Phosphorus in agricultural soils: drivers of its distribution at the global scale

Phosphorus (P) availability in soils limits crop yields in many regions of the World, while excess of soil P triggers aquatic eutrophication in other regions. Numerous processes drive the global spatial distribution of P in agricultural soils, but their relative roles remain unclear. Here, we combined several global data sets describing these drivers with a soil P dynamics model to simulate the distribution of P in agricultural soils and to assess the contributions of the different drivers at the global scale. We analysed both the labile inorganic P (P_ILAB), a proxy of the pool involved in plant nutrition and the total soil P (P_TOT). We found that the soil biogeochemical background corresponding to P inherited from natural soils at the conversion to agriculture (BIOG) and farming practices (FARM) were the main drivers of the spatial variability in cropland soil P content but that their contribution varied between P_TOT vs. P_ILAB. When the spatial variability was computed between grid cells at half‐degree resolution, we found that almost all of the P_TOT spatial variability could be explained by BIOG, while BIOG and FARM explained 38% and 63% of P_ILAB spatial variability, respectively. Our work also showed that the driver contribution was sensitive to the spatial scale characterizing the variability (grid cell vs. continent) and to the region of interest (global vs. tropics for instance). In particular, the heterogeneity of farming practices between continents was large enough to make FARM contribute to the variability in P_TOT at that scale. We thus demonstrated how the different drivers were combined to explain the global distribution of agricultural soil P. Our study is also a promising approach to investigate the potential effect of P as a limiting factor for agroecosystems at the global scale.     

云贵高原喀斯特坡耕地土壤微生物量 C、N、P空间分布

土壤微生物是地球生物演化进程中的先锋种类,具有重要的生态修复功能,但空间分布格局是否存在的争议很大。以云贵高原典型喀斯特坡耕地为对象,基于网格法取样,用经典统计学和地统计学综合分析了土壤微生物生物量的空间变异特征。结果表明,云贵高原喀斯特坡耕地土壤微生物生物量碳(C_mic)、磷(P_mic)以及碳氮比(C_mic/N_mic)适宜,氮(N_mic)的含量较低,变异均很大,空间自相关性明显,最佳拟合模型均为指数模型。块金值C₀较小(0.0016-0.0087),C₀/(C₀+C)均<25%(2.6%-10.2%),变程a较短(22.2-51.0 m),其强烈的空间变异主要由结构性变异引起。Kriging等值线图表明,C_mic、N_mic和C_mic/N_mic的高值区分布在坡的中上部,P_mic的高值区则在坡的中下部和坡脚。云贵高原喀斯特坡耕地土壤微生物不仅存在着小尺度的空间分布格局,而且不同土壤微生物属性的空间分布不同。     

Stochastic analysis of the relationships between saturated hydraulic conductivity variance and solute dispersion in heterogeneous soils

The effect of three‐dimensional heterogeneity of saturated hydraulic conductivity on the vertical transport of solutes in soils is examined by means of controlled numerical experiments. Saturated hydraulic conductivity, an important transport parameter that controls the dispersion of pollutants in heterogeneous soils, is assumed to be composed of a homogeneous mean value and a perturbation caused by the vertical variability of soil properties, producing a stochastic process in the mean flow direction. The spatial heterogeneity of porous soils is characterized by the variance and correlation scale of the saturated hydraulic conductivity in the transport domain. Numerical experiments are carried out to evaluate the extent of contaminant dispersion in Hawaiian Oxic soils when uncertainty exists as a result of the spatial heterogeneity of saturated hydraulic conductivity. Statistical analysis of the saturated hydraulic conductivity measurements on undisturbed soil cores from two locations in Hawaiian Oxic soils indicated two different soils with the same mean and different variances. The partial differential equations describing three‐dimensional transient flow and solute transport in soils with a random conductivity field were solved to evaluate the effect of these two variance levels on the transport of a contaminant plume originating from the surface. The significance of the variance on the spatial and temporal distribution of tracer concentrations is demonstrated using solute breakthrough curves at various depths in the soil profile. The longitudinal macrodispersivity resulting from tracer spreading in the heterogeneous soils with a finite local dispersivity is also analyzed. The analysis shows a similar solute dispersion behavior for the two variances. However, there is an overall reduction in the dispersion of solutes resulting from a uniform velocity field with the same mean. Macrodispersivity values in heterogeneous soils are proportional to the variance at smaller travel distances but converge to the same value at larger travel distances.     

Migration of fallout-radionuclides in the soil: effect of non-uniformity of the sorption properties on the activity–depth profiles

Experimentally observed activity–depth profiles of fallout radionuclides in the soil frequently exhibit a comparatively fast moving tail in soil layers below the peak concentration (tailing). Monte Carlo calculations on the basis of the convection-dispersion model show that this phenomenon can be explained by assuming that either the hydraulic properties of the soil (characterised by the diffusion/dispersion coefficient and pore water velocity) or the sorption properties of the soil (characterised by the distribution coefficient K _d ), or both, exhibit a horizontal variability according to a log-normal distribution. Modifications of the activity–depth profile due to a K _d value which decreases linearly with depth were examined by using a random walk approach, based also on the convection-dispersion model. In this case, however, a pronounced tailing effect of the activity–depth profile did not result. Interpretation and realistic modelling of an experimentally observed activity–depth profile which exhibits a tailing effect is thus not unambiguously possible without any additional information on the spatial variability of the hydraulic parameters and, independently, also for the sorption properties. Received: 5 January 2001 / Accepted: 1 May 2001     

Long-term effect of re-vegetation on the microbial community of a severely eroded soil in sub-tropical China

The long-term (18 years) effects of re-vegetating eroded soil on soil microbial biomass, community structure and diversity were investigated in a forest soil derived from Quaternary clay in the Red Soil Ecological Experimental Station of the Chinese Academy of Sciences. Large areas of land in this region of China have been subjected to severe soil erosion, characterised by the removal of the fertile surface soil and even the exposure of parental rock in some areas due to a combination of deforestation and heavy rainfall. The effects of planting eroded or uneroded soil with Pinus massoniana, Cinnamomum camphora or Lespedeza bicolor on the soil microbial community and chemical properties were assessed. Total soil microbial community DNA was extracted and bacterial 16 S rRNA gene fragments were amplified by PCR and analysed by terminal restriction fragment length polymorphism (T-RFLP). Microbial biomass carbon (C_mic) was measured by chloroform fumigation-extraction. Following the restoration there were significant increases in both C_mic and bacterial diversity (Shannon index), and significant changes in bacterial community structure. Erosion factors were significant only in minor dimensions suggesting that the restoration had been largely successful in terms of bacterial community structure. Compared with uneroded soil, C_mic recovered in L. bicolor and P. massoniana restored eroded plots and was significantly greater under these tree species than C. camphora, although soils in C. camphora restored plots displayed the highest bacterial diversity. The recovery of microbial biomass and diversity in the eroded plots was, to large extent, accompanied by the development of the same bacterial community structure as in the uneroded plots with erosion having relatively little effect on bacterial community structure.     

Temporal variability in soil microbial communities across land-use types

Although numerous studies have investigated changes in soil microbial communities across space, questions about the temporal variability in these communities and how this variability compares across soils have received far less attention. We collected soils on a monthly basis (May to November) from replicated plots representing three land-use types (conventional and reduced-input row crop agricultural plots and early successional grasslands) maintained at a research site in Michigan, USA. Using barcoded pyrosequencing of the 16S rRNA gene, we found that the agricultural and early successional land uses harbored unique soil bacterial communities that exhibited distinct temporal patterns. α-Diversity, the numbers of taxa or lineages, was significantly influenced by the sampling month with the temporal variability in α-diversity exceeding the variability between land-use types. In contrast, differences in community composition across land-use types were reasonably constant across the 7-month period, suggesting that the time of sampling is less important when assessing β-diversity patterns. Communities in the agricultural soils were most variable over time and the changes were significantly correlated with soil moisture and temperature. Temporal shifts in bacterial community composition within the successional grassland plots were less predictable and are likely a product of complex interactions between the soil environment and the more diverse plant community. Temporal variability needs to be carefully assessed when comparing microbial diversity across soil types and the temporal patterns in microbial community structure can not necessarily be generalized across land uses, even if those soils are exposed to the same climatic conditions.     

No short-term change in soil properties following four-fold litter addition in a Costa Rican rain forest

We experimentally manipulated forest floor litter to investigate the influence of litter quality and quantity on soil properties over the short-term (weeks to months) in a wet tropical forest in NE Costa Rica. The study included old growth forest on high fertility soils, old growth forest on low fertility soils, and secondary forest on intermediate fertility soils. Forest floor litter was removed from a 16 m² area and added to an adjacent 4 m² area in March 2003, resulting in a one to four-fold increase in the annual litter input to the forest floor. We created three addition, three removal and three control plots per forest type. We measured treatment effects on variation in soil moisture, temperature, pH, and Bray-1 P (plant available) over a 5-month period that captured the dry-wet season transition. Litter manipulation had no effect on any of the soil properties measured during the 5-month study period. Significant variability through time and a similar temporal pattern across the three forest stands suggest that climatic variability is driving short-term patterns in these soil properties rather than seasonal inputs of litter. In general, soils were warmer, drier and more basic with higher available P during dry season months. Even in wet tropical forests, small variability in climate can play an important role in soil dynamics over periods of weeks to months. Although litter manipulation did not influence soil properties over the 5-month study period, a longer lag may exist between the timing of litter inputs and the influence of that litter on soil properties, especially plant available P.     

Spatial patterns of the red palm weevil and applied entomopathogenic nematode Heterorhabditis bacteriophora

The distribution and abundance of red palm weevil (Rhynchophorus ferrugineus) within palm tree-infected stems at Ismaelia governorate, Egypt, were investigated. Taylor's power law was fit (P ≤ 0.01) to its larvae, pupae and/or adults. The slope value of this power law indicated the clumped distribution of these insect stages. Sample size optimisation needed to achieve a predetermined level of sampling error for the insect stages was calculated. Also, soil samples were taken after uniform application of entomopathogenic nematode Heterorhabditis bacteriophora strain EG₁ to the soil under citrus tree canopies at the rate of 10⁴ infective juveniles/400 cm² plots, and assayed for the nematode using the Galleria bait method. At the first and second sampling dates, the nematode displayed contagious distribution and attained mean insect mortality of 15.3 and 4, respectively. At the third date of sampling, the nematode showed random distribution according to chi-squared test and caused 1.8 mean insect mortality. Evaluation of H. bacteriophora EG₁ as a biocontrol measure for the weevil was discussed based on their investigated dispersion indices.     

Spatial variation of N2-fixation in field pea (Pisum sativum L.) at the field scale determined by the 15N natural abundance method

Grain legumes such as field pea are known to have high variability of yield and dinitrogen (N₂) fixation between seasons, but less is known about the yearly spatial variability within a field. The objective of this study was to improve the understanding of spatial field scale variability of field pea dry matter (DM) yield and nitrogen (N) acquisition from fixation and soil within a 10 ha farmer’s field. A 42 m systematic random grid providing 56 plant sampling locations across 10 ha supplemented by soil data provided from an existing database were used to determine whether the observed spatial variability could be explained by the variability in selected abiotic soil properties. All measured soil variables showed substantial variability across the field and the pea dry matter production ranged between 4.9 and 13.8 Mg ha^?1 at maturity. The percent of total N derived from the atmosphere (%Ndfa) at flowering, estimated using the ¹⁵N natural abundance method, ranged from 65% to 92% with quantitative N₂-fixation estimates from 93 kg to 202 kg N ha^?1. At maturity %Ndfa ranged from 26% to 81% with quantitative N₂-fixation estimates from 48 kg to 167 kg N ha^?1. Significant correlations were found between pea dry matter production and humus content, potassium content (collinear with humus) and total N in the 0–25 cm topsoil. No correlation was found between any individual soil property and %Ndfa or kg N fixed ha^?1. It was not possible to create a satisfactory global multi-regression model for the field dry matter production and N₂-fixation. A number of other models were tested, but the best was only able to explain less than 40% of the variance in %Ndfa using seven soil properties. Together with the use of interpolated soil data, high spatial variation of soil ¹⁵N natural abundance, a mean increase in pea ¹⁵N natural abundance of 1 δ unit between flowering and maturity and a reference crop decline of 1.3 δ¹⁵N unit over the same period increased noise of derived variables, making modeling of N₂-fixation difficult. Furthermore, complex interactions with other soil variables and biotic stresses not measured in this study may have contributed significantly to the variability of fixation and yield of pea within the field. Pea N₂-fixation obtained from two additional 10 ha farmer fields was in agreement with the other findings highlighting that N₂-fixation takes place under a range of physical and chemical soil properties and is controlled by local site specific conditions. In future studies addressing field scale variability we recommend that soil variables wherever possible should be measured in the same plots as the sampled crop. Sampling designs that optimize the use of a priori information about the field soil and landscape properties for positioning plots and that facilitate estimates of local variances should be considered.     

Landfarming Process Effects on Biochemical Properties of Petroleum-Contaminated Soils

The presence of petroleum contaminants in soil may be toxic to humans, plants, and soil microorganisms. Therefore, remediation of these compounds from the environment is vital. In this study, bioremediation of two petroleum-contaminated soils (S₁ and S₂) using a landfarming technique was evaluated. Investigation of the effect of this technique on biological and chemical properties of contaminated soil was also part of the goal. The results showed that about 50 and 57% of hydrocarbon contents were eliminated from soils S₁ and S₂ at the end of the experiment, respectively. Landfarming processes enhanced microbial respiration rate in both soils S₁ and S₂. Microbial biomass-nitrogen values in the landfarming plots were significantly (P < 0.05) higher than in the control plots (without landfarming operations). Urease activity increased by 21, 45, 26, and 23% in the landfarming plots as compared to the control plots for soil S₂ at the end of first to the 4th month of the experiment, respectively. There was also significant difference (P < 0.05) in soil pH values between the landfarming treatment and control. Soil electrical conductivity in the landfarming plots was lower than in the controls. Total organic matter and total nitrogen contents in the landfarming plots were significantly lower in comparison to the control plots. It appears that improving soil aeration and exposing new layers of soil to sunlight and air as a result of landfarming operations facilitated the degradation of petroleum hydrocarbons.     

Soil trace element changes during a phytoremediation trial with willows in southern Québec,Canada

This study determined the changes in trace elements (TE) (As, Cd, Cu, Ni, Pb, Zn) chemistry in the soils of a willow (“Fish Creek” – Salix purpurea, SV1 – Salix x dasyclados and SX67 – Salix miyabeana) plantation growing under a cold climate during a three-year trial. The soil HNO₃-extractable and H₂O-soluble TE concentrations and pools significantly decreased under most cultivars (Fish, SX67). Yet, TE changes showed inconsistent patterns and localized soil TE increases (Ni, Pb) were measured. Temporal changes in soil TE were also detected in control plots and sometimes exceeded changes in planted plots. Discrepancies existed between the amount of soil TE change and the amount of TE uptake by willows, except for Cd and Zn. Phytoremediation with willows could reduce soil Cd and Zn within a decadal timeframe indicating that they can be remediated by willows in moderately contaminated soils. However, the time needed to reduce soil As, Cu, Ni and Pb was too long to be efficient. We submit that soil leaching contributed to the TE decrease in controls and the TE discrepancies, and that the plantation could have secondary effects such as the accelerated leaching of soil TE.     

Simultaneous Simulations of Uptake in Plants and Leaching to Groundwater of Cadmium and Lead for Arable Land Amended with Compost or Farmyard Manure

The water budget of soil, the uptake in plants and the leaching to groundwater of cadmium (Cd) and lead (Pb) were simulated simultaneously using a physiological plant uptake model and a tipping buckets water and solute transport model for soil. Simulations were compared to results from a ten-year experimental field study, where four organic amendments were applied every second year. Predicted concentrations slightly decreased (Cd) or stagnated (Pb) in control soils, but increased in amended soils by about 10% (Cd) and 6% to 18% (Pb). Estimated plant uptake was lower in amended plots, due to an increase of K_d (dry soil to water partition coefficient). Predicted concentrations in plants were close to measured levels in plant residues (straw), but higher than measured concentrations in grains. Initially, Pb was mainly predicted to deposit from air into plants (82% in 1998); the next years, uptake from soil became dominating (30% from air in 2006), because of decreasing levels in air. For Cd, predicted uptake from air into plants was negligible (1–5%).     

Effects of plant species on phosphorus availability in a range of grassland soils

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using ³³P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot^–1) than ryegrass (1.1–15.6 mg pot^–1) from the soil except in the Temuka soil in which the level of available P (e.g., E _1min P_i, bicarbonate extractable P_i) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable P_i pool (E _{> 10m} P_i, reduced by 31.8% on the average) to the rapidly exchangeable P_i (E _1min-1d P_i, E _1d-10m P_i) pools in most soils. The values of R/r ₁ (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties.     

Differences in Belowground Heterogeneity Within a Restoration of a Dewatered Reservoir in Southwestern Wisconsin

Integrating belowground properties into ecological restoration is critical for understanding aboveground response. We investigated a recently dewatered basin in southwestern Wisconsin to test hypotheses related to the magnitude, heterogeneity, and spatial dependence of physicochemical properties between sediments and buried soils. A spatial grid was established in a 2.4‐ha basin to sample 7 physical and 13 chemical properties. Sediment depth (Sd) averaged 43 cm (std dev = 15) across the study area. The mean bulk density (BD) of the buried soils (1.41 g/cm³) was significantly greater than the sediments (0.97 g/cm³), which corresponded with a significantly higher soil moisture content (SMC) and hydraulic conductivity (K_s) of the sediments. Relative to one another, buried soils had significantly higher concentrations of Bray P, Cu, and Mn, whereas sediments had higher concentrations of Bray K, K, total phosphorus, Na, S, Al, and Ca and thus had a higher pH; Fe and B concentrations were similar between the sediments and the buried soil. Sediments were consistently less spatially heterogeneous than the buried soils. Using geostatistics, sediment BD and SMC displayed both small‐scale spatial structures and large‐scale trends, whereas Sd and the buried soil SMC displayed only small‐scale variability. Distance from the former dam was positively related to nine physicochemical properties and negatively related to only S, SMC, and the percentage of clay; no trends were found in the buried soils. These results demonstrate that damming, inundation, and dewatering due to dam removal yielded a spatially distinct and homogenized substrate relative to the soils that were buried.     

重度放牧对欧亚温带草原东缘生态样带土壤氮矿化及其温度敏感性的影响

以欧亚温带草原东缘生态样带为平台,以样带上未放牧和重度放牧配对样地为研究对象,开展重度放牧对欧亚温带典型草原土壤氮矿化及其温度敏感性的影响研究。结果表明:(1)在室内培养条件下,土壤氮积累量和土壤净氮矿化速率呈现出干燥度指数较大的样点显著大于干燥度指数最低的样点(P0.05)。在相对湿润的样点,土壤氮素矿化周转速率较快;(2)重度放牧对不同样点土壤氮积累量和土壤氮矿化速率的影响是不同的。在干燥度指数较高样点,重度放牧样地土壤铵态氮减少量和速率较未放牧样地低(P0.05),重度放牧显著降低了土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率(P0.05);在干燥度指数较低样点,重度放牧样地土壤铵态氮减少量和速率较未放牧样地高(P0.05),重度放牧对土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率影响不大(P0.05);(3)土壤氮矿化作用温度敏感性(Q_(10))变化范围在1.61—2.06,重度放牧对Q_(10)无显著影响。随着纬度的升高,Q_(10)呈升高趋势。Q_(10)与基质质量指数以及表观活化能与基质质量指数均呈显著的负相关关系(P0.05);(4)土壤硝态氮积累量、无机氮积累量、硝化速率、净氮矿化速率对重度放牧的响应比与干燥度指数呈极显著负相关(P0.01),重度放牧对欧亚温带典型草原土壤氮矿化的影响受气候条件(温度和降水)的调控。     

Soil and total ecosystem respiration in agricultural fields: effect of soil and crop type

A study was made of the effect of soil and crop type on the soil and total ecosystem respiration rates in agricultural soils in southern Finland. The main interest was to compare the soil respiration rates in peat and two different mineral soils growing barley, grass and potato. Respiration measurements were conducted during the growing season with (1) a closed-dynamic ecosystem respiration chamber, in which combined plant and soil respiration was measured and (2) a closed-dynamic soil respiration chamber which measured only the soil and root-derived respiration. A semi-empirical model including separate functions for the soil and plant respiration components was used for the total ecosystem respiration (TER), and the resulting soil respiration parameters for different soil and crop types were compared. Both methods showed that the soil respiration in the peat soil was 2–3 times as high as that in the mineral soils, varying from 0.11 to 0.36 mg (CO₂) m^–2 s^–1 in the peat soil and from 0.02 to 0.17 mg (CO₂) m^–2 s^–1 in the mineral soils. The difference between the soil types was mainly attributed to the soil organic C content, which in the uppermost 20 cm of the peat soil was 24 kg m^–2, being about 4 times as high as that in the mineral soils. Depending on the measurement method, the soil respiration in the sandy soil was slightly higher than or similar to that in the clay soil. In each soil type, the soil respiration was highest on the grass plots. Higher soil respiration parameter values (R_s0, describing the soil respiration at a soil temperature of 10°C, and obtained by modelling) were found on the barley than on the potato plots. The difference was explained by the different cultivation history of the plots, as the potato plots had lain fallow during the preceding summer. The total ecosystem respiration followed the seasonal evolution in the leaf area and measured photosynthetic flux rates. The 2–3-fold peat soil respiration term as compared to mineral soil indicates that the cultivated peat soil ecosystem is a strong net CO₂ source.     

Applying a solute transfer model to phytoextraction: Zinc acquisition by Thlaspi caerulescens

Phytoextraction is the removal of metals from contaminated soils into harvested plant tissues. The rate of phytoextraction is governed by both soil and plant characteristics. Most effort has focused on identifying appropriate plants for phytoextraction, but the benefits from this effort will be marginal unless the metals are in phytoavailable forms in the rhizosphere. The concentration of a metal in the rhizosphere can be estimated using solute transfer models that incorporate: the metal concentration in the bulk soil solution, the buffer power of the soil, diffusion coefficient for the metal, water movement, root size and morphology, and the rate of entry of metal into the roots. Here a solute transfer model is developed to predict the concentration of Zn in the rhizosphere solution ([Zn]_ext) of Thlaspi caerulescens, a hyperaccumulator species that could be exploited for Zn phytoextraction. The model predicts that Zn accumulation by T. caerulescens is sub-optimal when the Zn concentration in the bulk soil solution is <27 M. Such a high [Zn]_ext is rare in contaminated agricultural soils, but is possible in the metalliferous substrates where T. caerulescens is endemic. Sensitivity analyses indicate that Zn diffusion is more important than transpiration-driven mass flow for Zn delivery to the root, implying that management of soil physical and hydrological properties will improve phytoextraction. Sensitivity analyses also imply that strategies to enhance the Zn absorption power of the root will not necessarily be successful for enhancing phytoextraction per se. Thus, research into enhancing Zn availability and mobility in soil will be as important as understanding and manipulating Zn uptake by plants. In general, such models can be used to identify constraints to efficient phytoextraction (whether plant or soil) and to determine whether commercial phytoextraction is feasible.     

Effects of soil frost on nitrogen net mineralization, soil solution chemistry and seepage losses in a temperate forest soil

Freezing and thawing may alter element turnover and solute fluxes in soils by changing physical and biological soil properties. We simulated soil frost in replicated snow removal plots in a mountainous Norway spruce stand in the Fichtelgebirge area, Germany, and investigated N net mineralization, solute concentrations and fluxes of dissolved organic carbon (DOC) and of mineral ions (NH₄⁺, NO₃⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺). At the snow removal plots the minimum soil temperature was −5 °C at 5 cm depth, while the control plots were covered by snow and experienced no soil frost. The soil frost lasted for about 3 months and penetrated the soil to about 15 cm depth. In the 3 months after thawing, the in situ N net mineralization in the forest floor and upper mineral soil was not affected by soil frost. In late summer, NO₃⁻ concentrations increased in forest floor percolates and soil solutions at 20 cm soil depth in the snow removal plots relative to the control. The increase lasted for about 2–4 months at a time of low seepage water fluxes. Soil frost did not affect DOC concentrations and radiocarbon signatures of DOC. No specific frost effect was observed for K⁺, Ca²⁺ and Mg²⁺ in soil solutions, however, the Na⁺ concentrations in the upper mineral soil increased. In the 12 months following snowmelt, the solute fluxes of N, DOC, and mineral ions were not influenced by the previous soil frost at any depth. Our experiment did not support the hypothesis that moderate soil frost triggers solute losses of N, DOC, and mineral ions from temperate forest soils.     

Effects of community- and neighborhood-scale spatial patterns on semi-arid perennial grassland community dynamics

Although plant spatial patterns strongly influence community-structuring processes, few empirical studies have addressed pattern effects on perennial community dynamics. We tested the effects of community- and neighborhood-scale patterns in experimental semi-arid grassland communities comprising the stronger competitor crested wheatgrass (Agropyron cristatum) and the weaker competitor Snake River wheatgrass (Elymus wawawaiensis). Treatments consisted of community-scale patterns (Poisson random, regular, and aggregated) and neighborhood-scale patterns (Poisson random, small, and large aggregations) applied to 6.25-m² plots, with aggregations generated through simulated realizations of Neyman-Scott cluster processes. Two years of data were collected on aboveground biomass of both species, and variability in light (photosynthetically active radiation; PAR) was also quantified. We found that plant performance was strongly affected by community-scale spatial patterns and time, with additional effects of neighborhood-scale pattern in certain treatments. Mean biomass and relative growth rates of both species were highest in plots with community-scale regularity and random neighborhoods, suggesting a strong effect of pattern on competition that was magnified for the weaker competitor E. wawawaiensis, especially in the second year. There were also significant effects of treatment and time on variability of PAR, supporting past research on the importance of canopy patterns for light distribution near the soil surface. We observed more variable light environments in plots with community-scale aggregation, and variability also increased in the second year. Our research provides new information on the effects of plant patterns on community dynamics, with particular relevance for semi-arid perennial grasslands.     

The effect of afforestation with Scots pine (Pinus silvestris L.) of sandy post-arable soils on their selected properties. II. Reaction, carbon, nitrogen and phosphorus

Despite the extensive literature on the effect on soil properties of afforestation of former arable land, we still lack full understanding of whether the changes proceed in the same direction and at the same rate, and of how long is required to achieve a state of soil equilibrium typical of a natural forest ecosystem. Therefore, as part of a study comparing post-arable sandy soils (Dystric Arenosols) afforested with Scots pine (Pinus silvestris L.) with arable soils and soils of continuous coniferous forests, the range and direction of changes in pH, organic carbon (C_org), total nitrogen (N_tot), ammonium (N-NH₄) and nitrates (N-NO₃) in soil solution, total (P_tot) and available (P_av) phosphorus were determined. The studies were carried out in south-east Poland (51°30′-51°37′N, 22°20′-22°35′E). Ten paired sites of afforested soils (five with 14- to 17-year-old stands and five with 32- to 36-year-old stands) with adjacent cultivated fields, and five sites of continuous forest with present stands of ca. 130–150 years old were selected. Soil samples were taken from the whole thickness of master horizons and, in the case of the A horizon of the afforested soils, from three layers: 0–5 (A_0–5), 5–10 (A_5–10) and 10–20 cm (A_10–20). The cultivated soils in the Ap horizon showed higher pH (by ca. 1.0 unit), lower C_org and C:N, similar N_tot, lower N-NH₄, higher N-NO₃, higher P_tot and P_av contents compared with the Ah horizon of continuous forest soils. The results indicated decreased soil pH in the former plough layer of the afforested soils, with the greatest decrease observed in the 0–5 cm layer. In these soils, the C_org content was considerably higher in the A_0–5 layer, but lower in the two deeper layers and in the whole A horizon (0–20 cm) compared with the Ap horizon of the arable soils. The results indicate that the C_org content, after an initial phase of decline, again achieved a level characteristic of arable soils. The N_tot content in all layers of the A horizon of the afforested soils was lower than in the Ap horizon of the arable soils, and showed a reduction with stand age, especially in deeper layers. The C:N ratios in the mineral topsoil increased with stand age. N-NH₄ content increased and N-NO₃ decreased after afforestation. The P_tot and P_av contents in all layers and in the whole A horizon of the afforested soils, on stands of both ages, was lower than in the Ap of the cultivated soils. From the results, it could be concluded that, after more than 30 years of tree growth, the soils of the A horizon were still more similar to arable than to continuous forest soils with respect to C_org, P_tot and P_av. With respect to pH, N-NH₄ and N-NO₃, especially in the 0–5 cm layer, they were more similar to continuous forest soils than to cultivated soils, but with respect to N_tot and C:N ratio they were somewhere in between.