Timothy R. Cavagnaro

Roots, Nitrogen Transformations, and Ecosystem Services

This review considers some of the mechanistic processes that involve roots in the soil nitrogen (N) cycle, and their implications for the ecological functions that retain N within ecosystems: 1) root signaling pathways for N transport systems, and feedback inhibition, especially for NO(3)(-) uptake; 2) dependence on the mycorrhizal and Rhizobium/legume symbioses and their tradeoffs for N acquisition; 3) soil factors that influence the supply of NH(4)(+) and NO(3)(-) to roots and soil microbes; and 4) rhizosphere processes that increase N cycling and retention, such as priming effects and interactions with the soil food web. By integrating information on these plant-microbe-soil N processes across scales and disciplinary boundaries, we propose ideas for better manipulating ecological functions and processes by which the environment provides for human needs, i.e., ecosystem services. Emphasis is placed on agricultural systems, effects of N deposition in natural ecosystems, and ecosystem responses to elevated CO(2) concentrations. This shows the need for multiscale approaches to increase human dependence on a biologically based N supply.

Arbuscular mycorrhizas, microbial communities, nutrient availability, and soil aggregates in organic tomato production

Effects of arbuscular mycorrhzal (AM) fungi on plant growth and nutrition are well-known, but their effects on the wider soil biota are less clear. This is in part due to difficulties with establishing appropriate non-mycorrhizal controls in the field. Here we present results of a field experiment using a new approach to overcome this problem. A previously well-characterized mycorrhizal defective tomato mutant (rmc) and its mycorrhizal wildtype progenitor (76R MYC+) were grown at an organic fresh market tomato farm (Yolo County, CA). At the time of planting, root in-growth cores amended with different levels of N and P, were installed between experimental plants to study localized effects of mycorrhizal and non-mycorrhizal tomato roots on soil ecology. Whilst fruit yield and vegetative production of the two genotypes were very similar at harvest, there were large positive effects of colonization of roots by AM fungi on plant nutrient contents, especially P and Zn. The presence of roots colonized by AM fungi also resulted in improved aggregate stability by increasing the fraction of small macroaggregates, but only when N was added. Effects on the wider soil community including nematodes, fungal biomass as indicated by ergosterol, microbial biomass C, and phospholipid fatty acid (PLFA) profiles were less pronounced. Taken together, these data show that AM fungi provide important ecosystem functions in terms of plant nutrition and aggregate stability, but that a change in this one functional group had only a small effect on the wider soil biota. This indicates a high degree of stability in soil communities of this organic farm.

The role of arbuscular mycorrhizas in improving plant zinc nutrition under low soil zinc concentrations: a review

Many of the world’s soils are zinc (Zn) deficient. Consequently, many crops experience reduced growth, yield and tissue Zn concentrations. Reduced concentrations of Zn in the edible portions of crops have important implications for human Zn nutrition; this is a cause of global concern. Most terrestrial plant species form arbuscular mycorrhizas (AM) with a relatively limited number of specialized soil fungi. Arbuscular mycorrhizal fungi (AMF) can take up nutrients, including Zn, and transfer them to the plant, thereby enhancing plant nutrition. Under high soil Zn concentrations the formation of AM can also ‘protect’ against the accumulation of Zn in plant tissues to high concentrations. Here, a short review focusing on the role of AM in enhancing plant Zn nutrition, principally under low soil Zn concentrations, is presented. Effects of Zn on the colonisation of roots by AMF, direct uptake of Zn by AMF, the role of AM in the Zn nutrition of field grown plants, and emerging aspects of Zn molecular physiology of AM, are explored. Emergent knowledge gaps are identified and discussed in the context of potential future research.

Growth and nutritive value of cassava (Manihot esculenta Cranz.) are reduced when grown in elevated CO.

Global food security in a changing climate depends on both the nutritive value of staple crops as well as their yields. Here, we examined the direct effect of atmospheric CO(2) on cassava (Manihot esculenta Cranz., manioc), a staple for 750 million people worldwide. Cassava is poor in nutrients and contains high levels of cyanogenic glycosides that break down to release toxic hydrogen cyanide when damaged. We grew cassava at three concentrations of CO(2) (C(a): 360, 550 and 710 ppm) supplied together with nutrient solution containing either 1 mM or 12 mM nitrogen. We found that total plant biomass and tuber yield (number and mass) decreased linearly with increasing C(a). In the worst-case scenario, tuber mass was reduced by an order of magnitude in plants grown at 710 ppm compared with 360 ppm CO(2). Photosynthetic parameters were consistent with the whole plant biomass data. It is proposed that since cassava stomata are highly sensitive to other environmental variables, the decrease in assimilation observed here might, in part, be a direct effect of CO(2) on stomata. Total N (used here as a proxy for protein content) and cyanogenic glycoside concentrations of the tubers were not significantly different in the plants grown at elevated CO(2). By contrast, the concentration of cyanogenic glycosides in the edible leaves nearly doubled in the highest C(a). If leaves continue to be used as a protein supplement, they will need to be more thoroughly processed in the future. With increasing population density, declining soil fertility, expansion into marginal farmland, together with the predicted increase in extreme climatic events, reliance on robust crops such as cassava will increase. The responses to CO(2) shown here point to the possibility that there could be severe food shortages in the coming decades unless CO(2) emissions are dramatically reduced, or alternative cultivars or crops are developed.

Variations in the Chemical Composition of Cassava (Manihot esculenta Crantz) Leaves and Roots As Affected by Genotypic and Environmental Variation

The purpose of this study was to assess the quality of cassava cultivars, in terms of cyanogenic potential and composition of macro- and micronutrients, sampled from different locations in rural Mozambique. Total cyanide concentrations in fresh cassava tissues were measured using portable cyanide testing kits, and elemental nutrients were later analyzed from dried plant tissue. Variation in cyanogenic potential and nutrient composition occurred both among cultivars and across locations. The majority of cultivars contained >100 ppm total cyanide, fresh weight, and are therefore considered to be dangerously poisonous unless adequately processed before consumption. Leaf cyanogenic and nutrient content varied with plant water status, estimated using carbon isotope discrimination (δ(13)C). The colonization of roots of all cultivars by arbuscular mycorrhizal fungi was also quantified and found to be high, indicating that mycorrhizas could play a key role in plant nutrient acquisition in these low-input farming systems.

Arbuscular mycorrhizas modify tomato responses to soil zinc and phosphorus addition

Arbuscular mycorrhizas (AM) play an important role in plant P and Zn nutrition; however, relatively few studies have directly investigated the interactive effects of these nutrients on plants. Therefore, we undertook a glasshouse experiment to study the effects of Zn and P on AM formation and functioning. A mycorrhiza defective tomato mutant (rmc) and its mycorrhizal wild-type progenitor (76R) were used in this experiment. Plants were grown in soil amended with five Zn concentrations, ranging from deficient to toxic, and two levels of P addition. The addition of Zn and P to the soil over a range of concentrations had profound effects on plant growth and nutrition and mycorrhizal colonization. Mycorrhizal benefits were the greatest when plants were grown under low soil P and Zn. Furthermore, the effect of soil Zn supply on plant growth, nutrition, and AM colonization was strongly influenced by the concentration of P in the soil. Thus, studies of AM and Zn (or other nutrients of interest) should take into account the impact of soil P concentration on the role of AM in plant Zn acquisition, under both deficient and toxic soil Zn concentrations.

Arbuscular mycorrhizas enhance plant interception of leached nutrients

Arbuscular mycorrhizal fungi (AMF) can increase plant growth and nutrition. However, their capacity to reduce the leaching of nutrients through the soil profile is less well understood. Here we present results of an experiment in which the effects of forming arbuscular mycorrhizas (AM) on plant growth and nutrition, nutrient depletion from soil, and nutrient leaching, were investigated in microcosms containing the grass Phalaris aquatica L. Mycorrhizal and non-mycorrhizal plants were grown in a mixture of riparian soil and sand under glasshouse conditions. The formation of AM by P. aquatica significantly increased plant growth and nutrient uptake. Lower levels of NO3-, NH4+ and plant available P in both soil and leachate were observed in columns containing mycorrhizal root systems. These differences in nutrient interception were proportionally greater than the increase in root biomass of the mycorrhizal plants, compared with their non-mycorrhizal counterparts. Taken together, these data indicate that mycorrhizal root systems have an important, but previously little considered, role to play reducing the net loss of nutrients via leaching.

Arbuscular mycorrhizas modify plant responses to soil zinc addition

Zinc deficiency is one of the most commonly reported plant and human nutrient deficiencies worldwide. Conversely, Zn is also a common environmental contaminant, significantly reducing plant growth. These contrasting effects of Zn on plant growth and nutrition have been the focus of a considerable number of studies; however, most studies focus on plant responses to soil Zn concentration under either deficient or toxic concentrations, but not both. The formation of arbuscular mycorrhizas (AM) can increase plant Zn uptake under low soil Zn concentrations, and on the other hand, ‘protect’ plants against excessive Zn accumulation under high soil Zn conditions. Here we report the findings of an experiment in which we studied the response of AM formed by tomatoes under low, medium and high soil Zn conditions. To control for the formation of AM in this study we used a mycorrhiza defective tomato mutant and its mycorrhizal wildtype progenitor. While mycorrhizal colonization was not significantly impacted by soil Zn addition, the growth of plants and tissue Zn concentrations were. Together these data highlight the complex interactions between AM and Zn, and the utility of a genotypic approach for studying AM in this context.

Growth, nutrition, and soil respiration of a mycorrhiza-defective tomato mutant and its mycorrhizal wild-type progenitor

The effects of colonisation of roots by arbuscular mycorrhizal fungi (AMF) on soil respiration, plant growth, nutrition, and soil microbial communities were assessed using a mycorrhiza-defective tomato (Solanum lycopersicum L.) mutant and its mycorrhizal wild-type progenitor. Plants were grown in rhizocosms in an automated respiration monitoring system over the course of the experiment (79 days). Soil respiration was similar in the two tomato genotypes, and between P treatments with plants. Mycorrhizal colonisation increased P and Zn content and decreased root biomass, but did not affect aboveground plant biomass. Soil microbial biomass C and soil microbial communities based on phospholipid fatty acid (PLFA) analysis were similar across all treatments, suggesting that the two genotypes differed little in their effect on soil activity. Although approximately similar amounts of C may have been expended belowground in both genotypes, they may have differed in the relative C allocation to root construction v. respiration. Further, net soil respiration did not differ between the two tomato genotypes, but root dry weight was lower in mycorrhizal roots, and respiration of mycorrhizal roots per unit dry weight was higher than nonmycorrhizal roots. This indicates that the AM contribution to soil respiration may indeed be significant, and nutrient uptake per unit C expenditure belowground in this experiment appeared to be higher in mycorrhizal plants.

Effects of arbuscular mycorrhizas on ammonia oxidizing bacteria in an organic farm soil

Arbuscular mycorrhizal fungi (AMF) are potentially important in nutrient cycling in agricultural soils and particularly in soils managed for organic production; little is known, however, about the interrelationships between AMF and other members of soil microbial communities. Ammonia oxidizing bacteria (AOB) are a trophic group of bacteria having an enormous impact on nitrogen availability in soils and are expected to be influenced by the presence of AMF. In a field study, we utilized a unique genetic system comprised of a mycorrhiza defective tomato mutant (named rmc) and its mycorrhiza wild-type progenitor (named 76RMYC+). We examined the effect of AMF by comparing AOB community composition and populations in soil containing roots of the two tomato genotypes in an organically managed soil. Responses of AOB to soil N and P amendments were also studied in the same experiment. Phylogenetic analysis of cloned AOB sequences, derived from excised denaturing gradient gel electrophoresis (DGGE) bands, revealed that the organic farm soil supported a diverse yet stable AOB community, which was neither influenced by mycorrhizal colonization of roots nor by N and P addition to the soil. Real-time TaqMan polymerase chain reaction (PCR) was used to quantify AOB population sizes and showed no difference between any of the treatments. An alternative real-time PCR protocol for quantification of AOB utilizing SYBR green yielded similar results as the TaqMan real-time PCR method, although with slightly lower resolution. This alternative method is advantageous in not requiring the detailed background information about AOB community composition required for adaptation of the TaqMan system for a new soil.