Total, hot water extractable, and oxidation-resistant carbon in sandy hydromorphic soils-analysis of a 220-year chronosequence

Soil organic carbon stocks decrease after conversion of soils from pasture to cropland. It has been assumed that this applies especially to mineral hydromorphic soils. In this paper we evaluate hot-water extractable carbon (C_hwe) as a measure for detecting long-term changes in the SOM following land use change. Furthermore, we assess whether a treatment of the soils with NaOCl leads to the isolation of long-term stable C fractions. For these purposes, we established a chronosequence of sandy hydromorphic soils that have been converted from pasture to cropland at different periods of history. To gain further insight into the impacts of different types of land use on carbon sequestration, soils under forest, either afforested or permanent, were studied. Bulk density, total organic carbon (TOC), C_hwe, and NaOCl-resistant C were quantified in the surface soils of 72 Gleyic Podzols and Haplic Gleysols. The bulk density increased from 0.9 (±0.2) g cm^?3 to 1.4 (±0.1) g cm^?3 during the first 25 years after the conversion of the soils from permanent pasture to cropland. In the permanent pasture sites, the TOC concentration amounted to 35.4 (±12.1) g kg^?1. It decreased to 12.88 (±5.9) g kg^?1 during the first 46 years of cultivation (R²?=?0.71). In the permanent forest soils the TOC concentrations were significantly higher than in the soils that have been afforested. C_hwe concentrations of the chronosequence sites were linearly correlated to the TOC concentrations (R²?=?0.84), while permanent forest sites exhibited significantly higher C_hwe/TOC ratios. This shows that the determination of the C_hwe is a very promising measure for detecting changes in SOM dynamics following afforestation. In the permanent pasture sites, 14.3 (±5.38) g kg^?1 NaOCl-resistant C was measured, while 46 years after conversion, only 2.8 (±1.2) g kg^?1remained. No enrichment of NaOCl-resistant C was observed in the chronosequence, as NaOCl-resistant C decreased faster in the course of cultivation than the TOC. Therefore, we conclude that that the C fraction that resists the oxidation with NaOCl is not long-term stable in soils, and most probably, there is no such long-term stable C fraction in the soils under study.     

The fate of fertiliser P in soil under pasture and uptake by subterraneum clover – a field study using <Superscript>33</Superscript>P-labelled single superphosphate

Background and aims

Single superphosphate (SSP) is a major source of phosphorus (P) used in grazing systems to improve pasture production. The aim of this experiment was to determine the fate of fertiliser P in clover pastures under field conditions.

Methods

A procedure was developed to radiolabel SSP granules with a ³³P radiotracer, which was then applied to the soil surface (equivalent to ~12 kg P ha^?1) of a clover pasture. Recovery of fertiliser P was determined in clover shoots, fertiliser granules and soil fractions (surface layer: 0–4 cm and sub-surface layer: 4–8 cm).

Results

The P diffusion patterns of the ³³P-labelled SSP granules were not significantly different to those of commercial SSP granules (P?>?0.05). Recovery of fertiliser P in clover shoots was 30–35 %. A considerable proportion of the fertiliser P (~28 %) was recovered in the surface soil layer and was largely inorganic P.

Conclusions

Recovery of fertiliser P by clover plants was up to 35 % in the year of application. Much of the fertiliser P in soil fractions was inorganic P, which highlights the importance of inorganic P forms and dynamics in soils under clover pasture on a single season timeframe at these sites.

     

A farm-scale investigation of the organic matter composition and soil chemistry of Andisols as influenced by land use and management

Andisols are characterised by having abundant reactive Al in the form of short-range ordered (SRO) Al constituents and organo-Al complexes, which facilitates the accumulation of soil organic matter (OM). However, recent studies of New Zealand pastoral systems have reported loss of carbon (C) from Andisols when under intense management. This study compares the organic and inorganic chemistry of Andisols on two adjacent pasture sites under different pastoral management regimes (Paddock 2 being more intensively managed than Paddock 1), as well as under a nearby pine stand (Forest). Mean soil pH-H₂O in Forest (5.3) was significantly lower (P?<?0.05) than that in Paddock 1 (5.7), which itself was significantly lower (P?<?0.05) than in Paddock 2 (6.1). Soil C concentrations were significantly higher (P?<?0.05) in the soils under pasture than under pine (63.8 g C/kg), and C in Paddock 1 (98.1 g C/kg) was significantly higher (P?<?0.05) than in Paddock 2 (84.1 g C/kg). The ratio of Al in organo-Al complexes (as estimated with sodium pyrophosphate) to the sum of Al in both SRO and organo-Al complexes (Al_p/Al_o) was significantly smaller (P?<?0.05) as the alkalinity of the soils increased (0.38, 0.23, 0.16 for Forest, Paddock 1 and Paddock 2, respectively). At the molecular level, soils under Forest had a larger relative contribution of degraded products of plant polysaccharides than those under pasture, while these had a larger contribution of fresh (e.g. cellulose and cutan/suberan aliphatic structures) and N-rich OM (e.g., microbial fingerprints, denoting a high microbial activity). Dissolved organic C content in the rhizosphere of pasture species was similar between paddocks, but Paddock 2 had a significantly (P?<?0.05) greater contribution of organic acids of MW?<?500 Da and higher pH (6.8 vs. 6.2). The results (1) confirm the common enrichment in organic C of New Zealand top soils under pasture compared to those under pine, and (2) reveal that the changes in the soil chemistry associated with pasture management may weaken the ability of these soils to preserve OM.     

The ecosystem engineering role of the Neotropical crab <Emphasis Type="Italic">Cardisoma guanhumi</Emphasis> on mangrove soil properties

The land crab Cardisoma guanhumi is one of the most common species in mangroves of the American Atlantic coast and Caribbean islands however, studies of its effects on the physical and chemical soil properties are scarce. This study compares specific physicochemical properties of soil between C. guanhumi burrows (B) and adjacent zones (AZ), and provides the first insights on their role as an ecosystem engineer in mangroves. The study was conducted in an estuarine system dominated by Rhizophora mangle, located at the Río Chico estuary, Miranda state of Venezuela. Random soil samples were taken digging each burrow until reaching the bottom and at the same depth for AZ. Data analysis was carried out using Bayesian inference. Credible mean differences between B and AZ, were found for sand (B?=?26.53?±?10.76, AZ?=?17.25?±?5.7%), silt (B?=?73.16?±?10.77, AZ?=?82.42?±?5.69%), pH (B?=?8.71?±?0.36, AZ?=?9.12?±?0.30), soil organic matter (SOM, B?=?0.43?±?0.21, AZ?=?0.17?±?0.06%), total N (TN, B?=?786?±?232, AZ?=?529?±?107 µg g^?1), Mg (B?=?4.42?±?0.60, AZ?=?3.48?±?0.71 cmol_c kg^?1) and K (B?=?0.12?±?0.05 AZ?=?0.06?±?0.02 cmol_c kg^?1). Chemical variables as SOM, K, Mg and TN showed the highest values of effect size (>?1.4). With the exception of the pH, all chemicals variables—which were different between B and AZ—showed strong and decisive evidences of correlations with SOM. When SOM variable was controlled, the relationships between pH–TN, TN–K and Mg–K decreased, even though the correlation evidence between each pair remained. Differences in chemical contents found in B respect to AZ suggest that the activities of C. guanhumi (feeding, moulting, excretion and defecation) within their burrows promote the spatial heterogeneity of mangrove soils.     

Improving N2 fixation from the plant down: Compatibility of Trifolium subterraneum L. cultivars with soil rhizobia can influence symbiotic performance

Plant genotypes of Trifolium subterraneum L. (subterranean clover) were evaluated for differences in symbiotic N₂ fixation with soil rhizobia, with the long-term aim of using plant selection to overcome sub-optimal N₂ fixation associated with poorly effective soil rhizobia. Symbiotic performance (SP) was assessed for 49 genotypes of subterranean clover with each of four pure Rhizobium strains isolated from soil. Plants were grown in N free media in the greenhouse and their shoot dry weights measured and expressed as a percentage of dry weight with R. leguminosarm bv. trifolii WSM1325, the recommended commercial inoculant. Average SP with two Rhizobium strains (H and J) ranged from completely ineffective to 80% of potential for the subterranean clover genotypes. Two clover cultivars with high (cv. Campeda) and low (cv. Clare) SP values were investigated in more detail. Campeda typically fixed more N₂ than Clare when inoculated with 30 soil extracts (4.2 vs 2.4 mg N₂ fixed/shoot) and with 14 pure strains isolated from those soils (4.2 vs 2.2 mg N₂ fixed/shoot). The poor performance of Clare could be attributed to interruptions at multiple stages of the symbiotic association, from nodule initiation (less nodules), nodule development (small, white nodules), through to reduced nodule function (N₂ fixed/mg nodule) depending on the inoculation treatment. Through the careful use of subterranean clover genotypes by plant breeders it should be possible to make significant gains in the SP of future subterranean clover cultivars.     

Nutrient stocks and phosphorus fractions in mountain soils of Southern Ecuador after conversion of forest to pasture

Understanding pasture degradation processes is the key for sustainable land management in the tropical mountain rainforest region of the South Ecuadorian Andes. We estimated the stocks of total carbon and nutrients, microbial biomass and different P fractions along a gradient of land-uses that is typical of the eastern escarpment of the Cordillera Real i.e., old-growth evergreen lower montane forest, active pastures (17 and 50 years-old), abandoned pastures 10 and 20 years old with bracken fern or successional vegetation. Conversion of forest to pasture by slash-and-burn increased the stocks of SOC, TN, P and S in mineral topsoil of active pasture sites. Microbial growth in pasture soils was enhanced by improved availability of nutrients, C:N ratio, and increased soil pH. Up to 39 % of the total P in mineral soil was stored in the microbial biomass indicating its importance as a dynamic, easily available P reservoir at all sites. At a 17 years-old pasture the stock of NH₄F extractable organic P, which is considered to be mineralisable in the short-term, was twice as high as in all other soils. The importance of the NaOH extractable organic P pool increased with pasture age. Pasture degradation was accelerated by a decline of this P stock, which is essential for the long-term P supply. Stocks of microbial biomass, total N and S had returned to forest levels 10 years after pasture abandonment; soil pH and total P 20 years after growth of successional bush vegetation. Only the C:N ratio increased above forest level indicating an ongoing loss of N after 20 years. Soil nutrient depletion and microbial biomass decline enforced the degradation of pastures on the investigated Cambisol sites.     

Effect of conversion of sugarcane plantation to forest and pasture on soil carbon in Hawaii

It is well known that land use change can affect soil C storage of terrestrial ecosystems either by altering the biotic processes involved in carbon cycling or by altering abiotic processes such as carbon adsorption on soil minerals. Relatively few studies, however, have examined the dynamics of soil C pools after conversion of farmland to forest or pasture. We selected three pairs of secondary forests and pastures that originated from the same abandoned sugarcane (interspecific hybrids of Saccharum spp.) land in the wet tropics of Hawaii to examine whether forest or pasture converted from farmland is more effective in sequestering C in soils. We compared the soil C pool, soil chemistry, and stable C isotope ratios between the forests and pastures. We found that total soil C was greater (P?<?0.01) in forests than in the pastures 22 years after land conversion. The percentages of SOC₄ in the pastures were significantly higher than in the secondary forests in both soil layers. The percentages of SOC₃ in the pastures were lower than in the secondary forests in both soil layers. The net SOC₃ increase in the forest soils at 0–10 and 10–25 cm was 28.6?±?5.6 and 43.9?±?3.2 Mg ha^?1 while net SOC₄ increase in pasture soils at these respective depths was 18.8?±?2.2 and 26.1?±?2.7 Mg ha^?1. We found that the net increases of SOC₃ in both soil layers in the forest were greater (P?<?0.01) than the net increases of SOC₄ in the respective soil layers in the pasture. Aluminum saturation was greater (P?<?0.01) in the forests than the pastures in both soil layers. There was no difference in oxalate extractable Fe concentration between the forests and the pastures but oxalate extractable Al concentration in both soil layers was greater (P?<?0.05) in forests than the pastures. Our findings indicated that reforestation of abandoned sugarcane farmland in Hawaii is more effective in soil C increase and stabilization than conversion to pasture.     

Use of a 15N2 labelling technique to estimate exudation by white clover and transfer to companion ryegrass of symbiotically fixed N

Backgrounds and aims

N rhizodeposition by legumes leads to enrichment of N in soils and in companion plants. N rhizodeposition can be divided into two major components, root exudation and root senescence. Our aim was to quantify N root exudation in white clover (Trifolium repens L.) through an estimation of short-term N rhizodeposition and to assess its impact on N transfer to companion perennial ryegrass (Lolium perenne L.) grown in mixture with clover.

Method

¹⁵N₂ provided in the root atmosphere for 3 days was used to estimate transfer of symbiotically fixed nitrogen (SFN) to the growing medium by clover grown in pure stand and to ryegrass by clover grown in mixture for 2 months.

Results

The proportion of N rhizodeposited over the 3 days increased from 3.5 % of SFN in pure stand to 5.3 % in mixture. The ¹⁵N-enrichment of ammonium from the adhering substrate shows that a part of the rhizodeposited N was released in the form of ammonium. 4 % of the rhizodeposited N was taken up by ryegrass during the labelling period.

Conclusions

This study showed a significant contribution of root N exudation to the total N rhizodeposition of legumes and in the transfer of N to grasses.     

Phosphorus in pasture plants: potential implications for phosphorus loss in surface runoff

Phosphorus (P) loss from land can impair surface water quality. Losses can occur from soil and plant components. While it is known that P losses increase with soil P concentration, it is not known how losses from pasture plants vary with soil P concentration or between different forages. We examined total P and filterable reactive P (FRP) in water extracts of plant shoots, used as a measure of potential P loss to surface runoff, in different forage species relative to soil P concentration in field trials and a glasshouse experiment. The mean total P concentration of 16 forage species in grazed field plots was greater (P?<?0.01; LSD₀₅?=?117 mg kg^?1) in legumes (3,480 mg kg^?1) than for grasses (3,210 mg kg^?1). Total plant P concentrations of grasses and legumes increased with soil Mehlich-3 P concentrations in both glasshouse and field trials with concentrations close to 6,000 mg kg^?1 in arrowleaf clover at 680 mg kg^?1 Mehlich-3 soil P. FRP in water extracts of plant shoots increased relative to plant total P as soil Mehlich-3 P increased, with the greatest concentrations shown by crimson clover and arrowleaf clover. Analysis of water extracts of ryegrass and clover herbage from a field trial showed that while FRP was increasing, phytase-available-P decreased significantly from about 70% of filterable unreactive P at the lowest Mehlich-3 P concentrations, to close to zero at 200 mg kg^?1 Mehlich-3 P. The wide variation, and enrichment of FRP in water extracts and total P with increasing Mehlich-3 P among species, indicates that cultivar and site selection and sward management provide a potential option to mitigate P loss to surface waters.     

Estimates of seasonal nitrogen fixation of annual subterranean clover-based pastures using the15N natural abundance technique

Annual pasture legumes play a key role in ley farming systems of southern Australia, providing biologically fixed nitrogen (N) to drive the production of the pastures as well as subsequent crops grown in rotation. Seasonal inputs of biologically fixed N in shoot biomass of the subterranean clover (Trifolium subterraneum) component of grazed annual pastures were assessed using the¹⁵N natural abundance technique and appropriately timed sampling of herbage dry matter (DM) for N accumulation. At three study sites spanning a gradient across the Western Australian wheatbelt from 300 to 600 mm annual rainfall the performance of the clover and non-legume herbs and grasses was examined as paired comparisons involving two management treatments expected to give contrasting effects on pasture productivity, botanical composition and N₂ fixation. The proportion of clover N derived from atmospheric N₂ fixation (%Ndfa) ranged from 65 to 95% across sites, treatments and sampling times. Amounts of fixed N accumulated in clover shoot biomass ranged from 50 to 125 kg ha⁻¹, and paralleled trends in clover production. Substantial increases in pasture production in high yielding treatments generally occurred without decrease in %Ndfa, suggesting that N₂ fixation was essentially non-limiting to performance of the clover component. Seasonal profiles for accumulation of fixed N were skewed towards the late winter and spring period, particularly in low plant density pastures following a cereal crop. There were seasonal, site and treatment-specific effects on the proportion of clover and non-legume pasture components and consequently clover yield and N₂ fixation were variably affected by competition from non-legume species.     

Variation in seedling growth of 11 perennial legumes in response to phosphorus supply

Phosphorus (P) deficiency is a major problem for Australian agriculture. Development of new perennial pasture legumes that acquire or use P more efficiently than the current major perennial pasture legume, lucerne (Medicago sativa L.), is urgent. A glasshouse experiment compared the response of ten perennial herbaceous legume species to a series of P supplies ranging from 0 to 384 µg g^?1 soil, with lucerne as the control. Under low-P conditions, several legumes produced more biomass than lucerne. Four species (Lotononis bainesii Baker, Kennedia prorepens F.Muell, K. prostrata R.Br, Bituminaria bituminosa (L.) C.H.Stirt) achieved maximum growth at 12 µg P g^?1 soil, while other species required 24 µg P g^?1. In most tested legumes, biomass production was reduced when P supply was ≥192 µg g^?1, due to P toxicity, while L. bainesii and K. prorepens showed reduced biomass when P was ≥24 µg g^?1 and K. prostrata at ≥48 µg P g^?1 soil. B. bituminosa and Glycine canescens F.J.Herm required less soil P to achieve 0.5 g dry mass than the other species did. Lucerne performed poorly with low P supply and our results suggest that some novel perennial legumes may perform better on low-P soils.     

Land-use change in a tropical mountain rainforest region of southern Ecuador affects soil microorganisms and nutrient cycling

Over the past decades, the tropical mountain rainforest of southern Ecuador has been threatened by conversion to cattle pastures. Frequently, these pastures are invaded by bracken fern and abandoned when bracken becomes dominant. Changes in land-use (forest–pasture–abandoned pasture) can affect soil microorganisms and their physiological responses with respect to soil carbon and nutrient cycling. In situ investigations on litter decomposition and soil respiration as well as biogeochemical characterization of the soil were carried out to identify the driving factors behind. The conversion of forest to pasture induced a pronounced increase in CO₂–C effluxes to 12.2 Mg ha^?1 a^?1 which did not decrease after abandonment. Soil microbial activity and biomass showed a different pattern with lowest values at forest and abandoned pasture sites. With 3445 mg kg^?1 (0–5 cm) microbial biomass carbon (MBC by CFE-method), the active pasture had a more than three times higher value than forest and abandoned pasture, which was among the highest in tropical pasture soils. A shift in the microbial community structure (phospholipid fatty acid, PLFA) was also induced by the establishment of pasture land; the relative abundance of fungi and Gram-negative bacteria increased. PLFA fingerprints of the forest organic layer were more similar to pasture than to forest mineral soil. Chemical properties (pH value, exchangeable cations) were the main factors influencing the respective microbial structure. Bracken-invasion resulted in a decrease in the quantity and quality of above- and belowground biomass. The lower organic substance and nutrient availability induced a significant decline in microbial biomass and activity. After pasture abandonment, these differences in soil microbial function were not accompanied by pronounced shifts in the community structure and in soil pH as was shown for the conversion to pasture. A disconnection between microbial structure and function was identified. Similar soil CO₂–C effluxes between active and abandoned pasture sites might be explained by an underestimation of the effluxes from the active pasture site. All measurements were carried out between grass tussocks where fine-root density was about 2.6 times lower than below tussocks. Thus, lower proportions of root respiration were expected than below tussocks. Overall, soil microorganisms responded differently to changes in land-use from forest to pasture and from pasture to abandoned pasture resulting in pronounced changes of carbon and nutrient cycling and hence of ecosystem functioning.     

Bioavailability and crop uptake of trace elements in soil columns amended with sewage sludge products

In order to assess the potential impact of long-term sewage sludge application on soil health, the equivalent of about 25 years of agronomic applications of low-metal (`EQ') sewage sludge products were made to greenhouse soil columns. After a 6-year period of `equilibration', during which time successive crops were grown with irrigation by simulated acid rain, the plant-available quantities of trace elements were estimated in the soils by extraction with 0.01 M CaCl₂ at 90 °C, and measured directly by uptake into a crop of red clover (Trifolium pratense L.). Soil pH had a strong influence on the level of extractable and plant-available metals, and because the tested sludge products affected soil pH differently, pH was directly factored into the comparison of different sludge treatments with controls. CaCl₂-extractable levels of several metals (Cu, Zn, Mo), sulfur and phosphorus were found to be higher in the soils amended with organic-rich sludge products than in the control soils. However, extractable Cd and Ni were not significantly elevated by the sludge amendments, presumably because of the low total loading of these metals. Copper, Zn and Mo applied in the form of sludge ash had low soil extractability, suggesting that these trace metals were trapped in high-temperature mineral phases formed during sludge incineration, and resisted subsequent weathering in the soil environment. Extractable soil metals in the alkaline-stabilized sludge treatment were also generally low. Phytotoxicity from the sludge metal loadings (Zn≤125, Cu≤135 kg/ha), was not clearly indicated as long as soil pH was maintained in the 6–7 range by lime amendment. Nevertheless, unexplained depressions in yield were noted with some of the sludge products applied, particularly the dewatered and composted materials. On limed soil columns, the most consistent effect of sludge product amendment on red clover composition was a marked increase in plant Mo.     

Estimating base cation weathering rates in Canadian forest soils using a simple texture-based model

Mineral weathering is the primary source of long-term buffering capacity in soils and is important for forest nutrient sustainability. Regional assessments of weathering rates in Canada and the U.S. have employed an empirical clay-based Soil Texture Approximation (STA) owing to limited data availability, although the STA is rarely calibrated before application to a study area. Soil weathering rates estimated with the STA at 75 sites in Canadian forests (6–367 eq ha^?1 year^?1) were on average seven times lower than estimates obtained using the PROFILE model and when mineralogy was not available, the Analysis to Mineralogy model and parameter estimation (143–2,119 eq ha^?1 year^?1). Comparison with a catchment mass balance at a subset of sites in Ontario (n = 19) demonstrated the reliability of PROFILE weathering estimates. A revised (generalized) STA model for total base cation weathering was developed at the 75 study sites to incorporate soil silt content (%) and loss-on-ignition (LOI, %) (BC _w = (1.73 + 0.03 · silt ? 0.06 · LOI) · 1,000 · depth). The model performance (R _adj ²  = 73%) and relative bias (?1%) suggested that the revised STA may have broad application to forest soils in Canada but may not necessarily be suited to all soil texture classes.     

Deposition and decomposition of cattle dung and its impact on soil properties and plant growth in a cool-temperate pasture

Livestock dung provides an important direct pathway by which carbon and nutrients enter soils in pasture ecosystems and affects carbon and nitrogen cycling indirectly through changes in soil and plant properties. Here, we quantify dung deposition, decomposition, and the effects of dung on soil and plants in a Zoysia japonica grassland in Japan. We determined (1) the distribution of dung, (2) the mass loss rate of dung and the amount of carbon respired as CO₂, and (3) changes in soil properties and aboveground biomass of Z. japonica. Dung deposition was 4.0–9.7 g C and 0.4–1.0 g N m^?2 year^?1 and distributed patchily (Morishita’s I _δ  > 1). Most (71 %) of the carbon in dung deposited in June was lost within a single grazing period by aerobic decomposition, more than mass loss rate of Z. japonica litter in the first year (about 50 %), suggesting that grazing and defecation can accelerate carbon cycling compared with the typical litterfall–decomposition regime. Nitrogen in dung mass entered the soil as ammonium nitrogen and was nitrified. The spatiotemporal distribution of these processes corresponded to that of stimulated Z. japonica growth. These results suggested that dung deposition significantly affected the inorganic nitrogen status of soil and, therefore, the growth of Z. japonica. However, these effects were very restricted temporally (July–August) and spatially (within 10 cm from dung edge). Thus, such spatiotemporally restricted effects combined with the patchy distribution of dung may contribute to the heterogeneous structure of pasture ecosystems.     

Identification of genetic factors influencing salt stress tolerance in white clover (Trifolium repens L.) by QTL analysis

Allotetraploid (2n = 4x = 32) white clover (Trifolium repens L.) is the most commonly cultivated legume component of temperate pastures, sown in swards with a companion grass species. Genetic control of growth performance of white clover on saline land is highly important for dairy industries, due to increasing soil salinity problems. The objective of this study was to identify quantitative trait loci (QTLs) for salinity tolerance in terms of vegetative growth under stress. Two parental genetic maps consisting of 213 and 159 marker loci and spanning 1,973.0 and 1,837.6 cM, respectively, were constructed using simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers from a two-way pseudo-test cross F₁ population derived from pair-crossing of the Haifa₂ and LCL₂ genotypes. A total of 8 unique genomic regions on 8 linkage groups (LGs) of the Haifa₂ parental map and 6 unique regions on 5 LGs in the LCL₂ parental map were associated with plant growth under salt stress and relative growth under stress, as compared to control conditions. The results of this study indicate that salt tolerance in white clover is controlled by multiple QTLs, some at common locations, but each of limited magnitude. Location of these QTLs provides the genetic basis and potential for pyramiding of salt tolerance genes in breeding improvement.     

Nitrogen fixation and transfer in grass-clover leys under organic and conventional cropping systems

Background and aim

Symbiotic dinitrogen (N₂) fixation is the most important external N source in organic systems. Our objective was to compare symbiotic N₂ fixation of clover grown in organically and conventionally cropped grass-clover leys, while taking into account nutrient supply gradients.

Methods

We studied leys of a 30-year-old field experiment over 2 years in order to compare organic and conventional systems at two fertilization levels. Using ¹⁵N natural abundance methods, we determined the proportion of N derived from the atmosphere (PNdfa), the amount of Ndfa (ANdfa), and the transfer of clover N to grasses for both red clover (Trifolium pratense L.) and white clover (Trifolium repens L.).

Results

In all treatments and both years, PNdfa was high (83 to 91 %), indicating that the N₂ fixation process is not constrained, even not in the strongly nutrient deficient non-fertilized control treatment. Annual ANdfa in harvested clover biomass ranged from 6 to 16 g?N m^?2. At typical fertilizer input levels, lower sward yield in organic than those in conventional treatments had no effect on ANdfa because of organic treatments had greater clover proportions. In two-year-old leys, on average, 51 % of N taken up by grasses was transferred from clover.

Conclusion

Both, organically and conventionally cropped grass-clover leys profited from symbiotic N₂ fixation, with high PNdfa, and important transfer of clover N to grasses, provided sufficient potassium- and phosphorus-availability to sustain clover biomass production.     

A geographically extensive survey of hedgehogs (Erinaceus europaeus) in England

A hedgehog (Erinaceus europaeus) survey was conducted in four regions of west and south-west England during the summer of 2006. In each region, surveys were conducted in a series of 15–18 survey areas each of which contained 1–4 paired amenity grassland (recreation ground, sports field, and village green) and permanent pasture survey sites. Each pair of survey sites was surveyed for hedgehogs using night-time lamped searches during two separate survey periods. Located hedgehogs were sexed, weighed, and aged. The density of hedgehogs on individual sites was based on the maximum number of hedgehogs recorded during any one of the two surveys. The occurrence of hedgehogs on pasture (2 % of fields) was much lower than that on amenity grassland (26 % of fields), resulting in mean densities of hedgehogs that were significantly greater on amenity grassland than on pasture in all four regions. The density of hedgehogs was 0.47?±?0.09 ha^?1 on amenity grassland and 0.04?±?0.02 ha^?1 on pasture (mean?±?standard error). This study represents a geographically extensive and locally intensive hedgehog field survey in England and produces results that are spatially and temporally concurrent with wildlife surveys of another ecologically linked species, the badger (Meles meles).     

Potassium release characteristics, potassium balance, and fingermillet (Eleusine coracana G.) yield sustainability in a 27- year long experiment on an Alfisol in the semi-arid tropical India

Aims

In Alfisols, potassium (K) deficiency limits productivity, as these soils are poor in K-bearing minerals such as mica. As nutrient management practices greatly influence K nutrition of crops especially in the longer term, we evaluated the effects of 27 (1978–2004) years of cropping fingermillet (Eleusine coracana G.) under different manure and mineral fertilizer treatments on K release, balance and yield sustainability on K deficient Alfisols in the semi-arid tropical region of southern India.

Methods

Fingermillet (variety: PR-202) was grown each year under rainfed conditions with 5 different nutrient management treatments: control (no amendment), 10 Mg ha^?1 farm yard manure (FYM), 10 Mg ha^?1 FYM +50 % NPK, 10 Mg ha^?1 FYM +100 % NPK and 100 % NPK. Potassium release characteristics in the soil profile were determined using 1 N boiling HNO₃ (strong extracting solution), 0.01 M HCl (medium extracting solution) and 0.01 M CaCl₂ (mild extracting solution).

Results

Continuous cropping of Alfisols for 27 years resulted in a decrease in K supplying capacity due to soil K depletion through crop K uptake. In soils without K addition, inherent soil supply could not meet the K requirement of fingermillet; thus, a negative K balance following 27 years of cropping affected K nutrition of the crop in all the treatments. As a result, the highest sustainable yield index (SYI) was observed using an integrated nutrient supply (combined application of nutrients from organic and inorganic sources), and the lowest index was obtained without K additions.

Conclusion

For balanced nutrient management in cereal production systems, K nutrition needs urgent attention in the K deficient Alfisol region of southern India. Addition of any amount of organic manures available at field level offers an alternative strategy for maintaining soil K fertility to improve and sustain crop productivity.     

Soil-atmosphere exchange of nitrous oxide and methane in New Zealand terrestrial ecosystems and their mitigation options: a review

The two non-CO₂ greenhouse gases (GHGs) nitrous oxide (N₂O) and methane (CH₄) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N₂, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH₄ emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH₄ from the atmpsphere, as the global estimates confirm. Both the net uptake of CH₄ by soils and N₂O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO₂ GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N₂O fluxes and CH₄ uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N₂O–N ha^?1 year^{? 1}), intermediate in sheep-grazed pastures, (4–6 kg N₂O–N ha^?1 year^?1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N₂O–N ha^?1 year^?1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N₂O production. Generally, N₂O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH₄ uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N₂O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH₄ ha^?1 year^?1), intermediate in some pine forest soils (4–6 kg CH₄ ha^?1 year^?1), and lowest in most pasture (<1 kg CH₄ ha^?1 year^?1) and cropped soils (1.5 kg CH₄ ha^?1 year^?1). Afforestation /reforestation of pastures results in increases in soil CH₄ uptake, largely as a result of increases in soil aeration status and changes in the population and activities of methanotrophs. Soil CH₄ uptake is also seasonally dependent, being about two to three times higher in a dry summer and autumn than in a wet winter. There are no practical ways yet available to reduce CH₄ emissions from agricultural systems. The mitigation options to reduce gaseous emissions are discussed and future research needs identified.