Short- and long-term effects of biodiversity on soil nutrient concentrations in a semi-natural grassland: results from a 14-year experiment

Global biodiversity is declining at an alarming rate, which is likely to have important consequences on ecosystem functioning. Previous studies have shown that in the short term, higher plant biodiversity in grasslands is linked to lower soil nitrogen concentrations, particularly of nitrate, probably due to higher plant uptake. It is unknown, however, how this trend will develop in the long term. To establish long-term responses to experimental changes in biodiversity, long-term data in adequately high resolution is required to separate the long-term trend from seasonal variation in the data, and such data sets are still exceedingly rare. We present a data set of soil solution nitrogen and phosphorus concentrations collected every two weeks over 14 years after the establishment of an experimental grassland with varying levels of biodiversity. Analysis of this data allows us to determine a) whether the system has reached a new steady-state in soil nutrients after conversion from cropland soils to semi-natural grassland 15 years ago, and b) whether these steady-states are different for different levels of plant biodiversity. Furthermore, we expect to be able to detect c) the effects of extreme events (drought, flood) and d) temporal trends under different levels of plant biodiversity before the establishment of steady state. This will have important implications for our understanding of both the biodiversity-ecosystem functioning relationship and the nutrient dynamics of soils changing from previously fertilized systems to semi-natural grasslands. Our results might additionally have practical implications for the establishment and management of hay meadows

Stable hydrogen isotope ratios in crystal water of clay minerals

Hydrogen is the most abundant element in the Universe. But the utilization of the H isotopic composition (δH-2 value) of soil to elucidate biogeochemical processes or to serve as a palaeo climate proxy is still in its infancy. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. The δH-2 value of structural H in clay minerals – mainly from C-poor subsoils - has been studied since the 1970s. The δH-2 value of clay minerals mainly depends on (a) the average δH-2 value of ambient water at the site and time of formation, and on (b) the size of the equilibrium isotopic fractionation factor between water and clay mineral at the temperature of formation. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. Only nonexchangeable H in in structural water of minerals preserves its inherited δH-2 value and does not exchange with water at temperatures usually occurring in soil environments at the Earth’s surface. Nonexchangeable H is bound in crystal water, which integrates the δH-2 value of soil water over several millennia. This is in turn determined by palaeoclimatic variations of the precipitation’s δH-2 signal with distinguishable shifts e.g., from Pleistocene to Holocene. For a global data set, Ruppenthal (2014) reported a close correlation of bulk soil δH-2 values with those of the mean local precipitation and confirmed this for organic matter, while the clay fraction of soils was up to now not studied. We will adapt a steam equilibration method with water vapor of known H isotopic composition – formerly applied by Ruppenthal (2014) on SOM and bulk soil – to clay fractions and compare our results to the hitherto used heating treatments (200-250°C) under vacuum. We expect that the δH-2 signal of the clay fraction of Bt horizons will serve to differentiate soils developed in different climatic epochs (e.g., Holocene, last interstadial, last interglacial) by analyzing dated palaeo soil samples. To test the hypothesis that there is a similar global regression line of the δH-2 values in structural water of clay as up to now reported for bulk soils and soil organic matter, we will analyze the clay fraction in a global set of soil samples

Stable hydrogen isotope ratios in soil organic matter

Stable H isotope ratios are a promising indicator of OM transformation processes (Schimmelmann et al., 2006). δ2H values of bulk organic matter and of specific organic compounds can be used as ecological tracer and forensic tool if the proportion of H that readily exchanges with ambient moisture is accounted for (Wassenaar & Hobson, 1998). There are a few reports about the H isotope ratios in plant-soil systems illustrating that there is little knowledge of the controls of the isotopic composition of the non-exchangeable H fraction of bulk OM (Schimmelmann et al., 2006; Ruppenthal et al., 2015). The increasingly closer relationship between δ2H values of rainfall and of non-exchangeable H in OM (δ2Hn) in the order, plant – plant litter (above- and belowground) – soil along a climatic gradient (Ruppenthal et al., 2015) suggests that decomposition influences δ2Hn values in OM in a systematic way. However, there are knowledge gaps concerning the fractionation factors and the extent of incorporation of ambient water-H into the nonexchangeable fraction of H in OM during decomposition. Our research will focus on the mechanisms responsible for the strong correlation between δ2H values in rainfall and δ2Hn values of SOM. Therefore, our study aims to investigate (1) the incorporation of ambient water-H into the nonexchangeable H fraction in OM during decomposition by heterotrophic bacteria as model organisms and quantify apparent fractionation factors, (2) the extent of incorporation of ambient water into the nonexchangeable H fraction of OM by the soil microbial community under laboratory conditions, and (3) the extent to which H is incorporated into nonexchangeable OM pool from ambient water during decomposition of aboveground litter under field conditions. We will work with microcosms using two bacteria species and determine decomposition rates of litter. Steam equilibration (Ruppenthal et al., 2015) and TC/EA-IRMS are used as analytical tools. We expect that different decomposition rates because of differences in litter quality will be reflected by the extent of H incorporation from ambient water into the nonexchangeable H fraction of the products. Additionally, different litter types enriched in 2H will be buried in soil of forest stands. We hypothesize that the incorporation of 2H-depleted ambient water into 2H-enriched nonexchangeable H fraction of OM will depend on litter type, soil moisture/ temperature, and the heterotrophic activity during the experiment

Effect of iprodione fungicide application on deterioration rate of stored shelled corn

[[abstract]]The fungicide iprodione was applied with two different amounts of water carrier, 3.29 and 5.48 mL/kg wet corn, to study its effect on the allowable storage time (time to lose 0.5% of original dry matter) for shelled corn (Zea mays) at 18 and 22% moisture (wet basis) and 20蚓. Allowable storage time was determined by measuring the amount of carbon dioxide produced by corn samples and by using a carbohydrate respiration model to calculate dry matter loss. Performance of the fungicide was slightly better with the smaller amount of water carrier, but the difference in performance for the two levels of water carrier was not significant (p < 0.05). With the smaller amount of water carrier, the fungicide increased the allowable storage time about 45% for 18% moisture corn and about 16% for 22% moisture corn. At the end of the tests, the 22% moisture corn had higher levels of total damaged kernels than did the 18% moisture corn for all fungicide treatments

15N tracing to elucidate links between biodiversity and nitrogen cycling in a grassland experiment

Nitrogen (N) cycling is a fundamental ecosystem function of high complexity because N undergoes many transformations in soil and vegetation. The effect of biodiversity loss on ecosystem functions in general, and on N cycling in particular, was studied in several manipulative field experiments. To generate a comprehensive view of the influence of species richness on all major N transformations, we conducted laboratory incubations, in which we added 15N-labeled ammonium and nitrate to soil samples of the “Jena Experiment”, a manipulative large scale, long-term biodiversity experiment in grassland. The experimental site is located in Jena, Germany. The design consists of 4 blocks and 82 plots with 1-60 species and 1-4 functional groups (grasses, legumes, small herbs, tall herbs). Approx. 400 g of field-fresh soil was sampled from each plot of one of the 4 blocks and divided into three aliquots of 100 g each. In order to trace N turnover, we amended the incubations (in triplicate) either with 15N-labelled (98 at%) ammonium, nitrate, or with a mixture of both. The samples were incubated for two months at 20°C. Soil solution was extracted 1, 2, 4, 9 and 16 days after 15N application by percolating 100 mL of nutrient solution through each vessel. Concentrations of NH4-N, NO3-N and total N in the extracts were determined with colorimetric methods. The N-isotopic composition in nitrate was analyzed by isotope ratio mass spectrometry (IRMS) using the denitrifier method. Ammonium N isotope ratios were determined using the “hypobromite oxidation” method, in which ammonium-N is converted to nitrite followed by azide reaction to nitrous oxide and IRMS analysis. The results will be comprehensively evaluated in a quantitative context using the modelling approach of Müller et al. (2007) to determine the size of six N pools and the rates of nine N transformations. Links between N transformation rates, N-pool size and plant species richness will be verified with the help of ANOVA

Sodium demand of microorganisms in the phyllosphere and the organic layer of a tropical montane forest in south Ecuador

Recent studies raise the hypothesis that Na shortage restricts decomposition and affects the carbon cycle in tropical forests. When Na concentrations in soils are low and the stands are far off-coast, they do not receive substantial Na inputs from the atmosphere. Since terrestrial plants have low concentrations of Na, which is not considered as an essential element, the demand of soil fauna may not be covered. Yet, in contrast to animals, little is known of Na demands of fungi and phyllosphere microorganisms. We present results from a study on Na limitation in a montane forest ecosystem in South Ecuador, which is located on the eastern cordillera of the Andes. We tested the hypotheses that (1) the study area is characterized by low Na concentrations because of low deposition rates with incident precipitation (wind directions mainly from the Amazonian Basin), (2) decomposition processes are limited by fauna and fungal Na restrictions and (3) Na is retained in the canopy because of Na limitation of microorganisms in phyllosphere. Since 1998, we measure Na fluxes in rainfall, throughfall, stemflow, litter leachate, litterfall and organic layer in a microcatchment under an undisturbed lower montane rainforest. Results reveal comparatively low Na concentrations in the ecosystem and similar Na concentrations in throughfall and stemflow. Since Na fluxes are lower with throughfall than with incident rainfall, we conclude that Na is retained in the canopy. To explore the role of the phyllosphere in Na retention we sampled leaves covered by phyllosphere microorganisms and leaves without phyllosphere cover from several tree species, which were sprayed with a NaCl solution containing 0.5 mg L-1 Na, corresponding to the Na concentration in incident rainfall in our study area. Additionally, responses of litter decomposition to Na additions and the involved interaction of soil fungi and fauna were tested in a litterbag experiment at two sites (1000 and 2000 m a.s.l.). Results revealed enhanced decomposition rates following Na additions, though only in the presence of soil fauna. These results might have future ecosystem implications, since our time series showed that total Na deposition decreased within the past 15 years from ca. 40 kg ha-1 a-1 to 10 kg ha-1 a-1, suggesting a potential role of Na in regulating ecosystem processes

Base metal budgets of a small catchment in a tropical montane forest in South Ecuador

In a tropical montane rain forest in south Ecuador, the alkali and earth alkali metals Ca, Mg, K, and Na are supplied by weathering of the parent substrate consisting of phyllites and metasandstones and by atmospheric inputs. Phases of acid deposition are interrupted by alkalinization through episodic basic dust deposition. Although the biological productivity of most terrestrial ecosystems is thought to be N- and/or P-limited, there is increasing evidence that the essential plant nutrients K, Na, Mg and Ca can also limit biological functioning. We quantified biological and geochemical contributions to base metal fluxes and set up a metal budget of a ca. 9.1-ha large catchment from 1998 to 2013. The catchment is characterized by a high annual interception loss (28–50 %) and a low contribution of stem flow to throughfall. Mean total annual soil input (throughfall + stemflow + litterfall) was 13800 ± 1500 mg m-2 (Ca, mean ± SD), 19000 ± 1510 (K), 4690 ± 619 (Mg) and 846 ± 592 (Na) of which 22 ± 6 % (Ca), 45 ± 16 (K), 39 ± 10 (Mg) and 84 ± 33 (Na) were leached to soil horizons below the organic layer. The three nutrient metals Ca, K and Mg were thus to a large part retained in the biotic part of the catchment. The canopy budget of K was consistently and most pronouncedly negative. The canopy budgets of Ca and Mg were closely correlated and in most years negative, while the budget of Na was consistently positive, indicating net retention of this element in the canopy. The mineral soil retained 79–94 % of Ca, K and Mg, while Na was net released from the mineral soil. The size of mainly biologically controlled aboveground fluxes of Ca, K and Mg was 1-2 orders of magnitude larger than that of mainly geochemically controlled fluxes which are driven by sorption to soil and weathering. Annual net hydrological fluxes (bulk deposition – stream flow) were –66 ± 278 mg m-2 (Ca), 361 ± 421 (K), –188 ± 159 (Mg) and –1700 ± 587 (Na). If estimated dry deposition was included, the system accumulated 86 kg Ca ha-1 and 199 kg K ha-1, had a nearly balanced budget of Mg (+0.3 kg ha-1) and lost 56 kg of Na ha-1 in the last 15 years. The strongest driver of all budgets was the input flux into the various compartments