A simple model for estimating the contribution of nitrogen mineralization to the nitrogen supply of crops from a stabilized pool of soil organic matter and recent organic input

A simple model was developed to estimate the contribution of nitrogen (N) mineralization to the N supply of crops. In this model the soil organic matter is divided into active and passive pools. Annual soil mineralization of N is derived from the active pool. The active pool comprises stabilized and labile soil organic N. The stabilized N is built up from accumulated inputs of fresh organic N during a crop rotation but the labile N is a fraction of total N added, which mineralizes faster than the stabilized N. The passive pool is considered to have no participation in the mineralization process. Mineralization rates of labile and stabilized soil organic N from different crop residues decomposing in soil were derived from the literature and were described by the first-order rate equation dN/dt =-K*N, where N is the mineralizable organic N from crop residues andK is a constant. The data were groupedK ₁ by short-term (0–1 year) andK ₂ by long-term (0–10 years) incubation. Because the range of variation inK ₂ was smaller than inK ₁ we felt justified in using an average value to derive N mineralization from the stabilized pool. The use of a constant rate ofK ₁ was avoided so net N mineralization during the first year after addition is derived directly from the labile N in the crop residues. The model was applied to four Chilean agro-ecosystems, using daily averages of soil temperature and moisture. The N losses by leaching were also calculated. The N mineralization varied between 30 and 130 kg N ha^–1 yr^–1 depending on organic N inputs. Nitrogen losses by leaching in a poorly structured soil were estimated to be about 10% of total N mineralized. The model could explain the large differences in N- mineralization as measured by the potential N mineralization at the four sites studied. However, when grassland was present in the crop rotation, the model underestimated the results obtained from potential mineralization.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

How Physical Disturbance and Nitrogen Addition Affect the Soil Carbon Decomposition?

The decomposition of soil organic carbon (SOC) plays a critical role in regulating atmospheric CO₂ concentrations and climate dynamics. However, the mechanisms and factors controlling SOC decomposition are still not fully understood. Here, we conducted a 60 days incubation experiment to test the effects of physical disturbance and nitrogen (N) addition on SOC decomposition. N addition increased the concentration of NO₃^- by 51% in the soil, but had little effect on the concentration of NH₄⁺. N addition inhibited SOC decomposition, but such an effect differed between disturbed and undisturbed soils. In disturbed and undisturbed soils, application of N decreased SOC decomposition by 37% and 15%, respectively. One possible explanation is that extra N input suppressed microbial N mining and/or increased the stability of soil organic matter by promoting the formation of soil aggregates and incorporating part of the inorganic N into organic matter, and consequently decreased microbial mineralization of soil organic matter. Physical disturbance intensified the inhibition of N on SOC decomposition, likely because physical disturbance allowed the added N to be better exposed to soil microbes and consequently increased the availability of added N. We conclude that physical disturbance and N play important roles in modulating the stability of SOC.     

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO₃ ^?-N and total inorganic N concentrations than pasture soils, but substantial NO₃ ^?-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

The small-scale pattern of seepage water nitrate concentration in an N saturated spruce forest is regulated by net N mineralization in the organic layer

Soil net nitrogen (N) mineralization and nitrification as well as gross nitrification rates were studied in a forest soil within a 30?×?18m homogeneous plot located in an N saturated mature spruce stand at the Höglwald Forest (Bavaria, Germany) in order to explain the small-scale variation in nitrate (NO₃ ^?) concentration in seepage water. Seepage water was sampled below the main rooting zone in 40cm depth with suction cups over two periods at 20 measuring spots respectively. The sampling spots were uniformly distributed over the plot for both sampling periods, and represented the whole concentration range of seepage water NO₃ ^?concentrations measured within a close mesh of 121 suction cups. At each measuring spot soil net N mineralization, gross and net nitrification, heterotrophic soil respiration, extractable soil ammonium (NH₄ ⁺) and NO₃ ^?, and additional physical and chemical soil parameters were measured in the organic layer and correlated with the NO₃ ^? concentrations in seepage water. Furthermore, the effects of environmental parameters on N conversion processes were evaluated using multiple linear regression analysis. We found that the small-scaled variations in seepage water NO₃ ^? concentration were related to similar small-scaled variations in key processes of microbial N turnover rates in the organic layer. Within this study net N mineralization in the organic layer could explain 51–59% of the corresponding small-scale variation of nitrate concentrations in seepage water below the main rooting zone using a multiple linear regression model with stepwise procedure. In addition, we found that small-scale patterns of N turnover in the organic layer were strongly influenced by water content in the organic layer and the dry mass of organic matter.     

Increase in pH stimulates mineralization of ‘native’ organic carbon and nitrogen in naturally salt-affected sandy soils

Large amounts of terrestrial organic C and N reserves lie in salt-affected environments, and their dynamics are not well understood. This study was conducted to investigate how the contents and dynamics of ‘native’ organic C and N in sandy soils under different plant species found in a salt-affected ecosystem were related to salinity and pH. Increasing soil pH was associated with significant decreases in total soil organic C and C/N ratio; particulate (0.05–2 mm) organic C, N and C/N; and the C/N ratio in mineral-associated (<0.05 mm) fraction. In addition, mineral-associated organic C and N significantly increased with an increase in clay content of sandy soils. During 90-day incubation, total CO₂-C production per unit of soil organic C was dependent on pH [CO₂-C production (g kg⁻¹ organic C) = 22.5 pH – 119, R ² = 0.79]. Similarly, increased pH was associated with increased release of mineral N from soils during 10-day incubation. Soil microbial biomass C and N were also positively related to pH. Metabolic quotient increased with an increase in soil pH, suggesting that increasing alkalinity in the salt-affected soil favoured the survival of a bacterial-dominated microbial community with low assimilation efficiency of organic C. As a result, increased CO₂-C and mineral N were produced in alkaline saline soils (pH up to 10.0). This pH-stimulated mineralization of organic C and N mainly occurred in particulate but not in mineral-associated organic matter fractions. Our findings imply that, in addition to decreased plant productivity and the litter input, pH-stimulated mineralization of organic matter would also be responsible for a decreased amount of organic matter in alkaline salt-affected sandy soils.     

Effects of plant species diversity, atmospheric [CO2], and N addition on gross rates of inorganic N release from soil organic matter

A significant challenge in predicting terrestrial ecosystem response to global changes comes from the relatively poor understanding of the processes that control pools and fluxes of plant nutrients in soil. In addition, individual global changes are often studied in isolation, despite the potential for interactive effects among them on ecosystem processes. We studied the response of gross N mineralization and microbial respiration after 6 years of application of three global change factors in a grassland field experiment in central Minnesota (the BioCON experiment). BioCON is a factorial manipulation of plant species diversity (1, 4, 9 and 16 prairie species), atmospheric [CO₂] (ambient and elevated: 560 μmol mol^?1), and N inputs (ambient and ambient +4 g N m^?2 yr^?1). We hypothesized that gross N mineralization would increase with increasing levels of all factors because of stimulated plant productivity and thus greater organic inputs to soils. However, we also hypothesized that N addition would enhance, while elevated [CO₂] and greater diversity would temper, gross N mineralization responses because of increased and reduced plant tissue N concentrations, respectively. In partial support of our hypothesis, gross N mineralization increased with greater diversity and N addition, but not with elevated [CO₂]. The ratio of gross N mineralization to microbial respiration (i.e. the ‘yield’ of inorganic N mineralized per unit C respired) declined with greater diversity and [CO₂] suggesting increasing limitation of microbial processes by N relative to C in these treatments. Based on these results, we conclude that the plant supply of organic matter primarily controls gross N mineralization and microbial respiration, but that the concentration of N in organic matter input secondarily influences these processes. Thus, in systems where N limits plant productivity these global change factors could cause different long‐term ecosystem trajectories because of divergent effects on soil N and C cycling.     

Modelling the change in soil organic C and N and the mineralization of N from soil in response to applications of slurry manure

A computer simulation model of the turnover of organic matter in soil was adapted to simulate the change in soil organic C and N contents of soil during several years following annual additions of farm slurry to maize fields. The model proved successful in estimating the build-up of both C and N in soil and the leaching of N to ground-water in response to applications of slurry ranging from 50 to 300 tons per hectare per year. The model was then used to estimate the build-up of organic matter in soil under crops of fodder maize that were grown using the excess of manure produced during the last 20 years in the Netherlands. The build-up of organic matter from these applications was estimated to lead to about 70 kg extra nitrogen mineralized ha^-1 yr^-1. As a result of legislation manure applications have decreased and are expected to decrease further in the immediate future. Calculations suggest that after 10 years of manure applied at rates no longer exceeding the amount needed to replace the phosphorus removed by crops, the extra mineralization of N will still be between 45 and 60 kg ha^-1 yr^-1. If manure applications cease altogether then the extra mineralization will be about 25–30 kg N ha^-1 yr^-1.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Natural 15N abundance of vegetation and soil in the Kapalga savanna,Australia

Abstract The natural abundance of the stable isotope ¹⁵N was measured in different vegetation components and in the soil of a northern Australian savanna. Most of the vegetation was found to be ¹⁵N-depleted compared to atmospheric N₂. Herbaceous legumes, perennial grasses, tree legumes, non-legume trees and annual grasses exhibited mean δ¹⁵N of ? 1.7, ? 0.8, ? 0.7, 0.0 and + 0.3‰, respectively. These results are in good agreement with previous studies. Legumes exhibit slightly negative values, indicating that they are likely to be nitrogen-fixing plants. Non-legume plants have a δ¹⁵N close to zero, which could equally result from non-symbiotic fixation, soil organic matter mineralization, or fresh root litter mineralization. In contrast, soil organic matter was ¹⁵N-enriched. Values of δ¹⁵N increased with depth and were + 2.5, + 5.2 and +6.1‰ in the 0–10, 10–20 and 20–40cm layers, respectively. Soil organic matter δ¹⁵N shows a typical profile of mature soils.     

Soil organic matter as a potential net nitrogen sink in a fertilized cornfield,South Deerfield,Massachusetts, USA

Net nitrogen (N) mineralization in situ and N mineralization potential (N₀) over one complete year (1986–1987) were examined for a conventionally managed silage cornfield that received at least 235 kg fertilizer N ha^-1. Net N mineralization at the site, measured by sequential in situ polyethylene-bag incubations, totaled –54 kg N ha^-1 yr^-1, and –31 kg N ha^-1 over the May-to-August growing season. Nitrogen mineralization potential of the soil organic matter (SOM), measured by laboratory anaerobic incubations, was positive uniformly and varied with month of sample collection. The soil gained 72 kg inorganic N ha^-1 from April to October, principally because of a fall manuring, only 7 kg N ha^-1 from April to September. The in situ incubations, likely more representative of the balance between N mineralization and immobilization under N-fertilized conditions, suggest that SOM at the site is accumulating N.Contribution from the Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003, USA.Contribution from the Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003, USA.     

Nitrogen mineralization, denitrification, and nitrate ammonification by soil-feeding termites: a 15N-based approach

Soil-feeding termites are abundant and play important roles in the biogeochemical processes in tropical soils. Previous studies indicated that they preferentially utilize the peptidic components of soil organic matter as a nutrient resource. Here, we determined the corresponding mineralization fluxes and elucidated other N transformation processes that occur during soil gut passage using ¹⁵N tracer techniques. Termite-based rates of N mineralization by Cubitermes umbratus and Cubitermes ugandensis in soil microcosms amended with ¹⁵NH₄ ⁺ were 6.6 and 9.2 nmol N day^?1 (g fresh wt)^?1, which means that the soil peptides fuel about 20 and 40% of the respiratory activity of these insects. Considering the areal biomass of soil-feeding termites in humid savannahs, soil-feeding termites should mineralize about 3% of the total N in their food soil per year. In addition to producing ammonia from ingested ¹⁵NO₃ ^? at approximately 10% of the mineralization rate, C. umbratus also formed N₂ at similar rates. The formation of labelled N₂ in microcosms amended with ¹⁵NH₄ ⁺ seems to be at least partially due to nitrification activity in the soil; evidence for the formation of nitrate in the posterior hindgut remains inconclusive. However, the so far unexplained increase of ¹⁵N abundance in the ammonia pools of the posterior hindgut compartments manifests additional hitherto unknown metabolic processes in this gut region. Collectively, our results not only reinforce the concept of nitrogenous soil components as an important dietary resource for soil-feeding termites, but also allow us to predict that N mineralization and nitrate ammonification activities in the termite gut should positively affect the dynamics of N in tropical soil.     

Nitrogen addition alters mineralization dynamics of 13C‐depleted leaf and twig litter and reduces leaching of older DOC from mineral soil

Recent reviews indicate that N deposition increases soil organic matter (SOM) storage in forests but the undelying processes are poorly understood. Our aim was to quantify the impacts of increased N inputs on soil C fluxes such as C mineralization and leaching of dissolved organic carbon (DOC) from different litter materials and native SOM. We added 5.5 g N m^?2 yr^?1 as NH₄NO₃ over 1 year to two beech forest stands on calcareous soils in the Swiss Jura. We replaced the native litter layer with ¹³C‐depleted twigs and leaves (δ¹³C: ?38.4 and ?40.8‰) in late fall and measured N effects on litter‐ and SOM‐derived C fluxes. Nitrogen addition did not significantly affect annual C losses through mineralization, but altered the temporal dynamics in litter mineralization: increased N inputs stimulated initial mineralization during winter (leaves: +25%; twigs: +22%), but suppressed rates in the subsequent summer. The switch from a positive to a negative response occurred earlier and more strongly for leaves than for twigs (?21% vs. 0%). Nitrogen addition did not influence microbial respiration from the nonlabeled calcareous mineral soil below the litter which contrasts with recent meta‐analysis primarily based on acidic soils. Leaching of DOC from the litter layer was not affected by NH₄NO₃ additions, but DOC fluxes from the mineral soils at 5 and 10 cm depth were significantly reduced by 17%. The ¹³C tracking indicated that litter‐derived C contributed less than 15% of the DOC flux from the mineral soil, with N additions not affecting this fraction. Hence, the suppressed DOC fluxes from the mineral soil at higher N inputs can be attributed to reduced mobilization of nonlitter derived ‘older’ DOC. We relate this decline to an altered solute chemistry by NH₄NO₃ additions, an increased ionic strength and acidification resulting from nitrification, rather than to a change in microbial decomposition.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Soil Ca alters processes contributing to C and N retention in the Oa/A horizon of a northern hardwood forest

Recent studies on the effects of calcium (Ca) additions on soil carbon (C) cycling in organic soil horizons present conflicting results, with some studies showing an increase in soil C storage and others a decrease. We tested the legacy effects of soil Ca additions on C and nitrogen (N) retention in a long-term incubation of soils from a plot-scale field experiment at the Hubbard Brook Experimental Forest, NH, USA. Two levels of Ca (850 and 4250 kg Ca/ha) were surface applied to field plots as the mineral wollastonite (CaSiO₃) in summer of 2006. Two years after field Ca additions, Oa/A horizon soils were collected from field plots and incubated in the laboratory for 343 days to test Ca effects on C mineralization, dissolved organic carbon (DOC) export, and net N transformations. To distinguish mineralization of soil organic C (SOC) from that of more recent C inputs to soil, we incubated soils with and without added ¹³C-labeled sugar maple leaf litter. High Ca additions increased exchangeable Ca and pH compared to the control. While low Ca additions had little effect on mineralization of SOC or added litter C, high Ca additions reduced mineralization of SOC and enhanced mineralization of litter C. In litter-free incubations, δ¹³C of respired C was enriched in the high Ca treatment compared to the control, indicating that Ca suppressed mineralization of ¹³C-depleted SOC sources. Leaching of DOC and NH₄ ⁺ were reduced by Ca additions in litter-free and litter-amended soils. Our results suggest that Ca availability in these organic soils influences mineralization of SOC and N primarily by stabilization processes and only secondarily through pH effects on organic matter solubility, and that SOC binding processes become important only with relatively large alterations of Ca status.     

Nitrogen mineralization in a green manure-amended soil as influenced by cropping history and subsequent crop

A greenhouse experiment was conducted to determine the influence of cropping variables on nitrogen dynamics in a soil amended with green manure. Surface soil from various long-term spring wheat rotations was amended with¹⁵N-labelled legume green manure (Lathyrus tingitanus) and subsequently cropped (canola [Brassica napus] and spring wheat [Triticum aestivum]) or incubated without a crop for 56 days in a greenhouse. Nitrogen mineralization from both the indigenous soil N and from green manure was suppressed in cropped soil. Net N mineralization in the uncropped and cropped treatments averaged 73 and 43 mg kg⁻¹, respectively. This difference was attributed, in part, to enhanced biological immobilization in the rhizosphere. Previous cropping practices also had significant effect on N mineralization, largely by their influence on indigenous organic matter quality. These observations suggest that short-term N mineralization is favored by fallowing soil after green manure application whereas N retention in organic matter is favored by immediate cropping. Contribution 3878873     

蚂蚁筑巢对西双版纳热带森林土壤有机氮矿化的影响

蚂蚁作为生态系统工程师能够调节土壤微生物及理化环境,进而对热带森林土壤有机氮矿化速率及其时间动态产生显著影响。以西双版纳白背桐热带森林群落为研究对象,采用室内需氧培养法测定土壤有机氮矿化速率,比较蚁巢和非蚁巢土壤有机氮矿化速率的时间动态,揭示蚂蚁筑巢活动引起土壤无机氮库、微生物生物量碳及化学性质改变对有机氮矿化速率时间动态的影响。结果表明：（1）蚂蚁筑巢显著影响土壤有机氮矿化速率（P<0.01）,相较于非蚁巢,蚁巢土壤有机氮矿化速率提高了261%;（2）土壤有机氮矿化速率随月份推移呈明显的单峰型变化趋势,即6月最大（蚁巢1.22 mg kg^-1 d^-1、非蚁巢0.41 mg kg^-1 d^-1）,12月最小（蚁巢0.82 mg kg^-1 d^-1、非蚁巢0.18 mg kg^-1 d^-1）;（3）两因素方差分析表明,不同月份及不同处理对土壤有机氮矿化速率、NH₄-N及NO₃-N产生显著影响（P<0.05）,但对NO₃-N的交互作用不显著;（4）蚂蚁筑巢显著提高了无机氮库（NH₄-N与NO₃-N）、微生物生物量碳、有机质、水解氮、全氮及易氧化有机碳等土壤养分含量,而降低了土壤pH值;（5）回归分析表明,铵态氮和硝态氮对土壤有机氮矿化速率产生显著影响,分别解释87.89%、61.84%的有机氮矿化速率变化;（6）主成份分析表明NH₄-N、微生物生物量碳及有机质是影响有机氮矿化速率时间动态的主要因素,而全氮、NO₃-N、易氧化有机碳、水解氮及pH对土壤有机氮矿化速率的影响次之,且pH与土壤有机氮矿化速率呈显著负相关。总之,蚂蚁筑巢活动主要通过影响土壤NH₄-N、微生物生物量碳及有机质的状况,进而调控西双版纳热带森林土壤有机氮矿化速率的时间动态。研究结果将有助于进一步提高对土壤氮矿化生物调控机制的认识。     

Soil C and N dynamics within a precipitation gradient in Mediterranean eucalypt plantations

Soil C and N dynamics were evaluated in five eucalypt plantations within a precipitation gradient (500–2,000 mm) in Portugal. Soil physical and chemical properties, total and labile (particulate organic matter, hydrolyzable, hot water soluble and microbial) soil C and N pools, and C and N mineralization were measured to characterize the C and N dynamics and their controlling factors within this gradient. Contents of total and labile soil organic C and N were positively correlated with the mean annual precipitation. A similar relationship was observed for net N mineralization (anaerobic and long-term aerobic incubation), gross N mineralization (¹⁵N isotope dilution technique) and C mineralization. In contrast, rates of C and N mineralization (per unit of C and N) were higher in the driest sites due to their higher proportion of particulate organic matter C. Net and gross N mineralization were strongly correlated and showed similar controlling factors (mean annual precipitation, total and labile C and N and extractable P contents), suggesting that net N mineralization during long-term aerobic incubation reflects gross N transformations. Although, gross NO₃–N production and gross NO₃–N immobilization were observed in all sites, net nitrification in the drier sites was not observed in the first weeks of the study. Our results suggest that, under Mediterranean conditions, mean annual precipitation is the major factor determining the C and N dynamics in soils with Eucalyptus plantations.     

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

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