Hot-water Extractable Carbohydrates Research Articles

The effect of shrublands degradation intensity on soil organic matter-associated properties in a semi-arid ecosystem

Vegetation degradation can have significant effects on organic matter fractions as well as various soil characteristics. All these can lead to the changes in soil microbial communities, which are main drivers of nutrient cycles. This is especially important for mountainous ecosystems that are very sensitive and fragile habitats due to their climatic conditions, but less attention has been paid to them. Thus, Mirkola mountainous region (northern Iran) with semi-arid climate and vegetation being dominated by Crataegus and Berberis shrubs were investigated in this study. Sites with different intensities of vegetation degradation [light (60–70 % coverage), moderate (30–40 % coverage) and heavy (0–10 % coverage)] and also non-degraded control sites (90–100 % coverage) were selected. In order to avoid pseudoreplication, three plots, each with an area of 1 ha (100 m × 100 m), were used for each of the four studied habitats. Soil (in three soil depths at 0–10 cm, 10–20 cm and 20–30 cm) samples were collected from the corners and also the center (n = 5) of the plots using iron frames (30 × 30 cm). A total of 180 soil samples (4 habitats × 15 sample × 3 depths) were transferred to the laboratory. Labile and non-labile contents of soil organic matter (SOM) (C and N in soil particles and aggregates, C and N stocks, particulate and dissolved organic matter, hot-water extractable carbohydrate, C and N mineralization) were measured in each of these sites. Results showed that vegetation degradation might result in both labile and non-labile SOM losses. SOM and its associated properties were highest in the non-degraded sites, whereas, lower values were common under degraded areas. Based on the obtained data, vegetation cover can enhance the amount of organic matter entering the soil (especially in the surface layers) and formation of fertile islands in ecosystems. Accordingly, vegetation protection is emphasized to maintain stability and provide ecosystem services in mountainous semi-arid regions.

Effects of Different Vegetation Patches on Soil Functionality in the Central Iranian Arid Zone

This research aimed at evaluating how different vegetation patch types and inter-patch zone affect soil properties/functions and ecological processes. Five main types of patches and one inter-patch zone were identified in an arid rangeland ecosystem in the Ghamishloo National Park and Wildlife refuge, central Iran. Next, twenty-nine soil samples were collected from the patches and the inter-patch to measure soil texture, pH, electrical conductivity (EC), total nitrogen (TN), available phosphorus, bulk density (BD), soil organic matter (SOM), particulate organic matter (POM), basal soil respiration (BSR), hot-water extractable carbohydrates (HWEC), and aggregate stability (as quantified by mean weight diameter, MWD, of water-stable aggregates). At each six patch types and inter-patch zone, eleven soil indicators were measured along three 30-m transects to estimate soil stability, infiltration, and nutrient cycling indices as proposed by the landscape function analysis method. Results indicated that SOM, TN, MWD, HWEC, BSR, POM, and silt content varied significantly between the patch and inter-patch types. Moreover, the difference between shrub patch and inter-patch zone was significant for infiltration and nutrient cycling indices but not for soil stability. Substantial input of organic carbon is required in the soils of inter-patch zone and forb patch type to achieve greater soil functioning. Patches of shrubs, shrub-forbs, and grasses improved soil quality and ecosystem functioning more efficiently than forbs and inter-patch zone; hence, preserving these patches is recommended for conservational solutions in arid fragile ecosystems. The results highlighted the importance of SOM in evaluating functional status of arid rangelands.

Vertical distribution of different pools of soil organic carbon under long-term fertilizer experiment on rice-wheat sequence in mollisols of North India

ABSTRACT A profound consideration of soil organic carbon (SOC) and SOC pools as affected by the long-term use of fertilizer in a fixed crop sequence is essential to understand and maintain the health and productivity of soil. The dynamics of SOC pools, i.e. total soil organic carbon (TOC) and its different fractions under a rice-wheat cropping system subjected to various fertilizer management strategies over 42 years was investigated near Uttarakhand, India. The soil at the experimental site was an Aquic Hapludoll consisting of a poorly drained silty clay loam (1–1.5 m deep) overlying loamy to sandy sediments. Over 42 years, application of 100% NPK+ FYM (Farm Yard Manure) was the most effective management system in increasing organic C up to 0 to 60 cm soil depths under rice-wheat sequence. The combination of this integrated input application (100% NPK + 15 t ha−1 FYM) significantly built organic carbon fractions like total organic carbon, microbial biomass C (MBC), particulate organic C (POC), KMnO4 oxidizable C, Humic C, Fulvic C, Humin C, water-soluble carbon and hot water extractable carbohydrates at all four soil depths. Hence, application of FYM along with NPK resulted in a significant positive building up of all pools in the treatment at all four depths. Therefore, balance fertilization is panaceally important for sustaining improved soil health with balanced organic status and production potentiality of the soil for rice-wheat cropping sequence.

Tensile strength of mollisols of contrasting texture under influence of plant growth and crop residues addition

Tensile strength (TS) of soil aggregates is considered a sensitive and important indicator of the effects of the management practices on soil structure quality, which affects the seed germination and the initial crop growth. However, the influence of plant growth, crop residues addition and the produced aggregating agents on TS has not been widely studied. The objectives of this study were: i) to determine the specific effects of plant growth and different types, rates and location of crop residues in the aggregates tensile strength, and ii) to assess the relationship between the aggregating agents produced by plant growth and crop residues addition and the aggregate tensile strength of soils of contrasting texture. A greenhouse experiment was carried out in pots with a loamy soil (Typic Hapludoll, Santa Isabel series) and a silty-loamy soil (Typic Argiudoll, Esperanza series) under controlled conditions for 112 days. For each soil the following treatments were set up in triplicate: (i) soil type, (ii) with or without plant growth of wheat (Triticum aestivum L.), (iii) with or without residues addition, (iv) location of residues (surface vs. incorporated), (v) wheat vs. soybean (Glycine max L.) residues, and (vi) residue rates (6.3 and 15.7 g of dry matter per pot for wheat, and 6.3 and 18.8 g of dry matter per pot for soybean). Pots were exposed to wetting and drying (W/D) cycles. TS values and aggregating agent's content, i.e., total organic carbon (TOC), particulate organic carbon (POC), hot water extractable carbohydrates (HWEC), dilute acid extractable carbohydrates (DAC), total carbohydrates (TC), total glomalin-related soil protein (T-GRSP), and easily extractable glomalin-related soil protein (EE-GRSP) were measured. TS were significantly lower in the Typic Hapludoll (TS = 39.9 kPa) than in the Typic Argiudoll (TS = 61.6 kPa). TS values were significantly higher in the treatments with plants of wheat than in treatments without plants (49.5 vs. 30.3 kPa in the Hapludoll and 71.2 vs. 50.9 kPa in the Argiudoll). Plant growth increased TS through physical mechanisms, i.e. a greater number of drying-wetting cycles. Also, plant growth increased TS by producing aggregating agents. TS values were not directly affected by the addition of different types, rates and locations of crop residues. However, they increased the content of aggregating agents. Multiple regression analysis showed that TS was significantly related to soil type, TC and T-GRSP. TS increased with increasing TC and T-GRSP. These two variables explained 87% of the model variation. The obtained model provides a basis for understanding which are the most important aggregating agents and, consequently, which are the better management systems to improve o recover the structure quality of soils with different textures.

Tillage and fertilization practices affect soil aggregate stability in a Humic Cambisol of Northwest France

Reduced tillage and organic fertilizer application usually result in an increase in soil aggregate stability (AS). However, the magnitude of the effects can vary with soil properties and season. The aim of this study was to investigate AS dynamics over three seasons in a soil under various tillage and fertilization practices. The study was performed under three tillage practices (moldboard plowing (MP), surface tillage (ST) and no-tillage (NT)) and two types of fertilizer (poultry manure and mineral) seven and eight years after their establishment in Northwest France. AS was measured in three different seasons: spring, summer and winter. Soil properties that potentially influence AS such as organic carbon (OC), hot-water extractable carbohydrates (HWEC), water content (WC) and water repellency (WR) were also studied. On average, for all sampling dates, AS was 34% higher under NT than MP. Conversely, the effect of ST on AS varied with sampling date with values close to NT in mid-spring and summer, and values close to MP in early spring and winter. Poultry manure increased AS by an average of 12% regardless of sampling date or tillage practice. Variations in AS due to management practices were related to OC (r=0.92) and HWEC (r=0.88). Differences in AS between sampling dates were slightly greater than the effects of management practices. On average across management practices, AS increased by 47% from early spring to summer and decreased by 59% in winter. These variations were related to soil WC (r=−0.67) and WR (r=0.72) at time of sampling. We suggest that seasonal variations in AS were at least partly due to variations in WC which acted physically by modifying the water entry rate into the aggregates and slaking effects. In contrast, the long-term AS dynamics were related to the organic matter dynamics, which are controlled by management practices. Because of the predominant effect of climate on AS, we suggest measuring AS in winter and summer to better estimate the effects of management practices on soil erodibility in this region.

Effects of various organic amendments on organic carbon pools and water stable aggregates under a scented rice–potato–onion cropping system

Pools of organic carbon are quantified from the soil samples under scented rice crop from different soil layers (0–10, 10–20, and 20–30 cm) under 9 years’ long-term trials with five treatments in scented rice–potato–onion cropping system. These treatments were 100 % NPK (NPK), 50 % recommended NPK through mineral fertilizers + 50 % N as FYM (NPK + FYM), FYM + vermicompost (VC) + neem cake (NC) each equivalent to one-third of recommended N (FYM + VC + NC), 50 % N as FYM + biofertilizer for N + bone meal to substitute phosphorus requirement of crops + phosphate solubilizing bacteria (FYM + BFN + BM + PSB), FYM + vermicompost + neem cake each equivalent to 1/3rd of recommended N + PSB (FYM + VC + NC + PSB). SMBC (479 mg kg−1), HWEOC (373 mg kg−1), CWSCHO (235 mg kg−1), HWSCHO (839 mg kg−1), and ASCHO (180 mg kg−1) were found to be the highest in the soil treated with FYM + VC + NC + PSB and the lowest with NPK. The quantity of hot water-extractable carbohydrate content is highest amongst cold water, dilute acid and hot water extractable carbohydrate that decreases with the soil depth irrespective of treatments, except CWEOC. Soil microbial biomass carbon (SMBC) shows significant correlation with CWEOC (r = 0.60**), HWEOC (r = 0.94**), CWSCHO (r = 0.75**), HWSCHO (r = 0.83**), and ASCHO (r = 0.83**) that primed for better aggregate stability irrespective of soil layers up to 30 cm depth. This indicates that labile carbon pools, most specifically water-soluble carbon, carbohydrate, microbial biomass, could be a suitable indicator for evaluation of soil quality, particularly in relation to soil aggregation.

Natural Organic Compounds in Soil Solution: Potential Role as Soil Quality Indicators

This review focuses on the chemical nature of that fraction of already dissolved organic matter into soil solution, or extracted by mild extractants, which is truly readily available for microbial activity and, consequently, more sensitive than total soil organic matter to changes in management and/or environmental conditions. In particular, we deal with low molecular weight compounds such as monosaccharides, amino sugars and amino acids. Soil sampling strategy and extraction procedure, prior to analyses, are crucial to make comparable results among laboratories. Although soil management and climatic conditions may cause large variability, extractable organic C and N may indicate the amount of substrates available to microbial biomass. Hot water extractable carbohydrates are among the most sensitive indicators of various factors such as tillage, cropping system, soil inundation frequency, seasonality and wildfires. Moreover, hot water extracts mainly microbial carbohydrates, i.e. those more involved to bind soil aggregates. The turnover rate of free amino acids in soils is very high, ranging from 1 h to about 30 hours. This explains the usually very low amino acid-N extractable from soils. The pHvariable charge of amino acids favours their adsorption on soil colloids. Amino sugars concentration in soils (mainly glucosamine, galactosamine, mannosamine and muramic acid) is more useful indicators of microbial necromass than microbial living biomass since they tend to stabilize and accumulate in soil. Moreover, their use as specific biomarkers is questionable due the widespread presence among microorganisms and plants.

Profile distribution and seasonal dynamics of water-extractable carbohydrate in soils under mixed broad-leaved Korean pine forest on Changbai Mountain

Carbohydrate represents an important part of the soil labile organic carbon pool. Water soluble carbohydrate drives the C cycle in forest soil by affecting microbial activity and hot water extractable carbohydrate is thought related to soil carbon sequestration due to the association with soil aggregation. In a temperate forest region of northeast China, Changbai Mountain, we investigated the abundance, spacial distribution, and seasonal dynamics of cool and hot-water extractable carbohydrate in soils under mixed broad-leaved Korean pine forest. The concentrations of cool-water extractable carbohydrate (CWECH) in three soil layers (0–5, 5–10, 10–20 cm) ranged from 4.1 to 193.3 g·kg−1 dry soil, decreasing rapidly with soil depth. On an annual average, the CWECH concentrations in soils at depths of 5–10 and 10–20 cm were 54.2% and 24.0%, respectively, of that in the 0–5 cm soil layer. CWECH showed distinct seasonal dynamics with the highest concentrations in early spring, lowest in summer, and increasing concentrations in autumn. Hot-water extractable carbohydrate (HWECH) concentrations in three soil layers ranged from 121.4 to 2026.2 g·kg−1 dry soil, which were about one order of magnitude higher than CWECH. The abundance of HWECH was even more profile-dependent than CWECH, and decreased more rapidly with soil depth. On an annual average, the HWECH concentration in soils 10–20 cm deep was about one order of magnitude lower than that in the top 0–5 cm soil. The seasonality of HWECH roughly tracked that of CWECH but with seasonal fluctuations of smaller amplitude. The carbohydrate concentrations in cool/hot water extracts of soil were positively correlated with UV254 and UV280 of the same solution, which has implications for predicting the leaching loss of water soluble organic carbon.

Arbuscular Mycorrhizal Fungi Alter Plant Growth, Soil Aggregate Stability, and Rhizospheric Organic Carbon Pools of Citrus

The effects of an arbuscular mycorrhizal fungus, Glomus mosseae, on plant growth, soil aggregate stability, and rhizosphere carbon pools of young Citrus junos seedings were investigated with potted experiment in greenhouse. After three months of mycorrhizal inoculation, root colonization was 54.25%. Inoculation with G. mosseae significantly promoted plant height, stem diameter, leaf number, and shoot and root fresh weights. Colonization by G. mosseae significantly increased soil aggregate stability of the citrus rhizosphere through increase of mean weight diameter. G. mosseae could release a specific glycoprotein viz. glomalin into the rhizosphere as glomalin-related soil protein (GRSP). Meanwhile, mycorrhizal colonization was significantly positively correlated with two GRSP fractions. In stabilization of aggregate stability, in GRSP fractions only easy extractable -GRSP might contribute the role. The mycorrhizal symbiosis could increase soil organic carbon, hot-water extractable carbohydrates, and hydrolyzed carbohydrates concentrations, but the differences were not significant. Combined with the correlation analysis, it suggests that GRPS did not significantly regulate rhizospheric carbon pools, due to the short treated time (only 3 months).

Bulk soil and particle size-associated C and N under grazed and ungrazed regimes in Mountainous arid and semi-arid rangelands

Sheep grazing is known to influence soil organic carbon (SOC) storage and dynamics in rangelands. However, very little is known of grazing impacts on measurable SOC pools associated with primary particles, particulate organic matter (POM) and carbohydrates in the heavily grazed rangelands of Zagros Mountains, Iran. Bulk SOC, total nitrogen (N), POM and hot-water extractable carbohydrates (HWC) as well as different SOC and N fractions in particle-size separates were studied in natural mountainous rangelands of arid (Boroujen with 255 mm annual rainfall) and semi-arid (Sabzkouh with 860 mm annual rainfall) sites, Central Zagros. Two sheep grazing regimes including grazed and ungrazed (for 20–25 years) rangelands with four replicates were identified as the grazing treatments at each site. Soil samples (0–15 cm) were taken and analyzed for bulk SOC, total N, POM, HWC, and SOC and N associated with physical fractions. Bulk SOC contents were similar for both ungrazed and grazed regimes, while total N contents significantly decreased under grazed conditions. Bulk soil POM and HWC contents decreased considerably and clearly by sheep grazing, indicating that these fractions of total soil organic matter (SOM) pool may be suitable indicators for detecting the grazing effects on bulk SOC changes and storage in these arid and semi-arid ecosystems. Semi-arid rangeland sites contained more bulk SOC, total N and POM contents than arid rangeland sites. These differences were primarily due to the large differences in vegetation composition, annual rainfall and soil conditions between the two rangelands. After 20–25 years of grazing over 10–33 % of SOC and total N losses occurred in the sand- and clay-size fractions with 10 % increases in the silt fraction. This means sheep grazing increases the contribution of the silt fraction to bulk soil N. We found evidence that sheep grazing decreases soil POM and HWC pools, and the sand fraction C, suggesting a lower recent annual input of decomposable organic C in heavily grazed rangelands. Sheep grazing had no influence on the potential C mineralization of the bulk soil at the semi-arid site (Sabzkouh), but reduced C mineralization at the arid-site (Boroujen), indicating that sheep grazing may affect SOC dynamics by changes in substrate quality at the former, but by substrate quantity at the later. In brief, long-term sheep grazing can potentially lead to losses of both labile and no-labile SOM in these arid and semi-arid rangelands.

Stabilisation of soil against wind erosion by six saprotrophic fungi

Soils with biological crusts that consist of fungal hyphae, bacteria and other small organisms usually resist erosion. However, the processes by which soil organisms stabilise air-dry aggregates against wind erosion are not well understood. We used saprotrophic fungi to examine some of these processes in a sandy clay loam (Hypercalcic Calcarosol). Soil aggregates, wetted with distilled water or glucose solution, were inoculated with one of six fungi, and incubated in darkness at 24 °C for 7 d in petri dishes under sterile conditions. Abrasion resistance (taken as resistance to wind erosion), tensile strength of soil, hot-water extractable carbohydrates (HWEC), dispersion index, pH, and hyphal length density (HLD) were each measured across all treatments. In all treatments, stability (abrasion resistance) and tensile strength, were positively related to HLD. Such relationships have not been reported elsewhere. All fungi enlarged the aggregates of the soil by cross-linkage and entanglement of particles, but with different processes, or different intensity of the processes, between species (for the same amount of substrate). The skins seen in scanning electron micrographs of stabilised soil were probably extracellular polysaccharides also produced by the fungi. We propose that the ductile failure of disks of soil, particularly those inoculated with Mucor sp., under tensile stress was due to movement of enmeshed particles, whereas the brittle failure of disks of soil inoculated with the other fungal species was due to metabolites or dispersed clay on the surface of the hyphae which limited deformation.

Abundance and stability of belowground earthworm casts influenced by tillage intensity and depth

In temperate soils, there has been little study of the abundance and water stability of belowground earthworm casts under different tillage systems and soil depths. The aim of this study was to determine the effects of earthworm activity on soil aggregate stability under various tillage systems. Three tillage treatments were compared (moldboard plowing (MP), surface tillage (ST) and no-tillage (NT)). We present here an original method by which earthworm casts were quantified via image analysis at 2- and 12-cm depths. In addition, soil aggregate stability expressed as the mean weight diameter (MWD) and factors involved in soil aggregate stabilization such as soil organic carbon, hot-water extractable carbohydrate content (HWEC) and water repellency (WR) of aggregates were measured in earthworm casts and bulk soil. In superficial soil layers, the relative cast abundance was small and more abundant under NT than the ST and MP treatments. At 12 cm, the relative cast abundance doubled irrespective of the tillage treatment and tended to be higher with conservation tillage (i.e., NT and ST). Moreover, the MWD increased in casts compared to the bulk soil in this deeper layer. The increase in MWD in casts was explained in part by the increase in soil organic matter (carbon content and HWEC) and the WR. For the two depths, we found a positive relation between the MWD from the bulk soil and casts among the tillage treatments. We conclude from this study that casts located at a 2-cm depth did not participate in soil aggregate stabilization, while at 12 cm the greatest structural stability observed under conservation tillage was explained in part by the production of casts.

Effect of tillage system and straw management on organic matter dynamics

The choice of cultivation system in arable agriculture exerts a strong influence not only on soil health and crop productivity but also on the wider environment. Conservation tillage using non-inversion methods conserves soil carbon, reduces erosion risk and enhances soil quality. In addition, conservation tillage has been shown to sequester more carbon within the soil than inversion tillage, reducing carbon dioxide losses to the atmosphere. Stable, well structured topsoils that develop following long-term conservation tillage lead to more energy efficient systems due to the reduced power requirements for cultivation. Long-term experiments, e.g. more than 20 years, that confirm the impact of conservation tillage over an extended period are not common. Here we evaluate the impact of different tillage methods and winter wheat straw management, either incorporated or removed, on organic matter turnover and soil quality indicators. No-till, chisel and mouldboard ploughing was carried out for 23 years on a silty clay loam soil in South West England that was not considered suitable for non-inversion tillage due to weak soil structure. In order to assess the effect of contrasting cultivation and straw disposal method on soil carbon dynamics, a range of assays were conducted, including water extractable organic carbon, hot water extractable carbohydrate, microbial biomass carbon, activity of β-glucosidase and acid phosphatase enzymes, C sequestration and the natural abundance of 13C. Our results show that the soil organic carbon concentration in the topsoil was greater under no-till than mouldboard ploughing, while a reverse trend was observed in the lower depths. A 14–17% increase in soil organic carbon was observed in the topsoil for chisel plough and no-till treatments compared to mouldboard ploughing. Water extractable organic carbon was found to constitute only 1–7% of the microbial biomass carbon. Hot water extractable carbohydrate was one of the most sensitive indicators of soil quality and had a significant a negative correlation with bulk density and positive correlation with soil organic carbon microbial biomass carbon β-glucosidase and acid phosphatase. The choice of cultivation method exerted a major control on microbial and carbon dynamics. No-till and chisel ploughing maintained carbon in the soil surface horizons, which will benefit the stability of this weakly structured soil, but mouldboard ploughing distributed carbon more uniformly throughout the soil profile, particularly when straw was incorporated, hence leading to the retention of more carbon in the soil profile.

Soil management effects on aggregate stability and biological binding

In order to improve our understanding of soil aggregation, we have studied the relative importance of bonding and binding mechanisms, especially how they scale according to aggregate size and how they are influenced by farming system and different management options. Topsoil samples were collected from four arable sandy loam soils found as two pairs (FP1 and FP2) of neighbouring fields. One of the fields in FP2 had been grown for decades with annual cash crops without application of organic manures, while the other three fields had been managed with diversified crop rotations and manure dressings. Aggregates were segregated from the bulk soil by promoting brittle failure. The samples of soil structural units were fractionated to 4–8 mm, 0.5–1 mm and 0.063–0.25 mm aggregates during a process of air-drying with minimum energy input (e.g. short sieving times). We measured microbial biomass, ergosterol, clay dispersibility, hot-water extractable carbohydrates, and hyphal length. Generally, all four soils showed no significant differences among aggregate size classes in the content of microbial biomass, hot-water extractable carbohydrates and hyphal length. The FP2 soil grown with annual cash crops had significantly lower values for all soil attributes than its neighbouring soil, while a more complex pattern was observed for the FP1 soils. Our results do not indicate scaling according to aggregate size of the binding and bonding mechanisms studied. Results from the three fields with diversified crop rotations indicate satisfactory levels of bonding and binding agents for creation of stable aggregates. Exhaustion of soil organic matter as found in the cash crop system seems to change the way that clay particles interact with the biotic agents in aggregation.

Casts of Aporrectodea caliginosa (Savigny) and Lumbricus rubellus (Hoffmeister) differ in microbial activity, nutrient availability and aggregate stabilityThe 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

The microbial activity, nutrient availability and aggregate stability of casts of the endogeic earthworm, Aporrectodea caliginosa, and the epigeic species, Lumbricus rubellus, were compared with that of uningested soil. Casts were collected 48 hours after introducing earthworms of each species to containers which were filled with 15 cm of soil covered with a 5 cm layer of decomposing lucerne hay. Microbial biomass C, flourescein diacetate hydrolysis rate, aggregate stability and hot water-extractable carbohydrates tended to be lower in casts than uningested soil while the reverse was the case for light fraction C, basal respiration, metabolic quotient, exchangeable NH4+- and NO3 -N, Olsen P and phosphate-extractable SO42--S. Casts evidently contained a smaller, but more metabolically active, microbial community than soil. Values for all measured parameters were appreciable higher in casts of L. rubellus than those of A. caliginosa. This was attributed to the greater amounts of surface litter (lucerne hay) ingested by the former due to its epigeic habit.

Evaluating effects of sewage sludge and household compost on soil physical, chemical and microbiological properties

Recycling of organic wastes within agriculture may help maintain soil fertility via effects on physical, chemical and biological properties. Efficient use, however, requires an individual assessment of waste products, and effects should be compared with natural variations due to climate and soil type. An 11-month incubation experiment was conducted between April 1998 and March 1999, in which a sandy loam without or with anerobically digested sewage sludge (4.2 t dry matter (DM) ha −1) or household compost (17 t DM ha −1) was incubated under constant laboratory conditions at 10 °C, as well as in the field. The following properties were monitored: wet-stability of soil aggregates, clay dispersibility, hot-water extractable carbohydrates, resin-extractable P i, inorganic N, biomass C and N, PLFA profiles, FDA hydrolysis activity, β-glucosidase activity and CO 2 evolution. In general, effects of waste amendment were positive, but moderate compared to the dynamics observed in unamended soil, and mainly occurred in the first several weeks after amendment. The temporal dynamics of inorganic N, FDA hydrolysis activity, biomass C and PLFA composition appeared to be faster under the fluctuating climatic conditions in the field. To evaluate accumulated effects of repeated waste applications, soil was also sampled from a field trial, in which the sewage sludge and household compost had been applied at the same rates as in the incubation study for three consecutive years. Sampling took place after the final harvest, i.e. 5 months after the final waste application. Compost amendment had increased potentially mineralizable N by a factor of 1.8, and sludge amendment had increased the amount of resin-extractable P i by a factor of 1.6. However, there were no accumulated effects of waste amendment on the fraction of soil in wet-stable aggregates, or on the microbiological properties tested, which supported the observation from the incubation study that effects of organic wastes were transient.

Sensitive Indicators of Soil Organic Matter Sustainability in Orchard Floors of Organic, Conventional and Integrated Apple Orchards in New Zealand

ABSTRACT Major objectives of this study were to evaluate the effects of organic, conventional and integrated orchard fruit production systems on the quality and quantity of soil organic matter in orchard floors and to identify sensitive soil organic matter indicators. Seventeen different soil organic matter parameters were studied. These were soil aggregate stability (AS), total nitrogen (TN), total carbon (TC), carbon/nitrogen ratio, microbial biomass N (BN), microbial biomass C (BC), BC:BN; BC:TC; BN:TN; hot-water extractable C (HC), hot-water extractable carbohydrate (HCA), HCA:HC; humin N (hum N), humin C (hum C), hum C:TC; hum N:TN and potentially mineralizable N (Pot N). They were based largely on soil organic matter characteristics and fractions extracted sequentially by different extractants from soils collected from the alleys and treelines of organic, conventional, and integrated grassed-down apple orchard types from both commercial and experimental sites in Canterbury, New Zealand. A total of 12 apple orchards were investigated. Soil samples collected from these orchards (0–75 mm depth) were analysed for TC, TN, BC, BN, Pot N, AS and also sequentially extracted for labile and stable soil organic matter fractions using cold and hot water, acid mixtures (hydrochloric:hydroflouric acids, HC1:HF) and alkalis (sodium pyrophosphate and sodium hydroxide, Na4P207 and NaOH). Only TN, BN, and BN:TN showed significant interaction effects and all these were associated with soil N. Overall, combining orchards according to similar management systems, results obtained showed no significant differences between experimental and commercial orchards. However, sensitive indicators capable of distinguishing significant differences between different orchard management systems were BC, BC:TC and HCA:HC while HCA:HC distinguished significantly between treelines and alleys within each group of the same orchard type.

Influence of six crop species on aggregate stability and some labile organic matter fractions

The effect of the growth of barley, wheat, prairie grass, Italian ryegrass, white clover and lupin on the aggregate stability and related properties of a heavily cropped soil was investigated in a greenhouse experiment. For the non-leguminous crops, root mass and root length followed the order barley = wheat < prairie grass < Italian ryegrass. Less marked, but similar trends were found for microbial biomass C, cold and hot water-extractable carbohydrate content and aggregate stability. It was postulated that a higher root mass results in greater rhizodeposition of carbonaceous material and, therefore, a higher microbial biomass which in turn produces carbohydrate binding agents which increase aggregate stability. The hot water-extractable carbohydrate fraction was found to have a galactose plus mannose-to-arabinose plus xylose ratio of 2.1 confirming that it was predominantly of microbial origin. In comparison with the non-legumes, growth of white clover and lupin resulted in an unexpectedly high aggregate stability and to a lesser extent microbial biomass C content relative to their rather small root mass and length. Lupin, for example, had the highest aggregate stability of all the crops, while white clover had an aggregate stability similar to that of Italian ryegrass yet the two legumes had the lowest root length densities of all the crops studied. It was suggested that the rhizosphere microbial population of leguminous plants differed in some way to that of non-legumes (possibly due to the higher N content of rhizodeposited material) and that this contributed to the higher measured aggregate stability. A subsidiary experiment showed that fungal hyphal length in aggregates affected by lupin growth was four times that under wheat. There is a need for further research into aggregation in the rhizosphere of a wider range of legumes.

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.

Increasing the length of hyphae in a sandy soil increases the amount of water-stable aggregates

The contribution from increased lengths of saprophytic and mycorrhizal hyphae to the aggregation of a sandy soil was assessed in a glasshouse experiment. Increased length of mycorrhizal hyphae was encouraged by inoculation of Lolium rigidum with Scutellospora calospora and the effects of the roots on aggregation were restricted by containment within 38 μm mesh bags. The hyphae of saprophytic fungi were increased by the addition of glucose or straw substrates at 2.3 mg g −1. After incubation of the soils for five weeks, changes in > 2 mm water-stable aggregates were assessed in the root-free soil before and after drysieving through 8 mm mesh, to test the resistance of the aggregates to soil abrasion. Hyphal lengths and hot-water extractable carbohydrate C contents were measured in the > 2 mm water-stable fraction and non-aggregate ( < 2 mm) soil of the soils that were not dry-sieved. The treatments increased hyphal lengths from 3.2 m g −1 to 4–6.2 m g −1, but the hyphal lengths were not different between the treated soils ( P < 0.05) because much of the hyphal growth occurred only in the soil that was aggregated. The lengths of hyphae in the > 2 mm aggregates from the soils amended with straw and glucose were 2–5.7 m g −1 greater ( P < 0.05) than the lengths in non-aggregate soil (4–6 m g −1). Inoculation of non-amended soil increased the proportion of mycorrhizal roots by 340% ( P < 0.05) but had little effect on hyphal length in the non-amended soils. Water-stable aggregation was increased from 7–16 g kg −1 in the non-amended soils to 84–143 g kg −1 in the straw- and glucose-amended soils ( P < 0.05). Under a Scanning Electron Microscope, sand grains in the aggregates appeared to be linked together only by hyphae. In addition, the hot-water-extractable carbohydrate C contents of water-stable aggregates and non-aggregate soil were not different, indicating little involvement of microbial polysaccharides in stabilising the aggregates. Despite the strong evidence for hyphal involvement in stabilising aggregates, only the hyphal lengths in the whole soil of the non-amended soil and > 2 mm aggregate fraction of the soil amended with glucose were significantly correlated with aggregation ( P < 0.05). The amounts of dry-stable aggregates in 8 mm sieved soil were generally increased by 63–147% in the amended soils compared to non-amended soils and 27 to 33% of these aggregates were water-stable aggregates indicating that hyphae also contributed to the stabilisation of dry aggregates. Increases in aggregate stability could be largely attributed to the increased growth of hyphae in the sandy soil. Hyphae were able to contribute greatly to the stability of the sand aggregates because much of the soil was > 250 μm sand which favoured cross-linking of the sand grains by short lengths of hyphae.