Low Soil Nutrient Status Research Articles

Improving soil chemical properties and okra (Abelmoschus esculentus) yield through complementary applications of biochar, and organic and inorganic fertilizers

In spite of okra’s importance to food security in developing countries, its production is limited by low soil nutrient and health status. A screenhouse study examined soil chemical properties, okra growth and yield under complementary application of biochar, organic and inorganic fertilizer in an Ultisol. The treatments comprised 5 kg soil each amended with 5 t biochar ha−1, 5 t poultry manure compost (SPM) ha−1, urea at 56 kg N ha−1, urea (28 kg N ha−1) + 5 t biochar ha−1, 5 t ha−1 (2:1, PM: biochar) (SPMB), urea (28 kg N ha−1) + 5 t ha−1 (2:1, PM: biochar) (SPMBF2). The treatments, replicated thrice, were arranged in a completely randomized design. There were two sets of plants terminated at 50% flowering (dry matter yield (DMY) and nutrient uptake) and at 10 wk post-planting for growth and yield determination. Results revealed that soil chemical properties, okra growth and yield increased responsively to the organic amendments. The SPM, followed by SPMB and SPMBF, enhanced soil pH (0.18–0.19 unit), total N (79–100%), organic carbon (75–100%), available P (55–58%) and exchangeable cations over the control and others (p < 0.05). These treatments also increased the plant’s agronomic parameters: plant height (10–19%), number of leaves, leave area (21–31%), stem girth (33–51%), DMY (59–67%), and tissue Ca, Mg and K. The SPMBF2 – treated soil had the highest fruit yield (45 g pot−1). Conclusively, sole and complementary applications of biochar and compost improved soil chemical properties, and okra growth and yields.

Soil Physical and Chemical Properties as Affected by Bio-, Organic and Inorganic NPSB Fertilizers and Lime in Assosa Zone, Western Ethiopia

Background: Soil acidity, low soil nutrient status and low nitrogen fixing inoculants as a result of poor soil fertility management practices are the major constraints in soybean production in Assosa Zone, particularly in Assosa and Bambassi districts. Hence, this experiment was conducted to evaluate the effect of bio-, organic and inorganic NPSB fertilizers and lime on soil physico- chemical properties. Methods: During the period 2019-2020 factorial combinations of two bio-fertilizer inoculants [without bio-fertilizer (B1) and SB12 plus MAR1495 bio-fertilizer at their recommended rates of 500 g ha-1 (B2); two organic fertilizers without fresh cattle manure (M1) and fresh cattle manure at 10 t ha-1 (M2); two lime rates without lime (L1) and lime at 5 t ha-1 (L2); two inorganic NPSB fertilizers NPSB at 9.5-23-3.5-0.05 (F1) and NPSB at 19-46-7-0.1 (F2) at their recommended rates for soybean] were laid out in a randomized complete block design with three replications. Bulk density, pH, total nitrogen, available phosphorus, organic carbon and cation exchange capacity were collected and analyzed using SAS software version 9.1.3 and significant mean differences were separated using Duncan’s Multiple Range Test (DMRT). Result: Bulk density, pH, total nitrogen, available phosphorus, organic carbon and cation exchange capacity were highly affected (P less than 0.01) by the interactions of bio-, organic and inorganic fertilizers and lime. Finally, the interaction of SB12+MAR1495, 10t/ha fresh cattie manure, 5t/ha lime and NPSB at 19-46-7-0.1 improved the bulk density, pH, total nitrogen, available phosphorus, organic carbon and cation exchange capacity at both districts.

Effects of road and woodland type on the invasibility of woodlands invaded by Lantana camara in southern Africa

The susceptibility of a plant community to invasion is influenced by multiple factors including the characteristics of the invading species and the invaded environment but how road presence and woodland type affect invasibility in woodlands invaded by Lantana camara (hereafter, lantana) in southern Africa is not well known. We identified roads adjacent to miombo, Vachellia (hereafter, vachellia) and mixed woodlands. Parallel to each road, we laid two transects (200 × 30 m) and replicated them twice, one adjacent to the road and another in the interior. In each transect, we laid 5 plots (30 × 20 m), making a total of 60 plots (3 woodland types x 2 transects x 2 replicates x 5 plots). Our sampling plan yielded six woodland clusters: roadside vachellia, interior vachellia, roadside miombo, interior miombo, roadside mixed and interior mixed. Road presence increased invasibility in all clusters but was more pronounced in less diverse miombo and vachellia than in more diverse mixed woodland. Interior mixed woodland was 1.4 and 1.8 times more diverse than interior miombo and interior vachellia woodlands, respectively; whereas roadside mixed woodland was twice more diverse than roadside miombo and roadside vachellia. Invasibility in the interior mixed woodland was 19% and 24% lower than in the interior miombo and interior vachellia, respectively; whereas in the roadside mixed woodland, it was 19% and 21% lower than in roadside miombo and roadside vachellia, respectively. We found a negative diversity-invasibility relationship in all clusters. Vachellia woodlands had significantly higher soil C, N, P and K concentrations than miombo but were equally susceptible to lantana invasion. Collectively, our findings suggest that high diversity rather than low soil nutrient status might be more important in hindering plant invasion. We emphasize the importance of conserving native diversity in roadside habitats to reduce the success of invading plant species.

Assessing long-term nutrient and lime enrichment effects on a subtropical South African grassland

Nutrient enrichment influences grassland ecosystem structure, typically manifested by reduced species richness and increased productivity. Quantifying the long-term impacts of nutrient enrichment on grasslands contributes to understanding eutrophication effects on grassland, particularly for grasslands adapted to low soil nutrient status. Overextended time periods, nutrient enrichment may modify soil fertility. The Ukulinga Grassland Nutrient Experiment situated on a natural C4 grassland was set up in 1951 on the Ukulinga research farm, South Africa. Continuously applied treatments on plots measuring 2.7 × 9 m include combinations of nitrogen at 0, 7, 14 and 21 g m−2 per annum, phosphorus at 0 and 2.8 g m−2 per annum and lime at 0 and 225 g m−2 applied every five years. Nitrogen sources included ammonium sulphate (acidifying) and limestone ammonium nitrate (less acidifying). Grass species composition was influenced by both nitrogen forms. In contrast, forb species composition was more sensitive to LAN. We found evidence of nitrogen limitation on aboveground net-primary production. However, species richness (for grass and forbs) declined with increasing ammonium sulphate levels, owing to increased soil acidity and high aluminium concentrations. Aluminium toxicity can affect overall species composition by replacing Al-sensitive species with Al-tolerant species on site.

Comparative Effect of Poultry and Swine Manures on the Performance of White Guinea Yam (Dioscorea rotundata Poir) in an Ultisols Environment

White Guinea yam (Dioscorea rotundata Poir) is an important staple food in a number of African countries, including Nigeria and Ghana. However, the tuber yield in these areas is below its potential yield. The most probably constraint to low tuber yield is the low soil nutrient status. A field trial was conducted at the Experimental Farm of the Department of Crop Science, Faculty of Agriculture, University of Benin, Benin City, Nigeria, between April and November 2016 and 2017, to evaluate the effect of poultry and swine manures on the growth and tuber yield of white Guinea yam (D. rotundata) in an ultisols environment. The trial was laid out in a 2 × 3 split plot arrangement fitted into randomized complete block design with three replicates. The main plots were organic fertilizers (poultry and swine manures) and the sub-plots were represented by three rates (0, 300 and 450 kg N ha-1) of application. Data collection on growth characters were on vine length (cm), vine girth (cm), number of nodes, number of leaves and leaf area index. At harvest, tuber length (cm), tuber girth (cm), number of tubers per plant, tuber weight and tuber yield (t ha-1) were also evaluated. The results showed that growth and yield variables varied significantly (p &lt; 0.05) with organic fertilizer types and application rates. Poultry manure treated plants produced higher (p &lt; 0.05) number of nodes and leaves than swine manure treated plants. Application rates of 300 and 450 kg N ha-1 of poultry manure significantly increased vine length, vine girth, number of nodes and leaves as well as leaf area index. Tuber weight and yield were higher in plants treated with poultry manure with the application rates of 300 and 450 kg N ha-1 performed better than control in 2016, while the application rate of 300 kg N ha-1 had the highest tuber weight and yield in 2017. Application rates of 300 and 450 kg N ha-1 performed better than the control (0 kg N ha-1) in 2016. However, in 2017, application rate of 300 kg Nha-1 had the highest tuber weight and yield. The present study suggests that poultry manure at application rate of 300 kg N ha-1 could be adopted by white Guinea yam farmers for high productivity of the crop under intense and continuous cropping in humid ultisols environment.

Influence of Cow Dung Application on the Yield and Yield Components of Two Groundnut (Arachis hypogaea L.) Varieties in Sokoto, Semi-arid Zone of Nigeria

Abstract Groundnut (Arachis hypogaea L.) is a valuable crop for millions of small scale farmers around the tropics. Despite its importance, the production of the crop in semi-arid tropics is constrained by shorter rain duration and low soil nutrient status. The present studies was carried out during the 2007 and 2008 rainy seasons to study the influence of cow dung application on the yield and yield components of two groundnut varieties in the region. Treatments consisted of factorial combination of two groundnut varieties (Ex-Dakar and RMP-12) and three cow dung levels (0, 15 and 30 t ha−1) making a total of six treatment combinations. The treatments were laid out in a randomized complete block design replicated three times. The findings of this research revealed that Ex-Dakar variety performed better than RMP-12 in all the parameters measured. Application of cow dung increased the number of pods plant−1, pod yield and grain yield of the crop and was optimum at the rate of 15 t ha−1. Thus, Ex-Dakar variety could be suggested for increased groundnut production in the study area. Application of cow dung at the rate of 15 t ha−1 was optimum for groundnut production in the study area.

Enhanced biological N2 fixation and yield of faba bean (Vicia faba L.) in an acid soil following biochar addition: dissection of causal mechanisms

Acid soils constrain legume growth and biochars have been shown to address these constraints and enhance biological N2 fixation in glasshouse studies. A dissection of causal mechanisms from multiple crop field studies is lacking. In a sub-tropical field study, faba bean (Vicia faba L.) was cultivated in rotation with corn (Zea mays) following amendment of two contrasting biochars, compost and lime in a rhodic ferralsol. Key soil parameters and plant nutrient uptake were investigated alongside stable 15 N isotope methodologies to elucidate the causal mechanisms for enhanced biological N2 fixation and crop productivity. Biological N2 fixation was associated with plant Mo uptake, which was driven by reductions in soil acidity following lime and papermill (PM) biochar amendment. In contrast, crop yield was associated with plant P and B uptake, and amelioration of soil pH constraints. These were most effectively ameliorated by PM biochar as it addressed both pH constraints and low soil nutrient status. While liming resulted in the highest biological N2 fixation, biochars provided greater benefits to faba bean yield by addressing P nutrition and ameliorating Al toxicity.

Spatial gradient in nitrogen deposition affects plant species frequency in acidic grasslands.

Anthropogenic eutrophication impacts ecosystems worldwide. Here, we use a vegetation dataset from semi-natural grasslands on acidic soils sampled along a gradient in north-western Europe to examine the response of species frequency to nitrogen (N) deposition, controlling for the effects of other environmental variables. A second dataset of acidic grasslands from Germany and the Netherlands containing plots from different time periods was analysed to examine whether the results of the spatial gradient approach coincided with temporal changes in the abundance of species. Out of 44 studied species, 16 were affected by N deposition, 12 of them negatively. Soil pH and phosphorus (P) influenced 24 and 14 species, respectively, predominantly positively. Fewer species were related to the soil contents of NO3(-) or NH4(+), with no significant differences between the number of positive and negative effects. Whereas the temporal change of species was unrelated to their responses to pH, species responding negatively to N deposition, soil P and NO3(-) showed a significant decline over time in both countries. Species that were negatively affected by high N deposition and/or high soil P also showed a negative temporal trend and could be characterised by short stature and slow growth. The results confirm the negative role of N deposition for many plant species in semi-natural acidic grasslands. The negative temporal trends of species sensitive to high N deposition and soil P values clearly show a need for maintaining low soil nutrient status and for restoring the formerly infertile conditions in nutrient-enriched grasslands.

Factors Contributing to Citrus Mandarin Yield Decline in Dewathang Geog under Samdrup Jongkhar Dzongkhag

A study was conducted to determine the factors responsible for yield decline observed in Citrus reticulata Blanco (locally known as citrus mandarin) in Dewathang Geog under Samdrup Jongkhar Dzongkhag. Seventy two farmers were randomly selected and interviewed using a structured questionnaire survey. Twenty eight composite soil samples were collected from 14 different citrus mandarin orchards to evaluate soil nutrients. The Chinese fruit fly and the green stink bug were the major causes of fruit drop, causing significant yield loss in 97% of the orchards. Citrus greening disease has completely destroyed three orchards and was observed in 35% of the remaining orchards, posing a serious threat. About 82% of the citrus seedlings were propagated from backyard nurseries, which may have led to the supply of poor planting materials. Majority (86%) of the growers only applied farm yard manure, which may have contributed to the low soil nutrient contents recorded in the orchards. These growers do not practice proper management practices such as pruning, basin making, and spacing. Considering the poor management practices and low soil nutrient status, there is a need to train the citrus growers in soil fertility and good orchard management.

Rekomendasi Pemupukan Fosfor dan Potasium berdasarkan Analisis Hara Tanah pada Tanaman Sayuran

&lt;p&gt;ABSTRACT&lt;/p&gt;&lt;p&gt;&lt;br /&gt;Vegetables are important agricultural commodities. Productivity of vegetables in Indonesia is still low. One effort that can be done is an application of specific fertilizer recommendations. Fertilizer recommendation based on soil analysis is still rarely for vegetable crops and need further development. The purpose of this paper is to describe some fertilizer recommendations based on soil analysis for vegetable crops. Three stages to consider in the assessment of the research such as soil incubation, correlation test, calibration test and fertilizer &lt;br /&gt;recommendation statue. Application all this stages of soil method recommendation in Indonesia is still not widely applied. Two researches which were completed until the entire stage has been done for yard long beans and &lt;br /&gt;tomatoes. Recommendations for tomatoes on Inceptisols soil type with very low nutrient status of soil K was 180 kg K2O ha- 1, a low K soil nutrient status was 131.4 kg K2O ha-1, soil K nutrient status was 82.2 kg K 2O ha-1. Yard &lt;br /&gt;long bean that planted on Ultisol soil type with low soil P nutrient status was recommended by an application of 185.8 kg P2O5 ha-1, medium soil P nutrient status was added 174.9 kg P2O5 ha-1. Development of fertilizer recommendation based on soil testing to support agricultural development in Indonesia still has some problems and need some strategies for further research, application and dissemination in the future.&lt;/p&gt;&lt;p&gt;Key words: vegetables, soil test, fertilizer recommendation&lt;/p&gt;

Sexual differences in defensive and protective mechanisms ofPopulus cathayanaexposed to highUV‐B radiation and low soil nutrient status

Previous studies have indicated thatPopulus cathayanaRehder females are more sensitive and less tolerant to stressful environments than males, but it is still unknown whether there are sexual differences in defensive and protective traits under highUV‐B (HUVB) radiation and low soil nutrient status. In this study,P. cathayanawas employed as a model species to investigate sex‐related physiological and biochemical responses toUV‐B radiation under different soil nutrient conditions. Cuttings were exposed to twoUV‐B radiation regimes (ambientUV‐B radiation and decreasedUV‐B radiation) under two soil nutrient status (topsoil and deep soil) for 100 days over one growing season. Both HUVB radiation and low soil nutrient status induced greater decreases in plant growth, dry mass accumulation, gas exchange and leaf nitrogen use efficiency in females than in males, and greater increases in lipid peroxide and antioxidant enzyme activities, and secondary defense capacities in males than in females. Moreover, sexually different responses happened also in organelle ultrastructure. Our results showed that: (1) females suffered greater negative effects and exhibited lower defense capacities than did males under HUVB radiation, low soil nutrient status and their combination; (2) low soil nutrient status reduced plant's sensitivity to HUVB radiation by increasing allocation to defense and decreasing allocation to growth compared with high soil nutrient status. These results provide evidence that sexually different tradeoffs happen between growth and defense inP. cathayanaunder HUVB radiation and low soil nutrient status.

Effect of Planting Method on the Growth of Alnus glutinosa and Quercus petraea in Compacted Opencast Coal‐Mine Spoils, South Wales

AbstractLand degradation on officially ‘reclaimed’ opencast coal‐mine sites is a widespread problem in South Wales. This project explores methods suitable for use by community volunteers seeking to effect local environmental improvement by restoring geoecological self‐sustainability on lands that are commonly affected by extreme auto‐compaction and low soil nutrient status. This paper describes a formally established 7‐year experiment designed to assess the effect of three alternative tree planting strategies used in the forestation of such lands. These are notch planting (e.g. forestry), pit planting (e.g. parks and gardens) and trench planting (e.g. orchard terraces). The study explores tree survival and growth for two species commonly used in land reclamation contexts: alder [Alnus glutinosa (L.) Gaertn] and Welsh or Durmast Oak [Quercus petraea (Matt.) Liebl]. t‐testing of the results shows that survival and growth rates for both species tend to be significantly better for trench planting than for pit planting than for notch planting. Differences in mortality between pit‐planted and trench‐planted trees become less significant during the experiment while differences in growth increase with time. The conclusion is that providing a loosened, lower density, rooting substrate significantly improves both the growth and the survival rates of trees planted in compacted Welsh surface coal‐mine spoils and that trench planting is more effective than parks and gardens‐style pit planting, which is more effective than forestry‐style notch planting. Copyright © 2013 John Wiley &amp; Sons, Ltd.

A comparison of soil moisture relations between standing and clearfelled plots with burnt and unburnt harvest residue treatments of a clonal &lt;i&gt;eucalypt&lt;/i&gt; plantation on the Zululand Coastal Plain, South Africa

The effects of clearfelling and subsequent residue retention or burning on water and nutrient balances needs to be understood and quantified on forest sites that are sensitive to loss, so that the long-term sustainable productivity of such sites can be maintained and promoted. An experimental site was established in a clonal eucalypt compartment on the Zululand Coastal Plain, to compare changes in water fluxes through the mature undisturbed eucalypt stand with those after felling and re-planting, under 2 conditions: burning, and retention of the harvesting residues. The study was located in an area of high rainfall and high stand productivity, with sandy soils and low soil carbon and nutrient status; chosen so that the effects of intensive demands on water and nutrient fluxes on a potentially sensitive site could be investigated. This paper presents only the hydrological component of the study. Data collection included weekly determination of rainfall, throughfall, stemflow and soil moisture fluxes from the surface to a depth of 1 m. Drainage rates through the profile were established using time domain reflectometry probes while water drainage volumes were assessed using shallow plate lysimeters. Despite slow growth in the unfelled crop during the monitoring period (attributed to a pest infestation), soil moisture depletion remained rapid and drainage below 1 m remained low. Soil moisture was recharged within a few months after clearfelling, but became rapidly depleted as the canopy of new crop developed and approached canopy closure. A decreased wetting-front velocity and a marginally higher field capacity were proposed as evidence of pore clogging that appeared to occur during the inter-rotation period. The soil profile under the unburnt residue maintained a marginally higher soil moisture status and lower drainage than the soil profile under the burnt residue. Although soil moisture and drainage in the burnt and unburnt residue treatments became similar to the unfelled crop from canopy closure onwards, rainfall additions to soil moisture were depleted faster under the new crop during the first few months after canopy closure. Small differences in soil moisture status between the burnt and unburnt residue treatments presented here may not be sufficient to influence residue manage ment decisions. The length of the inter-rotation period and practice of residue burning may, however, need consideration where soil carbon and nutrient loss or displacement may negatively affect the sustainability of the site.

Multi‐stemmed trees in montane rain forests: their frequency and demography in relation to elevation, soil nutrients and disturbance

Summary Multi‐stemmed trees are a common component of tropical and temperate montane rain forests, where they co‐occur with single‐stemmed trees. We hypothesized that multi‐stemmed architecture should predominate in less productive sites (such as at high elevation or with low soil nutrient availability) and that, because it frequently results from sprouting, it should prevail in sites that are frequently disturbed. We also hypothesized that where multi‐stemmed architecture predominates, there should be lower rates of mortality and recruitment of individuals. We tested these hypotheses using data from permanent plots in tropical montane rain forests in Jamaica (14 years) and in temperate montane rain forests in New Zealand (19 years). The frequency of multi‐stemmed trees varied across plots in both forests (4–34% in Jamaica; 0–21% in New Zealand) along gradients of elevation and soil nutrients. Analyses examined the direct and indirect effects of the various environmental variables that are likely to drive site productivity and disturbance. We showed contrasting predictors of the frequency of multi‐stemmed trees in the tropical and temperate forests. Multi‐stemmed trees in Jamaican forests were most frequent on sites of low soil nutrient status (especially available soil phosphorus, P) whereas in New Zealand they were most frequent at high elevation sites, especially towards tree line. In both forests there was no relationship between multi‐stemmed tree frequency and slope. Turnover (the mean of mortality and recruitment rates) of multi‐stemmed trees in Jamaican forests was lowest on sites of low soil nutrient status (especially available P) but was unrelated to environmental predictors in New Zealand. In both forests, turnover rates of multi‐stemmed trees overall were 60% lower than those for co‐occurring single‐stemmed trees, offering support for the hypothesis that multi‐stemmed architecture favours persistence. Synthesis. Our study demonstrates that multi‐stemmed trees can predominate in low productivity sites in montane rain forests. Their low turnover compared with co‐occurring single‐stemmed trees confirms the importance of evaluating the persistence niche as a mechanism promoting woody species coexistence in forests.

Soil type, microsite, and herbivory influence growth and survival of Schinus molle (Peruvian pepper tree) invading semi-arid African savanna

Naturalization of Schinus molle (Anacardiaceae) has been observed in semi arid savanna of the Northern Cape Province of South Africa. However, with high dispersal ability, the species is expected to achieve greater densities and invade more widely. The study involved a field manipulation experiment over 14 months using a factorial block design to examine transplanted seedlings in different savanna environments. The experiments examine the effects of soil type (sandy and clay), microsite, and herbivores on seedling performance (establishment, growth and survival). Seedlings were grown in a greenhouse and individually transplanted into four treatment groups: in open grassland, under tree canopies, and with and without cages to exclude large herbivores (cattle and game). The same experiment was repeated in two different soil types: coarse sand and fine-textured clay soil. Results suggest that protection provided by canopies of large indigenous Acacia trees facilitates S. molle invasion into semi-arid savanna. In the field, S. molle seedlings performed considerably better beneath canopies of indigenous Acacia trees than in open areas regardless of soil type. Whether exposed or protected from large herbivores, no seedlings planted in open grassland survived the first winter. Although, seedlings grew better and had higher survival rates beneath tree canopies than in the open sites, exposure to large herbivores significantly decreased heights and canopy areas of seedlings compared with those protected from large herbivores. The effect was greater on clay soil than on sandy soil. The results suggest that low temperature (frost), and possibly inter-specific competition with grasses, may limit S. molle seedling establishment, survival and growth away from tree canopies in semi arid savannas. Low soil nutrient status and browsing may also delay growth and development of this species. The invasive potential of S. molle is thus greatest on fertile soils where sub-canopy microsites are present and browsing mammals are absent.

Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China

Responses of soil respiration (CO2 emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting) and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October 2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments (Control, no N addition; Low-N, 5 g N m−2 year−1; Medium-N, 10 g N m−2 year−1), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited significant positive exponential relationship with soil temperature and significant positive linear relationship with soil moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO2-C m−2 h−1) and 10% lower in medium-N plots (80 ± 4 mg CO2-C m−2 h−1) than in the control plots (89 ± 5 mg CO2-C m−2 h−1), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest, annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO2-C m−2 h−1) and 8% lower in medium-N plots (61 ± 3 mg CO2-C m−2 h−1) than in the control plots (66 ± 4 mg CO2-C m−2 h−1), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in the reforested tropical forests may vary depending on the status of human disturbance.

Effects of shifting cultivation on soil ecosystems in Sarawak, Malaysia

Experimental shifting cultivation was conducted at two sites, Balai Ringin (B. Ringin) and Sabal in Sarawak, Malaysia. The soils at the B. Ringin site were clayey with strongly acidic nature while those at the Sabal site were sandy with a very low nutrient status. The plots (10 x 10 m2) were burned with vegetation biomass (fuel) amounting to 0 (control), 100,200, and 300 Mg ha-1 for the cultivation of upland rice. At both sites, the fire severity was found to range from moderate to high for the treatments with 200 and 300 Mg ha-1 fuel, and from low to moderate for the treatment with 100 Mg ha-1 fuel. At the B. Ringin site, the content of total C (T-C) of the soils decreased during rice cultivation which could be ascribed to the rapid decomposition of soil organic matter (SOM) and the loss of SOM due to soil erosion. One year after burning, the T-C content returned to the initial level before burning, suggesting that SOM was replenished by the addition of fresh organic matter derived from rice straw and other dead plants. The effects of burning on soil microbial properties which tended to be more pronounced with the increase of the amount of fuel, persisted until 1 year after burning. Burning resulted in a yield of 850 kg ha-) of rice on the average, irrespective of the amount of fuel used. On the other hand, at the Sabal site, the T-C content decreased dramatically during rice cultivation. It was assumed that a portion of 80M was washed away by rainfall because of the sandy texture of soil and coarse fragments of 80M. Rice yield was very low due to the low nutrient status of soil and pest infestation

Megaherbivores influence trophic guilds structure in African ungulate communities.

We used a data set of ungulate censuses from 31 natural ecosystems from East and Southern Africa to test two hypotheses: (1) megaherbivores should dominate ungulate communities in ecosystems with high rainfall and low soil nutrient status because of their ability to survive on poor quality food resources, and (2) the abundance of megaherbivores affects the abundance of the mesoherbivores, distinguishing the different feeding guilds: mesograzers, mesobrowsers and mesomixed feeders. Two axes of a multivariate analysis (77% of the variance) discriminated the sites well, the first separating sites dominated by megaherbivores from those dominated by mesoherbivores, and the second representing a gradient between mesograzers and mesobrowsers. Our analysis shows (1) that megaherbivores can be considered to be a separate trophic guild and (2) that mesograzers and mesobrowsers respond differently to variation in their trophic environments. The metabolic biomass density of megaherbivores increased with annual rainfall, but was not related to soil nutrient status, and as predicted, megaherbivores comprised a larger proportion of the biomass of ungulate communities in ecosystems with high rainfall and low nutrient soils. The metabolic biomass density of mesoherbivores increased with rainfall and soil nutrient status. Within the mesoherbivores, the metabolic biomass density of mesograzers showed the same trend, and seemed unaffected by megaherbivores. Conversely, mesobrowsers and mesomixed feeders appeared to be unaffected by rainfall or soil nutrient status, but mesomixed feeders declined when megaherbivores were abundant. This suggests that megaherbivores may compete with the mesomixed-feeder species for food or they may alter the vegetation communities unfavourably. A similar analysis using elephants alone instead of megaherbivores as a group showed that both mesobrowsers and mesomixed feeders were affected significantly by elephant, which is consistent with the fact that most of the effect of megaherbivores on browse resources or woodland habitat is due to elephants. This study shows that the different trophic guilds within African ungulate communities react differently to environmental factors (rain and soil), and that megaherbivores, and particularly elephants, appear to compete with mesomixed feeders and mesobrowsers. These results are relevant for the understanding of the functioning of African ungulate communities and call for further testing with longitudinal data.

Multitrophic interactions between a Rhizoctonia sp. and mycorrhizal fungi affect Scots pine seedling performance in nursery soil.

• Interspecific variation in ectomycorrhizal fungal (ECMF) control of a root pathogenic uninucleate Rhizoctonia sp. (UnR) was identified in vitro and in planta. • Fungal-fungal and host-fungal interactions were assessed in direct confrontation and cell-free assays, the rhizosphere of Scots pine (Pinus sylvestris) seedling radicles and seedling mycorrhizospheres developed in N-limited nursery soil. • Isolates of Suillus bovinus inhibited UnR growth although no agar-diffusable fungicidal activity was detected. Presence of nonsymbiotic ECMF mycelia did not prevent UnR colonization of radicle apices and the onset of damping-off symptoms. Seedlings hosting S. bovinus mycorrhizas and extensive extramatrical mycelium showed vigorous and healthy shoot growth after a 168-d UnR challenge. Root biomass of Wilcoxina mikolae and Paxillus involutus colonized seedlings were negatively affected by both low soil nutrient status and UnR exposure. However, UnR was isolated from long and mycorrhizal short roots in all ECMF coinoculation treatments. • The differential responses highlight multitrophic host-fungal interaction dynamics that require further characterization in the development of 'efficient' UnR biological control solutions utilizing mycorrhizal fungal inoculants.

N and P Fertilization on Rangeland Production in Midwest Argentina

Low soil nutrient status may be the major limiting factor to forage production in rangelands of the Mendoza plains 4 years out of 10. We studied vegetation responses to annual applications of N and P on such rangelands. Fertilizer application rates were 0 or 25 N and 0 or 11 P (kg ha-1) in a factorial arrangement. Dry matter production of grasses and palatable shrubs and crude protein (CP) content of grasses were determined annually from 1996 to 1998. Experimental plots received rains of 189, 278, and 346 mm during the 3 study years compared to mean growing season rainfall of 258 mm. Forage production was increased by N+P fertilization only in 1998 (1,390 vs 980 kg ha-1, P < 0.05), producing 16.5 kg forage kg-1 N applied. Crude protein concentration was increased by N fertilization in 1997 (6.3 vs 5.3%, P < 0.05) and N+P application increased in 1998 (6.8 vs 5.7%, P < 0.05). Nitrogen and P application increased seasonal rain-use efficiency when the rainfall exceeded 300 mm. In 1998, the increase of grass production per kg N applied with and without P was 18.4 and 12.4 kg, respectively. The break-even point between rain and nutrients as the main primary production determinant on sandy soils in the central Mendoza plains is around 400 mm year-1 instead of 300 mm in other arid lands of the world. The value of meat increment derived from the N fertilization, with and without P application (US$ 0.07 ha-1 year-1 kg-1 N) was lower than the fertilizer cost (US$ 0.87 kg-1 N). A 5-fold increase in forage yields would be required to offset the cost of fertilizer. Fertilizer application did not increase forage production enough to be profitable for cattle production at present fertilizer and meat prices. DOI:10.2458/azu_jrm_v53i4_guevara