Distribution Of Soil Nutrients Research Articles

Soil Carbon and Nitrogen Dynamics During Conversion of Agricultural Lands to Natural Vegetation in Central Korea

Changes in land use can affect the distribution and cycling of soil organic matter and nutrients. Soil organic carbon (C) and nitrogen (N) concentrations, soil CO2 evolution, litter decomposition, and soil N availability under the three types of conversion of agricultural lands to natural vegetation (rice field conversion to forest, crop field conversion to shrub, and indigenous forest) in central Korea after 20 years of fallow were compared. There was no significant change in soil organic C and N concentrations among the three types at the three soil depths, and soil organic C and N increases were confined to the topsoil (0–10 cm). Soil CO2 evolution tracked seasonal soil temperature, however, there was no difference among conversion types. Annual soil CO2 evolution (Mg CO2/ha/yr) was 36.0±3.7 for the rice field conversion to forest site, 33.8±4.1 for the crop field conversion to shrub site, and 40.7±4.1 for the indigenous forest site, respectively. After decomposing for one year, 55.8% for the rice field conversion site, 64.9% for the crop field conversion site, and 66.3% for the indigenous forest site of the original leaf litter mass remained. The soil moisture condition seemed to limit the rate of decomposition in the study area. Resin concentration was higher at the rice field conversion site.

Long‐term, livestock‐mediated redistribution of nitrogen and phosphorus in an East African savanna

Summary 1. The effect of livestock on African rangelands has been a major focus of recent research, but little attention has been paid to the way livestock affects the distribution and availability of soil nutrients. In East African savannas, overnight containment of livestock in thorn-scrub corrals or ‘bomas’ concentrates large quantities of nutrients into small areas, potentially altering the landscape distribution of nitrogen (N ) and phosphorus ( P) in soils and plants. 2. This study was designed to (i) measure the density, turnover rates and soil nutrient concentrations of abandoned cattle bomas on nutrient-poor rangeland in central Kenya; (ii) determine whether long-term glades dominated by Cynodon plectostachyus are derived from abandoned bomas; and (iii) evaluate the effect of cattle bomas on the landscape-level distribution of N and P. 3. In the study area, glades (> 39 years old) averaged 0·71 ha in size and occurred at a density of 0·71 km − 2 . Abandoned bomas (1‐39 years since abandonment) averaged 0·39 ha and occurred at a density of 1·21 km − 2 . During 1961‐2000, no glades reverted to bushland vegetation, while 53 bomas were abandoned. 4. All characteristics of soils measured across a boma‐glade chronosequence indicated glades were indeed derived from abandoned bomas. Soil N, P and organic matter quality in the surface (0‐15 cm) layer were similar for glades and 30‐39-year-old bomas, but were significantly enriched relative to surrounding bushland. In contrast, at 40‐65 cm depth beneath bomas, glades and bushland, soil N was similar. The texture of surface soils from bomas, glades and bushland was similar, indicating glades were not derived from a unique parent material. 5. Leaves of C. plectostachyus from 12‐24-year bomas and long-term glades were enriched in P, calcium (Ca) and N relative to leaves of Cynodon dactylon from nearby bushland sites. In particular, P in boma and glade grass was above recommended levels for growing and lactating livestock, while P content of bushland grass was lower than recommended levels. 6. Cattle management via bomas exerts a greater effect on the distribution of P relative to N within the landscape. For cattle grazing an area of 20‐25 km 2 boma − 1 , an estimated 0·24‐0·30 g N m − 2 year − 1 is removed from the rangeland and deposited into bomas. Within 1·5 years of boma abandonment, 70% of this N is already lost from the manure and upper soil layer. Permanent N loss does occur via leaching, but the majority is probably volatilized and redistributed in rainfall. N deposition in rainfall (0·43 g N m − 2 year − 1 ) is more than sufficient to offset losses due to cattle grazing and deposition in bomas. In contrast, P deposited in bomas is more tightly retained, creating small P-enriched ‘hotspots’ while causing a permanent loss of the order of 0·021‐0·026 g P m − 2 year − 1 from the surrounding bushland landscape. 7. Synthesis and applications. Results indicate that abandoned bomas persist as nutrientenriched patches for at least four decades. Rangeland managers should recognize that the placement and relocation rate of current bomas influences the long-term distribution and availability of nutritionally important forage for livestock and wildlife. Future assessments of African rangeland stability should incorporate not only direct effects of

The relationship between animal foraging and nutrient patchiness in south-west Australian woodland soils

The woylie (Bettongia penicillata) was once common and abundant over the southern third of the Australian continent. Since European settlement the range of this rat-kangaroo has become reduced by more than 97%, and until the early 1990s, only 3 small natural populations remained, all in south-western Australia. These medium-sized (c. 1 kg) marsupials create a large number of diggings as they forage for the hypogeous fruiting bodies of ectomycorrhizal fungi upon which they feed. The effect of such foraging activity on the availability of plant nutrients in the vicinity of such diggings was evaluated in simulated digging experiments. Available nitrate, ammonium, and sulfur decreased significantly 3 years after diggings were constructed and had filled in, whereas phosphorus, potassium, iron, and organic carbon remained unchanged. The results suggest that preferential water infiltration via woylie diggings leads to a decrease in those soil nutrients that are susceptible to leaching and indicates that digging vertebrates may influence the distribution of surface soil nutrients.

Effect of tree canopy on the spatial distribution of soil nutrients in a Mediterranean Dehesa

In arid and semi-arid ecosystems, isolated trees have an important effect on soil properties, which can determine the structure and function of the herbaceous and soil fauna communities in these types of ecosystems. The effect of the presence of a tree canopy on the spatial properties of soil nutrients were studied in this work. The hypothesis stated that the effect of the tree on the soil will be different for nutrients with varied biogeochemical characteristics. Nutrients such as N, which is retained in the ecosystem by biological mechanisms, must have a spatial distribution that is different from P and K, involved in geochemical processes (they are part of soil minerals), and in turn have a different distribution from non essential elements for plants such as Na and Li. To test this hypothesis, soil samples were taken under and around a tree in a Mediterranean Dehesa ecosystem, characterized by the presence of isolated trees within a herbaceous matrix. The samples were taken every 2 m and then at 50 cm intervals in a regular grid of 20×20 m around a holm oak (Quercus ilex), in order to analyse the spatial properties, using geostatistical techniques. All nutrients showed different patterns of spatial distribution. Mineral-N (NH4+NO3), like soil organic matter, presented higher concentrations which coincided with the tree canopy. The highest P concentrations occupied a space which was beyond the canopy projection. Potassium showed a lower spatial structure, with high concentrations which coincided with the position of the holm oak trunk, while Na and Li showed a sparse spatial structure and their distribution seems to be independent of the tree position. These results support the hypothesis that the presence of a tree differentially affects the spatial distribution of the various nutrients, depending on their biogeochemical characteristics.

Soil phosphorus heterogeneity and mycorrhizal symbiosis regulate plant intra-specific competition and size distribution

We investigated the interactive effects of soil phosphorus (P) heterogeneity, plant density and mycorrhizal symbiosis on plant growth and size variability of Trifolium subterraneum. We set up mesocosms (trays 49×49cm and 12cm deep) with the same amount of available P, but distributed either homogeneously or heterogeneously, in randomly arranged cells (7×7cm each) with high or low available P. The trays were planted with either 1 or 4 seedlings of T. subterraneum per cell. Half of the trays were inoculated with spores of the mycorrhizal fungus Gigaspora margarita. We harvested the plants when leaves just started to overlap, 8 weeks after planting. Plants growing in high P cells had the lowest percentage infection, but the highest mean shoot and root biomass and root length. The mean size of the plants in each cell was determined mainly by local P concentration. However, in plants growing in high density, low P cells, ca. 20% of the variability in plant biomass was explained by the number of adjacent cells with high P. Patchy trays had the highest total shoot biomass, independently of mycorrhizal infection or plant density. Inoculated trays (M) had higher total shoot biomass and relative competition intensity (measured as reduction in plant biomass due to increased density) than non-inoculated trays (NM). Plant density reduced the plant response to mycorrhizal infection, and its effect was independent of P distribution. All populations growing in patchy trays, and low density mycorrhizal ones, had the highest plant-size inequality, presumably because patchy distribution of P and mycorrhizal infection increased competitive asymmetry. We conclude that mycorrhizal symbiosis has the potential to strongly influence plant population structure when soil nutrient distribution is heterogeneous because it promotes pre-emption of limiting resources.

Topographic Controls of Nitrogen, Sulfur, and Carbon Transport from a Tolerant Hardwood Hillslope

The lateral down-slope movement of water, NO3 -, NH4 +, SO4 2-, H+ and DOC through an ablation till was examined from 1987 to 1990 for a one hectaresoil catena on a steep hillslope with uniform forest cover at the Turkey Lakes Watershed (TLW), Ontario, Canada. Natural variation in the export of nutrients from the soil profile via soil water to Little Turkey Lake was assessed in relation to nutrient distribution in soil at different topographic positions.Subsurface throughflow exhibited dramatic differences in nutrientconcentrations and fluxes with slope position, largely reflectingthat of the soil horizons through which the water passed. GreaterNO3 -, SO4 2-, and DOC concentrations in subsurface water in the upper, well-drained hillslope were a reflection of enrichment by contact with more acidic, more developed podzols, and more favorable soil physical and biological conditions for NO3 - retention in solution.Nutrient inputs to the lake were strongly influenced by increaseddown-slope transport of water, and increased SO4 2-, N, and C retention in wetter, less-developed podzolic soils that characterize lower slope positions. An understanding of water movement and soil development variation withtopographic position was required to accurately estimate nutrient budgets for steep slopes at TLW.

Fine-scale spatial distribution of plants and resources on a sandy soil in the Sahel

We studied fine-scale spatial plant distribution in relation to the spatial distribution of erodible soil particles, organic matter, nutrients and soil water on a sandy to sandy loam soil in the Sahel. We hypothesized that the distribution of annual plants would be highly spatially autocorrelated and that this would be positively linked with the spatial distribution of erodible soil particles, soil organic matter and nutrients. Further, we hypothesized that larger vegetated patches (a relatively high vegetation cover at coarser spatial scale) will be stronger linked with erodible soil particles, organic matter and nutrients than smaller vegetated patches (a relatively low vegetation cover at coarser spatial scale). Generally, for `low cover' and `high cover' plots, spatially confined micro-elevations with a relatively high volume percentage of erodible very fine – medium sand (50–400 μm), were linked with a higher cover of annual plants. The robustness of vegetation patchiness (expressed as the proportion of the total variance accounted for by spatial dependent variance) was significantly higher for the `high cover' plots. For the `low cover' plots, higher vegetation cover was associated with higher elevations, soil moisture, and volume percentage of very fine to medium sand, but lower organic matter, total N and P. For the `high cover' plots, micro-elevations also consisted of a relatively high volume percentage of very fine to medium sand, and this was associated with dryer conditions and higher total N. Additionally, dryer conditions were weakly correlated with higher organic matter. So, micro-elevations were indirectly associated with dryer and more fertile conditions, which was opposite to what we found for the `low cover' plots.

Root distribution at various distances from clove trees growing in Indonesia

Efficient fertilizer application requires an understanding of the distribution of roots and soil nutrients in the soil profile. Cultural practices for clove trees in Indonesia has resulted in phosphorus (P) fertilizer being applied at the canopy edge; however, in these high P fixing soils efficient P fertilizer application should occur with the highest root densities. The objective of this study, therefore, was to determine the rooting distribution at various distances from the tree and soil depths for clove (Eugenia aromatica OK; variety Zanzibar) trees growing on an Andosol soil at Modoinding, Indonesia. Root distributions were determined to a 100-cm soil depth using soil cores at 0.5, 1.0 and 1.5 times the canopy radius for five 10-year-old clove trees grown on either level terrain or 23% slopes. Clove root length and weight densities decreased with soil depth and distance from the tree base. Fine clove roots (1 mm dia) comprised 72% of the total root length and was three to five times higher underneath the canopy than that outside the canopy. Roots were concentrated in the upper soil horizons; however, up to 36% of the total root length was found at a depth of 50–100 cm. Clove roots for trees growing at the level landscape position had the highest root length densities. Intercropped species root length densities were higher than clove root length densities at 1.5 times the canopy radius whereas intercropped root weight densities were higher than that for clove roots at both 1.5 and 1 times the canopy radius. Results suggest that fertilizer applications should be placed closer to the tree trunk rather than at the canopy edge to maximize P uptake by clove roots.

Soil Phosphorus and Potassium Mapping Using a Spatial Correlation Model Incorporating Terrain Slope Gradient

Variable-rate fertilizer application requires knowledge of the spatial distribution of soil nutrients within fields. Grid soil sampling might be used for acquiring this information, but is often too expensive for resolving spatial patterns in soil nutrients at the scale of precision fertilizer application. The objective of this study was to determine whether grid sampling efficiency can be improved using cokriging estimates with slope gradient as a secondary variable, which is easily obtained from high-resolution digital elevation models. Soils in two northern Montana wheat fields were sampled at the nodes of a 100-m diagonal grid. Soil test phosphorus and potassium maps were constructed with kriging and cokriging. Co-kriging uses the spatial correlation between two variables to predict for the less intensively sampled variable of interest, often with less estimation error than a univariate method such as kriging. The average estimation variance for cokriging compared to kriging was reduced for all values of the correlation considered. The additional complexity of cokriging might be justified provided a secondary variable exists that is spatially cross correlated with the primary variable of interest.

A three-dimensional spatial model for plant competition in an heterogeneous soil environment

The model described in this paper, the three-dimensional model for the interaction of plants and soil (3DMIPS), was developed to explore how the growth and competitive ability of plants is affected by the interactions between the size and scaling properties of their root systems and the spatial properties of the resources they consume. The main characteristics of the model are as follows. (1) The soil model simulates the spatiotemporal dynamics of temperature, water and solutes (nutrients) in a heterogeneous, 3-dimensional (3-D) soil profile. (2) The plant model has a 3-D simulation of root architecture and plasticity (the ability of certain plants to redirect root growth to areas with high soil nutrient concentration). Plant root architecture is a key component in the simulation of plant nutrient uptake and transpiration. (3) The model simulates the spatiotemporal dynamics of live root, dead root, live shoot, standing dead, and litter biomass and nutrients of individual plants. It simulates the translocation of biomass and nutrients between roots and shoots as a function of plant growth, nutrient uptake, defoliation, regrowth, senescence, and changes in environmental condition. The author presents two set of simulation results. The first set consist of model validation experiments where the author compares model results with data from experiments designed to test the model's ability to predict plant growth and competition in a heterogeneous soil nutrient environment. Model and experimental results were statistically similar in 297 of the 408 variables analyzed. Experimental and modeled variables that were statistically different, were nevertheless highly correlated, with the difference between model and experimental results averaging ±28%. The second set of simulations was designed to assess how plant growth can be affected by an interaction between root lateral spread and the supply rate and spatial distribution of soil nutrients. Results showed that the competitive ability, distribution, and performance of plants could be linked in part to interactions among the size, uptake rates, and scaling properties of their root systems and the supply rate and spatial distribution of the nutrients they utilize. Comparisons are made with the SPUR, PHOENIX, CENTURY, and GEM grassland models. Computer codes and instructions for the implementation of 3DMIPS are available from the author.

The distribution of soil nutrients with depth: Global patterns and the imprint of plants

To understand the importance of plants in structuring the vertical distributions of soil nutrients, we explored nutrient distributions in the top meter of soil for more than 10,000 profiles across a range of ecological conditions. Hypothesizing that vertical nutrient distributions are dominated by plant cycling relative to leaching, weathering dissolution, and atmospheric deposition, we examined three predictions: (1) that the nutrients that are most limiting for plants would have the shallowest average distributions across ecosystems, (2) that the vertical distribution of a limiting nutrient would be shallower as the nutrient became more scarce, and (3) that along a gradient of soil types with increasing weathering-leaching intensity, limiting nutrients would be relatively more abundant due to preferential cycling by plants. Globally, the ranking of vertical distributions among nutrients was shallowest to deepest in the following order: P > K > Ca > Mg > Na = Cl = SO4. Nutrients strongly cycled by plants, such as P and K, were more concentrated in the topsoil (upper 20 cm) than were nutrients usually less limiting for plants such as Na and Cl. The topsoil concentrations of all nutrients except Na were higher in the soil profiles where the elements were more scarce. Along a gradient of weathering-leaching intensity (Aridisols to Mollisols to Ultisols), total base saturation decreased but the relative contribution of exchangeable K C to base saturation increased. These patterns are difficult to explain without considering the upward transport of nutrients by plant uptake and cycling. Shallower distributions for P and K, together with negative associations between abundance and topsoil accumulation, support the idea that plant cycling exerts a dominant control on the vertical distribution of the most limiting elements for plants (those required in high amounts in relation to soil supply). Plant characteristics like tissue stoichiometry, biomass cycling rates, above- and belowground allocation, root distribu- tions, and maximum rooting depth may all play an important role in shaping nutrient profiles. Such vertical patterns yield insight into the patterns and processes of nutrient cycling through time.

The contribution of rabbits ( Oryctolagus cuniculus L.) to soil fertility in semi-arid Spain

Degradation of land in vulnerable areas can be significantly reduced by the maintenance or establishment of plant cover and diversity. Animals can facilitate plant diversity by grazing, by dispersing seeds or by contributing, through excreta, to the heterogeneous distribution of nutrients in soil. We investigated the latter property by examining the effects of rabbit (Oryctolagus cuniculus L.) dung deposition on soil properties in three adjacent plant communities at a semi-arid site in south-east Spain. Rabbit faecal pellets had concentrations of total N and P comparable to dung of stock animals, with K and Mg somewhat lower, although decomposition rates at this site are evidently very low. There was no significant difference in pH, but conductivity and concentrations of organic C, N (as NH4 + and NO3 –), K, P and Mg in soils from rabbit latrines were significantly greater relative to controls in each community. Barley plants grown as a bioassay of soil fertility had significantly greater total biomass, and lower root : shoot ratios in latrine compared to control soils. There were differences among communities in conductivity and concentrations of N, P and organic C under latrines which were reflected in the bioassay. Although latrines only comprised approximately 0.1% of the ground surface area in each community, they make significant localised contributions to soil fertility and may therefore be important in establishing and maintaining plant cover.

Soil Microclimate and Chemistry of Spruce–Fir Tree Islands in Northern Utah

The objective was to investigate differences in soil properties in high‐elevation tree islands composed of subalpine fir [Abies lasiocarpa (Hook.) Nutt.] and Engelmann spruce [Picea engelmannii Parry ex Engelm.] relative to semiarid meadows in Northern Utah. We compared snow depths, surface soil temperature and moisture, and soil chemical properties (0–30 cm) between tree islands and the surrounding meadow. Snow accumulated evenly around the tree islands. Snow depth differences developed during snowmelt when shaded areas to the north of the islands retained snow longer. Less snow accumulated inside and at the edge of the tree islands. In summer, canopy shading inside the islands slowed soil moisture loss and decreased the magnitude and fluctuation of soil temperatures relative to the exposed meadow. Tree islands affected O horizon mass and chemistry: 6 to 10 kg m−2 of O material, with higher macronutrient concentrations and lower C/N ratio, had accumulated under tree canopies; 3 kg m−2 in the island interior; and <1 kg m−2 in the meadow. Surface soils inside the tree islands had significantly higher C and N soil concentrations and higher C/N ratio. The pH of meadow soils was constant with depth (6.22–5.95). Below the trees, the pH in the upper soil was 0.1 to 0.3 units higher than in the meadow and decreased 0.8 to 0.9 pH units with depth. The presence of tree islands significantly modified the soil microclimate and nutrient distribution relative to the surrounding meadows.

Effect of plant growth on the distribution and forms of soil nutrients in the rhizosphere

The effect of plant growth on the distribution and forms of soil nutrients in the rhizosphere was studied by growing plants in rhizoboxes. An experiment was carried out in a growth chamber for 17 d with three treatments, i.e. maize (Zea mays L.), kidney bean (Phaseolus vulgaris L.), and no plants. After the experiment, soil samples were collected up to about 50 mm from the central compartment (C. C.) in which roots were allowed to grow, and analyzed for the soluble, exchangeable, and total amounts of nutrients. Plant samples were also analyzed for their nutrient contents. The pattern of nutrient distribution in the rhizosphere varied among the types and forms of elements. In both maize and kidney bean, the concentrations of soluble P, K, and N decreased significantly at the C.C., up to 4 mm and up to 10 mm from the C.C., respectively. The magnitude of their depletion reflected their rate of diffusion in soil. The concentration of exchangeable K decreased significantly up to 2 mm from theC.C., whereas those of exchangeable Ca and Mg increased within 1 mm from the C.C. The total amount of nutrients, however, did not change significantly for each element. The depletion of soluble P and K mainly around the C.C. indicated the presence of their diffusion toward the root surface. Their uptake was larger than the apparent decrease of their soluble forms, suggesting that their depletion was compensated by intensive replenishment from the soil solid phase. The concentrations of N, Ca, and Mg, on the other hand, markedly decreased up to the end of the rhizobox, indicating the presence of mass flow. The depletion was proportional to the rate of transpiration by plants.

BELOWGROUND CONSEQUENCES OF VEGETATION CHANGE AND THEIR TREATMENT IN MODELS

The extent and consequences of global land-cover and land-use change are increasingly apparent. One consequence not so apparent is the altered structure of plants belowground. This paper examines such belowground changes, emphasizing the interaction of altered root distributions with other factors and their treatment in models. Shifts of woody and herbaceous vegetation with deforestation, afforestation, and woody plant en- croachment typically alter the depth and distribution of plant roots, influencing soil nutrients, the water balance, and net primary productivity (NPP). For example, our analysis of global soil data sets shows that the major plant nutrients C, N, P, and K are more shallowly distributed than are Ca, Mg, and Na, but patterns for each element vary with the dominant vegetation type. After controlling for climate, soil C and N are distributed more deeply in arid shrublands than in arid grasslands, and subhumid forests have shallower nutrient dis- tributions than do subhumid grasslands. Consequently, changes in vegetation may influence the distribution of soil carbon and nutrients over time (perhaps decades to centuries). Shifts in the water balance are typically much more rapid. Catchment studies indicate that the water yield decreases 25-40 mm for each 10% increase in tree cover, and increases in transpiration of water taken up by deep roots may account for as much as 50% of observed responses. Because models are increasingly important for predicting the consequences of vegetation change, we discuss the treatment of belowground processes and how different treatments affect model outputs. Whether models are parameterized by biome or plant life form (or neither), use single or multiple soil layers, or include N and water limitation will all affect predicted outcomes. Acknowledging and understanding such differences should help constrain predictions of vegetation change.

The relationships between land use and soil conditions in the hilly area of the loess plateau in northern Shaanxi, China

Inappropriate land use is one of the main reasons for soil erosion and nutrient loss in the hilly loess area. In the Yangjuangou catchment, which has typical hill and gully topography, the effects of land use changes on soil erosion and the distribution of soil nutrients and moisture were studied at the three spatial scales of total catchment, transect and land use type. Between 1984 and 1996, forest and grassland increased by 36% and 5%, respectively, and slope farmland decreased by 43%. The land use changes decreased annual soil erosion by 24%. The amounts of total N, total P, available N, available P and organic matter of surface soil (0–20 cm) and of moisture in the 0–70 cm soil suggest that a farmland/grassland/forest land use structure from hill foot to hill top has a better capacity for soil conservation and retention of nutrients than other land use structures. The contents of soil nutrients are in the order forest>grassland>slope farmland, and those of soil water are forest<grassland<slope farmland.

Sampling strategies for mapping soil phosphorus and soil potassium distributions in cool temperate grassland.

Unlike the situation for arable soils, virtually nothing is known about the spatial dependencies of soil properties in cool temperate grassland or about what the optimal sampling strategies ought to be for mapping soil nutrient distributions in such situations. The aim of this study was to investigate the spatial variability in ‘plant-available’ (soil) phosphorus and potassium in a grass silage field in Northern Ireland and devise ‘optimal’ sampling strategies for mapping their distributions. Soil samples were collected from the field at 25 m intervals in a regular rectangular grid to provide a database of soil properties. Different data combinations were subsequently abstracted from this database for comparison purposes, and ordinary kriging used to produce interpolated soil maps. Soil potassium displayed greater spatial variability than soil phosphorus. In keeping with this observation, the results of three separate statistical procedures demonstrated that the optimal sample size for estimating the ‘true’ population means was about twice as large for soil potassium as for soil phosphorus. Optimal sampling strategies, however, related not just to sample size but to sample combination and field shape as well.

The effect of land use structure on the distribution of soil nutrients in the hilly area of the Loess Plateau, China

The irrational land use is one of the main reasons for the soil erosion and nutrienloss in the loess hilly area of China. In this project, 4 types of typical land use structure of sustain ment for about 15 years in the loess hill slope are selected to study the effect of land use structure on the distribution of soil nutrients. From hill bottom to hill top, the patterns of land use types are: grassland-slope farmland-forest, slope farmland-grassland-forest, terrace-grasslandforest and slope farmland-forest-grassland. By measuring the contents of the total N, total P, available N, available P and organic matter of soils, the results show that the land use structure types of slope farmland-grassland-forest and terrace-grassland-forest have a better capacity to maintain the soil nutrients.

Growth Consequences of Soil Nutrient Heterogeneity for two Old-field Herbs,Ambrosia artemisiifoliaandPhytolacca americana, Grown Individually and in Combination

Plant species can respond to small scale soil nutrient heterogeneity by proliferating roots or increasing nutrient uptake kinetics in nutrient-rich patches. Because root response to heterogeneity differs among species, it has been suggested that the distribution of soil resources could influence the outcome of interspecific competition. However, studies testing how plants respond to heterogeneity in the presence of neighbours are lacking. In this study, individuals of two species,Phytolacca americanaL. andAmbrosia artemisiifoliaL. were grown individually and in combination in soils with either a homogeneous or heterogeneous nutrient distribution. Above-ground biomass of individually grown plants of both species was greater when fertilizer was located in a single patch than when the same amount of fertilizer was distributed evenly throughout the soil. Additionally, both species proliferated roots in high-nutrient patches.A. artemisiifoliaexhibited larger root:shoot ratios, increased nitrogen depletion from nutrient patches, and a higher growth rate thanP. americana, suggestingA. artemisiifoliais better suited to find and rapidly exploit nutrient patches. In contrast to individually grown plants, soil nutrient distribution had no effect on final above-ground plant biomass for either species when grown with neighbours, even though roots were still concentrated in high nutrient patches. This study demonstrates that increased growth of isolated plants as a consequence of localized soil nutrients is not necessarily an indication that heterogeneity will affect interspecific encounters. In fact, despite a significant below-ground response, soil nutrient heterogeneity was inconsequential to above-ground performance when plants were grown with neighbours.

Soil macronutrient distribution in rotationally stocked kikuyugrass paddocks with short and long grazing periods

Grazing management affects plant growth and animal production and it may influence the redistribution and cycling of nutrients excreted in dung and urine. Unfortunately, the soil component of pasture systems has received little attention in most grazing trials, and when considered has been evaluated on pastures smaller than those used commercially. A naturalized kikuyu (Pennisetum clandestinum Hochst. ex Chiov.) grassland was grazed by heifers (Bos taurus) for 2 yr in Experiment 1 to determine the effects of two rotational stocking methods (short vs. long grazing periods) on soil distribution of extractable nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) in 4‐ha paddocks with natural shade. Additionally, in Experiment 2, kikuyugrass‐greenleaf desmodium (Desmodium intortum Urb.) paddocks were used to evaluate the effect of distance from waterers on soil nutrient distribution in rotationally stocked paddocks (long grazing periods) without shade in a cooler, higher elevation, environment. In Experiment 1, soil N, P, and K distribution did not differ between lengths of grazing period when assessed using a zonal soil sampling procedure with zones based on distance from shade and water sources. These nutrients, and in particular K, accumulated within 15 m of shade, but did not accumulate significantly around waterers. In Experiment 2, zonal soil sampling indicated that P, Mg, and especially K, accumulated within 15 m of the waterer. In both studies, extractable K data collected via a grid sampling regime and contour maps constructed from these data supported, in general, the conclusions made using zonal sampling. It is suggested that in this subtropical environment the magnitude of excretal N, P, and K accumulation is greater around shade than waterers, and that in paddocks without shade substantial amounts of P and K accumulate near the waterer. As with small paddocks/pastures, zonal soil sampling appears to be a practical sampling strategy for large paddocks (4 ha).