Soil nitrogen heterogeneity in a Dehesa ecosystem

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

秸秆在土壤内分解初期氮素矿化与固持的模拟测定

利用模拟软件Modelmaker对3种作物秸秆在土壤内分解初期氮素循环转化过程进行了模拟,取得了土壤铵态氮、硝态氮、微生物氮及其¹⁵N丰度等个变量模拟值和测定值的良好一致性.模型模拟对氮转化速率测定的结果表明,土壤微生物主要固持铵态氮,对硝态氮固持非常微弱.氮矿化主要发生于作物秸秆,腐殖质氮的矿化极其微弱.一级动力学方程对秸秆氮素矿化过程的描述优于零级动力学方程.微生物固持氮的再矿化过程落后于氮固持过程,假定再矿化不发生或认为再矿化与固持化同时进行可导致氮矿化与固持速率测定的严重误差.忽略氮硝化过程和挥发损失将导致氮矿化和固持速率的测定值偏低.净固持或净矿化的产生不仅与秸秆碳氮比有关,而且与秸秆在土壤内分解时间有关.     

Simulating the effects of N availability, straw particle size and location in soil on C and N mineralization

Predicting the C and N mineralization of straw added to soil is important for forecasting subsequent soil N availability during and between crop growth cycles. The decomposition module of the STICS model, parameterized under optimal conditions, was used to predict straw decomposition in sub-optimal conditions, i.e. when contact between soil and residue was poor (due to large size residues or surface placement) or when mineral N availability was restricted. The data used in the simulations were obtained from published studies of effects of residue size, location and N availability on C and N mineralization from straw under controlled laboratory conditions. We selected studies in which the dynamics of C and N mineralization were measured simultaneously. The dynamics of straw mineralization could be well predicted by the model under optimal conditions with standard parameter values as derived from measured C/N ratios of the residues, but not under sub-optimal conditions which required a new parameterization. A good fit could be obtained on these treatments by a marked reduction in the rate constants of residue and microbial biomass decomposition and a marked increase in the microbial biomass C/N ratio. Our results show the need to include in decomposition models routines for simulating effects of spatial heterogeneity of residue distribution, different particle sizes and limiting N availability.     

玉米与蚕豆秸秆配施对秸秆分解及土壤养分含量的影响

采用室内培养试验研究了禾本科作物玉米秸秆和豆科作物蚕豆秸秆单施及其不同比例配施后的秸秆分解及土壤养分含量.结果表明：单施玉米秸秆,土壤有机碳的矿化量和秸秆有机碳的矿化速率都较低,土壤矿质态氮被固持的时间也最长;玉米秸秆与蚕豆秸秆配合施用促进了秸秆有机碳和土壤固持矿质态氮的矿化.两种秸秆单施和配施均显著增加土壤微生物生物量碳、氮含量.禾本科作物秸秆与豆科作物秸秆配合施用,可以加快秸秆的分解,协调养分供应.     

The Effect of Phosphorus Availability on Decomposition Dynamics in a Seasonal Lowland Amazonian Forest

Once the weathering of parent material ceases to supply significant inputs of phosphorus (P), vegetation depends largely on the decomposition of litter and soil organic matter and the associated mineralization of organic P forms to provide an adequate supply of this essential nutrient. At the same time, the decomposition of litter is often characterized by the immobilization of nutrients, suggesting that nutrient availability is a limiting factor for this process. Immobilization temporally decouples nutrient mineralization from decomposition and may play an important role in nutrient retention in low-nutrient ecosystems. In this study, we used a common substrate to study the effects of native soil P availability as well as artificially elevated P availability on litter decomposition rates in a lowland Amazonian rain forest on highly weathered soils. Although both available and total soil P pools varied almost three fold across treatments, there was no significant difference in decomposition rates among treatments. Decomposition was rapid in all treatments, with approximately 50% of the mass lost over the 11-month study period. Carbon (C) and nitrogen (N) remaining and C:N ratios were the most effective predictors of amount of mass remaining at each time point in all treatments. Fertilized treatments showed significant amounts of P immobilization (P < 0.001). By the final collection point, the remaining litter contained a quantity equivalent to two-thirds of the initial P and N, even though only half of the original mass remained. In these soils, immobilization of nutrients in the microbial biomass, late in the decomposition process, effectively prevents the loss of essential nutrients through leaching or occlusion in the mineral soil.     

Incubation experiments on nitrogen mineralization in loess and sandy soils

Summary In aerobic incubation experiments, nitrogen mineralization was investigated in agricultural loess and sandy soils. Fresh, fieldmoist samples were used for incubation. Using an optimization procedure the N-mineralization was split into two nitrogen fractions: A resistant, slowly decomposable organic N-fraction (index rpm) and a fast decomposable N-fraction (index dpm).Loess- and sandy soils showed similar mean reaction coefficients for N-mineralization. The results also indicated that the amount of mineralizable nitrogen in the resistant N-fraction depended directly on clay content.Soil sampling at different times during crop growing period gave different mineralization amounts and courses.Effect of added plant residues on N-mineralization, was also studied by incubation. Variation of type and quantity of added residues changed the net N-mineralization in a characteristic way: Sugar beet leaves, added in minced form, caused an increase in mineralization; while straw caused a temporary immobilization, followed by remineralization.Incubation experiments on undisturbed soil columns showed nearly linear mineralization with time.This paper was presented in part at the 1983 Congress of the German Soil Science Society held at Trier.     

Soil C and N availability determine the priming effect: microbial N mining and stoichiometric decomposition theories

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter (SOM). Moreover, N input modifies the priming effect (PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural ¹³C labelling adding C₄‐sucrose or C₄‐maize straw to C₃‐soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects – microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K‐strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β‐glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r‐strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K‐ and r‐strategists were beneficial for priming effects, with an increasing contribution of K‐selected species under N limitation. Thus, the priming phenomenon described in ‘microbial N mining’ theory can be ascribed to K‐strategists. In contrast, ‘stoichiometric decomposition’ theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r‐strategists.     

Interactions of water,mulch and nitrogen on sorghum in Niger

We tested the hypothesis that plants only stimulate net mineralization of N when intense competition for N exists between plants and heterotrophs. Nitrogen mineralization in the soil used was insensitive to the range of moisture fluctuations that were inevitable during plant growth. Pots were planted to wheat (Triticum aestivum L.) or left unplanted and received no straw, straw added in one central layer, or straw added uniformly through the whole soil volume. Through the addition of¹⁵ N-labelled nitrate, initial soil inorganic N was increased to 17 g g^–1 in unplanted treatments and to 17 g g^–1 and 72 g g^–1 in planted treatments. Straw addition increased microbial immobilization of labelled N (soil inorganic N at planting), but did not reduce net mineralization of unlabelled soil N (soil organic N at planting), indicating that straw decomposers immobilized N early in the growth period. Plant growth did not reduce immobilization of N by straw decomposers. Net mineralization of N was not affected by plant growth at the low rate of N addition, but was reduced at the high rate of N addition. We conclude that the influence of wheat growth on net mineralization of N depends on soil N availability, with reductions in net mineralization at high N levels due to increased immobilization.     

Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest

The effects of the understory shrub Ledum palustre on soil N cycling were studied in a hardwood forest of Interior Alaska. This species releases high concentrations of phenolic compounds from green leaves and decomposing litter by rainfall. Organic and mineral soils sampled underneath L. palustre and at nearby non-Ledum sites were amended with L. palustre litter leachates and incubated at controlled conditions. We aimed to know (i) whether L. palustre presence and litter leachate addition changed net N cycling rates in organic and mineral soils, and (ii) what N cycling processes, including gross N mineralization, N immobilization and gross N nitrification, were affected in association with L. palustre. Our results indicate that N transformation rates in the surface organic horizon were not affected by L. palustre presence or leachate addition. However, mineral soils underneath L. palustre as well as soils amended with leachates had significantly higher C/N ratios and microbial respiration rates, and lower net N mineralization and N-to-C mineralization compared to no Ledum and no leachates soils. No nitrification was detected. Plant presence and leachate addition also tended to increase both gross N mineralization and immobilization. These results suggest that soluble C compounds present in L. palustre increased N immobilization in mineral soils when soil biota used them as a C source. Increases in gross N mineralization may have been caused by an enhanced microbial biomass due to C addition. Since both plant presence and leachate addition decreased soil C/N ratio and had similar effects on N transformation rates, our results suggest that litter leachates could be partially responsible for plant presence effects. The lower N availability under L. palustre canopy could exert negative interactions on the establishment and growth of other plant species.     

Decomposition of wheat straw and rye residues as affected by particle size

Effects of contact between the soil and crop residues on the processes of residue decomposition are still poorly understood. The objective of this study was to investigate the effects of residue particle size on the decomposition of wheat (Triticum aestivum L.) straw (C/N=270) and green rye (Secale cereale) residues (C/N=9). Residue particle size was used as a means to vary the contact between crop residues and the soil. Carbon mineralization was measured during 102 d for straw and 65 d for rye, on residues ranging in sizes from laboratory model (0.03 cm) to field-scale (10 cm). The soil was a silt (Typic Hapludalf) and the incubation was performed at 15 °C. The effects of particle size on C mineralization varied for the two residues. In the first two days of incubation, decomposition rate of rye increased with decreasing particle size but thereafter, the trend was reversed. In 65 days, 8% more C was decomposed in the 7-cm residues than in the 0.03-cm ones. For wheat straw, early decomposition (3–17 days) was faster for the small-sized particles (0.06 and 0.1 cm). Thereafter, the largest size classes (5 and 10 cm) decomposed faster. After 102 days, the very fine particles ( 0.1 cm) showed the greatest and the intermediate size classes (0.5 and 1 cm), the lowest amount of C mineralized. We hypothesized that greater availability and accessibility of N was responsible for the higher rates of decomposition observed for finely-ground wheat straw while a physical protection of finely ground residues was probably involved in the observed reverse effect for rye.     

The Role of Balsam Poplar Secondary Chemicals in Controlling Soil Nutrient Dynamics through Succession in the Alaskan Taiga

The vegetation mosaic of the Alaskan taiga is produced by patterns of disturbance coupled to well-defined successional patterns. In primary succession on river floodplains, one of the critical transitions in succession is that from thinleaf alder (Alnus tenuifolia) to balsam poplar (Populus balsamifera). This is the shift from a N2-fixing shrub to a deciduous tree. Through this transition there are major changes in N cycling including a decrease in N2-fixation, mineralization, and nitrification. Most models of plant effects on soil processes assume that these changes are caused by shifts in litter quality and C/N ratio. This paper reviews several studies examining the effects of balsam poplar secondary chemicals on soil nutrient cycling. Balsam poplar tannins inhibited both N2-fixation in alder, and decomposition and N-mineralization in alder soils. Other poplar compounds, including low-molecular-weight phenolics, were microbial substrates and increased microbial growth and immobilization, thereby reducing net soil N availability. Thus, substantial changes in soil N cycling through succession appear to have been mediated by balsam poplar secondary chemicals.     

In situ mineralization of nitorgen and phosphorus of arctic soils after perturbations simulating climate change

Seasonal net nitrogen (N) and phosphorus (P) mineralization was investigated at Abisko, Swedish Lapland in soils of a subarctic heath and in soils of a colder (by about 4° C), high altitude fellfield by (a) using in situ soil incubation in soils which had been shaded or subjected to two levels of increased temperature, combined with (b) reciprocal transplantation of soils between the two sites. Proportionally large and significant net seasonal mineralization of N, in contrast to non-significant P mineralization, was found in untransplanted and transplanted fellfield soil. In contrast, P was mineralized in proportionally large amounts, in contrast to low N mineralization, in the transplanted and untransplanted heath soil. The differences indicate that P was strongly immobilized in relation to N at the fellfield and that N was more strongly immobilized than P in the heath soil. The immobilization in both soils remained high even after a temperature change of 4–5° C experienced by transplanted soils. Air temperature increases of up to 4–5° C in greenhouses resulted in a soil temperature increase of 1–2° C and did not cause any extra increase of net N and P mineralization. The results suggest that soil temperature increases of up to 2° C, which are likely to occur by the end of the next century as an effect of a predicted 4–5° C rise in air temperature, have only small effects on net mineralization in at least two characteristic tundra soils. These effects are probably smaller than the natural fluctuation of plant available nutrients from site to site, even within the same plant community. A further soil temperature increase of up to 4–5° C may enhance decomposition and gross mineralization, but the rate of net mineralization, and hence the change of nutrient availability to the plants, depends on the extent of microbial immobilization of the extra nutrients released.     

Effect of rainfall pattern on nitrogen mineralization and leaching in a green manure experiment in South Rwanda

The effects of green manures, sorghum residues and farmyard manure on N dynamics and crop yields were studied during three dry and wet seasons on a Typic Sombriudox in South Rwanda. In addition, a resin core study was conducted within a 4-year green manure field experiment to follow the seasonal pattern of N mineralization and leaching after application of residues from Tephrosia vogelii, Sorghum bicolor, a mixture of both materials, and farmyard manure.During the dry season, topsoil (0–20 cm) mineral N remained constant. At the beginning of the wet season, the rainfall pattern determined N availability. With low rainfall intensities a mineralization flush occurred, doubling topsoil mineral N concentrations within 5 days after wetting. In contrast, under heavy rains at the onset of the rainy season, topsoil mineral N decreased by 50–70% within the first two weeks.The application of organic fertilizers has a strong influence on N availability, but the effects can be negated by heavy rainfall. Incorporation of leaves from Tephrosia vogelii (2.7 t dm ha^-1) and farmyard manure (7 t dm ha^-1) doubled the mineralization flush after the first rains. During the rest of the wet season, N release by the green manure was small, whereas the farmyard manure was found to mobilize N after a period of N immobilization. Incorporation of sorghum residues had only a small effect, while mixing the straw with green and farmyard manure immobilized N temporarily.Nitrogen leaching, measured by exchange resins at a depth of 20 cm, was increased up to 50% by the incorporation of green and farmyard manure. This points to rapid N translocation of easily mineralizable N. The additional incorporation of sorghum residues reduced N leaching of both materials significantly. Since rainfall is often unpredictable, the synchronization of N released from crop residues with crop N demand may require additional management practices.     

Phytotoxicity from plant residues

Summary Proximity of new wheat straw residues to sown wheat seed has an effect on germination, plant growth and ultimate yield. Decomposition of wheat straw may produce toxins or it may cause immobilization of nitrogen in, or applied to the soil. In pot experiments, it has been shown that germination of wheat was depressed when large amounts of straw were decomposed on the surface for up to 18 days; after 54 days it had no effect on germination. Immobilization of nitrogen occurred mainly when the straw was mixed with the soil, or when surface-rotted straw was ploughed into the soil just before seeding. The latter effect could not be overcome by the addition of mineral nitrogen. Part II, Plant and Soil 38, 347–361 (1973).     

Assessment of soil nitrogen and phosphorous availability under elevated CO2 and N-fertilization in a short rotation poplar plantation

Photosynthetic stimulation by elevated [CO₂] is largely regulated by nitrogen and phosphorus availability in the soil. During a 6 year Free Air CO₂ Enrichment (FACE) experiment with poplar trees in two short rotations, inorganic forms of soil nitrogen, extractable phosphorus, microbial and total nitrogen were assessed. Moreover, in situ and potential nitrogen mineralization, as well as enzymatic activities, were determined as measures of nutrient cycling. The aim of this study was to evaluate the effects of elevated [CO₂] and fertilization on: (1) N mineralization and immobilization processes; (2) soil nutrient availability; and (3) soil enzyme activity, as an indication of microbial and plant nutrient acquisition activity. Independent of any treatment, total soil N increased by 23% in the plantation after 6 years due to afforestation. Nitrification was the main process influencing inorganic N availability in soil, while ammonification being null or even negative. Ammonium was mostly affected by microbial immobilization and positively related to total N and microbial biomass N. Elevated [CO₂] negatively influenced nitrification under unfertilised treatment by 44% and consequently nitrate availability by 30% on average. Microbial N immobilization was stimulated by [CO₂] enrichment and probably enhanced the transformation of large amounts of N into organic forms less accessible to plants. The significant enhancement of enzyme activities under elevated [CO₂] reflected an increase in nutrient acquisition activity in the soil, as well as an increase of fungal population. Nitrogen fertilization did not influence N availability and cycling, but acted as a negative feed-back on phosphorus availability under elevated CO₂.     

In-situ方法在研究退化土壤氮库时空变化中的应用

利用原状土连续就地取样(sequentialcoringandin-situexposure)方法研究了澜沧江流域典型退化土壤的氮库营养动态变化过程,监测了矿质氮在时间和空间上的释放与固定、淋失与植物吸收消耗。结果表明人为干扰影响土壤氮矿质化,导致氮固定、淋失,引起养分衰减退化。从阔叶林转变为果园、坡耕地、桉树林和针叶林,矿质氮60d内平均衰减分别为51.51,29.64,26.84,16.40mg·kg-1,变异程度依次为21.5%、11.0%、14.2%、8.3%,氮固定分别为15.45,8.51,13.90,0.00mg·kg-1,淋失量则坡耕地最大,达44.50mg·kg-1,其次是针叶林和桉树林地,分别为38.41和25.30mg·kg-1。植物对土壤氮的吸收消耗为果园>坡耕地作物>桉树林>针叶林>阔叶林,利用形态以硝态氮为主。     

Soil nitrogen availability evaluations based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants

Summary Sudangrass [Sorghum sudanense (Piper) Stapf] was grown in a greenhouse pot experiment on 39 soils having a broad range of chemical and physical characteristics. Labelled N as sodium nitrate (9% excess N¹⁵) was applied at rates of 200 and 400 mg of N per pot (2kg of soil). After 6 weeks of growth, total N and N¹⁵ were determined on plant tops and roots and on the cropped soils. Maximum yield differed widely among the soils owing to variations in yield-limiting factors other than N. Despite the diversity of responses to N fertilizer, the experiment provided a meaningful basis for assessing soil nitrogen availability. Amounts of N taken up from soils were similar from pots receiving no fertilizer N and from pots receiving labeled N.Amounts of soil organic N mineralized during cropping plus the mineral N present initially in the soils correlated highly with amounts of soil N taken up by whole plants (tops and roots). Average recovery by whole plants of mineral N formed before and during the cropping period was about 85 per cent, a value corresponding closely to recovery of fertilizer N in this experiment. The similarity in recovery of N provided by soil and fertilizer suggests that mineral N from these sources comprised a common pool that behaved as an entity with respect to mineralization-immobilization relations or other reactions affecting N availability to plants.A-values, the amounts of soil N having an availability equivalent to that of applied fertilizer N, were similar for two levels of applied labeled N and for tops and whole plants. Moreover, A-values were similar to amounts of N mineralized before and during crop growth. This result is particularly significant, since amounts of N mineralized during crop growth were estimated from N mineralization potentials, taking into account the effects of temperature on the mineralization rate constant. Thus, the study provides preliminary evidence that the soil N mineralization potential offers a basis for reliably estimating amounts of soil N mineralized during selected periods of time under specified temperature regimes.     

Mineralization and distribution of nutrients in plants and microbes in four arctic ecosystems: responses to warming

Mineralization and nutrient distribution in plants and microbes were studied in four arctic ecosystems at Abisko, Northern Sweden and Toolik Lake, Alaska, which have been subjected to long-term warming with plastic greenhouses. Net mineralization and microbial immobilization were studied by the buried bag method and ecosystem pool sizes of C, N and P were determined by harvest methods. The highest amounts of organic N and P were bound in the soil organic matter. Microbial N and P constituted the largest labile pools often equal to (N) or exceeding (P) the amounts stored in the vegetation. Despite large pools of N and P in the soil, net mineralization of N and P was generally low during the growing season, except in the wet sedge tundra, and in most cases lower than the plant uptake requirement. In contrast, the microorganisms immobilized high amounts of nutrients in the buried bags during incubation. The same high immobilization was not observed in the surrounding soil, where the microbial nutrient content in most cases remained constant or decreased over the growing season. This suggests that the low mineralization measured in many arctic ecosystems over the growing season is due to increased immobilization by soil microbes when competition from plant roots is prevented. Furthermore, it suggests that plants compete well with microbes for nutrients in these four ecosystems. Warming increased net mineralization in several cases, which led to increased assimilation of nutrients by plants but not by the microbes.     

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The inverse texture hypothesis for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.     

Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE)

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO₂, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO₂ did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO₂ lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO₂ attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO₂, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.