Temperature‐sensitive biochemical 18^{18}O‐fractionation and humidity‐dependent attenuation factor are needed to predict δ 18^{18}O of cellulose from leaf water in a grassland ecosystem

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ$^{18}$O$_{cellulose}$) in a drought‐prone, temperate grassland ecosystem. A new allocation‐and‐growth model was designed and added to an $^{18}$O‐enabled soil–vegetation–atmosphere transfer model (MuSICA) to predict seasonal (April–October) and multi‐annual (2007–2012) variation of δ$^{18}$O$_{cellulose}$ and $^{18}$O‐enrichment of leaf cellulose (Δ$^{18}$O$_{cellulose}$) based on the Barbour–Farquhar model. Modelled δ$^{18}$O$_{cellulose}$ agreed best with observations when integrated over c. 400 growing‐degree‐days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ$^{18}$O$_{cellulose}$ (R$^{2}$ = 0.57) and Δ$^{18}$O$_{cellulose}$ (R$^{2}$ = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical $^{18}$O‐fractionation between water and substrate for cellulose synthesis (ε$_{bio}$, range 26–30‰) was temperature‐sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water $^{18}$O‐enrichment (1 – p$_{ex}$p$_{x}$, range 0.23–0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ$^{18}$O$_{cellulose}$ requires quantitative knowledge of climatic effects on p$_{ex}$p$_{x}$ and ε$_{bio}$

The 18O ecohydrology of a grassland ecosystem - predictions and observations

This research has been supported by the Deutsche Forschungsgemeinschaft (grant no. SCHN 557/9-1), the Agence Nationale de la Recherche (grant no. ANR-13-BS06-0005), and the European Commission (grant no. SOLCA 338264). This work was supported by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program.Peer reviewedPublisher PD

Arbuscular mycorrhiza mediated effects on growth, mineral nutrition and biological nitrogen fixation of Melilotus alba Med. in a subtropical grassland soil

Soil symbiotic microorganisms promote recruitment of legume forage species in grassland ecosystems by enhancing provision for nutrients. We studied the effects of arbuscular mycorrhizal fungi (AMF) on plant growth, mineral nutrition and biological nitrogen fixation of Melilotus alba Med., a forage legume with potential use in subtropical grasslands. Responses to AMF inoculation (a mix of Funneliformis mosseae, Rhizophagus irregularis and Simiglomus hoi) and phosphate fertilization were evaluated in plants growing for 12 weeks in a subtropical soil. All plants were previously inoculated with a suspension of Rhizobium meliloti. Plant responses were assessed in terms of mycorrhizal colonization, biomass production, mineral nutrition (P and N) and proportion of biological nitrogen fixation by 15N natural abundance method. The results showed that when soil phosphorus availability was very low, regardless of the degree of AMF colonization, the generation of mycorrhizae did not reach a significant impact on plant nutrition and biomass production. But under a relatively higher phosphorus provision, AMF symbiosis showed positive effects on phosphorus accumulation, nitrogen nutrition and biomass production, linked to an enhancement of biological nitrogen fixation. The results strengthen our understanding of how synergistic effects between belowground symbionts (in this case AMF + Rhizobium) could promote the recruitment of forage legumes in subtropical grassland soils.Fil: Hack, Claudina María. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias. Instituto Agrotecnico "Pedro M. Fuentes Godo"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste; ArgentinaFil: Porta, Miriam. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias. Instituto Agrotecnico "Pedro M. Fuentes Godo"; ArgentinaFil: Schäufele, Rudi. Universitat Technical Zu Munich; AlemaniaFil: Grimoldi, Agustin Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentin

Atmospheric CO2 mole fraction affects stand-scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO2 . Sunflower stands were grown at low (200 μmol mol(-1) ) or high CO2 (1000 μmol mol(-1) ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale (13) C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO2 (compared with low CO2 ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO2 (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO2 . Two main processes contributed to the reduction of CUE at high CO2 : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.The experiment was supported by the European Community’s Human Potential Program under contract HPRN-CT-1999- 00059, NETCARB, coordinated by Jaleh Ghashghaie

Root and Shoot Respiration of Perennial Ryegrass Are Supplied by the Same Substrate Pools: Assessment by Dynamic 13C Labeling and Compartmental Analysis of Tracer Kinetics1[OA]

The substrate supply system for respiration of the shoot and root of perennial ryegrass (Lolium perenne) was characterized in terms of component pools and the pools' functional properties: size, half-life, and contribution to respiration of the root and shoot. These investigations were performed with perennial ryegrass growing in constant conditions with continuous light. Plants were labeled with 13CO2/12CO2 for periods ranging from 1 to 600 h, followed by measurements of the rates and 13C/12C ratios of CO2 respired by shoots and roots in the dark. Label appearance in roots was delayed by approximately 1 h relative to shoots; otherwise, the tracer time course was very similar in both organs. Compartmental analysis of respiratory tracer kinetics indicated that, in both organs, three pools supplied 95% of all respired carbon (a very slow pool whose kinetics could not be characterized provided the remaining 5%). The pools' half-lives and relative sizes were also nearly identical in shoot and root (half-life < 15 min, approximately 3 h, and 33 h). An important role of short-term storage in supplying respiration was apparent in both organs: only 43% of respiration was supplied by current photosynthate (fixed carbon transferred directly to centers of respiration via the two fastest pools). The residence time of carbon in the respiratory supply system was practically the same in shoot and root. From this and other evidence, we argue that both organs were supplied by the same pools and that the residence time was controlled by the shoot via current photosynthate and storage deposition/mobilization fluxes

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C 3 grass—atmospheric drivers and physiological relations

The 18O enrichment (Δ18O) of leaf water affects the Δ18O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18O of bulk leaf water (Δ18OLW) and leaf sucrose (Δ18OSucrose). We grew Lolium perenne (a C3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol−1) as factors, and determined Δ18OLW, Δ18OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf), stomatal conductance (gs) and mesophyll conductance to CO2 (gm). The Δ18O of photosynthetic medium water (Δ18OSSW) was estimated from Δ18OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio). Δ18OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18Oe) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18OLW was a poor proxy for Δ18OSucrose, mainly due to opposite Δ18O responses of nonphotosynthetic tissue water (Δ18Onon-SSW) relative to Δ18OSSW, driven by atmospheric conditions

The allocation of assimilated carbon to shoot growth: in situ assessment in natural grasslands reveals nitrogen effects and interspecific differences

In grasslands, sustained nitrogen loading would increase the proportion of assimilated carbon allocated to shoot growth (A shoot), because it would decrease allocation to roots and also encourage the contribution of species with inherently high A shoot. However, in situ measurements of carbon allocation are scarce. Therefore, it is unclear to what extent species that coexist in grasslands actually differ in their allocation strategy or in their response to nitrogen. We used a mobile facility to perform steady-state 13C-labeling of field stands to quantify, in winter and autumn, the daily relative photosynthesis rate (RPR~tracer assimilated over one light-period) and A shoot (~tracer remaining in shoots after a 100 degree days chase period) in four individual species with contrasting morpho-physiological characteristics coexisting in a temperate grassland of Argentina, either fertilized or not with nitrogen, and either cut intermittently or grazed continuously. Plasticity in response to nitrogen was substantial in most species, as indicated by positive correlations between A shoot and shoot nitrogen concentration. There was a notable interspecific difference: productive species with higher RPR, enhanced by fertilization and characterized by faster leaf turnover rate, allocated ~20 % less of the assimilated carbon to shoot growth than species of lower productivity (and quality) characterized by longer leaf life spans and phyllochrons. These results imply that, opposite to the expected response, sustained nitrogen loading would change little the A shoot of grassland communities if increases at the species-level are offset by decreases associated with replacement of ‘low RPR-high A shoot’ species by ‘high RPR-low A shoot’ species.EEA RafaelaFil: Xiao, Ying Gong. Technische Universität München. Lehrstuhl für Grünlandlehre; AlemaniaFil: Berone, German Dario. Technische Universität München. Lehrstuhl für Grünlandlehre; Alemania. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Rafaela; Argentina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; ArgentinaFil: Agnusdei, Monica Graciela. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; ArgentinaFil: Rodríguez Palma, Ricardo Manuel. Universidad de República. Facultad de Agronomía. Departamento de Producción Animal y Pasturas. Estación Experimental en Salto; UruguayFil: Schäufele, Rudi. Technische Universität München. Lehrstuhl für Grünlandlehre; AlemaniaFil: Lattanzi, Fernando Alfredo. Technische Universität München. Lehrstuhl für Grünlandlehre; Alemani

A highly productive grass improves chemical and biological properties but does not aggregate stability in saline-sodic lowlands in Argentina

Remediation of saline-sodic soils used for cattle breeding is particularly challenging due to the limited alternatives. We hypothesize that introducing salt tolerant and productive forage in a lowland halophytic steppe, typical of saline-sodic soils, increases belowground biomass inputs and activity, generating a series of positive effects on soil biological and chemical properties, and aggregate stability – an accepted indicator of soil degradation resistance. Under natural environmental conditions, we found that the introduction of Panicum coloratum (panicum) increased belowground biomass almost three times, the abundance of mites tended to be greater and that of springtails was 9.4 kg−1 contrasting with none found in the halophytic steppe, after 7 years. The concentration of Ca2+ and Mg2+ increased 26% and 54%, respectively, and that of Na+ was reduced 31% compared with the halophytic steppe. Soil pH decrease 5% and electrical conductivity decreased 37% (changing from moderate to very slightly saline) in panicum compared to the halophytic steppe. However, in panicum, mineral-associated organic matter (MAOM) and aggregate stability decreased 22% and 26%, respectively. We concluded that, although biological and chemical properties improved, aggregate stability – an early indicator of soil recovery – decreased, which was likely determined by MAOM reduction in saline-sodic soils.Fil: Casas, Cecilia. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Recursos Naturales y Ambiente. Cátedra de Edafología; Argentina. Technische Universitat München; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Di Bella, Carla Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal. Cátedra de Forrajicultura; ArgentinaFil: Lattanzi, Fernando Alfredo. Instituto Nacional de Investigación Agropecuaria; Uruguay. Technische Universitat München; AlemaniaFil: Schwab, Magalí. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal. Cátedra de Forrajicultura; ArgentinaFil: Clavijo, Pilar. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal. Cátedra de Forrajicultura; ArgentinaFil: Schäufele, Rudi. Lehrstuhl Für Grünlandlehre, Tum; AlemaniaFil: Druille, Magdalena. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal. Cátedra de Forrajicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Grimoldi, Agustin Alberto. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal. Cátedra de Forrajicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentin