Responses of Degraded Tibetan Kobresia Pastures to N Addition

© 2017 John Wiley & Sons, Ltd.Kobresia pastures on the Tibetan Plateau are the largest alpine pastoral ecosystems. Kobresia pastures have experienced severe degradation in recent decades, inducing large nitrogen (N) losses from these ecosystems. This is particularly problematic, as it intensifies prevailing N limitation in these regions. Simultaneously, anthropogenic N deposition has increased across these ecosystems, but the fate of added N on variously degraded Kobresia pastures remains unclear. Kobresia pastures of three degradation stages were investigated: living, dying and dead root mats. High and very low (as a tracer) amounts of 15N-labelled ammonium nitrate (NH4NO3) were applied to root mats under controlled conditions. Leaching was simulated over 3 months, and 15N recovery was measured in the plant-soil system. N addition promoted aboveground biomass and foliar N content of Kobresia during the early growth period, indicating a short-term offset of N limitation. After 7-8 weeks, plant growth and 15N uptake were reduced in plants with initial N addition, reflecting a transition to N limitation induced by N uptake and leaching from soil. This limitation was also indicated by the strong decline of NO3- in leachates from living root mats compared with degraded root mats. Leaching N losses from dying and dead root mats increased 22 and 63 times, respectively, compared with those of living root mats. We conclude that N addition can facilitate plant growth in living root mats but contributes to N leaching in degraded pastures. This contribution to N leaching may weaken ecosystem recovery, increase NO3- loading of adjacent lower landscape parts and cause eutrophication of aquatic ecosystems

Carbon and Nitrogen Losses from Soil Depend on Degradation of Tibetan Kobresia Pastures

© 2016 John Wiley & Sons, Ltd.Degradation of Kobresia pygmaea pastures has strongly increased on the Tibetan Plateau over the last few decades and contributed to a high loss of soil organic carbon and nutrients. The pathways of carbon (C) and nitrogen (N) losses from degraded K. pygmaea pastures are still unclear, but this is a prerequisite to assess the recovery of Tibetan grasslands. We investigated the response of day- and nighttime CO2 efflux and leaching of dissolved organic C and N, NH4+ and NO3- from K. pygmaea root mats in three degradation stages: living root mat, dying root mat and dead root mat. Dying root mat had the highest C loss as CO2 and as leached dissolved organic carbon. This indicates K. pygmaea pastures shift from a C sink to a C source following plant death. In contrast, living root mat had the lowest daytime CO2 efflux (0·38±0·1μgCg-1h-1) because CO2 was assimilated via photosynthesis. Nighttime CO2 efflux positively correlated with soil moisture for living and dead root mats. It indicates that increasing precipitation might accelerate C losses due to enhanced soil organic carbon decomposition. Furthermore, dead root mat had the highest average NO3- loss (23±2·6mgNL-1) from leaching compared with other root mats. Consequently, leaching increases the negative impacts of pasture degradation on N availability in these often N limited ecosystems and thus impedes the recovery of K. pastures following degradation

Carbon and Nitrogen Losses from Soil Depend on Degradation of Tibetan Kobresia Pastures

Copyright © 2016 John Wiley & Sons, Ltd.Degradation of Kobresia pygmaea pastures has strongly increased on the Tibetan Plateau over the last few decades and contributed to a high loss of soil organic carbon and nutrients. The pathways of carbon (C) and nitrogen (N) losses from degraded K. pygmaea pastures are still unclear, but this is a prerequisite to assess the recovery of Tibetan grasslands. We investigated the response of day- and nighttime CO2 efflux and leaching of dissolved organic C and N, NH4+ and NO3- from K. pygmaea root mats in three degradation stages: living root mat, dying root mat and dead root mat. Dying root mat had the highest C loss as CO2 and as leached dissolved organic carbon. This indicates K. pygmaea pastures shift from a C sink to a C source following plant death. In contrast, living root mat had the lowest daytime CO2 efflux (0·38 ± 0·1 µg C g−1 h−1) because CO2 was assimilated via photosynthesis. Nighttime CO2 efflux positively correlated with soil moisture for living and dead root mats. It indicates that increasing precipitation might accelerate C losses due to enhanced soil organic carbon decomposition. Furthermore, dead root mat had the highest average NO3− loss (23 ± 2·6 mg N L−1) from leaching compared with other root mats. Consequently, leaching increases the negative impacts of pasture degradation on N availability in these often N limited ecosystems and thus impedes the recovery of K. pastures following degradation. Copyright © 2016 John Wiley & Sons, Ltd

Degradation mechanisms and consequences for SOC stocks for the world's largest alpine pastoral ecosystem on the Tibetan Plateau

Approximately 1.5 million km² of the Tibetan Plateau are covered with grasslands. Thereof one third is occupied by the world’s largest pastoral alpine ecosystem (Kobresia pastures). Paleo-records indicate the grazing-induced origin of this ecosystem since more than 8000 years or at least since yak domestication since 4000 years. Long-term moderate grazing by yak and sheep increased belowground C allocation of Kobresia pygmaea, caused the development of dense root-mats and finally lead to an accumulation of soil organic carbon (SOC) and nutrients such as nitrogen (N) and phosphorus (P) in the topsoil. These pastures, however, are increasingly affected by large-scale degradation caused by overgrazing of these highly sensitive ecosystems. Loss of the topsoil threatens several ecosystem functions: i.e. SOC and nutrient storage, biodiversity, provision of grazing-ground and supply of clean water for large parts of SE-Asia. Here, we present a conceptual model and results of degradation processes combining anthropogenic and natural amplifications. To evaluate losses of SOC and nutrients we synthesize field observations and surveys in the highlands and validates this with own analyses in the Kobresia core area. We show that drought- and frost-induced polygonal cracking opens the root-mats, already weakened by overgrazing. This initiates a dying of the Kobresia turf, extends the surface cracks, triggers soil erosion and promotes SOC mineralization and leaching losses. Soil erosion caused further high losses of SOC and nutrients from the topsoil (i.e. 0-10 cm: ~5.1 kg C m-2), whereas SOC loss beneath the surface cracks was primary caused by both, decreasing C-input and SOC mineralization (mineralization-derived SOC loss: ~2.5 kg C m-2). The root biomass decreased with intensity of pasture degradation and lower C input constrains the ecosystem recovery. A negative δ13C shift of SOC reflected intensive decomposition and corresponded to a relative enrichment of 13C depleted lignin components. In sum, degradation triggered high SOC loss (up to 70% of intact soil in 0-30 cm: ~7.6 kg C m-2) from this ecosystem with profound consequences for carbon sequestration, atmospheric CO2, water quality and ecosystem stability

Nitrogen pools and cycles in Tibetan Kobresia pastures depending on grazing

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Kobresia grasslands on the Tibetan Plateau comprise the world’s largest pastoral alpine ecosystem. Overgrazing-driven degradation strongly proceeded on this vulnerable grassland, but the mechanisms behind are still unclear. Plants must balance the costs of releasing C to soil against the benefits of accelerated microbial nutrient mineralization, which increases their availability for root uptake. To achieve the effect of grazing on this C-N exchange mechanism, a 15NH4+ field labeling experiment was implemented at grazed and ungrazed sites, with additional treatments of clipping and shading to reduce belowground C input by manipulating photosynthesis. Grazing reduced gross N mineralization rates by 18.7%, similar to shading and clipping. This indicates that shoot removal by grazing decreased belowground C input, thereby suppressing microbial N mining and overall soil N availability. Nevertheless, NH4+ uptake rate by plants at the grazed site was 1.4 times higher than at the ungrazed site, because plants increased N acquisition to meet the high N demands of shoot regrowth (compensatory growth: grazed > ungrazed). To enable efficient N uptake and regrowth, Kobresia plants have developed specific traits (i.e., efficient above-belowground interactions). These traits reflect important mechanisms of resilience and ecosystem stability under long-term moderate grazing in an N-limited environment. However, excessive (over)grazing might imbalance such C-N exchange and amplify plant N limitation, hampering productivity and pasture recovery over the long term. In this context, a reduction in grazing pressure provides a sustainable way to maintain soil fertility, C sequestration, efficient nutrient recycling, and overall ecosystem stability

Nitrogen Uptake in an Alpine Kobresia Pasture on the Tibetan Plateau: Localization by ¹⁵N Labeling and Implications for a Vulnerable Ecosystem

© 2015, Springer Science+Business Media New York. Grasslands are very important regionally and globally because they store large amounts of carbon (C) and nitrogen (N) and provide food for grazing animals. Intensive degradation of alpine grasslands in recent decades has mainly impacted the upper root-mat/soil horizon, with severe consequences for nutrient uptake in these nutrient-limited ecosystems. We used 15N labeling to identify the role of individual soil layers for N-uptake by Kobresia pygmaea—the dominating plant in the degraded Tibetan pasture ecosystems. We hypothesized a very efficient N-uptake corresponding mainly to the vertical distribution of living roots (topsoil > subsoil). We assume that K. pygmaea develops a very dense root-mat, which has to be maintained by small aboveground biomass, to enable this efficient N-uptake. Consequently, a higher N-investment into roots compared to shoots was hypothesized. The 15N recovery in whole plants (~70%) indicated very efficient N-uptake from the upper injection depths (0–5 cm). The highest 15N amounts were recovered in root biomass, whereby 15N recovery in roots strongly decreased with depth. In contrast, 15N recovery in shoots was generally low (~18%) and independent of the 15N injection depth. This clearly shows that the low N demand of Kobresia shoots can be easily covered by N-uptake from any depth. Less living root biomass in lower versus upper soil was compensated by a higher specific activity of roots for N-uptake. The 15N allocation into roots was on average 1.7 times higher than that into shoots, which agreed well with the very high R/S ratio. Increasing root biomass is an efficient strategy of K. pygmaea to compete for belowground resources at depths and periods with available resources. This implies high C-costs to maintain root biomass (~6.0 kg DM m−2), which must be covered by a very low amount of photosynthetically active shoots (0.3 kg DM m−2). It also suggests that Kobresia grasslands react extremely sensitively toward changes in climate and management that disrupt this above-/belowground trade-off mechanism

Fate of Organic and Inorganic Nitrogen in Crusted and Non-Crusted Kobresia Grasslands

© 2016 John Wiley & Sons, Ltd.A widespread pattern of the Tibetan plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non-crusted and crusted patches in Kobresia grasslands remains unknown. We reported on a field 15N-labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto soil surface. Crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil than plants in both patches 29days after the labeling. In non-crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly because of higher root biomass. Microorganisms in non-crusted patches were N-limited because of more living roots and competed strongly for N with roots. Inorganic N input to non-crusted patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant+soil) for inorganic N forms. Compared to non-crusted patches, microorganisms in crusted patches were more C-limited because of depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N-glycine than inorganic N. We conclude that overgrazing-induced crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N

Fate of Organic and Inorganic Nitrogen in Crusted and Non-Crusted Kobresia Grasslands

Copyright © 2016 John Wiley & Sons, Ltd.A widespread pattern of the Tibetan plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non-crusted and crusted patches in Kobresia grasslands remains unknown. We reported on a field 15N-labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29 days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto soil surface. Crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil than plants in both patches 29 days after the labeling. In non-crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly because of higher root biomass. Microorganisms in non-crusted patches were N-limited because of more living roots and competed strongly for N with roots. Inorganic N input to non-crusted patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant + soil) for inorganic N forms. Compared to non-crusted patches, microorganisms in crusted patches were more C-limited because of depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N-glycine than inorganic N. We conclude that overgrazing-induced crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N. Copyright © 2016 John Wiley & Sons, Ltd

Nitrogen but not phosphorus addition affects symbiotic N2 fixation by legumes in natural and semi‑natural grasslands located on four continents

The amount of nitrogen (N) derived from symbiotic N2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods We evaluated symbiotic N2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15N natural abundance method. Results N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion Our results reveal that N addition mainly impacts symbiotic N2 fixation via reduced biomass of legumes rather than changes in N2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N 2 fixation in grasslands, and these effects cannot be reversed by additional P amendment.EEA Santa CruzFil: Vázquez, Eduardo. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); AlemaniaFil: Vázquez, Eduardo. Swedish University of Agricultural Sciences. Department of Soil and Environment; SueciaFil: Schleuss, Per‑Marten. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); AlemaniaFil: Borer, Elizabeth T. University of Minnesota. Department of Ecology, Evolution, and Behavior; Estados UnidosFil: Bugalho, Miguel N. University of Lisbon. Centre for Applied Ecology “Prof. Baeta Neves” (CEABN-InBIO). School of Agriculture; Portugal.Fil: Caldeira, Maria. C. University of Lisbon. Forest Research Centre. School of Agriculture; Portugal.Fil: Eisenhauer, Nico. German Centre for Integrative Biodiversity Research; AlemaniaFil: Eisenhauer, Nico. Leipzig University. Institute of Biology; AlemaniaFil: Eskelinen, Anu. German Centre for Integrative Biodiversity Research; AlemaniaFil: Eskelinen, Anu. Physiological Diversity, Helmholtz Centrefor Environmental Research; AlemaniaFil: Eskelinen, Anu. University of Oulu. Ecology & Genetics; FinlandiaFil: Fay, Philip A. Grassland Soil and Water Research Laboratory (USDA-ARS); Estados UnidosFil: Haider, Sylvia. German Centre for Integrative Biodiversity Research; AlemaniaFil: Haider, Sylvia. Martin Luther University. Institute of Biology. Geobotany and Botanical Garden; AlemaniaFil: Jentsch, Anke. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); AlemaniaFil: Kirkman, Kevin P. University of KwaZulu-Natal. School of Life Sciences; SudáfricaFil: McCulley, Rebecca L. University of Kentucky. Department of Plant and Soil Sciences; Estados UnidosFil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina.Fil: Peri, Pablo Luis. Universidad Nacional de la Patagonia Austral; Argentina.Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina.Fil: Price, Jodi. Charles Sturt University. Institute for Land, Water and Society; Australia.Fil: Richards, Anna E. CSIRO Land and Water. Northern Territory; Australia.Fil: Risch, Anita C. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; SuizaFil: Roscher, Christiane. German Centre for Integrative Biodiversity Research; AlemaniaFil: Roscher, Christiane. Physiological Diversity, Helmholtz Centre for Environmental Research; AlemaniaFil: Schütz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; SuizaFil: Seabloom, Eric William. University of Minnesota. Dept. of Ecology, Evolution, and Behavior; Estados UnidosFil: Standish, Rachel J. Murdoch University. Harry Butler Institute; Australia.Fil: Stevens, Carly J. Lancaster University. Lancaster Environment Centre; Reino UnidoFil: Tedder, Michelle J. University of KwaZulu-Natal. School of Life Sciences; SudáfricaFil: Virtanen, Risto. University of Oulu. Ecology & Genetics; Finlandia.Fil: Spohn, Marie. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); AlemaniaFil: Spohn, Marie. Swedish University of Agricultural Sciences. Department of Soil and Environment; Sueci

Nitrogen but not phosphorus addition affects symbiotic N-2 fixation by legumes in natural and semi-natural grasslands located on four continents

Background and aims: The amount of nitrogen (N) derived from symbiotic N-2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known.Methods: We evaluated symbiotic N-2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the N-15 natural abundance method.Results: N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N-2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials.Conclusion: Our results reveal that N addition mainly impacts symbiotic N-2 fixation via reduced biomass of legumes rather than changes in N-2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N-2 fixation in grasslands, and these effects cannot be reversed by additional P amendment