Linking spatial metrics and fish catch reveals the importance of coastal wetland connectivity to inshore fisheries in Queensland, Australia

Many commercially important fish species use coastal marine environments such as mangroves, tidal flats and seagrass beds as nurseries or breeding grounds. The ecological importance of spatially connected habitats to conservation is well established for terrestrial environments. However, few studies have applied spatial metrics, including measures of structural connectivity to marine environments. We examined the relationship between catch-per-unit-effort for commercially caught species and the spatial patterning of mapped benthic habitat types along the coast of Queensland, Australia in their dominant fisheries (trawl, line, net or pot fisheries). We quantified the composition and spatial configuration of seascapes and calculated coastline length, number of estuaries, river length and geographical latitude using 12 metrics within ninety 30-nautical-mile grid cells, which supported inshore fish catch data from 21 species groups. Multiple regression analysis and non-metric multidimensional scaling plots indicated that ecological linkages may exist between geomorphic coastal features and nearshore fisheries production for a number of species groups. Connectivity indices for mangroves, salt marsh and channels explained the largest proportion (30–70%), suggesting the importance of connected tidal wetlands for fisheries. Barramundi (Lates calcarifer) catch-per-unit-effort was best explained by the number of wetland patches, mangrove connectivity and wetland connectivity (r² = 0.38, n = 28). Catch-per-unit-effort for the Gulf of Carpentaria was highly correlated with wetland connectivity, the number of estuaries and seagrass patch density (r = 0.57, n = 29). The findings could guide the spatial design of marine protected area networks to maintain ecosystem services and avoid potential disruption to connectivity caused by habitat removal or modification. Application of the same approach to analyses of finer spatial scales would enable catch information to be related to particular estuarine habitats and provide better understanding of the importance of habitat connectivity for fisheries.     

Viability and patch occupancy of a swamp rabbit metapopulation at the northern edge of its distribution

Swamp rabbits (Sylvilagus aquaticus) are state-endangered in Indiana, USA, and population decline has been attributed to habitat loss. We conducted pellet surveys as part of a long-term survey effort that has been conducted at approximate 10-year intervals over the last 40 years. We modeled patch occupancy and conducted a spatially-explicit population viability analysis (PVA). Although occupancy of individual patches varied over time, occupancy rate has been constant for the last 30 years, and Indiana swamp rabbits exist as a metapopulation that appears to be stable. Metapopulation dynamics were best characterized as being stationary, but area was an important factor in extinction rates; occupied patches (142 ± 37 ha) were significantly larger (P = 0.01) than unoccupied patches (79 ± 20 ha). We did not find strong support for models with colonization rates as a function of distance to neighboring patches, nor was distance to contiguous patches of habitat significantly different (P = 0.12) for occupied and unoccupied sites. Population viability analysis corroborated our findings based on occupancy modeling, and evaluation of the PVA model using occupancy data for the period 1985–2006 resulted in predictions that nearly matched our field observations (33% observed patch occupancy vs. 25% predicted patch occupancy). Population viability was most sensitive to reductions in survival and fecundity rates, but was otherwise robust to changes in parameters such as initial abundance and carrying capacity. Our findings provide novel insights into a poorly studied member of Sylvilagus and into species metapopulation dynamics at the edge of the range.     

Using a global VNIR soil-spectral library for local soil characterization and landscape modeling in a 2nd-order Uganda watershed

Combining global soil-spectral libraries with local calibration samples has the potential to provide improved visible and near-infrared (VNIR, 400–2500 nm) diffuse reflectance spectroscopy (DRS) soil characterization predictions than with either global or local calibrations alone. In this study, a geographically diverse “global” soil-spectral library with 4184 samples was augmented with up to 418 “local” calibration soil samples distributed across a 2nd-order Ugandan watershed to predict the amount of clay-size material (CLAY), soil organic carbon (SOC) and proportion of expansible 2:1 clays (termed “montmorillonite” or MT in the global library). Stochastic gradient boosted regression trees (BRT) were employed for model construction, with a variety of calibration and validation schemes tested. Using the global library combined with 13- and 14-fold cross-validation by local profile for CLAY and SOC, respectively, yielded dambo/upland RMSD values of 89/68 g kg^− 1 for CLAY (N = 429/410) and 4.2/2.6 g kg^− 1 for SOC (N = 272/105). These results were obtained despite the challenge of combining spectral libraries constructed using different spectroradiometers and laboratory reference measurements (total combustion vs. Walkley–Black, hydrometer vs. pipette). Using only the global library, a VNIR-derived index of MT content was significantly correlated with the square root of X-ray diffraction (XRD) MT peak intensity for local dambo soils (r² = 0.52, N = 59, p < 0.0001), an acceptable result given the semi-quantitative nature of the reference XRD method. Though VNIR predictions did not approach laboratory precision, for soil-landscape modeling VNIR characterization worked remarkably well for clay mineralogy, was adequate for mapping dambo “depth to 35% clay”, and was insufficiently accurate for SOC mapping.     

Role of soil mesofauna in dispersal of Coniothyrium minitans: transmission to sclerotia of Sclerotinia sclerotiorum

Coniothyrium minitans Campbell is a mycoparasite with proven biocontrol activity against Sclerotinia sclerotiorum (Lib.) de Bary in the field and glasshouse. It is known to spread from sites of application but the mechanisms of dispersal are unclear. As C. minitans has been recovered from collembolans collected during glasshouse trials, and numerous mites and insects are often associated with decaying S. sclerotiorum-infected plant material in the glasshouse, the mite Acarus siro L. and the collembolan Folsomia candida Willem were used to investigate the potential of soil mesofauna to disperse C. minitans. In an initial investigation, A. siro was found to transmit the mycoparasite from infected to uninfected sclerotia of S. sclerotiorum in moist sterile sand and non-sterile soil. Subsequently, a simple assay system to monitor transfer of C. minitans from colonised wheat grains to uninfected sclerotia of S. sclerotiorum was developed. Both A. siro and F. candida transmitted C. minitans at least 55 mm to sclerotia in soil at water potentials ranging from saturation to −3.6 MPa. Transmission by A. siro was greater in drier conditions (−0.25 to −3.6 MPa) as mites survived poorly in saturated soil. However, water potentials between saturation to −3.6 MPa had no effect on transmission by F. candida, although collembolans died after 18 d at water potentials of −5.4 MPa or drier. Generally, maximum dispersal occurred within 2 weeks. In soil lacking added arthropods, negligible spread of the mycoparasite was observed. These results suggest that soil mesofauna may be important in the dissemination of C. minitans.     

Organic matter addition, N, and residue burning effects on infiltration, biological, and physical properties of an intensively tilled silt-loam soil

Seventy years of different management treatments have produced significant differences in runoff, erosion, and ponded infiltration rate in a winter wheat (Triticum aestivum L.)–summer fallow experiment in OR, USA. We tested the hypothesis that differences in infiltration are due to changes in soil structure related to treatment-induced biological changes. All plots received the same tillage (plow and summer rod-weeding). Manure (containing 111 kg N ha⁻¹), pea (Pisum sativum L.), vine (containing 34 kg N ha⁻¹), or N additions of 0, 45 and 90 kg ha⁻¹ were treatment variables with burning of residue as an additional factor within N-treatments. We measured soil organic C and N, water stability of whole soil, water stable aggregates, percolation through soil columns, glomalin, soil-aggregating basidiomycetes, earthworm populations, and dry sieve aggregate fractions. Infiltration was correlated (r = 0.67–0.95) to C, N, stability of whole soil, percolation, and glomalin. Basidiomycete extracellular carbohydrate assay values and earthworm populations did not follow soil C concentration, but appeared to be more sensitive to residue burning and to the addition of pea vine residue and manure. Dry sieve fractions were not well correlated to the other variables. Burning reduced (p < 0.05) water stability of whole soil, total glomalin, basidiomycetes, and earthworm counts. It also reduced dry aggregates of 0.5–2.0 mm size, but neither burning nor N fertilizer affected total C or total N or ponded infiltration rate. Water stability of whole soil and of 1–2-mm aggregates was greater at 45 kg N ha⁻¹ than in the 0 and 90 kg N ha⁻¹ treatments. Zero N fertilizer produced significantly greater 0.5–2.0 mm dry aggregate fractions. We conclude that differences in infiltration measured in the field are related to relatively small differences in aggregate stability, but not closely related to N or residue burning treatments. The lack of an effect of N fertilizer or residue burning on total C and N, along with the excellent correlation between glomalin and total C (r = 0.99) and total N (r = 0.98), indicates that the major pool of soil carbon may be dependent on arbuscular mycorrhizal fungi.     

Soil quality indicators under continuous cropping systems in the Argentinean Pampas

Over the last two decades, soil cultivation practices in the southern Argentinean Pampas have been changing from a 7 year cash-crop production system alternated with 2–3 years under pasture, to a continuous cropping system. A better understanding of the impact of the period of time a field has been under continuous cropping on a broad spectrum of soil properties related to soil quality is needed to target for sustainable cropping systems. The objectives of this study were to: (i) assess the relationship between physical and chemical soil parameters related to soil quality and (ii) identify soil quality indicators sensitive to soil changes under continuous cropping systems in the Argentinean Pampas.

Correlation analysis of the 29 soil attributes representing soil physical and chemical properties (independent variables) and years of continuous cropping (dependent variable) resulted in a significant correlation (p < 0.05) in 78 of the 420 soil attribute pairs. We detected a clear relationship between hydraulic conductivity at tension h (K_h) and structural porosity (ρ_e); ρ_e being a simple tool for monitoring soil hydraulic conditions.

Soil tillage practice (till or no-till) affected most of the soil parameters measured in our study. It was not possible to find only one indicator related to the years under continuous cropping regardless of the cultivation practice. We observed a significant relationship between years under continuous cropping and K_h under no-till (NT) and wheat fallow (p < 0.001, R² = 0.70). Under these conditions, K₋₄₀ diminished as the number of years under continuous cropping increased.

The change in mean weight diameter (CMWD) was the only physical parameter related to the number of years under continuous cropping, explaining 36% of the variability in the number of years under continuous cropping (p < 0.001) The combination of three soil quality indicators (CMWD, partial R² = 0.38; slope of the soil water retention curve at its inflexion point (S), partial R² = 0.14 and cation exchange capacity (CEC), partial R² = 0.13) was able to explain, in part, the years under continuous cropping (R² = 0.65; p value > 0.001), a measure related to soil quality.     

Earthworm community structure on five English golf courses

A clear understanding of the size and structure of earthworm communities is important to sports turf facilities managers if they are to control the activity of earthworms within the soil. Earthworms are directly linked to a wide range of biogeochemical nutrient cycles, and are frequently described as ecosystem engineers. In this role they assist land managers in maintaining a healthy turf sward. Despite this, earthworm populations in sports turf were frequently suppressed using organochloride based vermicides prior to such compounds being banned in the UK. A survey of earthworm species diversity was carried out over 1 year, using mustard extraction at five golf courses in Bedfordshire and Buckinghamshire, UK, to investigate how species diversity varied temporally and was influenced by the physical environment within different course surfaces. Rank–abundance analysis indicated that four species of earthworm (Aporrectodea rosea, Lumbricus rubellus, Aporrectodea longa and Lumbricus terrestris) were more dominant in the community than any other. The endemic earthworm population number of the five study sites was found to differ significantly (p < 0.01) related to their geographical location, and the most diverse communities were found where the population was greatest. A very strong linear correlation was found between the number of earthworms recovered and the sand content of the soil (r² = 0.97). Significant differences in the size of the earthworm populations were recorded at different times of the year (p < 0.01). A distinct and non-linear relationship between species diversity and microbial biomass C was shown (p < 0.01).     

Effect of tillage systems on weed control, yield and fibre quality of upland (Gossypium hirsutum L.) and Asiatic tree cotton (G. arboreum L.)

Asiatic diploid (n = 13) cotton (Gossypium arboreum L.) is grown on Vertisols of central India with limited amounts of fertilizers and pesticides under rainfed conditions. In an earlier study it was established that reduced tillage (RT) systems improved productivity of tetraploid (n = 26) upland cotton (G. hirsutum L.). Such information is currently not available for the Asiatic cotton. Field studies were continued from 2002–2003 through 2004–2005, to determine the effect of tillage systems on weed control, yield and fibre quality. Tillage treatments continued for 6 years before this phase of the study. The experiment was conducted in a split plot design, with three tillage systems as main plots and combination of species (G. arboreum and G. hirsutum) and N rates (60 and 75 kg N ha⁻¹) as subplots. Conventional tillage (CT) involved mouldboard ploughing + four to five inter-row cultivations and was compared with two levels of RT. RT₁ being pre-emergence herbicide application with two inter-row cultivations by a bullock drawn hoe and RT₂ was only herbicide application with no inter-row cultivation. Weed density (monocot and dicot weeds) was significantly lower on the RT than on the CT plots. Consequently, the RT plots had accumulated less weed dry matter. Seed cotton yield was affected by tillage systems in 1 out of 3 years. In 2002–2003, the yield trend was: RT₁ > CT > RT₂. The tillage × species interaction was significant in 2002–2003 and 2004–2005 and combined-across-years. Averaged over years, Asiatic G. arboreum produced 8% less seed cotton with treatment RT₂ than with CT. Upland, G. hirsutum produced 118–134 kg ha⁻¹ additional seed cotton on the RT than with CT. Differences in maturity and rooting habit probably contributed to the two species differing in their tillage requirement. The Asiatic cottons were early maturing and are known to possess a deeper root system than the upland cotton. The tillage × N and species × N interactions were not significant. Average seed cotton yield with the 75 kg N was 15.7% more than the 60 kg N ha⁻¹ plots. Among fibre properties, fibre length was significantly better with treatment RT₁ than with the CT in 2 out of 3 years. In summary, seed cotton yield of upland G. hirsutum cotton was higher with RT system, whereas converse occurred with G. arboreum. There were no adverse effects of RT on fibre quality.     

A short-term investigation of trace gas emissions following tillage and no-tillage of agroforestry residues in western Kenya

Improved-fallow agroforestry systems are increasingly being adopted in the humid tropics for soil fertility management. However, there is little information on trace gas emissions after residue application in these systems, or on the effect of tillage practice on emissions from tropical agricultural systems. Here, we report a short-term experiment in which the effects of tillage practice (no-tillage versus tillage to 15 cm depth) and residue quality on emissions of N₂O, CO₂ and CH₄ were determined in an improved-fallow agroforestry system in western Kenya. Emissions were increased following tillage of Tephrosia candida (2.1 g N₂O-N ha⁻¹ kg N applied⁻¹; 759 kg CO₂-C ha⁻¹ t C applied⁻¹; 30 g CH₄-C ha⁻¹ t C applied⁻¹) and Crotalaria paulina residues (2.8 g N₂O-N ha⁻¹ kg N applied⁻¹; 967 kg CO₂-C ha⁻¹ t C applied⁻¹; 146 g CH₄-C ha⁻¹ t C applied⁻¹) and were higher than from tillage of natural-fallow residues (1.0 g N₂O-N ha⁻¹ kg N applied⁻¹; 432 kg CO₂-C ha⁻¹ t C applied⁻¹; 14.7 g CH₄-C ha⁻¹ t C applied⁻¹) or from continuous maize cropping systems. Emissions from these fallow treatments were positively correlated with residue N content (r = 0.62–0.97; P < 0.05) and negatively correlated with residue lignin content (r = −0.56, N₂O; r = −0.92, CH₄; P < 0.05). No-tillage of surface applied Tephrosia residues lowered the total N₂O and CO₂ emitted over 99 days by 0.33 g N₂O-N ha⁻¹ kg N applied⁻¹ and 124 kg CO₂-C ha⁻¹ t C applied⁻¹, respectively; estimated to provide a reduction in global warming potential of 41 g CO₂ equivalents. However, emissions were increased from this treatment over the first 2 weeks. The responses to tillage practice and residue quality reported here need to be verified in longer term experiments before they can be used to suggest mitigation strategies appropriate for all three greenhouse gases.     

Seal–fishery operational interactions: Identifying the environmental and operational aspects of a trawl fishery that contribute to by-catch and mortality of Australian fur seals (Arctocephalus pusillus doriferus)

Australian fur seals are known to interact directly (i.e. operational interactions) with trawlers fishing in the winter blue grenadier trawl fishery of western Tasmania, Australia. The purpose of this study was to identify the environmental and operational aspects of the fishery that were associated with increased numbers of seals observed at the surface. The incidence of net entry was determined in order to establish the effectiveness of the currently used Seal Exclusion Device (SED) at reducing by-catch and mortalities. In addition, the stage and depth at which seals were at greatest risk of entering the net at depth and becoming by-catch were also identified.

An increase in seal numbers observed at the surface is assumed to be proportional to the increased risk of by-catch and mortality incidences at trawlers. The most important subset of environmental and operational factors for predicting the number of seals observed at the surface both in the fishing ground in general and during fishing operations were identified using Generalised Linear Modelling. The final model yielded a significant result (R² = 0.63, P < 0.01, n = 149) and indicated that the number of seals observed increased when weather conditions deteriorated, particularly when barometric pressure decreased and when swell height and visibility increased. Vessel operations also influenced the number of seals observed; seal numbers increased when the number of nearby vessels and trawl frequency increased, but decreased when vessel speed increased. Seal numbers also increased as the distance from the nearest breeding colony and haul-out site decreased.

Seal numbers at the surface generally increased steadily during trawling operations (n = 475), although brief declines were noted during shooting and hauling phases. Sub-surface net interactions were also examined using a submersible video camera unit. Seal activity at depth was converse to that observed at the surface, with increased seal numbers noted during shooting and hauling, suggesting that seals dived to forage on fish in the submerged net.

All seal by-catch occurred during the day and almost half occurred during shooting. Mortalities were significantly higher during shooting compared with hauling (Fishers exact test: coef. = 0.65, P < 0.05). However, mortality rates were similar between tows with the Seal Exclusion Device (SED) attached and those without (Fishers exact test: coef. = 0.07, P < 0.99). Only one seal was detected entering and exiting the net mouth during monitored tows, suggesting that the recently observed reduction in by-catch levels (and mortalities) may not necessarily be attributed to the introduction of the Seal Exclusion Device. Seal by-catch recorded during hauling typically occurred when haul speeds exceeded minimum average swimming speeds for fur seals.

Recommendations based on these findings have been made to assist with the future management of fur seal populations that interact with trawl fisheries.     

Dynamics of soil hydraulic properties during fallow as affected by tillage

There is limited information on the effects of tillage practices on soil hydraulic properties, especially changes with time. The objective of this study was to evaluate on a long-term field experiment the influence of conventional tillage (CT), reduced tillage (RT) and no-tillage (NT) on the dynamics of soil hydraulic properties over 3 consecutive 16–18 month fallow periods. Surface measurements of soil dry bulk density (ρ_b), soil hydraulic conductivity (K(ψ)) at −14, −4, −1 and 0 cm pressure heads using a tension disc infiltrometer, and derived hydraulic parameters (pore size, number of pores per unit of area and water-transmission porosity) calculated using the Poiseuille's Law were taken on four different dates over the fallow period, namely, before and immediately after primary tillage, after post-tillage rains and at the end of fallow. Under consolidated structured soil conditions, NT plots presented the most compacted topsoil layer when compared with CT and RT. Soil hydraulic conductivity under NT was, for the entire range of pressure head applied, significantly lower (P < 0.05) than that measured for CT and RT. However, NT showed the largest mean macropore size (0.99, 0.95 and 2.08 mm for CT, RT and NT, respectively; P < 0.05) but the significantly lowest number of water-conducting pores per unit area (74.1, 118.5 and 1.4 macropores per m² for CT, RT and NT, respectively; P < 0.05). Overall, water flow was mainly regulated by macropores even though they represented a small fraction of total soil porosity. No significant differences in hydraulic properties were found between CT and RT. In the short term, tillage operations significantly increased K (P < 0.05) for the entire range of pressure head applied, which was likely a result of an increase in water-conducting mesopores despite a decrease in estimated mesopore diameter. Soil reconsolidation following post-tillage rains reduced K at a rate that increased with the intensity of the rainfall events.     

Structure and functioning of earthworm communities in woodland flooding systems used for drinking water production

Earthworms are known to influence water infiltration in soils, but most of the existing knowledge relates to grasslands and arable systems; little is known on the role of earthworms for water infiltration in forests. We studied earthworm populations and water infiltration rates in woodland flooding sites used for groundwater recharge and the production of drinking water. Intensive flooding may detrimentally affect earthworm populations and simultaneously result in clogging of the topsoil, with the latter being a common problem in groundwater recharge systems. However, clogging does not occur at our study site, the “Lange Erlen” (Basel, Switzerland) and total earthworm numbers and biomass in flooded sites exceeded those of non-flooded sites (+51% and +71%, respectively). Total earthworm numbers (r = 0.85***), numbers of endogeic (r = 0.64*) and epigeic (r = 0.81**) earthworms and numbers of two species (Lumbricus rubellus, r = 0.62* and Allolobophora chlorotica, r = 0.77**) significantly correlated with water infiltration rates. The results suggest that short-term flooding (max. 10 days) interrupted by longer recovery periods favor earthworm populations which likely contribute to the long-term (ca. 100 years) sustainability of the studied forest groundwater recharge system and thereby to effective and cost efficient drinking water production.     

基于Dual RNA-seq分析的褪黑素影响萝卜与链格孢菌互作的作用研究

由芸薹链格孢菌(Alternaria brassicae)严重影响萝卜(Raphanus sativus)生产,外源喷施褪黑素可减轻病症,但褪黑素同时存在于植物和微生物中,外源喷施时可对感病植株上寄主和病原物的生命活动均有影响,其介导寄主-病原菌互作的作用模式。为解析褪黑素参与调控病叶上萝卜和链格孢菌互作的生理和基因转录机制,本研究通过对感黑斑病萝卜叶面外施0~1 500 μmol·L^-1浓度褪黑素,检测病情指数变化,同时利用互作转录组分析0、500和1 500 μmol·L^-1褪黑素处理下萝卜和链格孢菌转录水平变化。结果显示,50~500 μmol·L^-1褪黑素可分别显著提高萝卜和链格孢菌双方的生长势和抗逆性,且500 μmol·L^-1表现最优,1 000和1 500 μmol·L^-1与500 μmol·L^-1相比作用较小,呈明显剂量依赖型模式,但500 μmol·L^-1褪黑素处理下,萝卜对链格孢菌抗性明显提升。Dual RNA-seq检出萝卜基因组reads比对率>88%,链格孢菌基因reads比对率≤0.06%,萝卜差异表达基因数显著高于链格孢菌检出数。对萝卜及链格孢菌生长及抗逆性调控相关基因表达模式进行验证,基因表达模式基本与表型反应一致。综上所述,褪黑素介导萝卜与链格孢菌的作用存在低浓度促进、高浓度抑制的剂量依赖性效应,500 μmol·L^-1浓度下褪黑素对双方均具生长促进作用,但褪黑素对寄主的影响超过对链格孢菌的影响,最终表现为诱导萝卜对链格孢菌抗性增强。本研究为褪黑素在萝卜黑斑病抑制中的应用提供了数据支撑和理论基础,并在转录水平上初步解析了褪黑素参与萝卜对链格孢菌抗性变化的作用模式。     

Soil carbon and nitrogen dynamics using stable isotopes in 19- and 10-year-old tropical agroforestry systems

Conversion of forests to agricultural land in the American tropics, through traditional agricultural practices such as shifting cultivation, has not been able to maintain stocks of soil organic carbon (SOC), and increasing population pressure has led to shortened fallow periods, causing further losses of soil fertility. However, land management practices such as agroforestry can provide a sustainable alternative to single cropping because of its ability to maintain or increase the SOC pool. This study quantified SOC and nitrogen (N) pools, gross SOC turnover, residue stabilization efficiency (RSE_AC) in the alley crop, soil δ¹³C partitioning, C₃-C abundance and δ¹⁵N dynamics in 19- and 10-year Gliricidia sepium and Erythrina poeppigiana alley cropping system. Each system was studied at two fertilizer levels (tree prunings only [−N or −A], and tree prunings plus chicken manure [+N], or Arachis pintoi as a groundcover [+A]), and was compared to a sole crop system. The SOC and N pools were significantly higher (p < 0.05) in the 19-year-old alley crop compared to the sole crop, but not significantly different (p < 0.05) in the 10-year-old system. Soil C and N (%) showed a similar trend as that of the SOC and N pools in both 19- and 10-year-old systems. Gross SOC turnover, to a 20 cm depth, ranged from 12 to 21 years in the 19-year-old alley crop compared to 50 years in the sole crop, and from 20 to 32 years in the 10-year-old alley crop compared to 106 years in the sole crop. The RSE_AC ranged from 10% to 58% in the 19-year-old system, and from 3% to 43% in the 10-year-old system. The δ¹³C signature of the soil shifted significantly (p < 0.05) towards that of C₃ vegetation in the alley crop due to the greater input of organic residues from tree prunings compared to the sole crop. The proportion of input from tree prunings only in the 19-year-old alley crop ranged from 14% to 20%, and from 9% to 11% in the 10-year-old system to a soil depth of 20 cm. The δ¹⁵N signature of the soil showed two patterns: that of the 19-year-old system being enriched in δ¹⁵N, and that of the 10-year-old system being depleted in δ¹⁵N compared to the sole crop. The addition of manure in the 19-year-old system has enriched the soil δ¹⁵N and in the 10-year-old system the soil was depleted due to the N₂-fixing groundcover A. pintoi.     

Cultivation effects on temporal changes of organic carbon and aggregate stability in desert soils of Hexi Corridor region in China

Sustainable agricultural use of cultivated desert soils has become a concern in Hexi Corridor in Gansu Province of China, because loss of topsoil in dust storms has been recently intensified. We chose four desert sites to investigate the effects of cultivation (cropping) on (i) soil organic C and its size fractions and (ii) soil aggregate stability (as a measure of soil erodibility). These parameters are of vital importance for evaluating the sustainability of agricultural practices.

Total organic C as well as organic C fractions in soil (coarse organic C, 0.1–2 mm; young organic C, 0.05–0.1 mm; stable organic C, <0.05 mm) generally increased with the duration of the cultivation period from 0 (virgin soil, non-cultivated) to more than 30 years (p < 0.05). Compared to total organic C in virgin soils (2.3–3.5 g kg⁻¹ soil), significantly greater values were found after 10 to >20 years of cultivation (6.2–7.1 g kg⁻¹ soil). The increase in organic C in desert soils following prolonged cultivation was mainly the consequence of an increase in the coarse organic C. The increase in total organic C in soil was also dependent on clay content [total organic C = 0.96 + 0.249 clay content (%) + 0.05 cultivation year, R² = 0.48, n = 27, p < 0.001]. This indicates that clay protected soil organic C from mineralization, and also contributed to the increase in soil organic C as time of cultivation increased.

There was a significant positive correlation between aggregate stability and total organic C across all field sites. The water stability of aggregates was low (with water-stable aggregate percentage 4% of dry-sieved aggregates of size 1–5 mm). There was no consistent pattern of increase in the soil aggregate stability with time of cultivation at different locations, suggesting that desert soils might remain prone to wind erosion even after 50 years of cultivation. Alternative management options, such as retaining harvested crop residues on soil surface and excluding or minimizing tillage, may permit sustainable agricultural use of desert soils.     

Soil aggregation in a laboratory experiment: Interactions between earthworms, woodlice and litter palatability

Interactions between soil invertebrate functional groups are still poorly understood. In this study, the effect of the soil dwelling earthworm Aporrectodea caliginosa and the woodlouse Porcellio scaber (litter inhabitant), alone or in combination, on soil aggregation was investigated in laboratory microcosms with litter of different quality (Quercus robur and Fagus sylvatica). After 4 months of incubation, the aggregate size distribution was measured using a dry-sieving method. A. caliginosa played an important role in the formation of large aggregates (>2 mm), whereas P. scaber greatly influenced litter disappearance. A. caliginosa individuals formed a greater number of large aggregates when fed with beech leaves compared to oak leaves even though they preferred the latter. In the presence of beech and P. scaber, A. caliginosa produced significantly fewer large aggregates than expected.     

Explaining movement decisions of forest rodents in fragmented landscapes

Functional connectivity is a measure of the interaction of landscape structure and a species’ dispersal ability to determine the degree to which a landscape facilitates movement among patches. Dispersal through an inhospitable matrix requires that a species is willing to enter the matrix and can successfully colonize another habitat patch. Many connectivity indices have been developed which incorporate various attributes of the landscape, but little empirical evidence of the accuracy of these indices is available. We studied the ability of white-footed mice (Peromyscus leucopus), eastern chipmunks (Tamias striatus), and southern flying squirrels (Glaucomys volans) to move through agricultural fields in west-central Indiana. Radio-collared animals were translocated into corridors that either were connected to or unconnected from an associated forest patch to determine their willingness to enter the matrix. Animals also were translocated into fields to determine how motivation to find resources influences movement through the matrix. All species demonstrated a strong motivation to find the forest. Animals were capable of moving through the matrix successfully, however, we were unable to determine whether they would do so willingly. Initial bearing was an important predictor for successfully reaching a forest patch, which has implications for modeling dispersal. Additionally, abiotic variables, such as temperature and precipitation, had a strong effect on latency to move from the release site. Although weather often correlates with seasonal migration, our study emphasizes the importance of weather in influencing short-term decisions on timing of movement.     

Effect of cultivation and within-field differences in soil conditions on feral Helianthus annuus growth in ridge-tillage maize

Differences in weed population dynamics with respect to within-field heterogeneity are not well documented despite increasing interest in site-specific management of agro-ecosystems. The focus of this study was to determine if mechanical weed management (cultivation) and/or soil factors help to explain observed within-field distributions of feral common sunflower (Helianthus annuus L.). The ridges and furrows created by the ridge–tillage system adds additional microsites to existing spatial heterogeneity for soil characteristics such as soil organic carbon (SOC) concentration. Experimental areas were selected on the basis of naturally high or low SOC concentration. Cultivation resulted in 100% mortality of H. annuus seedlings growing in the middle of furrows. Cultivation of pre-emergence herbicide treated and no-herbicide ridges resulted in small but statistically significant ( = 0.05) reductions in seedling survival. No differences were detected in H. annuus canopy height, stem diameter, stem length, or vegetative biomass between high and low SOC environments. Neither total reproductive biomass (P = 0.49) nor the biomass of flowers near physiological maturity (an estimate of fecundity; P = 0.59) were affected by SOC environment. Late season H. annuus lodging was observed to reduce reproductive biomass. Juvenile plants that survived mechanical weed control efforts grew and produced reproductive biomass similarly across SOC environments. The lack of difference in vegetative and reproductive characteristics between high and low SOC environments suggests that SOC (or the edaphic conditions associated with greater or lesser SOC level) was not critical in contributing to the observed distribution of H. annuus from juvenile to flowering stages of growth within well-fertilized, irrigated agricultural habitats.     

Interactions of an introduced shrub and introduced earthworms in an Illinois urban woodland: Impact on leaf litter decomposition

This study examined an ‘invasional meltdown’, where the invasion of a Midwestern woodland by an exotic shrub (Rhamnus cathartica L.P. Mill) and the invasion by Eurasian earthworms facilitated one another. Using a litterbag approach, we examined mass loss of four substrates (R. cathartica, Acer saccharum, Quercus rubra, and Quercus alba) along a gradient of Eurasian earthworm density and biomass throughout a 40.5 ha oak woodland in Glencoe, Illinois. Earthworm densities and biomass were greatest in patches where R. cathartica prevailed, and populations were lowest in an upland forest subcommunity within the woodland. At each of three points along this earthworm gradient, we placed replicated litterbags constructed either to permit or to deny access to the litter by earthworms. The treatments were, therefore, plot treatments (low, medium and high earthworm density and biomass) and litterbag treatments (earthworm access and earthworm excluded). We found that earthworms promoted a very rapid loss of litter from R. cathartica bags. Within 3 months greater than 90% of this litter was lost from the litterbags. Earthworm impacts on other substrates followed the sequence A. saccharum>Q. alba=Q. rubra. Effects of both litterbag and plot treatments were found within 3 months for A. saccharum but Quercus species were affected only after a year. We propose that the impact of earthworms on litter breakdown creates conditions that promote and sustain invasion by R. cathartica. Previous work has demonstrated that R. cathartica may alter soil properties in a way that promotes and sustains invasion by earthworms. These findings have implications for the restoration management of these systems, since the legacy of R. cathartica on soil properties and earthworm populations may persist even after the plant has been physically removed.     

Root–soil adhesion as affected by crop species in a volcanic sandy soil of Mexico

Field observations have shown that root residues maintain root-adhering soil for several months after harvest. The aim of this work was to compare post-harvest effect of Amaranthus hypochondriacus (amaranth), Phaseolus vulgaris (common bean) and Zea mays (maize) roots on root-adhering soil, aggregation and organic carbon content. The experimental site was located on a volcanic sandy soil (Typic Ustifluvent) in the Valley of Mexico. In 1999 and 2000, maize had the highest root mass (92 and 94 g m⁻²) and the highest root-adhering soil (9051 and 5876 g m⁻²) when a root–soil monolith of 0.20 m × 0.20 m × 0.30 m was excavated after harvest. In contrast, bean roots (2 and 5 g m⁻²) had only 347 and 23 g m⁻² of adhering soil per monolith in each year. Amaranth had intermediate values between maize and bean. Dry soil aggregate classes (<0.25, 0.5, 1, 2, 5 and >5 mm) were similarly distributed among the three species. The sum of the three soil macro-aggregates classes >1 mm was 0.1 g g⁻¹ in both years. Neither water stability of the 2–5 mm aggregates (0.05–0.09 g g⁻¹) nor soil organic C (SOC) in three aggregate classes (<0.25, 1–2 and >5 mm; mean 14.6 mg g⁻¹) was affected by species (P < 0.05) in either year. Observations of thin sections (10× and 40×) revealed absence of macro-aggregates under maize. Soil compaction was attributed to high mass of maize roots in the sampled soil volume. Root systems sampled after harvest had the capacity to maintain a well structured soil mass, which was proportional to root mass. Root-adhering soil measured in the field could be used to select species promoting soil adhesion by roots.