European Journal of Soil Science

Chúng tôi đã xem xét tác động của sự lắng đọng axit và các nguồn axit khác trong suốt 110–140 năm qua lên đất dưới rừng (Broadbalk và Geescroft Wildernesses) và đồng cỏ (Park Grass), bao gồm một số thí nghiệm cổ điển tại Trạm Thí nghiệm Rothamsted. Những thay đổi trong hóa học đất đã được theo dõi bằng cách phân tích một số mẫu lưu trữ độc đáo cho pH, cation cơ bản tan trong nước và có khả năng trao đổi, nhôm, sắt và mangan, độ acid trao đổi, khả năng trao đổi cation (CEC) và anion tan. Các cân bằng proton và dữ liệu lịch sử cho thấy tầm quan trọng của sự lắng đọng axit đối với sự axit hóa và những thay đổi đồng thời trong hóa học của đất. Giá trị pH của đất mặt ở Geescroft Wilderness đã giảm từ 6.2 xuống 3.8 kể từ năm 1883. Sự giảm pH của mảnh đất không được vôi hóa và không phân bón ở Park Grass ít hơn trong cùng một khoảng thời gian (từ pH 5.2 xuống 4.2), minh họa tác động đáng kể của tán cây rừng lên việc ngăn chặn các chất ô nhiễm có khả năng axit hóa. Tác động của sự gia tăng độ axit lên hóa học đất của Geescroft Wilderness được thấy qua việc giảm độ bão hòa cơ bản và CEC, với các cation cơ bản di chuyển xuống sâu trong hồ sơ đất. Các khoáng vật đất sét đang bị phong hóa không thể hồi phục, và Mn cùng với Al đang dần được thúc đẩy, đến nỗi hiện tại Al chiếm 70% của phức hợp trao đổi trong đất mặt. Ngay cả với sự giảm lắng đọng lưu huỳnh hiện tại, các tải trọng quan trọng đối với lưu huỳnh, nitơ và độ axit vẫn đang bị vượt quá. Những hệ sinh thái bán tự nhiên như vậy là không bền vững dưới bối cảnh ô nhiễm hiện tại.

Understanding the movement of cations in soil, particularly trace metals, is required in many applications such as phytoremediation and pollution control. A dynamic mechanistic model has been developed to describe the long‐term root uptake of a surface‐applied, strongly adsorbed, pollutant metal cation, such as radiocaesium, from soil. It consists of two submodels. The first calculates uptake per unit root length at a local scale over a root's lifetime, for various initial conditions. The second calculates cumulative uptake at a whole‐plant scale for the entire rooting depth as a function of time. The model takes into account the renewal of roots which are considered to have a limited lifetime. Root density may be a function of soil depth and a proportion of roots need not contribute to uptake. Recycling from decaying, or grazed, roots and shoots is considered.

Simulations show that removal of cations from soil is exaggerated unless some recycling by roots or shoots is considered or the entire root length does not contribute to uptake. Because of root turnover, uptake is not rapidly limited by diffusive flux of the cation from the bulk soil solution to the solution–root interface. Uptake is very sensitive to root architecture and plant physiology.

Những thí nghiệm thực địa dài hạn của Rothamsted, bắt đầu hơn 150 năm trước, cung cấp vật liệu độc đáo để nghiên cứu chu kỳ carbon trong tầng đất dưới bề mặt. Tổng hợp carbon hữu cơ, ¹⁴C và ¹³C đã được đo trên các hồ sơ đất từ những thí nghiệm này, trước và sau các thử nghiệm bom nhiệt hạch vào giữa thế kỷ 20. Bốn hệ thống quản lý đất đối nghịch đã được lấy mẫu: đất trồng hàng năm cho lúa mì mùa đông; rừng tái sinh trên đất chua; rừng tái sinh trên đất canxi; và đồng cỏ cũ. Tuổi trung bình của carbon phóng xạ từ tất cả các mẫu trước khi thử nghiệm bom trên đất trồng là 1210 năm (0–23 cm), 2040 năm (23–46 cm), 3610 năm (46–69 cm) và 5520 năm (69–92 cm). Carbon phóng xạ từ thử nghiệm bom nhiệt hạch có mặt ở toàn bộ hồ sơ trong tất cả các mẫu sau bom, mặc dù dưới 23 cm số lượng thấp và các phép đo carbon phóng xạ trước và sau khi thử nghiệm bom thường không khác biệt đáng kể. Giá trị δ¹³C tăng xuống dưới mặt cắt, từ −26.3‰ (lớp 0–23 cm, trung bình của tất cả các phép đo) đến −25,2‰ cho lớp 69–92 cm. Tỷ lệ C/N giảm theo độ sâu trong hầu hết các hồ sơ được lấy mẫu. Ngoại trừ các lớp đất bề mặt (0–23 cm) từ đồng cỏ cũ, phương trình hyperbola m = 152.1 − 2341/(1 + 0.264n) cung cấp độ khớp chặt chẽ với dữ liệu carbon phóng xạ từ tất cả các độ sâu, thời gian lấy mẫu và các địa điểm đã thử nghiệm, trong đó n là hàm lượng carbon hữu cơ của đất, tính bằng tấn/ha⁻¹, và m là hàm lượng carbon phóng xạ của đất, trong các đơn vị Δ¹⁴C, đã được điều chỉnh cho các biến đổi của lớp đất theo thời gian. Các loại đất đồng cỏ khác biệt gần như chắc chắn chứa than: một trong số chúng đã được chứng minh bằng ¹³C‐NMR để chứa 0.82% carbon than. Trong Phần 2 (số này) của cặp bài báo này, các phép đo carbon phóng xạ và tổng hợp carbon này được sử dụng để phát triển và kiểm tra một mô hình mới cho chu trình của carbon hữu cơ trong tầng đất dưới bề mặt.

The aim of this study was to evaluate the pattern of nitrous oxide (N₂O) and methane (CH₄) fluxes, and leaching losses of nitrate (NO₃⁻) and dissolved organic C (DOC), during a fallow–onion crop–fallow cycle in a Mediterranean area. The importance of the fallow (intercrop) period and the type of fertilizer were also evaluated. Goat and chicken manure (M) from an organic farm, digested pig slurry (DPS) and urea (U) were applied at a rate of 110 kg N ha⁻¹ and compared with a zero N treatment (Control). The crop period contributed more than each fallow period to the total N₂O emission (ranging from 70 to 85% of the total emission, depending on the treatment). The variability of rainfall during fallow periods affected N₂O emissions, with the highest fluxes observed in the second fallow, which was the wetter. Negative net fluxes of N₂O (0 to −0.4 mg N₂O‐N m⁻² day⁻¹) were mainly observed during the irrigation period and in fallow periods. The type of fertilizer had no effect on N₂O fluxes, but influenced the CH₄ oxidation. The largest CH₄ emission was from the manure treatment (2.4 mg CH₄‐C m⁻² day⁻¹) during the irrigation period. The lowest NO₃⁻ but highest DOC leaching rates were measured during the second fallow period from the manure treated plots (0.2 kg NO₃⁻‐N ha⁻¹ and 3.9 kg C ha⁻¹), which also had the highest drainage. The use of OM, therefore, seems to be a suitable method to reduce the environmental impacts associated with N leaching as well as increase the potential to denitrify NO₃⁻ in groundwater.

Plant roots influence the biological, chemical and physical properties of rhizosphere soil. These effects are a consequence of their growth, their activity and the exudation of organic compounds from them. In natural ecosystems, the linkages between inputs of carbon from plants and microbial activity driven by these inputs are central to our understanding of nutrient cycling in soil and the productivity of these systems. This coupling of plant and microbial productivity is also of increasing importance in agriculture, where the shift towards low‐input systems increases the dependence of plant production on nutrient cycling, as opposed to fertilizers. This review considers the processes by which plants can influence the cycling of nutrients in soil, and in particular the importance of organic inputs from roots in driving microbially mediated transformations of N. This coupling of plant inputs to the functioning of the microbial community is beneficial for acquisition of N by plants, particularly in low‐input systems. This occurs through stimulation of microbes that produce exoenzymes that degrade organic matter, and by promoting cycling of N immobilized in the microbial biomass via predation by protozoa. Also, plants increase the cycling of N by changes in exudation in response to nitrogen supply around roots, and in response to browsing by herbivores. Plants can release compounds in exudates that directly affect the expression of genes in microbes, and this may be an important way of controlling their function to the benefit of the plant.

It is often thought that the most important source of nitrogen for plants and microorganisms comes from amino acids and amino sugars when they are hydrolysed in acid conditions. We did a microcosm experiment to test the hypothesis. In the experiment spruce seedlings (Picea abies L. Karst) were grown for 145 days in soil taken from a podzol Oa horizon under a long‐term nitrogen fertilization experiment (control and N‐treated soil). Net changes in different pools of organic N were determined using standard fractionation (acid hydrolysis and pyrophosphate extraction). During the experiment the amino acid and amino sugar pools decreased significantly (14% and 15% for the control and 10% and 17% for the N treatment), whereas no significant change was observed in the non‐amino acid plus non‐amino sugar fraction. On a per organic C basis there was even a significant increase in the non‐amino acid plus non‐amino sugar fraction of 11% for the control and 8% for the N treatment. Pyrophosphate extractions suggest that amino acids or amino sugars associated with the humin fraction were more accessible to microbes and plants than those associated with the humic acid, fulvic acid and hydrophilic substances. The long‐term N fertilization (about 73 kg N ha⁻¹ was added annually as NH₄NO₃ during a 24‐year period) resulted in an enrichment of all major fractions of organic N, i.e. amino acids, amino sugars and non‐amino acids plus non‐amino sugars. This enrichment was largely the result of small increases in all of the amino acids rather than large increases in just a few.

Loss on ignition (LOI) is one of the most widely used methods for measuring organic matter content in soils but does not have a universal standard protocol. A large number of factors may influence its accuracy, such as furnace type, sample mass, duration and temperature of ignition and clay content of samples. We conducted a series of experiments to quantify these effects, which enabled us to derive (i) guidelines for ignition conditions (sample mass, duration and temperature), (ii) temperature‐specific soil organic matter (SOM) to soil organic carbon (SOC) conversion factors and (iii) clay content‐dependent correction factors for structural water loss (SWL). Bulk samples of a sandy soil (4% clay) and a silt loam soil (25% clay) were used to evaluate the effects of ignition conditions. Samples with a range of clay contents (0–50%) were used to quantify conversion and correction factors. Two furnaces, one without and one with pre‐heated air, did not show significant differences in terms of within‐batch LOI variability. In both furnaces less combustion occurred close to the door, which necessitated tray turning at half‐time as this reduced the standard deviation per batch significantly. Variation in mass loss declined exponentially with sample mass (range, 0.15–20 g). The LOI increased with duration at lower temperatures (≤ 550°C) for the sandy soil. At greater temperatures (600 and 650°C), no effect of duration was found. For the silt loam soil, LOI values increased with duration for each temperature, which was attributed to SWL. The SOM to SOC conversion factor decreased strongly with temperature at an ignition duration of 3 hours from 0.70 (350°C) to 0.57 (500°C) and stabilized around 0.55 between 550 and 650°C, indicating that at temperatures ≥ 550°C all SOM had been removed. The clay correction factor for SWL increased from 0.01 to 0.09 as the temperature of ignition increased from 350 to 650°C. To minimize within‐batch LOI variation we recommend a standard ignition duration of 3 hours, tray turning at half‐time, a sample mass ≥ 20 g and temperatures equal to or greater than 550 °C. To avoid over‐estimates of SOM through structural water loss, the presented SWL correction procedure should always be applied.

Several biochemical and molecular methods are used to investigate the microbial diversity and changes in microbial community structure in rhizospheres and bulk soils resulting from changes in management. We have compared the effects of plants on the microbial community, using several methods, in three different types of soils. Pots containing soil from three contrasting sites were planted with Lolium perenne (rye grass). Physiological (Biolog), biochemical (PLFA) and molecular (DGGE and TRFLP) fingerprinting methods were employed to study the change in soil microbial communities caused by the growth of rye grass. Different methods of DNA extraction and nested PCR on TRFLP profiles were examined to investigate whether they gave different views of community structure. Molecular methods were used for both fungal and bacterial diversity. Principal component analysis of Biolog data suggested a significant effect of the plants on the microbial community structure. We found significant effects of both soil type and plants on microbial communities in PLFA data. Data from TRFLP of soil bacterial communities showed large effects of soil type and smaller but significant effects of plants. Effects of plant growth on soil fungal communities were measured by TRFLP and DGGE. Multiple Procrustes analysis suggested that both methods gave similar results, with only soil types having a significant effect on fungal communities. However, TRFLP was more discriminatory as it generated more ribotype fragments for each sample than the number of bands detected by DGGE. Neither methods of DNA extraction nor the nested PCR had any effect on the evaluation of soil microbial community structure. In conclusion, the different methods of microbial fingerprinting gave qualitatively similar results when samples were processed consistently and compatible statistical methods used. However, the molecular methods were more discriminatory than the physiological and biochemical approaches. We believe results obtained from this experiment will have a major impact on soil microbial ecology in general and rhizosphere–microbial interaction studies in particular, as we showed that the different fingerprinting methods for microbial communities gave qualitatively similar results.