Proportion Of Water-stable Aggregates Research Articles

Effects of organic carbon and clay contents on structure‐related properties of arable soils with high clay content

AbstractUnderstanding of factors governing soil structural features is necessary for managing key processes affecting crop productivity and environmental impacts of agriculture, for example, soil water balance, aeration, and root penetration. Organic matter is known to act as a major binding agent in soil aggregation and thus constitutes a central pillar in soil structure formation. However, knowledge of the structural role of organic matter or carbon (OC) in soils highly rich in clay‐sized particles (<0.002 mm) is limited. In this study, the effects of clay and OC contents on aggregate stability, water holding capacity, near‐saturated hydraulic conductivity, total porosity, and pore size distribution were assessed in cultivated fields with high clay content located in private crop production farms in southern Finland. Significant positive correlations were found between OC content and proportion of water stable aggregates and specific pore sizes from the range of 30 μm up to 1 mm diameter determined by image analysis. Porosities on a smaller size range derived from water retention measurements likewise showed a positive correlation with OC in <0.2 μm sizes. On the range of 0.2–1 μm, a negative relationship was observed, which induced a negative effect of OC on soil plant available water reserves. In line with the positive correlation between OC and larger soil pores, free water, representing the amount of water that can be drained by gravity, exhibited a positive relationship with OC suggesting that OC content can enhance aeration of soils with high clay content. Compared to OC, clay content tended to have an adverse effect on soil structural properties. Clay correlated negatively with pores larger than 30 μm, free water content, and extrapolated field saturated hydraulic conductivity. Further, our imaging results showed how saturated hydraulic conductivity was controlled by pore morphology, and there was a power law relationship between the conductivity and critical pore diameter. in agreement with the percolation theory. Overall, the structural impacts and hydrological implications of OC and clay in heavy clay soils vary by pore size ranges and their emergent practical impacts are thus not straightforward.

Carbon stock, aggregate stability and hydraulic properties of soils under tillage, crop rotation and mineral fertiliser application in sub-humid Zimbabwe

Appropriate tillage and crop diversifications can improve soil quality leading to yield sustainability. Our objective was to quantify tillage, crop rotation and mineral fertiliser application effects on carbon sequestration, aggregation and soil water movement after two cropping cycles in the smallholder sector of Zimbabwe. Two split-plot experiments were set up at four sites on sandy, loamy and clayey soils. At experiment 1, crop rotation (maize-soya bean; continuous maize) was the main plot and mineral fertiliser ((NPKS (180 N + 30P2O5+30K2O+6.5SO3 kg ha−1); control (no fertiliser added)) was the sub-plot. At experiment 2, tillage (reduced, conventional) was the main plot and mineral fertiliser (NPKS; control) was the sub-plot. Soil samples collected from 0 to 0.2 m and 0.2–0.4 m layers were analysed for soil organic matter (SOM) content, bulk density and proportion of water stable aggregates. Saturated hydraulic conductivities (Ks), steady state infiltration rates (is) and soil sorptivities (Sp) were estimated from fitting field infiltration data into the Phillip model. SOM stocks (mean = 3.483 Mg ha−1) were significantly increased by reduced tillage at the sandy site and higher (p < 0.05) in 0–0.20 m than in 0.20–0.40 m layers at clayey sites. Proportion of water stable aggregates increased (p < 0.05) under reduced tillage compared with conventional tillage and under rotation compared with continuous maize system. Bulk densities were 11% lower (p < 0.05) in the 0–0.20 m than in 0.20–0.40 m layers. The estimated Ks(1 × 10−4-8x10−4 cm s−1) and is (7.08–55 × 10−4 cm s−1) were at least 100% higher (p < 0.05) under rotation compared with continuous maize whilst sorptivities (0.050–0.143 cm s−05) did not vary across the treatments. NPKS fertiliser reduced (p < 0.05) is by up to 1.8 fold compared with the control. Short term adoption of reduced tillage and maize-soya bean rotation can mitigate soil structural degradation; increase water recharging and increase carbon sequestration quicker in sands than in the buffering clays making the practices more relevant in the smallholder sector.

Effects of Symbiotic Fungi on Sugars and Soil Fertility and Structure-Mediated Changes in Plant Growth of Vicia villosa

Many terrestrial plants form reciprocal symbioses with beneficial fungi in roots; however, it is not clear whether Vicia villosa, an important forage and green manure crop, can co-exist with these fungi and how such symbiosis affects plant growth and soil properties. The aim of this study is to analyze the effects of inoculation with three arbuscular mycorrhizal fungi (AMF) such as Diversisporaspurca, Funneliformismosseae, and Rhizophagusintraradices and an endophytic fungus Serendipitaindica on plant growth, root morphology, chlorophyll and sugar levels, soil nutrients, and aggregate size distribution and stability in V. villosa plants. After 63 days of inoculation, the beneficial fungi colonized the roots with colonization rates of 12% to 92%, and also improved plant growth performance and root morphology to varying degrees, accompanied by the most significant promoted effects after R.intraradices inoculation. All AMF significantly raised chlorophylls a and b, carotenoids and total chlorophyll concentrations, along with a significant increase in leaf sucrose, which consequently formed a significantly higher accumulation of glucose and fructose in roots providing carbon sources for the symbionts. Root fungal colonization was significantly (p &lt; 0.01) positively correlated with chlorophyll compositions, leaf sucrose, and root glucose. In addition, inoculation with symbiotic fungi appeared to trigger a significant decrease in soil Olsen-P and available K and a significant increase in NH4-N, NO3-N, and glomalin-related soil protein levels, plus a significant increase in the proportion of water-stable aggregates at the size of 0.5–4 mm as well as aggregate stability. This improvement in soil aggregates was significantly (p &lt; 0.01) positively correlated with root fungal colonization rate and glomalin-related soil protein concentrations. The study concludes that symbiotic fungi, especially R. intraradices, improve the growth of V. villosa, which is associated with fungal modulation of sugars, soil fertility and root structural improvement.

Long-term impacts of topsoil depth and amendments on soil physical and hydrological properties of an Alfisol in central Ohio, USA

Soil hydrological properties and aggregate stability are strongly impacted by erosion and management practices. However, magnitudes of the erosion-induced changes in topsoil depth and the attendant alterations in soil properties are not well understood. Therefore, the present study was conducted on a long term (20 years) simulated study of topsoil depth and use of soil amendments to monitor changes in soil hydrological properties, and aggregate stability of an Alfisol at the Waterman Farm of The Ohio State University, Columbus, Ohio. The aim of the study was to compare long-term effects of soil amendments (synthetic fertilizer and organic compost) on soil physical and hydrological properties at varying soil depth. The experimental plots, comprising of five treatments, were laid out in Randomized Block Design and replicated thrice. Treatments were: (1) topsoil removed (20 cm deep), (2) undisturbed topsoil (intact topsoil); with two soil amendments: (a) synthetic fertilizer 150 kg nitrogen (N) ha−1 yr−1, (b) organic compost at 20 Mg ha−1 yr−1, and (3) a permanent grass field (as a benchmark plot). Soil properties, measured for 0–10 cm and 10–20 cm depth, were: texture, aggregate stability, geometric mean diameter (GMD) of aggregates, water retention properties, hydraulic conductivity (Ks), pore size distribution, and plant available water capacity (PAWC). Aggregate stability was the highest (87.9 and 84.7%) in the permanent grass at 0–10 cm and 10–20 cm depths, respectively. Among the cultivated treatments, compost- amended undisturbed plots (87.6 and86.9%) had the highest proportion of water stable aggregates (WSA) at 0–10 cm and 10–20 cm depths, respectively. However, the GMD of aggregates was the highest 4.1 mm (0–10 cm) and 3.5 mm (10–20 cm) for the topsoil removed and compost-amended treatment. Soil texture was silty clay loam in topsoil removed treatments, clay loam in the undisturbed treatment, and loam in permanent grass treatment, probably due to artificial removal of topsoil. Plant available water content was more in the disturbed and undisturbed compost-amended plots for both the 0–10 and 10–20 cm depths, respectively. The highest soil water volumetric content ranged from 0.37 to 0.25 m3 m−3 in the topsoil removed fertilizer added compared with 0.34 to 0.24 m3 m−3 in undisturbed compost added plots, respectively. However, the pore size distribution was not affected by treatments at the 0–10 cm depth. For the10-20 cm depth, an overall greater pore size distribution range of 0.04 to 0.33 m3 m−3 was observed in the permanent grass, and undisturbed compost amended treatments. Soil Ks (cm day−1) for 0–10 cm depth did not differ significantly across treatments. The data obtained enhances the understanding of the impacts of long-term use of amendments on soil water retention and aggregate stability of simulated topsoil removed and undisturbed field under no-till (NT) in corn (Zea mays)–soybean (Glycine max L. Merr.) rotation in the Eastern Corn Belt of the U.S.

Soil fragmentation and aggregate stability as affected by conventional tillage implements and relations with fractal dimensions

Due to a lack of accurate information regarding soil aggregate size distribution and stability, farmers in Sindh, Pakistan often apply excessive tillage, creating an overly fine seedbed that can lead to the depletion of soil organic matter (SOM) and make the soil more vulnerable to wind and water erosion. The current paper investigates the effects of conventional tillage implements on soil fragmentation and aggregate stability, along with how these factors relate to fractal dimensions. The study aims to: (i) achieve a better understanding of soil structure and establish which tillage implement provides the best soil structure and (ii) determine the function that best describes the proportion of small aggregates in the soil. Six tillage treatments were studied, including: moldboard plough + disk harrow two passes (MPDH2), disk plough + cultivator two passes (DPC2), cultivator two passes (C2), cultivator four passes (C4), cultivator six passes (C6), and minimum tillage (disk harrow one pass) (MT) for a clay loam soil. At each of two depths (0-0.15 m and 0.16-0.30 m), four soil samples from randomly selected locations in each plot were collected (3 blocks × 6 tillage × 2 depths × 4 samples, n = 144) with the help of a soil auger. Aggregate size distribution was measured by a dry sieving method, and aggregate stability was determined by a wet sieving method. The correlations between mean weight diameter (MWD), mass fractal dimension (Dm), and soil aggregation were investigated. The greatest proportions of small aggregates and the lowest proportions of intermediate aggregates were found under DPC2 followed by C6, while a greater proportion of intermediate aggregates and a lower proportion of small aggregates was observed under MT. However large aggregates (>25 mm) were negligible across all treatments. The highest values of Dm were found after DPC2, while the highest values of MWD were found after MT. The highest proportion of water stable aggregates (WSA) was found under DPC2, while the lowest proportion was found under MPDH2 followed by MT. Similarly, aggregate stability was greatest under DPC2. Across all tillage implements, the highest fuel consumption was recorded under C6 followed by MPDH2, while the lowest occurred under MT. The Dm showed a negative correlation with intermediate aggregates (5.0–25.0 mm) and a positive correlation with small aggregates (<5.0 mm), while MWD had a positive correlation with intermediate aggregates (5.0–25.0 mm) and a negative correlation with small aggregates (<5.0 mm). Similarly, Dm was negatively correlated with MWD. This suggests that farmers should adopt the technique of using a disk plough followed by two passes of the cultivator to achieve the best possible seedbed conditions in a clay loam soil. Higher values of Dm and lower values of MWD may be used to quantify fine seedbeds, with the converse being preferable for quantifying coarse seedbeds.

Evaluation Of Selected Soil Structural Properties of Different Land Uses In Oforola, Imo State, Nigeria

This study centered on evaluating selected soil structural properties of different land uses in Oforola, Imo State, Nigeria. Soil samples were collected from six different land-use types, namely, secondary forest, bush fallow, bare plot, cassa- va cultivated plot, oil palm plantation, and grassland vegetation. Soil auger was used to collect soil samples from each of the land use types at two (2) depths (0 – 15 cm and 15 – 30cm). The samples from each land use at different depths were composited, mixed thoroughly and sub-sample, and analyzed in the laboratory for selected physical and chemical parameters. The data were analyzed statisti- cally using GENSTAT software Ver. 8.2 in Completely Randomized Design (CRD). The soils of the study area were principally sandy, with sand accounting for more than 65% of the inorganic mineral fragment in the soil at 0 – 15 cm depth and 15 – 30 cm depth. Land use effect on the dispersion ratio (DR) was significant (P ≤ 0.05) at the 0 -15 cm and 15 – 30 cm depth. However, oil palm plantation recorded significantly (P ≤ 0.05) higher dispersion ratio at the 0 – 15 cm depth. Bulk density was highest (1.57 g/cm3) in grassland vegetation and lowest in secondary forest and bush fallow (1.25 g/cm3). The bulk density val- ues for bare plot, cassava cultivated plot and oil palm plantation were 1.51 g/ cm3, 1.35 g/cm3, and 1.29 g/cm3, respectively. The total porosity of the soil was significantly higher in secondary forest (51.84%). Total porosity values for bush fallow, Cassava cultivated plot, oil palm plantation, bare plot, and grassland veg- etation were 51.83%, 48.38%, 50.97%, 42.36%, and 40.08%, respectively. The proportion of water stable aggregates were significantly (P ≤ 0.05) influenced by the various land-use types. On the average, secondary forest and bush fallow had higher water-stable aggregates of various sizes compared to the cassava cultivat- ed plot that had moderate water-stable aggregates. The least water-stable aggre- gates of various sizes were recorded in grassland vegetation and oil palm planta- tion. The mean weight diameters were significantly (P ≤ 0.05) influenced by var- ious land-use types. Low mean weight diameter was observed for oil palm plan- tation, grassland vegetation and cassava cultivated plots. Based on the findings of the study, secondary forest and bush fallow land-use types possessed the most stable structural properties and also showed optimal impact compared to other land uses in Oforola soils, Imo State, Nigeria

Distribution of water-stable aggregates under soil tillage practices in a black soil hillslope cropland in Northeast China

Soil aggregates are the basic units of soil structure. Water-stable aggregates (WSAs) are greatly influenced by tillage practices. The main objective of our study was to assess the effects of different tillage practices on water aggregate stability on hillslope cropland in northern China and to identify the relationship between soil aggregates and soil structure stability. The study was conducted under on-farm conditions. Soils from plots with no tillage (NT), rotary tillage (RT), and conventional tillage (CT) treatments were sampled for 2 years (2016–2017) in the study area (Xiangyang village, Harbin city, Heilongjiang Province). Soil aggregates were collected, and the mean weight diameter (MWD) and geometric mean diameter (GMD) were calculated. The relationships between the soil aggregate variables and soil structural stability were determined using multivariate stepwise analysis. The results indicated that for macroaggregates, the MWD and GMD increased under NT and more readily formed WSAs > 5 mm. The effects of the different treatments on microaggregates were similar among the different slope positions. The variation in the proportion of macroaggregate across all the treatments was lower in 2017 than that in 2016. The reduction in soil macroaggregates under NT was much smaller than the reductions under RT and CT, suggesting that soil structure can be maintained under NT. The multivariate stepwise analysis showed that the proportion of WSAs > 5 mm were positively and significantly correlated with MWD and GMD and represented the predominant factor influencing soil structure according to the discriminant coefficients. The proportion of microaggregates also affected the MWD and GMD of the soil aggregates but had negative correlations with these variables. There were significantly positive correlations between the soil organic matter content and soil structure stability variables (macroaggregate content, MWD, and GMD). Overall, macroaggregates in soil likely play a key role in soil structure stability. The soil aggregate stability is also affected by slope position. Comparing RT and NT, soil structure can be better maintained with NT.

Rotation and manure amendment increase soil macro-aggregates and associated carbon and nitrogen stocks in flue-cured tobacco production

Flue-cured tobacco production in China is typically over-fertilised and mono-cropped. To understand how this agronomic management affects soil structure and organic matter, this study investigated the effect of rotation, fertilizer rate, and manure amendment on the proportion of water stable aggregates and aggregate-associated soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations and stocks in tobacco production. Two tobacco management systems (Tobacco monoculture and Tobacco-rice rotation) with four fertilizer treatments (0, 75, and 112 kg N/ha, and 60 kg N/ha + manure) were established in 1998. After 18 years, soil aggregation and aggregate-associated SOC and TSN were significantly affected by rotation and fertilizer management. Compared to tobacco monoculture and current fertilizer management, rotation and manure amendment increased macroaggregate (>250 μm) proportion and geometric mean diameter and decreased the proportion of microaggregates and silt-clay sized fractions (<250 μm). Simultaneously, rotation and manure amendment increased macroaggregate-associated SOC and TSN stocks at the expense of the microaggregate and silt-clay size class and their associated SOC and TSN stocks. Rotation and/or manure treatment can maintain satisfactory physico-chemical soil properties through macroaggregate stabilization in tobacco production, which contributes to conserving SOC and TSN stocks.

Effects of no-tillage systems on soil physical properties and carbon sequestration under long-term wheat–maize double cropping system

A thorough assessment of changes in soil quality associated with soil management practices is vital for the selection and establishment of sustainable agricultural management. The winter wheat (Triticum aestivum L.)–maize (Zea mays L.) double cropping system was used to study the integrated effects of a 9-year-old tillage coupled with fertilization on soil carbon sequestration and other physical properties in the Yellow River Delta (YRD). Three tillage systems were selected: no-tillage with straw cover plus recommended urea nitrogen rate (NTS), no-tillage with straw removed and manure applied plus recommended urea nitrogen rate (NTM), and conventional tillage with straw removed plus conventional urea nitrogen application rate (CT). There were three replicates of each treatment organized in a randomized block design. NTS and NTM treatments were found to result in a slightly decrease in the soil bulk density (BD), and significantly increased the proportion of water stable aggregates (WSA) (>2mm), as well as the water infiltration capacity. The proportion of water stable macroaggregates (>0.25mm), the mean weight diameter (MWD) and the geometric mean diameter (GMD) of aggregates in the 0–20cm layer were unchanged by NTS and NTM. The total soil organic carbon (SOC) stock at a depth of 0–60cm was not significantly different among the treatments. Both aggregate-associated SOC concentration and stocks (0–5cm) were significantly greater for NTS and NTM compared with CT, while CT led to a greater OC accumulation in the 20–60cm soil layer compared with NTS and in the 10–20cm compared with NTM. The mesoaggregate fraction (2–0.25mm) and its associated OC pool accounted for the highest percentages in the whole soil profile under the CT treatment. The NT system was found to have a positive effect on the investigated soil physical properties and increase soil carbon content in the soil surface layer. The CT system in conjunction with the wheat–maize double cropping system, however, improved soil aggregation in the soil profile (0–60cm) and also maintained a higher fraction of SOC in the subsoil compared with the NT systems.

Glomalin-related soil protein and water relations in mycorrhizal citrus (Citrus tangerina) during soil water deficit

Glomalin-related soil protein (GRSP), a glycoprotein of arbuscular mycorrhizal fungi (AMF) secreted into soil, governs the aggregate stability, but the role of GRSP in soil and plant water is sparsely studied. The 24-week-old red tangerine (Citrus tangerina) inoculated with Glomus etunicatum and G. mosseae were subjected to a soil drying for 12 days as soil water deficit (SWD). Length of SWD significantly reduced mycorrhizal colonization, soil hyphal length, and leaf and soil water potential (Ψ), but increased total GRSP (T-GRSP), easily extractable GRSP (EE-GRSP), and proportion of water-stable aggregates (WSAs) in >0.25 mm size, irrespective of AMF source. The AMF-inoculated seedlings showed significantly higher T-GRSP, EE-GRSP, and leaf/soil Ψ than non-AMF seedlings during SWD. A significantly positive correlation was observed for mycorrhizal colonization versus leaf or soil Ψ, and hyphal length versus leaf Ψ, suggesting that root intra- and extra-radical hyphae participated in water transport. Interestingly, in GRSP fractions, only T-GRSP was significantly positively correlated with 0.25–1 and >0.25 mm WSA and negatively with leaf and soil Ψ. These results revealed a strong glue function of T-GRSP (not EE-GRSP and hyphae) to alter the proportional distribution of WSA size, thereby aiding toward prevention of soil water loss for improving soil–plant water relations.

Soil aggregation and greenhouse gas flux after 15 years of wheat straw and fertilizer management in a no-till system

Mulching effect on carbon (C) sequestration depends on soil properties, mulch material, and the rate and duration of application. Thus, rate of soil C sequestration was assessed on a 15 year field study involving three levels of wheat straw at 0 (M0), 8 (M8) and 16 (M16) Mgha−1yr−1, at two levels (244kgNha−1yr−1, F1 and without, F0) of supplemental N. Soil C concentration was assessed in relation to aggregation and occlusion in aggregates of a silt loam Alfisol under a no-till (NT) and crop-free system in central Ohio. In comparison to control, soil organic carbon (SOC) concentration in the 0–10cm depth of bulk soil increased by 32% and 90% with M8 and M16 treatments with a corresponding increase in the SOC stock by 21–25% and 50–60%, respectively. With increase in rate of residue mulch, proportion of water stable aggregates (small macroaggregates, >250μm size) increased by 1.4–1.8 times and of microaggregates (53–250μm) by 1.4 times. Fertilizer N significantly increased the SOC concentration of small macroaggregates under M16 treatments only. Ultra-sonication showed that 12–20% of SOC occluded in the inter-microaggregate space of small macroaggergates, was a function of both mulch and fertilizer rates. Significantly higher and positive correlation of greenhouse gases (GHGs), CO2, CH4 and N2O flux was observed with C and N concentrations of small macroaggregates and also of the occluded fraction of small macroaggregates. The higher correlation coefficient indicated the latter to be prone to microbial attack. On the contrary, non-significant relationship with C and N concentrations of microaggregates indicate a possible protection of microaggregate C. The diurnal fluxes of CO2, CH4 and N2O were the lowest under bare soil and the highest under high mulch rate with added N, with values ranging from 1.51 to 2.31gm−2d−1, −2.79 to 3.15mgm−2d−1 and 0.46 to 1.02mgm−2d−1, respectively. Mulch rate affected the GHGs flux more than did the fertilizer rates. The net global warming potential (GWP) was higher for high mulch (M16) than low mulch (M8) rates, with values ranging from 0.46 to 0.57 Mg CO2 equivalent – Cha−1yr−1 (M8) and 1.98 to 3.05Mg CO2 equivalent – Cha−1yr−1 (M16). In general, mulch rate determined the effect of fertilizers. The study indicated that over long-term, a mulch rate between 8 and 16Mgha−1yr−1 may be optimal for Alfisols in Central Ohio.

Soil quality indicator responses to row crop, grazed pasture, and agroforestry buffer management

Soil enzyme activities and water stable aggregates have been identified as sensitive soil quality indicators, but few studies exist comparing those parameters within buffers, grazed pastures and row-crop systems. Our objective was to examine the effects of these land uses on the activities of selected enzymes (β-glucosidase, β-glucosaminidase, fluorescein diacetate (FDA) hydrolase, and dehydrogenase), proportion of water stable aggregates (WSA), soil organic carbon and total nitrogen content. Four management treatments [grazed pasture (GP), agroforestry buffer (AgB), grass buffer (GB) and row crop (RC)] were sampled in 2009 and 2010 at two depths (0 to 10- and 10 to 20-cm) and analyzed. Most of the soil quality indicators were significantly greater under perennial vegetation when compared to row crop treatments. Although there were numerical variations, soil quality response trends were consistent between years. The β-glucosaminidase activity increased slightly from 156 to 177 μg PNP g−1 dry soil while β-glucosidase activity slightly decreased from 248 to 237 μg PNP g−1 dry soil in GB treatment during 2 years. The surface (0–10 cm depth) had greater enzyme activities and WSA than sub-surface (10–20 cm) samples. WSA increased from 178 to 314 g kg−1 in row crop areas while all other treatments had similar values during the 2 year study. The treatment by depth interaction was significant (P < 0.05) for β-glucosidase and β-glucosaminidase enzymes in 2009 and for dehydrogenase and β-glucosaminidase in 2010. Soil enzyme activities were significantly correlated with soil organic carbon content (r ≥ 0.94, P < 0.0001). This is important because soil enzyme activities and microbial biomass can be enhanced by perennial vegetation and thus improve several other soil quality parameters. These results also support the hypothesis that positive interactions among management practices, soil biota and subsequent environmental quality effects are of great agricultural and ecological importance.

Tillage and green manure effects on Bt transgenic cotton ( Gossypium hirsutum L.) hybrid grown on rainfed Vertisols of central India

Transgenic Bt cotton hybrids, in India, now constitute more than 90% of the cotton area. Conservation tillage systems such as reduced tillage (RT) improve soil health and crop productivity. Field experiments were conducted to study the response of Bt cotton hybrids to the tillage methods in a split plot design for three years (2005–2006 to 2007–2008) with tillage practices as main plots (conventional tillage {CT}, RT with two inter-row cultivations {RT 1} and RT with no inter-row cultivation {RT 2}). In the sub-plot, in situ green manure (GM) was included along with 100 (GM + N 100), 80 (GM + N 80) and 60 kg N ha −1 (GM + N 60) and were compared to N alone (N 100). Growth, yield and fibre quality of Bt transgenic cotton hybrid (RCH-2 Bt) were monitored during the study. In all seasons, weed density and biomass were significantly lower in the RT treatments than in the CT treatments. Taller plants with more main stem nodes were produced on the RT plots (63.6–75.8 cm) than on the CT plots (58.1–70.2 cm). Thus, plants of RT treatment retained more bolls (60.8–62.0 m −2) than those of the CT treatment (52.4 m −2). Averaged over seasons, RT treatment had seed cotton yield (1717–1740 kg ha −1) significantly higher than the CT treatment (1489 kg ha −1). The treatments N 100, GM + N 100 and GM + N 80 (1687–1734 kg ha −1) did not differ and were significantly better than the GM + N 60 (1303 kg ha −1). Tillage × GM interaction was significant in two of three years and data combined over years. The RT plots with GM had significantly greater yield than the CT plots. Compared to the CT treatment, the RT plots had significantly greater proportion of water stable aggregates (48.6% vs. 54.4–56.0%) and mean weight diameter (0.47 mm vs. 0.49–0.51 mm). The GM plots had significantly higher WSA and MWD than those without. Further, soil moisture content was greater in the GM mulched plots up to 0.60 m depth compared to without GM treatment (CT + N 100). Favourable soil physical properties may have contributed to improved seed cotton yield in the RT treatments with GM than in the CT treatment. Fibre quality was not affected by either the tillage systems or GM. This study indicates that Bt transgenic cotton can be grown under RT systems with an in situ legume GM.

Soil enzyme activities and physical properties in a watershed managed under agroforestry and row-crop systems

The proportion of water-stable aggregates (WSA) and a diverse microbial activity influence soil quality, crop growth, nutrient retention, water infiltration, and surface runoff. The objective of the study was to test the hypothesis that permanent vegetative buffers increase WSA and contribute to increased soil enzyme activity. Soil samples (5 cm diameter and 10 cm long) from agroforestry (AG), grass buffer (GB), grass waterway (GWW) and crop (CS) areas were collected from summit, middle, and lower landscape positions at the Paired Watershed Study, near Novelty, MO. Water-stable aggregates (>250 μm diameter; wet-sieving method), soil carbon, and soil enzyme activity were determined and data were statistically analyzed. Soils under permanent vegetative buffers and GWW had significantly lower bulk density and more WSA than the crop areas. Soil carbon contents were highest in the GWW and lowest in the CS treatments. Fluorescein diacetate (FDA) hydrolase, β-glucosidase, and glucosaminadase enzyme activities were higher in AG, GB, and GWW soils than CS soils. Dehydrogenase activity differed between grass buffer or GWW and crop areas. The results of the study show that WSA, soil carbon, and functional diversity of enzyme activity increased due to establishment of buffers with trees and grass. It can be speculated that increased diversity of functional enzymes associated with cycling of key soil nutrients and improved soil physical properties may reduce nonpoint source pollution (NPSP) from row-crop agriculture watersheds thus improving overall environmental quality.

Compost Applications Increase Water‐Stable Aggregates in Conventional and No‐Tillage Systems

Agricultural practices that alter the soil organic matter (SOM) content are expected to cause changes in soil stability and aggregation. The objective of this study was to evaluate short‐term (&lt;2 yr) changes in water‐stable aggregates (WSA) in a silt‐loam soil under different management regimes. The interactive effects of tillage (no‐till and conventional tillage), crop rotations (continuous corn, corn‐soybean rotation) and composted cattle manure applications [0, 15, 30, and 45 Mg (wet weight) ha−1] on WSA were assessed in a factorial (tillage × crop rotation) split plot (compost) experiment. The proportion of WSA &gt;4 mm was greater in compost‐amended than unamended soils within 1 yr, and the mean weight diameter (MWD) of aggregates increased with increasing compost application rates. By the second year of the study, no‐till soils under continuous corn and the soybean phase of the corn‐soybean rotation had more WSA &gt;4 mm and a greater MWD than any crop rotation in conventionally tilled soils. Increasing the C input to soil increased the MWD of aggregates. The MWD of aggregates was related to the C content of soils under no‐till, but not conventional tillage, suggesting more physical stabilization of organic matter (OM) in no‐till than conventional tillage agroecosystems. Our findings indicate rapid improvements in aggregation of a silt‐loam in the first 2 yr after compost application and the adoption of no‐tillage practices.

Production and physico-chemical properties of surface casts from microchaetid earthworms in central Eastern Cape

The study was undertaken to estimate surface cast production by microchaetid earthworms and to compare the physical and chemical properties of the casts with the surrounding non-cast soil. An average of 104 (range 64–132) and 192 (range 182–222) t ha−1 of surface casts were produced at Alice and Dimbaza sites in central Eastern Cape during the period September 1995 to August 1996. The casts had an average height of 6.8 cm and diameter of 5.6 cm and had higher silt and clay (66.2 vs 62.9%) but lower sand (33.8 vs 37.1 %) sized partides than the non-cast surface soil. The casts also had a higher proportion of water stable aggregates > 0.5 mm (76.7 vs 67.2%), mean weight diameter, bulk density (1.55 vs 1.37 Mg m−3) and retained more water than the non-cast soil. Earthworm casts had significantly higher available Bray 1-P, exchangeable K, Ca, Mg and Na than the non-cast soil. Levels of organic C and pH were also elevated in earthworm casting compared to surrounding soil. The large amounts of casts that are deposited on the surface can play an important role in modifying the hydraulic properties at the soil surface and help minimise runoff and erosion. Because of the high concentration of nutrients in casts compared to non-cast soil, potential may exist for using the casts to supply plants with nutrients.

Comparison of the effects of latex and poly(DADMAC) on structural stability and strength of soil aggregates

Two types of high molecular weight polymers having contrasting charge properties and molecular shapes [latex and poly(DADMAC)] were applied at different rates to three Alfisols and a Mollisol to examine the effects of these polymers on soil strength and structural stability, and to identify some of the mechanisms by which these polymers act to stabilize soils. Wet aggregate stability, mechanically dispersible clay, and soil friability tests were used to assess treatment effects, which were found to be greatest in soils having a poor structural condition. Each polymer acted differently. Latex, which acted like a coat of paint (because of its inability to penetrate beyond the surface of aggregates), dramatically increased the proportion of water stable aggregates &gt;2 mm in all soils examined, and this was achieved without the traditional (expensive) use of aromatic oils and stabilizers. The latex coating produced a conglomeration of smaller soil aggregates, but did not extensively alter either the dispersibility of the aggregates in the face of mechanical action, or their strength and friability. Results indicate latex may be best suited to minimum tillage cropping systems. Poly(DADMAC), by contrast, was capable of entering most pores accessible to water. This enabled it to minimize clay dispersion, increase soil friability, and to modestly increase the size of water stable aggregates. In many ways, poly(DADMAC) would appear to be an ideal soil conditioner, although the economic suitability of this polymer and its effects on erosion control and plant growth have yet to be assessed.

EFFECTS OF CROPPING ON MACRO-AGGREGATION OF A MARINE CLAY SOIL

Field and growth chamber experiments were conducted to evaluate the short-term effects of selected crops on macro-aggregation of a Kamouraska clay soil. Under field conditions, the growth of barley (Hordeum vulgare L.) and alfalfa (Medicago sativa L.) for up to 2 yr resulted in increased macro-aggregate size and stability compared to a fallow control and to initial conditions. Under these two crops, the proportion of water-stable aggregates of the 2- to 6-mm fraction increased from 25% in May 1986 to 40% in September 1987 at the expense of the 0.25- to 1.0-mm fraction which decreased from 37% to 19% over the same period. Macro-aggregation after 2 yr was not different in corn (Zea mays L.) and fallow control. Seasonal variations in aggregate stability were significant but small and less important than the effects of cropping treatments. Results of the growth chamber experiment agreed with those obtained under field conditions suggesting that controlled environment experiments can be used to model cropping effects on the aggregation of this soil. Key words: Aggregation, soil structure, clay soil, corn, barley, alfalfa

Changes in soil aggregate water stability induced by wetting and drying cycles in non‐saturated soil

SummaryWetting and drying of remoulded soil resulted in water stable aggregation. The greatest proportions of water stable aggregates arose from wetting and drying in the −1 to −100 kPa range of matric water potential. The effect occurred with sterile and non‐sterile soil. but the proportion of water stable aggregates was less with sterile soil.The application of wetting and drying cycles in the laboratory to non‐tilled soil resulted in a steady decrease in the proportion of water stable aggregates. With tilled soil, the proportion of water stable aggregates first increased to a maximum and then decreased steadily with further wetting and drying cycles. However. with sterilized, tilled soil, only a steady decrease in the proportion of water stable aggregates was observed.Natural water content fluctuations in the field after tillage gave an increase in water stability to a maximum after a few days followed by a steady decrease. The similarity of this result to that obtained in the laboratory for tilled. non‐sterilized soil indicates that micro‐organisms were probably contributing to the observed short‐term changes in the water stability of aggregates in the field.