Higher Soil Organic Carbon Levels Research Articles

Changes in soil organic carbon and nitrogen stocks in organic farming practice and abandoned tea plantation

BackgroundThe restoration of conventional tea plantations and the adoption of organic farming practices could impact soil organic carbon (SOC) and nitrogen (N) stocks. This study investigated the soil properties, SOC and N contents and stocks, and their vertical distributions of a secondary forest restored from an abandoned conventional tea plantation and a converted organic tea plantation. An adjacent conventional tea plantation employing similar intermediate farming served as a comparison.ResultsWithin a 50-cm depth, the secondary forest exhibited a higher SOC stock of 115.53 ± 7.23 Mg C ha− 1 compared to 92.1 ± 8.54 Mg C ha− 1 for the conventional tea plantation. No significant differences in N stocks were seen between the two land uses. Significantly high SOC and N contents and stocks were found in the 0–10 cm layer of the secondary forest compared to the conventional tea plantation. No significant disparities in SOC and N stocks were found between the conventional and organic tea plantations within the 50 cm depth (92.1 ± 8.54 Mg C ha− 1 and 10.06 ± 1.01 Mg N ha− 1 vs. 97.47 ± 1.53 Mg C ha− 1 and 9.70 ± 0.10 Mg N ha− 1). However, higher levels of SOC and N contents and stocks were observed at a depth of 10 cm in the conventional tea plantation and below 10 cm in the organic tea plantation.ConclusionsThe C and N inputs derived from high litter production at the top soil strongly contributed to higher SOC and N contents and stocks in the secondary forest. The application of soybean amendments in the conventional tea plantation and the longer tea plantation age of the organic tea plantation influenced their distribution of SOC and N contents and stocks, respectively. Reverting a conventional tea plantation into a secondary forest contributed to C recovery and reaccumulation. The conventional tea plantation, employing similar intermediate farming practices, increased SOC and N contents and stocks in the surface soil compared to the organic tea plantation. However, adopting organic farming did not significantly increase SOC stocks compared to the conventional tea plantation.

Subsoil tillage enhances wheat productivity, soil organic carbon and available nutrient status in dryland fields

Tillage practices during the fallow period benefit water storage and yield in dryland wheat crops. However, there is currently no clarity on the responses of soil organic carbon (SOC), total nitrogen (TN), and available nutrients to tillage practices within the growing season. This study evaluated the effects of three tillage practices (NT, no tillage; SS, subsoil tillage; DT, deep tillage) over five years on soil physicochemical properties. Soil samples at harvest stage from the fifth year were analyzed to determine the soil aggregate and aggregate-associated C and N fractions. The results indicated that SS and DT improved grain yield, straw biomass and straw carbon return of wheat compared with NT. In contrast to DT and NT, SS favored SOC and TN concentrations and stocks by increasing the soil organic carbon sequestration rate (SOCSR) and soil nitrogen sequestration rate (TNSR) in the 0–40 cm layer. Higher SOC levels under SS and NT were associated with greater aggregate-associated C fractions, while TN was positively associated with soluble organic nitrogen (SON). Compared with DT, the NT and SS treatments improved soil available nutrients in the 0–20 cm layer. These findings suggest that SS is an excellent practice for increasing soil carbon, nitrogen and nutrient availability in dryland wheat fields in North China.

Carbon farming: Are soil carbon certificates a suitable tool for climate change mitigation?

Increasing soil organic carbon (SOC) stocks in agricultural soils removes carbon dioxide from the atmosphere and contributes towards achieving carbon neutrality. For farmers, higher SOC levels have multiple benefits, including increased soil fertility and resilience against drought-related yield losses. However, increasing SOC levels requires agricultural management changes that are associated with costs. Private soil carbon certificates could compensate for these costs. In these schemes, farmers register their fields with commercial certificate providers who certify SOC increases. Certificates are then sold as voluntary emission offsets on the carbon market.In this paper, we assess the suitability of these certificates as an instrument for climate change mitigation. From a soils' perspective, we address processes of SOC enrichment, their potentials and limits, and options for cost-effective measurement and monitoring. From a farmers’ perspective, we assess management options likely to increase SOC, and discuss their synergies and trade-offs with economic, environmental and social targets. From a governance perspective, we address requirements to guarantee additionality and permanence while preventing leakage effects. Furthermore, we address questions of legitimacy and accountability.While increasing SOC is a cornerstone for more sustainable cropping systems, private carbon certificates fall short of expectations for climate change mitigation as permanence of SOC sequestration cannot be guaranteed. Governance challenges include lack of long-term monitoring, problems to ensure additionality, problems to safeguard against leakage effects, and lack of long-term accountability if stored SOC is re-emitted. We conclude that soil-based private carbon certificates are unlikely to deliver the emission offset attributed to them and that their benefit for climate change mitigation is uncertain. Additional research is needed to develop standards for SOC change metrics and monitoring, and to better understand the impact of short term, non-permanent carbon removals on peaks in atmospheric greenhouse gas concentrations and on the probability of exceeding climatic tipping points.

Improving maize sustainability with partial replacement of N fertilizers by inoculation with Azospirillum brasilense

AbstractMaize (Zea mays L.) is a major cereal crop worldwide and demands N fertilizers, which can be replaced partially by plant growth‐promoting bacteria. In Brazil, inoculants carrying Azospirillum brasilense strains Ab‐V5 and Ab‐V6 have exponentially grown, but a quantitative study on the replacement of N fertilizer by inoculants is still missing. Under greenhouse conditions, we investigated the effects of A. brasilense cells and metabolites applied via seeds or leaf‐sprayed at the V3 growth stage on roots and confirmed strong benefits in traits that improve the uptake of water and nutrients. In the field, 30 trials were performed at 13 sites in Brazil over 10 yr, comparing noninoculated and inoculated maize receiving 0, 50, 75, and 100% (90 kg ha–1 of N) of N fertilizer (urea) applied as side‐dress 35 d after emergence. Overall, inoculation significantly increased grain yield by 3.1%. Increases were confirmed in lower and higher yield levels, subtropical and tropical conditions, and clayey and sandy soils, with low and higher soil organic carbon levels. Yield increases due to inoculation were statistically significant at all levels of N fertilizer, except for at 50%. Inoculation with the supply of 75% of N increased yield by 4.6% compared with the noninoculated counterpart and, noteworthy, did not differ statistically from the non‐inoculated control receiving 100% of N. These results confirmed the feasibility of replacing 25% of N‐fertilizer applied as side‐dress by seed inoculation with A. brasilense, besides the possibility of avoiding the emission of 4.95 million Mg of CO2‐e per year.

Surface soil organic carbon sequestration under post agricultural grasslands offset by net loss at depth

Post agricultural grasslands are thought to accumulate soil organic carbon (SOC) after cultivation cessation. The Conservation Reserve Program (CRP) in the U.S. is a wide-scale, covering approximately 8.9 Mha as of 2020, example of row-crop to grassland conversion. To date, changes in SOC stock in CRP lands have mostly been evaluated at local scales and focused on the surface 20–30 cm of the soil profile. Thus, we lack knowledge of SOC dynamics in CRP lands on a continental scale, especially in the subsurface soil, after agricultural cessation. The Rapid Carbon Assessment (RaCA) project is the most recent effort by the United States Department of Agriculture (USDA) to systematically quantify C stock in the 0–100 cm soil profile across the conterminous US. Here we analyzed data from RaCA to evaluate the SOC stocks of both surface and subsurface soil of the CRP on a continental scale. We found there was no difference in SOC stock between croplands and CRP lands when comparing the 0–100 cm soil profiles, which indicates that the C sequestration in CRP lands is insignificant overall. We did find that CRP lands have higher SOC stocks in the surface soil (0–5 cm). However, such higher SOC levels in surface (0–5 cm) soil were offset by the lower SOC stock in the subsurface (30–100 cm) of the CRP. We also found that CRP lands in humid and warm regions may have net soil C sequestration because they have much more SOC in the surface as compared with croplands in the same regions. Whether the lower SOC in the subsurface of CRP lands is caused by legacy effects or is a result of C losses needs to be verified by long-term repeated sampling in both surface and subsurface soil. This analysis highlights the importance of examining C dynamics in subsurface soil after agricultural cessation to accurately measure and improve C sequestration rates in CRP lands.

Crop production on heavily disturbed soils following crude oil remediation

AbstractCropland contaminated by crude oil typically requires remediation before it can meet pre‐contaminated productivity potentials. Remediation strategies focus on removing the contaminant, but they also often alter soil properties, which affects the capacity of the land to sustain agricultural production. This research describes crop production following the remediation of crude oil contaminated soil in North Dakota, USA using two different strategies, a modified land‐farming technique and ex situ thermal desorption. Wheat (Triticum aestivum L.) and field pea (Pisum sativum L.) were grown in soils treated by both of those techniques, as well as mixtures of each treated soil with native topsoil. Across three growing seasons of comparisons to native topsoil, crop production was 61 ± 20% lower in the modified land‐farm soils and 52 ± 25% lower in thermal desorption‐treated soils; these yield declines were likely associated with reduced soil organic carbon levels. Notably, the soil mixtures had higher soil organic carbon levels and matched the native topsoil in both yield and protein content, suggesting that soil mixing may be a promising tool in cropland soil reclamation. Overall, these findings reinforce that crop production on reclaimed land is linked with soil properties, so long‐term reclamation success requires a holistic approach to soil management.

Shift of millet rhizosphere bacterial community during the maturation of parent soil revealed by 16S rDNA high-throughput sequencing

Microorganisms, particularly bacteria, are important components of mature soil capable of high crop yields. However, changes in the diversity and abundance of bacteria during the maturation of parent soil have not been thoroughly elucidated to date. In this study, we determined the diversity of the bacterial community in millet rhizospheres from mellow soil (MS), immature soil (IS), and a mixture of MS and IS (MS-IS mixture) using high-throughput Illumina sequencing. We also analysed soil chemical properties, microbial carbon, microbial nitrogen and enzyme activities, and we correlated them with bacterial taxa. In different rhizosphere soil samples, higher bacterial diversity was found in MS than in IS or the MS-IS mixture. The same trends were observed for soil organic carbon (SOC), available nitrogen (AN), available phosphorus (AP), carbon to nitrogen ratio (C/N), soil microbial carbon, soil microbial nitrogen and soil enzyme activities. Compared to IS, MS harboured a higher abundance of two phyla (Proteobacteria and Actinobacteria) but a lower abundance of five phyla (Chloroflexi, Acidobacteria, Bacteroidetes, Nitrospirae and Firmicutes) and a higher abundance of 22 genera but a lower abundance of 13 genera. A redundancy analysis indicated that the higher abundance of seven genera (Caenimonas, Gemmatimonas, Lysobacter, Ramlibacter, Devosia, Mycobacterium and Streptomyces) was correlated with higher levels of SOC, AN and C/N. Phosphatase, invertase and urease were correlated with higher levels of SOC, AN and C/N. These findings suggest that higher abundance of some specific bacteria accelerates soil maturation by increasing soil chemical properties and soil enzyme activities, thereby leading to higher crop yields.

Spatial and seasonal distribution of carbon, nitrogen, phosphorus, and sulfur and their ecological stoichiometry in wetland soils along a water and salt gradient in the Yellow River Delta, China

Top soils (0–10 cm) were collected in three sampling belts during four seasons in 2014, including bare land (HN1), Calamagrostis epigeios (HN2), Typha orientalis (HN3), Phragmites australis (HN4), Tamarix chinensis (HN5) and Suaeda salsa (HN6) along a water and salinity gradient in the Yellow River Delta, China. Soil organic carbon (SOC), total nitrogen (TN), total phosphorous (TP), total sulfur (TS) and their ecological stoichiometry were measured to investigate their seasonal and horizontal distribution patterns, as well as their important influencing factors such as electric conductivity (EC) and water content (WC). Our results showed that the contents of SOC and TN exhibited similar changing tendency along the water and salinity gradient. The TP contents followed the order HN5 ≈ HN2 > HN3 ≈ HN6 > HN4 > HN1. TS levels generally increased with increasing salinity from HN1 to HN6. The higher levels of SOC and TP were mostly observed in October and August, respectively, while the seasonal variations in TN were heterogeneous under different plant covers. TS contents were lower in August compared with other sampling periods except for HN4. The mean values of the C/N, C/P and C/S ratios along a water-salinity gradient ranged from 26 to 72, 20 to 74, and 61 to 292, respectively. Generally, higher C/P ratios were observed in sampling sites with plant covers in October expect for HN1, whereas they were lower in January or August. SOC, TN and TP were significantly positively correlated with soil organic matter (SOM), silt, WC and cation exchange capacity (CEC) (p < 0.05), whereas TS showed a positive correlation with EC and cations content (p > 0.05). Bulk density (BD) had a great influence on C/N ratio, C/P ratio were mainly effected by SOM, EC and silt, while C/S ratio showed a significant negative correlation with BD, EC, K+, Na+, and Mg2+ (p < 0.05).

Dynamics and driving factors of the organic carbon fractions in agricultural land reclaimed from coastal wetlands in eastern China

Soils are an important pool for storing organic carbon. Differences in soil managements, the effect of land use, and other factors are often evaluated to explain the dynamics and storage potential of the total soil organic carbon (SOC). As the SOC pool is composed of several subpools with different degrees of stability and turnover rate, insights into the dynamics and driving factors of the functional pools are essential for evaluating carbon sequestration or emission. In China, coastal wetlands are undergoing intensive reclamation due to the ever-increasing population. However, the dynamics and identification of key drivers of the functional SOC pools of reclaimed soils remain largely unclear. In this study, the concentrations of SOC and permanganate-oxidizable carbon (POxC) were determined for soils with different reclamation durations and land uses in an intensified reclamation region in eastern China. A random forest (RF) model was used to identify the importance of the potential drivers of the SOC and POxC at 241 sites in this study area. The result indicated that both SOC and POxC increased significantly with reclamation duration until 65 years of land reclamation. The human-dominated land uses had significantly (p < 0.05) higher levels of SOC and POxC than the tidal flats. For lands with identical reclamation durations, the paddy soils were superior to the upland soils in terms of sequestering the SOC and POxC. Soil total nitrogen (STN), pH and soil total phosphorus (STP) were identified as the most important factors impacting the spatial pattern of SOC, while the three most important drivers of POxC were STN, STP and Cl in coastal reclaimed lands. The relatively higher POxC contents and POxC/SOC ratios of the reclaimed soils in this study compared to previous studies suggest that reclaimed soils are potentially under the great threat of global warming due to the high sensitivity of POxC to environmental changes. However, paddy management practices are recommended as an efficient approach for minimizing the potential negative impacts of global warming because of the larger POxC sequestration ability of paddy soils than that of other land uses.

Organic carbon and total nitrogen dynamics of reclaimed soils following intensive agricultural use in eastern China

In past decades, coastal tidal flats in China have experienced rapid and extensive agricultural reclamation due to the increasing pressure of population growth. With more coastal land likely reclaimed in the future, an increase in the understanding of the effects of the reclamation history and changes in land use on soil properties is essential. In this study, a total of 746 surface soil/sediment samples were collected from three study areas with different reclamation durations and land use patterns on the coasts of Dafeng, Rudong and Cixi in eastern China, covering a total area of 1926km2. The results showed that mean soil organic carbon (SOC) and soil total nitrogen (STN) in the three study areas differed slightly and ranged from 7.24 to 7.69gkg−1 and from 0.71 to 0.76gkg−1, respectively. SOC and STN increased significantly with an increase in reclamation duration in all of the study areas, except for the early stage of land conversion in Cixi and the late stage of conversion in Dafeng, which highlighted that the accumulation of SOC and STN was positively correlated with reclamation duration. In Dafeng and Rudong, land dominated by human uses had much higher levels of SOC and STN than those in the tidal flats. For the same duration of reclamation, paddy land had higher levels of SOC and STN than those in the upland, with the exception of the 10-year-old land in Cixi. Our results also indicated that reclaimed coastal soils can sequester considerable amount of OC through improved land management. The differences in the sequestration potential for OC and the dynamics of SOC and STN among the three chronosequences were attributed to specific land use patterns in each study area.

Archaeal and bacterial communities across a chronosequence of drained lake basins in Arctic Alaska.

We examined patterns in soil microbial community composition across a successional gradient of drained lake basins in the Arctic Coastal Plain. Analysis of 16S rRNA gene sequences revealed that methanogens closely related to Candidatus ‘Methanoflorens stordalenmirensis’ were the dominant archaea, comprising >50% of the total archaea at most sites, with particularly high levels in the oldest basins and in the top 57 cm of soil (active and transition layers). Bacterial community composition was more diverse, with lineages from OP11, Actinobacteria, Bacteroidetes, and Proteobacteria found in high relative abundance across all sites. Notably, microbial composition appeared to converge in the active layer, but transition and permafrost layer communities across the sites were significantly different to one another. Microbial biomass using fatty acid-based analysis indicated that the youngest basins had increased abundances of gram-positive bacteria and saprotrophic fungi at higher soil organic carbon levels, while the oldest basins displayed an increase in only the gram-positive bacteria. While this study showed differences in microbial populations across the sites relevant to basin age, the dominance of Candidatus ‘M. stordalenmirensis’ across the chronosequence indicates the potential for changes in local carbon cycling, depending on how these methanogens and associated microbial communities respond to warming temperatures.

Effects of Cultivation and Alternative Vineyard Management Practices on Soil Carbon Storage in Diverse Mediterranean Landscapes: A Review of the Literature

Managing carbon storage at the landscape level through emission reduction and carbon sequestration is emerging as a viable local response to atmospheric carbon loading from anthropogenic activities. The conversion of uncultivated land uses and land covers (LULCs) to arable or perennial cropping systems is widely recognized as resulting in significant decomposition of soil organic carbon (SOC). Minimizing conversion and advocating alternative management of these cultivated land uses have been identified as having the potential to minimize this loss and potentially sequester atmospheric carbon. However, effective landscape management requires a more rigorous understanding to inform local decision-making. This review of published studies within diverse Mediterranean landscapes found that cultivated areas contained roughly half of the SOC of uncultivated LULCs, with vineyards often containing the lowest observed SOC levels in a landscape. Mitigation through alternative management can result in higher SOC levels than conventional management, but the latter is likely to be a fraction of the C loss from initial cultivation. However, the majority of relevant studies relied on shallow standardized sampling depths and other protocols that have been demonstrated to lead to miscalculations of existing SOC stocks. Novel sampling techniques and emerging research opportunities have the potential to revolutionize our understanding of this question and support scientifically sound carbon-based landscape management.

Short Term Leguminous Trees-Tillage Interactions and Their Effect on Soil-Water Content in a Semi-Arid Agroforestry Parkland

Agricultural activities that encourage slashing, burning and ploughing greatly affect the soil structure and soil organic matter on which soil water retention depends. In this study, we hypothesized that inclusion of rotational leguminous tree species improves soil water retention in a semi-arid conservation agriculture system. In a study done in Kibwezi, semi-arid eastern Kenya, results showed that the amount of water retained in the different soil strata from plots with different tree species and tillage practices was highly significant (P = 0.032). Plots with planting basins and Gliricidia sepium and Faidherbia albida tree species retained more water in both the upper and lower strata. Plots with G. sepium tree species under planting basins and zero tillage under F. albida had significantly higher soil organic carbon levels than plots that were managed under ridges and ploughing (P = 0.002). On the other hand, bulk density in plots with planting basins and zero tillage and ridges ranged between 1.35 g/cm3 and 1.53 g/cm3. Conventional tillage plots had bulk density values of 1.65 g/cm3 and 1.72 g/cm3 in the upper and lower strata respectively. The time-dependent nature of rotational leguminous tree species on soil organic matter and soil water retention in the semi-arid conservation agriculture system highlights the importance of considering these species for improving organic carbon and water retention for improved crop production.

Can vegetation types work as an indicator of soil organic carbon? An insight from native vegetations in Nepal

Soil is the largest pool of carbon after the ocean. The assessment of soil organic carbon (SOC) stocks under different types of vegetations is critical for evidence-based implementation of soil carbon trading under different market mechanisms. Using data from 490 permanent soil sample plots with elevations ranging from 271m to 3238m above sea level, from three different watersheds in Nepal, this study aims to compare SOC under five different forest types and determine whether forest types could be indicator of SOC: Shorea robusta; mixed broadleaf; Schima-Castanopsis; pine; and Rhododendron-Quercus forests. In each vegetation type forests with dense canopies (≥70% canopy cover) have higher amounts of SOC than that of open canopy forests (<70% canopy cover). On average, with up to 30cm soil depth, dense canopy Rhododendron-Quercus forest has the highest amount of SOC (14,136gC/m2), followed by dense canopy mixed broadleaf forest (12,576gC/m2). One-way analysis of variance (ANOVA) indicates significant differences in soil organic carbon (SOC) amounts across the five forest types (df=485, F=17.299, p=0.000) and therefore forest types could be indicator of SOC. Moreover, within the similar altitudinal zone and topographic, edaphic and climatic environments: soils under the mixed species forests have higher amounts of SOC than that of single species dominated forests; and soils under forests with nitrogen fixing trees have a higher amount of SOC than those from other forests. Therefore, in addition to forest types these two conditions within the given vegetation types could work as sub-indicators of SOC. In comparison with global studies, all Nepalese forest types had much higher levels of SOC. Only two of the seven factors/indicators investigated (altitude and average age of dominant trees) were found to have significant impacts (r=0.33 to 0.64) on SOC and only in the dense and sparse canopy pine forests. The reasons of not having any impacts of all other factors on SOC in different vegetation types have been discussed.

Soil organic matter balances in organic versus conventional farming—modelling in field experiments and regional upscaling for cropland in Germany

The question of whether organic farming leads to higher soil organic matter (SOM) levels in arable soils compared with conventional farming is an ongoing debate. Building on several studies reported in the literature, we hypothesize that the impact on SOM levels is not an intrinsic characteristic of any farming system but is the result of the actual structure of the farming system, in particular, the composition and management of crop rotations, and the availability and utilization of organic manure. The SOM balances for organic versus conventional farming in Germany are compared by considering data on the structure of organic and conventional farming systems from agricultural census reports and then applying the SOM balance model HU-MOD. Preliminary testing confirmed the applicability of the model using a survey on soil organic carbon (SOC) change and SOM balances in four long-term field experiments in Germany and Switzerland and found that more positive SOM balances coincided with higher SOC levels. We therefore conclude that, where the SOM supply level of organic farming systems is higher than in conventional management, a shift from conventional to organic agriculture would increase SOM levels. Upscaling using agricultural census data in Germany, we found that SOM balances of organic farming were more positive than for conventional farming in the scenarios without consideration of animal manure application, but SOM balances for the two systems were not different where animal manure application rates were assumed to be at the current average rate for all cropped land. However, in fact, animal manure availability and application shows strong regional variations, and it is likely that this would affect the mean cropland SOM balance if it were possible to calculate it based on such spatially disaggregated data. We confirm the applicability of simple SOM balance models to compare the impact of farming systems and cropland structures on SOC levels. More work is needed to develop data inputs at a sufficient spatial and structural resolution to support more detailed evaluation.

Changes in soil organic matter indices following 32 years of different wheat production management practices in semi-arid South Africa

Soil organic matter (SOM) degradation is common in semi-arid regions due to frequent and intensive cultivation, removal of crop residues after harvesting and warmer environmental conditions. Therefore, we evaluated the effects of long-term wheat production management practices on organic matter content of a Plinthosol in semi-arid South Africa. The treatments included two methods of straw management (unburned and burned), three methods of tillage (no-tillage, stubble mulch and ploughing) and two methods of weeding (chemical and mechanical). Soil samples were collected in 2010 at various depths and analysed for soil organic carbon (SOC), soil total nitrogen (STN) and soil total sulfur (STS) as organic matter indices. Treatments where straw was not burned had greater STN and STS, but lower SOC levels than those where straw was burned. No-tillage had higher SOC levels than the stubble mulch and ploughing treatments only in the 0–50 mm soil layer. Below 100 mm soil depth, higher SOC levels were recorded in the ploughed plots. No-tillage and stubble mulch enhanced STN throughout the soil profile compared to ploughing. Ploughing and stubble mulch treatments had greater STS levels than no-tillage treatments in the upper 250 mm soil layer, and STS in the 0–450 mm soil layer was higher in mechanically weeded plots than in chemically weeded plots. Treatment combinations also showed some significant interactions on these indices, but lack of consistency made it difficult to single out the combination that was superior to others. However, to maintain or improve SOM of this Plinthosol priority should be given to no-tillage and stubble mulch management practices. Wheat grain yields over the 32 years trial period were significantly influenced by straw management and tillage methods, but not by weeding methods.

Correlations between environmental factors, the biomass of exotic annual grasses and the frequency of native perennial grasses

The species composition of temperate grasslands in the mid-north of South Australia has been radically altered from a system dominated by native perennial grasses to a system dominated by Mediterranean annual grasses. This study investigated the importance of chemical and physical soil characteristics, topographical features and climatic variables on the abundance of native and exotic grass species in nine ungrazed grasslands. Overall, climatic and other abiotic factors were highly variable. In addition, past management practices and original species composition are generally unknown, leading to further unexplained variation in the data. On a large spatial scale (among sites), the abundance of exotic annual grasses was positively correlated with mean annual rainfall, and on any scale, with finer soil textures and higher soil organic carbon levels. The most abundant annual grass, Avena barbata (Pott ex Link), was generally associated with soil factors denoting higher soil fertility. The abundance of native perennial grass species was not correlated with any environmental variables at any scale. The various native perennial grass species did not show clear associations with soil factors, although they tended to be associated with factors denoting lower soil fertility. However, at small spatial scales (within some sites) and among sites, the abundances of exotic annual and native perennial grasses were strongly negatively correlated. The results suggest that at the present time, rainfall and soil properties are important variables determining the abundance of annual grasses. The driving variables for the abundance of perennial grasses are less clear. They may be controlled by other factors or extreme rainfall events, which were not surveyed. In addition, they are likely to be controlled by competitive interactions with the annual grasses.

Sequestration of carbon and changes in soil quality under conservation tillage on light-textured soils in Australia: a review

Light-textured soils (&lt;35% clay) make up more than 80%, by area, of cropping soils in Australia. Many have inherent soil physical problems, e.g. hardsetting, sodicity and low organic carbon levels. Maintenance and improvement of soil organic carbon levels are crucial to preserving the soil structure and physical fertility of these soils.A review of field trials on conservation tillage (3–19 years duration) on these soils in southern Australia revealed that significantly higher soil organic carbon levels compared with conventional tillage were found only in the wetter areas (&gt;500 mm) and the differences were restricted to the top 2.5–10.0 cm. The average magnitude of the difference was lower than that reported in the USA. The lack of a positive response to conservation tillage is probably a reflection of a number of factors, namely low crop yield (due to low rainfall), partial removal of stubble by grazing and the high decomposition rate (due to the high temperature). There is evidence suggesting that under continuous cropping in the drier areas, the soil organic carbon level continues to decline, even under conservation tillage.Better soil structure and soil physical properties, namely macro-porosity, aggregate stability and higher infiltration have been reported under conservation tillage when compared with conventional tillage. However, little information on long-term changes of these properties under conservation tillage is available. As many of these soil qualities are associated directly or indirectly with soil organic carbon levels, the lack of significant increase in the latter suggests that many of these improvements may not be sustainable in the longer term, particularly in the drier areas. Continuous monitoring of long-term changes in the soil organic carbon and soil quality under conservation tillage in different agro-ecological zones is needed.