Extractable and dissolved soil organic nitrogen - A quantitative assessment

Extractable Organic N (EON) or Dissolved Organic Nitrogen (DON) pools are often analyzed to predict N mineralisation, N leaching, and to evaluate agricultural (nutrient) management practices. Size and characteristics of both pools, however, are strongly influenced by methodology. Quantifying the influence of methodology can increase the accuracy of soil tests to predict N mineralisation, improve model simulations, and can help to quantify the contribution of the EON and DON pools to soil N cycling. We estimated the relative impact of methodological, management, and environmental factors on EON and DON, using a meta-analysis approach based on 127 studies. Our results indicate that the EON and DON pools are neither similar in size nor controlled by the same factors. The influence of factors controlling EON generally decreased in the order of methodology (Δ10-2400%), followed by environment (Δ11-270%) and management (Δ16-77%). DON concentrations were primarily controlled by management factors: different land use and fertilisation caused a variation of 37-118%. Seasonal variations in DON concentrations were generally smaller than variations in EON, suggesting that high mineralisation and sorption rates buffer DON. The large range in EON as affected by different methodology emphasizes the importance of using appropriate and standardized methods for the determination of EON. The determination of DON can be useful to estimate leaching losses. EON, however, can be used to assess the impact of soil management practices on the turnover rate of labile soil organic matter pools.     

Forms of fertilizer nitrogen residues in soil after intercropping of maize-cowpea

Summary Under greenhouse and field conditions, after the harvest of maize-cowpea intercropping, soils were analysed for total, ammonium and organic N fractions and fertilizer ¹⁵N residues. Growing cowpea as the sole crop or in intercropping with maize results in increased relative amounts of the acid hydrolysable organic N fractions in soil. After sole cropping of maize 70% of the residual fertilizer N was found in the acid hydrolysable fraction while after intercropping it was 80%–92%. The fertilizer and soil N labelling with ¹⁵N in identical but alternate series provided information on the nitrogen fixed by cowpea and left in the soil as crop residues. Under field conditions the cowpea plant residues left after cropping contained 170 kg N ha^–1 in sole cropping and 105 kg N ha^–1 in intercropping with maize. The N assimilated by cowpea-Rhizobium symbiosis was mainly present in the acid hydrolysable forms, particularly in the -amino N fraction and ammonium N fraction.     

Availability of soil and fertilizer nitrogen to wetland rice following wheat straw amendment

Summary A pot experiment was conducted to study the availability of soil and fertilizer N to wetland rice as influenced by wheat straw amendment (organic amendment) and to establish the relative significance of the two sources in affecting crop yield. Straw was incorporated in soil at 0.1, 0.2, and 0.3% before transplanting rice. Inorganic N as ¹⁵N-ammonium sulphate was applied at 30, 60, and 90 g g^-1 soil either alone or together with wheat straw in different combinations. After harvesting the rice, the plant and soil samples were analyzed for total N and ¹⁵N. Straw incorporation significantly decreased the dry matter and N yield of rice, the decrease being greater with higher rates of straw. The reduction in crop yield following the straw incorporation was attributed mainly to a decrease in the uptake of soil N rather than fertilizer N. The harmful effects of organic matter amendment were mitigated by higher levels of mineral N addition. The uptake of applied N increased and its losses decreased due to the straw incorporation. Mineral N applied alone or together with organic amendment substantially increased the uptake of unlabelled soil N. The increase was attributed to a real added N interaction.     

Effect of extracellular-enzyme activities on solubilization rate of soil organic nitrogen

In a sandy soil containing ¹⁵N-labeled active (soluble and easily degradable) and non-labelled passive (recalcitrant) fractions of soil organic matter, the rate of net N mineralization (solubilization) was determined during a 55-day incubation at 25°C, 63% water-holding capacity and different levels of soil extracellular-enzyme activities. The active fraction of soil N was labelled by preincubation (at 5°C and 74% water-holding capacity for 6 months) of soil amended with ¹⁵N-labeled plant material. Increases in the activity of extracellular-enzymes in soil were induced by the addition of glucose and KH₂PO₄ at the beginning of the incubation. The results show that the contents of total soluble N (NO ₃ ^– –N+NH ₄ ⁺ –N + soluble organic N) were significantly higher in glucose-amended soil compared to the unamended soil. The increases in soluble N in soil amended with 1 and 2 mg glucose g^-1 dry soil corresponded to a mean rate of net solubilization of 7.9±1.4 and 18.8±0.7 nmol N g^-1 dry soil day^-1, respectively. The mean rate of net N solubilization (3.6±1.0 nmol N g^-1 dry soil day^-1) in unamended soil was significantly lower than those of glucose amended soils. The content of ¹⁵N in total soluble N in soil amended with 2 mg glucose, for example, was diluted from 3.11±0.08 atom% before the incubation to 2.77±0.03 atom% after 55 days. This indicates that 89% of soluble-N accumulated in soil by the end of the incubation originated from the active fraction of soil N and the rest, estimated at 11%, originated from the passive fraction. The activities of soluble and total proteases as well as the rate of N solubilization in the soil increased with the application of glucose. The activity of these extracellular enzymes was highly correlated with the rates of net N solubilization. Thus, increases in extracellular-enzyme activities in glucose-amended soils had a priming effect on the solubilization of ¹⁵N-labeled active and non-labeled passive fractions of soil organic N. It seems that the activity of extracellular-enzymes expressed in terms of total and soluble protease activities could be a rate-limiting factor in the processes of soil organic N solubilization.     

Mineralization of soil organic nitrogen solubilized by aqua ammonia fertilizer

Organic N solubilized by NH_3(aq) was extracted from ¹⁵N-labelled or unlabelled soil, concentrated and added to non-extracted soil, which was incubated under aerobic conditions at 27±1°C. Gross N mineralization, gross N immobilization, and nitrification in soils with or without addition of unlabelled soluble organic N were estimated by models based on the dilution of the NH ₄ ⁺ or NO _inf3 ^sup- pools, which were labelled with ¹⁵N at the beginning of incubation. Mineralization of labelled organic N was measured by the appearance of label in the mineral N pool. Although gross N mineralization and gross N immobilization were increased in two soils between day 0 and day 7 following addition of unlabelled organic N solubilized by NH_3(aq), there was no increase in net N mineralization. Solubilization of ¹⁵N-labelled organic N increased and the ¹⁵N enrichment of the soluble organic N decereased as the concentration of NH_3(aq) added increased. A constant proportion of approximately one-quarter of the labelled organic N added at different rates to non-extracted soil was recovered in the mineral N pool after an incubation period of 14 days, and the availability ratios calculated from net N mineralization data were 1.1:1 and 2.1:1 for 111 and 186 mg added organic-N kg^-1 soil, respectively, indicating that the mineralization of organic N was increased by solubilization.     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Availability of soil and fertilizer nitrogen to wheat (Triticum aestivum L.) following rice-straw amendment

Summary A pot experiment was conducted to study the N availability to wheat and the loss of ¹⁵N-labelled fertilizer N as affected by the rate of rice-straw applied. The availability of soil N was also studied. The straw was incorporated in the soil 2 or 4 weeks before a sowing of wheat and allowed to decompose at a moisture content of 60% or 200% of the water-holding capacity. The wheat plants were harvested at maturity and the roots, straw, and grains were analysed for total N and ¹⁵N. The soil was analysed for total N and ¹⁵N after the harvest to determine the recovery of fertilizer N in the soil-plant system and assess its loss. The dry matter and N yields of wheat were significantly retarded in the soil amended with rice straw. The availability of soil N to wheat was significantly reduced due to the straw application, particularly at high moisture levels during pre-incubation, and was assumed to cause a reduction in the dry matter and N yields of wheat. A significant correlation (r=0.89) was observed between the uptake of soil N and the dry matter yield of wheat with different treatments. In unamended soil 31.44% of the fertilizer N was taken up by the wheat plants while 41.08% of fertilizer N was lost. The plant recovery of fertilizer N from the amended soil averaged 30.78% and the losses averaged 45.55%     

Influence of tillage, crop rotation and nitrogen fertilization on soil organic matter and nitrogen under rain-fed Mediterranean conditions

The long-term effects of cropping systems and management practices on soil properties provide essential information for assessing sustainability and environmental impact. Field experiments were undertaken in southern Spain to evaluate the long-term effects of tillage, crop rotation and nitrogen (N) fertilization on the organic matter (OM) and mineral nitrogen (N_min) contents of soil in a rain-fed Mediterranean agricultural system over a 6-year period. Tillage treatments included no tillage (NT) and conventional tillage (CT), crop rotations were of 2 yr with wheat (Triticum aestivum L.)-sunflower (Helianthus annuus L.) (WS), wheat-chickpea (Cicer arietinum L.) (WP), wheat-faba bean (Vicia faba L.) (WB), wheat-fallow (WF), and in addition, continuous wheat (CW). Nitrogen fertilizer rates were 50, 100, and 150 kg N ha⁻¹. A split-split plot design with four replications was used. Soil samples were collected from a depth of 90 cm at the beginning of the experiment and 6 yr later. Soil samples were also collected from a depth of 30 cm after 4 yr. These samples, like those obtained at the beginning of the experiment, were subjected to comprehensive physico-chemical analyses. The soil samples that were collected 6 yr later were analyzed for OM, NH₄⁺---N and NO₃⁻---N at the 0–30, 30–60 and 60–90 cm soil depths. The tillage method did not influence the OM or N_min contents of the soil, nor did legume rotations increase the OM content of soil relative to CW. A longer period may have been required for differences between treatments to be observed owing to the small amount of crop residue that is returned to soil under rain-fed conditions of semi-arid climates. The WF rotation did not raise the N_min content of the soil relative to the other rotations. The consistent significant interaction between tillage and crop rotation testifies to the differential effect of the management system on the OM content and N status of the soil. The ammonium levels clearly exceeded those of NO₃⁻---N throughout the soil profile. The high N_min content of the soils reveals the presence of abundant N resources that should be borne in mind in establishing N fertilization schemes for crops under highly variable climatic conditions including scant rainfall such as those of the Mediterranean region.     

Evaluation of the 15Nitrogen Isotope Dilution Technique for Estimating Crop Nitrogen Uptake from Organic Residues

Abstract

A field experiment was conducted to test the new approach for estimating crop nitrogen (N) uptake from organic inputs. The soil was prelabeled with ¹⁵N by applying ¹⁵N fertilizer to sunflower crop (Helianthus annuus L. var. Viki). The ¹⁴N plots, which received unlabelled fertilizer, were also set up. At harvest, ¹⁵N labeled residues were added to the unlabeled soils at a rate of 73 kg N ha^?1 (direct technique) and unlabeled residues were added to the ¹⁵N‐labeled soils at the same rate (indirect technique). Control plots without residues were also established. All plots were sown with the wheat (Triticum aestivum L. var merchouch)–fababean (Vicia faba L.)–wheat (Triticum aestivum L. var merchouch) cropping sequence.

In the cropping sequence, the first, second and third crop derived respectively 12.01, 2.4, and 1.93 kg N ha^?1 from crop residues estimated by the direct method and 14.77, 3.3, and 1.85 kg N ha^?1 estimated by the indirect method. The results showed no significant difference between the two techniques, which suggests that the new soil prelabeling technique compares well with the direct technique.     

Fate of nitrogen derived from 15N-labeled plant residues and composts in rice-planted paddy soil

Pot experiments that lasted for 3 y were conducted to investigate the dynamics of nitrogen derived from plant residues (rice root, hull, straw, corn root, and rapeseed pod-wall), and composts (rice straw compost, cattle manure compost, and cattle manure sawdust compost), which were labeled with ¹⁵N. The rates of nitrogen uptake by rice (=N efficiency), denitrification, and immobilization derived from the organic materials incorporated before the first year of cultivation were investigated throughout 3 y of cultivation. At the end of the first year of cultivation, relatively high rates of N efficiency were obtained for rapeseed pod-wall (24.6%), rice straw (19.1%), and rice hull (18.6%), while corn root and cattle manure sawdust compost displayed a noticeably high denitrification rate. Corn root, cattle manure sawdust compost, rice hull, and rapeseed pod-wall exhibited remarkably high N mineralization rates ranging from 60 to 75% of the organic materials N applied. Cumulative rates of N efficiencies from the organic materials applied before the first year of cultivation fitted well to a first-order kinetic model and their asymptotes were compared among the organic materials. The asymptotic rates of N efficiency tended to depend on the rates at the end of the first year of cultivation.     

The contribution of soil organic matter fractions to carbon and nitrogen mineralization and microbial community size and structure

The aims of this study were to: (i) assess the impact of hay and fertilizer application on organic matter (OM) fractions (dissolved organic matter (DOM), light fraction organic matter (LFOM, <1.0 g cm⁻³), heavy fraction OM (HFOM, <1.7 g cm⁻³)), carbon (C) and nitrogen (N) cycling processes and microbial community size and structure, and (ii) quantify the role of OM fractions to C and N cycling. Soil was collected in 2001 from a field experiment to which grass hay (1996) and/or fertilizer (1995 and 1999) had previously been applied. DOM-C (P<0.05) and DOM-N (P=0.07) were significantly higher in control and fertilized soil than hay and hay+fertilized soil. LFOM and HFOM C and N contents and C/N ratios were significantly (P<0.05) higher in hay+fertilized and hay amended soil than in control and fertilized soil. Potentially mineralizable-N (PMN), microbial biomass-C (MB-C), microbial biomass-N (MB-N) and microbial respiration (CO₂) were not affected by fertilizer and/or hay application. Gross N mineralization (Gross Min) and gross nitrification (Gross Nit) rates were significantly (P<0.05) higher in fertilized, hay, hay+fertilized soil than control soil. However, there was no significant difference between treatments in gross N immobilization rates. Results reported here highlight the importance of a labile fraction of the DOM pool to N and C cycling as its removal significantly (P<0.05) reduced PMN, MB-N, Gross Min and Gross Nit compared with whole soil in most or all treatments. In soil where DOM+LFOM were removed PMN was significantly (P<0.05) lower, but MB-C, Gross Min and Gross Nit was significantly (P<0.05) higher than in DOM removed soil. This suggests that LFOM plays an important role as a sink for mineral-N. Total soil phospholipid fatty acid (PLFA) concentration was significantly (P<0.05) higher in hay amended than control, fertilized and hay+fertilized soil. Principal components analysis was able to clearly discriminate between control, fertilized, hay+fertilized and hay amended soil. Soil amended with hay or fertilizer had a microbial community structure which differed from that of the control or hay+fertilized soils. Redundancy analysis with Monte Carlo permutation tests revealed that PLFA profiles were strongly correlated to differences in Gross Min, Gross Nit, MB-N, MB-C, MB-C/N ratio, total soil C and total soil C/N ratio. The results of this research suggest that changes in microbial structure are related to aspects of soil C and N pools and cycling.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity     

Nitrogen availability to grain sorghum from organic and inorganic sources on sandy and clayey soil surfaces in a greenhouse pot study

In a greenhouse pot study, we examined the availability of N to grain sorghum from organic and inorganic N sources. The treatments were¹⁵N-labeled clover residues, wheat residues, and fertilizer placed on a sandy clay loam and loamy sand soil surface for an 8-week period. Soil aggregates formed under each soil texture were measured after 8 weeks for each treatment. Significantly greater ¹⁵N was taken up and recovered by grain sorghum in sandy clay loam pots compared with loamy sand pots. Greater ¹⁵N recovery was consistently observed with the inorganic source than the organic sources regardless of soil texture or time. Microbial biomass C and N were significantly greater for sandy clay loam soil compared with the loamy sand. Microbial biomass ¹⁵N was also significantly greater in the sandy clay loam treatment compared to the loamy sand. The fertilizer treatment initially had the greatest pool of microbial biomass ¹⁵N but decreased with time. The crop residue treatments generally had less microbial biomass ¹⁵N with time. The crop residues and soil texture had a significant effect on the water-stable aggregates formed after 8 weeks of treatments. Significantly greater water-stable aggregates were formed in the sandy clay loam than the loamy sand. Approximately 20% greater water-stable aggregates were formed under the crop residue treatments compared to the fertilizer only treatment. Soil texture seemed to be one of the most important factors affecting the availability of N from organic or inorganic N sources in these soils.Contribution from the MissouriAgricultural Experiment Station, Journal Series No.12131     

Origin of the nitrogen assimilated by soil fauna living in decomposing beech litter

We investigated the nitrogen source for main taxa of soil fauna in two beech forests of contrasted humus type using ¹⁵N-labelled beech litter and ¹⁵N analysis of soil fauna. ¹⁵N-labelled beech litter was deposited on the topsoil in December 2000 in four stands of different ages at Leinefelde (Germany) with mull humus and in one mature stand at Sorø (Denmark) with moder humus. The fate of the tracer isotope was measured in litter and soil, as well as in the soil fauna, and for each taxa, we calculated the proportion of N in the animal derived from the labelled substrate. Of the original N contained in the litter, 20-41% was lost after 9 months at Leinefelde, and only 10% at Sorø. This loss was counterbalanced by the incorporation of 24-31% external N at Leinefelde, and 31% at Sorø, partly originating from fungal colonisation of the added litter. The proportion of N assimilated from the labelled litter by the different soil animals varied in relation to their mobility and feeding preferences. Large and mobile soil animals, especially predators, derived on average less ¹⁵N because they were also able to feed outside the labelled litter boxes. Detritivores assimilated at most 15% of their nitrogen content at Leinefelde and 11% at Sorø from the decomposing labelled litter. The most labelled taxa at Leinefelde were small fungivorous and coprophagous species, mainly isotomid Collembola such as Isotomiella and Folsomia. At Sorø, best labelled taxa were saprophagous species such as Enchytraeidae, Glomeridae and Phthiracaroidea. These low rates of ¹⁵N assimilation indicate that fresh litter is not directly the main N source for soil animals. The results obtained suggest that soil fauna fed preferentially upon microorganisms colonising the litter at Leinefelde (mull) and from litter itself at Sorø (moder).     

Carbon and nitrogen dynamics in a forest soil amended with purified tannins from different plant species

Tannins are purported to be an important factor controlling nitrogen cycling in forest ecosystems, and the ability of tannins to bind proteins in protein-tannin complexes is thought to be the primary mechanism responsible for these effects. In this study, we examined the influence of well-characterized tannins purified from five different plant species on C and N dynamics of a forest soil A horizon. Tannic acid, a commonly used and commercially available hydrolyzable tannin (HT), and cellulose were also included for comparison. With the exception of tannins from huckleberry (Vaccinium ovatum), the amendments increased respiration 1.4-4.0 fold, indicating that they were acting as a microbial C source. Tannic acid was significantly more labile than the five purified tannins examined in this study. All treatments decreased net N mineralization substantially, through greater N immobilization and decreased mineralization. The six tannins inhibited gross ammonification rates significantly more than cellulose. This suggests that added tannins had effects in addition to serving as an alternative C source. Tannins purified from Bishop pine (Pinus muricata) were the only tannins that significantly inhibited potential gross nitrification rates, however, rates were low even in the control soil making it difficult to detect any inhibition. Differences in tannin structure such as condensed versus HTs and the hydroxylation pattern of the condensed tannin B-ring likely explain differences observed among the tannin treatments. Contrary to other studies, we did not find that condensed tannins were more labile and less inhibitory than HTs, nor that shorter chained tannins were more labile than longer chained tannins. In addition to supporting the hypothesis that reduced N availability in the presence of tannins is caused by complexation reactions, our data suggests tannins act as a labile C source leading to increased N immobilization.     

Dynamics of microbial biomass nitrogen as influenced by organic matter application in paddy fields

The effects of annual application of rice straw or cow manure compost for 17–20 y on the dynamics of fertilizer N and soil organic N in Gley paddy fields were investigated by using the ¹⁵N tracer technique during the rice cropping season. The chloroform fumigation-extraction method was evaluated to determine the properties of soil microbial biomass under submerged field conditions at the tillering stage before mid-summer drainage, with special reference to the fate of applied NH₄ ⁺-¹⁵N.

The transfer ratios from applied NH₄ ⁺-¹⁵N to immobilized N in soil and to uptake N by rice during given periods varied with the rice growth stages and were affected by organic matter application. The accumulated amounts of netmineralized soil organic N (net-M_j ), immobilized N (I_j ), and denitrified N (D_j ) during the cropping season were estimated to be 14.0–22.5, 6.3–11.2, and 3.4–5.3 g N m^-2, respectively. Values of net-M_j and I_j were larger in the following order: cow manure compost plot > rice straw plot > plot without organic matter application, and their larger increase by the application of cow manure compost contributed to a decrease of the D_j values, as compared with rice straw application.

Values of E _N extra extractable soil total N after fumigation, increased following organic matter application, ranging from 2.1 to 5.4 g N m^-2. Small residual ratios of applied ¹⁵N in the fraction E _N at the end of the given period indicated that re-mineralization of newly-assimilated ¹⁵N through the easily decomposable fraction of microbial biomass had almost ended. Thus, the applicability to paddy field soils of the chloroform fumigation-extraction method was confirmed.     

Manipulating the N release from N labelled celery residues by using straw and vinasses

The aim of this laboratory study was to investigate the effect of straw and vinasses on the nitrogen (N) mineralization-immobilization turnover of celery residues during two periods (each simulating a time period from autumn till spring) under laboratory conditions. During the first period (1-198 d), ¹⁵N-labelled celery residues (1.1 g dry matter (DM) kg⁻¹ soil) were incubated together with straw (8.1 g DM kg⁻¹ soil), aiming to immobilize the N released from celery residues, followed by an incorporation of vinasses (1.9 g DM kg⁻¹ soil) after 84 d, with a view to remineralizing the immobilized celery-N. During the second period (198-380 d), the experimental set-up was repeated, except that non-labelled celery residues were used. Total N, mineral N and their ¹⁵N enrichments as well as microbial biomass N were determined at regular time intervals. During both periods, mixing celery residues with straw significantly increased microbial biomass N (90.5 and 40.5 mg N kg⁻¹ extra compared to celery only treatment) and decreased the amount of mineral N (reduction of 56.1 and 45.9 mg N kg⁻¹ soil compared to celery only treatment) and the celery-derived mineral ¹⁵N (0% of mineral celery-derived ¹⁵N in straw treatment compared to 35% of mineral celery-derived ¹⁵N in celery only treatment). After maximum immobilization, a natural remineralization (without addition of vinasses) of 32.2 (at day 198) and 11.1 mg N kg⁻¹ soil (at day 380) occurred in the straw treatment, but the mineral N content remained significantly lower than in the celery only treatment during the complete experiment, and the amount of remineralized celery-¹⁵N was very low (5.4% of celery-derived ¹⁵N after 380 d). Vinasses caused no real priming effect, although it did slightly increase the amount of remineralized celery-¹⁵N (+6.4% of celery-derived ¹⁵N at day 380 compared to the straw treatment), probably due an apparent added N interaction caused by displacement reactions with the soil microbial biomass.     

The effect of mixing organic biological waste materials and high-N crop residues on the short-time N2O emission from horticultural soil in model experiments

Manipulating the N release from high-N crop residues by simultaneous mixing of these residues with organic biological waste (OBW) materials seems to be a possible method to reduce NO₃^– leaching. The aim of this study was to examine whether the incorporation of OBW materials together with a high-N crop residue (celery) had also an effect on N₂O emission from horticultural soil under short-term and optimised laboratory conditions. A sandy loam soil and celery residues were mixed with different OBW materials and brought into PVC tubes at 80% water-filled pore space and 15°C. Every 2.5 h, a gas sample was taken and analysed by gas chromatography for its N₂O concentration. The soil amended with only celery residues had a cumulative N₂O emission of 9.6 mg N kg^–1 soil in 50 h. When the celery residues were mixed with an OBW material, the N₂O emission was each time lower than the emission from the celery-only treatment (between 3.8 and 5.9 mg N kg^–1 soil during maximum 77 h), except with paper sludge (17.2 mg N kg^–1 soil in 100 h). The higher N₂O emission from the paper sludge treatment was probably due to its unusually low C:N ratio. Straw, green waste compost 1 (GWC1) and 2 (GWC2), saw dust, and tannic acid reduced the N₂O emission of the celery treatment by 40 to 60%. Although the N₂O reduction potential can be expected to be lower and with differing dynamics under field conditions, this study indicates that apart from reducing NO₃^– leaching, OBW application may at the same time reduce N₂O emissions after incorporation of high-N crop residues.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.