Nutrient Hotspots Research Articles

The Nature and Potential Causes of Nutrient Hotspots in a Sierra Nevada Forest Soil

A study of soil nutrient hotspots was conducted by sampling two forest soils both destructively and with resin-based methods. In con- junction with the soil sampling, O horizon interflow collectors were established near each plot. Based on previous work, we hypothesized that nutrient hotspots existed in the soils and that they would be more common for less abundant than more abundant nutrients. We also hypothesized that nutrient-enriched O horizon interflow was occurring at these sites and could cause hotspots where it enters the soil in preferential flow paths. The first hypothesis was only partially supported: Hotspots, as indicated by significant positive skew and statistical outliers, occurred not only for least abundant nutrients, but also for nearly all measured nutrients by various methods. The second hypothesis was supported: O horizon interflow did occur, was enriched in nutrients, and could well have caused some nutrient hotspots. We found one candidate for such a hotspot where extreme out- liers for all but one water-extractable nutrient was identified.

The coarse‐soil fraction is the main living space of fungal hyphae in the BhBs horizon of a Podzol

AbstractIn acidified forest soils, the coarse‐soil fraction is a potential nutrient source. Plant nutrient uptake from the coarse‐soil fraction is aided by ectomycorrhiza. Similarly, (recalcitrant) organic matter (OM) is an important nutrient source largely made plant‐available through (symbiotic) microorganisms, especially in the topsoil. We hypothesized that in a podzol profile, fungal hyphae would concentrate in nutrient hotspots, either OM or the coarse‐soil fraction. Absolute hyphal length, base saturation, and organic‐C content of a Podzol profile were determined in the fine‐earth and coarse‐soil fractions. In the fine‐earth fraction, hyphae were attracted by the organic‐C content and relative high base saturation. In the coarse‐soil fraction of the BhBs horizon, the absolute hyphal length exceeded the hyphal length in the fine earth by factor 3, yet C content and base saturation were lowest. We could not determine to what fungi the hyphae belonged. Most likely ectomycorrhiza, ericoid mycorrhiza and saprotrophic fungi dominate the upper soil layers of this profile and all utilize OM for nutrition. In the deeper mineral horizons and especially in the coarse‐soil fraction, ectomycorrhiza are better adapted than other fungi to harvest nutrients from inorganic sources. Additionally, favorable physical properties may explain the high amount of fungal hyphae in the coarse‐soil fraction of the BhBs horizon. Both the coarse‐soil fraction and deeper mineral soil horizons may play a more active role in microbial nutrient cycling than previously assumed.

Contrasting root behaviour in two grass species: a test of functionality in dynamic heterogeneous conditions

Root systems are highly plastic as they express a range of responses to acquire patchily distributed nutrients. However, the ecological significance of placing roots selectively in nutrient hotspots is still unclear. Here, we investigate under what conditions selective root placement may be a significant functional trait that determines belowground competitive ability. We studied two grasses differing in root foraging behaviour, Festuca rubra and Anthoxanthum odoratum. The plants were grown in stable and more dynamic heterogeneous environments, by switching nutrient patches halfway through the experiment. A. odoratum was a factor of two less selective in placing its roots into nutrient-rich patches than F. rubra. A. odoratum produced overall higher root length densities with higher specific root length than F. rubra and acquired more nutrients. A. odoratum appeared to be the superior competitor, irrespective of the nutrient dynamics. Our results suggest that root behaviour consisting of producing high root length densities at relatively low biomass investments can be a more effective foraging strategy than placing roots selectively in nutrient hotspots. When understanding the functionality of root traits among different species, specific root length may play a key role.

Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation

In savannas, the tree–grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.

Foliar nutrient and water content in subtropical tree islands: A new chemohydrodynamic link between satellite vegetation indices and foliar δ15N values

We examined the relationships between two satellite-derived vegetation indices and foliar δ 15N values obtained from dominant canopy species in a set of tree islands located in the Everglades National Park in South Florida, USA. These tree islands constitute important nutrient hotspots in an otherwise P-limited wetland environment. Foliar δ 15N values obtained from a previous study of 17 tree islands in both slough (perennially wet) and prairie (seasonally wet) locations served as a proxy of P availability at the stand level. We utilized five cloud-free SPOT 4 multispectral images (20 m spatial resolution) from different times of the seasonal cycle to derive two atmospherically corrected vegetation indices: the normalized difference vegetation index (NDVI) and the normalized difference water index (NDWI), averaged for each tree island. NDWI, which incorporates a shortwave infrared (SWIR) band that provides information on leaf water content, showed consistently higher linear fits with island foliar δ 15N values than did NDVI. In addition, NDWI showed greater variation throughout the seasonal cycle than did NDVI, and was significantly correlated with average water stage, which suggests that the SWIR band captures important information on seasonally variable water status. Tree islands in slough locations showed higher NDWI than prairie islands during the dry season, which is consistent with higher levels of transpiration and nutrient harvesting and accumulation for perennially wet locations. Overall, the results suggest that water availability is closely related to P availability in subtropical tree islands, and that NDWI may provide a robust indicator of community-level water and nutrient status.

Nutrient hot spots in some sierra Nevada forest soils

Re-examination of data distributions from several forested sites in the eastern Sierra Nevada Mountains shows consistent, positive outliers and skew for NH4+, NO3−, and mineral N in resin lysimeters and resin capsules, indicating that most values were low but hotspots of high N flux were present in most cases. Exact causes of these N flux hotspots is not known, but could include water flux hotspots (e.g., preferential flowpaths), microbial hotspots, and possibly the entry of N-enriched O horizon interflow. Soil and resin stake (PRS probe) data from one site (North Lake Tahoe) also showed consistent, positive outliers and skew for NH4+, NO3−, and mineral N, suggesting the presence of microbially produced hotspots. Bicarbonate-P data from soils and ortho-P data from PRS probes also showed highly positive skew and extreme outliers, but Bray (HCl/NH4F-extractable) P in soils did not. Other measured nutrients (extractable Mg2+, K+, SO42−, and Ca2+) also showed positive skew and outliers, but less so than NH4+, NO3−, and mineral N. Calcium stood out among measured nutrients as the most abundant nutrient with the least outliers and the lowest (sometimes negative) skew. The differences in distributions of NH4+, NO3−, and mineral N and those of Ca2+ may reflect relative abundance: the most abundant ion, Ca2+, shows little evidence of hotspots whereas the much less abundant ions, NH4+, NO3− consistently show evidence of hotspots. We hypothesize that the differing distributions of N and Ca reflect the relative biological competition for these nutrients and that positively-skewed distributions and hotspots will be characteristic of any other nutrient when it is in limited supply relative to biological demand.

Exploring the “nutrient hot spot” hypothesis at trees used by bats

Abstract Temperate bat species are well-known predators of nocturnal insects; however, their role in forest nutrient cycling is unclear. We tested the “nutrient hot spot” hypothesis, which suggests that colonial bats should create nutrient peaks in and around their roosts via guano deposits. We measured the mass and nitrogen content of guano deposited outside of roosts occupied by maternal colonies of 2 tree-roosting species, Myotis septentrionalis and M. sodalis. We assessed whether these measures were related to date, species, and bat abundance using least-squares regression. We then compared the expected amount of nitrogen deposited over a maternity season to the expected annual amount of nitrogen mineralized by a forest. Mass of guano deposited increased with bat abundance and corresponded to periods of parturition and lactation. Nitrogen mineralization near a large roost of bats can be 380% of that due to decay of leaf litter. Such nutrient peaks could influence patterns of forest vegetation by impac...

Exploring the “nutrient hot spot” hypothesis at trees used by bats

Exploring the “nutrient hot spot” hypothesis at trees used by bats

Relationships between local stand density and local species composition and nutrient content in the topsoil of pure and mixed stands of silver fir (Abies alba Mill.)

This study examined topsoil samples from the humic mineral horizon in four pure silver fir (Abies alba Mill.) and five mixed stands with beech (Fagus sylvatica L.), spruce (Picea abies L.; Karst.) and pine (Pinus sylvestris L.). In every stand, 60–70 sampling locations were chosen using a stratified random scheme that represented a gradient in local stand density and local species composition. One working hypothesis was that stand density and species composition affect the content of nutrients in the mineral humic soil horizon. Specifically, the analysis statistically tested for relationships between general and species-specific local stand density measures (basal area, sum of tree diameters weighted by distance, canopy openness) and the contents of total C, total N (in four stands), base cations (Ca2+, Mg2+, K+, Na+) and Mn2+ (in nine stands). A considerable variation in nutrient concentrations was observed; the coefficients of variation ranged from 0.21 to 0.38 for N, from 0.20 to 0.94 for the sum of base cations and from 0.56 to 0.95 for Mn. However, no clear relationship emerged between local stand density and the concentration of N and base cations; in five stands, manganese showed a negative correlation with local stand density. In the mixed fir-beech stand, significantly higher concentrations of Ca, K and Mn were found in the vicinity of beeches, and species-specific stand density explained up to 15, 29 and 34% of total variation in the content of these elements, respectively. Similar tendencies also appeared in the mixed stands with pine, although the relationships were weak. The results obtained for two fir-spruce stands were ambiguous. In sum, in the stands studied, small canopy gaps and inter-crown openings might not be considered ‘‘nutrient hot spots’’, and the admixed species, except for beech, had an insignificant effect on nutrient content in the humic mineral horizon.

HABITAT QUALITY AND HETEROGENEITY INFLUENCE DISTRIBUTION AND BEHAVIOR IN AFRICAN BUFFALO (SYNCERUS CAFFER)

Top-down effects of predators on prey behavior and population dynamics have been extensively studied. However, some populations of very large herbivores appear to be regulated primarily from the bottom up. Given the importance of food resources to these large herbivores, it is reasonable to expect that forage heterogeneity (variation in quality and quantity) affects individual and group behaviors as well as distribution on the landscape. Forage heterogeneity is often strongly driven by underlying soils, so substrate characteristics may indirectly drive herbivore behavior and distribution. Forage heterogeneity may further interact with predation risk to influence prey behavior and distribution. Here we examine differences in spatial distribution, home range size, and grouping behaviors of African buffalo as they relate to geologic substrate (granite and basalt) and variation in food quality and quantity. In this study, we use satellite imagery, forage quantity data, and three years of radio-tracking data to assess how forage quality, quantity, and heterogeneity affect the distribution and individual and herd behavior of African buffalo. We found that buffalo in an overall poorer foraging environment keyed-in on exceptionally high-quality areas, whereas those foraging in a more uniform, higher-quality area used areas of below-average quality. Buffalo foraging in the poorer-quality environment had smaller home range sizes, were in smaller groups, and tended to be farther from water sources than those foraging in the higher-quality environment. These differences may be due to buffalo creating or maintaining nutrient hotspots (small, high-quality foraging areas) in otherwise low-quality foraging areas, and the location of these hotspots may in part be determined by patterns of predation risk.

Decomposition of organic matter and nutrient mineralisation in wood ant (Formica rufa group) mounds in boreal coniferous forests of different age

Wood ants (Formica rufa group) are dominating ecosystem elements of the boreal region due to their wide and abundant occurrence. They collect and concentrate organic material from the surrounding forest floor by building large above-ground mounds. These mounds have higher temperature and lower water content than the surrounding forest floor. We studied how these peculiar environmental conditions affected mass loss and carbon (C), nitrogen (N), phosphorus (P) and potassium (K) mineralisation of organic matter in boreal Norway spruce (Picea abies L. Karst.)-dominated mixed forest stands of four different age classes (5-, 30-, 60-, and 100-year-old) situated in eastern Finland using the litter bag technique. Norway spruce needle litter was incubated in inhabited and abandoned wood ant mounds as well as on the surrounding forest floor. We expected decomposition to be extremely slow due to the dryness of the mounds. Mass losses inside inhabited mounds were lower compared to the surrounding forest floor (on average 30 vs 50% after 2 years) but not as low as we expected, which might be a result of ant and microbial activity in the mounds. Decomposition in the abandoned mounds proceeded similarly as on the forest floor. Nutrient mineralisation proceeded more slowly in the ant mounds than on the surrounding forest floor. Mineralisation occurred for all studied nutrients in the ant mounds, except for N, which net amount remained stable during the years of the experiment. When wood ant mounds are abandoned and their porous and dry structure is no longer maintained by the ants, their decomposition is accelerated, and nutrients may be available for uptake by plants, although the nutrient mineralisation seems still to remain lower compared to the surrounding forest floor. However, eventually the mound material will be decomposed and nutrients mineralised, thus providing a nutrient hot spot increasing the heterogeneity of forest floor nutrient availability.

Outdoor pig fattening at two Swedish organic farms—Spatial and temporal load of nutrients and potential environmental impact

In Sweden, outdoor organic pig production is gaining interest. However, the excretory behaviour of pigs may create plant nutrient hotspots in outdoor areas, increasing the environmental impact. This study examined fluxes and balances of N, P, K, Cu and Zn at pen level, and determined the effects of the excretory behaviour of fattening pigs on nutrient load, manure distribution and N, P, K, Cu and Zn concentration in soil within pens at two farms with different outdoor systems (mobile and stationary). A pen in the mobile system had about 72 pigs ha −1 and in the stationary system about 91 pigs ha −1. The average pen balance in the mobile system was 270 kg N, 60 kg P, 110 kg K, 0.4 kg Cu and 1.3 kg Zn ha −1 and in the stationary system 205 kg N, 57 kg P, 99 kg K, 0.4 kg Cu and 1.5 kg Zn ha −1. The smaller net accumulation of nutrients in the stationary system was due to about 30% of excrement nutrients being excreted indoors. A substantial proportion of nutrients (43–95%) from one pig group was found to be concentrated in an area of arable land representing 4–24% of the total pen area. In the mobile system the major part of the defecating was deposited on the hut, feeding and drinking sub-areas. The manure mapping also revealed pig behaviour of avoiding defecation in certain zones. In the stationary system, the concentration of exchangeable P and K in soil (0–30 cm depth) in areas preferred for excretion was more than four-fold higher and the concentration of mineral-N (0–90 cm depth) was about eight-fold higher than in other areas of the pen by the end of the fattening period. Preferred areas for excretion within the pen were affected both by the present pig groups and by previous pig groups 4 years back in time. In the mobile system, the concentration of mineral-N in soil was about three-fold higher in preferred excretion areas compared with other areas. Neither of the two outdoor systems succeeded in avoiding excessive point loads of N within the pen. The flexibility of the mobile outdoor system has to be further improved so that no harmful point loads of nutrients can occur. In the stationary system, a nutrient management technique for collecting the manure on the preferred excretion areas on arable land needs to be developed.

Limited costs of wrong root placement in Rumex palustris in heterogeneous soils

Nutrient hot spots in the soil have a limited life span, but the costs and benefits for root foraging are still underexposed. We assessed short-term costs that may arise when a nutrient-rich patch induces root proliferation, but then rapidly disappears. Rumex palustris plants were grown with a homogeneous or a heterogeneous nutrient application. After root proliferation in a nutrient-rich patch, nutrient supply was switched from homogeneous to heterogeneous, and vice versa, or the patch location was changed. R. palustris proliferated its roots in the rich patch. After switching, the relative growth rates of the roots were adjusted to the novel pattern of nutrient availability. However, the changes in local root biomass lagged behind the rapid shift in nutrient supply, because the root mass realized in specific sectors could not be rapidly relocated. Despite this, R. palustris did not exhibit costs of switching in terms of biomass or nitrogen uptake. Our data suggest that rapid shifts in uptake rate and redistribution of nitrogen within the plant may have lowered the costs of incorrect root placement.

The importance of nutrient hot-spots in the conservation and management of large wild mammalian herbivores in semi-arid savannas

Extensive studies in the Serengeti showed a strong link between patchy herbivory and forage quality, and the factors that determine forage patch selection at different scales have been evaluated in various models. The purpose of this study was to examine to what extent favoured forage patches in the Lowveld of South Africa are determined by forage quality and how important these are as forage resources. Understanding the factors that determine herbivore distribution will provide insights into how the utilization of forage by herbivores affects and is influenced by the ecosystem and thereby improve our abilities to conserve and manage these systems. The Kruger National Park is a large wildlife conservation area, with stratified rainfall and soil nutrient patterns. Tuft utilization and number of faecal deposits were used to determine favoured forage patches within these stratifications. On the low-nutrient granites, the use of grass tufts on the crests of hillslopes was about a quarter that on sodic sites, and a third of that of termite mounds. On the high-nutrient basalts, utilization of crests was about a third of that of termite mounds and about a quarter of the utilization of sodic sites in the wet and growth season, but towards the end of the dry season all patches were utilized to a similar extent. On sodic sites, shorter grass grazers accounted for almost ten times more faecal deposits than other species, although there was evidence of all the other large herbivore groups also utilizing these sites. The foliar nitrogen and phosphorus concentrations from termite mounds and sodic patches were up to twice that found on the crests in the wet season, especially on the granites, indicating that forage quality may indeed play an important role in determining favoured forage patches. Furthermore, only the patches on the sodic sites and termite mounds produced foliage of sufficient quality to support reproduction and maintenance of body condition. These patches thus form key resource areas that determine animal condition and hence dry season survival. These findings have important management consequences: firstly in predicting the number of animals that may be supported by an area; secondly, because these nutrient hot-spots are so intensely utilized by herbivores, they will be the first to show degradation, and monitoring programs should thus include these areas. Appropriate monitoring designs will detect degradation in these areas in time to take appropriate management actions that would avoid irreversible system changes.

Maintaining tree islands in the Florida Everglades: nutrient redistribution is the key

The Florida Everglades is an oligotrophic wetland system with tree islands as one of its most prominent landscape features. Total soil phosphorus concentrations on tree islands can be 6 to 100 times greater than phosphorus levels in the surrounding marshes and sloughs, making tree islands nutrient hotspots. Several mechanisms are believed to redistribute phosphorus to tree islands: subsurface water flows generated by evapotranspiration of trees, higher deposition rates of dry fallout, deposition of guano by birds and other animals, groundwater upwelling, and bedrock mineralization by tree exudates. A conceptual model is proposed, in which the focused redistribution of limiting nutrients, especially phosphorus, onto tree islands controls their maintenance and expansion. Because of increased primary production and peat accretion rates, the redistribution of phosphorus can result in an increase in both tree island elevation and size. Human changes to hydrology have greatly decreased the number and size of tree islands in parts of the Everglades. The proposed model suggests that the preservation of existing tree islands, and ultimately of the Everglades landscape, requires the maintenance of these phosphorus redistribution mechanisms.

Burrow Dynamics of the Red Land Crab Gecarcoidea Natalis (Brachyura, Gecarcinidae) in Rain Forest on Christmas Island (Indian Ocean)

Abstract This study quantified the burrow dynamics of the gecarcinid land crab Gercarcoidea natalis (Pockock, 1888), the most common land crab in rain forest on Christmas Island, Indian Ocean. Mean burrow density remained relatively stable over a two-year period, fluctuating between 1.4 and 1.6 burrows/m2. Burrows were long-lived, stable structures, with a mean turnover time estimated at least 4.4 ± 1.6 y, and probably greater than 5 y. Burrow entrances were also highly stable in space. Two thirds of burrow entrances were repositioned within an area of 20 cm2 or less, and 83% of entrances moved within less than 60 cm2. Red crabs accumulate litter around their burrow entrances and create nutrient hotspots in the soil around burrow entrances, whose stability and longevity suggested that germinating seedlings may derive some benefit from germinating in the immediate vicinity of crab burrows. The rate of burrow turnover is slow, causing minimal soil disturbance at the study site with little, if any, impact on...

Laboratory and field-based studies of abundances, small-scale patchiness, and diversity of gymnamoebae in soils of varying porosity and organic content: evidence of microbiocoenoses.

Soil samples (varying in granularity) from four natural sites were cultured in microcosms to determine small-scale patchiness in abundance and diversity of gymnamoebae. Eighty grams of the same thoroughly mixed soil, either moistened with distilled water (- nutrients) or supplemented with an equivalent vol. of organically enriched water (+ nutrients), were placed in covered glass jars and incubated for 14 d (25 degrees C). Abundances (number/gram soil) were assessed in each of 3 core samples (5-10 mm apart). Assay precision was estimated to be +/- 4%. Abundances were similar in the 3 closely-spaced samples, but occasional samples had higher abundances, probably representing localized enriched sites ("nutrient hot spots"). Diversity within the triplicate, closely spaced samples varied substantially. Mean abundance and diversity of amoebae were consistently higher in organically enriched soil and in soil of increasing granularity. Field samples collected directly from two of the sites showed similar patterns of abundance and diversity as found in the experimental studies, indicating substantial small-scale compartmentalization of soil protist communities. These data provide evidence of soil eukaryotic microbiocoenoses and indicate that soil microfauna may encounter wide variations in resources and prey communities as they migrate within small distances of several millimeters or less.

Soil microarthropod contribution to forest ecosystem processes: the importance of observational scale

Investigations of the role of microarthropods (Acari and Collembola) in organic matter decomposition and nutrient cycling have shown that they may contribute to primary productivity in nutrient poor conditions. The potential of microarthropods to affect other ecosystem properties, such as above ground plant diversity or succession, lags somewhat. In this contribution we demonstrate: (1) that the effect on the mobilization of nutrients promoted by microarthropods must be measured at the microhabitat scale appropriate to the scale of the faunal activity, and (2) that small changes in the structure of microarthropod assemblages can have significant effects on the local mobilization of nutrients. In the first of two experiments we measured the nutrients leaching from field mesocosms containing litter and mineral soil, with and without fauna. After eight months, the C:N ratios of the litter differed significantly indicating that the fauna were effective in altering the decomposition rate. However, the patterns of release over time and the concentration of the measured nutrients differed little between the two sets of mesocosms. In a second experiment microarthropod assemblages, which differed only slightly, were introduced into laboratory microcosms and the nutrient fluxes were measured over a ten week period. Significant differences were detected in the concentration of nitrogen, K and Mg leached and in CO2 evolved. We suggest that when the potential influence of microarthropods on ecosystem properties is being assessed, specific knowledge of the relevant details of interactions at the smallest scale must be considered. These details can be incorporated or dismissed when interactions on the next level of the ecological hierarchy are examined. Using such analysis we suggest that the creation of soil nutrient hot-spots by microarthropods may have implications for maintaining plant species of lowered competitive ability in a given system.

The Influence of Soil Fertility on the Ecology of Southern African Dry Savannas

In dry savannas, soil fertility has a controlling patterns of herbivory, the activities of prehistoric man and influence on the slope of the relation between annual nutrient cycling through trees and termite mounds. While rainfall and annual aboveground herbaceous production. It the base-richness of the parent material is initially impor- also influences many other aspects of their structure and tant in determining soil fertility, biological activities involv- function; such as the species composition, morphology, ing phosphorus, nitrogen and carbon appear to be important forage chemistry and degree and type of herbivory. In dry in the creation and maintenance of localized areas of savannas there is a wide range in soil fertility status, related enhanced soil fertility on base-poor substrates, and could at a regional scale to differences in parent material and the provide a mechanism for the long-term stability of such age of the land surface. At a landscape scale, catenal pro- nutrient hot-spots. cesses result in large fertility contrasts between ridgetop and bottomland positions. There are several other processes