Fate of N and C from labelled plant material: Recovery in perennial ryegrass–clover mixtures and in pore water of the sward

The belowground C and N dynamics leading to organic and inorganic N leaching from perennial ryegrass–clover mixtures are not well understood. Based on the hypothesis that four different plant materials would degrade differently, a 16 months field experiment was conducted to determine (i) the source strength of labelled plant residues in dissolved inorganic N (DIN) and dissolved organic N (DON) in pore water from the plough layer, and (ii) the plant uptake of organically bound N. Litterbags containing ¹⁴C- and ¹⁵N-labelled ryegrass or clover roots or leaves were inserted into the sward of a ryegrass–clover mixture in early spring. The fate of the released ¹⁴C and ¹⁵N was monitored in harvested biomass, roots, soil, and pore water percolating from the plough layer. No evidence of plant uptake of dual-labelled organic compounds from the dual-labelled residues could be observed. N in pore water from the plough layer during autumn and winter had a constant content of dissolved organic N (DON) and an increasing content of dissolved inorganic N (DIN). A positive correlation between aboveground clover biomass harvested in the growth season and total-N in pore water indicated that decaying roots from the living clover could be a major source of the 10 kg N ha⁻¹ being lost with pore water during autumn and winter. The presence of ¹⁵N in pore water shifted from the DON fraction in autumn to the DIN fraction in late winter, with strong indications that ¹⁵N originated from the living ryegrass. However, ¹⁵N in pore water originating from plant residues only constituted 1.5% of the total dissolved N from the plough layer.     

Patterns of natural N in soils and plants from chemically and organically fertilized uplands

Diagnostic tests for organic production of crops would be useful. In this study, the difference in natural ¹⁵N abundances (δ¹⁵N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using δ¹⁵N as a marker of organic produce. Twenty samples each of soils and plants were collected from each field in early summer after applying fertilizer or compost. The δ¹⁵N of fertilizers and composts was −1.6±1.5‰ (n=8) and 17.4±1.2‰ (n=10), respectively. The δ¹⁵N of total soil-N was significantly (P<0.05) higher in CU fields (8.8±2.0‰) than in FU fields (5.9±0.7‰) due to long-term continuous application of ¹⁵N-enriched compost, as indicated by a positive correlation (r=0.62) between N content and δ¹⁵N of total soil-N. The NO₃⁻ pool of CU soils (11.6±4.5‰) was also significantly (P<0.05) enriched in ¹⁵N compared to FU soils (4.7±1.1‰), while the ¹⁵N contents of NH₄⁺ pool were not different between both soils. Compost application resulted in ¹⁵N enrichment of plants; the δ¹⁵N values were 14.6±3.3‰ for CU and 4.1±1.7‰ for FU fields. These results showed that long-term application of compost resulted in a significant ¹⁵N-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that δ¹⁵N could serve as promising indicators of organic fertilizers application when used with other independent evidence. However, further studies under many conditions should be conducted to prepare reliable δ¹⁵N guidelines for organic produce, since the δ¹⁵N of inorganic soil-N and plant-N are influenced by various factors such as soil type, plant species, the rate of N application, and processes such as mineralization, nitrification, and denitrifcation.     

Assessing nitrogen transfer from legumes to associated grasses

A new method for directly assessing the transfer of N from legumes to associated grasses was developed and tested. Legume plants were labelled with ¹⁵N by foliar absorption and N transfer was estimated from the difference in ¹⁵N concentration between grasses associated with the labelled legumes and those from unlabelled control plots or pots. In a pot experiment, 2.2% of the N in subterranean clover (Trifolium subterraneum L.) was transferred to annual ryegrass (Lolium rigidum Gaud.) over a 29-day period whereas in a 36-day field study there was no measurable transfer of N from subterranean clover or lucerne (Medicago sativa L.) to ryegrass.     

Root size fractions of ryegrass and clover contribute differently to C and N inclusion in SOM

Grass-clover mixtures are essential in many low-N-input cropping systems, but the importance of various root fractions for the below-ground N dynamics are not well understood. This may be due to the difficulties of studying root longevity and turnover in situ in mixtures. The present field study, investigated (1) the development in root biomass over two growing seasons and (2) the turnover of dual ¹⁵N- and ¹⁴C-labelled ryegrass and white clover root material. Litter bags containing various dual-labelled plant materials were incubated in cylinders inserted in the topsoil of a young ryegrass-clover ley. Disappearance of ¹⁴C and ¹⁵N from the litter bag material were studied for 1 year following incubation. Four times during two growing seasons, roots were divided into two classes: large roots, retained on a 1-cm sieve, and small roots, passing a 1-cm sieve but retained on a 100-µm sieve. Large root biomass increased during the two growing seasons, and small root biomass increased during the growing seasons but decreased during autumn and winter. White clover roots lost ¹⁴C and ¹⁵N almost twice as fast as ryegrass roots. The disappearance pattern of ¹⁴C and ¹⁵N from dual-labelled ryegrass and white clover roots and the C and N contents of the recovered root material indicate that large roots are determining soil C pool build-up, whereas small roots determine soil N pool build-up.     

Effects of White Clover Inclusion on Turf Characteristics,Nitrogen Fixation,and Nitrogen Transfer from White Clover to Grass Species in Turf Mixtures

Abstract

An irrigated field trial was conducted to test the effects of white clover in three turfgrass species (perennial ryegrass, Kentucky bluegrass, and creeping bentgrass) on color, clipping yield, and botanical composition and to estimate nitrogen (N)₂ fixation and N transfer from white clover to associated turfgrass species under different N‐fertilization conditions in 1999–2002.

Nitrogen fertilizers significantly increased color ratings in all observations. Grass–white clover mixtures had better color ratings than pure grass at all sampling dates and seasonal averages in unfertilized conditions. Fertilized pure grass plots yielded significantly more than control plots in all turfgrass species. Nitrogen fertilization did not affect clipping yield greatly in turfgrass–white clover mixtures. Nitrogen application significantly decreased white clover percentage in the harvested clippings in second and third year.

Nitrogen fertilization increased tissue N concentration positively in all turfgrass species grown alone. In contrast, N fertilization did not greatly affect tissue N concentration of either turfgrass species or white clover in the mixtures. Nitrogen fixation of white clover was estimated as 24.6, 30.7, and 33.8 g m^?2 year^?1 in perennial ryegrass, Kentucky bluegrass, and creeping bentgrass, respectively. The total estimated N₂ fixation gradually decreased with increasing N fertilization. Nitrogen transfer from white clover to the associated turfgrass varied from 4.2 to 13.7% of the total N that the white clover fixed annually.     

Leaching of dissolved organic and inorganic nitrogen from legume-based grasslands

Leaching of dissolved inorganic N (DIN) and dissolved organic N (DON) is a considerable loss pathway in grassland soils. We investigated the white clover (Trifolium repens) contribution to N transport and temporal N dynamics in soil solution under a pure stand of white clover and white clover-ryegrass (Lolium perenne) mixed stand. The temporal white clover contribution to N leaching was analysed by ¹⁵N incorporation into DIN and DON in percolating soil solution collected at 25-cm depth following white clover ¹⁵N leaf labelling that was applied at different times during the growing season. The white clover contribution to N transport in the soil profile was investigated over 2 years by analysing ¹⁵N in DIN and DON in percolating soil solution collected at 25-, 45- and 80-cm depth following ¹⁵N leaf labelling of white clover. The results showed that clover was a source of both DIN and DON. White clover autumn deposition contributed the most to N leaching. The leaching of DIN from the white clover in pure stand exceeded that of the mixed stand and confirmed that leaching of DIN is a function of N loadings and N demand. The DON leaching was unaffected by the presence of a companion grass, suggesting that the DON leaching from our grassland derived from the lysis of soil microbial biomass living on recent white clover deposits. White clover contributed to the leaching of DIN and DON at all depths, and the fact that the contents of DI ¹⁵N and DO ¹⁵N did not change with depth indicated that surplus of DIN and DON, formed in the uppermost soil layer, was transported in the soil profile.     

Rhizodeposition of nitrogen by red clover,white clover and ryegrass leys

Correct assessment of the rhizodeposition of N in grassland is essential for the evaluation of biological N₂-fixation of legumes, for the total N balance of agro-ecosystems, and for the pre-cropping value of grasslands. Using a leaf-feeding technique by which plants were ¹⁵N labelled while growing in mezotrons in the field, the rhizodeposition of N by unfertilised red clover, white clover and perennial ryegrass growing in pure stands was shown to amount to 64, 71 and 9 g N m⁻², respectively, over two complete growing seasons. The corresponding values for red clover and white clover growing in mixtures with ryegrass were 89 and 32 g N m⁻², respectively. The rhizodeposited N compounds, including fine roots, constituted more than 80% of the total plant-derived N in the soil, and in all cases exceeded the amount of N present in stubble. In the mixtures of red clover–ryegrass and white clover–ryegrass and the pure stands of red clover, white clover and ryegrass, respectively, the rhizodeposition constituted a 1.05, 1.52, 1.26, 2.21 and 2.77 fold increase over the total N in the shoots harvested during the two production years. In pure stands and mixtures of clover, 84 and 92%, respectively, of this N derived from biological N₂ fixation. It is concluded that rhizodeposition provides a very substantial input of N to the legume-based grassland systems with great consequences for ecosystem N balance and turnover. Furthermore, the amount of atmospheric-derived N in the rhizodeposits may exceed that in the harvested shoots.     

Carbon flow from C-labeled clover and ryegrass residues into a residue-associated microbial community under field conditions

Microbial colonization of soil-incorporated, ¹³C-labeled, crimson clover and ryegrass straw residues was followed under western Oregon field conditions from late summer (September) to the following early summer (mid-June) by measuring the ¹³C content of phospholipid fatty acid (PLFA) extracted from residues recovered from soil. Residue type influenced the rate of appearance of specific PLFA during early decomposition, with branch chain bacterial PLFA (i15:0, a15:0, i16:0) appearing on clover and ryegrass residues in October and November, respectively. By April, additional PLFA (16:1ω5, 16:1ω7, cy17:0, 18:0, 18:1ω9) had appeared on both residues. Between April and June, microbial community structure shifted again with significant increases (cy17:0, 18:0, 18:1ω9), and decreases (18:1ω7+10Me18:0) detected in the quantities of specific PLFA on both residue types. In the case of clover, the PLFA-C was derived primarily from residue C (85-100%), whereas in the case of ryegrass, both residue C (57-66%), and soil C contributed substantially to the PLFA-C.     

Long-term management impacts on soil carbon and nitrogen dynamics of grazed bermudagrass pastures

Managed pastures have potential for C and N sequestration in addition to providing forage for livestock. Our objectives were to investigate changes in soil organic C (SOC) and soil organic N (SON) concentrations and mineralizable C and N in cattle (Bos indicus) grazed bermudagrass [Cynodon dactylon (L.) Pers.] pastures up to 32 y after establishment. Management included low- and high-grazing intensity, fertilization, and winter overseeding with annual ryegrass (Lolium multiflorum Lam.) and clover (Trifolium sp.). Soil (0-15 cm) was sampled 7, 15, 26, and 32 y after establishment of Coastal and common bermudagrass pastures. No significant differences in SOC or SON concentrations were observed between Coastal and common bermudagrass pastures. Grazing strategies played important roles in C and N sequestration, as high-grazing intensity resulted in a lower increase in SOC and SON concentrations over time compared to low-grazing intensity. Increases in SOC were observed up to 26 y, while increases in SON were observed up to 32 y after establishment of bermudagrass pastures. Soil organic C increased 67 and 39% from 7 to 26 y at low-grazing intensity for bermudagrass+ryegrass and bermudagrass+clover pastures, respectively. SOC and SON concentrations did not increase beyond 15 y after bermudagrass establishment at high-grazing intensity. An exception was the Coastal bermudagrass+ryegrass pastures, which exhibited higher SON at 32 y than at 7 y at both grazing intensities. By 32 y, SON increased 83 and 45% in Coastal bermudagrass+ryegrass pastures at low- and high-grazing intensity, respectively, compared to 7 y. The introduction of clover to pastures decreased SOC and SON relative to ryegrass at high- but not at low-grazing intensity. Potentially mineralizable C increased from 7 to 15 y, while mineralizable N increased from 7 to 32 y. Potentially mineralizable N was also greater for bermudagrass+clover than bermudagrass+ryegrass pastures. Long-term increases in SOC and SON concentrations suggest that managed and grazed pastures have strong potential for C and N sequestration.     

Carbon distribution and variations in nitrogen-uptake between catch crop species in pot experiments

Chicory (Cichorium intybus L.) and perennial ryegrass (Lolium perenne L.) are seen as suitable catch crops species in Sweden. Pot experiments were conducted to study C distribution and variations in nitrogen uptake between several varieties of chicory and perennial ryegrass for comparison.A soil amended with Ca(¹⁵NO₃) (109 and 145 mg N kg⁻¹ soil) and glucose (2.5 g C kg⁻¹ soil) was incubated for 10 days to promote the immobilization of added ¹⁵N; therefore, N was supplied to plants through the remineralization of the immobilized ¹⁵N. In experiment 1 four varieties of chicory and one variety of perennial ryegrass were grown for 60 days in greenhouse conditions. In experiment 2, only two varieties of chicory and one ryegrass were grown in soil with high-N rate of fertilization. In the later experiment, pots were moved from greenhouse to a growth chamber with ¹⁴CO₂ atmosphere for a pulse labelling of the plants 7–10 days before harvest.At both levels of N supply, dry weights of taproots were higher in the chicory cultivars Cassel and Fredonia than in cultivars Puna and Salsa. The opposite was found for dry weights of small roots. There were significant differences in N uptake between chicory varieties. Cassel and Fredonia together with the ryegrass were significantly more effective in securing nitrate than the other two varieties. Significantly higher amounts of labelled-N were found in taproots of Cassel than in Puna. The opposite trend was found for small roots. Similar results were measured for amounts of radioactivity (kBq pot⁻¹) of newly fixed C transferred to roots. Amounts of labelled-N measured in soil residues for both crop species were significantly higher at the low level of N supply than at the high level of N. There was no significant increase in plant uptake of soil-N (native-N) either between chicory varieties or between chicory and ryegrass, when the high level of N was supplied.The importance of these results is discussed in relation to the suitability of chicory species as catch crop and as plant material for breeding.     

Comparison of the difference method and 15N technique for studying the fate of nitrogen from plant residues in soil

We compared the dynamics of net mineralization of nitrogen (N) derived from white clover material (Ndfc) as measured by the difference and the ¹⁵N methods in a pot experiment with a sandy loam (15°C and pF 2.4) planted with Italian ryegrass. On day 22, mineralized Ndfc (soil mineral N plus plant N uptake) was 5.8% and 1.3% of added N for the ¹⁵N and the difference methods, respectively. The discrepancy was reduced on day 43. On day 64, the relationship was reversed, and on day 98 the values given by the two methods were 22.8% and 29.5%, respectively. The results obtained by the two methods were linearly correlated (r = 0.987) and, on average, did not differ significantly. Nevertheless, the different temporal patterns led to appreciably different parameter values as estimated by fitting of a reparameterized Richards model. On day 22, clover amendment reduced mineralized N derived from soil (Ndfs) by 3.4 mg N pot^–1. The reason for this was that the clover amendment led to a reduction in plant growth and uptake of Ndfs, most likely because of allelopathy, while mineral Ndfs did not increase correspondingly. Clover-induced Ndfs in the microbial biomass of 5.1 mg N pot^–1 suggested that the mineral Ndfs not taken up by plants had been reimmobilized. Towards the end of the experiment, clover-induced Ndfs in the biomass declined to 1.5 mg N pot^–1, while mineralized Ndfs due to clover amendment increased to 5.1 mg N pot^–1. The results strongly suggested that this increase was caused by a real stimulation of humus N mineralization by clover amendment rather than by isotope displacement or pool substitution. Received: 5 May 1997     

Can differences in 15N natural abundance be used to quantify the transfer of nitrogen from legumes to neighbouring non-legume plant species?

Natural variations in the stable isotope ¹⁵N are often exploited in studies of N cycling in ecosystems. Lower ¹⁵N natural abundance in non-legume plants growing in association with legumes, compared with the non-legume grown alone in pure stands have been observed in cropping, forage, and agroforestry systems. Such observations have frequently been attributed to the transfer of biologically-fixed nitrogen (N) from the legume to the companion non-legume, and various methodologies have been employed to calculate the extent of the N transfer. While some of these ¹⁵N natural abundance-based estimates of N transfer were within the range previously reported using equivalent ¹⁵N-enriched techniques (<20% of non-legume plant N and <10 kg N ha⁻¹ derived from fixed N contributed by neighbouring legumes), many of the values obtained using natural abundance were much higher (30%–83% of the non-legume N derived from fixed N representing up to 30–40 kg N ha⁻¹) than generally measured by ¹⁵N-enriched methods; with even greater estimates being determined where data were available to allow N transfer to be re-calculated on the basis of total legume N rather than fixed N (42% to >100%, and up to 110 kg N ha⁻¹ per year). This review raises concerns about the assumptions behind the natural abundance approach, and provides some alternative interpretations for the observed differences in natural ¹⁵N abundance between plants grown in the presence and absence of legumes. It was concluded that simple comparative measures of non-legume δ¹⁵N alone cannot provide a quantitative estimate of N transfer between plant species if the dominant source and the isotopic identity of the transferred N cannot be validated, and if the extent of any isotopic fractionation associated with relevant N transformations occurring during transfer cannot be defined. To date this information is not forthcoming. There is a need to greatly improve our understanding of the transfer processes before the real value of the δ¹⁵N technology can be realized. In the first instance this will primarily be achieved by carefully executed experiments under controlled conditions, and in the field, employing both ¹⁵N natural abundance and enrichment approaches so estimates of transfer can be compared, and the data interrogated using modelling approaches to explore isotopic fractionation.     

Green Manuring with Clover and Ryegrass Catch Crops Undersown in Small Grains: Effects on Soil Mineral Nitrogen in Field and Laboratory Experiments

Abstract

In three field trials in southern Norway, Italian ryegrass (Lolium multiflorum Lam.), white clover (Trifolium repens L.) or subterranean clover (T. subterraneuni L.) was undersown in spring grain at three N fertilizer rates and ploughed under in late October as a green manure for a succeeding spring grain crop. The content of topsoil (0-20 cm) mineral nitrogen was determined during the growth of the grain crop, after grain harvest and after ploughing. In addition, mineralization of nitrogen and carbon was measured in green-manured soil incubated at 15°C and controlled moisture conditions. During grain crop growth, ryegrass tended to reduce soil mineral N compared with the other treatments. After grain harvest, in a small-plot experiment where extra nitrate was added, ryegrass reduced soil nitrate N (0-18 cm) from 4.2 to 0.4 g m^?2 within 13 days, while the clovers had negligible effect compared with bare soil. Up to 9.4 g N m^?2 was present in above-plus below-ground ryegrass biomass at ploughing. In incubated ryegrass soil, there was a temporary net N immobilization of up to 0.9 g N m^?2 as compared with unamended soil. In clover-amended soil, mineral N exceeded that in unamended soil by up to 5 g N m^?2.     

Response of turfgrass to urea-based fertilizers formulated to reduce ammonia volatilization and nitrate conversion

Stabilized urea fertilizers are currently being marketed for use in turfgrass, as a more efficient alternative to standard urea that minimizes adverse impacts on the environment. These fertilizers have been evaluated for reducing N losses and increasing grain yield in crop plants, but their effects in turf are not well characterized. The efficacy of two stabilized urea fertilizers containing urease and nitrification inhibitors, N-(n-butyl) thiophosphoric triamide and dicyandiamide or butenedioc-methylenesuccinic acid copolymer, in reducing N losses was studied for a 56-day period in a mixed stand of Kentucky bluegrass (Poa pratensis L.) and perennial ryegrass (Lolium perenne L.) using ¹⁵?N-enriched fertilizers. Turf responded to a 49-kg ha^?1 N input with increased color, quality, and biomass production. No benefit of nitrification and urease inhibitors compared to urea was observed for clipping production, N use efficiency, or turfgrass color and quality. Though the efficacy of urease and nitrification inhibitors has been demonstrated both in the laboratory and for row crops, inhibitors appear to be of limited value for enhancing N use efficiency in turf.     

Distribution and fate of 13C-labeled root and straw residues from ryegrass and crimson clover in soil under western Oregon field conditions

Annual ryegrass (Lolium multiflorum Lam.) and crimson clover (Trifolium incarnatum L.) were pulse-labeled with ¹³C-CO₂ in the field between the initiation of late winter growth (mid-February) and through flowering and seed formation (late May). Straw was harvested after seed maturation (July), and soil containing ¹³C-labeled roots and root-derived C was left in the field until September. ¹³C-enriched and ¹³C-unenriched straw residues of each species were mixed in factorial combinations with soil containing either ¹³C-enriched or ¹³C-unenriched root-derived C and incubated in the field for 10 months. The contributions of C derived from straw, roots, and soil were measured in soil microbial biomass C, respired C, and soil C on five occasions after residue incorporation (September, October, November, April, and June). At straw incorporation (September), 25–30% of soil microbial biomass C was derived from root C in both ryegrass and clover treatments, and this value was sustained in the ryegrass treatment from September to April but declined in the clover treatment. By October, between 20 and 30% of soil microbial biomass C was derived from straw, with the percentage contribution from clover straw generally exceeding that from ryegrass straw throughout the incubation. By June, ryegrass root-derived C contributed 5.5% of the soil C pool, which was significantly greater than the contributions from any of the three other residue types (about 1.5%). This work has provided a framework for more studies of finer scale that should focus on the interactions between residue quality, soil organic matter C, and specific members of the soil microbial community.     

Utilisation by white clover and ryegrass of sulphur from 35s‐Labelled Gypsum

Abstract

This investigation reports the uptake of S from a surface application of ³⁵S‐labelled gypsum by a ryegrass‐white clover mixture sward and by a pure ryegrass stand, each growing at three levels of N in the field. Nitrogen stimulated ryegrass growth, reducing the contribution of white clover to the total yield, whereas S did not influence the yield of either species. Gypsum, while not increasing the total S in the white clover, contributed 23 to 50% of the total S concentration. In contrast, gypsum increased the total S in the ryegrass. The level of N nutrition did not alter the fertiliser S in white clover, but depressed the total S in the ryegrass. Nitrogen enhanced the fertiliser S in ryegrass at the first harvest, however, at the second harvest N depressed the fertiliser S.

Recovery of applied S was increased by N, reaching a maximum value of 19.8% by two harvest, and was decreased with increasing rate of gypsum. Without ‐N the white clover accounted for 50% and 27% of the S recovery by the mixture at the 1st and 2nd harvests respectively, the proportion dropping to less than 20% for each harvest at a high level of N.

There was no apparent competitive advantage of ryegrass over white clover when grown in association although the data indicated a greater ability by ryegrass to absorb S from a surface application. Under conditions of incipient S deficiency the reduction in the contribution of white clover to production with increasing N supply was considered to be due to factors other than the availability of S in the environment.     

Carbon allocation to roots,rhizodeposits and soil after pulse labelling: a comparison of white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis> L.) and perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.)

Organically managed farm areas in Denmark are expanding and typically contain clover-grass leys that are known to stimulate accumulation of organic matter in arable soils. We compared the C allocation to roots and soil from clover and grass, and determined for how long assimilated C remained mobile in these plant-soil systems. Pots with perennial ryegrass, white clover or a mixture of both were pulse-labelled with ¹⁴CO₂, and harvested for analyses after 4, 11, 20, and 30 days. ¹⁴C losses by shoot respiration stopped within 4 days and after this incubation time the input of assimilated ¹⁴C to below-ground compartments was greater in grass (52%) than in clover (36%). During the next 4 weeks, ¹⁴C allocation below ground increased in grass (up to 75% at day 30), but remained constant in clover (37% at day 30). In the grass/clover mixture, the below-ground fraction increased to 50% at day 30. In clover, ¹⁴C was incorporated sooner into stable plant and soil pools and less was released in rhizodeposition than in grass. This was confirmed by the ¹⁴C in the soil microbial biomass that decreased fastest in the clover treatment. Root-derived C compounds of clover probably decomposed faster than those from grass. The larger size and specific activity of the soil microbial biomass in the mixed treatment suggested a stimulating effect of the two plant species on substrate utilisation by the microbial community. This study showed that a 2- to 3-week distribution period is needed before sampling for quantitative estimates of C allocation.     

Nitrogen transfer between high- and low-quality leaves on a nutrient-poor Oxisol determined by N enrichment

It has been proposed that the C/N ratio, or quality, of litter or mulch mixtures affects N release. Although total N release from these mixtures and the effects on soil N are relatively well understood, a mechanistic understanding of the interactions between litter species with respect to their N release is still lacking. This study examines decomposition and N dynamics in mixtures of high-quality leguminous mulch, gliricidia [Gliricidia sepium (Jacq.) Kunth. ex Walp.] with a C/N ratio of 13, and low-quality cupuaçu [Theobroma grandiflorum (Wild. ex Spring) Schumann] litter with a C/N ratio of 42, which occur in combination in agroforestry systems. Ratios of 100:0, 80:20, 50:50, 20:80, 0:100 of fresh ¹⁵N-enriched gliricidia leaves and senescent cupuaçu leaves, totaling the same dry weight of 6.64 t ha⁻¹, were applied to an Oxisol and sampled at 6, 14, 38, and 96 days after application. After more than 40% of the N in the gliricidia leaves had been released and the microbial biomass N reached its peak, a significant increase in available soil N occurred at day 14, which was more pronounced with greater amounts of gliricidia in the leaf mixture. However, relative to the N applied in the leaf mixture, there was no significant difference in available soil N with greater proportions of gliricidia. Total N release from the mixtures corresponded to the total N applied by gliricidia. Until day 38, cupuaçu C mineralization was significantly faster in the presence of the highest proportion of gliricidia compared to lower proportions. This faster C mineralization of more than 0.5% per day, however, did not increase total C loss or N release from cupuaçu leaves after 96 days. The use of ¹⁵N tracers identified an N transfer from gliricidia leaves and the soil to cupuaçu leaves and consequently, a lower N release from gliricidia to the soil in the presence of cupuaçu leaves. Though we expected that available N in the soil would also decrease with greater amounts of cupuaçu litter in the mixture, our results indicated an additive effect of the two species on N release and soil mineral N, with gross interactions between them canceling net interactive effects. Therefore, N release of leaf mixtures behaved as predicted from a calculated sum of individual release patterns, in spite of a transfer of N from the high- to the low-quality leaves.     

Fractionation of N2O isotopomers during production by denitrifier

Isotopomer ratios of N₂O, which include intramolecular ¹⁵N-site preference in addition to conventional isotope ratios for N and O in NNO (we designate N^α and N^β for the center and end N atom, respectively, in the asymmetric molecule), reflect production and consumption processes of this greenhouse gas. Therefore, they are useful parameters for deducing global N₂O budget. This paper reports the first precise measurement of ¹⁵N-site preference in N₂O produced by two species of denitrifying bacteria, Pseudomonas fluorescens (ATCC 13525) and Paracoccus denitrificans (ATCC 17741).Cultures were incubated in a batch mode with a liquid medium that contains KNO₃ as unique nitrogen supply under acetylene/helium (10% v/v) atmosphere at 27 °C. Enrichment factors for ¹⁵N in bulk nitrogen in N₂O (average for N^α and N^β) fluctuated in a few tens permil showing a slight difference between the species. In contrast, ¹⁵N-site preference (difference in isotope ratios between N^α and N^β) showed nearly constant and distinct value for the two species (23.3±4.2 and −5.1±1.8‰ for P. fluorescens and P. denitrificans, respectively). The site preference was also measured for N₂O produced by inorganic reactions (nitrite reduction and hydroxylamine oxidation); a unique value (about 30‰ for the both reactions) was obtained. These results and those recently reported for nitrifying bacteria suggest that ¹⁵N-site preference in N₂O can be used to identify the production processes of N₂O on the level of bacterial species or enzymes involved.     

Use of 13C and 15N natural abundance techniques to characterize carbon and nitrogen dynamics in composting and in compost-amended soils

Isotope fractionation during composting may produce organic materials with a more homogenous δ¹³C and δ¹⁵N signature allowing study of their fate in soil. To verify this, C, N, δ¹³C and δ¹⁵N content were monitored during nine months covered (thermophilic; >40 °C) composting of corn silage (CSC). The C concentration reduced from 10.34 to 1.73 g C (g ash)⁻¹, or 83.3%, during composting. Nitrogen losses comprised 28.4% of initial N content. Compost δ¹³C values became slightly depleted and increasingly uniform (from −12.8±0.6‰ to −14.1±0.0‰) with composting. Compost δ¹⁵N values (0.3±1.3 to 8.2±0.4‰) increased with a similar reduced isotope variability.The fate of C and N of diverse composts in soil was subsequently examined. C, N, δ¹³C, δ¹⁵N content of whole soil (0-5 cm), light (<1.7 g cm⁻³) and heavy (>1.7 g cm⁻³) fraction, and (250-2000 μm; 53-250 μm and <53 μm) size separates, were characterized. Measurements took place one and two years following surface application of CSC, dairy manure compost (DMC), sewage sludge compost (SSLC), and liquid dairy manure (DM) to a temperate (C₃) grassland soil. The δ¹³C values and total C applied (Mg C ha⁻¹) were DM (−27.3‰; 2.9); DMC (−26.6‰; 10.0); SSLC (−25.9‰; 10.9) and CSC (−14.0‰; 4.6 and 9.2). The δ¹³C of un-amended soil exhibited low spatial (−28.0‰±0.2; n=96) and temporal (±0.1‰) variability. All C₄ (CSC) and C₃ (DMC; SSLC) composts, except C₃ manure (DM), significantly modified bulk soil δ¹³C and δ¹⁵N. Estimates of retention of compost C in soil by carbon balance were less sensitive than those calculated by C isotope techniques. One and two years after application, 95 and 89% (CSC), 75 and 63% (SSLC) and 88 and 42% (DMC) of applied compost C remained in the soil, with the majority (80-90%) found in particulate (>53 μm) and light fractions. However, C₄ compost (CSC) was readily detectable (12% of compost C remaining) in mineral (<53 μm) fractions. The δ¹⁵N-enriched N of compost supported interpretation of δ¹³C data. We can conclude that composts are highly recalcitrant with prolonged C storage in non-mineral soil fractions. The sensitivity of the natural abundance tracer technique to characterize their fate in soil improves during composting, as a more homogeneous C isotope signature develops, in addition to the relatively large amounts of stable C applied in composts.