differences in SOC stocks in the upper 30cm soil layer as a whole were found among pasture, Eucalyptus, and Pinus treatments. By contrast, SOC in the AB and Bt1 horizons was significantly higher under afforested sites than native pasture (P<0.001 and P<0.039, respectively). Soil δ13C in the afforested treatments (0–30cm) reflects the contribution of the new vegetation (Eucalyptus or Pinus) to SOC. Net accumulation of new SOC from the planted trees in the top 15cm soil layer was equivalent to 0.20MgCha−1yr−1 for Eucalyptus and 0.30MgCha−1yr−1 for Pinus. Assuming initial declines in SOC following afforestation have largely ended and young C sequestration rates continue at the same rate, SOC stocks in the top 15cm layer would reach those found under pasture after 15 and 11years for Eucalyptus and Pinus plantations, respectively. Measurements of additional sites afforested with Eucalyptus and Pinus of different ages are needed to draw firm conclusions about the net SOC balance in these afforested pasture soils.

Microbial biomass, fungal and bacterial residues, and their relationships to the soil organic matter C/N/P/S ratios

The interrelations between the microbial biomass, total N, organic P, and organic S in the accumulation of soil organic C (SOC) and the contribution of fungal and bacterial residues to SOC, based on amino sugar data, were investigated in the current study. The soils had been developed under humid temperate, arid sub-tropical, and tropical climatic conditions and were used as arable, grass, and forest land. They covered a wide range in soil pH, in salinity as well as in the contents of clay and SOC. An increased microbial biomass C/N ratio due to nutrient limitation is not reflected by any increase in SOM elemental C/N/P/S ratio within the total nutrient pool or within the organic fraction. Increased SOC/total N and SOC/organic S ratios reduced the contribution of microbial biomass C to SOC, whereas neutral soil pH and high clay contents had positive effects. An increased formation of microbial biomass C also enlarged the contribution of microbial residue C to SOC, which accounted on average for 48% SOC in the neutral arable and moderate acidic grassland soils and for 30% in the saline arable and strongly acidic forest soils. The fungal C to bacterial C ratio increased with increasing acidity from 0.9 in the saline arable soils to 4.5 in the strongly acidic forest soils. Consequently, the relationship of the fungal C to bacterial C ratio was not simply related to the contribution of microbial residues to SOC. The lower the ergosterol to fungal GlcN ratio, the higher the contribution of microbial residues to SOC. This means the higher the contribution of arbuscular mycorrhizal fungi and the lower the contribution of saprotrophic fungi to the microbial community, the more fungal residues can be sequestered to SOC.

Humus, nitrogen and energy balances, and greenhouse gas emissions in a long-term field experiment with compost compared with mineral fertilisation

Compost fertilisation is one way to close material cycles for organic matter and plant nutrients and to increase soil organic matter content. In this study, humus, nitrogen (N) and energy balances, and greenhouse gas (GHG) emissions were calculated for a 14-year field experiment using the model software REPRO. Humus balances showed that compost fertilisation at a rate of 8 t/ha.year resulted in a positive balance of 115 kg carbon (C)/ha.year. With 14 and 20 t/ha.year of compost, respectively, humus accumulated at rates of 558 and 1021 kg C/ha.year. With mineral fertilisation at rates of 29–62 kg N/ha.year, balances were moderately negative (–169 to –227 kg C/ha.year), and a clear humus deficit of –457 kg C/ha.year showed in the unfertilised control. Compared with measured soil organic C (SOC) data, REPRO predicted SOC contents fairly well with the exception of the treatments with high compost rates, where SOC contents were overestimated by REPRO. GHG balances calculated with soil C sequestration on the basis of humus balances, and on the basis of soil analyses, indicated negative GHG emissions with medium and high compost rates. Mineral fertilisation yielded net GHG emissions of ~2000 kg CO2-eq/ha.year. The findings underline that compost fertilisation holds potential for C sequestration and for the reduction of GHG emissions, even though this potential is bound to level off with increasing soil C saturation.

Dependence of Soil Organic Carbon on available Iron and Manganese Concentrations in Submerged Rice Soils

Dependence of Soil Organic Carbon on available Iron and Manganese Concentrations in Submerged Rice Soils Effect of soil submergence on changes in soil organic C (SOC) and DTPA-extractable Fe and Mn and their interrelationship was studied in three texturally-different soils viz. loamy sand, clay loam and silty clay traditionally grown to rice-wheat system. Laboratory incubation experiments were conducted for 120 d under controlled temperature and moisture conditions. Soil organic C increased with incubation period up to 30-45 d and declined sharply thereafter. Similarly, concentration of DTPA-Fe was highest at 30 d and decreased sharply thereafter. However, DTPA-Mn concentration was the highest at 15 and 30 d of incubation, under submerged and field capacity moisture regimes, respectively. Soils incubated under submerged conditions exhibited higher SOC, DTPA-Fe, and DTPAMn than those incubated at field capacity moisture. In all the three soils under submerged conditions SOC, DTPA-Fe and Mn were higher at 40o C followed by 30o C and 20o C temperature. Irrespective of soil and incubation temperature, values of these variables did not change with incubation of air dry soil. The concentration of DTPAFe and Mn showed positive relationship with SOC concentration at submerged and field capacity moisture regimes. Increase in SOC with submergence was considered to be due to an increase in DTPA-Fe and Mn suggesting the need for using some chelating agents to remove their interference in SOC determination by wet digestion method..

Soil Organic Carbon Stocks in Deep Soils at a Watershed Scale on the Chinese Loess Plateau

Core Ideas:SOC stocks under three land uses were measured to a soil depth of 500 cm.SOC stocks in the 0‐200 cm soil layer were significantly the lowest under shrubland.Significantly higher SOC stocks occurred in the loamy than in the sandy soils.Land use, soil texture, and water significantly affected SOC stock to depths of 500 cm.A considerable amount of SOC is stored in deep soils on the Chinese Loess Plateau.An accurate evaluation of soil organic C (SOC) stocks is important to C management and to understanding fully the role of soils in the C cycle. However, SOC stocks in deep soils and the factors that affect them have been largely ignored. We measured SOC stocks and other soil properties to a depth of 500 cm (n= 73) under three land uses in the Lao Ye Man Qu watershed on the Chinese Loess Plateau. Similar patterns in the vertical distributions of SOC stocks were found under cropland, grassland, and shrubland. However, for the 0‐ to 200‐cm soil layer, SOC stocks were significantly lower under shrubland than under either cropland or grassland. The SOC stocks under cropland, grassland, and shrubland in the 0‐ to 100‐cm layer were 2.64 ± 0.67, 2.50 ± 0.69, and 1.99 ± 0.22 kg m−2, respectively; in the 0‐ to 500‐cm layer they were 8.34 ± 2.26, 8.37 ± 2.01, and 7.26 ± 1.00 kg m−2, respectively. Significantly higher SOC stocks occurred in loamy than sandy soils (P&lt; 0.01), and they were lower at the mid than at the upper and lower slope positions. However, SOC stocks were similar in sunlit and shaded soils. Pearson correlation analysis indicated that land use, soil texture, and soil water content significantly affected SOC stocks in each 100‐cm soil layer to a depth of 500 cm. Effects of topographic features and pH varied with increasing depth. A considerable amount of SOC was stored in deep soils, indicating the SOC content dependence on time and depth. This information is essential for evaluating C fluxes, estimating C stocks, and for managing the C cycles in regions around the world with deep soils.

Stand Age and Productivity Control Soil Carbon Dioxide Efflux and Organic Carbon Dynamics in Poplar Plantations

Establishment of hybrid poplar (HP) plantations has been advocated as an effective method for sequestering CO2 from the atmosphere. However, how carbon (C) cycling in HP plantations changes during stand development under different soil fertility and climatic conditions is poorly understood. We studied the dynamics of soil respiration (Rs) and its heterotrophic (Rh) and autotrophic (Ra) components and soil organic C (SOC) content in HP plantations of different stand ages established on soils with different productivity (Class 1 vs. Class 3 soils) in Alberta (AB) and Ontario (ON), Canada. The Rs was the highest for AB-Class 1 (high-productivity) soils, followed by AB-Class 3 and ON-Class 3 (low-productivity) soils. Annual Rs, Rh, and Rh/Rs decreased in the first 3 yr in AB-Class 1 and 6 yr in AB-Class 3 soils after plantation establishment and increased thereafter, but they increased with stand age after plantation establishment in ON-Class 3 soils. The SOC content in the AB-Class 1, AB-Class 3, and ON-Class 3 sites decreased in the first 3, 5, and 1.5 yr, respectively, after plantation establishment and then increased, reaching the pre-establishment level after 6, 10, and 3 yr, respectively. From this first attempt to understand C cycling across multiple sites with different soil fertility levels, we conclude that stand age and soil productivity play a significant role in affecting C cycling, and such effects should be considered in regional and global C cycling models, especially when assessing the C sequestration potential of HP plantations.

Short-term responses of soil organic carbon and carbon pool management index to different annual straw return rates in a rice–wheat cropping system

Crop straw return is recommended as an effective management practice to alter soil organic carbon (C) content. Understanding the effects of straw return rate on changes of soil organic C (SOC) is helpful for improving soil quality and sustaining crop productivity. To determine the short-term responses of SOC content and grain yield to different annual straw return rates, a 2-year field experiment with a random block design was carried out in a rice–wheat rotation system. Seven gradient rates of straw return, including 0%, 25%, 50%, 75% and 100% of total straw from previous crop returned in rice and wheat seasons, and 100% of straw returned only in rice or wheat season were set. Both rice and wheat yields were investigated and SOC pools and C pool management index (CPMI) were measured. The soil total organic C (TOC) and labile organic C fraction contents were higher under the straw return treatments compared to the no straw return treatment (0%S) at a 0–21 soil depth. The 50% annual straw return rate (50%S) had significantly higher soil TOC, dissolved organic C (DOC), easily oxidizable C (EOC), and microbial biomass C (MBC) contents than the 0%S treatment at a 0–21cm depth. All of the straw return treatments had a significantly higher DOC content than the 0%S treatment at a 0–21cm depth, except for the 100% only rice straw return treatment (100%RS). Compared to the 0%S, the 25%S and 50%S treatments had a significantly higher SOC lability (L), lability index (LI), C pool index (CPI) and CPMI at a 0–21cm soil depth. With the increase of straw return rate, the crop yields first increased and then decreased, and the 50%S treatment had the highest crop yields. There were significant and positive correlations between the soil TOC and the annual crop yield. We concluded that the 50% annual straw return rate is the best option for improving SOC sequestration and crop productivity in the rice–wheat rotation system of China.

Valuing Supporting Soil Ecosystem Services in Agriculture: A Natural Capital Approach

Soil biodiversity through its delivery of ecosystem functions and attendant supporting ecosystem services—benefits soil organisms generate for farmers—underpins agricultural production. Yet lack of practical methods to value the long‐term effects of current farming practices results, inevitably, in short‐sighted management decisions. We present a method for valuing changes in supporting soil ecosystem services and associated soil natural capital—the value of the stock of soil organisms—in agriculture, based on resultant changes in future farm income streams. We assume that a relative change in soil organic C (SOC) concentration is correlated with changes in soil biodiversity and the generation of supporting ecosystem services. To quantify the effects of changes in supporting services on agricultural productivity, we fitted production functions to data from long‐term field experiments in Europe and the United States. The different agricultural treatments at each site resulted in significant changes in SOC concentrations with time. Declines in associated services are shown to reduce both maximum yield and fertilizer‐use efficiency in the future. The average depreciation of soil natural capital, for a 1% relative reduction in SOC concentration, was 144 € ha−1 (SD 47 € ha−1) when discounting future values to their current value at 3%; the variation was explained by site‐specific factors and the current SOC concentration. Moreover, the results show that soil ecosystem services cannot be fully replaced by purchased inputs; they are imperfect substitutes. We anticipate that our results will both encourage and make it possible to include the value of soil natural capital in decisions.

Spatial Modeling of Organic Carbon in Degraded Peatland Soils of Northeast Germany

Spatial variation of C stocks within peatlands is an overall challenge for monitoring global C cycle processes, which is critical for responding to climate change induced by greenhouse gases (GHGs). The objective of this study was to evaluate the ability of high-resolution, minimally invasive sensor data to predict spatial variation of soil organic C (SOC) stocks within highly degraded peatland soils in northeast Germany. Within the Rhin-Havelluch, a paludification mire that has been cultivated and drained for about 300 yr, seven fields were sampled by soil cores up to 2 m in depth, nine points for each field. Soil horizons were examined for dry bulk density, soil organic C content (SOCC), and thickness to calculate SOC stocks and to test for relationships with overall peat thickness, elevation, and electrical conductivity (ECa). Elevation was determined by light detection and ranging (LiDAR) and ECa by an EM38DD, both producing maps of high resolution (1 m). Soil organic C density (SOCd) was related to elevation, ECa, and peat thickness. Based on these relationships, maps of SOCd were produced. Within field variation of SOCd was high, which could be modeled by use of the covariate maps. If available, ECa maps can improve the prediction of SOCd based on elevation. Modeling peat thickness based on sensor data needs additional research, but seems to be a valuable covariate in digital soil mapping.

Long-term tillage impacts on soil organic matter components and related properties on a Typic Argiudoll

Soil organic matter affects a number of soil processes and properties. A better understanding of soil-profile distribution of organic matter components and related soil properties under long-term tillage systems is thus needed. The objective of this study was to evaluate the impacts of 33 years no-till (NT), double disk (DD), chisel (CH), and plow tillage (PT) under corn (Zea mays L.)–soybean (Glycine max L.) rotation on soil organic C (SOC), particulate organic matter (POM), pH, and wet aggregate stability to 100cm soil depth on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudolls) in eastern NE. After 33 years, NT and DD management increased SOC by 1.2 times and mean weight diameter (MWD) of aggregates by 2 times compared with CH and PT at the 0–10cm depth. At the 0–20cm, NT had 1.1 times higher SOC concentration than CH and PT. When compared with data collected 24 years prior to this study, SOC at the 0–20cm increased by 12.5% across NT, DD, and CH and by 2.7% for PT. No-till had 5 times higher total POM concentration than PT, 4.7 times higher than CH, and 2.4 times higher than DD at the 0–10cm depth. However, at the 10–20cm, PT had higher POM than other tillage systems, which is most probably due to mixing and burial of residues at the bottom of the plow layer. Soil pH did not differ among tillage treatments at the 0–10cm, but it differed in this order: PT>CH>DD>NT at the 10–20cm and PT=CH=DD>NT at the 20–40cm depth. The lower pH under NT, DD, and CH in deeper soil depths may be due to the limited or no lime mixing in these systems compared with PT. When compared with data (pH 5) collected 33 years prior to this study, soil pH increased by 0.9 in NT, 1.4 in DD, 1.5 in CH, and 1.9 units in PT at 0–20cm depth, probably due to surface application and incorporation of lime. Overall, 33 years of NT increased near-surface soil organic matter components and soil aggregation compared with the PT.

Consequences of minimum soil tillage on abiotic soil properties and composition of microbial communities in a shallow Cambisol originated from fluvioglacial deposits

A long-term field experiment was run for 12 years to evaluate the impact of minimum tillage (MT) compared to conventional mouldboard ploughing (CT) on soil chemical, physical and microbial properties in a shallow Cambisol formed over fluvioglacial deposits of Drava river in Slovenia. Significant differences between MT and CT were found in vertical distribution of soil organic C (SOC) and nutrients (total N and plant available potassium); under MT, concentrations decreased from the soil surface to the lower layers, as opposed to CT which maintained rather uniform distribution down to the ploughing depth. MT in comparison with CT also increased the proportion of water-stable 2–4-mm-sized aggregates (80.9 and 61.3 %, respectively), water holding capacity (24.8 and 22.2 %, respectively) and plant available water (13.4 and 10.3 %, respectively) in the upper 0–10-cm soil layer. Bulk density, porosity, the proportion of water-stable 1–2-mm-sized aggregates and infiltration rate showed no significant differences between the tillage treatments. SOC content in the upper 0–10-cm soil layer was not significantly different between MT and CT (1.60 ± 0.07 and 1.45 ± 0.05 %, respectively), as well as the overall stock in the investigated soil profile (0–60 cm) remained unaffected (57.4 ± 0.8 and 59.1 ± 2.2 t ha−1, respectively). Microbial biomass, estimated by the total soil DNA, was higher in MT than CT in the 0–10-cm layer. Furthermore, a positive linear dependence of microbial biomass on SOC content was observed. Fingerprinting of bacterial, fungal and archaeal communities indicated that microbial community composition changed by long-term MT, whereas changes in microbial diversity were not detected for any domain. The most pronounced shifts in the composition were found for bacterial communities in the 10–20-cm layer, while the composition of fungal communities slightly changed in the upper 0–10 cm of MT soil. The composition of archaeal communities was not affected by the tillage or by the soil depth. Our results indicate that MT generates modest changes in soil structure and soil water retention properties and could support measures against erosion, drought and nutrient leaching. Considering increased microbial biomass in the topsoil of MT and shifts in microbial diversity, the impacts of MT on soil microbiome are also evident and need to be further investigated to identify the affected functional traits.

Response of soil carbon dioxide fluxes, soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

AbstractBiochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO2) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO2 fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO2 fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO2 fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO2 fluxes and MBC content, respectively. Both soil CO2 fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO2 fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO2 flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.

Impacts of long-term chemical and organic fertilization on soil puddlability in subtropical China

Soil puddlability measures the susceptibility of a soil to puddling, and can influence transplantation and the growth of rice plants. The effects of chemical fertilizers and organic amendments on soil puddlability of paddy soils are poorly understood. This study used two 26-year field experiments (1) to compare the effects of chemical and organic fertilization on soil puddlability by measuring sinkage resistance and hard clod content after puddling, (2) to characterize physical properties of hard clods and (3) to explain the change in soil puddlability. Each of the two experiments consisted of nine treatments of chemical fertilizers alone or in combination with organic amendments. The sinkage resistance and the content of hard clods were higher in the treatments with chemical fertilization alone than in the treatments with organic amendments. The sinkage resistance was positively correlated with the content of hard clods and negatively correlated to content of soil organic C (SOC) and mean weight diameter (MWD). The bulk density, water sorptivity and apparent porosity were similar among individual hard clods from different treatments, suggesting that the hard clods were formed under the same processes. The formation of hard clods was likely attributed to the breakdown of the compacted topsoil by puddling tillage, which formed due to clogging pores by fine particles produced during previous puddling tillage and due to shrinkage upon drying during rice growth period. Compared with the organic amendment treatments, the chemical fertilization treatments contained more and larger hard clods, indicating that the compacted topsoil was thicker due to higher soil dispersibility due to N fertilization and lower SOC content in the chemical fertilization treatments than in the organic amendment treatments. The study also suggests that continuous input of organic C at an annual rate of >2.5Mgha−1 is needed to maintain SOC content and soil structure under chemical fertilization in the study region.

23-year manure and fertilizer application increases soil organic carbon sequestration of a rice–barley cropping system

Fertilizer application can potentially change soil organic C (SOC) sequestration of croplands. However, how fertilization affects SOC sequestration and soil C fractions has not been sufficiently investigated. Here, we studied this issue in a rice–barley system of eastern China after 23-year repeated organic and chemical fertilizer amendments. The fertilization treatments were as follows: unfertilized control (CK), pig manure (M), chemical fertilizer NPK (NPK), and pig manure combined with NPK (MNPK). Our results showed that M, NPK, and MNPK significantly increased 25, 13, and 30 % of SOC content as compared with CK. Compared with CK, the three fertilizations, especially manure addition, significantly increased the content of SOC fractions such as microbial biomass C, dissolved organic C, labile organic C, and particulate organic C. Compared with the initial SOC stock at 0–20-cm depth (41.2 Mg C ha−1), M, NPK, and MNPK increased 11, 1, and 16 % of SOC storage, respectively, whereas CK decreased 9 % of SOC pool. The SOC sequestration was positively correlated with the cumulative C input (root residue-C + manure-C) (SOC sequestration = 0.21 × C input − 5.67, R 2 = 0.88, P < 0.001). This linear relationship suggested that (i) the soil had not reached a SOC saturation, (ii) the conversion rate from input-C to SOC was 21 %, and (iii) a C input threshold of 1.17 Mg C ha−1 year−1 was needed for maintaining the initial SOC level. Therefore, manure application is recommended for promoting SOC sequestration in the rice–barley cropping system of eastern China.

Cattle Manure Application Reduces Soil Compactibility and Increases Water Retention after 71 Years

Current interest in integrating livestock with crop production systems warrants further understanding of the impacts of manure application on soil physical processes and properties such as soil's susceptibility to compaction (compactibility) and water retention. We quantified the 71-yr cumulative impacts of beef (Bos Taurus L.) cattle manure (0 and 27 Mg ha−1 yr−1) and inorganic N fertilizer (0, 90, and 180 kg N ha−1 yr−1) application to conventionally tilled and irrigated corn (Zea mays L.) on soil compactibility, water retention, available water, and soil organic C (SOC) on a Tripp very fine sandy loam (coarse-silty, mixed, superactive, mesic Aridic Haplustoll) at the centennial Knorr-Holden plots of the University of Nebraska's Panhandle Research and Extension Center, Scottsbluff, NE. Manure application improved soil properties at the 0- to 30-cm depth, but inorganic fertilization had no effects. Across N levels, manure application reduced maximum Proctor bulk density by 6% and increased Proctor soil critical water content (water content at which maximum compactibility occurs) by about 17%, indicating that manured soils can be slightly less susceptible to compaction and trafficked at higher water content without being excessively compacted compared with nonmanured soils. Manured plots also retained 18% more soil water at −0.033 MPa and 21% more at −1.5 MPa potential, resulting in about 16% more plant available water compared with nonmanured plots. The SOC concentration increased two-fold with manure addition and was negatively correlated (r > −0.77) with soil compactibility and positively (r > 0.72) with water retention. Overall, 71-yr manure application reduced compactibility and increased water retention in this semiarid soil.

Soil Carbon Accumulation under Switchgrass Barriers

The benefits of grass barriers or hedges for reducing offsite transport of non‐point‐source water pollutants from croplands are well recognized, but their ancillary benefits on soil properties have received less attention. We studied the 15‐yr cumulative effects of narrow and perennial switchgrass (Panicum virgatum L.) barriers on soil organic C (SOC), total N, particulate organic matter (POM), and associated soil structural properties as compared with the cropped area on an Aksarben silty clay loam (fine, smectitic, mesic Typic Argiudoll) with 5.4% slope in eastern Nebraska. Five switchgrass barriers were established in 1998 at ∼38‐m intervals parallel to the crop rows in a field under a conventional tillage and no‐till grain sorghum [Sorghum bicolor (L.) Moench]–soybean [Glycine max (L.) Merr.]–corn (Zea mays L.) rotation. Compared with the cropped area, switchgrass barriers accumulated about 0.85 Mg ha−1 yr−1 of SOC and 80 kg ha−1 yr−1 of total soil N at the 0 to 15 cm soil depth. Switchgrass barriers also increased coarse POM by 60%. Mean weight diameter of water‐stable aggregates increased by 70% at 0 to 15 cm and by 40% at 15 to 60 cm, indicating that switchgrass barriers improved soil aggregation at deeper depths. Large (4.75–8 mm) macroaggregates under switchgrass barriers contained 30% more SOC than those under the cropped area. Switchgrass‐induced changes in SOC concentration were positively associated with aggregate stability (r = 0.89***) and porosity (r = 0.47*). Overall, switchgrass barriers integrated with intensively managed agroecosystems can increase the SOC pool and improve soil structural properties.

Increase in soil stable carbon isotope ratio relates to loss of organic carbon: results from five long-term bare fallow experiments.

Changes in the (12)C/(13)C ratio (expressed as δ(13)C) of soil organic C (SOC) has been observed over long time scales and with depth in soil profiles. The changes are ascribed to the different reaction kinetics of (12)C and (13)C isotopes and the different isotopic composition of various SOC pool components. However, experimental verification of the subtle isotopic shifts associated with SOC turnover under field conditions is scarce. We determined δ(13)C and SOC in soil sampled during 1929-2009 in the Ap-horizon of five European long-term bare fallow experiments kept without C inputs for 27-80 years and covering a latitudinal range of 11°. The bare fallow soils lost 33-65% of their initial SOC content and showed a mean annual δ(13)C increase of 0.008-0.024‰. The (13)C enrichment could be related empirically to SOC losses by a Rayleigh distillation equation. A more complex mechanistic relationship was also examined. The overall estimate of the fractionation coefficient (ε) was -1.2 ± 0.3‰. This coefficient represents an important input to studies of long-term SOC dynamics in agricultural soils that are based on variations in (13)C natural abundance. The variance of ε may be ascribed to site characteristics not disclosed in our study, but the very similar kinetics measured across our five experimental sites suggest that overall site-specific factors (including climate) had a marginal influence and that it may be possible to isolate a general mechanism causing the enrichment, although pre-fallow land use may have some impact on isotope abundance and fractionation.

Biomass and carbon storage in an age-sequence of Cyclobalanopsis glauca plantations in southwest China

Million-hectare plantations of Cyclobalanopsis glauca were planted in southwest China to restore the degraded vegetation. Although many studies of these plantations have been conducted, from the genetic scale to regional scales, the role of stand age in regional carbon (C) cycling remains unclear. In this study, we examined the biomass and C storage of the main ecosystem components in five C. glauca plantation stands in Guangxi Province, southwest China, that ranged in age from 8 to 70 years. The ecosystem and tree biomass increased with increasing stand age. On average, the highest C concentration was found in tree branches, whereas the lowest C concentration was found in the underground parts of herb. C concentration in trees, shrubs, herbs, and litter were age-dependent. C storage in C. glauca plant biomass in stands aged 8, 20, 40, 50, and 70 years was 40.45, 78.25, 90.41, 94.46, and 114.69Mgha−1, respectively. Soil organic C (SOC) storage in the top 1m of soil in plantations aged 8, 20, 40, 50, and 70 years was 70.38, 90.84, 84.39, 47.91, and 107.99Mgha−1, respectively, with SOC concentration decreasing with increasing soil depth. Total C storage in the plantation ecosystem averaged 110.83, 169.09, 174.80, 142.37, and 222.68Mgha−1 in the 8-, 20-, 40-, 50-, and 70-year-old plantations, respectively, with most of the C storage in the aboveground biomass and soil rather than in the belowground biomass. Our results suggest that successional development as temporal factor influencing C storage in plantations are important considerations for fully realizing the ecological value of the C. glauca plantations in southwest China.