•A meta-analysis was conducted to elucidate the impact of grassland degradation on SOC stocks.•SOC in the topsoil of non-degraded and degraded soils were compared.•Degradation reduced SOC by 16% in ...dry climates compared to 8% in wet climates.•SOC depletion was more pronounced in sandy soils with a SOC depletion of 10% relative to 1% in clayey soils.•Results underscore the need to establish effective rehabilitation strategies.
Grasslands occupy about 40% of the world’s land surface and store approximately 10% of the global soil organic carbon (SOC) stock. This SOC pool, in which a larger proportion is held in the topsoil (0–0.3m), is strongly influenced by grassland management. Despite this, it is not yet fully understood how grassland SOC stocks respond to degradation, particularly for the different environmental conditions found globally. The objective of this review was to elucidate the impact of grassland degradation on changes in SOC stocks and the main environmental controls, worldwide, as a prerequisite for rehabilitation. A comprehensive meta-analysis was conducted using 55 studies with 628 soil profiles under temperate, humid, sub-humid, tropical and semi-arid conditions, to compare SOC stocks in the topsoil of non-degraded and degraded grassland soils. Grassland degradation significantly reduced SOC stocks by 16% in dry climates (<600mm) compared to 8% in wet climates (>1000mm) and Asia was the most affected continent (−23.7%). Moreover, the depletion of SOC stock induced by degradation was more pronounced in sandy (<20% clay) soils with a high SOC depletion of 10% compared to 1% in clayey (≥32% clay) soils. Furthermore, grassland degradation significantly reduced SOC by 14% in acidic soils (pH≤5), while SOC changes were negligible for higher pH. Assuming that 30% of grasslands worldwide are degraded, the amount of SOC likely to be lost would be 4.05Gt C, with a 95% confidence between 1.8 and 6.3Gt C (i.e. from 1.2 to 4.2% of the whole grassland soil stock). These results by pointing to greater SOC losses from grasslands under dry climates and sandy acidic soils allow identification of grassland soils for which SOC stocks are the most vulnerable, while also informing on rehabilitation measures.
Context
Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in ...savannas influences tree-grass-soil interactions.
Objectives
This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil.
Methods
Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m × 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa.
Results
We found that an increase in tree density along the encroachment gradient decreased grass richness (GR) by 53% in the intensively woody-encroached grassland. Higher tree density and canopy cover led to a 90% dominance of
Panicum maximum
grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer.
Conclusions
This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas.
According to the World Resources Institute (2000), a relative increase of carbon (C) stocks in world soils by 0.4% per year would be sufficient to compensate all anthropogenic greenhouse gas ...emissions. Several land management practices such as the suppression of tillage in agroecosystems and livestock exclusion in grasslands had initially been thought to store more carbon into the soil, but recent research puts this into question. In a context where finding effective C sequestration methods is urgent, the main objective of this study was to assess the ability of an innovative grassland management practice based on high density and short duration (HDSD) grazing to sequester atmospheric C into soils. The study was performed in a degraded communal rangeland in South Africa where soil organic C (SOC) depletion ranged from 5 to 95% depending on the degradation level, which varied from non-degraded (ND; with grass above ground coverage, Cov of 100%), degraded (D1; 50<Cov<75%), D2 (25<Cov<50%) and HD (highly degraded: Cov<5%). The ability of HDSD (1200cows ha−1 for 3days a year) to replenish SOC stocks was compared to four commonly used strategies: (1) livestock exclosure (E); (2) livestock exclosure with topsoil tillage (ET); (3) livestock exclosure with NPK fertilization (2:3:3, 22 at 0.2tha−1) (EF); (4) annual burning (AB); all treatments being compared to traditional free grazing control. A total of 540 soil samples were collected in the 0–0.05m soil layer for all treatments and degradation intensities. After two years, topsoil SOC stocks were significantly increased under EF and HDSD, by an average of 33.4±0.5 and 12.4±2.1g C m2 y−1, respectively. In contrast, AB reduced SOC stocks by 3.6±3.0gCm2y−1, while the impact of E and ET was not significant at P<0.05. HDSD replenished SOC stocks the most at D1 and D2 (6.7 and 7.4%y−1) and this was explained by grass recovery, i.e. a significant increase in soil surface coverage by grass and grass production. HDSD is cost-effective, and thus has great potential to be widely adopted by smallholder farmers.
•We assessed the ability of high density and short duration (HDSD) grazing to sequester atmospheric C into soils;•HDSD was compared to livestock exclosure; top-soil tillage and NPK fertilization;•After two years, SOC stocks were significantly increased under fertilization and HDSD;•HDSD replenished SOC stocks by as much as 7.4%y−1.
Editorial: Soil-microbial interactions Dlamini, Phesheya; Sekhohola-Dlamini, Lerato M; Cowan, A Keith
Frontiers in microbiology,
05/2023, Letnik:
14
Journal Article
Mountainous rangelands provide key ecosystem goods and services, particularly for human benefit. In spite of these benefits, mountain grasslands are undergoing extensive land-cover change as a result ...of woody plant encroachment. However, the influence of topographic and soil factors on woody plant encroachment is complex and has not yet been studied comprehensively. The aim of this review was to establish current knowledge on the influence of topographic and soil factors on woody plant encroachment in mountainous rangelands. To find relevant literature for our study on the impact of topographic and soil factors on woody plant encroachment in mountain rangelands, we conducted a thorough search on ScienceDirect and Google Scholar using various search terms. Initially, we found 27,745 papers. We narrowed down the search to include only 66 papers published in English that directly addressed the research area. The effect of slope aspect and slope position on woody plant encroachment is complex and dynamic, with no universal consensus on their impact. Some studies found higher woody plant encroachment on the cooler slopes, while others found increased woody plant encroachment on the warmer slopes. Slope gradient has a significant impact on woody plant encroachment, with steeper slopes tending to have more woody plant encroachment than gentle slopes. Soil texture and depth are important soil factors affecting woody plant encroachment. Coarse-textured soils promote the growth of woody plants, while fine-textured soils limit it. The effect of soil depth on woody plant encroachment remain unclear and requires further research. Soil moisture availability, soil nutrient content and soil microbial community are influenced by topography, which in turn affect the woody plant growth and distribution. In conclusion, the spread of woody plants in mountainous rangelands is a complex and dynamic process influenced by a range of factors. Further research is needed to fully understand the mechanisms behind these interactions and to develop effective strategies for managing woody plant encroachment in mountainous rangelands.
ABSTRACTSoil carbon dioxide (CO2) fluxes are a critical component in understanding carbon sequestration. In sub-Saharan Africa, empirically measured CO2 emissions data from diverse land-use systems ...is limited. Soil CO2 emission rates were measured in the Limpopo Province, South Africa for 12 months at two-week intervals in natural systems (forest and shrubland) and commercially managed orchards (avocado and citrus) to establish seasonal dynamics of soil CO2 emissions across these land-use systems. The results showed a variation in emission rates with the variation depending on the season. In the spring and winter, soil CO2 emission rates in citrus were four times higher than in the shrubland due to higher moisture levels. However, in the summer season, the forest emission rates were 40% higher than in citrus due to higher soil organic carbon content. Organic carbon stocks were higher in the forest (1.19 kg/m2) compared to the other land uses. This study revealed differences in soil CO2 emission rates among land-use systems, with the cumulative amount of CO2 emitted over a 12-month period following the order: forest (39.3 tons/ha) > citrus (36.1 tons/ha) > shrubland (28.1 tons/ha) > avocado (26.9 tons/ha). Thus, understanding the emission patterns from various ecosystems can inform strategies for mitigating greenhouse gas emissions.
In South Africa (SA), smallholder farmers contribute significantly to food production and play an essential role in the nation’s food and nutritional security. However, there is a lack of basic ...understanding of the spatial variability of soil nutrients and their controlling factors in these smallholdings, which subsequently hinders their agricultural production. In this work, we assessed the spatial variability and structure of key soil nutrients required by banana fruit, identified their factors of control, and delineated management zones in a smallholder farm. We used a regular grid (50 m × 50 m) to collect a total of 27 composite samples from the 0–30 cm depth interval and analysed for soil physicochemical properties. Our classical statistics results indicated that phosphorus (P), potassium (K), calcium (Ca) and zinc (Zn) varied highly, while magnesium (Mg) and total nitrogen (TN) varied moderately across the plantation. On the other hand, geostatistics revealed that P and K were strongly spatially dependent (implying a good structure), while Mg and Zn were moderately spatially dependent (indicating a moderate structure) across the banana plantation. Soil Ca and TN contents were found to be weakly spatially dependent (meaning there was no structure) across the farm. The spatial prediction maps showed that P, Mg and Zn contents were high in the northeast part (underlain by Valsrivier) and low in the northwest part (underlain by Westleigh) of the banana plantation farm. Similarly, K and Ca were low in the northwest part (underlain by Westleigh), but they were high in the south to southwest portion (underlain by Glenrosa) of the farm. Soil TN was high in the west part (underlain by Westleigh) and low in the east-northeast part (underlain by Valsrivier) across the plantation. Three management zones (MZs) were delineated for soil P, K and Ca, while for other nutrients (Mg, Zn and TN), two MZs were delineated. The results of this study provide baseline information for site-specific management of fertilisers to supplement soil nutrients in the field to improve banana productivity.
Malt barley is typically grown in dryland conditions in South Africa. It is an important grain after wheat, but little is known about its water requirements and, most importantly, how it responds to ...water stress. Determining when water stress sets in and how malt barley responds to water deficit during its growing season is crucial for improved management of crop water requirements. The objectives of this study were to evaluate the response of transpiration (T), stomatal conductance (SC), and leaf water potential (LWP) to water stress for different growth stages of malt barley and to characterise water stress to different levels (mild, moderate, and severe). This was achieved by monitoring the water stress indicators (soil- and plant based) under greenhouse conditions in well-watered and water-stressed lysimeters over two seasons. Water stress was characterised into different levels with the aid of soil water content 'breaking points' procedure. During the first season, at the end of tillering, flag leaf, and milk/dough growth stages, which represent severe water stress, plant available water (PAW) was below 35%, 56%, 14%, and 36%, respectively. LWP responded in accordance to depletion of soil water during the growing season, with the lowest recorded value to -5.5 MPa at the end of the milk/dough growth stage in the first season. Results also show that inducing water stress resulted in high variability of T and SC for both seasons. In the second season, plants severely stressed during the anthesis growth stage recorded the least total grains per pot (TGPP), with 29.86 g of grains. The study suggests that malt barley should be prevented from experiencing severe water stress during the anthesis and milk/dough stages for optimum malt barley production. Quantification of stress into different levels will enable the evaluation of the impact of different levels of stress on the development, growth, and yield of barley.
Mountainous grassland landscapes are severely threatened by the proliferation of shrub encroachment. So far, it remains unclear though how slope aspect coupled with land disturbances linked to the ...proximity of homesteads to communal grazing sites affects the distribution, density and structure of the encroaching shrubs in these grassland ecosystems. In this study, we investigated the role that slope aspect plays in determining the density and structure of an encroaching shrub species
Euryops floribundus
N.E. Br in three communal grazing sites located at varying distances from rural homesteads in a semi-arid mountainous grassland in the Eastern Cape Province of South Africa. Three sites were strategically chosen in Cala communal grazing lands in an encroached mountainous grassland landscape that depicted north and south-facing slope aspects. The selected sites were Tsengiwe; a site located in close proximity to homesteads at a distance of less than 100 m, Upper Mnxe situated at an intermediate distance ranging from 200 to 800 m away from homesteads and Manzimdaka, which was located furthest from homesteads at a distance greater than 1600 m. In each site and corresponding slope aspect, the density and structure of the shrub
E. floribundus
were evaluated on 36 randomly distributed plots, yielding a total of 108 plots across all sites. We found that shrub density was significantly higher (50%) on the north-facing slope compared to the south-facing slope in Tsengiwe, the site located near homesteads. Shrub height was significantly higher (53 and 17%) on the north-facing slope compared to the south-facing slope at Upper Mnxe and Manzimdaka, which were located at intermediate and furthest distances from the homesteads. Notably, shrub height was significantly lower (37%) in the north-facing slope compared to the south-facing slope in Tsengiwe. Following a similar pattern to shrub height, total stem number was significantly higher (20 and 85%) in the north-facing slope compared to the south-facing slope at Upper Mnxe and Tsengiwe, respectively. Shrub crown area was higher (33 and 11%) in the north-facing slope compared to the south-facing slope at Upper Mnxe and Manzimdaka, respectively. A strong positive relationship was established between the height of shrubs and their longest crown diameter, shortest crown diameter and shrub crown area across all sites, suggesting that the investigated shrub species
E. floribundus
employs its structural characteristics to survive, and thrive and this was more evident in the north-facing slope. The results highlight the importance of considering the proximity of homesteads into account in ecological studies, and puts emphasis on improved understanding of the vegetation patterns shaped by shrub encroachment in mountainous grasslands, which is crucial in the development of effective land management strategies.
Land degradation is recognized as a main environmental problem that adversely depletes soil organic carbon (SOC) and nitrogen (SON) stocks, which in turn directly affects soils, their fertility, ...productivity and overall quality. While it is expanding worldwide at rapid pace, quantitative information on the impact of land degradation on the depletion of SOC and SON stocks remains largely unavailable, limiting the ability to predict the impacts of land management on the C losses to the atmosphere and associated global warming. The main objective of this study was to evaluate the consequences of a decrease in grass aerial cover on SOC and SON stocks. A degraded grassland showing an aerial cover gradient from 100% (Cov100, corresponding to a non-degraded grassland) to 50–75% (Cov75), 25–50% (Cov50) and 0–5% (Cov5, corresponding to a heavily degraded grassland), was selected in South Africa. Soil samples were collected in the 0.05m soil layer at 48 locations along the aerial cover gradient and were subsequently separated into the clay+silt (2–20μm) and sand (20–2000μm) fractions, prior to total C and N analysis (n=288). The decline in grass aerial cover from 100% to 0–5% had a significant (P<0.05) impact on SOC and SON stocks, with losses by as much as 1.25kgm−2 for SOC and 0.074kgm−2 for SON, which corresponded to depletion rates of 89 and 76%, respectively. Furthermore, both the C:N ratio and the proportion of SOC and SON in the silt+clay fraction declined with grass aerial cover, which was indicative of a preferential loss of not easily decomposable organic matter. The staggering decline in SOC and SON stocks raises concerns about the ability of these acidic sandy loam soils to sustain their main ecosystem functions. The associated decrease in chemical elements (e.g., Ca by a maximum of 67%; Mn, 77%; Cu, 66%; and Zn, 82%) was finally used to discuss the mechanisms at stake in land degradation and the associated stock depletion of SOC and SON stocks, a prerequisite to land rehabilitation and stock replenishment.
•We evaluate the impact of degradation; decrease in grass cover on SOC stocks•Sigmoidal decrease of SOC stock due to linear decrease in grass cover•Degradation also decreased SOC in the silt+clay particle size fraction.•SOC decrease accompanied by decreasing soil bulk density and aggregate stability
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