Showing papers by "Sven Jonasson published in 2008"

Site-dependent N uptake from N-form mixtures by arctic plants, soil microbes and ectomycorrhizal fungi

Abstract: Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH 4 + –N and NO 3 − –N, with one N form labelled with 15N at a time, and in the case of glycine, also labelled with 13C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH 4 + - and NO 3 − –15N, respectively, recovered in the plant–soil system, and the major part of 15N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized 15N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO 3 − . Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms.

Plant and Microbial Uptake and Allocation of Organic and Inorganic Nitrogen Related to Plant Growth Forms and Soil Conditions at Two Subarctic Tundra Sites in Sweden

Abstract: In order to follow the uptake and allocation of N in different plant functional types and microbes in two tundra ecosystems differing in nutrient availability, we performed a 15N-labeling experiment with three N forms and followed the partitioning of 15N label among plants, microorganisms and soil organic matter. At both sites the deciduous dwarf shrub Betula nana and the evergreen Empetrum hermaphroditum absorbed added 15N at rates in the order: NH4+ > NO3− > glycine, in contrast to the graminoid Carex species which took up added 15N at rates in the order NO3− > NH4+ > glycine. Carex transported a high proportion of 15N to aboveground parts, whereas the dwarf shrubs allocated most 15N to underground storage. Enhanced 13C in Betula nana roots represents the first field evidence of uptake of intact glycine by this important circumpolar plant. Plant and microbial uptake of label was complementary as plants took up more inorganic than organic N, while microbes preferred organic N. Microbes initially...

Uptake of pulse injected nitrogen by soil microbes and mycorrhizal and non-mycorrhizal plants in a species-diverse subarctic heath ecosystem

Abstract: 15N labeled ammonium, glycine or glutamic acid was injected into subarctic heath soil in situ, with the purpose of investigating how the nitrogen added in these pulses was subsequently utilized and cycled in the ecosystem. We analyzed the acquisition of 15N label in mycorrhizal and non-mycorrhizal plants and in soil microorganisms, in order to reveal probable differences in acquisition patterns between the two functional plant types and between plants and soil microorganisms. Three weeks after the label addition, with the 15N-forms added with same amount of nitrogen per square meter, we analyzed the 15N-enrichment in total soil, in soil K2SO4 (0.5 M) extracts and in the microbial biomass after vacuum-incubation of soil in chloroform and subsequent K2SO4 extraction. Furthermore the 15N-enrichment was analyzed in current years leaves of the dominant plant species sampled three, five and 21 days after label addition. The soil microorganisms had very high 15N recovery from all the N sources compared to plants. Microorganisms incorporated most 15N from the glutamic acid source, intermediate amounts of 15N from the glycine source and least 15N from the NH4+ source. In contrast to microorganisms, all ten investigated plant species generally acquired more 15N label from the NH4+ source than from the amino acid sources. Non-mycorrhizal plant species showed higher concentration of 15N label than mycorrhizal plant species 3 days after labeling, while 21 days after labeling their acquisition of 15N label from amino acid injection was lower than, and the acquisition of 15N label from NH4 injection was similar to that of the mycorrhizal species. We conclude that the soil microorganisms were more efficient than plants in acquiring pulses of nutrients which, under natural conditions, occur after e.g. freeze–thaw and dry–rewet events, although of smaller size. It also appears that the mycorrhizal plants in the short term may be less efficient than non-mycorrhizal plants in nitrogen acquisition, but in a longer term show larger nitrogen acquisition than non-mycorrhizal plants. However, the differences in 15N uptake patterns may also be due to differences in leaf longevity and woodiness between plant functional groups.

Nitrogen Uptake During One Year in Subarctic Plant Functional Groups and in Microbes After Long-Term Warming and Fertilization

Abstract: For the first time in an arctic long-term warming and fertilization experiment, the short-term (days) and longer-term (month and year) nitrogen (N) uptake and allocation in plants, microbes, and soil pools were studied, with 15N-labeling of an organic nitrogen form, glycine. The long-term warming and fertilization had no marked effect on soil inorganic N content, but both dissolved organic N (DON) and plant biomass did increase after fertilization. Soil microbes initially immobilized most of the added 15N, but in the following months, they lost two-thirds, while label concentration in plants increased. After a year, however, the 15N recovered in microbes was still 10-fold higher than that in the plant biomass, showing the high importance of soil microbes in nutrient retention in arctic ecosystems, irrespective of the impact of long-term warming or fertilization. The effects of the treatments on the uptake of label by deciduous shrubs and evergreens paralleled that of their N pool sizes, suggesting that their N uptake potential was unaffected by long-term warming and fertilizer addition. Mosses and herbs had high uptake potential but in fertilized plots they took up less 15N, that is, they were N saturated. The fraction of 15N in microbes tended to decrease after fertilization, but this was an effect of higher N pool dilution after 1 month and a year, and not due to lower initial uptake. Although the concentration of soil inorganic N did not change after fertilization, both increased DON and the results of the 15N label addition showed that the N availability in the ecosystem had increased. By contrast, warming had little effect on soil N pools and microbial 15N uptake, and, hence, had no detectable effects on 15N accumulation.

Ecosystem partitioning of 15N-glycine after long-term climate and nutrient manipulations, plant clipping and addition of labile carbon in a subarctic heath tundra

Abstract: Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied 15N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added 15N label. In all treatments, microbes immobilized a major part of the added 15N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved 15N (TD15N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.