Soil organic carbon (SOC) is heterogeneous and consists of multiple pools differing in physical and chemical properties. SOC stocks are expected to increase in response to nitrogen (N) enrichment. ...However, the comprehensive understanding and general conclusions about the responses of different SOC pools such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to N addition are still lacking, in spite of the fact that this knowledge is of valuable for the accuracy of predictions of the vulnerability of SOC to N deposition. Here, we conducted a meta-analysis to explore the effects of N addition on POC and MAOC, as well as their relative proportions in bulk SOC. We found that N addition increased POC (20.6%) to a greater extent than MAOC (4.5%). Consequently, N addition significantly increased the proportion of POC in bulk SOC by 10.4%, but significantly decreased the proportion of MAOC in bulk SOC by 2.6%. The response of POC to N addition was negatively correlated with the response of pH and microbial biomass carbon (MBC), while the response of MAOC to N addition was positively correlated with the response of pH and MBC. Notably, the positive effect of N addition on the proportion of POC in bulk SOC decreased as the initial proportion of POC increased. Inversely, the negative effect of N addition on the proportion of MAOC in bulk SOC increased with the initial proportion of MAOC. Overall, our findings suggest that although N could enhance SOC stocks, the larger increment of POC would make the SOC pools more susceptible to future global changes. Our results also highlight the need to explicitly incorporate the differential responses of POC and MAOC into ecosystem models to improve predictions of SOC stock responses to global change.
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●N addition increased the POC fractions to a greater extent than MAOC.●N addition decreased the proportion of MAOC in bulk SOC.●Changes in MAOC were positively correlated with changes in pH and MBC.●The response of SOC stability was linked to the initial proportion of MAOC.●Long-term N addition increased the vulnerability of bulk SOC.
Agroforestry systems, i.e., agroecosystems combining trees with farming practices, are of particular interest as they combine the potential to increase biomass and soil carbon (C) storage while ...maintaining an agricultural production. However, most present knowledge on the impact of agroforestry systems on soil organic carbon (SOC) storage comes from tropical systems. This study was conducted in southern France, in an 18-year-old agroforestry plot, where hybrid walnuts (Juglans regia × nigra L.) are intercropped with durum wheat (Triticum turgidum L. subsp. durum), and in an adjacent agricultural control plot, where durum wheat is the sole crop. We quantified SOC stocks to 2.0m depth and their spatial variability in relation to the distance to the trees and to the tree rows. The distribution of additional SOC storage in different soil particle-size fractions was also characterized. SOC accumulation rates between the agroforestry and the agricultural plots were 248±31kgCha−1yr−1 for an equivalent soil mass (ESM) of 4000Mgha−1 (to 26–29cm depth) and 350±41kgCha−1yr−1 for an ESM of 15,700Mgha−1 (to 93–98cm depth). SOC stocks were higher in the tree rows where herbaceous vegetation grew and where the soil was not tilled, but no effect of the distance to the trees (0 to 10m) on SOC stocks was observed. Most of the additional SOC storage was found in coarse organic fractions (50–200 and 200–2000μm), which may be rather labile fractions. All together our study demonstrated the potential of alley cropping agroforestry systems under Mediterranean conditions to store SOC, and questioned the stability of this storage.
•Additional SOC storage rate was 350±41kgCha−1yr−1 at 0–100cm.•About 75% of additional SOC was located at 0–30cm.•Tree rows accounted for more than 50% of additional SOC storage.•Most additional SOC was made of particulate organic matter (>50μm).
•We studied soil organic C and land use in the upper Blue Nile Basin, Ethiopia.•There is more recalcitrant organic matter in shrub- than in grass- or croplands.•It is more difficult to sequester C ...into an eroded than a non-eroded soil.•Conversion to shrubland shows high potential to restore recalcitrant soil C.•Current management at the sites of croplands and grasslands is not sustainable.
The purpose of this study was to characterize the soil organic carbon dynamics associated with four land-uses (cropland, grassland, shrubland, and forestland) in the upper Blue Nile Basin of the Ethiopian Highlands. We collected diverse biophysical data to allow spatial variability of soil organic carbon and factors contributing to this variation to be determined statistically, and used well established simulation models to interpret the data, predict long-term carbon dynamics and determine the potential for improvements in soil quality and mitigation of greenhouse gas emissions. The spatial variation in soil organic carbon in the 0–20cm soil depth was significant across study areas (P<0.01, 21.6gkg−1–37.8gkg−1), and between land-uses (P<0.05, 27.9gkg−1 in cropland and 43.0gkg−1 soil in forestland). In a multiple linear regression model, among the 11 explanatory variables used, four (total nitrogen, shrubs, trees and land use) showed a significant positive effect (P<0.01), while three (impact of grazing, impact of erosion and clay) showed a significant negative effect (P<0.01). Simulations using the assumption of steady state to estimate carbon dynamics suggested that plant inputs from croplands are generally lower than grass or shrublands, resulting in build-up of recalcitrant organic matter in shrublands compared to grass or croplands. Erosion results in a decline in both the absolute and relative amounts of carbon in recalcitrant pools, but the more active pools remained unchanged, meaning that the overall activity of soil organic matter is increased when the soil is eroded, and it becomes more difficult to sequester soil carbon. More rapid decrease in soil organic carbon is likely to be due to increased erosion, persistent removal of organic materials, grazing pressures, and higher rate of decompositions in cropland and grazing land-uses. Conversion to shrubland shows high potential to quickly restore eroded soils and build up a pool of recalcitrant soil carbon, suggesting that management of land using periodic 20 year exclosures, preventing cropping and grazing and allowing shrubland succession, could be beneficial in restoring degraded soils. Simulations using a calibration approach, rather than assuming steady state, corroborated these findings, suggesting that after 30 years of current management, croplands will deplete soil organic carbon by 5.6, 7.1 and 7.2tha−1, and grasslands by 3.5, 3.9 and 2.7tha−1, while shrublands will build-up soil organic carbon by 0.6, 0.1 and 1.3tha−1 at study sites. At one study site, forests, will further increase soil organic carbon by 6.7tha−1 after 30 years. The significant positive impact of shrubs and trees on soil organic carbon suggests the need to focus on introduction of agroforestry systems in crop and grasslands.
Nitrogen (N) fertilization affects the rate of soil organic carbon (SOC) decomposition by regulating extracellular enzyme activities (EEA). Extracellular enzymes have not been represented in global ...biogeochemical models. Understanding the relationships among EEA and SOC, soil N (TN), and soil microbial biomass carbon (MBC) under N fertilization would enable modeling of the influence of EEA on SOC decomposition. Based on 65 published studies, we synthesized the activities of α-1,4-glucosidase (AG), β-1,4-glucosidase (BG), β-d-cellobiosidase (CBH), β-1,4-xylosidase (BX), β-1,4-N-acetyl-glucosaminidase (NAG), leucine amino peptidase (LAP), urease (UREA), acid phosphatase (AP), phenol oxidase (PHO), and peroxidase (PEO) in response to N fertilization. The proxy variables for hydrolytic C acquisition enzymes (C-acq), N acquisition (N-acq), and oxidative decomposition (OX) were calculated as the sum of AG, BG, CBH and BX; AG and LAP; PHO and PEO, respectively. The relationships between response ratios (RRs) of EEA and SOC, TN, or MBC were explored when they were reported simultaneously. Results showed that N fertilization significantly increased CBH, C-acq, AP, BX, BG, AG, and UREA activities by 6.4, 9.1, 10.6, 11.0, 11.2, 12.0, and 18.6%, but decreased PEO, OX and PHO by 6.1, 7.9 and 11.1%, respectively. N fertilization enhanced SOC and TN by 7.6% and 15.3%, respectively, but inhibited MBC by 9.5%. Significant positive correlations were found only between the RRs of C-acq and MBC, suggesting that changes in combined hydrolase activities might act as a proxy for MBC under N fertilization. In contrast with other variables, the RRs of AP, MBC, and TN showed unidirectional trends under different edaphic, environmental, and physiological conditions. Our results provide the first comprehensive set of evidence of how hydrolase and oxidase activities respond to N fertilization in various ecosystems. Future large-scale model projections could incorporate the observed relationship between hydrolases and microbial biomass as a proxy for C acquisition under global N enrichment scenarios in different ecosystems.
•A meta-analysis was conducted based on 65 published nitrogen fertilization experiments.•Nitrogen fertilization stimulated hydrolases associate with C and P but depressed oxidases..•Nitrogen fertilization has no significant effect on hydrolases associated with N.•A significantly positive relationship was found between response ratios of hydrolases associate with C and microbial biomass.•Future model projections could consider the above relationship for different ecosystems.
Soil organic carbon (SOC) is an important component in agricultural soil, and its stock is a major part of global carbon stocks. Estimating the SOC distribution and storage is important for improving ...soil quality and SOC sequestration. This study evaluated the SOC distribution different land uses and estimated the SOC storage by classifying the study area by land use in a small watershed on the Loess Plateau. The results showed that the SOC content and density were affected by land use. The SOC content for shrubland and natural grassland was significantly higher than for other land uses, and cropland had the lowest SOC content. The effect of land use on the SOC content was more significant in the 0–10
cm soil layer than in other soil layers. For every type of land use, the SOC content decreased with soil depth. The highest SOC density (0–60
cm) in the study area was found in shrublandII (
Hippophae rhamnoides), and the other land uses decreased in the SOC density as follows: natural grassland
>
shrublandI (
Caragana korshinskii)
>
abandoned cropland
>
orchard
>
level ground cropland
>
terrace cropland
>
artificial grassland. Shrubland and natural grassland were the most efficient types for SOC sequestration, followed by abandoned cropland. The SOC stock (0–60
cm) in this study was 23,584.77
t with a mean SOC density of 4.64 (0–60
cm).
► Estimating SOC distribution and storage is important for improving soil quality and SOC sequestration. ► SOC distribution and storage for different land uses were evaluated in a small watershed. ► Shrubland and natural grassland were the most efficient land use types for SOC sequestration. ► The SOC stock (0–60
cm) was 23,584.77
t with a mean SOC density of 4.64 (0–60
cm).
The response of soil organic carbon (SOC) decomposition to global warming is a potentially major source of uncertainty in climate prediction. However, the magnitude and direction of SOC cycle ...feedbacks under climate warming remain uncertain because of the knowledge gap about the global‐scale spatial pattern and temperature sensitivity (Q10) mechanism of SOC decomposition.
Here, we collected data of Q10 and corresponding soil variables from 81 peer‐reviewed papers using laboratory incubation to explore how Q10 varied among different ecosystems at the global scale and whether labile and recalcitrant SOC pools had equal Q10 values.
Q10 with a global average of 2.41 substantially varied among different ecosystems, ranging from the highest in cropland soils (2.76) and the lowest in wetland soils (1.84). Hump‐shaped correlations of Q10 values with the maximum at SOC = 190 g/kg and the minimum at clay = 37% were observed. However, the main influencing factors of Q10 differed among various ecosystems. Q10 values showed a clear decrease with increasing incubation temperature but no significant decrease above 25°C. In general, labile SOC was less sensitive than recalcitrant SOC to warming. Structural equation model analyses showed that total N and SOC accounted for 53% and 46%, respectively, of the variation in Q10 of labile SOC and recalcitrant SOC. This finding suggested that Q10 values of labile and recalcitrant SOC pools had different controlling factors.
Our findings highlighted the importance of Q10’s variations in ecosystem types and the response of recalcitrant SOC to warming in predicting the soil C cycling and its feedback to climate change. Therefore, ecosystem type and difference in Q10 of labile and recalcitrant SOC should be considered to precisely predict the soil C dynamics under global warming.
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•Soil water under the PM treatment was higher than that under the SM treatment.•Compared with CK, plastic film mulching and straw mulching increased MBC and MBN concentrations.•Compared with CK, ...straw mulching increased DOC and DON, but plastic film mulching decreased DOC and DON.•The average water use efficiencies under mulching practices were higher than that under the CK treatment.
A field experiment was conducted in the Loess Plateau of Northwest China to study the effects of plastic film mulching and straw mulching on soil water, soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon (MBC) and nitrogen (MBN), dissolved organic carbon (DOC) and nitrogen (DON), crop yield and water use efficiency under winter wheat (Tricicum aestivum L.)—summer maize (Zea mays L.) double-cropping system conditions using the following three cultural practices: (i) traditional plough with no mulching (CK), (ii) traditional plough with plastic film mulching (PM), and (iii) traditional plough with straw mulching (SM). Soil water contents were measured by the gravimetric method. SOC was determined using the dichromate oxidation method. TN was analyzed by the Kjeldahl method. MBC and MBN were determined using the chloroform fumigation extraction method. DOC and TDN were determined following Jones’ procedures proposed by Jones and Willett (2006). The results showed that soil water was higher under the PM treatment than under the SM treatment and mainly changed in the upper 60 cm soil layer. Compared with the CK treatment, the concentrations of SOC and TN under the SM treatment were increased by 16.9% and 7.7% at the 0–10 cm soil depth, respectively, and the PM treatment had the similar SOC and TN concentrations. Compared with the CK treatment, soil C:N ratio was increased under the SM treatment by 6.2% (P < 0.05), and that under the PM treatment was decreased by 5.2% (P < 0.05) after three years. The concentrations of MBC under the PM and SM treatments were significantly increased by 42.0% and 24.1%, respectively, and MBN under the PM treatment was significantly increased by 5.6% at 0–10 cm soil depth after the maize season. Compared with the CK treatment, DOC was significantly increased by 21.0% under the SM treatment and decreased by 13.1% under the PM treatment, and DON was significantly increased by 10.5% under the SM treatment and decreased by 4.3% under the PM treatment at the 0–10 cm soil depth after the maize season. Relative changes of labile soil organic carbon and nitrogen fractions were more sensitive than that of SOC and TN. The relative decline or increase of labile soil organic carbon and nitrogen fractions was on average almost 13.6% for the mulching practices. Compared with the CK treatment, the average maize yields under the PM and SM treatments were increased by 26.4% and 9.8%, and the average wheat yields under the PM and SM treatments were increased by 21.3% and 7.4%, respectively. The average water use efficiencies under the PM and SM treatments were 24.5%, 8.8% in winter wheat and 22.9%, 6.3% in summer maize higher than that under the CK treatment, respectively. Our results suggested that plastic film mulching could be used as an effective practice to improve low soil quality with adequate nitrogen and increase crop yield and water use efficiency in the Loess Plateau, China.
Root exudates are well-known ‘labile’ sources of soil carbon that can prime microbial activity. Recent investigations suggest that the stability of labile carbon inputs in soil mostly depends upon ...the physical, chemical, and biological properties of the surroundings. Here, we propose that, in some ecosystems, such as forests and grasslands, root exudates can function as a source of soil organic carbon (SOC) that can be stabilized through various mechanisms leading to long-term sequestration. Increasing soil carbon sequestration is important for capturing atmospheric CO2 and combating climate change issues. Thus, there is an urgent need to preserve existing ecosystems and to adopt strategies such as afforestation, reforestation, and establishment of artificial grasslands to foster carbon sequestration through higher root exudate inputs in the soil.
Soil and plants are pivotal to the processes important for maintaining the integrity of biogeochemical cycles, such as the carbon cycle. Over the past few decades, anthropogenic activities have disturbed the atmospheric carbon cycle, leading to severe CO2 emissions into the atmosphere.Soil carbon sequestration by plant root exudates is an important means for net removal of CO2 content from the atmosphere.The rhizosphere environment in natural ecosystems, such as forests and grasslands, can help to stabilize root exudates in soil, while conditions in croplands do not appear favorable to stabilize root exudates as a soil organic carbon (SOC) source.Thus, preserving forests and grasslands with plant species secreting a high amount of carbon compounds might increase the SOC content in the soil of these ecosystems.
Despite the recognized importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, at a global scale, there has been no quantification for cropland, grassland, ...and forest ecosystems. To address this knowledge gap, the contents of fungal and bacterial necromass were estimated based on glucosamine and muramic acid contents in cropland (986 samples), grassland (278 samples), and forest (452 samples) soils. On an average, microbial necromass C contributed 51%, 47%, and 35% to the SOC in cropland, grassland, and forest soils, respectively, in the first 20 cm of topsoil. The contribution of microbial necromass to SOC increased with soil depth in grasslands (from 47% to 54%) and forests (from 34% to 44%), while it decreased in croplands (from 51% to 24%). The microbial necromass accumulation coefficient (the ratio between necromass and living microbial biomass C) was higher in soil from croplands (41) and grasslands (33) than in forest (20) soils. These results suggest that the turnover of living microbial biomass is faster in grassland and cropland soils than in forest soils, where the latter contains more partially decomposed plant residues. Fungal necromass C (>65% of total necromass) had consistently higher contributions to SOC than bacterial necromass C (32–36%) in all soils due to i) a larger living fungal biomass than bacterial biomass, and ii) fungal cell compounds being decomposed slowly and, thus able to persist longer in soil. The ratio of fungal:bacterial necromass C increased from 2.4 to 2.9 in the order of croplands < grasslands < forests, because fungi are the principal decomposers of complex substrates dominant in grassland and, especially, in forest soils. The ratios of bacterial:microbial necromass and bacterial:fungal necromass in cropland soils are larger than those in grassland and forest soils. This result indicates that the relative contribution of fungal necromass to total microbial necromass is lowest in cropland among the three land uses. Moreover, fungal and bacterial necromass increased with the total living microbial C and N contents. Lower temperatures and soil pH (e.g., in temperate and boreal ecosystems) stimulate fungal and bacterial necromass accumulation. These findings highlight the fact that shifts in the bacterial:fungal necromass ratio and the microbial necromass contribution to SOC are ecosystem-specific and depend on climate. In conclusion, microbial necromass contributes to approximately half of the SOC in cropland and grassland soils, and only 35% in forest soils; whereas, two-thirds of microbial necromass are of fungal origin.
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•Bacterial:fungal necromass ratio in cropland soil is larger than that in grassland and forest soils.•Half of soil organic matter under cropland and grasslands originates from microbial necromass.•Two-thirds of microbial necromass originates from fungi.•Fungal and bacterial necromass increases with total living microbial C and N.•Lower temperature and soil pH stimulate the accumulation of microbial necromass.
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