Differential effects of legume species on the recovery of soil microbial communities, and carbon and nitrogen contents, in abandoned fields of the Loess Plateau, China

Are there universal soil responses to cover cropping? A systematic review

Cover cropping is commonly acknowledged to promote soil health in agriculture. However, contradictory findings on the benefits of cover crops for soil health, crop productivity, economic and ecological factors, as well as the influence of inherent soil parameters on such benefits exist in the scientific literature. Here, we critically assessed evidence of cover crop benefits through a systematic review of the published literature. To access relevant papers, we searched the literature for cover crops and soil health indicators using Scopus (1996-2020), ScienceDirect (1996-2020) and Google scholar (1970-1996) with specific keywords and combinations. Only English research papers including experimental plots and control groups were considered. We analyzed 102 unique peer-reviewed papers and 1494 corresponding unique plots encompassing various cover crops, soil textures, climates, management systems and experimental duration (1-3 years, 4-6 years, 7-10 years and over 10 years). Strong evidence suggests that cover crops can enhance soil structure and promote soil health by improving soil physical and chemical properties, including saturated hydraulic conductivity (mean net change of 105.6 %), total organic carbon (10.1 %), and total nitrogen (20.2 %). On the other hand, cover crops exhibit weak effects on properties like bulk density and microporosity with fairly low values of net change. In most cases, cover crops increase the soil carbon content, including microbial biomass carbon (19.5 %) and particulate organic carbon (49.5 %). In this systematic review, we found limited studies on the effect of cover crops on soil health as influenced by soil texture, regional climate, rainfall and duration of the cover crop practices. The paucity of long-term regional systematic research of soil physics, chemistry and biology makes it difficult to forecast future implications of cover crops on soil health indicators.

Influence of cover crops at the four spheres: A review of ecosystem services, potential barriers, and future directions for North America

Cover crops have been studied for over a century, but the recognition of a complex interaction of cover crop on the Earth's biosphere, lithosphere, hydrosphere, and atmosphere is relatively recent. Furthermore, previously published cover crop research has largely focused on evaluating cover crop impacts on subsequent crop yield. Understanding the cover crop-induced benefits on soil organic carbon (SOC) sequestration, nitrous oxide (N₂O) emissions, wind and water erosion, weed control, and soil microbial communities has gained considerable attention in the last few decades, which is crucial to make progress towards developing sustainable agricultural production systems. New research is continuously published to gain a comprehensive understanding of the multiple ecosystem services provided by cover crops. Here, in this review, we aimed to (a) summarize current knowledge related to cover crop impacts on agroecosystem functioning and explore the potential mechanisms responsible for those effects, and (b) identify the key factors limiting the adoption of cover crops into agroecosystems and the conspicuous knowledge gaps in cover crop research. Overall, the review results suggest that cover crops increased subsequent crop yield, increased SOC storage, increased weed suppression, mitigated N₂O emissions, reduced wind and water erosion, suppressed plant pathogens, and increased soil microbial activity and wildlife biodiversity. However, the magnitude of benefits observed with cover crops varied with cover crop type, location, and the duration of cover cropping. Notably, cover crop termination methods, designing crop rotations to fit cover crops, additional costs associated with cover crop integration, and uncertainty related to economic returns with cover crops are some of the major barriers limiting the adoption of cover crops into production systems, particularly in North America. In addition to long-term effects, future research on cover crop agronomy, breeding cover crop cultivars, and interactive effects of cover crops with other sustainable land management practices is needed.

Characterization of hydrocarbon degrading microorganisms from Glycine max and Zea mays phytoremediated crude oil contaminated soil

Microbe-plant partnership in phytoremediation involves a synergistic interaction that leads to degradation of contaminants. The identification and characterization of these microorganisms is fundamental in environmental management. This study is aimed at investigating the influence of Glycine max and Zea mays on microbial make-up and differentiation of soil bacterial and fungal isolates in crude oil contaminated soil. We employed conventional technique of microbial isolation and gene sequencing to evaluate the microbial composition in crude oil contaminated soil. The microorganisms were isolated from crude oil contaminated soil (0%, 4%, 8%) and were identified using 16S rRNA gene (for bacteria) and Internal Transcribed Spacer (ITS) gene (for fungi). We observed a change in the microbial cell density with respect to treatment conditions implying a shift in microbial dynamics to total hydrocarbon utilizing bacteria as the dominant microbes. The sequence data revealed five bacteria strain; Klebsiella aerogenes strain 77, Klebsiella aerogenes strain UISO178, Salmonella enterica strain ABUH7, Klebsiella aerogenes strain M242 and Enterobacter sp. NCCP-607 and three fungi strains; Galactomyces geotrichum strain CBS, Aspergillus niger strain YMCHA73 and Trichoderma virens isolate A701. Annotation analysis using FGENESB and gene scan revealed proteins involved in various metabolic processes and hydrocarbon utilization. GHOSTKOLA output revealed several genetic elements and pathways such as DnaA, PYG, mrcA, environmental, cellular and genetic information processing and degradation enhancers. Our findings show that G. max and Z. mays in association with bacteria can enhance ecosystem restoration of crude oil contaminated soil.

Ammonia Oxidizing Prokaryotes Respond Differently to Fertilization and Termination Methods in Common Oat's Rhizosphere

Cover crops (CC) have demonstrated beneficial effects on several soil properties yet questions remain regarding their effects on soil microbial communities. Among them, ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) have a key role for N cycling in soil and their responses in the rhizosphere of terminated CC deserve further investigation. A greenhouse experiment was established to assess N fertilization (with or without N) and termination methods (glyphosate, mowing, and untreated control) of common oat (Avena sativa L.) as potential drivers of AOA and AOB responses in the rhizosphere. The abundance of amoA genes was determined by quantitative real-time PCR (qPCR), the community structure was assessed with Illumina amplicon sequencing of these genes, while the function was assessed from potential nitrification activity (PNA). While N fertilization had no influence on AOA, the termination method significantly increased amoA gene copies of AOA in mowed plants relative to glyphosate termination or the untreated control (1.76 and 1.49-fold change, respectively), and shifted AOA community structure (PERMANOVA, p<0.05). Ordination methods indicated a separation between AOA communities from control and glyphosate-terminated plants relative to mowed plants for both UniFrac and Aitchison distance. Converserly, N fertilization significantly increased AOB abundance in the rhizosphere of mowed and control plants, yet not in glyphosate-treated plants. Analyses of community structure showed that AOB changed only in response to N fertilization and not to the termination method. In line with these results, significantly higher PNA values were measured in all fertilized samples, regardless of the termination methods. Overall, the results of this study indicated that bacterial and archaeal nitrifiers have contrasting responses to fertlization and plant termination methods. While AOA were responsive to the termination method, AOB were more sensitive to N additions, although, the stimulative effect of N fertilization on amoA_AOB abundance was dependent on the termination method.

A prolonged dry season and nitrogen deposition interactively affect CO2 fluxes in an annual Mediterranean grassland

Mediterranean annual grasslands are species-diverse ecosystems of high economic and ecological value. CO₂ and water fluxes in these grasslands are triggered by the first rains in autumn, after a long hot and dry summer. Climate change scenarios project altered rainfall patterns, such as prolonged dry season into the autumn, while simultaneously nitrogen (N) deposition is increasing globally. However, how these global change drivers will interact to affect Mediterranean grassland CO₂, water fluxes and productivity is still unclear. In a greenhouse experiment, we subjected the seedbank of an annual Mediterranean grassland to a factorial treatment, by prolonging the dry season by 0 days (i.e. no autumn drought), 50 days and 100 days and crossing these drought treatments with two levels of N deposition: no N and N addition. A delayed onset of the rain season, i.e., a prolonged dry season, induced lower CO₂ and water fluxes throughout the growing season and a lower aboveground biomass by the end of the study period. However, N addition attenuated the effects on NEE, Reco and GPP, but did not affect aboveground biomass or functional group composition. A prolonged dry season also lowered the productivity of forbs, the dominant functional group in our grassland. Our results anticipate important effects of interacting global change drivers on Mediterranean grassland functioning.

Effects of nitrogen and phosphorus fertilization on soil carbon fractions in alpine meadows on the Qinghai-Tibetan Plateau

In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization at 5, 10, 15 g m-2 yr-1 (N5, N10, N15, P5, P10, P15, N5P5, N10P10, and N15P15) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0-20 cm and 20-40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m-2 yr-1 decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N5, N10, P5, and P10 in 0-20 cm, and increased in N10 and P5 while decreased in other treatments in 20-40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO2) in 0-20 cm but increased qCO2 in 20-40 cm. Fertilization increased soil microbial functional diversity (except N15) but decreased cumulative C mineralization (except in N15 in 0-20 cm and N5 in 20-40 cm). Soil organic C (SOC) decreased in P5 and P15 in 0-20 cm and for most of the fertilization treatments (except N15P15) in 20-40 cm. Overall, these results suggested that soils will not be a C sink (except N15P15). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m-2 yr-1 may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.