The soil microbial community is a key indicator to evaluate the soil health and productivities in agricultural ecosystems. Monoculture and conversions of forests to tea plantations have been widely applied in tea plantation globally, but long-term monoculture of tea plantation could lead to soil degradation and yield decline. Understanding how long-term monoculture systems influence the soil health and ecosystem functions in tea plantation is of great importance for soil environment management. In this study, through the comparison of three independent tea plantations across eastern China composed of varying stand ages (from 3 to 90 years after conversion from forest), we found that long-term tea monoculture led to significant increases in soil total organic carbon (TOC) and microbial nitrogen (MBN). Additionally, the structure, function, and co-occurrence network of soil bacterial communities were investigated by pyrosequencing 16S rRNA genes. The pyrosequencing analysis revealed that the structures and functions of soil bacterial communities were significantly affected by different stand ages, but sampling sites and land-use conversion (from forest to tea plantation) had stronger effects than stand age on the diversity and structure of soil bacterial communities. Soil bacterial diversity can be improved with increasing stand ages in tea plantation. Further RDA analysis revealed that the C and N availability improvement in tea plantation soils led to the variation of structure and function in soil bacterial communities. Moreover, co-occurrence network analysis of soil bacterial communities also demonstrated that interactions among soil bacteria taxa were strengthened with increasing stand age. Our findings suggest that long-term monoculture with proper managements could be beneficial to soil ecosystems by increasing the C and N content and strengthening bacterial associations in tea plantations. Overall, this study provides a comprehensive understanding of the impact of land-use change and long-term monoculture stand age on soil environments in tea plantation.
Tag: Soil organic carbon
Recovery after volcanic ash deposition: vegetation effects on soil organic carbon, soil structure and infiltration rates
Volcanic eruptions of pyroclastic tephra, including the ash-sized fraction (< 2 mm; referred to as volcanic ash), have negative direct impacts on soil quality. The intensity (deposit thickness, particle-size distribution) and frequency (return period) of tephra deposition influence soil formation. Vulnerability and subsequent recovery (resilience) of the plant-soil system depend on land-uses (vegetation and management). Few previous studies covered the whole deposition-recovery cycle. We investigated the volcanic ash deposition effects on soil properties and their recovery across land-uses on a densely populated volcanic slope. We measured the canopy cover and volcanic ash thickness six years after the 2014 Mt. Kelud eruption in four land-use systems: remnant (degraded) forests, complex agroforestry, simple agroforestry, and annual crops. Each system was monitored in three landscape replicates (total 12 plots). For the soil recovery study, we measured litter thickness, soil texture, Corg, soil C stocks, aggregate stability, porosity, and soil infiltration in three different observation periods (pre-eruption, three, and six years after eruption). Post-eruption volcanic ash thickness varied between land-use systems and was influenced by the plots slope position rather than canopy cover. The average soil texture and porosity did not vary significantly between the periods. Surface volcanic ash and soil layers initially had low aggregate stability and limited soil infiltration, demonstrating hydrophobicity. While Corg slowly increased from low levels in the fresh volcanic ash, surface litter layer, aggregate stability, and soil infiltration quickly recovered. Different land-use management resulted in different recovery trajectories of soil physical properties and function over the medium to long term after volcanic ash deposition.
Grazing exclosures increase soil organic carbon stock at a rate greater than “4 per 1000” per year across agricultural landscapes in Northern Ethiopia
The establishment of grazing exclosures is widely practiced to restore degraded agricultural lands and forests. Here, we evaluated the potential of grazing exclosures to contribute to the”4 per 1000” initiative by analyzing the changes in soil organic carbon (SOC) stocks and sequestration (SCS) rates after their establishment on degraded communal grazing lands in Tigray region of Ethiopia. We selected grazing areas that were excluded from grazing for 5 to 24 years across the three agroecological zones of the region and used adjacent open grazing lands (OGLs) as control. Soil samples were collected from two depths (0-15 cm and 15-30 cm) and SOC and aboveground C stocks were quantified in both exclosures and OGLs. The mean SOC stock and SCS rate in exclosures (0-30 cm) were 31 Mg C ha-1 and 3 Mg C ha-1 year-1, which were respectively 166% and 12% higher than that in the OGLs, indicating a positive restoration effect of exclosures on SOC storage. With increasing exclosure age, SOC stock and SCS rate increased in the exclosures but decreased in the OGLs. Higher SOC stock and SCS rate were recorded in 0-15 cm than in 15-30 cm. The relative (i.e., to the SOC stock in OGLs) rates of increase in SOC stocks (70-189‰ year-1) were higher than the 4‰ year-1 and were initially high due to low initial SOC stock but declined over time after a maximum value of SOC stock is reached. Factors such as aboveground biomass, altitude, clay content and precipitation promoted SOC storage in exclosures. Our study highlights the high potential of exclosure for restoring SOC in the 0-30 cm soil depth at a rate greater than the 4‰ value. We argue that practices such as grazing exclosure can be promoted to achieve the climate change mitigation target of the “4‰” initiative.
Particulate macronutrient exports from tropical African montane catchments point to the impoverishment of agricultural soils
Agricultural catchments in the tropics often generate high concentrations of suspended sediments following the conversion of natural ecosystems. The eroded fine particles are generally enriched with carbon (TC) and nutrients (TN and TP) originating from the topsoil of agricultural land. Sediment-associated TC, TN and TP are an important loss to the terrestrial ecosystem and tightly connected to an increase in riverine particulate TC and nutrient export. Soil nutrient depletion can limit crop growth and yields, whereas an excess of nutrients in streams can cause eutrophication in freshwater systems. Streams in East Africa, with widespread land conversion from forests to agriculture, are expected to receive high loads of sediment-associated TC, TN and TP. In this study, we assess the effect of land use on particulate TC, TN and TP concentrations. Suspended sediments (time-integrated, manual-event-based and automatic-event-based sediment samples) were analysed for TC, TN and TP concentrations collected at the outlet of a natural montane forest (35.9), a tea-tree plantation (33.3) and a smallholder agriculture (27.2 km2) catchment in western Kenya during a wet sampling period in 2018 and a drier sampling period in 2019. Particulate TC, TN and TP concentrations were up to 3-fold higher (p<0.05) in the natural forest catchment compared to fertilized agricultural catchments. However, because of higher sediment loads from the smallholder agriculture catchment, the total sediment-associated loads of TC, TN and TP were higher compared to the natural forest and tea-tree plantation catchment. The higher C:N ratios in the natural forest suggest that the particulate nutrients are of organic origin due to tighter nutrient cycles, whereas lower ratios in both agricultural catchments suggest a mixture of mineral and organic sediment sources. The findings of this study imply that with the loss of natural forest, the inherent soil fertility is progressively lost under the current low fertilization rates and soil management strategies.
Carbon stocks in Miombo Woodlands: Evidence from over 50 years
Miombo woodlands are extensive dry forest ecosystems in central and southern Africa covering ≈2.7 million km2. Despite their vast expanse and global importance for carbon storage, the long-term carbon stocks and dynamics have been poorly researched. The objective of this paper is to present and summarize the evidence gathered on above- and belowground (root and soil) carbon stocks of miombo woodlands from the 1960s to mid-2018 through a review. We analyzed data to answer: (1) What is the range of aboveground and belowground carbon stocks found in miombo woodlands over the last six decades? (2) Are there differences in carbon stocks based on land-management categories? (3) Does precipitation influence aboveground carbon stocks in old-growth miombo? (4) Do differences in cover type, age and region influence carbon stocks? (5) How does previous land-use affect carbon stocks in re-growth miombo? A literature review protocol was used to identify 56 publications from which quantitative data on aboveground and soil carbon pools were extracted. We found that the mean aboveground carbon stock in old-growth miombo was 30.83±16.76 Mg C ha-1 (range 1.48—107.24 Mg ha-1). Old-growth miombo had an average calculated root carbon stock of 16.49±9.18 Mg C ha-1 (range 0.8—57.81 Mg ha-1). Soil carbon stocks in old-growth miombo varied widely, between 8.75 and 134.6 Mg C ha-1 while in re-growth miombo they varied between 10.73 and 52.2 Mg C ha-1. It must be noted these soil data are given only for information; they inconsistently refer to varying soil depths and are thus difficult to interpret. The wide range reported suggests a need for further studies, much more systematic in methods and reporting. Other limitations of the dataset include the lack of systematic sampling and lack of data in some countries, viz. Angola and Democratic Republic of the Congo.
Edaphic and climatic factors affecting phenology of naturally growing calotropis procera in semi-arid regions of Kenya
Background and Objective: Cultivating Calotropis procera for fiber supply to the textile industry can improve the livelihoods of communities in arid and semi-arid regions. This study determined edaphic and climatic factors affecting phenological traits of C. procera in the semi-arid regions of Kenya. Materials and Methods: Repeated measure research design was used with multistage sampling technique to monitor activity indices, number of flowers and fruits and phenophase intensities. Climatic and edaphic factors of study sites were also monitored. Data was analyzed using linear, Poisson log linear regression based on Generalized Estimation Equation (GEE) and Mixed Analysis of Variance (ANOVA). Results: High Soil Organic Carbon (OC) content (3%) and exchangeable Na (112.5 ppm) at (0-20) cm soil depth were recorded in Tharaka. High mean monthly rainfall (160.37 mm) was recorded in Makueni. Flowering activity indices in (June-August, 2018) were 64.97% and 69.6% in Tharaka and Makueni, respectively . Available P, average monthly rainfall and temperature had significant association with flowering and fruiting activity indices (p<0.05). The mean number of flowers and fruits per stem were significantly associated with soil available P, exchangeable Na and OC content (p<0.05). Though edaphic factors were not significantly associated with phenophase intensities of C. procera, average monthly rainfall and temperature were positively and negatively associated with phenophase intensities, respectively . Conclusion: Available P, exchangeable Na, available K and OC content noticeably affect phenological traits of naturally growing C. procera. Rains and temperatures are critical climatic factors affecting phenological traits of C. procera.
Exploring the potential of ground-penetrating radar (GPR) to measure the extent of chronic disturbance in peatlands
Peatlands are accumulations of partially decayed organic soil that cover approximately 3% of Earth’s surface and have been shown to serve essential environmental and ecological functions such as sequestering carbon, purifying water, and providing habitat for organisms. However, peatlands are threatened by pressures from agriculture, urban development, mining, and climate change. Geophysical methods have been used in peatlands to determine peat volume and carbon stocks (e.g., Comas et al., 2017), observe differences in humification and water content (e.g., Ulriksen, 1982), guide engineering projects (e.g., Jol and Smith, 1995), learn about subsurface greenhouse gas dynamics (Wright and Comas, 2016), observe seasonal variations in pore water salinity (Walter et al., 2018), and assess hydrological processes (Hare et al., 2017). Among various geophysical methods, ground penetrating radar (GPR) is arguably the most popular for studying peat properties given the method’s sensitivity to variations in water content and ability to resolve major structural properties within the peat at high spatial resolution. Though less widely applied, frequency-domain analysis of GPR may also yield useful information.
Assessing biogeochemical and human-induced drivers of soil organic carbon to inform restoration activities in Rwanda
Land restoration is of critical importance in Rwanda, where land degradation negatively impacts crop productivity, water, food and nutrition security. We implemented the Land Degradation Surveillance Framework in Kayonza and Nyagatare districts in eastern Rwanda to assess baseline status of key soil and land health indicators, including soil organic carbon (SOC) and soil erosion prevalence. We collected 300 topsoil (0–20 cm) and 281 subsoil (20–50 cm) samples from two 100 km2 sites. We coupled the soil health indicators with vegetation structure, tree density and tree diversity assessments. Mean topsoil organic carbon was low overall, 20.9 g kg−1 in Kayonza and 17.3 g kg−1 in Nyagatare. Stable carbon isotope values (d13CV-PDB ) ranged from −15.35 to −21.34 ‰ indicating a wide range of plant communities with both C3 and C4 photosynthetic pathways. Soil carbon content decreased with increasing sand content across both sites and at both sampling depths and was lowest in croplands compared to shrubland, woodland and grasslands. Field-saturated hydraulic conductivity (Kfs) was estimated based on infiltration measurements, with a median of 76 mm h−1 in Kayonza and 62 mm h−1 in Nyagatare, respectively. Topsoil OC had a positive effect on Kfs, whereas pH, sand and compaction had negative effects. Soil erosion was highest in plots classified as woodland and shrubland. Maps of soil erosion and SOC at 30-m resolution were produced with high accuracy and showed high variability across the region. These data and analysis demonstrate the importance of systematically monitoring multiple indicators at multiple spatial scales to assess drivers of degradation and their impact on soil organic carbon dynamics.
Carbon Storage Potential of Silvopastoral Systems of Colombia
Nine Latin American countries plan to use silvopastoral practices—incorporating trees into grazing lands—to mitigate climate change. However, the cumulative potential of scaling up silvopastoral systems at national levels is not well quantified. Here, we combined previously published tree cover data based on 250 m resolution MODIS satellite remote sensing imagery for 2000–2017 with ecofloristic zone carbon stock estimates to calculate historical and potential future tree biomass carbon storage in Colombian grasslands. Between 2000 and 2017, tree cover across all Colombian grasslands increased from 15% to 18%, with total biomass carbon (TBC) stocks increasing from 0.41 to 0.48 Pg. The range in 2017 carbon stock values in grasslands based on ecofloristic zones (5 to 122 Mg ha−1) suggests a potential for further increase. Increasing all carbon stocks to the current median and 75th percentile levels for the respective eco-floristic zone would increase TBC stocks by about 0.06 and 0.15 Pg, respectively. Incorporated into national C accounting, such Tier 2 estimates can set realistic targets for silvopastoral systems in nationally determined contributions (NDCs) and nationally appropriate mitigation actions (NAMAs) implementation plans in Colombia and other Latin American countries with similar contexts.
Controls on timescales of soil organic carbon persistence across sub-Saharan Africa
Given the importance of soil for the global carbon cycle, it is essential to understand not only how much carbon soil stores but also how long this carbon persists. Previous studies have shown that the amount and age of soil carbon are strongly affected by the interaction of climate, vegetation, and mineralogy. However, these findings are primarily based on studies from temperate regions and from fine-scale studies, leaving large knowledge gaps for soils from understudied regions such as sub-Saharan Africa. In addition, there is a lack of data to validate modeled soil C dynamics at broad scales. Here, we present insights into organic carbon cycling, based on a new broad-scale radiocarbon and mineral dataset for sub-Saharan Africa. We found that in moderately weathered soils in seasonal climate zones with poorly crystalline and reactive clay minerals, organic carbon persists longer on average (topsoil: 201 ± 130 years; subsoil: 645 ± 385 years) than in highly weathered soils in humid regions (topsoil: 140 ± 46 years; subsoil: 454 ± 247 years) with less reactive minerals. Soils in arid climate zones (topsoil: 396 ± 339 years; subsoil: 963 ± 669 years) store organic carbon for periods more similar to those in seasonal climate zones, likely reflecting climatic constraints on weathering, carbon inputs and microbial decomposition. These insights into the timescales of organic carbon persistence in soils of sub-Saharan Africa suggest that a process-oriented grouping of soils based on pedo-climatic conditions may be useful to improve predictions of soil responses to climate change at broader scales.