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.     

Indices of dissolved organic nitrogen, ammonium and nitrate across productivity gradients of boreal forests

Organic nitrogen (N) uptake, rather than solely inorganic N (DIN), is considered a significant pathway for plant nutrition, especially in arctic, alpine and boreal ecosystems. Assays of plant-available N in these ecosystems might therefore be improved with measures of dissolved organic N (DON). We examined DON and DIN abundance from an in situ 5-week incubation across plant associations that represent the widest range in site potential in southern boreal forests of British Columbia, Canada. The supply of N from forest floors and mineral soils (20 cm depth) was measured separately and then combined (kg ha⁻¹) to facilitate comparisons of sites. DON was the predominant form of extractable N, and was increasingly supplemented, rather than replaced, by NH₄⁺ and NO₃⁻ on productive sites. The amount of DIN produced in the soils was very low, perhaps too small to support forest needs, and the correlation of DIN to asymptotic stand height (a measure of site potential) was significant but nonlinear. The combined amount of DON+DIN was considered a more effective index of plant-available N because it was strongly significant as a linear correlation to stand height and more typical of annual forest N uptake. The relative shift in N forms, from a predominance of DON to progressively greater ratios of DIN:DON, was consistent with the current paradigm of N forms across gradients of N availability, although the actual amounts of DON increased, rather than decreased, with site potential. Based on this, we suggest organic N uptake has the potential to contribute to plant nutrition across the entire productivity gradient of soils in southern boreal forests. Although other N indices were effective in characterizing forest productivity, a combined assay of DON+DIN production could provide new insights into functional differences in plant-available N.     

Soil acidification used as a management strategy to reduce nitrate losses from agricultural land

pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO₃⁻) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO₃⁻ concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH₄⁺ which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO₃⁻ leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops.     

Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards

Several studies have focused on the formation and losses of dissolved organic matter in forest systems, whereas a limited number have dealt with this aspect in agricultural soils. The purpose of this study was to estimate the leaching of dissolved organic carbon (DOC) and nitrogen (DON), with focus on the period after cultivating grass-clover swards. Grass-clovers were ploughed in the spring prior to sowing cereals followed by either catch crops or bare soil. The concentrations of DOC and DON decreased with soil depth and ranged at 90-cm soil depth between 7 and 21 mg C L⁻¹ and between 1 and 3 mg N L⁻¹, respectively, in a sandy loam soil, and between 16 and 63 mg C L⁻¹ and between 1 and 10 mg N L⁻¹, respectively, in a coarse sandy soil. The resulting DOC/DON ratios were in the range between 2 and 42, with higher values in the coarse sandy soil than in the sandy loam soil. The total percolation was 218 mm in the sandy loam soil and 596–645 mm in the coarse sandy soil, which resulted in an annual leaching of 22–40 kg DOC ha⁻¹ year⁻¹ and 3–4 kg DON ha⁻¹ year⁻¹ in the sandy loam soil, and 174–310 kg DOC ha⁻¹ year⁻¹ and 10–31 kg DON ha⁻¹ year⁻¹ in the coarse sandy soil. It was shown that higher amounts of DOC were lost by leaching under the catch crops than from bare soil, that losses of DON were higher from bare soil than from soils with catch crops and that DON contributed significantly to the total N loss. Thus, DON needs to be taken into account in N-balance calculations.     

Contribution of dissolved organic nitrogen to N leaching from four German agricultural soils

Dissolved organic nitrogen (DON) substantially contributes to N leaching from forest ecosystems. However, little is known about the role of DON for N leaching from agricultural soils. Therefore, the aim of our study was to quantify the contribution of DON to total N leaching from four agricultural soils. Concentrations and fluxes of DON and mineral N were monitored at two cropped sites (Plaggic Anthrosols) and two fallow plots (Plaggic Anthrosol and Gleyic Podzol) from November 1999 till May 2001 by means of glass suction plates. The experimental sites were located near the city of Münster, NW Germany. Median DON concentrations in 90 cm depth were 2.3 mg l^—1 and 2.0 mg l^—1 at the cropped sites and 1.6 mg l^—1 and 1.3 mg l^—1 at the fallow sites. There was only a slight (Anthrosols) or no (Gleyic Podzol) decrease in median DON concentrations with increasing depth. Total N seepage was between 19 kg N ha^—1 yr^—1 and 46 kg N ha^—1 yr^—1 at the fallow sites and 16—159 kg N ha^—1 yr^—1 at the cropped sites. For the fallow plots, DON seepage contributed 10—21 % to the total N flux (4—5 kg DON ha^—1 yr^—1), at the cropped sites DON seepage was 6—21 % of the total N flux (6—10 kg DON ha^—1 yr^—1). Thus, even in highly fertilized agricultural soils, DON is a considerable N carrier in seepage that should be considered in detailed soil N budgets.     

Evidence for recycling of N from plants to soil during the growing season

In spite of the known below-ground biomass production of plant roots that concurrently introduce significant amounts of carbon and nitrogen into the soil, the effects of these inputs on N cycling in the soil–plant system are seldom considered. Here, we report on two field experiments carried out between 1995 and 1997 at the FAM Research Station Scheyern: (1) a N-turnover experiment to determine the N fluxes derived from ¹⁵N-labeled clover residues incorporated into the plough layer of defined plots, and (2) a root production experiment to assess the above (shoot) and below ground (gross and net root) biomass production of winter wheat in different fields, but nearby the ¹⁵N plots. An initial 50% decrease in soil organic ¹⁵N at 0–20-cm soil depth was recorded between fall, 1996 (incorporation of clover straw) and spring, 1997 (138 days after incorporation), which was then followed by a period of stability in ¹⁵N levels in the soil organic N until the harvest of winter wheat (286 days after incorporation). This stability may be explained in two ways: (a) actual stability of clover-derived ¹⁵N remaining in the second phase, e.g., due to recalcitrant compounds or microbial immobilization; or (b) apparent stability, e.g., because the actual mineralization of clover-derived ¹⁵N in the soil was compensated by secondary inputs of organic ¹⁵N (recycling). Further results showed that the first explanation was unlikely, as (1) between 138 and 286 days after clover incorporation, the mean ¹⁵N signature in soil mineral N was 2.1 at.%, indicating a persistent mineralization of clover residues; and (2) a decrease in soil microbial biomass ¹⁵N occurred in the second phase, indicating a continued N turnover in the soil. The amount of clover-derived ¹⁵N accumulated below the plough layer at 20–110-cm soil depth (11.5%) between early spring and the harvest of wheat also corroborated the return of mineralized ¹⁵N into the soil being due to the root N inputs by winter wheat. Based on the depth distribution of winter wheat net root biomass (root production experiment) and on soil organic ¹⁵N depth distribution (¹⁵N-turnover experiment), the root N input into soil was estimated to be 282 kg ha⁻¹, equivalent to 54% of total net N assimilation of winter wheat. Thus, the results of this study give substantial evidence for a N loop between soil and growing plants, whereby a part of the net mineralized N taken up by plants is continuously returned into the soil by their roots. The implications of this N loop for the interpretation of ¹⁵N experiments and for plant nutrition are discussed.     

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.     

Dissolved organic carbon and nitrogen in precipitation,throughfall, soil solution,and stream water of the tropical highlands in northern Thailand

Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land‐use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L^–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L^–1 and 0.9–1.2 mg N L^–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral‐soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial‐derived hydrophilic compounds. At least at the cabbage‐cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic‐N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.     

Dissolved Organic Nitrogen Concentrations and Ratios of Dissolved Organic Carbon to Dissolved Organic Nitrogen in Throughfall and Soil Waters in Norway Spruce and Scots Pine Forest Stands Throughout Norway

Dissolved organic nitrogen (DON) plays an important ecological role in forest ecosystems, and its concentration is related to that of dissolved organic carbon (DOC). We investigated DON concentrations and ratios of DOC to DON in throughfall and soil waters in 16 Norway spruce and two Scots pine forest stands sampled at weekly intervals between 1996 and 2006. The stands are all included in the ICP Forests Level II monitoring program and are located throughout Norway. DON concentrations were significantly and positively related to DOC concentrations in throughfall (r ²?=?0.72, p?<?0.0001) and soil water at 5, 15, and 40 cm (r ²?=?0.86, 0.32, and 0.84 and p?<?0.0001, 0.04, and <0.0001, respectively). At most sites, the annual median DOC/DON ratio in throughfall ranged from 20.3 to 55.5, which is lower than values in soil water, which ranged from 24.5 to 81.3, gradually decreasing with soil depth. DON concentrations varied seasonally in throughfall at many plots and in soil water at 5-cm depth at one plot only, with higher values in the growing season, but there was no noticeable seasonality at greater depth. The ratios of DOC/DON in soil water were significantly positively related to the C/N ratio in soil at the same depth. Above-ground litter input was the main factor having a significant, negative relationship to DOC/DON in soil water at all depths studied. This might reflect the effect of site conditions on both DOC/DON ratios and litter quantity.     

The amount,but not the proportion,of N2 fixation and transfers to neighboring plants varies across grassland soils

ABSTRACT

Biological nitrogen fixation (BNF) is an important nitrogen source for both N₂-fixers and their neighboring plants in natural and managed ecosystems. Biological N fixation can vary considerably depending on soil conditions, yet there is a lack of knowledge on the impact of varying soils on the contribution of N from N₂-fixers in mixed swards. In this study, the amount and proportion of BNF from red clover were assessed using three grassland soils. Three soil samples, Hallsworth (HH), Crediton (CN), and Halstow (HW) series, were collected from three grassland sites in Devon, UK. A pot experiment with ¹⁵N natural abundance was conducted to estimate BNF from red clover, and the proportion of N transferred from red clover to the non-N₂ fixing grass in a grass-clover system. The results showed that BNF in red clover sourced from atmosphere in the HH soil was 2.92 mg N plant^?1, which was significantly lower than that of the CN (6.18 mg N plant^?1) and HW (8.01 mg N plant^?1) soils. Nitrogen in grass sourced from BNF via belowground was 0.46 mg N plant^?1 in the HH soil, which was significantly greater than that in CN and HW soils. However, proportionally there were no significant differences in the percentage N content of both red clover and grass sourced from BNF via belowground among soils, at 65%, 67%, 65% and 35%, 27%, 31% in HH, CN, and HW, respectively. Our observations indicate that the amount of BNF by red clover varies among grassland soils, as does the amount of N sourced from BNF that is transferred to neighboring plants, which is linked to biomass production. Proportionally there was no difference among soils in N sourced from BNF in both the red clover plants and transferred to neighboring plants.     

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.     

Synthesis of Nitrogen Pools and Fluxes from European Forest Ecosystems

To investigate which ecosystem parameters determine the risk and magnitude of nitrate leaching we compiled data from published and unpublished sources on dissolved inorganic nitrogen (DIN: NO₃ ^-) in throughfall, DIN leaching loss in runoff or seepage water, and other ecosystem characteristics from 139 European forests. Not all data were available for all sites: 126 sites had at least one year's data on DIN inputs and DIN leaching loss; 40-50 sites had some data on soil chemistry and/or vegetation pools of N. DIN inputs in throughfall range between <1 and about 70 kg N ha-1 yr-1 and the losses with seepage or runoff range between <1 and 50 kg N ha-1 yr-1. Retention of N within the ecosystem increases with increasing DIN deposition and increasing proportion of NH₄ ⁺ in deposition. The amount of N in needles and litterfall shows a significant linear relationship with throughfall deposition of DIN, whereas the C:N ratio of the organic (OH) horizon is uncorrelated to the level of throughfall-DIN flux. About 50% of the variability in DIN leaching loss can be explained by the flux of DIN in throughfall. Alternatively, about 60% of the variability in DIN leaching loss can be explained in a two-variable multiple regression combining the C:N ratio of the organic soil and the pH of the mineral soil. The survey data suggest that leaching of DIN from forest ecosystems in Europe is related in part to current DIN deposition and in part to the longer-term internal ecosystem N status as reflected in the chemistry of the humus and acidification status of the soil.     

Nitrogen Leaching of Two Forest Ecosystems in a Karst Watershed

Karst watersheds are a major source of drinking water in the European Alps. These watersheds exhibit quick response times and low residence times, which might make karst aquifers more vulnerable to elevated nitrogen (N) deposition than non-karst watersheds. We summarize 13 years of monitoring NO ₃ ^? , NH ₄ ⁺ , and total N in two forest ecosystems, a Norway spruce (Picea abies (L.) Karst.) forest on Cambisols/Stagnosols (IP I) and a mixed beech (Fagus sylvatica L.) spruce forest on Leptosols (IP II). N fluxes are calculated by multiplying concentrations, measured in biweekly intervals, with hydrological fluxes predicted from a hydrological model. The total N deposition in the throughfall amounts to 26.8 and 21.1 kg/ha/year in IP I and IP II, respectively, which is high compared to depositions found in other European forest ecosystems. While the shallow Leptosols at IP II accumulated on average 9.2 kg/ha/year of N between 1999 and 2006, the N budgets of the Cambisols/Stagnosols at IP I were equaled over the study period but show high inter-annual variation. Between 1999 and 2006, on average, 9 kg/ha/year of DON and 20 kg/ha/year of DIN were output with seepage water of IP I but only 4.5 kg/ha/year of DON and 7.7 kg/ha/year of DIN at IP II. Despite high DIN leaching, neither IP I nor IP II showed further signs of N saturation in their organic layer C/N ratios, N mineralization, or leaf N content. The N budget over all years was dominated by a few extreme output events. Nitrate leaching rates at both forest ecosystems correlated the most with years of above average snow accumulation (but only for IP I this correlation is statistically significant). Both snow melt and total annual precipitation were most important drivers of DON leaching. IP I and IP II showed comparable temporal patterns of both concentrations and flux rates but exhibited differences in magnitudes: DON, NO ₃ ^? , and NH ₄ ⁺ inputs peak in spring, NH ₄ ⁺ showed an additional peak in autumn; the bulk of the annual NO ₃ ^? and DON output occurred in spring; DON, NO ₃ ^? , and NH ₄ ⁺ output rates during winter months were low. The high DIN leaching at IP I was related to snow cover effects on N mineralization and soil hydrology. From the year 2004 onwards, disproportional NO ₃ ^? leaching occurred at both plots. This was possibly caused by the exceptionally dry year 2003 and a small-scale bark beetle infestation (at IP I), in addition to snow cover effects. This study shows that both forest ecosystems at Zöbelboden are still N limited. N leaching pulses, particularly during spring, dictate not only annual but also the long-term N budgets. The overall magnitude of N leaching to the karst aquifer differs substantially between forest and soil types, which are found in close proximity in the karstified areas of the Northern Limestone Alps in Austria.     

Plant availability and leaching of 15N-labelled mineral fertilizer residues retained in agricultural soil for 25 years: A lysimeter study

Background

A high use-efficiency of fertilizer N remains essential to sustain high crop productivity with low environmental impact. However, little is known on the long-term lability of mineral fertilizer N.

Aims

To quantify crop uptake and leaching of ¹⁵N-labelled mineral fertilizer that has been retained in an agricultural soil for 25–30 years in crops with variable growing season.

Methods

A field plot received ¹⁵N-labelled mineral fertilizers over a period of 5 years and was then kept under arable cropping for 12 years. After relocation to 16 lysimeters, the topsoil grew set-aside grassland for the next 13 years. Then crop uptakes and leaching losses of ¹⁵N remaining in soil was tested over a 2-year period by either converting set-aside grass to production grassland, or by replacing it with spring barley (+/− autumn cover crop) or vegetation-free fallow. All treatments received unlabelled mineral N fertilizers.

Results

Crop uptake and leaching of ¹⁵N were generally highest in the first test year after termination of the set-aside. The leaching of residual ¹⁵N in soil declined in the order: vegetation-free soil (4.7%), spring barley (1.9%), spring barley + cover crop (0.7%) and production grassland (0.2%). Corresponding losses for the second leaching period were 2.7%, 0.9%, 0.4% and 0.06%. There was a fixed relationship between leaching losses of ¹⁵N and total N.

Conclusions

After residing in soil for 25–30 years, the lability of labelled mineral N fertilizer residues appeared slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses.     

Alleviation of P limitation makes tree roots competitive for N against microbes in a N-saturated conifer forest: A test through P fertilization and 15N labelling

Chronic N deposition to forests may induce N saturation and stand decline, leading to reduced ecosystem N retention capacity, triggered by a shift from N limitation of trees to limitation by another nutrient. We conducted a ¹⁵N soil labelling experiment in non-fertilized and P-fertilized plots at two elevations in an N-saturated Mediterranean-fir (Abies pinsapo) forest in southern Spain which shows P limitation symptoms. Root-exclusion was applied to identify the relative contributions of roots (plus mycorrhizal fungi) uptake, and heterotrophic immobilization by free-living microbes, to N retention. Overall ¹⁵N recovery from the litter, 0–15-cm soil and root-uptake components was c.a. 35% higher in P-fertilized than in non-fertilized plots at both elevations. In non-fertilized plots, soil was the biggest sink for added ¹⁵N. Phosphorus fertilization increased the competitive ability of tree roots for soil N resulting in equal importance of the autotrophic (roots plus associated mycorhizal fungi) and heterotrophic (free-living microbes) components with respect to total ¹⁵N recovery in P-fertilized plots. Phosphorus addition increased litter and soil N immobilization only if roots had been excluded. By combining in situ fertilization, root-exclusion and isotope labelling we have demonstrated that reduced N retention capacity and dominance of soil microbial over plant immobilization in a N-saturated forest results from a shift from N to P limitation of trees, while alleviation of P limitation makes tree roots and associated mycorrhizal fungi competitive for N against free soil microorganisms.     

Microbial biomass,N mineralization,and the activities of various enzymes in relation to nitrate leaching and root distribution in a slurry-amended grassland

High rates of cattle slurry application induce NO _inf3 ^sup- leaching from grassland soils. Therefore, field and lysimeter trials were conducted at Gumpenstein (Austria) to determine the residual effect of various rates of cattle slurry on microbial biomass, N mineralization, activities of soil enzymes, root densities, and N leaching in a grassland soil profile (Orthic Luvisol, sandy silt, pH 6.6). The cattle slurry applications corresponded to rates of 0, 96, 240, and 480 kg N ha^-1. N leaching was estimated in the lysimeter trial from 1981 to 1991. At a depth of 0.50 m, N leaching was elevated in the plot with the highest slurry application. In October 1991, deeper soil layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) from control and slurry-amended plots (480 kg N ha^-1) were investigated. Soil biological properties decreased with soil depth. N mineralization, nitrification, and enzymes involved in N cycling (protease, deaminase, and urease) were enhanced significantly (P<0.05) at all soil depths of the slurry-amended grassland. High rates of cattle slurry application reduced the weight of root dry matter and changed the root distribution in the different soil layers. In the slurry-amended plots the roots were mainly located in the topsoil (0–10 cm). As a result of this study, low root densities and high N mineralization rates are held to be the main reasons for NO _inf3 ^sup- leaching after heavy slurry applications on grassland.     

Potassium leaching from cut grassland and from urine patches

In grassland farming, especially on coarse‐textured soils, K can be a critical element. On these soils, the actual K management as well as fertilizer history to a large extent determine the leaching of K. The effects of four fertilizer regimes on the nutrient balances and leaching of K from grassland grown on a sandy soil were investigated. The swards differed in the source and level of N input and K fertilizer: no fertilizer N + 166 kg K ha^?1 year^?1 (Control), 320 kg inorganic N ha^?1 + 300 kg K ha^?1 year^?1 (MIN 320), 320 kg N + 425 kg K ha^?1 year^?1 in form of cattle slurry (SLR 320) and a grass–clover sward + 166 kg K ha^?1 year^?1 (WCL 0) without any inorganic N input. In a second experimental phase, cores from these swards were used in a mini‐lysimeter study on the fate of K from urine patches. On cut grassland after 6 years K input minus removal in herbage resulted in average K surpluses per year of 47, 39, 56 and 159 kg K ha^?1 for the Control, MIN 320, WCL 0 and SLR 320, respectively. Related leaching losses per year averaged 7.5, 5, 15 and 25 kg K ha^?1. Losses of urinary‐K through leaching were 2.2–4.5 and 5.7–8.4% of the K supplied in summer and autumn applications, respectively. Plant and soil were the major sinks for K from fertilizer or urine. High levels of exchangeable K in the soil and/or large and late fertilizer or urine applications stimulated leaching of K.     

Contribution of dissolved organic nitrogen deposition to total dissolved nitrogen deposition under intensive agricultural activities

For elucidating the atmospheric deposition contribution of dissolved organic nitrogen (DON) to the total dissolved nitrogen (TDN) deposition rate, dissolved inorganic nitrogen (DIN: NH₄ ⁺ + NO₃ ^–) and DON deposition rates were annually and monthly estimated during 4 and half-yr monitoring period in an experimental multi-farm under intensive agricultural activities of N fertilizer use and animal husbandry in Central Japan. Annual NH₄ ⁺, DON and NO₃ ^– deposition rates in bulk and wet deposition data accounted for 48%, 32% and 20% of TDN deposition, respectively, which indicated that this area is strongly affected by the intensive agricultural activities. The DIN and DON deposition rates were respectively estimated at 21.6 and 10.1 kg N ha^?1 yr^?1, which ranked high in a worldwide regional data set. Consequently, this area has been exposed to a large amount of N deposition including DON with N fertilizer input. The difference between bulk and wet deposition rates (NH₄ ⁺ and DON) is one of important factors controlling the N deposition in this area. Monthly DON deposition showed positive correlations with DIN and NH₄ ⁺ deposition rates, respectively, with a significant linear regression curve. The linear regression curve of our monthly data (n = 127) indicates the same trend as the worldwide annual data set (n = 31).     

Nitrogen availability and leaching during the terrestrial phase in a várzea forest of the Central Amazon floodplain

Availability and leaching of dissolved inorganic N (DIN = NH₄⁺ + NO₃^-) in soil were measured in a periodically flooded forest of the Central Amazon floodplain (várzea) during one terrestrial phase. Special emphasis was on the effects of a legume and a non-legume tree species. NH₄⁺-N accounted for more than 85% of DIN even at the end of the terrestrial phase although it decreased throughout the experimental period. While extractable NO₃^–-N was always low in the soil (less than 15% of DIN), the amount of leached NO₃^–-N was in the same range as NH₄⁺-N. Under the legume trees mean DIN contents of the topsoil were higher than under the non-legume trees. DIN leaching from the topsoil (0–20 cm) was significantly higher under the legume trees than at the other sites, also indicating a higher N availability. Therefore, despite considerable leaching legume trees may be an important source of N supporting a high biomass production of the várzea forest.     

Microbial activity differentially regulates the vertical mobility of nitrogen compounds in soil

Alongside nitrate, dissolved organic nitrogen (DON) represents a significant N loss pathway in many agroecosystems. To better understand the factors controlling DON leaching in soil we followed the vertical movement of ¹⁵N-labeled NO₃⁻, NH₄⁺, alanine and trialanine in packed soil columns in response to a simulated rainfall event. We show that in autoclaved (sterile) soil where sorption is assumed to be the dominant regulating factor, leaching followed the series NO₃⁻ > trialanine > alanine > NH₄⁺. In the non-sterile packed soil columns, the rapid rate of NO₃⁻ leaching was unaffected whilst the movement of the amino acid, peptide and NH₄⁺ was almost completely prevented due to microbial immobilization. Our results support the view that (1) DON loss from agricultural soils occurs mainly in the form of recalcitrant compounds (e.g. humic DON) rather than in the form of labile low MW DON (e.g. oligopeptides and amino acids), and (2) that although nitrate was bioavailable, it was not a preferred N form for the C-limited microbial biomass.