Pedogenic Thresholds and Soil Process Domains in Basalt-Derived Soils

Pedogenic thresholds occur where soil properties change abruptly and/or nonlinearly with a small increment in environmental forcing; soil process domains are the regions between thresholds where soils change much more gradually across a large range of environmental forcing. We evaluated thresholds and domains in basalt-derived soils on two rainfall gradients in Hawaii—one from 260 to 3,540 mm/y precipitation on 150,000-year-old substrate, the other from 600 to 3,760 mm/y on 4,100,000-year-old substrate. We identified thresholds associated with the initiation of biological uplift of nutrients at about 700 mm/y on the younger substrate, the depletion of primary minerals at about 2,100 mm/y on the younger and about 900 mm/y on the older substrate, and the initiation of anoxic conditions and associated Fe mobility at about 2,500 mm/y on the older substrate. These thresholds delineated process domains characterized by pedogenic carbonate accumulation and wind erosion (dry young substrate); by weathering and biological uplift of nutrients (intermediate rainfall young substrate and dry old substrate); by surface Fe enrichment and nutrient depletion (wet young substrate and intermediate rainfall old substrate); and by Fe mobilization and loss (wet old substrate). Soils on the older substrate were more highly weathered, lower in total and available P, and characterized by more crystalline clays than otherwise comparable soils on the younger substrate. Prior to European contact, Hawaiian cultivators developed an intensive rainfed agricultural system in the weathering/biological uplift domain on the younger substrate; we suggest that only this domain could support indigenous agricultural intensification in upland soils.     

What controls plant nutrient use in high elevation ecosystems?

The importance of rock-derived mineral nutrients (P, K, Mn, Mg, and Ca) in plant physiological function is well established. However, one important and relatively unexplored question is whether or not the same rules of plant nutrient use efficiency apply to these essential elements even if they are not limiting to primary production. We examined conifer growth and nutrient use dynamics across sites with contrasting geologies (sedimentary and volcanic) that vary in both rock-derived mineral nutrient and N availability. Differences in bedrock geochemistry generally corresponded to differences in available soil nutrients, such that the volcanic site tended to have greater available nutrients. Foliar nutrient concentrations reflected both differences in bedrock chemistry and indices of available soil nutrients for P, K, and Mn. Aboveground biomass production did not follow expected patterns and was greater for trees growing on low nutrient sites, but only with respect to the annual woody increment. Fine litter production did not differ between sites. Finally, we found evidence for trade-offs between two commonly examined components of nutrient use efficiency (NUE): nutrient productivity (A _n) and mean residence time of nutrients. However, we did not find evidence for higher plant NUE in soils with lower nutrient availability for N or rock-derived nutrients.     

Erosion and the Rejuvenation of Weathering-derived Nutrient Supply in an Old Tropical Landscape

Studies of long-term soil and ecosystem development on static geomorphic surfaces show that old soils become depleted in most rock-derived nutrients. As they are depleted, however, static surfaces also are dissected by fluvial erosion. This fluvial erosion leads to colluvial soil transport on the resulting slopes, which in turn can rejuvenate the supply of weathering-derived nutrients to plants. We evaluated the influence of erosion and consequent landscape evolution on nutrient availability along a slope on the Island of Kauai, near the oldest, most nutrient-depleted site on a substrate age gradient across the Hawaiian Islands. Noncrystalline minerals characteristic of younger Hawaiian soils increased from 3% of the soil on the static constructional surface at the top of the slope to 13% on the lower slope, and the fraction of soil phosphorus (P) that was occluded (and hence unavailable) decreased from 80% to 56% at midslope. Foliar nitrogen and P concentrations in Metrosideros polymorpha increased from 0.82% and 0.062% to 1.13% and 0.083% on the constructional surface and lower slope, respectively. The increase in foliar P over a horizontal difference of less than 250 m represents nearly half of the total variation in foliar P observed over 4.1 million years of soil and ecosystem development in Hawaii. The fraction of foliar strontium (Sr) derived from weathering of Hawaiian basalt was determined using ⁸⁷Sr:⁸⁶Sr; it increased from less than 6% on the constructional surface to 13% and 31% on lower slope and alluvial positions. Erosional processes increase both nutrient supply on this slope and the fine-scale biogeochemical diversity of this old tropical landscape; it could contribute to the relatively high level of species diversity observed on Kauai.     

Sulphate,dissolved organic carbon,nutrients and terminal metabolic products in deep pore waters of an intertidal flat

This study addresses deep pore water chemistry in a permeable intertidal sand flat at the NW German coast. Sulphate, dissolved organic carbon (DOC), nutrients, and several terminal metabolic products were studied down to 5 m sediment depth. By extending the depth domain to several meters, insights into the functioning of deep sandy tidal flats were gained. Despite the dynamic sedimentological conditions in the study area, the general depth profiles obtained in the relatively young intertidal flat sediments of some metres depth are comparable to those determined in deep marine surface sediments. Besides diffusion and lithology which control pore water profiles in most marine surface sediments, biogeochemical processes are influenced by advection in the studied permeable intertidal flat sediments. This is supported by the model setup in which advection has to be implemented to reproduce pore water profiles. Water exchange at the sediment surface and in deeper sediment layers converts these permeable intertidal sediments into a “bio-reactor” where organic matter is recycled, and nutrients and DOC are released. At tidal flat margins, a hydraulic gradient is generated, which leads to water flow towards the creekbank. Deep nutrient-rich pore waters escaping at tidal flat margins during low tide presumably form a source of nutrients for the overlying water column in the study area. Significant correlations between the inorganic products of terminal metabolism (NH₄ ⁺ and PO₄ ³⁻) and sulphate depletion suggest sulphate reduction to be the dominant pathway of anaerobic carbon remineralisation. Pore water concentrations of sulphate, ammonium, and phosphate were used to elucidate the composition of organic matter degraded in the sediment. Calculated C:N and C:P ratios were supported by model results.     

Sedimentary and biofacies records in calciturbidites at the Devonian-Carboniferous boundary in Moravia (Moravian-Silesian Zone, Middle Europe)

Summary At the Devonian/Carboniferous boundary, major climatic and oceanographic changes influenced sedimentation on carbonate platforms and in peri-platfrom asreas. Three deep-water carbonate successions in Moravia, which were selected to represent different paleotectonic settings, have been studied with the aim of testing the influence of eustatic, climatic and tectonic controls on sedimentation and conodont paleoecology and taphonomy. On the slopes of the wide carbonate platforms of the Moravian Karst Development (Lesní lom and Grygov sections), an exemplary highstand shedding systems developed in the upper Famennian (expansa Zone), marked by a pronounced thickness of their respective calciturbidite successions and an abundance of shallow-water skeletal grains.Palamatolepis— andBispathodus-dominated conodont assemblages contain an admixture ofPolygnathus representing a transported, near-shore component. The eustatic sea-level fall in the praesulcata Zone and the lowstand conditions at the D/C boundary resulted in a decline of carbonate platform production and condensed deposition or nondeposition. In the Lesní lom section, a condensed sequence of turrbiditic calcarenites and shales (Middle praesulcata—lowermost sulcata Zone) was followed by lime mud calciturbidites (sulcata and duplicata Zones). In the conodont assemblages, the first event in the Lower praesulcata Zone was associated with the reduction of ‘mesopelagic’Palmatopic and a bloom of epipelagicPolygnathus communis. The second event in the Middle praesulcata Zone corresponds to the onset of polygnathidprotogranthodid biofacies, indicating a carbonate slope environment. In the Grygov section, a pronounced thickening and upward-coarsening succession of tubiditic calcilutites through calcarenites and intraclast breccias, with poor palmatolepid-bispathodid connodont assemblages (expansa Zone), indicates a progradation of the calciturbidite system associated with sea-level highstand. After a break in sedimentation, covering the interval from the Lower praseulcata to the base of Lower crenulata Zone, thick-bedded, fine-grained calciturbidites were deposited in the Lower crenulata Zone, and are associated with poor, mixed assemblages where siphonodellids and polygnathids predominate. At the isosticha-Upper crenulata/Lower typicus boundary, coasre grained, turbiditic calcarenites and breccias rich in clastic quartz grains and mixed conodont assemblages with reworked Frasnian and Famennian conodonts indicate a deep erosion of the source area, presumably due totectonic uplift (relative lowstand). In the Jesenec section, on the flanks of the volcanic seamount (the Drahany Development), a deep-water Upper Famennian condensed succession of calciturbidites and presumably winnowed pelagic limestones is marked by conodont assemblages of palmatolepid-bispathodid biofacies. More proximal calciturbidites with mixed deep-water and shallowwater conodonts prograde at the top of the Upper Famennian succession (Middle to Upper expansa Zone). A striking hiatus, covering the interval from the Early preaesulcata to the base of Lower crenulata Zone, resulted from extreme condensation and submarine bottom current erosion due to sea-level lowstand in the late Famennian and early Tournaisian. The renewed middle Tournaisian calciturbidite sedimentation with strong evidence of erosion at the source area indicates global eustatic rise and tectonic uplift of the Drahany Development seamounts (relative lowstand). The earlier occurrence of the uplift in the Jesenec area, relative to the Grygov section, shows the advance of tectonic processes over time in the Moravian-Silesian basin (orogenic polarity) as a consequence of Variscan orogenic movements.     

Nutrient dynamics on a precipitation gradient in Hawai'i

We evaluated soil and foliar nutrients in five native forests in Hawai'i with annual rainfall ranging from 500 mm to 5500 mm. All of the sites were at the same elevation and of the same substrate age; all were native-dominated forests containing Metrosiderospolymorpha Gaud. Soil concentrations of extractable NO₃-N and PO₄-P, as well as major cations (Ca, Mg, and K), decreased with increasing annual precipitation, and δ¹⁵N values became more depleted in both soils and vegetation. For M.polymorpha leaves, leaf mass per area (LMA) and lignin concentrations increased significantly, while δ¹³C values became more depleted with increasing precipitation. Foliar phosphorus, and major cation (Ca, Mg, and K) concentrations for M.polymorpha all decreased significantly with increasing precipitation. For other native forest species, patterns of LMA, δ¹³C, and δ¹⁵N generally mirrored the pattern observed for M. polymorpha. Decreasing concentrations of available rock-derived nutrients in soil suggest that the effect of increased rainfall on leaching outweighs the effect of increasing precipitation on weathering. The pattern of decreased foliar nutrient concentrations per unit leaf area and of increased lignin indicates a shift from relatively high nutrient availability to relatively high carbon gain by producers as annual precipitation increases. For nitrogen cycling, the pattern of higher inorganic soil nitrogen concentrations in the drier sites, together with the progressively depleted δ¹⁵N signature in both soils and vegetation, suggests that nitrogen cycling is more open at the drier sites, with smaller losses relative to turnover as annual precipitation increases. Received: 24 March 1997 / Accepted: 19 September 1997     

Linking chronosequences with the rest of the world: predicting soil phosphorus content in denuding landscapes

Results from chronosequences from the arctic to the tropics show that phosphorus (P) availability, total P, and the fraction of bedrock-derived P remaining in soil diminishes as soils age. Thus we predict that ecosystems mantling old substrates are likely to have low available P. Yet there are myriad examples in the biogeochemical literature where the results from chronosequences are used to argue the reverse, and ecosystems observed to be P poor are assumed to mantle an old substrate. This premise is difficult to test, for while the concept of substrate age is useful on uneroded surfaces that formed at a particular time, it becomes obscured in denuding landscapes, where substrate ages instead reflect the rates of rock weathering, denudation and mixing of dust into soil. Here we explore this premise as it relates to one of the most ubiquitous assumptions in the biogeochemical literature: that the differences in nitrogen (N) and P cycling between temperate and tropical regions are driven by gradients in substrate age. We build a conceptual framework for quantifying the fraction of parent material P remaining in soil ([SoilP]/[RockP]), by estimating P inputs (rock weathering and dust deposition) and outputs (P leaching). We parameterize our model with spatially explicit (0.5°) estimates of global denudation, weathering zone thickness, and P deposition. To test the assumption that latitudinal gradients in P status are the result of soil age, we apply a single P loss rate, derived from a humid tropical system in the Hawaiian Islands, to our spatially explicit map of soil residence times. Surprisingly, in this formulation, we find only a modest latitudinal gradient in soil P depletion, with mean depletion values in the humid tropics <2× greater than in the previously unglaciated humid temperate zone. This small latitudinal gradient in P depletion is unlikely to be sufficient to drive the observed differences in tropical vs. temperate ecosystem stoichiometry (e.g. trends in foliar and litter N:P). Thus our results suggest that, to the extent P depletion is greater in the tropics, the appropriate conceptual model for attributing causation may not be one of a chronosequence where time is the primary driver of P loss. We hypothesize that the covariation of inferred P availability with latitude may be strongly controlled by latitudinal changes in rates of P leaching and occlusion, rather than gradients in substrate age.     

Dust as a Nutrient Source for Fynbos Ecosystems,South Africa

Abstract Fynbos is the main vegetation of the Cape Floristic Region, a biodiversity hotspot that occurs in southwestern South Africa. A major question concerning the fynbos ecosystem is how it supports abundant and diverse vegetation on soils derived from nutrient-poor bedrock. In addition to marine aerosols (recycled sea salts), geochemical analyses reported here suggest that dust (aeolian) deposition represents a significant source of nutrients (for example, K, Ca and Zn) to the fynbos ecosystem. Headwater portions of the Boontjies River sub-catchment near the Cederberg Mountains support mountain fynbos communities that are entirely underlain by the Peninsula Formation, a quartz arenite with greater than 98 wt% SiO₂. Fynbos soils in these areas are composed of quartzose sand with 3–6 wt% kaolinitic clay and 1–2 wt% organic carbon. The minor amount of feldspar and mica minerals in the bedrock (0.5 wt% Al₂O₃) suggests an aeolian source for much of the clay minerals in the soil. The isotope composition of soluble Pb and Sr from fynbos vegetation and soils indicates a mixture of anthropogenic and terrigenous sources, most likely from washout of combusted petrol and dust from the arid interior particularly in association with Berg Wind events. Approximate mass balance calculations indicate that washout of aerosols provides an important source of nutrients such as Ca, K, P, Fe, Mn and Zn which the fynbos ecosystem is highly effective in retaining.     

Managing soils for long-term productivity

Meeting the goal of long-term agricultural productivity requires that soil degradation be halted and reversed. Soil fertility decline is a key factor in soil degradation and is probably the major cause of declining crop yields. There is evidence that the contribution of declining soil fertility to soil degradation has been underestimated. <br>Sensitivity to soil degradation is implicit in the assessment of the sustainability of land management practices, with wide recognition of the fact that soils vary in their ability to resist change and recover subsequent to stress. The concept of resilience in relation to sustainability requires further elaboration and evaluation.<br>In the context of soil degradation, a decline in soil fertility is primarily interpreted as the depletion of organic matter and plant nutrients. Despite a higher turnover rate of organic matter in the tropics there is no intrinsic difference between the organic matter content of soils from tropical and temperate regions. The level of organic matter in a soil is closely related to the above and below ground inputs. In the absence of adequate organic material inputs and where cultivation is continuous, soil organic matter declines progressively. Maintaining the quantity and quality of soil organic matter should be a guiding principle in developing management practices.<br>Soil microbial biomass serves as an important reservoir of nitrogen (N), phosphorus (P) and sulphur (S), and regulates the cycling of organic matter and nutrients. Because of its high turnover rate, microbial biomass reacts quickly to changes in management and is a sensitive indicator for monitoring and predicting changes in soil organic matter. Modelling techniques have been reasonably successful in predicting changes in soil organic matter with different organic material inputs, but there is little information from the tropics. <br>Nutrient depletion through harvested crop components and residue removal, and by leaching and soil erosion accentuates the often very low inherent fertility of many soils in the tropics. An integrated approach involving inorganic and organic inputs is required where animal and plant residues are returned, as far as practicable. Chemical fertilizers alone cannot achieve long-term productivity on many soils and organic material inputs are required to maintain soil organic matter levels and crop productivity. A major research effort is required to develop improved strategies for halting and reversing soil degradation if long-term productivity is to be secured. <br>     

福建红壤区不同侵蚀强度马尾松林地土壤营养元素变化特征

采用野外调查和室内分析的方法对福建红壤区不同侵蚀强度马尾松林地土壤营养元素动态进行研究,以揭示红壤侵蚀区不同侵蚀强度对马尾松林地土壤营养元素的影响.结果表明:除速效磷以外,侵蚀强度显著改变了土壤有机质、碱解氮、全钾、速效钾的含量以及土壤pH.土壤有机质和碱解氮在中度侵蚀马尾松林地含量最高,随着侵蚀强度的增加植被覆盖减少,土壤有机质和碱解氮显著降低.土壤全钾和速效钾含量随着侵蚀强度的增加而显著增加,土壤速效磷随着侵蚀强度的增加没有显著的变化.土壤pH值随着土壤侵蚀强度和土壤深度的增加而显著增加.皮尔逊相关关系分析表明:有机质与碱解氮、全钾和pH显著相关.灰色关联度分析表明:除侵蚀强度对营养元素和pH有显著影响外,坡度对土壤营养元素和pH影响较大;覆盖度对有机质、碱解氮和速效磷影响较大.因此,植被恢复和植被生境的综合治理是红壤侵蚀区生态恢复与重建的有效途径.     

Occurrence of Tuber melanosporum in relation to soil surface layer properties and soil differentiation

An intensive survey was carried out on a 12-year-old experimental truffle bed of Tuber melanosporum Vitt. located in the central Apennines. The aim of the investigation was to relate the presence and carpophore production of T. melanosporum to changes in soil structure, aeration and fertility — expressed in terms of 0.25–2.00 mm aggregate fraction, total organic carbon, DTPA-extractable Mn and host plant height — and to determine if these modifications, whenever present, could be ascribed to soil differentiation within the truffle bed. The occurrence of pianelli — i.e. areas with little herbaceous ground cover created by T. melanosporum — showed a close relationship with host plant height and aeration of soil surface layers. Where pianelli occurred, the height of symbiont trees increased and the content of reduced Mn, indicating the presence of a well-aerated soil environment, decreased. The variation of host plant height was attributable not only to the increased absorption of nutrients related to the ectomycorrhizal partnership, but also to soil differentiation. The soils of the investigated area were characterized by a relatively low slope gradient, a rigid framework of gravel and a homogeneous physico-chemical behaviour, due to the predominance of Ca among exchangeable bases. In these environmental conditions, T. melanosporum was present in the rather thick soil belonging to Typic Rendolls, whereas it was absent in the area characterized by thin Lithic Rendolls. In the latter case, the plant cover was probably too scarce to protect T. melanosporum from summer dryness, and consequently the more resistant T. aestivum species prevailed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contribution of the Western Australian wheatbelt termite,Drepanotermes tamminensis (Hill), to the soil nutrient budget

The role of soil modification by the mound-building termite,Drepanotermes tamminensis (Hill), was studied during 1991 in the Durokoppin Nature Reserve, Western Australia. Soil chemical parameters were quantified for ‘soils’ in nests and for surrounding soil in both a Wandoo (Eucalyptus capillosa) woodland and a Casuarina (Allocasuarina campestris) shrubland plot. All ‘soils’ in nests were more acidic than the surrounding soil within each study plot. Generally, nutrient levels in the nested soils were higher than the un-nested soil within each study plot and were also higher in the woodland than in the shrubland plot. Depending on the nuttient concerned, the nested soil contained between 0.3 and 21.9% of the total nutrient load per hectare within each study plot. The quantities of nutrients per hectare in termite mounds were higher in the woodland than in the shrubland plot. It is concluded that mounds of this species of termite form a significant bank of nutrients, although time for release of such nutrients depends on the degree of erosion and on the longevity of mounds.     

Nitrogen and phosphorus budgets for a tropical watershed impacted by agricultural land use: Guayas, Ecuador

Large-scale changes in land use are occurring in many tropical regions, with significant impacts on nitrogen and phosphorus biogeochemistry. In this study we examine the relationships between land use, anthropogenic nutrient inputs, and riverine nutrient exports in a major agricultural watershed of the Pacific coast of South America, the Guayas River basin of Ecuador. We present comprehensive nutrient budgets for nitrogen (N) and phosphorous (P) for the Guayas River basin and 10 sub-watersheds. We quantify the four major anthropogenic nutrient fluxes into and out of the region: N and P fertilizer application, N fixation by leguminous crops, net import/export of N and P in agricultural products (food and feed), and atmospheric deposition. We also estimate inputs of N from biological N fixation in forests and of P from weathering sources in soils and bedrock. The sum of these sources represents net inputs of N and P to each watershed region. Overall, synthetic fertilizers are the largest input to the Guayas Basin for N (53%) and P (57%), and the largest outputs are N and P in crops. Losses of N and P in river export account for 14–38% of total N and P inputs, and there is significant accumulation of N and P, or unmeasured forms of N and P export, in most of the sub-basins. Nutrient balances are indicative of the sustainability of land use practices in a region, where a negative balance of N or P indicates nutrient depletion and subsequent loss of soil fertility, yield, and economic viability. Although the nutrient balance of the entire Guayas Basin is positive, there are negative or near zero balances in two sub-watersheds with extensive banana, coffee and permanent crops. In these basins, degradation of soil quality may be occurring due to these net nutrient losses. Our data show that nutrients are leaving the basin primarily as export crops, with riverine losses of nutrients smaller than crop exports. Nonetheless, there is a direct relationship between nutrient inputs and river outputs, suggesting that agricultural management practices in the basin may have direct impacts on N and P delivery to the highly productive Guayas estuary.     

Nitrogen-based symbioses,phosphorus availability,and accounting for a modern world more productive than the Paleozoic

Evolution of high-productivity angiosperms has been regarded as a driver of Mesozoic ecosystem restructuring. However, terrestrial productivity is limited by availability of rock-derived nutrients such as phosphorus for which permanent increases in weathering would violate mass balance requirements of the long-term carbon cycle. The potential reality of productivity increases sustained since the Mesozoic is supported here with documentation of a dramatic increase in the evolution of nitrogen-fixing or nitrogen-scavenging symbioses, including more than 100 lineages of ectomycorrhizal and lichen-forming fungi and plants with specialized microbial associations. Given this evidence of broadly increased nitrogen availability, we explore via carbon cycle modeling how enhanced phosphorus availability might be sustained without violating mass balance requirements. Volcanism is the dominant carbon input, dictating peaks in weathering outputs up to twice modern values. However, times of weathering rate suppression may be more important for setting system behavior, and the late Paleozoic was the only extended period over which rates are expected to have remained lower than modern. Modeling results are consistent with terrestrial organic matter deposition that accompanied Paleozoic vascular plant evolution having suppressed weathering fluxes by providing an alternative sink of atmospheric CO₂. Suppression would have then been progressively lifted as the crustal reservoir's holding capacity for terrestrial organic matter saturated back toward steady state with deposition of new organic matter balanced by erosion of older organic deposits. Although not an absolute increase, weathering fluxes returning to early Paleozoic conditions would represent a novel regime for the complex land biota that evolved in the interim. Volcanism-based peaks in Mesozoic weathering far surpass the modern rates that sustain a complex diversity of nitrogen-based symbioses; only in the late Paleozoic might these ecologies have been suppressed by significantly lower rates. Thus, angiosperms are posited to be another effect rather than proximal cause of Mesozoic upheaval.     

Quantitative effects of vegetation cover on wind erosion and soil nutrient loss in a desert grassland of southern New Mexico,USA

Wind is a key abiotic factor that influences the dynamics of arid and semiarid systems. We investigated two basic relationships on vegetation manipulation (grass cover reduction) plots at the Jornada Experimental Range in southern New Mexico: (1) wind erosion rates (horizontal mass flux and dust emission) versus vegetative cover, and (2) nutrient loss versus vegetative cover. The results indicate that wind erosion rates and nutrient loss by dust emission are strongly affected by plant cover; however, the importance of shrubs and grasses in reducing dust flux may not be equal. The dramatic increase of wind erosion between 75% grass cover reduction and 100% grass cover reduction suggests that sparsely distributed mesquites are relatively ineffective at reducing wind erosion and nutrient loss compared to grasses. Comparisons of nutrients between surface soils and wind blown dust indicate that aeolian transport is a major cause for the loss of soil nutrients in susceptible environments. We found that increased aeolian flux over three windy seasons (March 2004–July 2006) removed up to 25% of total organic carbon (TOC) and total nitrogen (TN) from the top 5 cm of soil, and about 60% of TOC and TN loss occurred in the first windy season (March–July 2004). The balance between net loss of nutrients by aeolian processes and the addition of nutrients by biotic processes changed from negative (net loss) to positive (net accumulation) between 50% grass cover reduction and 25% grass cover reduction. The estimated lifetime of surface soil TOC and TN of about 10 years on the plot with 100% grass cover reduction indicates that impacts of wind erosion on soil resources can occur on very short timescales.     

Assessment of phosphorus leaching losses from arable land

Phosphorus (P) losses from soil to water by erosion and surface runoff have been much studied and quantified. However, P losses by leaching have received much less attention, mainly because, until recently, the quantities involved were not considered to be of environmental significance. Furthermore, P leaching losses, unlike P losses from erosion or surface runoff were not believed to be related to rates of P addition, as inorganic fertilizer or manures. Here we report results from a number of field and laboratory experiments, designed to assess the significance of P leaching losses from soil to water. Annual cumulative total P losses in drainage waters from four UK field sites ranged from about 0.03 to 5 kg P ha⁻¹ during 2001–2002. Molybdate reactive P ranged from 45–57%, soluble organic P from 10–13% and particulate P from 29–45% of total P on the two sites (Broadbalk and Woburn) where they could be accurately measured. The proportions of these different P forms were comparable in all treatments, including drainage waters from the unfertilised soils and soils receiving long-term applications of farmyard manure or inorganic fertilizer. In all soils, there was indication of an Olsen- (0.5 M NaHC0₃-, pH 8.5) extractable P concentration, (termed the Change-Point), where P measured in field drainage waters or in laboratory soil extracts of 0.01 M CaCl₂ began to increase linearly as Olsen-P increased. There was also some agreement between drainage water-P or CaCl₂-P and the Olsen-P concentration where the Change-Point occurred. This suggests that CaCl₂-extractable P may provide an approximate indicator of soil P concentrations above which significant quantities of P may be lost by leaching under field conditions. There were positive linear relationships between soil dithionate-extractable Al and soil organic C with the Change-Point: [Change Point = [(0.049)[Al3+] minus (9.2)(% organic C)] accounting for 93% of the variance in the data. If this relationship holds under further testing it could well be a useful predictor of Change-Points in different soils. Phosphate sorption isotherms were used to study the soil P concentrations above which P was at risk of moving from soil to water. They showed that soil solution P concentrations were significantly lower between pH 6.9–7.2 than between pH 7.7–8.1, with implications for P loss from soil to water.     

Response of changes in soil nutrients to soil erosion on a purple soil of cultivated sloping land

Severe soil erosion of cultivated sloping land in hilly areas of Sichuan, China, has resulted in deterioration of soil quality, and therefore has an adverse impact on crop production. A hillslope of 110 m in length was selected with a slope steepness of 10.12% where the soils were classified as Regosols. Soil samples for determining ¹³⁷Cs, soil organic matter (SOM), total N, P, K, available N, P, K and particle size fraction were collected at 10 m intervals along a transect of the hillslope. Loss of soil nutrients owing to soil erosion was studied by using ¹³⁷Cs technique, and the relationships between ¹³⁷Cs-derived soil redistribution rates and soil nutrients were established over the cultivated sloping land in hilly areas of Sichuan, China (30o26′N, 104o28′E). The values of SOM, total N, available N, P, K and the soil particle fractions of size < 0.002 mm were smaller at upper and middle slope positions where ¹³⁷Cs inventories were lower (i.e., soil erosion rates were higher) than at downslope positions where ¹³⁷Cs inventories were higher (i.e., soil erosion rates were lower). The lowest ¹³⁷Cs inventories were found at the hilltop, showing that besides erosion owing to water flow, tillage also contributed to soil losses, and intensive tillage was mostly responsible for severe erosion at upper slope positions. There were significant differences in SOM, total N, available N, P, K and the soil particle fractions of size < 0.002 mm between different slope segments, and these properties were significantly correlated with slope length. These soil properties were also significantly correlated with ¹³⁷Cs inventories, indicating that both ¹³⁷Cs and nutrient concentrations varied with topographical changes. The variation in soil properties was strongly influenced by erosion-induced soil redistribution, and therefore ¹³⁷Cs inventories mirroring soil redistribution rates would be considered as an integrated indicator of soil quality.     

Nature of interference potential of leaf debris of Ageratum conyzoides

The present study investigated the allelopathic interference of leaf debris of Ageratum conyzoides (billy goat weed; Asteraceae)—a weed of cultivated land—against rice (Oryza sativa). Seedling length and dry weight of rice were significantly reduced (16–20%) in soil from A. conyzoides infested fields compared to the soil from an area devoid of the weed. It indicated the presence of certain phytotoxins in the A. conyzoides infested soil. To explore the possible contribution of the weed in releasing these phytotoxins, growth studies involving leaf debris extracts and amended soils (prepared by incorporating leaf debris—5, 10, 20 g kg⁻¹ soil, w/w, or its extracts—0.5%, 1.0% and 2.0%, v/v) were conducted. The growth of rice was severely inhibited in A. conyzoides leaf debris- and debris extract-amended soils compared to unamended control soil. A significant amount of water-soluble phenolics, the potent phytotoxins, was found in the A. conyzoides infested soil, leaf debris, and debris-amended soils. These phenolics were identified as gallic acid, coumalic acid, protocatechuic acid, catechin and p-hydroxybenzoic acid. Among these, protocatechuic acid was in the maximum amount (35.72%) followed by coumalic acid (33.49%) and these two accounted for >69% of total phenolic compounds. Further, there was a significant increase in the available nutrient content in soil amended with A. conyzoides leaf debris thus ruling out the possibility of any resource depletion upon residue incorporation and their negative role in causing growth reduction. Based on the observations, the present study concludes that leaf debris of A. conyzoides deleteriously affects the early growth of rice by releasing water-soluble phenolic acids into the soil environment and not through soil nutrient depletion.     

Soil organic carbon budget and fertility variation of black soils in Northeast China

Black soils in Northeast China are characteristic of high soil organic carbon (SOC) density and were strongly influenced by human activities. Therefore, any change in SOC pool of these soils would not only impact the regional and global carbon cycle, but also affect the release and immobilization of nutrients. In this study, we reviewed the research progress on SOC storage, budget, variation, and fertility under different scenarios. The results showed that the organic carbon storage of black soils was 646.2 TgC and the most potential sequestration was 2887.8 g m⁻². According to the SOC budget, the net carbon emission of black soils was 1.3 TgC year⁻¹ under present soil management system. The simulation of CENTURY model showed that future climate change and elevated CO₂ concentration, especially the increase of precipitation, would increase SOC content. Furthermore, fertilization and cropping sequence obviously influenced SOC content, composition, and allocation among different soil particles. Long-term input of organic materials such as manure and straw renewed original SOC, improved soil structure and increased SOC accumulation. Besides, soil erosion preferred to transport soil particles with low density and fine size, decreased recalcitrant SOC fractions at erosion sites and increased activities of soil microorganism at deposition sites. After natural grasslands were converted into croplands, obvious variation of soil chemical nutrients, physical structure, and microbial activities had taken place in surface and subsurface soils, and represented a degrading trend to a certain degree. Our studies suggested that adopting optimal management such as conservation tillage in black soil region is an important approach to sequester atmospheric CO₂ and to slow greenhouse effects.     

Rapid, plant-induced weathering in an aggrading experimental ecosystem

To evaluate whether rates of weathering of primary minerals are underestimated in watershed mass-balance studies that fail to include products of weathering accumulating in plants and in developing soil, changes in the calcium and magnesium content of vegetation and soil fractions were measured in large, monitored lysimeters (sandbox ecosystems) at Hubbard Brook Experimental Forest, New Hampshire. Weathering was evaluated over 4–8 yr in sandboxes planted with red pine (Pinus resinosa Ait.) and kept mostly free of vegetation (nonvegetated). Three mass-balance equations were used that cumulatively include (a) Ca and Mg in precipitation inputs and drainage outputs, (b) accumulation of Ca and Mg in vegetation, and (c) changes in products of weathering in soils. Soil products were evaluated with an extraction process designed to avoid removing ions from primary minerals. Relative to the input-output equation, the estimated rate of weathering increased 2.4 (Ca) and 1.8 (Mg) times when accumulation of Ca and Mg in pine biomass was accounted for, and 8 (Ca) and 23 (Mg) times when changes in soil products were also included. Weathering estimates that included accumulation in vegetation and soil products were 261 (Ca) and 92 (Mg) kg ha^-1 yr^-1 in the pine sandbox. These rates were 10 (Ca) and 18 (Mg) times higher than the rates in the nonvegetated sandbox, which were not significantly greater than zero. This study raises the possibility that weathering can play a significant role in the release of nutrients available to plants over short periods. Faster rates like this become extremely important where managers are trying to balance nutrients available to plants from precipitation and weathering release with outputs including harvest removals.