Increase In Soil Organic C Research Articles

Carbon sequestration in a bamboo plantation: a case study in a Mediterranean area

In the Mediterranean region, despite bamboo being an alien species that can seriously alter plant and animal biocoenosis, the area occupied by bamboo plantations continues to increase, especially for the purpose to sequester carbon (C). However, the C dynamics in the soil–plant system when bamboo is grown outside its native area are poorly understood. Here we investigated the C mitigation potential of the fast-growing Moso bamboo (Phyllostachys edulis) introduced in Italy for climate-change mitigation. We analyzed aboveground (AGB) and belowground (as root/shoot ratio) biomass, litter and soil organic C (SOC) at 0–15- and 15–30-cm depths in a 4-year-old bamboo plantation in comparison with the former annual cropland on which the bamboo was established. To have an idea of the maximum C stored at an ecosystem level, a natural forest adjacent the two sites was also considered. In the plantation, C accumulation as AGB was stimulated, with 14.8 ± 3.1 Mg C ha–1 stored in 3 years; because thinning was done to remove culms from the first year, the mean sequestration rate was 4.9 Mg C ha–1 a–1. The sequestration rates were high but comparable to other fast-growing tree species in Italy (e.g., Pinus nigra). SOC was significantly higher in the bamboo plantation than in the cropland only at the 0–15 cm depth, but SOC stock did not differ. Possibly 4 years were not enough time for a clear increase in SOC, or the high nutrient uptake by bamboos might have depleted the soil nutrients, thus inhibiting the soil organic matter formation by bacteria. In comparison, the natural forest had significantly higher C levels in all the pools. For C dynamics at an ecosystem level, the bamboo plantation on the former annual cropland led to substantial C removal from the atmosphere (about 12 Mg C ha–1 a–1). However, despite the promising C sequestration rates by bamboo, its introduction should be carefully considered due to potential ecological problems caused by this species in overexploited environments such as the Mediterranean area.

Effects of the combined application of organic and chemical nitrogen fertilizer on soil aggregate carbon and nitrogen: A 30-year study

To understand the long-term effects of combined organic and chemical nitrogen fertilization on soil organic C (SOC) and total N (TN), we conducted a 30-year field experiment with a wheat–maize rotation system on the Huang-Huai-Hai Plain during 1990–2019. The experimental treatments consisted of five fertilizer regimes: no fertilizer (control), chemical fertilizer only (NPK), chemical fertilizer with straw (NPKS), chemical fertilizer with manure (NPKM), and 1.5 times the rate of NPKM (1.5NPKM). The NPK, NPKS, and NPKM treatments had equal N inputs. The crop yields were measured over the whole experimental duration. Soil samples were collected from the topsoil (0–10 and 10–20 cm) and subsoil (20–40 cm) layers for assessing soil aggregates and taking SOC and TN measurements. Compared with the NPK treatment, the SOC and TN contents increased significantly in both the topsoil (24.1–44.4% for SOC and 22.8–47.7% for TN) and subsoil layers (22.0–47.9% for SOC and 19.8–41.8% for TN) for the organically amended treatments (NPKS, NPKM and 1.5NPKM) after 30 years, while no significant differences were found for the average annual crop yields over the 30 years of the experiment. The 0–10 cm layer of the NPKS treatment and the 20–40 cm layer of the NPKM treatment had significantly higher macroaggregate fraction mass proportions (19.8 and 27.0%) than the NPK treatment. However, the 0–10 and 20–40 cm layers of the 1.5NPKM treatment had significantly lower macroaggregate fraction mass proportions (–19.2 and –29.1%) than the control. The analysis showed that the higher SOC and TN in the soil of organically amended treatments compared to the NPK treatment were related to the increases in SOC and TN protected in the stable fractions (i.e., free microaggregates and microaggregates within macroaggregates), in which the contributions of the stable fractions were 81.1–91.7% of the increase in SOC and 83.3–94.0% of the increase in TN, respectively. The relationships between average C inputs and both stable SOC and TN stocks were significantly positive with R2 values of 0.74 and 0.72 (P<0.01) for the whole 40 cm soil profile, which indicates the importance of N for soil C storage. The results of our study provide key evidence that long-term combined organic and chemical nitrogen fertilization, while maintaining reasonable total N inputs, benefited soil C and N storage in both the topsoil and subsoil layers.

Soil biotic associations play a key role in subsoil C mineralization: Evidence from long-term tillage trial in the black soil of Northeast China

Quantifying the potential of soil carbon (C) mineralization improves our understanding of changes in soil health in agricultural ecosystems. Previous studies rarely linked the trait of interactions inherent in soil communities to soil C mineralization rate (Cmin). This study provides field-based evidence of tillage-driven changes in biological associations and their impacts on soil Cmin. Data obtained from a long-term (15 years) conservation tillage field experiment in Northeast China, and showed that the soil Cmin (per unit soil mass) and specific Cmin (per unit soil C) were 79% and 33% higher in topsoil (0–5 cm) and 27% and 24% lower in subsoil (5–20 cm) under no tillage with residue (NT) compared to conventional tillage with residue removal (CT). Meanwhile, in topsoil, NT strengthened biotic associations in June and August; in subsoil, a more complex network was found in CT. In topsoil, soil Cmin was not related to soil biotic associations, but was positively (P < 0.05) correlated with soil organic C (SOC) and certain biotic properties (e.g., MBC (microbial biomass carbon), microbial phospholipid fatty acid (PLFA) biomass and the relative abundance of AMF (arbuscular mycorrhizal fungi) and PP (plant-parasites)). Among them, the increases in SOC under NT exerted a positive effect on soil Cmin directly or indirectly through mediating biotic biomass properties (MBC, the PLFA biomass of all microbial groups and BF biomass). In contrast, in subsoil, soil Cmin was positively (P < 0.05) correlated with soil biotic associations, SOC and the relative abundance of PP. Notably, the increase in soil Cmin value was mainly achieved by strengthening biological associations. These findings provide novel insights into the role of soil biota in C mineralization, highlighting the diverse mechanisms underlying soil Cmin in response to tillage systems at different soil depths.

Contribution of roots to soil organic carbon: From growth to decomposition experiment

Carbon (C) derived from roots, rhizodeposition of living roots and dead root decomposition, plays a critical role in soil organic C (SOC) sequestration. Recent studies suggest that root inputs exert a disproportionate influence on SOC formation, and therefore, it is necessary to test separately the effects during root growth (i.e. rhizodeposition) and dead root decomposition (i.e. belowground litter) undernaturalconditions. A field experiment was carried out in grasslands with three typical plants: Stipa bungeana (St.B), Artemisia sacrorum (Ar.S), and Thymus mongolicus (Th.M) to differentiate the effects of root growth vs decomposition on C inputs by using in-growth soil cores and litter bag methods. For root growth experiment, the SOC content increased (2.5–3.8 g·kg−1) with root biomass, especially in soil under Ar.S. C input increased mineral-associated organic C (MAOC%) from 57% to 65%. The SOC δ13C was consistent with roots δ13C, indicating the roots were the primary source of SOC. The root decomposition experiment showed that the increase in SOC (0.74–2.6 g·kg−1) was highest at 90 days. Root decomposition rate was fast before 45 days, and SOC increased with the MAOC% during this period. After 45 days, particulate organic C % (POC%) was raised and was higher than in the control soil. The increase in SOC under root growth (2.5–3.8 g·kg−1) after one year was greater as compared to the rate under root decomposition (-0.4–0 g·kg−1). It suggested that C released during root growth was more effectively retained in the soil than that caused by dead root decomposition. The rise in MAOC during root growth and decomposition mainly explains the increase in SOC. Our results provided direct field based evidence illustrating the specific contribution of root growth and decomposition to SOC.

Light to moderate long-term grazing enhances ecosystem carbon across a broad climatic gradient in northern temperate grasslands

Grasslands are globally abundant and provide many ecosystem services, including carbon (C) storage. While grasslands are widely subject to livestock grazing, the influence of grazing on grassland ecosystem C remains unclear. We studied the effect of long-term livestock grazing on C densities of different ecosystem components in 110 northern temperate grasslands across a broad agroclimatic gradient in Alberta, Canada. These grasslands stored 50 to 180 t ha−1C in live and dead vegetation, as well as soil C to 30 cm depth, with the majority as soil organic C (SOC). The mulch layer comprised a large amount of C (~18 t ha−1C) especially within humid grasslands. Although grazing reduced C densities in litter mass, total ecosystem C was 8.5 % greater under grazing (127.8 t ha−1) compared to those non-grazed (117.8 t ha−1), primarily due to increases in SOC and roots. Increases in SOC were consistently observed in the 0–15 cm layer across all climatic conditions, with changes in SOC of the 15–30 cm layer inversely related to aridity. A structural equation model revealed that increased SOC under grazing was indirectly attributed to increases in eudicot rather than graminoid biomass. In addition, SOC increased with graminoid quality (i.e., a reduced carbon to nitrogen ratio), which together with elevated eudicots, increased litter and mulch C, and ultimately enhanced SOC densities. When applied to spatial maps of habitat type and land use (livestock grazing) activity across the region, an area of ~3.8 M ha of grassland was projected to contain an additional 17.1 M t of C under grazing, primarily in mesic grasslands, worth an estimated $3.1 B (Cdn.) under current C valuation guidelines in Canada. Overall, these results highlight the importance of grasslands for C storage and establishing policies that maintain and promote their sustainable use, including light to moderate grazing.

Improving crop nutrition, soil carbon storage and soil physical fertility using ramial wood chips

Ramial wood chips (RWC) amendment has great potential in sustainable agriculture; however more data is needed to assess its effect on soil fertility and carbon (C) storage. In this study, we investigated the effect of a single application of RWC amendment on a silty clay loam soil. During the 5 year experiment, we measured biomass production, grain yields and crop nutrient uptake. At the end of the experiment, we measured soil micro- and macronutrients, soil organic C (SOC) content and thermal stability, microbial biomass C (Cmic), and organic C and total N contents in soil particle size fractions. Soil physical properties, including structural porosity, air and water capacity, were also measured. Neither crop biomass production nor grain yields were affected by RWC. However, RWC was found to affect nutrient uptake, with improved N, P and Mg uptakes for the 2nd and 3rd crops after RWC amendment, and decreased Mn, Fe and Zn uptakes in the second half of the study period. The initially low SOC content increased by 10%, mainly in the mineral-associated organic matter fraction, resulting in a higher SOC stability. The increase in SOC following RWC amendment decreased the bulk density and increased the easily available water capacity due to a larger structural porosity. The increased porosity in the 15–30μm diameter range was ascribed to a change in SOC quality. In conclusion, RWC amendment improved macronutrient uptake in the short term, but decreased micronutrient uptake in the medium term. RWC increased SOC content and positively affected SOC quality, thus improving soil physical properties including water capacity and aeration.

Soil nutrient contents and stoichiometry within aggregate size classes varied with tea plantation age and soil depth in southern Guangxi in China

Abstract. Soil ecological stoichiometry offers a tool to explore the distribution, cycling, limitation, and balance of chemical elements in tea plantation ecosystems. This study aimed to explore how soil organic C (OC) and nutrient contents (total N (TN), total P (TP), Ca2+, Mg2+, Fe2+, and Mn2+) as well as their stoichiometric ratios (C/N, C/P, N/P, Ca/Mg, and Fe/Mn) varied with tea plantation age (8, 17, 25, and 43 years) and soil depth (0–10, 10–20, 20–40, and 40–60 cm) within aggregates in southern Guangxi in China. Our results showed that tea plantation age and soil depth significantly affected soil nutrient stoichiometry in different sizes of aggregates. Among different ages of tea plantations, soil OC, TN, and TP contents as well as C/N, C/P, and N/P ratios significantly decreased as the soil depth increased. In addition, soil Ca2+ and Mg2+ contents were significantly lower in the surface soil layer than the deeper soil layer, whereas soil Fe2+ and Mn2+ contents showed opposite trends, and no significant differences were detected in Ca/Mg and Fe/Mn ratios among different soil depths. At the 0–40 cm soil depth, continuous planting of tea corresponded to increases in soil OC, TN, Fe2+, and Mn2+ contents, whereas soil Ca2+ and Mg2+ contents significantly decreased over time. During the process of tea growth, the losses of soil Ca2+ and Mg2+, especially Ca2+ (as indicated by the decrease in the soil Ca/Mg ratio), led to soil acidification, which reduced Fe2+ absorption and enhanced Mn2+ uptake by tea plants (as indicated by the increase in the soil Fe/Mn ratio). In general, tea plantation age affected the variations of soil nutrient contents and stoichiometry, and such effects were more obvious at the 0–40 cm soil depth, in contrast to the 40–60 cm soil depth.

Partial Substation of Organic Fertilizer With Chemical Fertilizer Improves Soil Biochemical Attributes, Rice Yields, and Restores Bacterial Community Diversity in a Paddy Field.

Conventional farming systems are highly reliant on chemical fertilizers (CFs), which adversely affect soil quality, crop production and the environment. One of the major current challenges of current agriculture is finding ways to increase soil health and crop yield sustainably. Manure application as a substitute for CF is an alternative fertilization strategy for maintaining soil health and biodiversity. However, little is known about the complex response of soil bacterial communities and soil nutrients to manure and CFs application. This study reports the response of soil nutrients, rice yield, and soil microbial community structure to 2 years of continuous manure and CFs application. The study consisted of six treatments: no N fertilizer control (Neg-Con); 100% CF (Pos-Con); 60% cattle manure (CM) + 40% CF (High-CM); 30% CM + 70% CF (Low-CM); 60% poultry manure (PM) + 40% CF (High-PM), and 30% PM + 70% CF (Low-PM). We used high-throughput sequencing of 16S ribosomal RNA gene amplicons to characterize the soil bacterial communities. Results revealed that the addition of manure significantly altered the soil bacterial community composition and structure; and enhanced the relative abundance of phyla Proteobacteria, Chloroflexi, Firmicutes, Acidobacteria, and Planctomycetes. Organic fertilizer treatments, particularly high CM and PM had the highest measured soil bacterial diversity of all treatments. Similarly, integrated application of manure and CFs increased the soil biochemical traits [i.e., pH, total N (TN), soil organic C (SOC), microbial biomass N (MBN), and microbial biomass C (MBC)] and rice grain yield. Average increases in SOC, TN, MBN, and MBC were 43.66, 31.57, 24.34, and 49.45%, respectively, over the years in the High-PM compared with Pos-Con. Redundancy analysis showed that the dominant bacteria phyla were correlated with soil pH, SOC, TN, and microbial biomass, but the relative abundance of Proteobacteria was strongly correlated with environmental factors such as soil pH, SOC, TN, and MBC. We employed a structural equation model to examine the relationship between microbial biomass, soil nutrients and grain yield among treatments. This analysis supported the hypothesis that soil nutrient content and availability directly affect rice grain yield while soil bacteria indirectly affect grain yield through microbial biomass production and nutrient levels. Overall, the findings of this research suggest that the integrated application of CF and manure is a better approach for improving soil health and rice yield.

Effect of Integrated Organic–Inorganic Amendments on Leaf Physiological and Grain Starch Viscosity (Rapid Visco-Analyzer Profile) Characteristics of Rice and Ultisols Soil Quality

Current farming systems are highly reliant on chemical fertilizers (CF), which negatively affect soil health, the environment, and crop productivity. Substituting organic fertilizer for chemical fertilizer (CF) is an important agricultural practice that improves soil health and crop productivity and is eco-friendly. To explore the effects of organic fertilizer in the form of cattle manure (CM) or poultry manure (PM) combined with CF on soil properties, leaf physiological traits, and grain physiochemical characteristics of rice, a 2-year field experiment was conducted in a dual cropping system with six treatments: Neg-CF (no N fertilizer control), Pos-CF (100% CF), High-CM (60% CM + 40% CF), Low-CM (30% CM + 70% CF), High-PM (60% PM + 40% CF), and Low-PM (30% PM + 70% CF). The results exhibited that the adding of organic manure significantly improved soil chemical traits such as soil organic C (SOC), total N (TN), and available N (AN). Similarly, applying the combined fertilizer led to significant increases in the leaf net photosynthetic rate (Pn), SPAD values, and rice grain nutritional and cooking characteristics according to Rapid Visco-Analyzer (RVA) profile. Average increases in SOC, TN, Pn, starch content (SC), and amylose content (AC) were 16%, 12%, 9%, 7%, 9%, and 12%, respectively, across the year in the Low-PM compared to the Pos-CF treatment. Moreover, the manure amendments significantly altered the RVA profile attributes, including peak viscosity, tough viscosity, final viscosity, and the setback and pasting temperatures compared to Pos-CF. Linear regression analysis revealed that SOC and TN were positively associated with leaf Pn and SPAD values. Further, leaf Pn and SPAD were positively correlated with rice grain SC and AC. The correlation heat map analyses revealed a positive correlation in the RVA profile between the soil properties and leaf physiological traits. These correlation analyses showed that the increase in soil chemical traits and leaf physiological activities played a significant role in higher rice grain nutritional and cooking quality. Overall, the findings of this study show that the integrated use of organic fertilizers and CF in rice fields enhances soil functionality and the quality of rice on a sustainable basis.

Organic amendments affect soil organic carbon sequestration and fractions in fields with long-term contrasting nitrogen applications

Organic amendments are widely used to both manage soil organic C (SOC) and improve soil fertility in agro-ecosystems. However, crop production and the mechanisms of SOC sequestration following organic amendment may be affected by long-term differential N fertilizations, hence driving the dynamics in SOC fractions in cultivated fields. Our 3-yr (2016–2019) field experiment evaluated the responses of SOC sequestration and SOC fractions to organic amendments in soils with long-term contrasting N applications in a winter wheat monoculture system. We compared wheat straw return (St) and farmyard manure application (Fm) in soils with either long-term (2002–2019) N absence (-N) or normal N application (+N). Both St and Fm increased SOC stocks in the 0–20-cm layer, but Fm increased it more than St did. However, St exhibited a greater SOC sequestration rate in +N soils than that in –N soils, whereas Fm exhibited both a lower SOC sequestration rate and efficiency in +N soils than in –N soils. SOC increased across all fractions in all soils after both St and Fm. However, compared with –N soils, +N soils exhibited a lower SOC increase, specifically in the very labile fraction after St, but in the non-labile fraction after Fm. St, Fm, and +N all increased the soil C management index (CMI), but only +N increased wheat yield and its sustainable yield index (SYI). Those relationships indicated that both straw and manure amendments were responsible for both SOC sequestration and increasing the CMI, and N input was fundamentally responsible for an increased SYI and associated plant-C input. Consequently, straw return was expected to be more effective at sequestering SOC when applied to soil having had adequate long-term N application, whereas farmyard manure application was likely more effective when applied in N-deficient soil. Our field study findings provide empirical evidence that soil N availability affects SOC sequestration capacities following organic amendments, and that impacts strategies aimed at further increasing soil fertility improvements via organic amendment, especially considering that different agro-ecosystems are subject to different fertilization regimes.

Soil carbon sequestration potential and the identification of hotspots in the eastern Corn Belt of the United States

AbstractSoil C sequestration is a significant CO2 mitigation strategy, but precise assessments of sequestration require spatially explicit modeling of potential changes in soil organic C (SOC) in response to soil, climate, land condition, and management interactions. We assessed the SOC sequestration potential of the eastern Corn Belt (ECB) in the United States (Indiana, Kentucky, Maryland, Michigan, Ohio, Pennsylvania, and West Virginia) in response to the adoption of conservation farming practices and land use change (pasture and forestation) using the SOCRATES model. Input data was provided through an intersection of the State Soil Geographic database, National Land Cover Database, and a PRISM (https://prism.oregonstate.edu) climate surface. At the end of the 20th century, the 15.3 Mha of cropped soils in the ECB contained 632 Tg C, an estimated reduction of 52% since the introduction of agriculture in the mid 1800s. Complete adoption of no‐tillage practices on prime cropland would potentially recover 147 Tg SOC over 20 yr, whereas a continuation of conventional tillage would produce a loss of 35 Tg SOC over that period. Sequestration hotspots (&gt;500 Gg increase in SOC) under no‐tillage cover 2.3 Mha providing 28 Tg C over 20 yr. The conversion of marginal (nonprime) agricultural lands to forests would yield an additional 13 Tg C in SOC and 381 Tg C in aboveground biomass. The rehabilitation of minelands to forest would yield an additional 4 Tg C in SOC and 42 Tg C in biomass. Opportunities to sequester C in the ECB via tillage and reforestation are substantial and should be incorporated into regional and national climate change mitigation strategies.

Assessing the effectiveness of agricultural conservation practices in maintaining soil organic carbon under contrasting agroecosystems and a changing climate

AbstractThe potential of conservation practices to maintain soil organic C (SOC) and reduce climate change impacts on yields is unknown. This study aimed to validate the DayCent model with observed yield data and the CQESTR model with measured SOC in two agroecosystems, and predict the best practices to sustain SOC under projected climate change. Data were from a conventional tillage (CT) wheat (Triticum aestivum L.)–fallow rotation without N or with 135 kg N ha–1 fertilizer, and no‐till (NT) experiments, NTA (NTA0W‐F and NTA135W‐F) and NTB with a wheat–pea (Pisum sativum L.) cover crop (NTB0W‐P and NTB135W‐P) in Oregon; and crop rotations with CT and NT silage or grain corn (Zea mays L.)–soybean [Glycine max (L.) Merr.]–alfalfa (Medicago sativa L.) with or without manure or stover removal or a cover crop in Pennsylvania. Measured and CQESTR‐simulated SOC were significantly (p &lt; .0001) correlated (r = .90). In Oregon, predicted SOC to 60 cm increased under NTA135W‐F and NTB135W‐P at 0.05 and 0.08 Mg ha–1 yr–1 under projected climate change, respectively. The NT management under dryland production provided a limited SOC increase. In Pennsylvania, predicted SOC to 1 m decreased at 0.07 Mg ha–1 yr–1 in corn–soybean under CT or NT with stover removal, but increased by 0.71 Mg ha–1 yr–1 in dairy forage NT and manure added under climate change. The responses of SOC to climate change were affected by management, biomass type, edaphic properties, local climate, and agroecosystem.

Strength, swelling and compressibility of unsaturated sugarcane soils

Large-scale sugarcane plantation is characterized by successive and disordered traffic of heavy machinery and equipment over the cycle, which can cause damage to the soil structure. As the stress induced by traffic exceeds the soil load-bearing capacity, it results in soil compaction. However, the mechanisms and key variables that govern the compressive characteristics in agricultural unsaturated soils to support those applied loads without plastic deformations (compaction) is not fully understood. To investigate this process, we measured the strength, swelling and compressibility characteristics of unsaturated soils with contrasting texture from sugarcane fields. Undisturbed soil cores were sampled in nine cultivated sugarcane fields, equilibrated at a matric potential of –100 hPa and subjected to confined uniaxial compression tests. Precompression stress (σp), compression (λ) and swelling (κ) indexes were extracted from the soil compression curves. Soil organic C (SOC), degree of compactness, total porosity and particle size fractions were also measured and correlated to the compressive parameters σp, λ and κ. Principal components, correlation and multiple regression models were used to the data analyses. Overall, reductions in soil strength and increases in the soil compressibility and elasticity occurred with the increase in SOC, whereas increases in the degree of compactness resulted in higher soil strength and lower soil compressibility and elasticity. Furthermore, our results revealed that silt + clay content considerably increased soil compressibility. In combined scenarios for a range of soil texture, SOC and degree of compactness at matric potential at around field capacity (i.e. –100 hPa), the maximum σp observed was approximately 200 kPa, which is a much lower load-bearing capacity than the estimated stress for some trucks and trailers used in sugarcane harvest (∼600−800 kPa). Our study suggests that soil compaction is eminent in sugarcane fields at matric potential close to field capacity, and can be aggravated in clayey soils with high SOC content and low degree of compactness. Therefore, urgent strategies of traffic control are necessary to prevent or mitigate soil compaction induced by mechanized sugarcane operations.

Effects of field scale in situ biochar incorporation on soil environment in a tropical highly weathered soil.

Biochar has been proven as a soil amendment to improve soil environment. However, mechanistic understanding of biochar on soil physical properties and microbial community remains unclear. In this study, a wood biochar (WB), was incorporated into a highly weathered tropical soil, and after 1 year the in situ changes in soil properties and microbial community were evaluated. A field trial was conducted for application of compost, wood biochar, and polyacrylamide. Microstructure and morphological features of the soils were characterized through 3D X-ray microscopy and polarized microscopy. Soil microbial communities were identified through next-generation sequencing (NGS). After incubation, the number of pores and connection throats between the pores of biochar treated soil increased by 3.8 and 7.2 times, respectively, compared to the control. According to NGS results, most sequences belonged to Anaerolinea thermolimosa, Caldithrix palaeochoryensis, Chthoniobacter flavus, and Cohnella soli. Canonical correlation analysis (CCA) further demonstrated that the microbial community structure was determined by inorganic N (IN), available P (AP), pH, soil organic C (SOC), porosity, bulk density (BD), and aggregate stability. The treatments with co-application of biochar and compost facilitated the dominance of Cal. palaeochoryensis, Cht. flavus, and Coh. soli, all of which promoted organic matter decomposition and ammonia oxidation in the soil. The apparent increases in IN, AP, porosity, and SOC caused by the addition of biochar and compost may be the proponents of changes in soil microbial communities. The co-application of compost and biochar may be a suitable strategy for real world biochar incorporation in highly weathered soil.

Soil organic carbon sequestration when converting a rainfed cropping system to irrigated corn under different tillage systems and N fertilizer rates

AbstractThe aim of this study was to evaluate the impact of 21 years of tillage and N fertilization and the conversion from a rainfed to an irrigated cropping system on soil organic C (SOC). The study was carried out in northeastern Spain in a long‐term tillage and N rate field experiment established in 1996 under barley rainfed conditions, which in 2015 was converted into irrigation with corn. Three types of tillage (conventional tillage, CT; reduced tillage, RT; no‐tillage, NT) and three mineral N fertilization rates (0, 60, and 120 kg N ha−1 under barley, and 0, 200, and 400 kg N ha−1 under corn) were compared. Annual C‐inputs as aboveground crop residues and annual SOC sequestration rate (∆SOCrate) (0–40 cm depth) were calculated in three different periods (P1, P2 and P3) under rainfed (‐R) and irrigated (‐I) conditions (P1‐R, from 1996 to 2009; P2‐R, from 2009 to 2015; P3‐I, from 2015 to 2017). At the end of P3‐I, particulate organic C (POC) was measured from the 0–5, 5–10, 10–20, 20–30, and 30–40 cm depths. Averaged over all treatments, ∆SOCrate was 492, 222, and 969 kg C ha−1 yr−1 for P1‐R, P2‐R, and P3‐I, respectively. In P1‐R and P3‐I, C‐input explained 70% of the variability of ∆SOCrate. In P1‐R, ∆SOCrate followed the order NT &gt; RT &gt; CT, while for N rate, order was high &gt; medium &gt; 0. In P3‐I at the highest N rate, ∆SOCrate followed the order NT &gt; RT &gt; CT. In P2‐R, ∆SOCrate did not show differences between tillage and/or N rate treatments. The increase in SOC after conversion from a rainfed to an irrigation system was mainly explained by POC, which was increased by 75% compared to the previous rainfed period. The modification of the cropping system through the introduction of irrigation and adequate crop management practices under no‐tillage and adjusted N fertilizer rates can contribute to the sequestration of large amounts of atmospheric CO2.

Variation in rhizosphere priming and microbial growth and carbon use efficiency caused by wheat genotypes and temperatures

Living roots can influence microbial decomposition of soil organic matter, which has been referred to as the rhizosphere priming effect (RPE). Both microbial carbon efficiency (CUE) and microbial growth and turnover rates are associated with microbial decomposition and respiration of soil-derived C, but their linkage to the RPE remains poorly understood. Here we used a natural 13C tracer method to determine the RPE in soils planted with two wheat genotypes (249 or IAW2013) grown at high (30/24 °C during day/night) and low temperature (25/17 °C during day/night). We also determined microbial CUE, growth and biomass turnover rate using a substrate-independent H218O labeling method. The RPE varied from −2 to +455%, with significant effects of genotype, sampling date and their interaction with temperature. Compared to the unplanted control, microbial biomass C and growth/turnover rate were both enhanced in planted pots, with an average increase of 17% and 70%, respectively. Microbial CUE was lowest in pots planted with IAW2013 at low temperature, but there were no significant main effects of planting and temperature. Microbial biomass growth/turnover rate together with CUE accounted for 83% of the variation in soil-derived CO2, with a relatively larger contribution of microbial biomass growth/turnover rate (52%) than CUE (31%). Furthermore, using linear regression, we demonstrated that the RPE was significantly positively related to microbial biomass growth/turnover rate. No net soil organic C (SOC) loss or gain was detected, indicating that any increase in SOC due to increased microbial growth/turnover was counteracted by C loss caused by a higher RPE during the relatively short time of planting. These findings suggest that microbial biomass turnover associated with growth could control the loss of SOC with planting. We highlight the importance of plant-induced changes in microbial CUE and biomass growth/turnover for long-term soil C dynamics.

Shrub encroachment increases soil carbon and nitrogen stocks in temperate grasslands in China

AbstractShrub encroachment has significant impacts on carbon (C) and nitrogen (N) dynamics of grassland ecosystems, but, how it influences soil organic C (SOC) and soil total N (STN) stocks of grasslands is not well documented, especially in deep soils. Here, we examined these effects by investigating the SOC density (SOCD) and the STN density (STND) at depths of up to 5 m in shrub patches and grassy matrix at 18 grassland sites in Inner Mongolia, China. Our results revealed that 44.0–54.1% of SOC and 65.5–74.8% of STN were stored in soil layers of 100 cm to 500 cm, and that shrub encroachment resulted in a significant accumulation of SOC and STN in shrub patches. The mean SOCD at 0–500 cm soil layers in shrub patches and grassy matrix was 11.34 and 8.39 kg C m−2, respectively, and the mean STND was 2.45 and 2.13 kg N m−2, respectively. These differences between the shrub patches and grassy matrix were equivalent to an increase in the mean SOC and STN stocks of 2.95 kg C m−2 and 0.32 kg N m−2, respectively. Increases in SOCD and STND mainly occurred in 30–300 cm soil layers, and varied between typical grassland and desert grassland. The changes in SOCD and STND exhibited significant negative correlations with temperature and pH and positive correlations with precipitation and plant biomass. Our findings suggest that shrub encroachment can significantly accumulate SOC and STN in deep soils in temperate grasslands, which helps to understand soil C and N dynamics in shrub‐encroached grasslands.

Soil moisture drives microbial controls on carbon decomposition in two subtropical forests

Knowledge of microbial mechanisms is critical to understand Earth's biogeochemical cycle under climate and environmental changes. However, large uncertainties remain in model simulations and predictions due to the lack of explicit parameterization of microbial data and few applications beyond the laboratory. In addition, most experimental and modeling studies of warming-induced changes in soil carbon (C) focus on temperature sensitivity, neglecting concomitant effects of changes in soil moisture. Soil microbes are sensitive to moisture, and their responses can dramatically impact soil biogeochemical cycles. Here we represent microbial and enzymatic functions in response to changes in moisture in the Microbial-ENzyme Decomposition (MEND) model. Through modeling with long-term field observations from subtropical forests, we demonstrate that parameterization with microbial data in addition to respiration fluxes greatly increases confidence in model simulations. We further employ the calibrated model to simulate the responses of soil organic C (SOC) under multiple environmental change scenarios. The model shows significant increases in SOC in response to decreasing soil moisture and only minor changes in SOC in response to increasing soil temperature. Increasing litter inputs also cause a significant increase in SOC in the pine forest, whereas an insignificant negative effect is simulated in the broadleaf forest. We also demonstrate the co-metabolism mechanism for the priming effects, i.e., more labile inputs to soil could stimulate microbial and enzymatic growth and activity. Our study provides strong evidence of microbial control over soil C decomposition and suggests the future trajectory of soil C may be more responsive to changes in soil moisture than temperature, particularly in tropical and subtropical environments.

More replenishment than priming loss of soil organic carbon with additional carbon input

Increases in carbon (C) inputs to soil can replenish soil organic C (SOC) through various mechanisms. However, recent studies have suggested that the increased C input can also stimulate the decomposition of old SOC via priming. Whether the loss of old SOC by priming can override C replenishment has not been rigorously examined. Here we show, through data–model synthesis, that the magnitude of replenishment is greater than that of priming, resulting in a net increase in SOC by a mean of 32% of the added new C. The magnitude of the net increase in SOC is positively correlated with the nitrogen-to-C ratio of the added substrates. Additionally, model evaluation indicates that a two-pool interactive model is a parsimonious model to represent the SOC decomposition with priming and replenishment. Our findings suggest that increasing C input to soils likely promote SOC accumulation despite the enhanced decomposition of old C via priming.

Simulated Soil Organic Carbon Changes in Maryland Are Affected by Tillage, Climate Change, and Crop Yield.

The impact of climate change on soil organic C (SOC) stocks in no-till (NT) and conventionally tilled (CT) agricultural systems is poorly understood. The objective of this study was to simulate the impact of projected climate change on SOC to 50-cm soil depth for grain cropping systems in the southern Mid-Atlantic region of the United States. We used SOC and other data from the long-term Farming Systems Project in Beltsville, MD, and CQESTR, a process-based soil C model, to predict the impact of cropping systems and climate (air temperature and precipitation) on SOC for a 40-yr period (2012-2052). Since future crop yields are uncertain, we simulated five scenarios with differing yield levels (crop yields from 1996-2014, and at 10 or 30% greater or lesser than these yields). Without change in climate or crop yields (baseline conditions) CQESTR predicted an increase in SOC of 0.014 and 0.021 Mg ha yr in CT and NT, respectively. Predicted climate change alone resulted in an SOC increase of only 0.002 Mg ha yr in NT and a decrease of 0.017 Mg ha yr in CT. Crop yield declines of 10 and 30% led to SOC decreases between 2 and 8% compared with 2012 levels. Increasing crop yield by 10 and 30% was sufficient to raise SOC 2 and 7%, respectively, above the climate-only scenario under both CT and NT between 2012 and 2052. Results indicate that under these simulated conditions, the negative impact of climate change on SOC levels could be mitigated by crop yield increases.