Initial soil organic carbon concentration influences the short-term retention of crop-residue carbon in the fine fraction of a heavy clay soil

Among factors controlling decomposition and retention of residue C in soil, effect of initial soil organic C (SOC) concentration remains unclear. We evaluated, under controlled conditions, short-term retention of corn residue C and total soil CO₂ production in C-rich topsoil and C-poor subsoil samples of heavy clay. Topsoil (0–20 cm deep, 31.3 g SOC kg^?1 soil) and subsoil (30–70 cm deep, 4.5 g SOC kg^?1 soil) were mixed separately with ¹³C–¹⁵N-labeled corn (Zea mays L.) residue at rates of 0 to 40 g residue C kg^?1 soil and incubated for 51 days. We measured soil CO₂–C production and the retention of residue C in the whole soil and the fine particle-size fraction (<50 μm). Cumulative C mineralization was always greater in topsoil than subsoil. Whole-soil residue C retention was similar in topsoil and subsoil at rates up to 20 g residue C kg^?1. There was more residue C retained in the fine fraction of topsoil than subsoil at low residue input levels (2.5 and 5 g residue C kg^?1), but the trend was reversed with high residue inputs (20 and 40 g residue C kg^?1). Initial SOC concentration affected residue C retention in the fine fraction but not in the whole soil. At low residue input levels, greater microbial activity in topsoil resulted in greater residue fragmentation and more residue C retained in the fine fraction, compared to the subsoil. At high residue input levels, less residue C accumulated in the fine fraction of topsoil than subsoil likely due to greater C saturation in the topsoil. We conclude that SOC-poor soils receiving high C inputs have greater potential to accumulate C in stable forms than SOC-rich soils.     

Biochemical characterization of urban soil profiles from Stuttgart, Germany

The knowledge of biochemical properties of urban soils can help to understand nutrient cycling in urban areas and provide a database for urban soil management. Soil samples were taken from 10 soil profiles in the city of Stuttgart, Germany, differing in land use—from an essentially undisturbed garden area to highly disturbed high-density and railway areas. A variety of soil biotic (microbial biomass, enzyme activities) and abiotic properties (total organic C, elemental C, total N) were measured up to 1.9 m depth. Soil organic matter was frequently enriched in the subsoil. Microbial biomass in the top horizons ranged from 0.17 to 1.64 g C kg⁻¹, and from 0.01 to 0.30 g N kg⁻¹, respectively. The deepest soil horizon at 170-190 cm, however, contained 0.12 g C kg⁻¹ and 0.05 kg N kg⁻¹ in the microbial biomass. In general, arylsulphatase and urease activity decreased with depth but in three profiles potentially mineralizable N in the deepest horizons was higher than in soil layers directly overlying. In deeply modified urban soils, subsoil beside topsoil properties have to be included in the evaluation of soil quality. This knowledge is essential because consumption of natural soils for housing and traffic has to be reduced by promoting inner city densification.     

Spatial and vertical analysis of soil properties in a Mediterranean vineyard soil

Soil physical properties influence vineyard behaviour; therefore, the knowledge of their spatial variability is essential for making vineyard management decisions. Little work has been conducted at high spatial resolution on soil properties at depths lower than 0.30 m which is of special relevance to perennial crops. The objectives of this work were to (i) analyse the spatial and vertical variability of soil depth, particle size fractions and water‐holding capacity (WHC) by geostatistical techniques; (ii) study the causes of the variability, with additional information from classical soil sampling; and (iii) assess the significance of WHC through its relationship with vine vigour. The work was carried out in a vineyard of eight hectares within the D.O.Ca. Rioja (northern Spain). Soil variability was determined via grid sampling at three depth ranges (0–0.30, 0.30–0.60 and 0.60–0.90 m). A conventional soil survey provided additional information on soil variability. Clay, sand and silt fractions, soil organic matter content, WHC and pruning weight were determined. Most soil properties had strong or moderate spatial dependence, with the exceptions of sand at 0.30–0.60 m and silt in the topsoil. Topography and soil erosion caused the spatial variability of soil depth and contributed to the spatial distribution of particle size fractions in the topsoil, while the heterogeneity of parent material influenced the spatial pattern of soil properties at 0.60–0.90 m. The WHC and soil depth spatial distributions related well to that of vine vigour, demonstrating the importance of knowing the spatial variability of these soil properties.     

玉米秸秆覆盖与深翻两种还田方式对黑土有机碳固持的影响

秸秆还田是实现东北黑土肥力提升与保障区域生态环境安全的有效措施。明确玉米秸秆覆盖与深翻还田下土壤有机碳(SOC, Soil Organic Carbon)的变化及其在团聚体中的固持特征,对于揭示秸秆还田后黑土有机碳的稳定机制与固碳潜力具有重要意义。该研究基于黑土区中部6 a定位试验,选择常规种植(CK)、秸秆覆盖还田(SM, Stovers Mulching)和秸秆深翻还田(SI, Stovers Incorporation)3个处理,对0～10、10～20、20～30及30～40 cm土层SOC含量、容重、水稳性团聚体分布及团聚体中有机碳(OC, Organic Carbon)含量进行了分析与测定,并对各处理年均碳投入量、SOC储量与土壤固碳速率等进行了估算。与CK相比,SM处理显著增加了0～10 cm土层SOC含量,增幅为22.4%,但对10～40cm土层SOC含量无显著影响;SI处理显著增加了0～40cm土层SOC含量,增幅为18.1%～41.5%,以20～30cm的增幅最突出。与SM处理相比,SI处理0～10 cm土层SOC储量显著低于前者,而20～30 cm土层SOC储量反之。6 a间,SM处理耕层(0～20 cm)与亚耕层(20～40 cm)土壤固碳速率分别为1.34和0.77 Mg/(hm2·a),SI处理为0.85和1.74 Mg/(hm2·a)。秸秆不同还田方式显著改变了0～40 cm土层团聚体分布及其中OC含量。与CK相比,SM显著增加了耕层大团聚体(0.25 mm)比例与平均质量直径(MWD, Mean Weight Diameter),SI显著提高了0～40 cm土层团聚体MWD,且对10～40 cm土层团聚结构的改善作用优于SM;SM处理显著增加了0～10 cm土层2和0.053 mm粒级团聚体OC含量,SI处理不仅增加了0～10 cm土层2 mm粒级团聚体OC含量,也显著提高了10～40 cm土层各粒级团聚体OC含量。在黑土区,秸秆覆盖还田对SOC的提升主要集中于表层,秸秆深翻还田促进了0～40cm土层SOC积累与土壤团聚结构的改善。     

Relationships between spatial pattern of Locusta migratoria manilensis eggpods and soil property variability in coastal areas

Spatial distribution of Locusta migratoria manilensis eggpods and soil properties (water content at 5 cm depth, salinity, organic matter and pH) was studied by integrating geostatistical analysis and GIS techniques. During 2 years of surveys over the entire study area (6000 ha), extensive data (292 regularly grids with 450-m intervals), coupled with intensive data (2601 regularly grids of 0.5-m separation) were used to characterize spatial patterns of L. m. manilensis eggpods and soil property variability and to explore the relationship between them. Semivariograms indicated the eggpods and four soil properties showed high spatial heterogeneity. The spatial distribution of eggpods, at distances ranging from 50-452 m in spatial autocorrelation, was best described using the spherical model. Spatial autocorrelation in total spatial heterogeneity in soil water content at 5 cm, salinity, organic matter and pH were 76.15, 78.04, 57.19 and 61.85%, respectively, and the scales of spatial heterogeneity were 621, 594, 1014 and 1368 m, respectively. GIS assessment maps of eggpods and soil properties, derived by block kriging, displayed patterns of the locust eggpods and soil property variability at an area-wide scale. Most eggpods (66.27% and 72.24% for 2002 and 2003, respectively) were found at the areas with low salinity (<2.0% approximately), suitable water content at 5 cm (10.1-20.0%). No eggpods were found at such areas with high soil water content at 5 cm (>30%) and salinity (>3%). In a way, the spatial distribution pattern of locust eggpods mainly depended on the soil heterogeneity at the study area. t-Tests indicated that sites between with eggpods and without eggpods were significantly different only in soil water content at 5 cm depth and salinity. The results may provide useful information on sampling in the field, forecasting and monitoring locust plague and reclaiming coastal locust breeding areas in China.     

不同秸秆还田方式对白浆土土壤养分及大豆产量的影响

为确定秸秆还田方式对白浆土土壤养分及作物产量的影响,试验设置了普通翻耕的对照处理以及秸秆覆盖还田、心土还田和秸秆焚烧的3种还田方式。3年的试验结果表明:在耕层部分(0~20 cm),普通翻耕处理区土壤氮素和有机质含量测定值最低,而土壤磷素和钾素含量最高;在白浆层(20~40 cm),秸秆心土还田处理的土壤碱解氮、全氮、有效磷、全磷和有机质含量最高;在淀积层(40~60 cm),不同秸秆还田方式,氮素、磷素和有机质含量变化差异较小,钾素表现为土壤下层含量比表层高。两年的数据显示秸秆心土还田处理大豆产量最高,说明秸秆心土还田对土壤地力提升效果明显,利于大豆增产。     

黏土夹层位置对黄河泥沙充填复垦土壤水分入渗的影响

在中国东部地区,土地复垦的一项重要任务是将采煤沉陷地复垦为耕地,复垦后的耕地生产力水平应接近损毁前的水平。然而,采用传统的黄河泥沙一次性充填后覆盖一定厚度土壤层的复垦方式,复垦后土壤持水性差,生产力水平低。夹层式充填复垦能够有效改善传统充填复垦土壤的水分特性,该文研究夹层位置对黄河泥沙填复垦土壤水分入渗过程的影响,共设计当地普通农田土壤剖面(CK1),传统"上土下沙"土壤剖面构型(CK2)及5个夹层式土壤剖面构型处理T1～T5:在60 cm厚的黄河泥沙充填层中的不同位置夹20 cm厚心土层(黏土层),夹层距离表土距离分别为50 cm(T1),55 cm(T2),60 cm(T3),65 cm(T4)及70 cm(T5)。通过室内入渗试验,分析不同位置设置夹层后土壤水分入渗特性及含水率分布情况,优选适应于该地区的夹层式土壤剖面构型。结果表明:入渗率随着夹层深度的增加呈先增加后减小的趋势,即夹层位置距离土表55cm为一临界深度,此时重构土壤的入渗率最低、湿润锋的运移速度最慢、阻水效果最强,但考虑到研究区强降雨天气,易形成地表径流。当心土夹层位置距离土表60cm时,更接近普通农田土壤水分入渗特性,是黄河泥沙夹层式充填复垦的理想选择。该研究对深入探讨黄河泥沙夹层式充填复垦内部作用机理和指导滨黄河地区采煤沉陷地的土地复垦具有重要意义。     

Temporal change in spatial variability of soil respiration on a slope of Japanese cedar (Cryptomeria japonica D. Don) forest

Although information regarding the spatial variability of soil respiration is important for understanding carbon cycling and developing a suitable sampling design for estimating average soil respiration, it remains relatively understudied compared to temporal changes. In this study, soil respiration was measured at 35 locations by season on a slope of Japanese cedar forest in order to examine temporal changes in the spatial distribution of soil respiration. Spatial variability of soil respiration varied between seasons, with the highest coefficient variation in winter (42%) and lowest in summer (26%). Semivariogram analysis and kriged maps revealed different patterns of spatial distribution in each season. Factors affecting the spatial variability were relief index (autumn), soil hardness of the A layer (winter), soil hardness at 50 cm depth (spring) and the altitude and relief index (summer). Annual soil respiration (average: 39 mol m⁻² y⁻¹) varied from 26 mol m⁻² y⁻¹ to 55 mol m⁻² y⁻¹ between the 35 locations and was higher in the upper part of the slope and lower in the lower part. The average Q₁₀ value was 2.3, varying from 1.3 to 3.0 among the locations. These findings suggest that insufficient information on the spatial variability of soil respiration and imbalanced sampling could bias estimates of current and future carbon budgets.     

Land-use change affects stocks and stoichiometric ratios of soil carbon,nitrogen, and phosphorus in a typical agro-pastoral region of northwest China

Purpose

The impacts of land-use change on dynamics of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the subsoil (>?30 cm) are poorly understood. This study aims to investigate whether the effects of land-use change on stocks and stoichiometric ratios (R_CN, R_CP, and R_NP) of SOC, TN, and TP can be different between topsoil (0–30 cm) and subsoil (30–60 cm) in the Ili River Valley, northwest China.

Materials and methods

Soil samples (0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm) were collected from a pasture (PT), a 27-year-old cropland (CL) converted from PT, and a 13-year-old poplar (Populus tomentosa Carr.) plantation (PP) converted from CL. SOC, TN, and TP concentrations and soil bulk density were determined to calculate stocks and stoichiometric ratios (molar ratios) of SOC, TN, and TP.

Results and discussion

Conversion from PT to CL led to substantial losses in SOC, TN, and TP pools in both topsoil and subsoil, and the reduction rates in subsoil (13.8–24.7%) were higher than those in topsoil (8.5–17.3%), indicating that C, N, and P pools in subsoil could also be depleted by cultivation. Similar to topsoil, significant increases in SOC, TN, and TP stocks were detected after afforestation on CL in subsoil, although the increase rates (31.2–56.2%) were lower than those in topsoil (47.8–69.1%). Soil pH and electrical conductivity (EC), which generally increased after conversion from PT to CL while decreased after CL afforestation, showed significant negative correlations with SOC, TN, and TP, suggesting that cultivation might lead to soil degradation, whereas afforestation contributed to soil restoration in this area. Significant changes in C:N:P ratios in topsoil were only detected for R_NP after conversion from CL to PP. By contrast, land-use change significantly altered both R_CN and R_NP in the subsoil, demonstrating that the impacts of land-use change on R_CN and R_NP were different between topsoil and subsoil. The significant relationship between soil EC and R_NP suggested that R_NP might be a useful indicator of soil salinization.

Conclusions

Stocks of SOC, TN, and TP as well as R_CN and R_NP in subsoil showed different responses to land-use change compared to those in topsoil in this typical agro-pastoral region. Therefore, it is suggested that the effects of land-use change on dynamics of SOC, TN, and TP in subsoil should also be evaluated to better understand the role of land-use change in global biogeochemical cycles.

     

Bioturbation on wildfire-affected southeast Australian hillslopes: Spatial and temporal variation

The importance of bioturbation as an agent of soil and geomorphological change is well known but few observations have been made of spatial and temporal variations in bioturbation rates. We quantified variations in surface bioturbation by ants (particularly Aphaenogaster longiceps) and vertebrates in the sandstone terrain of the Blue Mountains, southeast Australia. Following wildfire during the period late 2001–early 2002, we monitored thirty-three 5 m² plots positioned in six different slope units and in two catchments affected by different wildfire severities. Measurements were made seasonally for six years. Overall, mean rates of ant mounding and surface scraping by vertebrates were similar (246 ± 339 g m^− 2 yr^− 1 and 274 ± 179 g m^− 2 yr^− 1, respectively). However, rates varied substantially according to slope unit, showing a marked maximum for both ant mounding and total bioturbation on footslopes. Possible reasons for this spatial variation are discussed. A complex response to various soil and ecological factors such as soil texture, soil moisture and vegetation patterns is the most likely explanation. Associated estimates of topsoil (0–30 cm depth) turnover times, based on ant mounding rates alone, ranged from 300 to 100,000 years for different slope units. In contrast to previous findings, wildfire severity did not seem to affect bioturbation, possibly because of ant survival in deep nests and spatial patchiness of fire severity. There was likewise no clear link between temporal changes in bioturbation and fire severity; high rates in the first two years after wildfire were followed by lower rates for all burn severity types. There was also seasonal variability that was not directly related to rainfall. The results substantiate the importance of bioturbation in modifying soil characteristics and influencing soil erosion, especially following a major disturbance event like wildfire.     

Impact of ecologically different earthworm species on soil water characteristics

A laboratory experiment was performed to assess the impact of ecologically different earthworm species on soil water characteristics, such as soil tension, water content, and water infiltration rate. Three earthworm species (Lumbricus rubellus, Aporrectodea caliginosa, Lumbricus terrestris) were exposed in soil columns (diameter 30 cm, height 50 cm) for 100 days with a total fresh earthworm biomass of 22.7 ± 0.4 g per column, each in duplicate. Each column was equipped with tensiometers at 10 and 40 cm and FD-probes at 10 cm depth, to continuously measure the temporal development of soil tension and soil moisture. Additionally, 30 g of sieved and rewetted horse manure was placed on the soil surface as a food source. Precipitation events (10 mm) were simulated at day 28 and day 64. At the end of the experiment the water infiltration rate and the runoff at 55 cm depth were determined.The results showed considerable evidence, that ecologically different earthworms modify soil water characteristics in different ways. The anecic L. terrestris and the endogeic A. caliginosa showed the tendency to enhance the drying of the topsoil and subsoil. Their intensive and deep burrowing activity might enhance the exchange of water vapor due to a better aeration in soil. In contrast, the epigeic L. rubellus tended to enhance the storage of soil moisture in the topsoil, which might be linked to lower rates of litter loss from soil surface and thus a thicker litter layer remaining. A. caliginosa led to considerable higher water infiltration rates and faster water discharges in the subsoil, relative to the other species, probably due to a high soil dwelling activity.     

Microbial community composition shapes enzyme patterns in topsoil and subsoil horizons along a latitudinal transect in Western Siberia

Soil horizons below 30 cm depth contain about 60% of the organic carbon stored in soils. Although insight into the physical and chemical stabilization of soil organic matter (SOM) and into microbial community composition in these horizons is being gained, information on microbial functions of subsoil microbial communities and on associated microbially-mediated processes remains sparse. To identify possible controls on enzyme patterns, we correlated enzyme patterns with biotic and abiotic soil parameters, as well as with microbial community composition, estimated using phospholipid fatty acid profiles. Enzyme patterns (i.e. distance-matrixes calculated from these enzyme activities) were calculated from the activities of six extracellular enzymes (cellobiohydrolase, leucine-amino-peptidase, N-acetylglucosaminidase, chitotriosidase, phosphatase and phenoloxidase), which had been measured in soil samples from organic topsoil horizons, mineral topsoil horizons, and mineral subsoil horizons from seven ecosystems along a 1500 km latitudinal transect in Western Siberia. We found that hydrolytic enzyme activities decreased rapidly with depth, whereas oxidative enzyme activities in mineral horizons were as high as, or higher than in organic topsoil horizons. Enzyme patterns varied more strongly between ecosystems in mineral subsoil horizons than in organic topsoils. The enzyme patterns in topsoil horizons were correlated with SOM content (i.e., C and N content) and microbial community composition. In contrast, the enzyme patterns in mineral subsoil horizons were related to water content, soil pH and microbial community composition. The lack of correlation between enzyme patterns and SOM quantity in the mineral subsoils suggests that SOM chemistry, spatial separation or physical stabilization of SOM rather than SOM content might determine substrate availability for enzymatic breakdown. The correlation of microbial community composition and enzyme patterns in all horizons, suggests that microbial community composition shapes enzyme patterns and might act as a modifier for the usual dependency of decomposition rates on SOM content or C/N ratios.     

Soil microbial community structure affected by 53 years of nitrogen fertilisation and different organic amendments

The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments, including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure, sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge treatment (89 ± 22 nmol PLFA g dw⁻¹). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data, the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2.     

Using Semivariogram and Moran's I Techniques to Evaluate Spatial Distribution of Soil Micronutrients

Spatial distributions of micronutrients in soils of Shouguang were evaluated using semivariogram and Moran's index (Moran‘s I) techniques to compare difference and veracity of these two spatial analysis methods. A total of 601 topsoil (0–20 cm) and 155 deep subsoil (150–200 cm) samples were collected on a symmetrical grid in the regional geochemical survey of soils in Shandong Province, and copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) concentrations were analyzed and compared. The results showed significant spatial correlations of micronutrients in Shouguang soils, and the spatial correlation degree was greater in topsoil than in deep subsoil. In topsoil and deep subsoil, the spatial correlation distance for each element obtained using the semivariogram technique was 20–60 km, whereas with Moran's I technique, the positive autocorrelation distance was 20–25 km and the negative autocorrelation distance was 25–55 km. The spatial autocorrelation degree was significant (P ≤ 0.05) for every micronutrient except deep subsoil Zn. Moran's I technique was able to distinguish between positive and negative autocorrelations and the results of semivariogram analysis gave the sum of the positive and negative autocorrelations. This study shows that Moran's I is more accurate and meaningful than semivariogram analysis for spatial autocorrelation of some soil attributes. These results provide the theoretical foundation for the application of spatial analysis methods, and Moran's I in particular, in environmental research.     

中国东南部红壤施用石灰和石膏后底土酸性的减缓

Application of lime or gypsum is a common agricultural practice to ameliorate soils with low pH which prohibits crop production. Its integrated effect on soil properties in a red soil derived from Quaternary red clay in Southeast China is discussed in this paper. Application of gypsum in the topsoil without leaching raised soil pH and promoted the production of soil NH₄, but lime addition had a contrary effect. Generally, application of lime and/or gypsum has little effect on soil electrical properties. Gypsum had a little effect on soil exchange complex and its effect went down to 30 cm in depth. The effect of lime reached only to 5 cm below its application layer. With leaching, Ca transferred from top soil to subsoil and decreased exchangeable Al in subsoil. Gypsum application led to a sharp decrease in soil exchangeable Mg but had no effect on K.     

Loss of labile organic carbon from subsoil due to land-use changes in subtropical China

Topsoil carbon (C) stocks are known to decrease as a consequence of the conversion of natural ecosystems to plantations or croplands; however, the effect of land use change on subsoil C remains unknown. Here, we hypothesized that the effect of land use change on labile subsoil organic C may be even stronger than for topsoil due to upward concentration of plantations and crops root systems. We evaluated soil labile organic C fractions, including particulate organic carbon (POC) and its components [coarse POC and fine POC], light fraction organic carbon (LFOC), readily oxidizable organic carbon, dissolved organic carbon (DOC) and microbial biomass down to 100 cm soil depth from four typical land use systems in subtropical China. Decrease in fine root biomass was more pronounced below 20 cm than in the overlying topsoil (70% vs. 56% for plantation and 62% vs. 37% for orchard. respectively) driving a reduction in subsoil labile organic C stocks. Land use changes from natural forest to Chinese fir plantation, Chinese chestnut orchard, or sloping tillage reduced soil organic C stocks and that of its labile fractions both in top and subsoil (20–100 cm). POC reduction was mainly driven by a decrease in fine POC in topsoil, while DOC was mainly reduced in subsoil. Fine POC, LFOC and microbial biomass can be useful early indicators of changes in topsoil organic C. In contrast, LFOC and DOC are useful indicators for subsoil. Reduced proportions of fine POC, LFOC, DOC and microbial biomass to soil organic C reflected the decline in soil organic C quality caused by land use changes. We conclude that land use changes decrease C sequestration both in topsoil and subsoil, which is initially indicated by the labile soil organic C fractions.     

Use of in-growth soil cores in mesh bags for studies of relations between soil compaction and root growth

Using in-growth soil cores in cylindrical mesh bags, the effects of 3 soil compaction treatments on growth of crop roots were studied in a sandy soil. The bags were inserted after crop emergence in holes (70 mm diameter; 60 cm depth) augered in the soil in crop row interspaces. In 1984 (with rapessed), at all sampling dates, root biomass in the inserted cores decreased with increased compaction of the plough layer (0–25 cm) as well as the subsoil (25–60 cm). Root biomass in the subsoil was low. In 1985 (with wheat), the effects of compaction in the subsoil were similar, although root biomass was greater than in 1984. However, in the plough layer there were significant differences in root biomass on only one sampling date. The mesh bag technique should be a useful complement to other field methods in studies of relations between physical soil characteristics or tillage treatments and root growth.     

Subsoil rhizosphere modification by chickpea under a dry topsoil: implications for phosphorus acquisition

Chickpea (Cicer arietinum L.) roots exude carboxylates. While chickpea commonly grows where the topsoil dries out during crop growth, the importance of carboxylate exudation by the roots and mobilization of soil P from below the dry topsoil has not been examined. The study investigates the response of carboxylate exudation and soil P mobilization by this crop to subsoil P fertilizer rate. In constructed soil columns in the glasshouse, the P levels (high, low, and nil P) were varied in the well‐watered subsoil (10–30 cm), while a low level of P in the dry topsoil (0–10 cm) was maintained. At flowering, rhizosphere carboxylates and rhizosphere soil from topsoil and subsoil roots were collected separately and analyzed. The concentration of total carboxylates per unit rhizosphere mass in the subsoil was nearly double that of the topsoil. Plants depleted sparingly soluble inorganic P (P_i), NaOH‐P_i, and HCl‐P_i, along with the labile P_i (water soluble and NaHCO₃‐Pi). The P depletion by plants was greater from the subsoil than the topsoil. The study concluded that depletion of sparingly soluble P from the chickpea rhizosphere in the subsoil was linked with the greater levels of carboxylates in the rhizosphere. These findings indicate that chickpea, with its deep rooting pattern, can increase its access to subsoil P when the topsoil dries out during crop growth by subsoil rhizosphere modification.     

Tillage and cover effects on soil microbial properties and fluometuron degradation

This research concerns the influence of no tillage (NT) or conventional tillage (CT) and a ryegrass (Lolium multiforum Lam.) cover crop in a cotton (Gossypium hirsutum L.) production system on soil and ryegrass microbial counts, enzyme activities, and fluometuron degradation. Fluorescein diacetate hydrolysis, aryl acylamidase, and colony-forming units (CFUs) of total bacteria and fungi, gram-negative bacteria, and fluorescent pseudomonads were determined in soil and ryegrass samples used in the degradation study. Fluometuron (14C-labelled herbicide) degradation was evaluated in the laboratory using soil and ryegrass. The CT and NT plots with a ryegrass cover crop maintained greater microbial populations in the upper 2 cm compared to their respective no-cover soils, and CT soils with ryegrass maintained greater bacterial and fungal CFUs in the 2–10 cm depth compared to the other soils The highest enzymatic activity was found in the 0–2 cm depth of soils with ryegrass compared to their respective soils without ryegrass. Ryegrass residues under NT maintained several hundred-fold greater CFUs than the respective underlying surface soils. Fluometuron degradation in soil and ryegrass residues proceeded through sequential demethylation and incorporation of residues into nonextractable components. The most rapid degradation was observed in surface (0 to 2 cm) soil from CT and NT–ryegrass plots. However, degradation occurred more rapidly in CT compared to NT soils in the 2 to 10 cm depth. Ryegrass cover crop systems, under NT or incorporated under CT, stimulated microbiological soil properties and promoted herbicide degradation in surface soils.     

典型绿洲区土壤盐分的空间变异特征

针对绿洲区存在的土壤盐化制约干旱区农业可持续发展问题,应用面域土壤信息调研采集,结合地统计学与GIS技术对新疆典型绿洲区土壤盐分空间变异特征进行研究。研究结果表明:0～30cm耕层土壤盐分呈现强变异性,30cm以下土层呈现中等变异。各土层土壤盐分均呈现中等空间自相关性,空间相关距离在24～28km范围内。各等级盐化土在不同深度土层的分布方位基本一致。各土层非盐化土面积均占主导地位,都占研究区总面积的69%以上;其次为轻度盐化土面积,占研究区总面积的19%～30%;各土层中度盐化土面积都在3.4%以下;重度盐化土仅在0～30cm土层有极少量分布。随着土层深度的增加,非盐化土和中度盐化土的面积逐渐减小,轻度盐化土面积逐渐增大。本研究对于指导干旱区绿洲农业生产、保障区域土壤资源可持续合理利用具有重要的理论价值和明确的实际应用前景。