Cover Crops and Soil Phosphorus Availability

Plants affect soil phosphorus (P) solubility through root exudates, but studies are lacking on species used as relay or cover crops in tropical environments. We evaluated the effect of cover crops on soil phosphorus (P) availability in an oxisol. Ruzigrass (Brachiaria ruziziensis), pearl millet (Pennisetum glaucum), peanut (Arachis hypogaea), crambe (Crambe abyssinica), and sorghum (Sorghum bicolor) were grown in pots with soil. Phosphorus uptake, soil inorganic and organic P, maximum P adsorption capacity, and plant root systems were assessed. When root length density is high, the efficiency of P uptake is low due to root competition. Crambe results in greater soil P availability, while peanut and sorghum decrease the soil maximum P adsorption capacity, probably by exuding or stimulating microbial production of organic acids and phenolic compounds. Hence, crambe, peanut, and sorghum are species that may be of interest to increase P use efficiency in cropping systems.     

Management of Nitrogen-Rich Legume Cover Crops as Mulch in Traditional Olive Orchards

A low-input agricultural system needs a natural source of nitrogen (N). Legume species can fix great amounts of N that can be subsequently used by a nonlegume crop. In this study three legume cover crops were grown in traditional olive orchards in northeastern Portugal from October 2009 to May 2010, and the aboveground biomass was mechanically destroyed and left on the ground as a mulch. In the following growing season, from October 2010 to May 2011, two nitrophilic plant species were grown in circular microplots of 154 mm surrounded by polyvinyl chloride rings to assess the soil N availability. The N fixed by the legume cover crops, estimated by the difference technique, was shown to vary from 79.7 to 187.5 kg N ha^?1. The nitrophilic plant species identified a small peak of soil available N in the autumn of 2010, probably resulting from the mineralization of the root system of the legume species. In the next spring, the increase of soil-available N in the plots where the legume cover crops had been grown, in comparison to the control plot, was residual. The great amounts of N present in the mulched materials seem to disappear without having entered the soil. Mulching with high-N content biomass may be troublesome due to the high risk of N losses probably by ammonia (NH₃) volatilization.     

Intercropping promotes the ability of durum wheat and chickpea to increase rhizosphere phosphorus availability in a low P soil

Cereal–legume intercropping can promote plant growth (i.e. facilitation) through an increase in the amount of phosphorus (P) taken up, especially in low P soils. The aim of this study was to test the hypothesis that these positive interactions are supported by rhizosphere processes that increase P availability, such as root-induced pH changes. In neutral and alkaline soils legumes are assumed to increase inorganic P availability by rhizosphere acidification due to N₂ fixation which benefit to the intercropped cereal. Growth, P uptake, changes in inorganic P availability and pH in the rhizosphere of intercropped species were thus investigated in a greenhouse pot experiment with durum wheat and chickpea either grown alone or intercropped. We used a neutral soil from a P fertilizer long-term field trial exhibiting either low (−P) or high (+P) P availability. Phosphorus availability was increased in the rhizosphere of both species, especially when intercropped in −P. Such increase was associated with alkalization. Rhizosphere pH changes could not fully explain the observed changes of P availability though. Low rates of N₂ fixation may explain why no rhizosphere acidification was observed. Increases in P availability did not lead to enhanced P uptake but growth promotion was observed for durum wheat intercropped with chickpea in −P soil. Our hypothesis of an increase in inorganic P availability in intercropping as a consequence of root-induced acidification by the legume was not validated, and we suggested that root-induced alkalization was involved instead, as well as other root-induced processes. Thus, the cereal through rhizosphere alkalization may also enhance P uptake and growth of the intercropped legume. Facilitation can thus occur in both ways.     

Effects of various cover crops after peas on nitrate leaching and nitrogen supply to succeeding winter wheat or potato crops

In organic farming systems, it has been demonstrated that grain pulses such as peas often do not enhance soil N supply to the following crops. This may be due to large N removals via harvested grains as well as N‐leaching losses during winter. In two field‐trial series, the effects of legume (common vetch, hairy vetch, peas) and nonlegume (oil radish) cover crops (CC), and mixtures of both, sown after peas, on soil nitrate content, N uptake, and yield of following potatoes or winter wheat were studied. The overall objective of these experiments was to obtain detailed information on how to influence N availability after main‐crop peas by adapting cover‐cropping strategies. Cover crops accumulated 56 to 108 kg N ha^–1 in aboveground biomass, and legume CC fixed 30–70 kg N ha^–1 by N₂ fixation, depending on the soil N supply and the length of the growing period of the CC. Nitrogen concentration in the aboveground biomass of legume CC was much higher and the C : N ratio much lower than in the nonlegume oil radish CC. At the time of CC incorporation (wheat series) as well as at the end of the growing season (potato series), soil nitrate content did not differ between the nonlegume CC species and mixtures, whereas pure stands of legume CC showed slightly increased soil nitrate content. When the CC were incorporated in autumn (beginning of October) nitrate leaching increased, especially from leguminous CC. However, most of the N leached only into soil layers down to 1.50 m and was recovered more or less by the following winter wheat. When CC were incorporated in late winter (February) no increase in nitrate leaching was observed. In spring, N availability for winter wheat or potatoes was much greater after legumes and, after mixtures containing legumes, resulting in significantly higher N uptake and yields in both crops. In conclusion, autumn‐incorporated CC mixtures of legumes and nonlegumes accomplished both: reduced nitrate leaching and larger N availability to the succeeding crop. When the CC were incorporated in winter and a spring‐sown main crop followed even pure stands of legume CC were able to achieve both goals.     

Can tropical grasses grown as cover crops improve soil phosphorus availability?

Tropical grasses grown as cover crops can mobilize phosphorus (P) in soil and have been suggested as a tool to increase soil P cycling and bioavailability. The objective of this study was to evaluate the effect of tropical grasses on soil P dynamics, lability, desorption kinetics and bioavailability to soya bean, specifically to test the hypothesis that introducing grass species in the cropping system may affect soil P availability and soya bean development according to soil P concentration. Three grass species, ruzi grass (Urochloa ruziziensis ), palisade grass (Urochloa brizantha ) and Guinea grass (Megathyrsus maximus ), were grown in soils with contrasting P status. Soya bean was grown after grasses to assess soil P bioavailability. Hedley P fractionation, microbial biomass P, phytase‐labile P and the diffusive gradient in thin films were determined, before and after cultivation. It was found that grasses remobilized soil P, reducing the concentration of recalcitrant P forms. The effect of grasses on changing the P desorption kinetics parameters did not directly explain the observed variation on P bioavailability to soya bean. Grasses and microorganisms solubilize recalcitrant organic P (P_o) forms and tropical grasses grown as cover crops increased P bioavailability to soya bean mainly due to the supply of P by decomposition of grass residues in low‐P soil. However, no clear advantages in soya bean P nutrition were observed when in rotation with these grasses in high‐P soil. This study indicates that further advantages in soya bean P nutrition after tropical grasses may be impeded by phytate, which is not readily available to plants.     

Differential Soil Acidity Tolerance of Tropical Legume Cover Crops

In tropical regions, soil acidity and low soil fertility are the most important yield‐limiting factors for sustainable crop production. Using legume cover crops as mulch is an important strategy not only to protect the soil loss from erosion but also to ameliorate soil fertility. Information is limited regarding tolerances of tropical legume cover crops to acid soils. A greenhouse experiment was conducted to determine the differential tolerance of 14 tropical legume cover crops to soil acidity. The acidity treatments were high (0 g lime kg^?1 soil), medium (3.3 g lime kg^?1 soil), and low (8.3 g lime kg^?1 soil). Shoot dry weight of cover crops were significantly affected by acidity treatments. Maximum shoot dry weight was produced at high acidity. Jack bean, black mucuna, and gray mucuna bean species were most tolerant to soil acidity, whereas Brazilian lucern and tropical kudzu were most susceptible to soil acidity. Overall, optimal soil acidity indices were pH 5.5, hydrogen (H)+ aluminum (Al) 6.8 cmol_c kg^?1, base saturation 25%, and acidity saturation 74.7%. Species with higher seed weight had higher tolerance to soil acidity than those with lower seed weight. Hence, seed weight was associated with acidity tolerance in tropical legume species.     

Phosphorus Amendment of a Lead-Spiked Soil with Low Phosphorus Availability: Roles of Phosphorus on Soil and Plant Lead

Phosphorus (P) is both a macronutrient for plants and an effective amendment to reduce lead (Pb) toxicity in soil. Thus, in Pb-polluted soil with low P availability, P will act as a nutrient as well as a Pb-immobilizing agent. However, this has not been fully investigated. A soil with 2.50 mg kg^?1 Olsen P was spiked with soluble Pb and then amended with superphosphate to examine the effect of P on soil Pb availability and ryegrass (Lolium perenne L. cv. Aubisque) growth. It was found that P/Pb = 2 increased ryegrass yield by 804% and decreased root Pb concentration and soil diethylenetriaminepentaacetic acid (DTPA)–extractable Pb concentration by 25.6% and 1.0%, respectively. As P amendment increased to P/Pb = 4, both plant yield and root Pb concentration declined compared with P/Pb = 2. Results of the sequential extraction indicated that the proportion of carbonate phase Pb decreased, while that of the manganese oxide phase increased as P was added. The proportion of residual Pb was little affected by the amendment. The results suggest that in soils with low P availability and high Pb availability, availability of soil Pb and root concentration of Pb are less affected, whereas the toxicity of Pb is greatly depressed by the P amendment; P/Pb = 2 is high enough to alleviate the stresses of low P availability.     

Responses of soil nematode community to monoculture or mixed culture of a grass and a legume forage species in China

Mixed cultivation of fast-growing grasses and nitrogen (N)-fixing legumes for forage production is widely considered effective for obtaining sustained high forage yields without depleting soil N levels. However, the effects of monoculture and mixed culture of these species on soil food webs are poorly understood. In this study, soil nematode communities were examined as indicators of the soil food web structure of monoculture and mixed culture of grass and legume at three N levels, i.e., 338 (low), 450 (moderate), and 675 (high) kg N ha^-1 year^-1, across 2 years in wet and dry seasons, using the grass Paspalum wetsfeteini and the legume Medicago sativa (alfalfa), both commonly cultivated worldwide. Repeated-measures analysis of covariance showed that compared with grass monoculture, legume monoculture and grass-legume mixture increased abundances of herbivorous, bacterivorous, and fungivorous nematodes in the soil food web under the low and moderate N fertilization levels. Principal response curve results showed that the abundance of Helicotylenchus, a plant parasite, was significantly higher under legume monoculture than other planting systems at the low N fertilization level. Structural equation model analysis indicated that the legume increased bacterivore abundance, while increasing N fertilization decreased omnivore abundance. The legume might increase the quantity and quality of food resources for soil biota, resulting in the bottom-up control of soil nematode communities. Our results indicate that targeted control of a soilborne pathogen, Helicotylenchus, is required in alfalfa-based planting systems. In addition, high inorganic N application, which is detrimental to legume-rhizobia symbiosis, nullified the otherwise positive effects of legumes on soil nematodes.     

Soil Factors Controlling Arsenic Availability for Silene vulgaris

The arsenic (As) resistance of Silene vulgaris is studied on two Mediterranean soils, spiked with As and grown under semicontrolled conditions. The results showed that this species accumulates high amounts of As in roots, but both transfer and translocation factors were far from the values found in the As hyperaccumulator species. Arsenic availability in both tested soils is related to soil parameters as follows: Phosphorus (P) > iron oxides > pH. Silene plants studied did not show visual symptoms of As toxicity except for growth reductions at the greater As doses and in the soil with greater P availability; this would indicate the As resistance ability of this plant population.     

Legume residue influence arbuscular mycorrhizal colonisation and P uptake by wheat

Legumes have been shown to increase growth and P uptake of the following cereal. This could, in part, be due to nutrients released by the decomposing legume residues. To investigate the effect of P added with legume residues on wheat growth, P uptake and arbuscular mycorrhizal (AM) colonisation, a number of experiments were conducted with different legume residues added to a soil with low P availability under conditions in which N was not limiting. Young and mature faba bean shoots (FYS, FMS) and mature chickpea shoots (CP) were added to soil at different rates (0.5–2%, w/w) with the P concentration being the greatest in the young faba bean shoots and least in the mature chickpea residues. Other treatments included addition of inorganic P at different rates (0–80 mg P kg⁻¹). Available P, growth and P uptake and AM colonisation of wheat were measured after 6 weeks. As expected, inorganic P addition increased growth and P uptake but decreased AM colonisation. The effect of the residues was more complex. AM colonisation was not correlated with available P in the soil amended with residues, whereas there was significant negative correlation between available P and AM colonisation in the treatments with inorganic P. Addition of FYS increased wheat shoot growth and P uptake and decreased AM colonisation. However, FMS and CP addition not only decreased wheat growth and P uptake but also AM colonisation despite low soil P availability. It is concluded that addition of some legume residues can improve the growth of subsequent cereals, but others have a negative effect on wheat growth and AM colonisation which cannot be explained solely by soil P availability.     

Soil carbon as affected by cover crops under no‐till under tropical climate

In tropical, low‐fertility soils, crop yields are dependent on soil carbon, and cropping systems under no‐till can increase soil C stocks. Plant residues supplied by cover crops in no‐till systems may improve aggregate stability and soil carbon, which may be further increased with the introduction of a legume in the cropping system. This research studied the effects of cover crops in rotation with soybean under no‐till on soil carbon and nitrogen, in Botucatu, Brazil, for 3 yr. The cover crops were millet (Penninsetum americanum Leek), cober crop (Sorghum bicolor × Sorghum sudanense) and sunn hemp (Crotalaria juncea L.), grown in the spring. Fallow without cover crops was used as a control. Grain sorghum (Sorghum bicolor L. Moench) and soybean (Glycine max (L.) Merril) were grown in fall–winter and summer, respectively. Generally, cover crops increased soil carbon contents, but soil N was only increased by sunn hemp in the particulate organic C fraction. An increase in the labile carbon fraction in the topsoil layers was closely related to cover crop root development. Fallow in spring should not be recommended in degraded soils with lowcarbon stock. Labile‐fractioned soil organic carbon and total carbon levels are more efficiently increased by grasses than by legumes in the short term, and grasses cropped in spring increase soil C/N ratio. Conversely, the introduction of a legume (sunn hemp) maintained a more stable C/N ratio, that is around 10, which would be more effective in increasing soil C in the long term.     

Effects of nitrogen feeding for extraradical mycelium of Rhizophagus irregularis maize symbiosis incorporated with phosphorus availability

In terrestrial ecosystems, plants are frequently in symbiosis with arbuscular mycorrhizal fungi (AMF) with mineral nutrients and photosynthesis carbon exchanges in between. This research sought to identify the effects of phosphorus (P) levels on the nitrogen (N) uptake via extraradical mycelium (ERM) and the mycorrhizal growth response (MGR) of maize plants within the AMF symbiosis. Pots were separated into root compartments and hyphae compartments (HCs) with two layers of a 30‐μm mesh membrane and an air gap in between, where only hyphae could pass through, to avoid both N diffusion and root growth effects. Maize plants were inoculated with Rhizophagus irregularis with different N fertilization in HCs under two different P fertilization levels. Our results indicated that a strong increase in MGR with low‐P fertilization. The same tendency was not observed with high‐P fertilization, although both had a large increase in P concentration as a potential source of growth in shoot tissue of mycorrhizal plants. Substantial effects (10.5% more N) were observed in the case of high‐P availability for the host plants from ERM fed with N, whereas under low‐P conditions ERM may prioritize P uptake rather than N uptake. The AM fungi increase the uptake of N and P, which are most limiting in the soil with fewer forces from soil resources. In addition, there was still more P accumulated than N due to the high N for ERM with high‐P supply. Low N in HCs corresponded with a lower colonization rate in roots but with high hyphae density in HCs; this result suggest that N and P availability might change the ratio of extraradical to intraradical hyphae length.     

Rhizosphere Phosphorus Solubility and Plant Uptake as Affected by Crop in a Clay Soil from the Central Plateau Region of Haiti

Phosphorus (P) is commonly a limiting nutrient affecting crop yields in tropical cultivated systems, and a high P-sorption capacity in many of these soils can decrease the effectiveness of fertilizer use. Smallholder farmers, such as those in rural Haiti who do not have sufficient access to P fertilizers, may benefit from crops that are efficient at scavenging for P. To evaluate commonly grown and potentially useful plants for this purpose, a pot study was conducted in a controlled-environment chamber set to approximate ambient May–June conditions in Haiti's Central Plateau with seven treatments (six legume and one grass species) using soil from an Ustalf taken in the region. Velvet bean (Mucuna pruriens [L.] DC) produced the largest amount of biomass, though no difference in P uptake among treatments was observed. Phosphorus uptake and rhizosphere P were positively correlated, indicating plants with a larger capacity to solubilize P took up more P.     

Application of low‐phosphorus‐containing legume residues reduces extractable phosphorus in a tropical Ultisol

Application of legume green manure (GM) is suggested to be effective in increasing the availability of native soil phosphorus (P) and the dissolution and utilization of phosphate rock (PR)‐P by food crops. Experiments were conducted to study the dynamics of extractable P (P extracted by Bray‐1‐extracting solution) of an Ultisol amended with or without GM residues of contrasting P concentrations in the absence of growing plants. In two separate experiments, GM residues of Aschynomene afraspera (a flood‐tolerant legume) and of Crotalaria micans (upland) with varying P concentrations were added to an acidic soil amended with PR‐P or triple superphosphate (TSP) in plastic bottles. Soil moisture was brought to field capacity of the soil in the upland experiment and saturated with distilled water in the lowland setup. This was done to simulate aerobic upland and anaerobic lowland soil conditions in the relevant plastic bottles. Only P concentration of the residues added varied, while lignin and C : N ratios were similar. A temperature of 25°C was maintained throughout the experiment. Changes in soil extractable Bray‐1‐P were measured at the end of the incubation period (60 or 80 d). In the aerobic soils, extractable P in the combined PR+GM or TSP+GM treatments was significantly lower than in the PR‐ or TSP‐ treated soils. The amendment with GM residues alone significantly increased Bray‐1‐P over the unamended control in the case of the inorganic P‐fertilized GM residues. The trend in extractable P was similar in the soils incubated under anaerobic conditions. However, in the case of PR, concentrations of P extracted by Bray‐1 solution did not significantly change in the presence or absence of GM. The results suggest that the incorporation of GM residues with low P concentration does not lead to a net P release in upland or lowland soils. These results have implications for nutrient cycling in farming systems in W Africa as most of the soils are poor and very low in available P.     

Incorporation of catch crop residues does not increase phosphorus leaching: a soil column experiment in unsaturated conditions

Some studies suggest that incorporation of catch crop residues leads to increased availability of P to plants. However, little information is available on how this affects P leaching in soils with a high P load. We tested the effect of catch‐crop residue incorporation at the end of winter on the P leaching potential in a soil column experiment under unsaturated conditions using a typical sandy loam soil of NW Europe characterized by a high P load. We sampled the catch crops white mustard (Sinapis alba L.), Italian ryegrass (Lolium multiflorum L.), black oats (Avena strigosa L.) and a perennial ryegrass‐white clover mix (Lolium perenne L.‐Trifolium repens L.) from a field trial on catch crops and soil from the plots where they were grown. Plant biomass was incorporated taking account of the differences in conditions of the plant material at the end of winter and the biomass yield of each catch crop. Incorporation of catch‐crop residues decreased P leaching compared to the fallow treatment probably through immobilization of soil P during catch crop residue decomposition. The exception was black oats, where the leaching of P was the same as for fallow soil. We observed clear differences in C/N, C/P, water soluble and total P concentration, and biodegradability between the tested catch crops, which seemed to affect the P leaching. We conclude that the incorporation of catch crop residues under typical soil and weather conditions and agricultural practices of NW Europe does not increase the potential P leaching losses.     

PHOSPHORUS NUTRITION AND MYCORRHIZAL GROWTH RESPONSE OF BROADLEAF AND NARROWLEAF BIRDSFOOT TREFOILS

In Argentina narrowleaf trefoil (Lotus glaber) predominates over broadleaf trefoil (Lotus corniculatus). Researchers have suggested this predominance occurs because L. glaber grows better at low levels of P availability. I studied the effect of increasing phosphorus (P) nutrition and mycorrhizal growth response of Lotus corniculatus and Lotus glaber in a soil of low available P. L. corniculatus was more efficient in P utilization than L. glaber and produced larger yields of shoot tissue per unit of P. The critical P concentration, measured as a percentage of P in the shoot required to achieve 90% of maximum shoot yield, was 0.22 in L. corniculatus and 0.28 in L. glaber. The roots of both two species were heavily infected by arbuscular micorrhizae (AM), and both mycorrhizal and nonmycorrhizal plants responded strongly to added P in soil. This suggests that both species are at no advantage or disadvantage whether mycorrhizal or nonmicorrhizal. The fraction of roots colonized by AM fungi differed between species at low levels of added P but was similar at high levels of added P. The specific root length (cm g^?1) of mycorrhizal plants was greatest in L. corniculatus when P was insufficient to achieve maximum growth. The lower critical P concentration and the higher specific root length at low levels of P nutrition may be why L. corniculatus is superior to L. glaber in soils low in P. Therefore, the predominance of L. glaber compared with L. corniculatus in Argentinean soils is not due to ability of L. glaber to grow better in soil at low levels of P availability.     

Nodulation and growth of black locust (Robinia pseudoacacia) on a desurfaced soil inoculated with a local Rhizobium isolate

Nodulation and nitrogen fixation of black locust (Robinia pseudoacacia L.), a legume tree broadly used in Argentina for urban and agricultural afforestation, was studied in hydroponic culture. The development of seedlings inoculated with a local strain of Rhizobium, highly specific for R. pseudoacacia, was also compared with respect to non-inoculated but N-fertilised seedlings. This strain produced fast nodulation and high crop yield and leaf N content. Already nodulated plants with the local Rhizobium strain were assayed for growth in a greenhouse pot experiment with soil from a field where topsoil has been removed for industrial purposes, whilst pots with non-desurfaced soil from the same field were used as control. Non-inoculated plants were also grown in either control or desurfaced soil. Inoculated plants developed better than non-inoculated plants in desurfaced soil, and in control soil as well, suggesting that the symbiosis was able to overcome the nutrient limitation of the desurfaced soil. Non-inoculated plants were nodulated by native soil born Rhizobium, either in control or desurfaced soil, but they showed low final nitrogen leaf content and low nitrogen fixation activity, suggesting that native rhizobia were ineffective.     

Effect of phosphorus management in rice–mungbean rotations on sandy soils of Cambodia

Nitrogen (N) and phosphorus (P) deficiencies are key constraints in rainfed lowland rice (Oryza sativa L.) production systems of Cambodia. Only small amounts of mineral N and P or of organic amendment are annually applied to a single crop of rainfed lowland rice by smallholder farmers. The integration of leguminous crops in the pre‐rice cropping niche can contribute to diversify the production, supply of C and N, and contribute to soil fertility improvement for the subsequent crop of rice. However, the performance of leguminous crops is restricted even more than that of rice by low available soil P. An alternative strategy involves the application of mineral P that is destined to the rice crop already to the legume. This P supply is likely to stimulate legume growth and biological N₂ fixation, thus enhancing C and N inputs and recycling N and P upon legume residue incorporation. Rotation experiments were conducted in farmers' fields in 2013–2014 to assess the effects of P management on biomass accumulation and N₂ fixation (δ¹⁵N) by mungbean (Vigna radiata L.) and possible carry‐over effects on rice in two contrasting representative soils (highly infertile and moderately fertile sandy Fluvisol). In the traditional system (no legume), unamended lowland rice (no N, + 10 kg P ha^?1) yielded 2.8 and 4.0 t ha^?1, which increased to 3.5 and 4.7 t ha^?1 with the application of 25 kg ha^?1 of urea‐N in the infertile and the moderately fertile soil, respectively. The integration of mungbean as a green manure contributed up to 9 kg of biologically fixed N (17% Nfda), increasing rice yields only moderately to 3.5–4.6 t ha^?1. However, applying P to mungbean stimulated legume growth and enhanced the BNF contribution up to 21 kg N ha^?1 (36% Nfda). Rice yields resulting from legume residue incorporation (“green manure use”–all residues returned and “grain legume use”–only stover returned) increased to 4.2 and 4.9 t ha^?1 in the infertile and moderately fertile soil, respectively. The “forage legume use” (all above‐ground residues removed) provided no yield effect. In general, legume residue incorporation was more beneficial in the infertile than in the moderately fertile soil. We conclude that the inclusion of mungbean into the prevailing low‐input rainfed production systems of Cambodia can increase rice yield, provided that small amounts of P are applied to the legume. Differences in the attributes of the two major soil types in the region require a site‐specific targeting of the suggested legume and P management strategies, with largest benefits likely to accrue on infertile soils.     

Plant heavy metal concentrations and soil biological properties in agricultural serpentine soils

Abstract

Soils developed on serpentinitic rocks have serious limitations for agriculture. They have high levels of magnesium (Mg) and heavy metals [copper (Cu), manganese (Mn), nickel (Ni), and chromium (Cr)] and are deficient in some macronutrients. In parts of Northwestern Spain, serpentine soils have been subjected to intensive management, based on the use of manure and harvesting residues. Although these practices have allowed the growth of crops, plants may have accumulated high amounts of metals. This study was carried out to assess the effect of the management practices on the uptake of heavy metals by crops, and to analyze the relationship between the concentrations of these metals in plants, and soil properties. Moderate levels of Ni and Mn and low levels of Cr and Cu were found in soil extractable fractions of these metals. In spite of this, analysis of plant tissues revealed high levels of Cr and Ni and moderate contents of Mn. Concentrations of Mn and Ni in foliage were correlated to soil extractable contents, whereas simple linear regression between concentration of Cr in plants and the soil‐extractable Cr showed a poor relationship, possibly because the availability of this metal, as Cr(VI), is determined by temporal environmental conditions. To assess the effects of the management on the uptake of heavy metals by plants, a complementary bioassay experiment was carried out in the laboratory in which Festuca rubra and Agrostis stolonifera were sown on serpentine soil with low organic matter content, and amended with peat and/or lime. This experiment confirmed that there is a reduction in heavy metal concentration in plants after organic amendment and suggested that the lower metal availability is partly due to the higher soil microbial activity, produced as a consequence of addition of organic matter.