Controls on soil carbon storage and turnover in German landscapes

Subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC contents. However, measures for enhancing SOC storage, such as no-tillage, agroforestry and application of animal manure, commonly focus on topsoils. In this dissertation, the option of actively burying SOC-rich soil material through tillage practices was studied for the first time assessing SOC sequestration in deep ploughed arable and forest soils as well as in fossil ridge and furrow cropland under forest with buried topsoils. Assessed deep-ploughed cropland soils contained on average 42±18% more SOC than the reference plots down to 100 cm. In contrast, total SOC stocks in deep ploughed forest soils were not significantly higher than in their respective reference soils. The ‘newly formed’ topsoil of the deep ploughed soils still contained less SOC than the reference topsoil, 14% and 37% under cropland and under forest, respectively. On average, the fraction of mineralised SOC assessed in one-year laboratory incubations, was 32% lower in incubated buried than in reference topsoils. Sandy cropland buried topsoil stripes and reference topsoils had the lowest mineralisable SOC fraction. Buried SOC was partly to fully preserved, most effectively in sandy cropland soils, but selective preservation of certain, most stabile SOC fractions and preferential decomposition of labile fractions, such as the commonly most easily decomposable free light fraction, could not explain SOC stability. Land use history of many of the studied sandy sites as heath was identified as an important aspect concerning SOC stabilisation. Medieval ridge-and-furrow cultivation led to the burial of former topsoil under ridges. SOC content was 0.4-0.9 g kg-1 higher at ridges than at reference positions, indicating long-term preservation of former topsoil SOC by burial under ridges, although enhanced SOC stocks at ridges due to carbon burial could not be significantly confirmed for all sites. In conclusion, deep ploughing led to enhanced subsoil SOC contents and stocks compared to a reference plot in forests and cropland. Deep ploughing increased the storage space for SOC by mechanically translocating topsoil material into the subsoil. SOC accumulation in the new topsoil horizons additionally determine the effect of deep ploughing on full profile SOC stocks.

Soil carbon content and stability are primarily influenced by the stabilization of particulate organic matter (POM) and mineral-associated organic matter (MAOM). Despite extensive research on the stabilization processes of POM and MAOM carbon components under various land-use types, the investigation into stabilization processes of soil carbon remains limited in saline–alkali soils. Therefore, we collected soil samples from different positions of saline–alkali drainage ditches at four reclamation times (the first, seventh, fifteenth, and thirtieth year) to determine their carbon content and physicochemical properties. Moreover, POM and MAOM fractions were separated from soil samples, and Fourier transform infrared spectra (FTIR) were used to investigate changes in their chemical composition. The results showed that with increasing reclamation time, the soil total carbon and soil organic carbon (SOC) contents significantly increased from 14 to 15 and 2.9 to 5.5 g kg−1, respectively. In contrast, soil inorganic carbon content significantly decreased from 11 to 9.6 g kg−1. Notably, the changes in soil carbon components following the increasing reclamation time were primarily observed in the furrow sole at a depth of 20–40 cm. While the SOC content of the POM fraction (SOCPOM) decreased significantly, the SOC content of the MAOM fraction (SOCMAOM) increased significantly. These alterations were largely dominated by drainage processes after reclamation instead of a possible conversion from SOCPOM to SOCMAOM. FTIR results revealed that MAOM was greatly influenced by the reclamation time more than POM was, but the change in both POM and MAOM contributed to an increase in soil carbon stability. Our findings will deepen the comprehension of soil carbon stabilization processes in saline–alkali drainage ditches after reclamation and offer a research framework to investigate the stability processes of soil carbon components via alterations in POM and MAOM fractions.

Immediate and long-term effects of tillage practices with crop residue on soil water and organic carbon storage changes under a wheat-maize cropping system

<p>Improvements in management practices can prevent the decline of soil organic carbon (SOC) storage caused by conventional tillage practice in Northeast China. Density and size fractionation can track the transformation of plant residue into SOC and its location in soil matrix. We used a long-term field study in China to evaluate these changes as a result of improved management involving tillage and cropping systems. Experimental treatments included no-till (NT) and moldboard ploughing (MP) under monoculture maize (Zea mays L.) (MM) and maize-soybean (Glycine max Merr.) rotation (MS); these were compared to the traditional management involving conventional tillage (CT) under MM. An incubation study was conducted to evaluate mineralization and the biodegradability of SOC. The soils were also physically fractionated by density (light fraction, LF) and size (sand, silt, clay). With improved management, the SOC storage in the clay showed the largest increase across all fractions. This increase was greater for MS than MM. The NTMS treatment resulted in a decline in silt-OC storage compared to CTMM. The SOC mineralization (mg CO<sub>2</sub>-C g<sup>-1</sup> soil) was affected by tillage and driven by LF-OC and was observed in the order: NTMM (2.06) > MPMM (1.72) ≈ NTMS (1.71) > CTMM (1.52) ≈ MPMS (1.41). Both cropping and depth affected the biodegradability of SOC. Considering the plough layer (0-20 cm), treatments under MM had larger proportion of biodegradable SOC than under MS. We conclude that the significant differences in SOC storage in physical fractions and SOC biodegradation were caused by differences in soil management.</p>

Using the data compiled from China's second national soil survey and an improved method of soil carbon bulk density, we have estimated the changes of soil organic carbon due to land use, and compared the spatial distribution and storage of soil organic carbon (SOC) in cultivated soils and noncultivated soils in China. The results reveal that ∼ 57% of the cultivated soil subgroups (∼ 31% of the total soil surface) have experienced a significant carbon loss, ranging from 40% to 10% relative to their noncultivated counterparts. The most significant carbon loss is observed for the non‐irrigated soils (dry farmland) within a semiarid/semihumid belt from northeastern to southwestern China, with the maximum loss occurring in northeast China. On the contrary, SOC has increased in the paddy and irrigated soils in northwest China. No significant change is observed for forest soils in southern China, grassland and desert soils in northwest China, as well as irrigated soils in eastern China. The SOC storage and density under noncultivated conditions in China are estimated to ∼ 77.4 Pg (1015 g) and ∼ 8.8 kg C m−2, respectively, compared to a SOC storage of ∼ 70.3 Pg and an average SOC density of ∼ 8.0 kg C m−2 under the present‐day conditions. This suggests a loss of ∼ 7.1 Pg SOC and a decrease of ∼ 0.8 kg C m−2 SOC density due to increasing human activities, in which the loss in organic horizons has contributed to ∼ 77%. This total loss of SOC in China induced by land use represents ∼ 9.5% of the world's SOC decrease. This amount is equivalent to ∼ 3.5 ppmv of the atmospheric CO2 increase. Since ∼ 78% of the currently cultivated soils in China have been degraded to a low/medium productivities and are responsible for most of the SOC loss, an improved land management, such as the development of irrigated and paddy land uses, would have a considerable potential in restoring the SOC storage. Assuming a restoration of ∼ 50% of the lost SOC during the next 20–50 years, the soils in China would absorb ∼ 3.5 Pg of carbon from the atmosphere.

Widespread adoption of conservation practices is increasingly encouraged to improve soil organic carbon (SOC) storage and mitigate climate change. However, soil texture and mineralogy cause variable SOC responses under conservation tillage. The role of minerals and organic carbon composition in soil aggregation and SOC stabilization remains insufficiently understood. This study evaluated the long‐term effects of conservation tillage (no‐tillage with straw return, NTS) versus conventional tillage (plough tillage with straw removal, CT) on carbon storage across Phaeozems, Calcaric Cambisols, and Calcic Luvisols. Compared to CT, NTS increased the annual average C sequestration rate by 15.3%–76.7% and SOC storage by 10.2%–28.4% in different soil types. NTS also increased macroaggregate percentage and mean weight diameter (MWD), resulting in 17.8%–28.3% larger macroaggregate‐associated SOC and total nitrogen (TN) contents. Notably, the aromatic‐C/aliphatic‐C ratio under NTS increased in bulk soil and macroaggregates, which were positively correlated with larger amorphous iron (Feo) content, Ca 2+ concentration, and specific surface area in different treatments. Phaeozems exhibit the largest SOC storage, along with larger Feo and Ca 2+ contents and aromatic‐C/aliphatic‐C ratio. However, NTS led to the greatest increases in MWD and SOC storage in Calcaric Cambisols, and the greatest enhancement of microbial biomass carbon in Calcic Luvisols. PLS‐PM analysis indicated that although the aromatic‐C/aliphatic ratio directly enhances aggregated stability, Feo and Ca 2+ promoted MWD indirectly by facilitating greater aromatic‐C and polysaccharide‐C. Overall, conservation tillage promoted selective binding of Feo and Ca 2+ to SOC functional groups, thus enhancing soil aggregation and SOC physico‐chemical protection, with calcareous soils showing a stronger response.

Forests worldwide store large quantities of carbon (C), particularly in soils as soil organic C (SOC). In northeastern China, two dominant forest types, secondary mixed forest (MF) and larch plantation forest (LF), cover extensive areas. However, we lack an understanding of the patterns and the mechanisms of SOC storage and stabilization in MF and LF, especially in deep soil layers. This research aims to illustrate the vertical distribution and mineral protection of SOC over soil depth; we also used δ13C values of soil fractions to evaluate SOC stability. Samples from the surface litter (Oi), organic layer (Oa+e), and 0–40 cm mineral soils were collected from both MF and LF plots. We used two different methods to separate bulk soils into distinguished fractions: (1) macro- and micro-aggregates and the non-aggregated fraction, and (2) particulate organic matter (POM) and mineral-associated organic matter (MAOM). The C concentrations, C stocks, and δ13C of all soil fractions were determined. Our findings were as follows: (1) SOC was mainly stored in mineral soils and was 13.6% lower in LF (8609 ± 1180 g C m−2) than MF (9969 ± 2084 g C m−2). (2) In both MF and LF, the SOC stock was mainly stored in aggregates (averaged 92.7%); macroaggregates dominated in the surface layers (Oa+e layer and 0–10 cm) but microaggregates dominated in the deep layers (10–20 cm and 20–40 cm). In mineral soils, MAOM was the dominant fraction of the C stock (averaged 81.6%). (3) The proportion of C distributed in microaggregates and MAOM increased from Oa+e to the 20–40 cm layer. (4) The C/N ratios and δ13C values of MAOM were smaller and heavier compared to those of POM. Our study demonstrated that in both forests, aggregate formation and mineral association predominantly contributed to SOC storage, and large stocks of SOC were distributed in the deep soil. The increasing proportion of SOC in microaggregates and MAOM along the soil depth, most likely derived from microbial turnover and microbial necromass, influenced SOC stability in both forest types.

Estuarine wetlands exhibit significant interaction between fresh and salt water, with long-term carbon sequestration capability. We set up 60 sampling sites in the reed wetlands of the fresh-salt water interaction zone of the Yellow River Estuary, covering four different zones of the weak-intensity fresh-salt water interaction zone (WIZ), medium-intensity fresh-salt water interaction zone (MIZ), high-intensity interaction fresh-salt water zone (HIZ) and strong-intensity fresh-salt water interaction zone (SIZ). We investigated how fresh-salt water interaction affected the spatial variation of soil organic carbon (SOC) storage. The results showed that the area of reed wetland accounted for 17.8% of the total area of the fresh-salt water interaction zone the Yellow River Estuary, which mainly distributed in the WIZ and MIZ. The SOC content of reed wetland in the fresh-salt water interaction zone ranged from 1.09 to 3.65 g·kg-1, the SOC density was between 1.85-5.84 kg·m-2, and the SOC storage was (17.32±3.64)×104 t. The SOC content and SOC density decreased with increasing fresh-salt water interaction. There were significant differences in surface SOC content between different subzones of the fresh-salt water interaction zone. The surface SOC content decreased significantly with the increases of fresh-salt water interaction intensity. SOC density was positively correlated with SOC, TN, NH4+-N, and biomass, but negatively correlated with salt ions, soil bulk density, pH, and EC. SOC storage in the 0-30 cm soil layer accounted for 50.9%-64.2% of that in the 0-60 cm soil layer, while SOC storage in the 0-60 cm soil layer occupied 19.1%-37.7% of that in the 0-400 cm soil layer. The results could provide a scientific basis for accurately evaluating SOC storage of estuarine wetlands, improving carbon sink function and wetland management.

BackgroundThe contribution of long-term fertilization to soil organic carbon (SOC) storage has been of great concern. To assess the effects of long-term fertilization on SOC storage and stability in top and sub-soil layers, soil samples were collected from a 29-year field experimental station in a typical paddy soil in southern China. The SOC storage of whole soil and SOC fractions was quantified at three soil depths (0–20, 20–40, 40–60 cm) under four treatments: no fertilization (Control), a combination of nitrogen, phosphorus and potassium (NPK), double the rates of NPK (2NPK), NPK plus manure (NPKM).ResultsThe increase of Cinput-total was significantly higher than that of SOC storage among different treatments (p < 0.05), indicating that soil fixation of exogenous carbon is limited. Besides, the SOC accumulation and sequestration rates patterned as NPKM > 2NPK > NPK, and these rates were higher at 0–20 cm depth as compared to other depth intervals. Furthermore, for the whole profile, the SOC storage of active pool was higher in the Control (39.6 t C ha−1) than in other treatment (36.2 t C ha−1, p < 0.05). Whereas, fertilization increased the SOC storage of passive pool, ranked as NPKM > 2NPK≈NPK > Control (p < 0.05), indicating that fertilization, especially organic combined with inorganic fertilization, improved SOC stability. From the perspective of soil layers, the difference of SOC storage among treatments for passive pool was mainly resulted from the difference at surface soil, and for active pool were the deeper layers. Additionally, manure application increased the difference among soil layers.ConclusionThis study concluded that non-fertilized treatment could improve the SOC storage of active pool especially in deep soil layers, while fertilization especially manure application could improve the SOC storage and stability in surface soil and increased the difference among soil layers.Graphical

Soil organic carbon (SOC) is a crucial indicator of soil quality and fertility. However, excessive nitrogen (N) application, while increasing Moso bamboo yield, may reduce SOC content, potentially leading to soil quality issues. The impact of N on SOC and its active fraction in Moso bamboo forests remains underexplored. Investigating these effects will elucidate the causes of soil quality decline and inform effective N management strategies. Four N application gradients were set: no nitrogen (0 kg·hm−2·yr−1, N0), low nitrogen (242 kg·hm−2·yr−1, N1), medium nitrogen (484 kg·hm−2·yr−1, N2), and high nitrogen (726 kg·hm−2·yr−1, N3), with no fertilizer application as the control (CK). We analyzed the changes in SOC, active organic carbon components, and the Carbon Pool Management Index (CPMI) under different N treatments. The results showed that SOC and its active organic carbon components in the 0~10 cm soil layer were more susceptible to N treatments. The N0 treatment significantly increased microbial biomass carbon (MBC) content but had no significant effect on SOC, particulate organic carbon (POC), dissolved organic carbon (DOC), and readily oxidizable organic carbon (ROC) contents. The N1, N2, and N3 treatments reduced SOC content by 29.36%, 21.85%, and 8.67%, respectively. Except for POC, N1,N2 and N3 treatments reduced MBC, DOC, and ROC contents by 46.29% to 71.69%, 13.98% to 40.4%, and 18.64% to 48.55%, respectively. The MBC/SOC ratio can reflect the turnover rate of SOC, and N treatments lowered the MBC/SOC ratio, with N1 &lt; N2 &lt; N3, indicating the slowest SOC turnover under the N1 treatment. Changes in the Carbon Pool Management Index (CPMI) illustrate the impact of N treatments on soil quality and SOC sequestration capacity. The N1 treatment increased the CPMI, indicating an improvement in soil quality and SOC sequestration capacity. The comprehensive evaluation index of carbon sequestration capacity showed N3 (−0.69) &lt; N0 (−0.13) &lt; CK (−0.05) &lt; N2 (0.24) &lt; N1 (0.63), with the highest carbon sequestration capacity under the N1 treatment and a gradual decrease with increasing N fertilizer concentration. In summary, although the N1 treatment reduced the SOC content, it increased the soil CPMI and decreased the SOC turnover rate, benefiting soil quality and SOC sequestration capacity. Therefore, the reasonable control of N fertilizer application is key to improving soil quality and organic carbon storage in Moso bamboo forests.

Marshes in high plateaus are critical in governing carbon cycle because of their huge but labile soil organic carbon (SOC) storage, which of even a small proportion of carbon contained in SOC can lead to high variations in the atmospheric CO2 levels. The Yarlung Zangbo River (YZR) is the world's highest river, flowing from west to east across Tibet. There are widespread alpine marshes and meadows along the YZR basin, with huge SOC storage but extremely vulnerable to climate change. However, few studies were carried out to explore the potential effects of environmental factors on SOC pool along the YZR. In the present work, effects of climate, plant biomass, soil nutrients, and molecular features of organic matter on SOC storage were systematically evaluated. Results indicated that SOC contents varied from 0.86% to 29.68%, with an average of 7.29%, and an estimated 5887 Gg (109g) of carbon was stored in marsh soils along the YZR. Soil accretion rates ranged from 1.66 to 2.73 mm yr−1, while carbon sequestration rates ranged from 43.65 gC m−2 yr−1 to 125.4 gC m−2 yr−1, with an average of 75.89 gC m−2 yr−1. It was estimated that 34.58 Gg of carbon was sequestrated by wetlands along the YZR every year. The abundance of lignin‐derived components, total nitrogen, and soil moisture contents (SWC) were the main drivers promoting SOC contents. SOC storage was significantly related to mean annual temperature (Temp) but decreased with the ratio of annual precipitation (Pre) to Temp. Compared with the chemical decomposition resistance of organic matter, more SOC variations arose from climate, and climate warming had adverse impacts on SOC pool. SWC were proved to be the most powerful promoter of SOC increase. Rising temperatures are expected to have a detrimental impact on SOC storage when the climate is warming in the future.

Soil is the largest pool of terrestrial organic carbon in the biosphere and interacts strongly with the atmosphere, climate and land cover. Remote sensing (RS) and geographic information systems (GIS) were used to study the spatio-temporal dynamics of croplands and soil organic carbon density (SOCD) in the Sanjiang Plain, to estimate soil organic carbon (SOC) storage. Results show that croplands increased with 10,600.68 km2 from 1992 to 2012 in the Sanjiang Plain. Area of 13,959.43 km2 of dry farmlands were converted into paddy fields. Cropland SOC storage is estimated to be 1.29 ± 0.27 Pg C (1 Pg = 103 Tg = 1015 g) in 2012. Although the mean value of SOCD for croplands decreased from 1992 to 2012, the SOC storage of croplands in the top 1 m in the Sanjiang Plain increased by 70 Tg C (1220 to 1290). This is attributed to the area increases of cropland. The SOCD of paddy fields was higher and decreased more slowly than that of dry farmlands from 1992 to 2012. Conversion between dry farmlands and paddy fields and the agricultural reclamation from natural land-use types significantly affect the spatio-temporal patterns of cropland SOCD in the Sanjiang Plain. Regions with higher and lower SOCD values move northeast and westward, respectively, which is almost consistent with the movement direction of centroids for paddy fields and dry farmlands in the study area. Therefore, these results were verified. SOC storages in dry farmlands decreased by 17.5 Tg·year−1 from 1992 to 2012, whilst paddy fields increased by 21.0 Tg·C·year−1.

Seven artificial alfalfa grasslands (aged 3, 5, 6, 7, 8, 11, 12 a) and an abandoned land plot were selected to examine the changes in soil organic carbon content, active organic carbon content, soil organic carbon storage and active organic carbon storage in the 0–200 cm soil layer and their correlation with soil nutrients, soil water content, soil bulk density, alfalfa cover, density, height and aboveground biomass. The results show that the soil organic carbon content was between 2.02 and 4.54 g kg–1 within depths of 0–200 cm, which was higher than that in the abandoned land. The soil organic carbon storage in the 0–40 cm layers accounted for, on average, 29.50 ± 7.13% of the total storage in the 0–200 cm soil layers in the 7-year-old alfalfa grasslands and the abandoned land, and the 0–40 cm layer was the main layer of soil organic carbon storage. The soil organic carbon storage in 12 a grassland was the highest, and the abandoned land had the smallest reserve, with values of 10.22 and 5.06 kg m–2, respectively. In addition, carbon storage was, on average, 1.84 kg m–2 higher in 7-year alfalfa artificial grassland than in abandoned farmland. The proportion of soil AOC storage to SOC storage ranged from 30.24 to 58.11%; the proportion in 5-year grassland was the lowest, while it was the highest in the abandoned land. SOC storage had significant positive correlations with TN and soil active organic carbon storage, and it had negative correlations with soil bulk density at the 0.01 level (p < 0.01).

Land use effects on organic carbon storage in soils of Bavaria: The importance of soil types

Long-term effects of tillage and straw management on soil organic carbon, crop yield, and yield stability in a wheat-maize system