Items

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  Do Cover Crops Impact Labile C More Than Total C? Data Synthesis

  The potential of cover crops (CC) to increase soil organic C (SOC) concentration can be inconsistent, but labile SOC is considered to be more sensitive to management than total SOC. This leads to two questions: Do CCs impact labile SOC more than total SOC? Do CCs increase labile SOC more rapidly than total SOC? This review compares CC impacts on labile and total SOC based on CC studies reporting both parameters up to Dec 31, 2022. Labile and total SOC concentrations were measured in 31 CC study locations. Cover crops increased labile SOC concentration in 58% (18 of 31) and had no effect in 42% (13 of 31) of locations, suggesting CCs do not increase labile SOC in all cases. Within the 18 locations, CCs increased labile SOC without increasing total SOC only in 19% (6 of 31 locations), while in the rest (12 of 31) of locations, CCs increased both labile and total SOC. Thus, CCs increased labile SOC more rapidly than total SOC only in one-fifth of cases. Also, the few studies that monitored changes in labile SOC with time found CCs do not always increase labile more rapidly than total SOC. In the 12 locations where CCs increased both labile and total SOC, CCs increased labile SOC by 54 ± 30% and total SOC by 23 ± 10%, indicating CCs can increase labile SOC by about two times compared with total SOC in some locations. Increased CC biomass production and reduced residue decomposition can increase labile SOC. Overall, CCs increase labile SOC in most cases but may not always increase labile SOC more rapidly than total SOC although more CC studies monitoring changes in SOC pools with time are needed to better understand CC impacts on SOC fractions under different CC management scenarios and climatic conditions.
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  Carbon sequestration capacity in no-till soil decreases in the long-term due to saturation of fine silt plus clay-size fraction

  The capacity of soils to stabilize carbon (C) may decrease over time, limiting the potential of no-till soil to act as a C sink in the long-term. Our objectives were to evaluate the effects of long-term no-till cropping systems on (i) C storage in soil, (ii) C stabilization in the fine silt plus clay-size (<20 μm) fraction and its relationship with the decrease of C saturation deficit (CSD) in this fraction, and (iii) on C accumulation in labile fractions of soil organic matter (SOM) in 0–2.5, 2.5–5, 5–10 and 10–20 cm layers of a subtropical Acrisol. The study was based on a long-term (36 years) no-till experiment where five cropping systems, with variable annual C inputs, were assessed: [i] bare-soil, [ii] black oat/maize, [iii] black oat + vetch/maize + cowpea, [iv] lablab + maize and [v] pigeon pea + maize. Cropping systems including maize and tropical legumes (lablab, pigeon pea and cowpea) with high C input led to the highest C storage in the top layers (up to 10 cm depth) of this no-till soil. Also, a decrease of CSD in fine silt plus clay-size fraction was observed in all soil layers to 20 cm depth, but the most expressive impact on CSD occurred in the topsoil (0–2.5 cm), where the capacity to further stabilize more carbon decreased by 90–97% when compared to bare soil. Considering the full C saturation level of the silt plus clay-size fraction and the current C contents in the soil, the remaining capacity of C sequestration up to 20 cm was estimated as ranging from 22.5 to 32.8 Mg C ha−1, and much of it (58–75%) was in the 10–20 cm layer. Our results highlight the importance of diversified cropping systems with high input (quantity and quality) crop residues to C sequestration in soil. Moreover, although the mineral-associated SOM of the top layer reached a C stabilization limit, C accumulation continues in non-saturated labile fractions, and in non-saturated fine silt plus clay-size fraction in deeper layers of subtropical no-till soils.
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  Effect of the intensification of cropping sequences on soil organic carbon and its stratification ratio in contrasting environments

  In environments where continuous agriculture leads to soil organic carbon (SOC) depletion, intensification practices (i.e. polyculture, cover crops (CC), and crop fertilization) have been suggested as strategies to improve crop residue inputs which, in turn, can increase SOC storage. However, SOC dynamics are regulated by a complex interplay of climatic and soil conditions. The objective of our study was to assess how intensification practices affect SOC, particulate organic carbon (POC) and SOC stratification ratio (SRSOC) as compared to soybean [Glycine max (L.) Merr.] monoculture, in soils with contrasting soil properties and climate. The experiment was carried out in four long term experiments (>10 yr) located in areas with contrasting environments. The surface soil textures ranged from sandy-loam to silty-clay and clay-loam, initial SOC (0–20 cm) from 34.5 to 67.8 Mg ha−1, mean air temperature: 14.0–18.9 °C, annual precipitation: 719.8–886.1 mm. Five treatments were evaluated: soybean monoculture (SB), soybean monoculture fertilized with phosphorus (P) and sulfur (S) (SBPS), CC/PS-fertilized soybean (SBPS/CC), nitrogen (N)-fertilized CC/PS-fertilized soybean (SBPS/CCN) and NPS-fertilized crop rotation (ROTNPS). Intensification of crop sequences (SBPS/CC, SBPS/CCN and/or ROTNPS) increased SOC and POC at 0–5 cm and in SRSOC in most sites as compared to SB. All treatments showed SOC depletion as compared to the beginning of the experiment. However, the magnitude of SOC lost during 10 years was 26–65% lower when intensified crop sequences were applied as compared with SB. Carbon input and environment characteristics influenced the impact of intensification practices on the analyzed variables. However, this effect was mostly associated with the ratio between SOC at the beginning of the experiment and the SOC of pristine soil (degradation status). The intensification practices evaluated were not sufficient to reverse the tendency of agricultural soils to lose SOC, but they slowed the rate of this degradation process.
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  Soil organic carbon is affected by organic amendments, conservation tillage, and cover cropping in organic farming systems: A meta-analysis

  Meta-analysis is often used to compare how soil health differs between organic and conventional farming systems. However, the burgeoning primary literature on organic farming now allows direct evaluation of the best management practices (BMPs) within organic farming systems on soil health improvements. Therefore, the main objective of this meta-analysis was to investigate the effect of BMPs, such as organic amendments, conservation tillage, and cover cropping, on soil health within organic farming systems. We focused on two principal soil health metrics: soil organic carbon (SOC) and microbial biomass carbon (MBC) concentrations. On average, adoption of BMPs increased depth-weighted SOC and MBC concentrations by 18 and 30 %, respectively, relative to organically-managed control groups. Among BMPs, organic amendments and conservation tillage practices showed net positive effect on soil health with 24 and 14 % increase in depth-weighted SOC concentrations, respectively. Although cover cropping did not have an overall influence on SOC concentrations, we found a temporal trend such that cover cropping significantly increased SOC concentrations after 5 years of its adoption. This indicates that the soil health benefits from BMPs accrue over time and highlights the need of long-term adoptability of BMPs to achieve agricultural sustainability. Future primary articles that focus on under-researched cropping practices in organic systems (e.g., crop rotation length and diversity, biochar addition) and the additive effects of multiple BMPs on soil health, will add to the synthesizable evidence base. Therefore, this meta-analysis confirms the soil health benefits of adopting BMPs within organic farming systems, identifies critical knowledge gaps, and provides directions for future organic farming research.