Modeling long-term crop response to fertilizer and soil nitrogen

A simple nitrogen balance model to calculate long-term changes in soil organic nitrogen, nitrogen uptake by the crop and recovery of applied nitrogen, is presented. It functions with time intervals of one year or one growing season. In the model a labile and a stable pool of soil organic nitrogen are distinguished. Transfer coefficients for the various inputs of nitrogen are established that specify the fractions taken up by the crop, lost from the system, and incorporated in soil organic nitrogen. It is shown how input data, model parameters and initial pool sizes can be derived and how the model can be used for calculating long-term changes in total soil organic nitrogen and uptake by the crop. For nitrogen applied annually as fertilizer or organic material the time course of nitrogen uptake and recovery of applied nitrogen is calculated. To test the sensitivity of the model, calculations have been performed for different environmental conditions with higher or lower risks for losses. The model has also been applied to establish fertilizer recommendations for a certain target nitrogen uptake by the crop. Finally, for agricultural systems where periods of cropping alternate with peroids of green fallow the time course of nitrogen uptake by the crop is calculated.     

Root-induced increases in soil pH and nutrient availability to field-grown cereals and legumes on acid sandy soils of Sudano-Sahelian West Africa

A field experiment with millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata L.) and groundnut (Arachnis hypogeae L.) was conducted on severely P-deficient acid sandy soils of Niger, Mali and Burkina Faso to measure changes in pH and nutrient availability as affected by distance from the root surface and by mineral fertiliser application. Treatments included three rates of phosphorus (P) and four levels of nitrogen (N) application. Bulk, rhizosphere and rhizoplane soils were sampled at 35, 45 and 75 DAS in 1997 and at 55 and 65 DAS in 1998. Regardless of the cropping system and level of mineral fertiliser applied, soil pH consistently increased between 0.7 and two units from the bulk soil to the rhizoplane of millet. Similar pH gradients were observed in cowpea, but pH changes were much smaller in sorghum with a difference of only 0.3 units. Shifts in pH led to large increases in nutrient availability close to the roots. Compared with the bulk soil, available P in the rhizoplane was between 190 and 270% higher for P-Bray and between 360 and 600% higher for P-water. Exchangeable calcium (Ca) and magnesium (Mg) levels were also higher in the millet rhizoplane than in the bulk soil, whereas exchangeable aluminium (Al) levels decreased with increasing pH close to the root surface. The results suggest an important role of root-induced pH increases for crops to cope with acidity-induced nutrient deficiency and Al stress of soils in the Sudano-Sahelian zone of West Africa. This revised version was published online in June 2006 with corrections to the Cover Date.     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Observation of soil grown roots of rye-grass shows that an approximately cylindrical volume of soil, the root hair cylinder, is densely occupied by root hairs. Estimates are given of the concentration of labile and solution potassium within the root hair cylinder during experiments measuring potassium uptake from two soils by single roots. Calculations, using a diffusion model, suggest that labile potassium concentrations may be reduced to between 99.3 and 53 per cent of the initial, depending on the diffusion characteristics of the soil and nutrient demand by the root. Of the total potassium absorbed by a root in 4 days, the proportion which is supplied from within the root hair cylinder is small (0.8 to 6.3 per cent) indicating that diffusion to the root from the soil outside the root hair cylinder is of paramount importance. When root demand is high, diffusion appears to limit uptake to between 71 and 59 per cent of that which roots of comparable physiology would be expected to absorb from stirred solution of the same concentration. Nevertheless, the presence of root hairs is calculated to have enhanced uptake by up to 77 per cent compared with roots without hairs because they virtually increase the root diameter. Diffusion does not appear to be a limiting factor when root demand is low and hairs can then add little to the efficiency of the root system in potassium absorption.     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Measurements were made of the diffusion of P³²-labelled phosphate to single roots of onion, leek and rye-grass growing in an Upper Greensand sandy loam (UGS) and a Coral Rag Clay (CRC) to which different amounts of phosphate had been added. Concentration-dependent diffusion coefficients for phosphate ions in the soils were calculated from phosphate desorption isotherms in calcium chloride. The experimental uptake by roots of known dimensions was compared with supply expected by diffusion to a cylindrical model root of the same dimensions. Allowance was made for absorption by the root hairs on rye-grass roots. Phosphate absorption by a cm length of intact root was found to continue for at least 16 days for onion, 10 days for leek and 5 days for rye-grass. Over a wide range of conditions (phosphate concentrations, soils, plant species), experimental uptake was close to the maximum calculated to be possible for the diffusion model except on one soil at a high level of phosphate. Although the concentration of phosphate in the soil solution at the root boundary appeared to be reduced to a small fraction of the initial concentration, because of the extreme non-linear form of the desorption isotherm less than 1/2 of the P³² exchangeable pool of P was considered to contribute to diffusion. Phosphate uptake by rye grass could only be accounted for if the root hairs were active. Although only a small fraction of the uptake is derived from inside the root hair cylinder, this increases the efficiency of the central root 2.3 fold by providing a zone close to the central root through which phosphate moves very readily.     

Nutrient accumulation and distribution of wheat genotypes in response to waterlogging and nutrient supply

The effect of soil waterlogging and nutrient supply on plant nutrient accumulation and distribution was investigated for two genotypes of winter wheat (Triticum aestivum L.) differing in waterlogging resistance, Bayles and Savannah. Plants were grown in waterlogged or drained sand and fertilized with half-strength or full-strength Hoagland's solution.Waterlogging reduced the concentrations of N, P, K, Mg, and Zn in leaves and stems and increased the concentrations of those elements in the root system. The effects were greater for waterlogging-sensitive Bayles than for waterlogging-resistant Savannah. Higher concentrations of Fe and Mn were found in waterlogged plants compared to the control plants for sensitive Bayles. Waterlogging increased the proportion of N and Zn in the root system and decreased that of K in stems for Bayles. The proportion of Fe increased in leaves and stems for Bayles and Savannah under waterlogged conditions, but to a greater extent for Bayles. Doubling the concentrations of all major and minor nutrient elements supplied to the waterlogged rooting medium improved plant nutrient status and enhanced plant dry matter production.     

Spatial heterogeneity in soil nutrient supply modulates nutrient and biomass responses to multiple global change drivers in model grassland communities

Changes in the atmospheric concentration of carbon dioxide ([CO₂]), nutrient availability and biotic diversity are three major drivers of the ongoing global change impacting terrestrial ecosystems worldwide. While it is well established that soil nutrient heterogeneity exerts a strong influence on the development of plant individuals and communities, it is virtually unknown how nutrient heterogeneity and global change drivers interact to affect plant performance and ecosystem functioning. We conducted a microcosm experiment to evaluate the effect of simultaneous changes in [CO₂], nutrient heterogeneity (NH), nutrient availability (NA) and species evenness on the biomass and nutrient uptake patterns of assemblages formed by Lolium perenne, Plantago lanceolata and Holcus lanatus. When the nutrients were heterogeneously supplied, assemblages exhibited precise root foraging patterns, and had higher above‐ and belowground biomass (average increases of 32% and 29% for above‐ and belowground biomass, respectively). Nutrient heterogeneity also modulated the effects of NA on biomass production, complementarity in nitrogen uptake and below: aboveground ratio, as well as those of [CO₂] on the nutrient use efficiency at the assemblage level. Our results show that nutrient heterogeneity has the potential to influence the response of plant assemblages to simultaneous changes in [CO₂], nutrient availability and biotic diversity, and suggest that it is an important environmental factor to interpret and assess plant assemblage responses to global change.     

Tissue nutrient concentrations in freshwater aquatic macrophytes: high inter-taxon differences and low phenotypic response to nutrient supply

1. The elemental composition and stoichiometry of aquatic plants has often been suggested to reflect the nutrient enrichment of aquatic habitats. However, the relationship is often weak. Moreover, uncertainties remain in the relevance of laboratory derived critical plant tissue nutrient concentrations to maximum yield or growth rates in the field.
2. Aquatic vascular plants and bryophytes, overlying water and sediment samples were collected to test whether freshwater aquatic macrophytes: (i) show tissue nutrient deficiencies when growing in oligotrophic freshwater habitats, and (ii) have strict homeostatic stoichiometry.
3. Plant nutrient concentrations were significantly related to total inorganic nitrogen (or nitrate), total dissolved phosphorus and sediment total phosphorus. However, these relationships were weak. Virtually all the variance in plant tissue nutrient concentrations, however, could be explained by species (taxon) identity.
4. Critical tissue nutrient concentrations for 95% maximum yield or 95% maximum growth rate in aquatic angiosperms, determined from laboratory bioassays, suggested that nutrients should not limit yield in wild aquatic macrophytes. However, there were a substantial number of samples where potential growth rate limitation was possible, particularly due to phosphorus.
5. Strict C : N : P stoichiometric ratios were found for both vascular plants and bryophytes, suggesting little scope for plants as indicators of nutrient enrichment, but provide robust stoichiometric data for studies on ecosystem metabolism and nutrient cycling.     

Meta-analysis of maize yield response to woody and herbaceous legumes in sub-Saharan Africa

A number of studies have tested the effect of woody and herbaceous legumes on soil fertility and maize yields in sub-Saharan Africa. However, their effects on maize productivity are much debated because results have been variable. A meta-analysis was conducted with the aim of evaluating the evidence in support of yield benefits from woody and herbaceous green manure legumes. A total of 94 peer-reviewed publications from West, East and southern Africa qualified for inclusion in the analysis. Maize yield from herbaceous green manure legumes (54 publications), non-coppicing legumes (48 publications), coppicing woody legumes (10 publications), natural fallows (29 publications), and fully fertilized monoculture maize (52 publications) were compared. Mixed linear modelling using yield differences (D) and response ratios (RR) indicated that the response to legumes is positive. The mean yield increase (D) over unfertilized maize was highest (2.3 t ha^?1) and least variable (CV?=?70%) in fully fertilized maize, while it was lowest (0.3 t ha^?1) and most variable (CV?=?229%) in natural fallows. The increase in yield over unfertilized maize was 1.6 t ha^?1 with coppicing woody legumes, 1.3 t ha^?1 with non-coppicing woody legumes and 0.8 t ha^-1 with herbaceous green manure legumes. Doubling and tripling of yields relative to the control (RR > 2) was recorded in coppicing species (67% of the cases), non-coppicing legumes (45% of the cases), herbaceous green manure legumes (16% of the cases) and natural fallows (19% of the cases). However, doubling or tripling of yields occurred only in low and medium potential sites. Amending post-fallow plots with 50% of the recommended fertilizer dose further increased yields by over 25% indicating that legume rotations may play an important role in reducing fertilizer requirements. Except with the natural fallow, the 95% confidence intervals of D and RR were higher than 1 and 0, respectively indicating significant and positive response to treatments. Therefore, it is concluded that the global maize yield response to legumes is significantly positive and higher than unfertilized maize and natural vegetation fallows.     

Effects of soil density on yield and fertilizer requirement of sugar beet

Three experiments (1971–3) on medium-textured soils of low organic-matter content examined the effects of soil density on seedling emergence and yield of sugar beet, and the interactions between soil density and the requirement for nitrogen (N) and phosphate (P₂O₅) fertilizers. Three soil densities were produced by rolling and harrowing, dutch harrowing twice, and power harrowing once; each was tested with 75, 150 and 225 kg ha^-1 N and 0, 100 and 200 kg ha^-1 P₂O₅. Most seedlings emerged on the medium-density treatment produced by dutch harrowing twice. The crop in the least dense soil treatment, produced by power harrowing once, consistently yielded slightly more sugar than the medium-density treatment, which yielded significantly more than the most dense treatment. In dense soil, created by rolling and harrowing, there was a response to more fertilizer N than the less dense ones; the crop did not respond to phosphate fertilizer. Mechanical impedance, not toxic concentrations of ethylene in the soil atmosphere, appeared to be the cause of reduced yield on the dense soils; these reductions were considerable at dry soil bulk densities in excess of 1·5 g ml^-1.     

Release of nutrient elements from soil as affected by fertilizer and moisture treatments

Summary Maximum amounts of Fe and Mn were released from a clay soil after 6 weeks incubation while maximum release of NH₄-N, NO₃-N, P and K occurred after 9 weeks. The interaction of potassium chloride and waterlogging resulted in the highest release of nutrient ions from the soil.     

A method to optimize N-application in relation to soil supply of N,and yield of potato

Mathematical models of crop growth can provide estimates of the potential yield of potato, and also the minimum, critical N-concentration required, [N_c], to attain that yield. Efficient use of nitrogen requires that the crop incorporates sufficient nitrogen to attain its potential yield and that excess uptake is avoided. Predictions of the rate of supply of nitrogen from the soil are imprecise and so it has been difficult to estimate accurately the required application of fertilizer-N. Our work has shown the feasibility of using the growing crop as a monitor of the rate of supply of N from the soil. Using a low initial application rate of N at planting and monitoring uptake rate, we can estimate the contribution from the soil, and couple that information with estimates of yield and the related [N_c] to give an estimate of the requirement for supplementary applied-N. The method can be seen, therefore, as a means to determine the size of a second or subsequent part of a split application of fertilizer. This approach avoids much of the uncertainty over the fate of applied nitrogen and should offer growers the double benefits of economic use of fertilizer and of minimizing leaching losses. Further, by tailoring applications of N-fertilizer to the crop's requirements the grower will be better able to ensure the quality considerations in his crop.     

Cotton root and shoot response to localized supply of nitrate,phosphate and potassium: Split-pot studies with nutrient solution and vermiculitic soil

Vertical stratification of plant-available K in vermiculitic soil profiles contributes to a late-season K deficiency that limits cotton (Gossypium hirsutum L.) yields on affected soils. Split-root solution culture and split-pot soil experiments were conducted to determine whether root distribution and cultivar differences in root extension in these stratified profiles result from a compensatory response to localized enrichment with NO₃-N, PO₄-P, and/or K in the root zone. Compensatory root growth was greatest in response to localized NO₃-N enrichment. For two cultivars examined in solution culture, 74% of new root development occurred in the half-pot providing 90% of the total NO₃-N supply. Only 60% of cultivar root development occurred in the half-pot providing 90% of the PO₄-P. No compensatory root growth was observed in response to localized K enrichment. In the split-pot system, the proportion of total root surface area developing in a half-pot was highly correlated with localized soil NO₃-N levels (r²=0.81), while increased K availability in one half of the root zone did not affect root distribution. Mean soil NO₃-N supply to the whole root system determined shoot N accumulation (r²=0.97). Shoot K accumulation was not related to soil K availability but was strongly correlated with mean root surface area density (r²=0.86). Cultivar Acala GC510, known to be less sensitive to K deficiency than Acala SJ-2, had significantly larger root diameter in all nutrient-supply environments. Under conditions of K stress, Acala GC510 had increased root branching and allocated greater dry matter to roots relative to shoots than Acala SJ-2. The results demonstrate that K acquisition by cotton is strongly influenced by the quantity and distribution of NO₃-N in the root zone through its effects on root proliferation, and that distinct cultivar differences associated with crop performance on low K soils can be detected in short-term, solution culture growth systems.     

Adaptations and biomass production of two grasses in response to waterlogging and soil nutrient enrichment

We analysed the response of two grass species, Danthonia montevidensis and Paspalum dilatatum to waterlogging, soil-nutrient enrichment and the combination of both factors. Waterlogging did not affect total biomass of D. montevidensis, but it slightly promoted growth of P. dilatatum. Most analysed variables showed no significant interaction between fertilization and waterlogging. Therefore, waterlogging does not produce a detrimental effect either in the growth of these species or in their response capacity to stimulating growth factors, such as fertilization.     

Spatial and temporal heterogeneity in soil nutrient supply measured usingin situ ion-exchange resin bags

Summary Data are presented which illustrate the range of ion values obtained from soil solutions eluted fromin situ ion exchange resin bags in grazed and ungrazed grassland soils sampled in the summer and early autumn. Overall, higher levels of cations were being supplied in both the grazed and ungrazed plots in the autumn compared with during the summer. Variation in ion levels reflected spatial heterogeneity in ion supply in these soils. This variation was correlated with the distribution and abundance of the dominant plants and soil surface microtopography. The use ofin situ ion exchange resin bags allow an understanding of short-term temporal and spatial heterogeneity in ion supply.     

Enhancement of artemisinin concentration and yield in response to optimization of nitrogen and potassium supply to Artemisia annua

Background and Aims

The resurgence of malaria, particularly in the developing world, is considerable and exacerbated by the development of single-gene multi-drug resistances to chemicals such as chloroquinone. Drug therapies, as recommended by the World Health Organization, now include the use of antimalarial compounds derived from Artemisia annua – in particular, the use of artemisinin-based ingredients. Despite our limited knowledge of its mode of action or biosynthesis there is a need to secure a supply and enhance yields of artemisinin. The present study aims to determine how plant biomass can be enhanced while maximizing artemisinin concentration by understanding the plant''s nutritional requirements for nitrogen and potassium.

Methods

Experiments were carried out, the first with differing concentrations of nitrogen, at 6, 31, 56, 106, 206 or 306 mg L⁻¹ being applied, while the other differing in potassium concentration (51, 153 or 301 mg L⁻¹). Nutrients were supplied in irrigation water to plants in pots and after a growth period biomass production and leaf artemisinin concentration were measured. These data were used to determine optimal nutrient requirements for artemisinin yield.

Key Results

Nitrogen nutrition enhanced plant nitrogen concentration and biomass production successively up to 106 mg N L⁻¹ for biomass and 206 mg N L⁻¹ for leaf nitrogen; further increases in nitrogen had no influence. Artemisinin concentration in dried leaf material, measured by HPLC mass spectroscopy, was maximal at a nitrogen application of 106 mg L⁻¹, but declined at higher concentrations. Increasing potassium application from 51 to 153 mg L⁻¹ increased total plant biomass, but not at higher applications. Potassium application enhanced leaf potassium concentration, but there was no effect on leaf artemisinin concentration or leaf artemisinin yield.

Conclusions

Artemisinin concentration declined beyond an optimal point with increasing plant nitrogen concentration. Maximization of artemisinin yield (amount per plant) requires optimization of plant biomass via control of nitrogen nutrition.Key words: Artemisia, fertigation, malaria, nitrogen, nutrition, potassium