The importance of exchangeable cations and resin-sink characteristics in the release of soil phosphorus

The significance of exchangeable cations in the release of phosphorus by sequential extraction with water was evaluated in 11 acid (_pH 5.0–6.3) New Zealand soils contrasting in P status and P retention. The release of P from Na-saturated soil exceeded that from the original Ca-dominated soils by up to four-fold. Possible explanations for the larger P release in the Na system include: (i) desorption of P induced by increased surface negative potential associated with the exchange of Na for Ca/Mg, and/or (ii) accelerated dissolution of Ca phosphate compounds or complexes resulting from the creation of a sink for Ca.
The potential of a series of anion- and cation-exchange resin systems (AER and CER, respectively) as sinks for labile soil P was also examined. For all soils studied, P extracted by AER-HCO₃ 3 and that removed by sequential extraction with water. Also, the amounts of P extracted by AER-OH/CER-H and NaCl/ H₂O were closely correlated ( r = 0.95**), suggesting similar release mechanisms. The results obtained indicate that charge-balancing cations, particularly Ca which is the predominant exchangeable cation in the majority of soils, exerts a more significant control on soil P equilibria in acid soils than is commonly recognized.     

Distribution of exchangeable cations in a soil catena in the area of Baer mounds

The soil cover of the Baer mounds in Astrakhan oblast consists of brown solonetzes on the tops of the mounds, solonetzic brown and meadow brown soils on the slopes, and solonchaks at the footslopes. The exchangeable sodium percentage in the brown soils on the tops and slopes varies from 7 to 42%. However, the morphological features of the solonetzic process are weakly pronounced in these soils, except for some cementation of the soil mass and the poorly developed columnar structure in the B horizon. This is probably related to the coarse soil texture and the relatively low exchange capacity (<10 meq/100 g of soil). The portion of exchangeable sodium reaches 2–9%. A direct correlation between the sodium concentrations in the water extracts and in the soil exchange complex is clearly seen.     

On the 80th birthday of A.G. Bondarev

Jubilee

On the 80th birthday of A.G. Bondarev     

The role of moisture content in the stability of soil aggregates from a temperate silty soil to raindrop impact

Aggregate stability to drop impact is of fundamental importance to many aspects of the soil system, in particular rainsplash erosion. The role of moisture content is considered by the use of a single drop simulator on soil aggregates of known moisture content selected from the Hamble series. Results indicate a negative logarithmic relationship between time to breakdown and specific moisture content. In addition the variability of the results drastically declines with increasing moisture content. The results are explained in terms of distribution of moisture within the aggregates prior to experimentation.     

The importance of soil acidity,moisture, exchangeable cation pools and organic matter solubility to the cationic composition of beech forest (Fagus sylvatica L.) soil solution

Humus horizons of dystric cambisols were sampled six times during 1990–1992 at 66 points along a beech forest transect in Scania, s. Sweden. Cation concentrations of soil solutions obtained by centrifugation of sifted samples at field moisture were related to pH, DOC, exchangeable pools of the cations and soil moisture. Soil solution Al was speciated in free ionic (easily reacting) Al_r and organically complexed Al_org. Two or three variables accounted for a large share (70–90 %) of the cation variability between sampling points. Exchangeable soil pools were the most important variables for K, Mg. Ca, and Mn and contributed more when calculated on C. E. C. than on soil dry weight. Some function of pH was also of importance to most cation concentrations. Al_r correlated well with both Al_org(+) and pH(-). Soil moisture was positively related to DOC and K, negatively to H-ion concentration. pH measured by different methods were closely correlated (r = 0.93–0.97), pH_kcl and pH being ca. 0.5 unit lower, pH ca. 0.3 unit higher than soil solution pH, which varied between 3.5 and 5.6.     

The role of tephra covers on soil moisture conservation at Haleakala's crater (Maui, Hawai'i)

The influence of tephra covers on soil water was studied in Haleakala (Maui, Hawai'i) during two summers; eight sites with tephra layers and silverswords (Argyroxiphium sandwicense DC.) were sampled at 2415–2755 m. At each site, eight paired-sample sets were obtained in bare soils and under adjacent tephra, at three depths. Tephra were sharply separated from underlying soils and showed prominent vertical stratification. Tephra clast size-distribution was assessed by photosieving and on interstitial-gravel samples; stones included 45.6% cobbles, 29.4% pebbles, and 25% blocks.Moisture content increased with depth in both positions, but soils below tephra had more water at all depths than exposed areas. Surface soils beneath tephra contained 83% more water than bare ground. Soils at 5–10 cm had  106% greater moisture under rocks, but only  70% at 10–15 cm. Differences between plots were statistically significant ( p < 0.001) for surface soils, but less pronounced for subsoils. Soils above 2650 m had greater water content than at lower elevations, and moisture disparity between sample pairs increased with altitude.All soils were coarse, with  20% gravel and  94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during  one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5–6 cm) tephra layers were nearly as effective as thicker (9–15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.     

A comparison of buffered and unbuffered ammonium salts to determine exchangeable base cations in acid soils

Abstract

In soil samples from two study sites in southern Norway, exchangeable cations were determined using two different ammonium (NH₄)‐salts as extractant. As expected, the cation exchange capacity (CEC) determined in 1M ammonium acetate (NH₄OAc), buffered at pH 7.0 was higher than the CEC measured in ammonium nitrate (NH₄NO₃). By contrast, the amount of exchangeable calcium (Ca), magnesium (Mg), and barium (Ba) was lowest in the NH₄OAc extract, in particular in the upper soil horizons high in organic matter (O‐ and E‐horizon). This suggests that NH₄ in 1M NH₄OAc does not compete effectively with multivalent base cations. The relatively high levels of exchangeable base cations in NH₄NO₃ could not be explained by increased weathering. An increase in selectivity of especially divalent cations may explain the relatively low amount of exchangeable base cations extracted by NH₄OAc, as this involves increased deprotonation and thus a higher negative charge.     

A review of the role of excess soil moisture conditions in constraining farm practices under Atlantic conditions

The Less Favoured Area (LFA) scheme is a major element of the EU Rural Development Policy, aimed at supporting farming in areas with natural handicaps or low soil productivity. It has been in place since 1975 and accounts for 14% of total Community funding. In 2003, the European Court of Auditors recommended that the socio‐economic criteria on which the current scheme is based be replaced by biophysical criteria. Reviews of the proposals suggest that in Atlantic climates of Northwest Europe, the new criteria do not delineate adequately areas where agricultural productivity is constrained by the biophysical environment and that such areas are instead demarcated by the occurrence of excess soil moisture conditions. In this paper, we review the impact of excess soil moisture conditions on the sustainability of farming systems and their role in constraining strategic and tactical farm management practices. In particular, we review the scientific evidence on the impact of excess soil moisture conditions on herbage growth, herbage utilization, farm operations and environmental sustainability. On the basis of this, we propose an additional biophysical criterion for the new delineation of LFAs, namely the length of time that soil water is in excess of field capacity (‘field capacity days’). While there is no clear threshold for field capacity days above which agricultural sustainability is acutely constrained, the evidence reviewed in this paper suggests that the sustainability of intensive livestock farming and tillage systems is particularly challenging in scenarios where the 80 percentile of field capacity days exceeds 220–230 days.     

A six year study of earthworm (Lumbricidae) populations in pasture woodland in southern England shows their responses to soil temperature and soil moisture

There are very few studies on the effects of temporal changes in soil properties on ecosystem engineers in UK soils. This study addresses this lack by presenting earthworm diversity data from a six-year seasonality study comprising 72 monthly samples from the litter and soil of pasture woodland in the New Forest, southern England. These data were analysed in the context of soil moisture and soil temperature, key factors affecting earthworm abundance, and factors likely to be strongly affected by future climate change. The data for the whole period were analysed using non-parametric regression and an additive model used to separate within-year and between-year effects. Seasonal patterns are present for all the common species, generally with a maximum in March and a minimum in September. A majority of the five commonest species show a strong decline in abundance during the two extremely dry periods (2002–2003 and 2006). In sharp contrast, the same species showed a relative increase during the very wet summer and autumn of 2007. There was, however, no significant overall trend in either the climate data or the earthworm species data. The epigeic species, Dendrobaena octaedra, showed the largest decline in the driest months which caused a crash to the point where there were no adults sampled during the four dry summer months of 2003. A second congeneric species, Dendrobaena attemsi, also epigeic, appears to have invaded the woodland during the six year period and is increasing rapidly in abundance. This may indicate the start of a shift in the distribution of the two species, as D. octaedra generally has a northern European distribution and is frost-tolerant, while D. attemsi has a southern European distribution and is more drought-tolerant. In contrast, the very wet summer of 2007 seems to have damped the usual periodic seasonal oscillations in earthworm numbers. Endogeic worm species do not show the D. octaedra seasonal pattern as obviously, probably because these species are able to move more freely through the soil and because they are able to aestivate. These changes are likely to be due to a combination of human movement of earthworms (e.g. D. attemsi) and variations in local climate.     

The effects of soil moisture variability on the vegetation pattern in Mediterranean abandoned fields (Southern Spain)

The demographic pressure decrease in Southern Spanish Mediterranean mountainous areas in the mid twentieth century led to the abandonment of agriculture and rupture in the geo-ecosystem balance which had existed until then. Since then, different phases of recovery have been put into action to return the landscape to its earlier natural condition according to climate and soil degradation state after the abandon. In Mediterranean climatic conditions (between subhumid and semi-arid regimes), degraded soil recovery has followed different tendencies rendering the landscape in heterogenic and complex one. This heterogeneity has manifested in the vegetation pattern of abandoned fields. In this paper, we analyze the state of three abandoned fields situated under different Mediterranean climatic conditions from the recovery point of view by means of monitoring the effects of spatial and temporal variability of soil moisture in the vegetation pattern over a period of two years sampling (Nov. 2002–Nov. 2004). The results showed that: i) more annual rainfall volume did not guarantee success in the biological recovery of the system due to the influence of other factors such as the degradation state of the soil post-abandonment or the steep slope gradient; ii) soil moisture variability tended to play a more important role in affecting vegetal cover in semi-arid conditions; and iii) in dry climatic conditions the system demonstrated greater signs of recovery (greater biodiversity).     

The response of Aporrectodea rosea and Aporrectodea trapezoides (Oligochaeta: Lubricidae) to moisture gradients in three soil types in the laboratory

The question of whether the response of earthworms to soil moisture is governed by their reaction to soil wetness (moisture content) or to soil water energy (matric suction) was examined in two species of earthworm using moisture gradients in three contrasting soil types with clay contents varying from 4 to 39%. Gravimetric moisture gradients ranging over 5–30% were established in horizontal cores comprising 12 or 14 sections containing loosely packed soil. Earthworms were introduced to each section at the beginning of each experiment. The earthworms moved from sections containing dry soil into adjacent sections containing moister soil. Clear effects were evident after 6 h but these became more obvious after 96 h. For the earthworm Aporrectodea rosea, the threshold soil mositure level at which earthworms were induced to move away from dry soil was a matric suction of about 300 kPa (pF 3.4) and was independent of soil type. In contrast, for A. trapezoides, the threshold soil moisture varied with soil type (sandy loam 15 kPa, loam 25 kPa, clay 300 kPa). We conclude that, for the earthworm A. rosea, matric suction and not water content of soil provided the cue by which the earthworm recognized dry soil. For A. trapezoides, there was an interaction between matric suction and soil type in which the response of A. trapezoides to soil moisture varied with soil texture and the threshold for avoidance of dry soil ranged from a matric suction of 300 kPa (20% w/w) in clay to 15 kPa (10% w/w) in sandy loam.     

Dedicated to the 100th birthday of Professor Dr. Dr. h. c. mult. Eduard von Boguslawski

Identifying the optimum resolution where differences in corn (Zea mays L.) grain yields are detectable could theoretically improve nitrogen (N) management, thereby resulting in economic and environmental benefits for producers and the public at large. The objective of this study was to determine the optimum resolution for prediction of corn grain yield using indirect sensor measurements. Corn rows, 15–30 m long, were randomly selected at three locations where the exact location of each plant was determined. In 2005 and 2006, four of eight rows at each location were fertilized with 150 kg N ha^?1 as urea ammonium nitrate (28% N). A GreenSeeker? optical sensor was used to determine average Normalized Difference Vegetation Index (NDVI) across a range of plants and over fixed distances (20, 40, 45.7, 60, 80, 91.4, 100, 120, 140, 160, 180, 200, 220, and 240 cm). Individual corn plants were harvested and grain yield was determined. Correlation of corn grain yield versus NDVI was evaluated over both increasing distances and increasing number of corn plants. Then, the squared correlation coefficients (r_cc ²) from each plot (used as data) were fitted to a linear plateau model for each resolution treatment (fixed distance and number of corn plants). The linear-plateau model coefficient of determination (r_lp ²) was maximized when averaged over every four plants in 2004 and 2006, and over 11 plants in 2005. Likewise, r_lp ² was maximized at a fixed distance of 95, 141, and 87 cm in 2004, 2005, and 2006, respectively. Averaged over sites and years, results from this study suggest that in order to treat spatial variability at the correct scale, the linear fixed distances should likely be <87 cm or <4 plants as an optimum resolution for detecting early-season differences in yield potential and making management decisions based on this resolution.     

The effects of changes in soil moisture on nitrogen cycling in acid wetland types of the New Jersey Pinelands (USA)

Wetlands are subject to changes in soil moisture as a result of both short-term seasonal climate variations and long-term changes in regional water resource management, both of which can modify the dynamics of ground and surface water inputs. In the New Jersey Pinelands, forested wetlands that differ in both plant communities and soil structure occur along a topographic and hydrological gradient associated with an unconfined aquifer. Proposed groundwater withdrawals may affect water content of soils along this gradient. We hypothesized that prolonged changes in soil moisture would alter net nitrogen mineralization and nitrification rates in proportion to the amount of moisture change, and that these changes would be similar for the different soils along the drainage catena. Soils from two catenary sequences of wetlands, including pine-dominated (driest landscape position), hardwood-dominated, and Atlantic white-cedar-dominated (wettest landscape position) communities were used in long-term laboratory incubations (36 weeks). Production of NH₄⁺-N, NO₃⁻-N, and dissolved organic N were measured under two sets of conditions: constant moisture levels of 100%, 60% and 30% water-holding capacity (WHC), and fluctuating moisture levels (alternating 2 week periods at 100% and 30% WHC). In soils from most of the wetlands, we observed increases in net mineralization and nitrification when constant low-moisture conditions were established, but not under fluctuating conditions. Contrary to expectations, responses to the drying treatments varied between wetland types and between replicate wetlands of each type. Under constant-moisture conditions, nitrification increased more in cedar swamps than in either type of pine wetland. Under all conditions, soils from all the wetlands within one of the catenas produced more inorganic and organic soluble N than did the wetlands from the other catena, suggesting that area-wide effects are as important as wetland type in regulating production of soluble N. Within both catenas, pine-hardwood wetlands generated more soluble N under all moisture conditions than did either pine-dominated or cedar wetlands. Our results suggest that changes in soil moisture due to management of water resources will affect N cycling in wetland soils, but that the magnitude of the effects, and the potential for large releases of nitrate, will depend on the specific soil properties of affected wetlands.     

Biodiversity in soil ecosystems: the role of energy flow and community stability

In a series of community food webs from native and agricultural soils, we modeled energetics and stability, and evaluated the role of the various groups of organisms and their interactions in energy flow and community stability. Species were aggregated into functional groups based on their trophic position in the food webs. Energy flow rates among the groups were calculated by a model using observations on population sizes, death rates, specific feeding preferences and energy conversion efficiencies. From the energetic organization of the communities we derived the strengths of the mutual effects among the populations. These interaction strengths were found to be patterned in a way that is important to community stability. The patterning consisted of the simultaneous occurrence of strong top down effects at lower trophic levels and strong bottom up effects at higher trophic levels. These patterns resulted directly from the empirical data we used to parameterize the model, as we found no stabilizing patterns with random but plausible parameter values. Also, the impact of each individual interaction on community stability was established. This analysis showed that some interactions had a relatively strong impact on stability, whereas other interactions had only a small impact. These impacts on stability were neither correlated with energy flow nor with interaction strength. Comparison of the seven food webs showed that these impacts were sometimes connected to particular groups of organisms involved in the interaction, but sometimes they were not, which might be due to different trophic positions in the food webs. We argue that future research should be directed to answer the question which energetic properties of the organisms form the basis of the patterning of the interaction strengths, as this would improve our understanding of the interrelationships between energetics, community stability, and hence the maintenance of biological diversity.     

Behaviour of arbuscular-mycorrhizal fungi on Vigna luteola growth and its effect on the exchangeable (32P) phosphorus of soil

 The effect of the inoculation of Glomus mosseae and Scutellospora fulgida, singly or in mixed inocula, was tested on the growth and mycorrhizal characteristics of Vigna luteola. Soil depletion by mycorrhizal inoculation was determined as exchangeable ³²PO₄. Five treatments were performed: non-inoculated control (C); inoculated with a suspension of microorganisms free of mycorrhizae (M); inoculated with S. fulgida plus microorganisms (Sf+M); inoculated with G. mosseae plus microorganisms (Gm+M) and a treatment inoculated with both S. fulgida and G. mosseae plus microorganisms (Sf+Gm+M). G. mosseae was the most efficient fungus in promoting growth of V. luteola. This fungus produced higher shoot dry weight, P uptake in shoots (P_shoot) and P_shoot/arbuscules ratio than the S. fulgida inoculum, even though percent arbuscular-mycorrhizal (AM) colonization was similar for the two single AM-inoculated treatments. The highest value of isotopically exchangeable P was recorded for treatment M (P<0.05). In comparison with M, Gm+M treatment reduced the different P pools more than Sf+M treatment. C_p values were reduced to half by Gm+M and Sf+M treatments and were minimum for the combined treatment. Pools A (exchangeable phosphate between 1 min and 1 day) and B (exchangeable phosphate between 1 day and 3 months) were reduced by Gm+M treatment in contrast to Sf+M, which left them unchanged. The depression of isotopically exchangeable P for all pools obtained for the Sf+Gm+M treatment suggested that specific characteristics of fungi caused differences in P absorption, which in addition was altered by the possible interactions among them, when multiple inocula were used in a soil low in native P. Received: 29 January 1999