Field inoculation effectiveness of native and exotic arbuscular mycorrhizal fungi in a Mediterranean agricultural soil

In sustainable agriculture, arbuscular mycorrhizal (AM) fungal inoculation in agronomical management might be very important, especially when the efficiency of native inocula is poor. Here, we assessed the effect of native and exotic selected AM fungal inocula on plant growth and nutrient uptake in a low input Trifolium alexandrinum-Zea mays crop rotation. We evaluated the effects of four exotic AM fungal isolates on T. alexandrinum physiological traits in greenhouse. Then, the field performances of T. alexandrinum inoculated with the exotic AMF, both single and mixed, were compared to those obtained with a native inoculum, using a multivariate analysis approach. Finally, we tested the residual effect of AM fungal field inoculation on maize as following crop. Multivariate analysis showed that the field AM fungal inoculation increased T. alexandrinum and Z. mays productivity and quality and that the native inoculum was as effective as, or more effective than, exotic AM fungal isolates. Moreover, the beneficial effects of AMF were persistent until the second year after inoculation. The use of native AMF, produced on farm with mycotrophic plants species, may represent a convenient alternative to commercial AM fungal inocula, and may offer economically and ecologically important advantages in sustainable or organic cropping systems.     

Effects of different management practices on arbuscular mycorrhizal fungal diversity in maize fields by a molecular approach

As obligate mutualistic symbionts, arbuscular mycorrhizal fungi (AMF) colonize the roots of many agricultural crops, and it is often claimed that agricultural practices are detrimental to AMF. As a result, agroecosystems impoverished in AMF may not get the fully expected range of benefits from these fungi. Using molecular markers on DNA extracted directly from soil and roots, we studied the effects of different management practices (tillage and N fertilization) on the AMF communities colonizing an experimental maize field in Central Italy. Our molecular analysis based on three different nuclear rRNA regions (18S, 28S and ITS) allowed us to assess AMF biodiversity. Glomeraceae members were the main colonizer, and they co-occurred with Gigasporaceae and Paraglomus regardless of the management practices applied. Diversisporaceae and Entrophosporaceae members were instead detected in the N-fertilized soils and in the untreated soil, respectively. The results obtained indicated that the general AMF assemblages structure and composition in the maize field plots appear to be primarily influenced by N fertilization and, to a lesser extent, by tillage. This study also validates the usefulness of multiple molecular markers to consolidate and refine the assessment of the environmental AMF diversity.     

Differential growth response of Curculigo orchioides to native arbuscular mycorrhizal fungal (AMF) communities varying in number and fungal components

The study reports diversity of arbuscular mycorrhizal fungal (AMF) species in the rhizosphere of an endangered anticancerous herb – Curculigo orchioides Gaertn. in its natural habitat. A total of 18 species of AMF, belonging to three genera (Acaulospora, Glomus and Gigaspora) were recorded, with Glomus microcarpum being the most abundant species type. The AMF species composition across the study sites appeared to be influenced by soil pH rather than soil P and vegetation. Acaulospora laevis spores were restricted to sites where the soil pH was acidic. The effectiveness of these native AMF species on growth performance of C. orchioides plants was compared under experimental conditions. In general, the mycorrhizal plants were superior in most of the evaluated parameters, but the extent to which the growth of mycorrhizal plants was influenced varied with the inocula used. The plants inoculated with mixed consortia containing maximum AMF species richness exhibited improved growth in comparison to consortia containing lower AMF diversity and monospecies cultures. The variable plant responses observed with any two consortia having same species richness in the present study could be due to variable component AMF species and their relative abundance. These results emphasize the need to protect the below-ground diversity of AMF and recommend their usage for restoration practices.     

Influence of arbuscular mycorrhizal fungi on storage metabolites,mineral nutrition,and nitrogen-assimilating enzymes in potato (Solanum tuberosum L.) plant

Mycorrhizae are fungal symbionts forming mutualistic relationship with plant roots. In this study, surface-sterilized potato tubers were sown in earthen pots filled with sterile soil. Half of the pots were inoculated with sterilized arbuscular mycorrhizal fungi (AMF) spores and sterilized inoculated maize root fragments, and another half without any AMF inoculation representing control. Inoculation was done twice 3 days before sowing the tubers and on the onset of seedling emergence. Plants, along with their controls, were sampled at 20-day intervals upto 80 days after tuber initiation. The tubers of potato are shown to present a higher level of metabolites and mineral nutrition in the mycorrhizal inoculated compared to the non-inoculated. The results showed that AMF has a potential in enhancing potato production by increasing the storage metabolites, mineral nutrition in tubers and nitrogen assimilating enzymes in plant.     

Influence of organic manures on arbuscular mycorrhizal fungi associated with Vigna unguiculata (L.) Walp. in relation to tissue nutrients and soluble carbohydrate in roots under field conditions

 The interaction of plant nutrients, root-soluble carbohydrate availability and arbuscular mycorrhizal (AM) fungi was examined in field grown cowpea [Vigna unguiculata (L.) Walp.]. Plant nutrients were altered through application of farmyard (cow dung, sheep manure) and green (sunnhemp, pongamia) manures. Organic amendments increased plant growth, AM fungal colonization, soluble carbohydrate concentration in roots, and spore numbers. Percent total colonization, root length with vesicles and spore numbers in soil were negatively correlated with the concentration of soluble carbohydrates within roots, which in turn were related to tissue nutrient levels. However, a positive correlation existed between soluble carbohydrate concentrations within root and root length with arbuscules. But the mycorrhizal parameters were related more to plant nutrient level and their ratios, indicating that tissue nutrients have another level of control in addition to their effect on soluble carbohydrate concentration in roots. Increased AM colonization due to organic amendment significantly reduced nutrient imbalances. The strong relationship between colonization and root-soluble carbohydrate concentration levels validates the basic assumption that mycorrhizal fungi act as a 'strong sink' for photosynthates. This study indicates that the host influences AM colonization by regulating the formation of AM fungal structures and spore formation via availability of root carbohydrates. Received: 15 January 1999     

Effects of mineral and organic fertilizers on the microfauna in a high-altitude reafforestation trial

Summary Protozoa (testate amoebae, ciliates), small metazoa (rotifers, nematodes), and soil enzymes (catalase, cellulase) were investigated in a reafforested fertilized site at the alpine timberline. Side-dressings of mineral and organic fertilizers were applied alone (90 g NPK; 90, 180, 300, and 450 g dried bacterial biomass per spruce seedling) and in combination with magnesite (90 g NPK+300 g Mg; 90, 180, and 300 g bacterial biomass + 300 g Mg each; 30 g dried fungal biomass + 270 g Mg). One-third of each quantity was applied in 1986, and two-thirds in 1988. None of the treatments caused a significant decrease in the biological parameters investigated in comparison with untreated controls. The soil life was more or less stimulated depending on the amount of organic material contained in the fertilizers and the quantity; 180–270 g organic material per seedling proved to be the most effective. Dried bacterial biomass increased the pH by about 0.5 units, catalase activity by about 70%, and the number of ciliates and nematodes by 150–400%. The ciliate biomass and the number of ciliate species were likewise increased. The organomineral fertilizers increased pH by up to two units and also stimulated the soil life, but where the organic content was less than 180 g per seedling, efficiency decreased markedly. The least biological activity was observed in the control soil and in soil fertilized with NPK. Testaceans, rotifers, and cellulolytic activities were only slightly (insignificantly) affected by the treatments. A pooled evaluation of the data (organic versus organomineral versus mineral treatments) and community analyses showed that the organic fertilizer caused a more pronounced increase in the soil life and greater changes in community structure than the mineral combinations. Two years after application of the fertilizers, the differences between the treatments and the unfertilized controls had diminished.     

Bacterial and fungal growth in soil heated at different temperatures to simulate a range of fire intensities

The intensity of a fire is an important factor determining the recovery of soil microorganisms after a forest fire, since it can alter the quality and quantity of carbon sources. Recovery of the microbial community was studied in a Mediterranean pine forest soil subjected to different temperatures to simulate the short-term effects of fire intensity on bacterial and fungal growth, estimated using leucine incorporation for bacteria and acetate incorporation into ergosterol for fungi. Soil samples were heated for 15 min at 50, 80, 120, 200, 300, 400 and 500 °C. After inoculation with fresh soil, and adding water to achieve 60% WHC, the soils were incubated at 20 °C for 21 days. Bacterial growth was initially inhibited in the samples heated above 50 °C (totally inhibited ≥ 200 °C), but recovered within days to levels much higher than the control, except for the samples heated at 500 °C, where growth remained low throughout the incubation period due to the destruction of most of the organic matter. After the first week of incubation, the bacterial response decreased to values close to, but still above, that of the control. Samples heated at 200 °C showed the highest cumulative bacterial growth. Fungal growth was initially lower than in the control in all the heated samples (totally inhibited ≥ 200 °C). Fungal growth recovered slowly during incubation in soils heated at ≤ 300 °C, but the cumulative growth in heated soils did not exceed that in the control. No fungal growth was observed in samples heated at the two highest temperatures. Soil respiration was initially totally inhibited in soil heated at ≥ 200 °C, but recovered rapidly in all soils; the highest respiration being observed already 1 day after inoculation. This is the first time both fungal and bacterial growth has been directly estimated in heated soils. High soil pH favouring bacteria can explain these results, but the differences in fungal and bacterial responses suggest a competitive interaction between these groups.     

Effects of different phosphatase-producing fungi on growth and nutrition of mung beans [Vigna radiata (L.) Wilczek] in an arid soil

Summary The efficiency of different phosphatase-producing fungi in arid soils was studied in pot experiments in order to evaluate the effect of these organisms on the enhancement of rhizosphere enzyme activities, the uptake of different nutrients, dry matter production, and grain yields of the mung bean. A significant increase in phosphatase (acid and alkaline), nitrogenase, and dehydrogenase activities was observed upon inoculation. Inoculation with phosphatase-producing fungi significantly increased dry matter production and grain yields compared with uninoculated controls. In general, there was a significant improvement in the uptake of N, P, K, Ca, Mg, Fe, and Zn while no effect on Na, Mn, and Cu was recorded. A significant positive correlation was observed between the activities of various enzymes in the rhizosphere on the one hand and N and P uptake, dry matter production, and grain yields of the mung bean on the other. Aspergillus rugulosus was the best of the phosphatase-producing fungi that were tested, followed by A. terreus. The experiment confirmed that phosphatase-producing fungi have a significant effect on growth and nutrient uptake in the mung bean and indicated that these fungi are particularly important in arid soils.     

Interactions between the arbuscular mycorrhizal fungus Glomus intraradices and the plant growth promoting rhizobacteria Paenibacillus polymyxa and P. macerans in the mycorrhizosphere of Cucumis sativus

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and bacteria from the genus Paenibacillus (P. macerans and P. polymyxa) were examined in a greenhouse pot experiment with Cucumis sativus with and without organic matter amendment (wheat bran). P. polymyxa markedly suppressed AM fungus root colonization irrespective of wheat bran amendment, whereas P. macerans only suppressed AM fungus root colonization in combination with wheat bran amendment. Dual inoculation with P. macerans and G. intraradices in combination with wheat bran amendment also caused severe plant growth suppression. Inoculation with G. intraradices was associated with increased levels of dehydrogenase activity and available P in the growth substrate suggesting that mycorrhiza formation accelerated the decomposition of organic matter resulting in mobilization of phosphorus. Inoculation with both Paenibacillus species increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the AM fungus biomarker 16:1ω5, which was reduced, though not significantly. Similarly, inoculation with G. intraradices increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the Gram-positive bacteria biomarker 15:0 anteiso, which was overall decreased by G. intraradices inoculation. In combination with wheat bran amendment G. intraradices inoculation caused a 39% reduction in the amount of 15:0 anteiso in the treatment with P. polymyxa, suggesting that G. intraradices suppressed P. polymyxa in this treatment. In conclusion, plant growth promoting species of Paenibacillus may have suppressive effects of AM fungi and plant growth, especially in combination with organic matter amendment. The use of an inert plant growth media in the present study allowed us to study rhizosphere microbial interactions in a relative simple substrate with limited interference from other soil biota. However, the results obtained in the present work mainly show potential interactions and should not be directly extrapolated to a soil situation.     

Effects of farmyard manure and inorganic fertilizer on the dynamics of soil microbial biomass in a tropical dryland agroecosystem

Changes in the soil microbial biomass following applications of farmyard manure and inorganic fertilizer, alone and in combination, were studied for two annual cycles in a rice-lentil crop sequence grown under rainfed tropical dryland conditions. During the two annual cycles the microbial biomass C range (g g^-1) was 146–241 (x = 204), 191–301 (245), 244–382 (305), and 294–440 (365) in control, fertilizer, manure and manure+fertilizer plots, respectively. The corresponding ranges for microbial biomass N (g g^-1) were 16.5–21.0 (19.5), 20.4–38.2 (26.0), 23.0–34.6 (27.0) and 26.2–42.4 (33.3), and for microbial biomass P (g g^-1) 4.4–8.2 (7.0) 6.0–11.2 (9.6), 11.2–22.0 (17.0), and 10.0–25.4 (18.3). The maximum increase in the microbial biomass, due to these inputs was observed under the manure+fertilizer treatment followed, in decreasing order, by manure alone and fertilizer alone. Within individual crop periods the levels of microbial biomass decreased sharply from the seedling to the flowering stage and then increased slightly with crop maturity. The maximum levels of microbial biomass C and P were observed during the summer fallow. The maximum accumulation of microbial biomass N occurred in the early rainy season, immediately after the soil amendments. Microbial biomass C, N, and P were positively related to each other throughout the annual cycle.     

Effects of long-term organic and mineral fertilizers on bulk density and penetration resistance in semi-arid Mediterranean soil conditions

Soil aggregation is of great importance in agriculture due to its positive effect on soil physical properties, plant growth and the environment. A long-term (1996-2008) field experiment was performed to investigate the role of mycorrhizal inoculation and organic fertilizers on some of soil properties of Mediterranean soils (Typic Xerofluvent, Menzilat clay-loam soil). We applied a rotation with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) as a second crop during the periods of 1996 and 2008. The study consisted of five experimental treatments; control, mineral fertilizer (300-60-150 kg N-P-K ha⁻¹), manure at 25 t ha⁻¹, compost at 25 t ha⁻¹ and mycorrhiza-inoculated compost at 10 t ha⁻¹ with three replicates. The highest organic matter content both at 0-15 cm and 15-30 cm soil depths were obtained with manure application, whereas mineral fertilizer application had no effect on organic matter accumulation. Manure, compost and mycorrhizal inoculation + compost application had 69%, 32% and 24% higher organic matter contents at 0-30 cm depth as compared to the control application. Organic applications had varying and important effects on aggregation indexes of soils. The greatest mean weight diameters (MWD) at 15-30 cm depth were obtained with manure, mycorrhiza-inoculated compost and compost applications, respectively. The decline in organic matter content of soils in control plots lead disintegration of aggregates demonstrated on significantly lower MWD values. The compost application resulted in occurring the lowest bulk densities at 0-15 and 15-30 cm soil depths, whereas the highest bulk density values were obtained with mineral fertilizer application. Measurements obtained in 2008 indicated that manure and compost applications did not cause any further increase in MWD at manure and compost receiving plots indicated reaching a steady state. However, compost with mycorrhizae application continued to significant increase (P < 0.05) in MWD values of soils. Organic applications significantly lowered the soil bulk density and penetration resistance. The lowest penetration resistance (PR) at 0-50 cm soil depth was obtained with mycorrhizal inoculated compost, and the highest PR was with control and mineral fertilizer applications. The results clearly revealed that mycorrhiza application along with organic fertilizers resulted in decreased bulk density and penetration resistance associated with an increase in organic matter and greater aggregate stability, indicated an improvement in soil structure.     

Effect of mycorrhizal inoculation and soil restoration on the growth of Pinus halepensis seedlings in a semiarid soil

An experiment was carried out to evaluate the growth of mycorrhizal Pinus halepensis seedlings planted in a semiarid soil amended with urban refuse in southeast Spain. Three fungal species were used: Pisolithus tinctorius, Rhizopogon roseolus, and Suillus collinitus. After 8 months, inoculated seedlings grown under controlled conditions did not differ significantly from controls with regard to plant height and nutrient assimilation. Other features such as root development and stem dry weight showed that the plants grew better in the absence of mycorrhizal inoculation. The mycorrhizal seedlings and the controls were planted in three experimental plots treated with urban refuse (0, 6, and 12 Kg m^-2). After 1 year of growth under field conditions the results showed that the type of fungus inoculated significantly influenced P. halepensis development. This effect varied with the dose of urban refuse. Plant growth was encouraged by the application of refuse but only at the lowest dose. Under these conditions P. tinctorius was the most effective fungus and R. roseolus yielded poorer plant development. The highest application of urban refuse led to notably worse results and a significant decrease in seedling growth compared to controls. In the control plot (without refuse) S. collinitus was the most effective fungus in plant growth improvement. The smallest application of urban refuse had a positive effect on the assimilation of N, P, and K in seedlings inoculated with P. tinctorius and S. collinitus.     

Effects of different sources of organic matter on soil aggregate formation and stability: A laboratory study on a Lithic Rhodoxeralf from Turkey

In this study the influences of three different organic sources on aggregate formation and stability in soil were investigated.Our approach involves establishing plot experiment in the greenhouse with a Lithic Rhodoxeralf and a variety of carbon sources having specific chemical properties applied to the soil of individual plots. Treatments include K-Humate (KH, 25, 50 and 100 kg/ha), Concentrated Plant Extract (CPE, 50, 100, 200 kg/ha) and Molasses (M, 50, 100, 200 kg/ha). After a seven-month incubation period, aggregate formation and their stability were determined and their correlation to different C sources was developed.The effect of KH on aggregate formation was significant for 2–1 mm aggregates (P < 0.05) and 0.5–0.25 mm aggregates (P < 0.01). The effect of CPE on aggregate formation was significant for 0.5–0.25 aggregates (P < 0.05), while the M had no significant effect on aggregate formation in any aggregate size. The effect of KH on aggregate stability was significant for 8–4 mm and 1–0.5 mm aggregates (P < 0.01). The effect of CPE on aggregate stability was significant for 0.5–0.25 mm aggregates (P < 0.05), while the M was significant for 8–4 mm aggregates (P < 0.05).     

Effects of arbuscular mycorrhizal fungi on the growth and zinc uptake of trifoliate orange (Poncirus trifoliata) seedlings grown in low-zinc soil

The effects of the arbuscular mycorrhizal (AM) fungi, Glomus intraradices and G. versiforme, on growth and zinc (Zn) uptake were investigated in trifoliate orange (Poncirus trifoliata) seedlings exposed to low-Zn soil. Low-Zn decreased growth, levels of leaf chlorophyll, soluble protein and sugar, and soil enzymatic activities, and pH in 0–2 cm rhizosphere soil. Low-Zn soil also decreased mineral nutrients (including Zn) concentrations in the shoots and roots. Glomus intraradices especially, significantly enhanced plant biomass, leaf soluble protein and sugar concentrations, root viability, acid phosphatase, catalase, invertase and urease activities, and easily extractable glomalin content in 0–2 cm and 2–4 cm rhizosphere soil. It also increased concentrations of Zn, phosphorus, potassium and magnesium in the shoots and roots, while decreased the soil pH. Arbuscular mycorrhizal fungi, especially G. intraradices, has the potential to improve growth and Zn uptake of triofoliate orange seedlings grown in low-Zn soil.     

Effects of tillage, organic resources and nitrogen fertiliser on soil carbon dynamics and crop nitrogen uptake in semi-arid West Africa

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053–0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha⁻¹ against a maximum of 146 kg N ha⁻¹ in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa.     

The influence of tillage on the proportion of organic carbon and nitrogen in the microbial biomass of medium-textured soils in a humid climate

Summary Microbial biomass C and N respond rapidly to changes in tillage and soil management. The ratio of biomass C to total organic C and the ratio of mineral N flush to total N were determined in the surface layer (0–5 cm) of low-clay (8–10%), fine sandy loam, Podzolic soils subjected to a range of reduced tillage (direct drilling, chisel ploughing, shallow tillage) experiments of 3–5 years' duration. Organic matter dynamics in the tillage experiments were compared to long-term conditions in several grassland sites established on the same soil type for 10–40 years. Microbial biomass C levels in the grassland soils, reduced tillage, and mouldboard ploughing treatments were 561, 250, and 155 g g^-1 soil, respectively. In all the systems, microbial biomass C was related to organic C (r=0.86), while the mineral N flush was related to total N (r=0.84). The average proportion of organic C in the biomass of the reduced tillage soils (1.2) was higher than in the ploughed soils (0.8) but similar to that in the grassland soils (1.3). Reduced tillage increased the average ratio of mineral N flush to total soil N to 1.9, compared to 1.3 in the ploughed soils. The same ratio was 1.8 in the grassland soils. Regression analysis of microbial biomass C and percent organic C in the microbial biomass showed a steeper slope for the tillage soils than the grassland sites, indicating that reduced tillage increased the microbial biomass level per unit soil organic C. The proportion of organic matter in the microbial biomass suggests a shift in organic matter equilibrium in the reduced tillage soils towards a rapid, tillage-induced, accumulation of organic matter in the surface layer.     

Effectiveness of arbuscular mycorrhizal colonization at nursery-stage on growth and nutrition in wetland rice (Oryza sativa L.) after transplanting under different soil fertility and water regimes

In order to analyze the effectiveness of colonization by arbuscular mycorrhizal fungi (AMF) at the nursery stage on the growth and nutrient concentration of wetland rice after transplanting, the experiments were conducted under glasshouse conditions using two types of soil, namely (i) sterilized paddy soil (PS) and (ii) sterilized paddy soil diluted with sterilized Andosol subsoil 5 times (DS) under two water regimes, (i) flooded conditions changed to non-flooded conditions 30 d before harvest (F-NF) and (ii) continuous flooding (CF) up to harvest. Treatments consisting of mycorrhizal inoculation (+AMF) and non-inoculation ( — AMF) were applied only at the nursery stage when the seedlings were produced under dry nursery (60% moisture of maximum water holding capacity) conditions.

Seedlings grown in PS showed a significantly higher biomass yield and nutrient concentrations than in DS. At 90 and 105 d after transplanting, the mycorrhizal plants showed a higher biomass than non-mycorrhizal plants in PS whereas there were no differences in DS except for roots. Mycorrhizal colonization at the transplanting stage was higher in DS than in PS. However, after transplanting opposite results were obtained, the level in PS being relatively higher than in DS. Grain yield and P concentration of unhulled grain and shoots in PS were higher in the +AMF treatments than in the -AMF treatments under both water regimes. Contents of micronutrients (Zn, Cu, Fe, and Mn) were higher in the +AMF plants than in the -AMF ones at all growth stages up to maturation irrespective of soil fertility and water regimes. These results suggest that AMF inoculation at the nursery-stage was beneficial for wetland rice after transplanting to flooded conditions in terms of growth promotion and increase of nutrient concentrations.     

Effects of cropping systems on nitrogen,phosphorus and potassium forms and soil organic carbon in a Gray Luvisol

The effects of up to 23 years of agricultural cropping of a boreal forest soil on soil organic carbon (SOC) and N, P, and K pools were studied. The cropping systems studied were: (a) continuous barley, (b) continuous forage bromegrass, (c) continuous forage legume, and (d) barley/grass-legume forage rotation. Continuous bromegrass increased while other cropping systems decreased SOC in the surface soil. Kjeldahl N in soil approximately followed the trend in SOC. The net gain in N under continuous grass was attributed mostly to nonsymbiotic N fixation. Changes in SOC content appeared to be also influenced by cropping and tillage frequencies. Changes in fixed (intercalary) ammonium were small. There was no measurable change in total P, in part, because input was only slightly higher than crop offtake. Organic P increased under continuous bromegrass, and tended to decrease under continuous legume. The C/N and C/P ratios of soil organic matter decreased slightly with cropping. Exchangeable K (K_ex) was decreased by cropping systems containing a legume crop to a greater extent than those without a legume crop. Most of the decrease occurred in the 0–15 cm depth. Nitric acid extractable K was not affected by cropping. Since net loss of K_ex to 30 cm depth was substantially less than crop offtake, it is suggested that subsoil K reserves and matrix K were supplying a major portion of the crops' K requirement. It is concluded that the effects of cropping systems on SOC, N, P and K are influenced by crop type, and cropping and tillage frequencies.     

Effects of liming on organic matter decomposition and phosphorus extractability in an acid humic Ranker soil from northwest Spain

Summary A laboratory incubation experiment was carried out over 17 weeks to determine the effect of liming on soil organic matter. The amount of lime as calcium hydroxide [Ca(OH)₂] required to completely neutralise exchangeable Al was found to be five times the standard lime requirement. This large amount of lime had a limited overall effect on the short-term stability of soil organic matter, causing the release of 1300 g g^-1 of C (1.7% total soil C) above the control during the incubation. Liming may have altered the potential availability of soil organic matter and organic P, as shown by a marked reduction in the extractability of soil organic P with sodium bicarbonate and sodium hydroxide. The latter was unlikely to be due to the formation of calclium-P artefacts, and may be attributed to the combined chemical effects of added calcium hydroxide and precipitation of exchangeable Al on the nature and solubility of soil organic constituents and organomineral complexes. The addition of lime increased the degradation of added oak leaf litter by 50%, from 3.2 to 4.7 mg g^-1, as determined by CO₂ evolution. The enhanced litter degradation indicated increased microbial activity in limed soil, but this improvement had only minor effects on the stability of native organic matter. This study highlights the need for further research into the relationships between the chemical nature of organic P in soil and the physical, chemical, temporal, and agronomic factors that control its turnover and availability.     

The effect of disturbance by Phellinus weirii on decomposition and nutrient mineralization in a Tsuga mertensiana forest

Summary Microbial biomass in the upper 7 cm of soil and needle decomposition on the forest floor were measured seasonally for 10 months in a mountain hemlock (Tsuga mertensiana) old-growth forest and in a regrowth forest after Phellinus weirii, a root-rot pathogen infection, had caused disturbance. The microbial biomass was higher in the old-growth forest soil than in the regrowth forest soil. However, T. mertensiana needle decomposition rates were higher in the regrowth than in the old-growth forest. Total N, Ca, Fe, Cu, and Zn concentrations in needles increased during the 1st year of decomposition in both the old and the regrowth forests, but P, K, Mg, Mn, and B concentrations decreased. N, P, K, Mg, Cu, and Zn concentrations were lower in regrowth than in old-growth decomposing needles. During mineralization, needles in the regrowth forests released more N, P, and K as a result of higher needle decomposition rates. Our results suggest that higher needle decomposition rates increased the mineralization of N, P, and K, which may lead to increased soil fertility and faster tree growth rates in the regrowth forest.