Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N₂ fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with ¹⁵NH₄ ¹⁵NO₃ to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.     

Soil pH changes associated with lupin and wheat plant materials incorporated in a red–brown earth soil

Both alkalization and acidification of soil occurred when shoot and root materials from lupin and wheat were incubated in a red–brown earth soil, but with three different starting pH values, during a 70-day period. The response of soil pH change to the addition of organic matter depended on the type of plant materials and starting pH. The net effect of addition of lupin and wheat shoots to acid soils (pH<5) caused soil pH to increase, the addition of lupin roots to soils caused soil pH to decrease slightly, whilst with a higher pH soil (6.5) the wheat straw and lupin shoots raised pH and pH was unchanged for soil with addition of lupin roots. The ash alkalinity of plant materials and the mineralization of organic N are major reasons for the soil pH increase, and the nitrification of mineralized N results in soil pH to decrease. Whilst the data given here would suggest the likelihood of soil acidification occurring, particularly on poorly buffered soil given the inevitable influence of legume root materials, the overall directions of soil pH change in a cropping system that is legume-based will be very much influenced by the balance of many factors associated with the soil and plant system.     

Root exudates: a pathway for short-term N transfer from clover and ryegrass

The short-term transfer of nitrogen (N) from legumes to grasses was investigated in two laboratory studies. One study was done in pots where the roots of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were allowed to co-exist, and a second study was performed using a micro-lysimeter system designed to maintain nutrient flow from the clover to the grass, whilst removing direct contact between the root systems. The ¹⁵N-dilution technique was used to quantify the transfer of N between species. Levels of ammonia and amino acids were measured in root exudates. The amounts of N transferred were in the same order of magnitude in both the pot and micro-lysimeter experiments. In the micro-lysimeter experiment, 0.076 mg of N were transferred per plant from clover to ryegrass during the course of the experiment. Ammonium exudation was much higher than amino acid exudation. The most abundant amino acids in both clover and ryegrass root exudates were serine and glycine. However, there was no correlation between the free amino acid profile of root extracts and exudates for both plant species: Asparagine was the major amino acid in clover roots, while glutamine, glutamate and aspartate were the major amino acids in ryegrass roots. Comparison of exudates obtained from plants grown in non-sterile or axenic conditions provides evidence of plant origin of ammonium, serine and glycine.     

Absorption and Distribution of Copper in Plants with Sufficient or Deficient Supplies

Wheat, red clover and ryegrass were grown in flowing solutionculture with sufficient (+ Cu) and deficient (–Cu) suppliesof copper. The rates of Cu absorption (µg g^–1 dryroot day^–1) did not differ greatly between species ineither treatment. Wheat plants, when transferred from the –Cu to the +Cu treatment, absorbed Cu at a much slower rate thanthose which had remained throughout in the + Cu treatment. Inall plants considerable proportions of the absorbed Cu wereretained in the roots, even when the plants were Cu-deficient,and the concentration in roots usually exceeded that in anypart of the shoots in both treatments. Transferring wheat plantsfrom the +Cu to the –Cu treatment decreased the concentrationin all plant parts except old leaves; similarly, transferringfrom the –Cu to +Cu treatment increased the concentrationin all parts of the shoots, execept old leaves, and in the roots. Lolium perenne, Trifolium pratense, Triticum aestivum, ryegrass, red clover, wheat, absorption, copper, flowing solution culture     

Uptake by wheat plants and turnover within soil fractions of residual N from leguminous plant material and inorganic fertilizer

Summary The availability and turnover in different soil fractions of residual N from leguminous plant material and inorganic fertilizer was studied in a pot culture experiment using wheat as a test crop. Plants utilized 64% of the residual fertilizer N and 20% of the residual legume N. 50–60% of the N taken up by plants was recovered in grain and 4–8% in roots. After harvesting wheat up to 35% and 38% of the residual legume N and fertilizer N, respectively was found in humic compounds. A loss of humus N derived from legume and fertilizer was found during wheat growth but the unlabelled N increased in this fraction. Biomass contained 6% and 8% of the residual legume and fertilizer N, respectively when both were available. The mineralizable component contained upto 28% of both the residual legume and residual fertilizer N. Only a small percentage of the soil N (3–4%) was observed in biomass whereas the mineralizable component accounted for 7–14% of the soil N. In this fraction legume derived N increased during wheat growth whereas unlabelled N increased in both the mineralizable component and microbial biomass. Some loss of N occurred from residual legume and fertilizer N. Nevertheless, a positive total N balance was observed and was attributed to the addition of unlabelled N in the soil-plant system by N₂ fixation. The gain in N was equivalent to about 38% of the plant available N in the soil amended with leguminous material. The additional N was concentrated mainly in the mineralizable fraction and microbial biomass, although some addition was also noted in humus fractions.     

Nitrogen-limitation and invasive sweetclover impacts vary between two Great Plains plant communities

Yellow sweetclover is an exotic herbaceous legume common in the Great Plains of the US. Although woody legumes have been shown to affect ecosystem processes through nitrogen (N) fixation (i.e., they can be considered “transformers” sensu Richardson et al. (2000)), the same has not been shown for short-lived herbaceous species. The objectives of this study were to (1) quantify the effects of yellow sweetclover on N mineralization and nitrification and (2) assess the effects of N fertilization on two plant communities, badlands sparse vegetation and western wheatgrass prairie. We used in situ (in wheatgrass prairie) and laboratory incubations (for both plant communities) to assess N dynamics at sites with high and low sweetclover cover in the two plant communities. We found that both N mineralization and nitrification were higher in the high sweetclover plots in the sparse plant community, but not in the wheatgrass prairie. To assess fertilization effects and determine if nutrients or water were limiting at our sites, we conducted a field experiment with five resource addition treatments, (1) N, (2) N + water, (3) water, (4) phosphorus, and (5) no addition. Water was limiting in the wheatgrass prairie but contrary to expectation, N was not. In contrast, N was limiting in the sparse community, where a fertilization effect was seen in exotic forbs, especially the toxic invader Halogeton glomeratus. Our results emphasize the contingent nature of plant invasion in which effects are largely dependent on attributes of the recipient vegetation.     

Forage production, N uptake, N2 fixation, and N recovery of berseem clover grown in pure stand and in mixture with annual ryegrass under different managements

In Mediterranean countries, forage grasses and legumes are commonly grown in mixture because of their ability to increase herbage yield and quality compared with monocrop systems. However, the benefits of intercropping over a monocrop system are not always realized because the efficiency of a grass–legume mixture is strongly affected by agronomic factors. The present study evaluated productivity, N₂ fixation, N transfer, and N recovery of berseem clover (Trifolium alexandrinum) grown in pure stand and in mixture with annual ryegrass (Lolium multiflorum) under high or low defoliation frequencies and varying plant arrangements (sowing in the same row or in alternating rows). On average, the berseem–ryegrass mixtures resulted in a greater yield and N yield than the monocrops. When mixed together, ryegrass was more efficient than berseem at absorbing soil N, increasing the reliance of berseem on N₂ fixation. Both defoliation management and plant arrangement affected forage yield and the quality of the mixture, modifying the proportion of the two components, the N content of the forage, and the symbiotic N₂ fixation of the legume. Reducing the proximity between plants of the two species may benefit the weaker component of the mixture. No apparent transfer of fixed N from berseem to ryegrass was detected.     

Indigenous legumes biomass quality and influence on C and N mineralization under indigenous legume fallow systems

Non-cultivated N₂-fixing indigenous legumes can be harnessed to enhance soil fertility replenishment of smallholder farms. Understanding N release patterns of biomass generated by such legumes is key in managing N availability to crops. Nitrogen and C mineralization patterns of indigenous legume species, mainly ofTephrosia andCrotalaria genera, and of soils sampled at termination of 1- and 2-year indigenous legume fallows (indifallows)were investigated in leaching tube incubations under laboratory conditions. With the exception ofTephrosia longipes Meisn (2.4%) andCrotalaria cylindrostachys Welw.ex Baker (1.8%), all indigenous legumes had >2.5% N. Total polyphenols and lignin were <4% and 15%, respectively, for all species.Crotalaria pallida (L.) andEriosema ellipticum Welw.ex Baker mineralized >50% of the added N in the first 30 days of incubation. Similar to mixed plant biomass from natural weed fallow,C. Cylindrostachys immobilized N during the 155-day incubation period. Indifallow fallow biomass reached peak N mineralization 55 days after most legumes had leveled off. Carbon release by legume species closely followedN release patterns,with mostCrotalaria species releasing >500 mg CO₂-C kg^?1 soil. Soils sampled at termination of fallows reached peak N mineralization in the first 21 days of incubation, with indifallows mineralizing significantly (P<0.05) more N than natural fallows. Application of mineral P fertilizer to indifallows and natural fallows increased C and N mineralization relative to control treatments. It was concluded that (i) indigenous legumes generate biomass of high quality within a single growing season, (ii) the slow N release of biomass generated under indifallow systems suggests that such fallows can potentially be manipulated to enhance N availability to crops, and (iii) N and C mineralization of organic materials in sandy soils is likely controlled by availability of P to the soil microbial pool.     

Legumes regulate grassland soil N cycling and its response to variation in species diversity and N supply but not CO2

Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17‐year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO₂ and ambient and enriched (+4 g N m^?2 year^?1) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four‐species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four‐species plots containing legumes compared to legume‐free four‐species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N‐fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO₂ nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

Effects of earthworms on biomass production,nitrogen allocation and nitrogen transfer in wheat-clover intercropping model systems

The effects of earthworms (Lumbricidae) on plant biomass production and N allocation in model intercropping systems of winter wheat and white clover were evaluated in two pot experiments. Wheat and wheat-clover mixtures were grown in a low-organic loam soil, earthworms were added at densities comparable to field population densities and the experiments were terminated 48 and 17 d after earthworm introductions. In both experiments, earthworms significantly increased the biomass and N uptake of wheat while they had generally no effects on clover. As a result, earthworm activity increased the proportion of wheat biomass in the total plant biomass of the mixture. Nitrogen budgets of the experiment lasting 48 d indicated that additional N in the system made available by earthworm activity was primarily taken up by the wheat. Earthworms also affected intra-plant N allocation in wheat which had significantly higher shoot:root N ratios when earthworms were present. When clover was labelled with ¹⁵N in the experiment which lasted 17 d, endogeic earthworms significantly reduced the amounts of ¹⁵N excess transferred from living or decomposing clover roots to accompanying wheat plants. Earthworms assimilated small quantities of ¹⁵N tracer from decomposing clover roots but not from living clover roots. The results of these model experiments suggest that earthworms can affect the balance between intercropped cereals and legumes by altering intra- and inter-plant N allocation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Potential to improve transfer of N in intercropped systems by optimising host-endophyte combinations

Possibilities for improving N transfer from N2-fixing plants to non-N2-fixing plants by mycorrhiza have been investigated. Initially, the genetic variability with respect to N uptake was assessed by screening five varieties of chicory (Cichorium intybus L.), four of peas (Pisum sativum L.) and three of red clover (Trifolium pratense L.) in combination with eight isolates of arbuscular mycorrhizal fungi. The most promising plant - fungi combinations identified through the cultivar screening were used to optimise conditions for N transfer between intercropped N2-fixing plants (peas and clover) and non-N2-fixing chicory. In the first experiment, the recovery of fixed legume N was investigated using three cultivars, of chicory intercropped with pea variety, and inoculated with one of four mycorrhizal isolates. Roots of the N2-fixing pea and the non-N2-fixing chicory were separated by a root-free soil layer in a three-compartment container. A section of the legume roots was forced to grow into a separate compartment which received four split applications of ¹⁵N. The percentage of N in the chicory derived from transfer ranged between 3% and 50%. In a second experiment one chicory variety was intercropped with one red clover variety and inoculated with four mycorrhizal isolates respecetively. A harvest regime was chosen in which the shoots were harvested from intercropped plants at 3,4.5 and 6 months of age. At three months the percentage of N in the chicory derived from transfer ranged between 15% and 18% and at a plant age of 4.5 months from 46 to 77%. At six months the percentage of N in the chicory roots derived from transfer of legume N ranged from 20 to 34% and varied with fungal isolate. Our results show that there is potential for improving N transfer in intercropped plant systems through the methodological selection of suitable plant and mycorrhizal partners.     

Impact of residue quality on the C and N mineralization of leaf and root residues of three agroforestry species

A laboratory incubation experiment with ¹⁵N labeled root and leaf residues of 3 agroforestry species (Leucaena leucocephala, Dactyladenia barteri and Flemingia macrophylla) was conducted under controlled conditions (25 C) for 56 days to quantify residue C and N mineralization and its relationship with residue quality.No uniform relation was found between the chemical composition of the above and below residues. The leucaena and dactyladenia roots contained more lignin (8 and 26% respectively) and less N (2.0 and 1.0% respectively) than the respective leaves (2 and 13% lignin and 2.9 and 1.4% N, respectively), whereas the differences between the lignin and N contents of the flemingia leaves and roots were not significant (4.6 and 3.0% lignin and 2.63 and 2.68% N, respectively). The leucaena leaves contained more polyphenols than the roots (6.4 and 3.6%), while the polyphenol content of the leaves and roots of the other residues was similar (5.0 and 5.1% for dactyladenia and 4.0 and 3.5% for flemingia).Three patterns of N mineralization could be distinguished. A first pattern, followed by residues producing the highest amounts of CO₂, showed an initial immobilization of soil derived N, followed by a net release of both soil and residue derived N after 7 days of incubation. A second pattern, followed by the flemingia leaf residues which produced intermediate amounts of CO₂ and had an intermediate quality, showed no significant immobilization of soil derived N, and significant mineralization of residue N. A third pattern, followed by both low quality dactyladenia residues, showed a low release of residue derived N and a continued inmobilization of soil derived N.Residue C mineralization was significantly (p<0.05) correlated with the residue lignin content, C-to-N ratio, and polyphenol-to-N ratio. The proportion of residue N mineralized (immobilized) after 56 days of incubation was significantly correlated with the residue N content (p<0.01) and the C-to-N ratio (p<0.05). The relations were quadratic, rather than linear. The ratio of the proportion of residue N mineralized (immobilized) over the proportion of residue C mineralized after 56 days was highly significantly correlated with the lignin content (p<0.01) and C-to-N (p<0.001), lignin-to-N (p<0.01), polyphenol-to-N (p<0.01) and (lignin+polyphenol)-to-N ratios (p<0.01) in a linear way. This indicates that due to the low availability of the residue C, relatively less N is immobilized for the very low quality residues ((lignin+polyphenol)-to-N ratio: 29.7) than for the residues with a relatively higher quality ((lignin+polyphenol)-to-N ratios between 3.3 and 12.5).     

The effect of drought on C and N stable isotopes in different fractions of leaves, stems and roots of sensitive and tolerant beech ecotypes

Beech seedlings from 11 German climatic provenances were exposed to a realistically timed drought treatment in a greenhouse experiment. The stable isotope composition of carbon (C) and nitrogen (N) was analysed in pooled bulk material of roots, stems and leaves, as well as in the aqueous extracts and starch fractions. The delta 13C values increased in bulk samples (BS) of roots, stems and leaves by drought, although no leaf growth occurred during the experimental period. A clear drought effect on delta 13C in aqueous extracts was detected in leaves. In aqueous extracts of stems and roots as well as in starch fractions of all organs, abundance of delta 13C also tended to be increased by drought, but this effect was not statistically significant. For both delta 13C and delta 15N, enrichment was observed from the site of uptake/ source to the site of use/sink. A gradient for delta 13C in all fractions from leaves (-29.49, -28.89 and -27.85 per thousand) to stems (-28.81, -27.48 and -26.98 per thousand) and to roots (-27.60, -26.37 and -26.48 per thousand) was detected in BS, aqueous extracts and starch, respectively. An opposite gradient for delta 15N was found in BS: 1.59 per thousand, 1.84 per thousand and 3.05 per thousand in roots, stems and leaves, respectively. delta 15N was neither affected by drought in the BS nor in aqueous extracts, but an effect of provenance was observed. Particularly in roots and stems, drought-sensitive provenances showed the strongest shifts in delta 13C induced by drought and the lowest delta 15N values. In the present experiment, delta 13C values were more affected by the environmental factor drought, while delta 15N values were more affected by the genetic factor provenance.     

Plant effects on soil N mineralization are mediated by the composition of multiple soil organic fractions

Despite the topic of soil nitrogen (N) mineralization being well-studied, very few studies have addressed the relative contribution of different plant and soil variables in influencing soil N mineralization rates, and thus the supply of inorganic N to plants. Here, we used data from a well-studied N-limited grassland to address the relative effects of six plant and soil variables on net and on gross rates of soil N mineralization. We also addressed whether plant effects on soil N mineralization were mediated by changes in C and N concentrations of multiple soil organic matter (SOM) fractions. Regression analyses show that key plant traits (i.e., plant C:N ratios and total root mass) were more important than total C and N concentrations of bulk soil in influencing N mineralization. This was mainly because plant traits influenced the C and N concentration (and C:N ratios) of different SOM fractions, which in turn were significantly associated with changes in net and gross N mineralization. In particular, C:N ratios of a labile soil fraction were negatively related to net soil N mineralization rates, whereas total soil C and N concentrations of more recalcitrant fractions were positively related to gross N mineralization. Our study suggests that changes in belowground N-cycling can be better predicted by simultaneously addressing how plant C:N ratios and root mass affect the composition and distribution of different SOM pools in N-limited grassland systems.     

Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation,N fertilizer recovery,N transfer and water use efficiency

Seasonal variation in N₂ fixation, N transfer from clover to ryegrass, and soil N absorption in white clover–ryegrass swards were investigated under field conditions over three consecutive years. The plots were established with different seeding ratios of clover and ryegrass and contrasting fertilizer N ranging from 3 to 72 kg ha^-1 year^-1.An initially poor clover population needed at least one growing season to reach the same yield output as an initially well established clover population. The clover content of the sward decreased by the annual application of 72 kg N ha^-1 but not by smaller N dressings.The total amount of atmospherically derived N in clover growing in mixture with ryegrass was, on average over the three years equal to 83, 71, 68 and 60 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively. The proportion of atmospherically derived N declined with increasing N application, but never became smaller than 80% of total clover N. The proportion of atmospherically derived N in a pure stand white clover amounted to 60–80% of the total N content, equivalent to 109, 110, 103 and 90 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively.Only small amounts of atmospherically derived N was transferred to the associated ryegrass during the first production year, while in each of the following years up to 21 kg ha^-1 was transferred. The average amount of N transferred from clover to ryegrass was equivalent to 3, 16 and 31% of the N accumulated in ryegrass in the first, second and third production year, respectively. Expressed relative to the total amount of fixed N₂ in the clover–ryegrass mixture, the transfer amounted to 3, 17 and 22% in the first, second and third production year, respectively. Thus transfer of atmospherically derived N from clover contributed significantly to the N economy of the associated ryegrass.The clover–ryegrass mixture absorbed constantly higher amount of soil derived N than the pure stands of the two species. Only 11% of the total accumulated fertilizer N and soil derived N in the mixture was contained within the clover component. Lower water use efficiencies for the plants grown in mixture compared to pure stands were mainly related to the increased N uptake in the mixture, with the subsequent increase in growth compared to the pure stands.It is concluded that positive interactions between clover and ryegrass growing in mixture ensure a more efficient fixation of atmospheric N₂ and absorption of fertilizer N and soil derived N than pure stands of the same species.     

Soil restoration under pasture after topsoil removal: Some factors influencing C and N mineralization and measurements of microbial biomass

Factors influencing rates of C and N mineralization of soil and plant materials, and the reliability of different procedures for estimating microbial biomass, were measured in a soil (Typic Dystrochrept) that had been restored under grazed pasture in a temperate environment for 10–11 years after 20 cm of the original topsoil had been removed by stripping. Rates of net N mineralization were appreciably lower, but CO₂-C production higher, in the stripped than in the unstripped soil. These activities were not influenced directly by levels of soil mineral-N, but they were influenced by differences in plant composition. Herbage and litter, and roots, from the stripped plots were generally mineralized more readily to CO₂-C, but more slowly to net mineral-N, than were the corresponding materials from the unstripped plots. Rates of mineralization of herbage and litter, or roots, were mainly indistinguishable in stripped and unstripped soil, whereas rates of mineralization of all standing dead material were lower in stripped soil. Measurements of extractable-C flush, and of CO₂-C flush (using a fumigated soil control) and mineral-N flush by fumigation-incubation procedures, indicated that microbial biomass in stripped soil had recovered to at least 88 percent of the levels in unstripped soil. Substrate-induced respiration also generally indicated high levels of recovery of microbial biomass. The fumigation-incubation procedure appeared to under-estimate microbial biomass markedly in stripped soil when unfumigated soil controls were used; the used of a large soil inoculum (20 percent w/w) only sometimes overcame this problem. Possible reasons for apparent anomalies in estimation of microbial C are discussed.     

Competition between siratro and kleingrass for15N labelled mineralized nitrogen

Summary Isotope dilution provides a method for measuring plant competition for mineral N and transfer of biologically fixed N from a legume to a grass. A plant growth medium was enriched with¹⁵N, and used to grow Siratro (Macropitilium atropurpureum D.C. Urb.) and Kleingrass 75 (Panicum coloratum L.) in 20 liter pots for 98 days in a glasshouse. The plants were grown in pure stand and in mixtures. When grown in 50∶50 mixture the grass obtained 59% of the labelled N and the legume obtained 41%. The grass produced nearly as much root mass as the legume even though biomass of the shoots were less than half that of the legume. Reducing the proportion of either plant species in the mixture reduced the proportion of the mineralized N absorbed by that species. The shoots of the grass were significantly more enriched (1.166 atom%¹⁵N excess) than the roots (1.036). The grass received 12% of its N as biologically fixed N from the legume.     

Soil nitrogen heterogeneity in a Dehesa ecosystem

The C mineralization and N transformations during the decomposition of sunflower stalks (Helianthus annuus L.) and wheat straw (Triticum aestivum L.) with and without addition of (NH₄)₂SO₄ (27.53 atom% ¹⁵N) were studied in a Vertisol. Soil samples were incubated under aerobic conditions for 224 days at 22 °C. The plant residues were added at a rate of 5.2 g kg^-1 soil. Nitrogen was applied at a rate of 50.7 mg N kg^-1 soil. Carbon dioxide emission and inorganic N content in soil were periodically determined. Gross N immobilization and remineralization were calculated on the basis of the isotopic dilution technique. At the end of the incubation period a ¹⁵N balance was established. Respectively, 68 and 45% of the applied residue-C mineralized from the sunflower stalks and wheat straw after 224 days. Both crop residues caused losses of up to 25% of added ¹⁵N after 224 days of incubation. These ¹⁵N losses were about three times larger than in the control soil, and were probably due to denitrification. The net immobilization of soil derived N following residue incorporation was largest in the case of wheat straw, depleting all soil inorganic N. In the wheat straw treatment with added (NH₄)₂SO₄ soil inorganic N remained available, resulting in an enhanced initial C mineralization and N immobilization compared to the treatment without added N. In the case of the sunflower stalks, the high inorganic N content of the stalks suppressed the effects of N addition on C mineralization and N immobilization/mineralization. Gross N immobilization amounted to 31.9 and 28.2 mg N g^-1 added C after 14 days for wheat straw and sunflower stalks, respectively. At the end of the incubation, about 35% of the newly immobilized N was remineralized in both plant residue treatments. Gross N immobilization plotted against decomposed C suggests that fairly uniform C-N relationships exist during the decomposition of divers C substrates. The results demonstrate that low fertilizer N use efficiencies may be expected in a wheat-sunflower cropping system with incorporation of crop residues, as the fertilizer N applied becomes largely immobilized in the soil organic fraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.