Enzyme kinetics inform about mechanistic changes in tea litter decomposition across gradients in land-use intensity in Central German grasslands.

Grassland ecosystems provide important ecosystem services such as nutrient cycling and primary production that are affected by land-use intensity. To assess the effects of land-use intensity, operational and sensitive ecological indicators that integrate effects of grassland management on ecosystem processes such as organic matter turnover are needed. Here, we investigated the suitability of measuring the mass loss of standardized tea litter together with extracellular enzyme kinetics as a proxy of litter decomposition in the topsoil of grasslands along a well-defined land-use intensity gradient (fertilization, mowing, grazing) in Central Germany. Tea bags containing either green tea (high-quality litter) or rooibos tea (low-quality litter) were buried in 5 cm soil depth. Litter mass loss was measured after three (early-stage decomposition) and 12 months (mid-stage decomposition). Based on the fluorescence measurement of the reaction product 4-methylumbelliferone, Michaelis-Menten enzyme kinetics (Vmax: potential maximum rate of activity; Km: substrate affinity) of five hydrolases involved in the carbon (C)-, nitrogen (N)- and phosphorus (P)-cycle (ß-glucosidase (BG), cellobiohydrolase (CBH), cellotriohydrolase (CTH), 1,4-ß-N-acetylglucosaminidase (NAG), and phosphatase (PH)) were determined in tea litter bags and in the surrounding soil. The land-use intensity index (LUI), summarizing fertilization, mowing, grazing, and in particular the frequency of mowing were identified as important drivers of early-stage tea litter decomposition. Mid-stage decomposition was influenced by grazing intensity. The higher the potential activity of all measured C-, N- and P-targeting enzymes, the higher was the decomposition of both tea litters in the early-phase. During mid-stage decomposition, individual enzyme parameters (Vmax of CTH and PH, Km of CBH) became more important. The tea bag method proved to be a suitable indicator which allows an easy and cost-effective assessment of land-use intensity effects on decay processes in manged grasslands. In combination with enzyme kinetics it is an appealing approach to identify mechanisms driving litter break down.

Spatial variation in dissolved phosphorus and interactions with arsenic in response to changing redox conditions in floodplain aquifers of the Hetao Basin, Inner Mongolia.

Increasing numbers of studies have reported groundwater with naturally high phosphorous (P) and arsenic (As) concentrations, which can potentially threaten the environment and human health. However, the cycling of P and its interactions with As in groundwater under changing redox conditions remain largely unknown. In this study, 83 groundwater samples and 14 sediment samples were collected from the Hetao Basin, Inner Mongolia, for systematic hydrogeochemical investigation and complementary geochemical evaluation. The results showed that P cycling in floodplain aquifers was tightly constrained by redox conditions. Under oxic/suboxic conditions, mineralization of organic matter and weathering of P-bearing minerals were the two dominant processes that mobilized considerable amounts of P in groundwater. When redox conditions became reducing, Fe(III)-oxide reduction dominated, resulting in enrichment of both P and As in groundwater. In Fe(III)-reducing conditions, secondary Ca/Fe(II)-minerals might serve as an important sink for P. When redox conditions became SO42--reducing, preferential adsorption and incorporation of P over As on Fe(II)-sulfides might constrain the As immobilization pathway, resulting in immediate retardation of P and hysteretic immobilization of As. This P-immobilization pathway in natural aquifers has not been described before. This study provides novel insights into P cycling and As enrichment in groundwater systems. Understanding the roles of Fe(II)- and S(-II)-minerals in the immobilization of and interaction between P and As in response to SO42- reduction may help to inspire effective in-situ remediation of contaminated groundwater, in which P and As coexist and remain mobile for decades or longer.

Above- and belowground biodiversity jointly tighten the P cycle in agricultural grasslands.

Experiments showed that biodiversity increases grassland productivity and nutrient exploitation, potentially reducing fertiliser needs. Enhancing biodiversity could improve P-use efficiency of grasslands, which is beneficial given that rock-derived P fertilisers are expected to become scarce in the future. Here, we show in a biodiversity experiment that more diverse plant communities were able to exploit P resources more completely than less diverse ones. In the agricultural grasslands that we studied, management effects either overruled or modified the driving role of plant diversity observed in the biodiversity experiment. Nevertheless, we show that greater above- (plants) and belowground (mycorrhizal fungi) biodiversity contributed to tightening the P cycle in agricultural grasslands, as reduced management intensity and the associated increased biodiversity fostered the exploitation of P resources. Our results demonstrate that promoting a high above- and belowground biodiversity has ecological (biodiversity protection) and economical (fertiliser savings) benefits. Such win-win situations for farmers and biodiversity are crucial to convince farmers of the benefits of biodiversity and thus counteract global biodiversity loss.

Phosphorus Pool Composition in Soils and Sediments of Transitional Ecotones under the Influence of Agriculture.

Transitional ecotones such as vegetated buffer strips, stream banks, and streambeds retain phosphorus (P) in the immediate surroundings of farmland. Yet the fate of P in these ecotones remains unclear. Our objectives were to (i) test the difference in the P pool composition of soil and sediment between sites surrounded by agriculture and forestry and (ii) test whether specific P pools differ among transitional ecotones. Phosphorus pools (by a modified Hedley fractionation scheme) and the degree of P saturation (DPS) were determined in 33 soil and sediment samples from eight farmland and three forest sites. At farmland sites, total P in soil and sediment was more than twofold higher as compared to forestry sites. The proportion of labile inorganic P (Pi) and the DPS were significantly larger in transitional ecotones close to farmland. We further used normalized values for comparing the respective ecotones at the sites. The deviation of each transitional ecotone relative to the respective site average revealed that the normalized total P concentration and proportions of labile and moderately labile Pi were significantly smaller in bed sediment adjacent to farmland as compared to respective stream bank and buffer strip soil, whereas the stable Ca-Pi proportion was larger. The results reflected a decreased Pi sorption capacity in combination with Pi desorption and transfer of Pi into secondary Ca-Pi minerals in bed sediment. In summary, the influence of agriculture increases labile P pools in soil and sediment, which are then subject to a succession of dynamic processes resulting in a partial loss of Pi to the aqueous phase as well as fixation of Pi in the Ca-Pi pool.

Biogeochemical phosphorus cycling in groundwater ecosystems - Insights from South and Southeast Asian floodplain and delta aquifers.

The biogeochemical cycling of phosphorus (P) in South and Southeast Asian floodplain and delta aquifers has received insufficient attention in research studies, even though dissolved orthophosphate (PO43-) in this region is closely linked with the widespread contamination of groundwater with toxic arsenic (As). The overarching aim of this study was to characterize the enrichment of P in anoxic groundwater and to provide insight into the biogeochemical mechanisms underlying its mobilization, subsurface transport, and microbial cycling. Detailed groundwater analyses and in situ experiments were conducted that focused on three representative field sites located in the Red River Delta (RRD) of Vietnam and the Bengal Delta Plain (BDP) in West Bengal, India. The results showed that the total concentrations of dissolved P (TDP) ranged from 0.03 to 1.50 mg L-1 in groundwater, with PO43- being the dominant P species. The highest concentrations occurred in anoxic sandy Holocene aquifers where PO43- was released into groundwater through the microbial degradation of organic carbon and the concomitant reductive dissolution of Fe(III)-(hydr)oxides. The mobilization of PO43- may still constitute an active process within shallow Holocene sediments. Furthermore, a sudden supply of organic carbon may rapidly decrease the redox potential, which causes an increase in TDP concentrations in groundwater, as demonstrated by a field experiment. Considering the subsurface transport of PO43-, Pleistocene aquifer sediments represented effective sinks; however, the enduring contact between oxic Pleistocene sediments and anoxic groundwater also changed the sediments PO43--sorption capacity over time. A stable isotope analysis of PO43--bound oxygen indicated the influences of intracellular microbial cycling as well as a specific PO43- source with a distinct isotopically heavy signal. Consequently, porous aquifers in Asian floodplain and delta regions proved to be ideal natural laboratories to study the biogeochemical cycling of P and its behavior in groundwater environments.

An inter-laboratory comparison of gaseous and liquid fumigation based methods for measuring microbial phosphorus (Pmic) in forest soils with differing P stocks.

In an inter-laboratory trial, gaseous ("CFE") and liquid fumigation ("Resin") based methods for measuring microbial phosphorus (Pmic) were compared, based on the analysis of soil samples from five forests, which differ in their P stocks. Both methods reliably detected the same Pmic gradient in the different soils. However, when the individual recovery rates of spiked P were taken into account, the "CFE" based methods consistently generated higher Pmic values (factor 2) compared to the "Resin" based approaches.

Plant diversity shapes microbe-rhizosphere effects on P mobilisation from organic matter in soil.

Plant species richness (PSR) increases nutrient uptake which depletes bioavailable nutrient pools in soil. No such relationship between plant uptake and availability in soil was found for phosphorus (P). We explored PSR effects on P mobilisation [phosphatase activity (PA)] in soil. PA increased with PSR. The positive PSR effect was not solely due to an increase in Corg concentrations because PSR remained significant if related to PA:Corg . An increase in PA per unit Corg increases the probability of the temporal and spatial match between substrate, enzyme and microorganism potentially serving as an adaption to competition. Carbon use efficiency of microorganisms (Cmic :Corg ) increased with increasing PSR while enzyme exudation efficiency (PA:Cmic ) remained constant. These findings suggest the need for efficient C rather than P cycling underlying the relationship between PSR and PA. Our results indicate that the coupling between C and P cycling in soil becomes tighter with increasing PSR.

Determinants of Acidobacteria activity inferred from the relative abundances of 16S rRNA transcripts in German grassland and forest soils.

16S rRNA genes and transcripts of Acidobacteria were investigated in 57 grassland and forest soils of three different geographic regions. Acidobacteria contributed 9-31% of bacterial 16S rRNA genes whereas the relative abundances of the respective transcripts were 4-16%. The specific cellular 16S rRNA content (determined as molar ratio of rRNA : rRNA genes) ranged between 3 and 80, indicating a low in situ growth rate. Correlations with flagellate numbers, vascular plant diversity and soil respiration suggest that biotic interactions are important determinants of Acidobacteria 16S rRNA transcript abundances in soils. While the phylogenetic composition of Acidobacteria differed significantly between grassland and forest soils, high throughput denaturing gradient gel electrophoresis and terminal restriction fragment length polymorphism fingerprinting detected 16S rRNA transcripts of most phylotypes in situ. Partial least squares regression suggested that chemical soil conditions such as pH, total nitrogen, C : N ratio, ammonia concentrations and total phosphorus affect the composition of this active fraction of Acidobacteria. Transcript abundance for individual Acidobacteria phylotypes was found to correlate with particular physicochemical (pH, temperature, nitrogen or phosphorus) and, most notably, biological parameters (respiration rates, abundances of ciliates or amoebae, vascular plant diversity), providing culture-independent evidence for a distinct niche specialization of different Acidobacteria even from the same subdivision.

Biodiversity effects on plant stoichiometry.

In the course of the biodiversity-ecosystem functioning debate, the issue of multifunctionality of species communities has recently become a major focus. Elemental stoichiometry is related to a variety of processes reflecting multiple plant responses to the biotic and abiotic environment. It can thus be expected that the diversity of a plant assemblage alters community level plant tissue chemistry. We explored elemental stoichiometry in aboveground plant tissue (ratios of carbon, nitrogen, phosphorus, and potassium) and its relationship to plant diversity in a 5-year study in a large grassland biodiversity experiment (Jena Experiment). Species richness and functional group richness affected community stoichiometry, especially by increasing C:P and N:P ratios. The primacy of either species or functional group richness effects depended on the sequence of testing these terms, indicating that both aspects of richness were congruent and complementary to expected strong effects of legume presence and grass presence on plant chemical composition. Legumes and grasses had antagonistic effects on C:N (-27.7% in the presence of legumes, +32.7% in the presence of grasses). In addition to diversity effects on mean ratios, higher species richness consistently decreased the variance of chemical composition for all elemental ratios. The diversity effects on plant stoichiometry has several non-exclusive explanations: The reduction in variance can reflect a statistical averaging effect of species with different chemical composition or a optimization of nutrient uptake at high diversity, leading to converging ratios at high diversity. The shifts in mean ratios potentially reflect higher allocation to stem tissue as plants grew taller at higher richness. By showing a first link between plant diversity and stoichiometry in a multiyear experiment, our results indicate that losing plant species from grassland ecosystems will lead to less reliable chemical composition of forage for herbivorous consumers and belowground litter input.

Environmental factors affect Acidobacterial communities below the subgroup level in grassland and forest soils.

In soil, Acidobacteria constitute on average 20% of all bacteria, are highly diverse, and are physiologically active in situ. However, their individual functions and interactions with higher taxa in soil are still unknown. Here, potential effects of land use, soil properties, plant diversity, and soil nanofauna on acidobacterial community composition were studied by cultivation-independent methods in grassland and forest soils from three different regions in Germany. The analysis of 16S rRNA gene clone libraries representing all studied soils revealed that grassland soils were dominated by subgroup Gp6 and forest soils by subgroup Gp1 Acidobacteria. The analysis of a large number of sites (n = 57) by 16S rRNA gene fingerprinting methods (terminal restriction fragment length polymorphism [T-RFLP] and denaturing gradient gel electrophoresis [DGGE]) showed that Acidobacteria diversities differed between grassland and forest soils but also among the three different regions. Edaphic properties, such as pH, organic carbon, total nitrogen, C/N ratio, phosphorus, nitrate, ammonium, soil moisture, soil temperature, and soil respiration, had an impact on community composition as assessed by fingerprinting. However, interrelations with environmental parameters among subgroup terminal restriction fragments (T-RFs) differed significantly, e.g., different Gp1 T-RFs correlated positively or negatively with nitrogen content. Novel significant correlations of Acidobacteria subpopulations (i.e., individual populations within subgroups) with soil nanofauna and vascular plant diversity were revealed only by analysis of clone sequences. Thus, for detecting novel interrelations of environmental parameters with Acidobacteria, individual populations within subgroups have to be considered.

Nitrogen and phosphorus budgets in experimental grasslands of variable diversity.

Previous research has shown that plant diversity influences N and P cycles. However, the effect of plant diversity on complete ecosystem N and P budgets has not yet been assessed. For 20 plots of artificially established grassland mixtures differing in plant diversity, we determined N and P inputs by bulk and dry deposition and N and P losses by mowing (and subsequent removal of the biomass) and leaching from April 2003 to March 2004. Total deposition of N and P was 2.3 +/- 0.1 and 0.2 +/- 0.01 g m(-2) yr(-1), respectively. Mowing was the main N and P loss. The net N and P budgets were negative (-6.3 +/- 1.1 g N and -1.9 +/- 0.2 g P m(-2) yr(-1)). For N, this included a conservative estimate of atmospheric N(2) fixation. Nitrogen losses as N(2)O were expected to be small at our study site (<0.05 g m(-2) yr(-1)). Legumes increased the removal of N with the harvest and decreased leaching of NH(4)-N and dissolved organic nitrogen (DON) from the canopy. Reduced roughness of grass-containing mixtures decreased dry deposition of N and P. Total dissolved P and NO(3)-N leaching from the canopy increased in the presence of grasses attributable to the decreased N and P demand of grass-containing mixtures. Species richness did not have an effect on any of the studied fluxes. Our results demonstrate that the N and P fluxes in managed grassland are modified by the presence or absence of particular functional plant groups and are mainly driven by the management.