Soil Organic Carbon Restoration Research Articles

Long-term impact of farming systems on soil organic carbon restoration in sandy soil in the southeastern US

Long-term impact of farming systems on soil organic carbon restoration in sandy soil in the southeastern US

How does national SOC monitoring on agricultural soils align with the EU strategies? An example using five case studies

AbstractSoil functioning contributes to the delivery of a vast range of ecosystem goods and services, and ecosystem health is therefore reflected by the capacity of the soil to perform underlying functions. Soil organic carbon (SOC) is a key indicator for soil quality as it is an integral driver of many soil functions and associated ecosystem services. Across the globe, SOC stocks are declining due to expanding agriculture and unsustainable practices. Awareness of the fact that soil is a non‐renewable resource and its functioning important for all life on Earth is increasing, especially among policymakers. As such, goals for the preservation and restoration of SOC are formulated in policies under the European Green Deal. However, the evaluation of these goals at the European level is hampered by a non‐harmonized diversity in national SOC monitoring strategies. While some SOC indicators can be useful for the evaluation of most policy goals (i.e., baseline and potential SOC stocks), additional and contrasting SOC data are often required for the evaluation of the goals formulated by the different EU directives. This study provides an overview of five ongoing SOC monitoring programmes across Europe and discusses how national programmes may be aligned to evaluate goals at the EU level. Five countries with very different soil monitoring programmes were included in a case study to illustrate the potential for harmonization and standardization of SOC assessment. Based on this study, we conclude that SOC monitoring strategies can be harmonized, but not standardized. We further suggest five sampling strategies that have potential for harmonization under the proposed Directive on Soil Monitoring and Resilience.

Should we consider integrated crop–livestock systems for ecosystem services, carbon sequestration, and agricultural resilience to climate change?

AbstractContemporary agricultural systems can be generalized as highly specialized operations that are fueled by mechanization; supplied with external nutrients to maximize production; crops protected by petrochemicals to fight against weed, disease, and insect pressures; and livestock protected by therapeutics to ward off virus and bacterial infections when managed in confinement. Such specialized systems have led to low levels of diversity, elevated environmental risks from contamination, loss of soil organic matter, ecological instability, and limited adaptability to climate change. More diversified farming systems are possible, but research required to characterize them in a holistic manner as an alternative to contemporary, specialized systems remains challenging to fund and sustain over time, primarily because they require more labor, management skills, and accessible markets to achieve additional ecological, environmental, and social goals. We share some perspectives as to (1) how specialized systems became the norm and (2) what changes could be made to reverse some ecological risks and environmental declines associated with specialization, acknowledging there is no panacea. Strong evidence exists for perennial forages to restore soil organic carbon (C) and nitrogen, but system‐level analyses of the net balance in greenhouse gas emissions remain to be characterized in the myriad of potential integrated crop–livestock systems that might be deployed across the diversity of edaphic, environmental, and socio‐economic conditions. We suggest there are abundant opportunities for more sustainable agricultural production to sequester soil organic C, reduce greenhouse gas emissions, and develop more climate‐resilient agricultural systems that will be needed in a future dominated by climate change issues.

Enhancing Organic Carbon Content in Tropical Soils: Strategies for Sustainable Agriculture and Climate Change Mitigation

Background: Tropical soils are characterized by low soil organic carbon (SOC) contents, which can negatively impact soil fertility, water retention, overall agricultural productivity, and food security. Objective: This paper aimed to review and synthesize the current body of literature on the restoration of SOC in tropical soils. Methods: A total of about 152 related articles were downloaded from electronic journal databases using search keywords, such as organic carbon (OC), conservation tillage, intercropping, biochar, and tropical soil. We discussed conservation tillage practices, such as reducing soil disturbance and respiration from the soils, as a means to promote soil OC sequestration. We also highlighted the importance of crop residue retention, which not only increases organic matter inputs and soil nutrients but also promotes soil water retention and reduces soil erosion. Results: Cover crops and crop rotation are identified as effective practices to cover bare ground during planting seasons, reduce erosion, and prevent nutrient losses through leaching and runoff. Additionally, we review the role of fertilizer application, manure and compost application, intercropping, agroforestry, and biochar as strategies to enhance OC content in tropical soils. We highlight the benefits of incorporating organic amendments, such as manure and compost, to improve soil structure and water-holding capacity. Furthermore, we discuss the potential of biochar, a carbon-rich material produced from biomass pyrolysis, as a strategy to sequester OC in tropical soils. Conclusion: Overall, this review provides insights into various strategies that can be implemented to increase the OC content of tropical soils, which can have multiple benefits for soil health, agricultural productivity, and climate change mitigation. Further research and implementation of these practices can contribute to mitigating climate change, conserving soil resources, and promoting sustainable agriculture in tropical regions.

Crop residue retention increases greenhouse gas emissions but reduces chemical fertilizer requirement in a vegetable-rice rotation

Retaining crop residues on site after harvesting can maintain soil fertility, reduce fertilizer requirements and restore soil organic carbon on intensively cultivated land. However, how crop residue retention and reduced chemical fertilizer application affect greenhouse gas emissions and crop yield in vegetable-rice rotations is poorly understood. We studied the effect of chemical fertilizer application alone (control, without straw retention), straw retention with 100 % nitrogen fertilizer application (STF) or with three levels of reduced chemical fertilizer application (SFR1, SFR2 and SFR3) in a vegetable-rice rotation in a mesocosm system. In the vegetable season, crop yield was maintained but the agronomic efficiency (AE) was higher in SFR1, SFR2 and SFR3 as compared to STF and the control. In the rice season under flooded condition, the methanogens were higher in SFR1, SFR2 and SFR3 compared to the control, but not different between STF and SFR1. The STF showed higher methane and nitrous oxide emissions, but carbon dioxide emissions and AE remained unaffected compared to the control in the vegetable-rice rotation. The SFR2 and SFR3 treatments maintained crop yields, increased AE, and decreased N2O emissions as compared with STF, but the CH4 emissions were 29.8 % lower in SFR2 than SFR3 in the vegetable-rice rotation. We concluded that straw retention with reduced chemical fertilizer application can maintain crop yield in a vegetable-rice rotation but has the potential to increase CH4 emissions in the rice cultivation season.

A global meta-analysis of soil organic carbon in the Anthropocene

Anthropogenic activities profoundly impact soil organic carbon (SOC), affecting its contribution to ecosystem services such as climate regulation. Here, we conducted a thorough review of the impacts of land-use change, land management, and climate change on SOC. Using second-order meta-analysis, we synthesized findings from 230 first-order meta-analyses comprising over 25,000 primary studies. We show that (i) land conversion for crop production leads to high SOC loss, that can be partially restored through land management practices, particularly by introducing trees and incorporating exogenous carbon in the form of biochar or organic amendments, (ii) land management practices that are implemented in forests generally result in depletion of SOC, and (iii) indirect effects of climate change, such as through wildfires, have a greater impact on SOC than direct climate change effects (e.g., from rising temperatures). The findings of our study provide strong evidence to assist decision-makers in safeguarding SOC stocks and promoting land management practices for SOC restoration. Furthermore, they serve as a crucial research roadmap, identifying areas that require attention to fill the knowledge gaps concerning the factors driving changes in SOC.

Soil carbon dynamics under organic farming: Impact of tillage and cropping diversity

Ecological indicators are herculean contrivance for assessing management practices' impacts on environmental changes. Soil organic carbon (SOC) potentially regulates the agricultural sustainability. Unfortunately, the SOC has been widely degraded through unsustainable land uses and agricultural practices. Hence, the efficacy of conservation tilling and diversified cropping were assessed in terms of restoration of SOC and associated soil properties in fixed plots, which are related with farm productivity and other ecosystem services under organic production systems of the Indian Himalayas. Three tillage management were selected as treatments of the study: i) conventional tilling (CT), ii) reduced tilling (RT), and iii) Zero tilling (ZT) applied to four diversified cropping systems [CS1- maize–black gram–toria, CS2 maize–black gram–buckwheat, CS3- maize–rajmash–toria and CS4 maize–rajmash–buckwheat]. The ZT had the highest SOC content (19.58 g kg−1) as well as the C pool (25.24 Mg ha−1) at a soil depth of 0–10 cm. On the contrary, ZT had the lowest ρb (1.29 Mg m3) and soil penetration resistance (1.32 MPa) at 0–5 cm and 5–10 cm soil depth. Concerning SOC partitioning, regardless of soil depth, it had been evident that more SOC were allocated in the active pool over the passive pool. Out of the diversified cropping systems, the CS2 produced a considerably higher total carbon pool of 24.98, and 23.0 Mg ha−1 at 0–10 cm, and 10–20 cm soil depths, respectively, and active and passive C pools. Hence, abolition of tillage and cultivation of legume embedding cropping systems resulted as a sustainable management system under organic farming for SOC restoration and soil quality improvement in the Himalayan ecosystem. Thus, the study suggested that the cultivation of CS2 under ZT may be promoted for efficient land resource management planning in the study region of the Indian Himalayas.

Soil organic carbon and nitrogen storage estimated with the root-zone enrichment method under conventional and conservation land management across North Carolina

Agriculture is a globally dominating land use, so efforts to restore soil organic carbon (C) and nitrogen (N) lost through historical degradation could have enormous benefits to production and the environment, particularly by storing an organic reserve of nutrients in soil and avoiding the return of a small portion of biologically cycling C to the atmosphere. Estimates of soil organic C and N storage from conservation agricultural management are still limited when considered in proportion to the large diversity of environmental and edaphic conditions. A study was undertaken to determine the total, baseline, and root-zone enrichment stocks of soil organic C and N as affected by land use on 25 research stations distributed throughout North Carolina. Root-zone enrichment of organic matter is that portion influenced by contemporary management, and baseline is that portion dominated by pedogenesis. These fractions were compared with more traditional estimation procedures. Soil organic C and N were strongly negatively associated with sand concentration. Although physiographic region influenced overall soil C and N contents, variations in soil type and research station management within a region were equally influential. Soil organic C and N stocks were strongly affected by land use, which did not interact with the soil textural effect. Across the 25 research station locations, root-zone enrichment of soil organic C followed the order (<i>p</i> &lt; 0.01) conventional-till cropland (11.1 Mg C ha⁻¹) &lt; no-till cropland (21.5 Mg C ha⁻¹) &lt; grassland (29.6 Mg C ha⁻¹) &lt; woodland (38.6 Mg C ha⁻¹). Root-zone enrichment of total soil N followed a similar order, except grassland and woodland effects were reversed. Root-zone enrichment provided an integrated soil-profile assessment and a more targeted response of soil organic C and N change than did more traditional paired land use approaches, primarily due to separation of a variable pedogenic influence among sites. These point-in-time results gave a clear indication that conservation agricultural management approaches will foster surface soil organic C and N restoration across a diversity of soil types in the southeastern United States.

Effects of Farmland Conversion to Orchard or Agroforestry on Soil Organic Carbon Fractions in an Arid Desert Oasis Area

In southern Xinjiang province, northwest China, farmland is undergoing rapid conversion to orchards or agroforestry. This has improved land-use efficiency but has also caused drastic ecological changes in this region. This study investigated the effects of farmland conversion to orchard or agroforestry on soil total organic carbon (TOC) and several soil labile fractions: readily oxidizable carbon (ROC), light fraction organic carbon (LFOC), and dissolved organic carbon (DOC). Soil samples were collected from seven cropping treatments: a monocultured wheat field (Mono), a 5-year-old jujube orchard (5 J), a 5-year-old jujube/wheat alley cropping system (5 JW), a 10-year-old jujube orchard (10 J), a 10-year-old jujube/wheat alley cropping system (10 JW), a 15-year-old jujube orchard (15 J), and a 15-year-old jujube/wheat alley cropping system (15 JW). The results show that the ROC concentrations varied from 0.17 ± 0.09 g/kg to 2.35 ± 0.05 g/kg across all land-use types and soil depths studied. It was higher in the 0–10 cm and 10–20 cm layers of treatment 10 JW than in other treatments and significantly greater than in the Mono treatment. The highest value of DOC was reached at 593.04 mg/kg in the 15 JW treatment at 0–10 cm. Labile organic carbon decreased with increasing depth in all treatments. The proportion of ROC and LFOC to TOC decreased with increasing soil depth. In all treatments, the ratio of DOC to TOC generally decreased initially and then increased again with increasing depth. Correlation analysis showed that ROC, LFOC, and DOC were closely correlated with TOC (p &lt; 0.01). The ROC, LFOC, and DOC concentrations were significantly correlated with each other (p &lt; 0.01). Following conversion of farmland to jujube orchard or agroforestry, the content and activity of soil organic carbon tended to increase due to augmentation of plant residues. Thus, jujube orchards and agroforestry systems are effective methods to restore soil organic carbon.

Building Climate Change Adaptation and Resilience through Soil Organic Carbon Restoration in Sub-Saharan Rural Communities: Challenges and Opportunities

Soil organic carbon (SOC) is widely recognised as pivotal in soil function, exerting important controls on soil structure, moisture retention, nutrient cycling and biodiversity, which in turn underpins a range of provisioning, supporting and regulatory ecosystem services. SOC stocks in sub-Saharan Africa (SSA) are threatened by changes in land practice and climatic factors, which destabilises the soil system and resilience to continued climate change. Here, we provide a review of the role of SOC in overall soil health and the challenges and opportunities associated with maintaining and building SOC stocks in SSA. As an exemplar national case, we focus on Tanzania where we provide context under research for the “Jali Ardhi” (Care for the Land) Project. The review details (i) the role of SOC in soil systems; (ii) sustainable land management (SLM) techniques for maintaining and building SOC; (iii) barriers (environmental, economic and social) to SLM implementation; and (iv) opportunities for overcoming barriers to SLM adoption. We provide evidence for the importance of site-specific characterisation of the biophysicochemical and socio-economic context for effective climate adaptation. In particular, we highlight the importance of SOC pools for soil function and the need for practitioners to consider the type of biomass returns to the soil to achieve healthy, balanced systems. In line with the need for local-scale site characterisation we discuss the use of established survey protocols alongside opportunities to complement these with recent technologies, such as rapid in situ scanning tools and aerial surveys. We discuss how these tools can be used to improve soil health assessments and develop critical understanding of landscape connectivity and the management of shared resources under co-design strategies.

Variation of soil carbon and nitrogen storage in a natural restoration chronosequence of reclaimed temperate marshes

Wetland restoration significantly affects ecosystem carbon and nitrogen cycles by changing soil physicochemical properties and plant properties. To reveal the effect of wetland restoration on soil organic carbon (SOC) and total nitrogen (TN) storage, we measured the SOC and TN content and calculated their storage in a restoration chronosequence (1, 4, 8, 13, 17, 27 years) and compared them with those in a soybean field and a natural marsh, all in Sanjiang Plain, northeast China. Our results showed that the SOC and TN content and storage in the investigated sites increased with restoration time. However, the restoration rate was faster in the first 8 years but slowed down after 8 years of restoration. The SOC and TN content and storage decreased with soil depths in the eight sites. After 27 years of restoration, the carbon and nitrogen storage in the restored site was 2.8 and 1.9 times that in the soybean field, respectively. However, the SOC and TN storage in the 27-year restored site was 45.3% and 35.1% lower than that in the natural marsh, respectively. The SOC and TN content, bulk density, moisture content, clay, and plant belowground biomass significantly impact the SOC and TN storage (P < 0.01). This study indicates that marsh restoration contributes to the accumulation of SOC and TN, but the restoration of SOC and TN storage to the level of natural marshes may take decades.

Soil Organic Carbon Sequestration and Active Carbon Component Changes Following Different Vegetation Restoration Ages on Severely Eroded Red Soils in Subtropical China

Degraded soil has a high carbon sink potential. However, the carbon sequestration capacity and efficiency of comprehensive control measures in soil erosion areas are still not fully understood, and this information is essential for evaluating the effects of adopted restoration measures. The objective of this study was to determine the restoration of soil organic carbon and active carbon components under the impact of soil erosion measures and reforestation following different restoration ages. A small watershed with four typical restored plots following the same control measures (combination measures with horizontal bamboo burl-groove + replanting trees, shrubs and grasses) but different restoration ages (4 years, 14 years, 24 years and 34 years) and two reference plots (bare land (carbon-depleted) and nearby undisturbed forest (carbon-enriched)) in subtropical China was studied. The results showed that the soil organic carbon contents at a 1 m soil depth and the dissolved organic carbon and microbial biomass carbon concentrations in the upper 60 cm of soils of the four restored lands were higher than those in the bare land. Furthermore, the restored lands of 4 years, 14 years, 24 years and 34 years had soil organic carbon stocks in the 1 m soil depth of 22.83 t hm−2, 21.87 t hm−2, 32.77 t hm−2 and 39.65 t hm−2, respectively, which were higher than the bare land value of 19.86 t hm−2 but lower than the undisturbed forestland value of 75.90 t hm−2. The restored forestlands of 34 years of ecological restoration also had a high potential of being a soil organic carbon sink. Compared to the bare land, the restored lands of 4 years, 14 years, 24 years and 34 years had soil organic carbon sequestration capacities of 2.97 t hm−2, 2.01 t hm−2, 12.91 t hm−2 and 19.79 t hm−2, respectively, and had soil organic carbon sequestration rates of 0.74 t hm−2 a−1, 0.14 t hm−2 a−1, 0.54 t hm−2 a−1 and 0.58 t hm−2 a−1, respectively. Our results indicated that the combined measures of horizontal bamboo burl-groove and revegetation could greatly increase carbon sequestration and accumulation. Suitable microtopography modification and continuous organic carbon sources from vegetation are two main factors influencing soil organic carbon recovery. Combination measures, which can provide suitable topography and a continuous soil organic carbon supply, could be considered in treating degraded soils caused by water erosion in red soil areas.

Can no‐till restore soil organic carbon to levels under natural vegetation in a subtropical and tropical Typic Quartzipisamment?

AbstractSandy soils are widely distributed in Brazil and occur in most states. However, they have serious limitations for agricultural use due to low natural fertility, low water retention capacity, low resilience, and high susceptibility to erosion. Management of these soils through a no‐till (NT) system following its basic principles (i.e., elimination of plowing, diversification of crop rotation, retention of crop residues, and maintenance of permanent soil cover) is a practice that can restore the soil organic carbon (SOC) stock, and restore productivity and economic viability of these soils. We hypothesized that: (a) we can recover the carbon (C) stock in Typic Quartzipisamment in subtropical and tropical climates to the level of native vegetation; (b) the adoption of NT during the initial phase, in conjunction with intensive cropping systems with high plant biomass input, is an efficient strategy to recover SOC in Typic Quartzipisamment soils; and (c) long‐term NT (over 20 years—the maintenance phase), and low/medium input of plant biomass, is an efficient strategy for SOC recovery in Typic Quartzipisamment soils. Thus, the present study aimed at assessment of the recovery of SOC stocks in soil profile (0–1.0 m depth) in a Typic Quartzipisamment in subtropical and tropical climates, in comparison with the neighboring soil under native vegetation (NV). The sites selected for this study are located in two Brazilian grain‐producing regions: (a) Manoel Viana City—Rio Grande do Sul State (MV—site 1) that represents the subtropical climate condition, and (b) Luiz Eduardo Magalhães City—Bahia State (LEM—site 2), which represents the tropical climate. Soil samples were collected from soil profile for 0–1.0 m depth. At site 1, the predominant crop rotation over the years was wheat/soybean/oats + forage turnip/soybean. At site 2, the crop rotation includes cultivation in two periods designated as the crop and off‐seasons, and the succession consisted of soybean/Brachiaria/cotton/millet/soybean/maize‐Brachiaria. In both sites, soils under the neighboring NV were also sampled. SOC content was determined by the dry combustion method using an elemental C and N analyzer. Soil C stock was calculated based on the equivalent soil mass. Approximately 31 and 23% of the SOC stock was stored in the surface at 0–0.20 m, compared with 69 and 77% in 0.20–1.0 m layer of a Typic Quartzipisamment with high and low/medium input of crop residues, respectively. The Typic Quartzipisamment with low/medium input of crop residues fully recovered the stock of C in the surface soil layer (0–0.05 m). The Typic Quartzipisamment with high input of crop residues fully recovered the stock of C in the cultivated (0–0.20 m) and also in deeper soil layers (0–0.40 m). Thus, adoption of NT associated with intensification of cultivation systems and high addition of biomass‐C is an effective strategy for C restoration in a Typic Quartzipisamment, while also playing a crucial role in restoring ecosystem productivity, soil quality, and environment.

Trade-off between ‘new’ SOC stabilisation from above-ground inputs and priming of native C as determined by soil type and residue placement

Due to the geographical expanse of grasslands with depleted organic matter stocks, there has been growing interest in the management of these ecosystems for C sequestration to help mitigate climate change. It is generally accepted that management practices intending to increase forage production (e.g. decreasing grazing density) result in increased soil C stocks by increasing the return of biomass inputs to the soil organic carbon (SOC) pool. However, the contribution of C inputs to stable SOC versus GHG losses, and how this is affected by soil properties, remains largely unknown, particularly within subtropical biomes. To investigate the role of soil texture and mineralogy on SOC stabilisation, we identified three different soil types with varying physical properties in close proximity (< 2 km2) to each other. We used isotopically labelled plant material (13C), placed on the soil surface versus incorporated within the mineral soil, to trace the fate of fresh residue inputs into SOM fractions that differed in their degree of protection and mechanistic interactions with the soil matrix. Weekly GHG measurements (CO2, N2O and CH4) were taken to understand the overall GHG balance resulting from C inputs (i.e. SOC accrual versus GHG losses in CO2 equivalents). In finer textured soils with a greater smectite content, SOC accrual was greater but was significantly outweighed by GHG losses, primarily from native SOC priming. The incorporation of residue within the soil increased residue-derived SOC accrual by 4- to 5-fold, whilst also suppressing the priming of native SOC. This improved understanding of how soil texture and residue placement affect the global warming mitigation potential of subtropical grassland soils will be important in determining identifiable regions that should be targeted for SOC restoration efforts by increasing C inputs.

A review of soil carbon dynamics resulting from agricultural practices

Literature related to the carbon cycle and climate contains contradictory results with regard to whether agricultural practices increase or mitigate emission of greenhouse gases (GHGs). One opinion is that anthropogenic activities have distinct carbon footprints – measured as total emissions of GHGs resulting from an activity, in this case, “agricultural operations”. In contrast, it is argued that agriculture potentially serves to mitigate GHGs emissions when the best management practices are implemented. We review the literature on agricultural carbon footprints in the context of agricultural practices including soil, water and nutrient management. It has been reported that the management practices that enhance soil organic carbon (SOC) in arid and semi-arid areas include conversion of conventional tillage practices to conservation tillage approaches. We found that agricultural management in arid and semi-arid regions, which have specific characteristics related to high temperatures and low rainfall conditions, requires different practices for maintenance and restoration of SOC and for control of soil erosion compared to those used in Mediterranean, tropical regions. We recommend that in order to meet the global climate targets, quantification of net global warming potential of agricultural practices requires precise estimates of local, regional and global carbon budgets. We have conducted and present a case study for observing the development of deep soil carbon profile resulting from a 10-year wheat-cotton and wheat-maize rotation on semi-arid lands. Results showed that no tillage with mulch application had 14% (37.2 vs 43.3 Mg ha−1) higher SOC stocks in comparison to conventional tillage with mulch application. By implementing no tillage in conjunction with mulch application, lower carbon losses from soil can mitigate the risks associated with global warming. Therefore, it is necessary to reconsider agricultural practices and soil erosion after a land-use change when calculating global carbon footprints.

Forest understories controlled the soil organic carbon stock during the fallow period in African tropical forest: a 13C analysis

Soil organic carbon (SOC) dynamics after slash-burn agriculture are poorly understood in African tropical forest, though recent studies have revealed C4 grass invasion as a forest understory influences SOC dynamics after deforestation. This study aimed to quantify the relative SOC contribution of C4 and C3 plants separately through the sequential fallow periods of forest (cropland, or 4–7, 20–30, or >50 years of fallow forest) in the tropical forest of eastern Cameroon. We evaluated the SOC stock and natural 13C abundance for each layer. The SOC stock was largest in 4–7 years fallow forest (136.6 ± 8.8 Mg C ha−1; 100 cm depth, and C4:C3 = 58:42), and decreased with increasing fallow period. SOC from C4 plants was larger in the 4–7 and 20–30 years fallow forests (57.2–60.4 ± 5.8 Mg C ha−1; 100 cm depth), while it clearly decreased in >50 years fallow forest (35.0 ± 4.1 Mg C ha−1; 100 cm depth), resulting in the smallest SOC in this mature forest (106.4 ± 12.9 Mg C ha−1; 100 cm depth). These findings indicate that C4 grass understories contributed to the SOC restoration during early fallow succession in the tropical forest of eastern Cameroon.

Soil organic carbon recovery in tropical tree plantations may depend on restoration of soil microbial composition and function

Soil organic carbon (SOC) supports essential functions in terrestrial biomes and global biogeochemical cycles, and tropical tree plantations are often called upon to reverse deforestation-induced SOC loss. Yet the comparative efficacy of different plantation types and associated drivers of SOC restoration remain unclear. Theory suggests that higher chemical and spatial heterogeneity of plant litter should promote greater efficiency of soil microbial communities involved in SOC formation, so we hypothesised that more species-diverse tree plantations should be more effective in accelerating recovery of SOC. To test this, we compared developmental recovery of SOC and soil microbial communities between monoculture (Swietenia macrophylla King, mahogany) and highly diverse and mostly native species plantations (termed “rainforestation”). All plantation types, which were aged 15 to 20 years, only restored the composition of the soil microbial community to 20–30% of the reference, selectively logged old-growth rainforest. Contrary to our hypothesis, mahogany plantations, but not rainforestation, restored SOC and microbial function. Rainforestation shifted soil microbial composition and the composition of the understory vegetation closer to reference conditions. Soil microbial composition at all plantation sites was correlated with plant composition and functional traits, and better explained variation in SOC than land use. In particular, soil fungal PLFA biomass displayed a strong positive correlation with topsoil SOC concentration. This suggests that belowground restoration with tropical reforestation is slow relative to typical rotation times of tropical plantations (15–20 years). We conclude that reliable and rapid restoration of SOC may depend on interventions both above and below ground to re-instate the soil microbial community. This may require careful selection of plant species in combination with microbial inoculations.

Degraded lands and their management for rehabilitation and improving crop productivity

Soil degradation causes decline in its productivity, deterioration in vegetative cover, qualitative, and quantitative decline of soil and water resources and pollution of air is widespread in India. In recent decades, the problem is greatly aggravated because of country’s increasing population, requiring marginal areas to be brought under the cultivation to meet the growing food demand. Poverty and natural resources degradation forced individuals to search for more land for food, fodder and fibre production. The main causes of degradation due to direct/indirect human intervention are deforestation and removal of natural vegetation, overgrazing, and converting forest lands to arable land, cultivating steep slopes and degrading marginal lands, other agriculture related activities and over exploitation of the vegetation for domestic purposes. Similarly, the removal or in-situ burning of crop residues, minimum addition of organic manures, and intensive cultivation are the major causes of the depletion of soil organic carbon (SOC). Potential threats to the conservation of soil resources are soil erosion both by water and air, salinization/ alkalinity, acidity, organic carbon (OC) losses, nutrient imbalance, pollution/contamination by toxic substances, and soil sealing and capping. To arrest the process of degradation, urgent measures viz., indigenous soil and land conservation practices, conservation farming, watershed management including land and water management technology, water harvesting, Integrated plant nutrient supply, (IPNS), physical, mechanical, and biological approach, soil amendments, restoration of SOC, aforestation and agro-forestry, farming system approach, and organic farming, etc. are required for rehabilitating productivity of degraded soils so that more food could be produced to provide livelihood and environmental security to the ever increasing Indian population.

Macroaggregation and soil organic carbon restoration in a highly weathered Brazilian Oxisol after two decades under no-till

Conclusions based on studies of the impacts of soil organic carbon (SOC) fractions and soil texture on macroaggregation and SOC stabilization in long-term (>20years) no-till (NT) fields remain debatable. This study was based on the hypothesis that the amount and frequency of biomass-C input associated with NT can be a pathway to formation of macroaggregates and to SOC buildup. The objectives were to: 1) assess the macroaggregate distribution (proportional mass, class mass) and the SOC and particulate organic carbon (POC) stocks of extra-large (8–19mm), large (2–8mm) and small (0.25–2mm) macroaggregate size classes managed for two decades by NT, and 2) assess the recovery of SOC stocks in extra-large macroaggregates compared to adjacent native vegetation (Andropogon sp., Aristida sp., Paspalum sp., and Panicum sp.). The crop rotation systems were: soybean (Glycine max L.), maize (Zea mays L.) and beans (Phaseolus vulgaris L.) in summer; and black oat (Avena strigosa Schreb), white oat (Avena sativa), vetch (Vicia sativa L.), black oat.+vetch (Avena strigosa Schreb+vetch) and wheat (Triticum aestivum L.) in winter. The experimental was laid out as 2×2 randomized block factorial with 12 replicates of a NT experiment established in 1997 on two highly weathered Oxisols. The factors comprised of: (a) two soil textural types: clay loam and sandy clay, and (b) two sampling depths: 0–5 and 5–20cm. The three classes of macroaggregates were obtained by wet sieving, and the SOC content was determined by the dry combustion method. The extra-large macroaggregate classes in 0–20cm depth for sandy clay (SdC) and clay loam (CL) Oxisol represented 75.2 and 72.4% of proportional mass, respectively. The SOC and POC stocks among macroaggregate classes in 0–5 and 5–20cm depths decreased in the order: 8–19mm>2–8mm ≈ 0.25–2mm. The SdC plots under soybean/maize at 3:1 ratio recovered 58.3%, while those at 1:1 ratio (high maize frequency) in CL recovered 73.1% of SOC stock in the extra-large macroaggregates compared with the same under native vegetation for 0–20cm depth. Thus, partial restoration of the SOC stock in original extra-large macroaggregate confirms the hypothesis that NT through higher maize cultivation frequency can be a pathway to fomation of macroaggregates and SOC buildup.

Soil carbon debt of 12,000 years of human land use

Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes "grazing" and "cropland" contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts.