Jennifer A. J. Dungait

Temperature sensitivity of soil respiration rates enhanced by microbial community response

Soils store about four times as much carbon as plant biomass, and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide. Short-term experiments have shown that soil microbial respiration increases exponentially with temperature. This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change. The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted.

Forest contraction in north equatorial Southeast Asia during the Last Glacial Period

Today, insular Southeast Asia is important for both its remarkably rich biodiversity and globally significant roles in atmospheric and oceanic circulation. Despite the fundamental importance of environmental history for diversity and conservation, there is little primary evidence concerning the nature of vegetation in north equatorial Southeast Asia during the Last Glacial Period (LGP). As a result, even the general distribution of vegetation during the Last Glacial Maximum is debated. Here we show, using the stable carbon isotope composition of ancient cave guano profiles, that there was a substantial forest contraction during the LGP on both peninsular Malaysia and Palawan, while rainforest was maintained in northern Borneo. These results directly support rainforest “refugia” hypotheses and provide evidence that environmental barriers likely reduced genetic mixing between Borneo and Sumatra flora and fauna. Moreover, it sheds light on possible early human dispersal events.

Advances in the understanding of nutrient dynamics and management in UK agriculture

Current research on macronutrient cycling in UK agricultural systems aims to optimise soil and nutrient management for improved agricultural production and minimise effects on the environment and provision of ecosystem services. Nutrient use inefficiencies can cause environmental pollution through the release of greenhouse gases into the atmosphere and of soluble and particulate forms of N, P and carbon (C) in leachate and run-off into watercourses. Improving nutrient use efficiencies in agriculture calls for the development of sustainable nutrient management strategies: more efficient use of mineral fertilisers, increased recovery and recycling of waste nutrients, and, better exploitation of the substantial inorganic and organic reserves of nutrients in the soil. Long-term field experimentation in the UK has provided key knowledge of the main nutrient transformations in agricultural soils. Emerging analytical technologies, especially stable isotope labelling, that better characterise macronutrient forms and bioavailability and improve the quantification of the complex relationships between the macronutrients in soils at the molecular scale, are augmenting this knowledge by revealing the underlying processes. The challenge for the future is to determine the relationships between the dynamics of N, P and C across scales, which will require both new modelling approaches and integrated approaches to macronutrient cycling.

Interactions among agricultural production and other ecosystem services delivered from European temperate grassland systems.

Global demand for food is increasing as is the recognition that this must be achieved with minimal negative impacts on the environment or other ecosystem services (ESs). Here we develop an understanding of the relationships among ESs delivered within temperate agricultural grassland systems in lowland Europe. We reviewed the refereed literature on pair-wise interactions between nine different ESs. These were agricultural production, climate regulation, air quality regulation, water quality regulation, hydrological regulation, soil erosion regulation, nutrient cycling, biodiversity conservation, and landscape quality. For each pair, we sought information on how each ES responds to changes in the other. Each interaction was assigned to one of five categories: (i) no direct relationship between the driving ES on the responding ES, (ii) the driving ES has a negative impact on the responding ES, (iii) the driving ES has a positive impact on the responding ES, (iv) the evidence of direction of effect is inconclusive, because of either inadequate information or contradictions in the literature, and (v) there is no current evidence in the current literature for a relationship. Negative relationships resulted only from the effects of increasing the intensity of agricultural production on other ESs. Available evidence infers that erosion regulation and good nutrient cycling were the only two driving ESs shown to enhance agricultural production implying that their protection will enhance our ability to meet future food needs. In order for agriculture to become more sustainable, we need to develop agricultural methods that can minimize the negative impacts of these win–lose relationships.

Interspecific variation in bulk tissue, fatty acid and monosaccharide δ13C values of leaves from a mesotrophic grassland plant community

The leaves of 37 grass, herb, shrub and tree species were collected from a mesotrophic grassland to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values of leaves from one plant community. The leaf tissue mean bulk delta(13)C value was -29.3 per thousand. No significant differences between tissue bulk delta(13)C values with life form were determined (P=0.40). On average, C(16:0), C(18:2) and C(18:3) constituted 89% of leaf tissue total fatty acids, whose delta(13)C values were depleted compared to whole leaf tissues. A general interspecific (between different species) trend for fatty acids delta(13)C values was observed, i.e. delta(13)C(16:0)<delta(13)C(18:2)<delta(13)C(18:3), although these values ranged widely between species, e.g. C(16:0) (-34.7 per thousand, Alisma plantago-aquatica; -44.0 per thousand, Leucanthemum vulgare), C(18:2) (-33.3 per thousand, Acer campestre; -44.2 per thousand, L. vulgare;) and C(18:3) (-34.3 per thousand, Bellis perennis; -41.8 per thousand, Plantago lanceolata). Average relative abundances of leaf monosaccharides arabinose, xylose, mannose, galactose and glucose were 12%, 13%, 5%, 12% and 54%, respectively. Mean delta(13)C values of these monosaccharides were -26.6 per thousand (arabinose), -27.2 per thousand (xylose), -30.9 per thousand (mannose), -30.0 per thousand (galactose) and -29.0 per thousand (glucose). The general relationship between individual monosaccharide delta(13)C values, delta(13)C(arabinose)>delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), was consistently observed. Therefore, we have shown (i) diversity in compound-specific delta(13)C values contributing to leaf bulk delta(13)C values; (ii) interspecific variability between bulk and compound-specific delta(13)C values of leaves of individual grassland species, and (iii) trends between individual fatty acid and monosaccharide delta(13)C values common to leaves of all species within one plant community.

Quantification of dung carbon incorporation in a temperate grassland soil following spring application using bulk stable carbon isotope determinations

Herbivore dung constitutes a substantial input of C to temperate grassland soils, and its fate must be determined in order to fully understand nutrient cycling in this ecosystem. This experiment used changes in bulk δ13C values of the 0–1 cm and 1–5 cm soil horizons of a dung-treated temperate grassland soil to approximate percentage applied dung C incorporation over 372 days. Natural abundance 13C-labelled C4 dung (δ13C − 12.6‰) and C3 dung (δ13C − 31.3‰) were produced in a monitored diet switch from ryegrass silage (δ13C − 30.1‰) to maize silage (δ13C − 11.6‰). The dung was applied to a C3 grassland (δ13C 0–1 cm − 29.9‰, 1–5 cm − 30.6‰), and dung remains and soil cores from beneath the treatments were sampled at intervals. δ13C values were used to estimate a maximum of 12% applied dung C incorporation in the top 5 cm of the soil after 112 days, which declined to around 8% at the end of the experiment. A significant increase in percentage applied dung C was observed in the top 1 cm of soil, compared with the 1–5 cm horizon, after a substantial rain event after 30 days. However, results of forage fibre analyses of the two dung types revealed significant differences in composition which may affect subsequent calculations of percentage dung incorporation based on bulk δ13C values.

Catchment source contributions to the sediment-bound organic matter degrading salmonid spawning gravels in a lowland river, southern England.

The ingress of particulate material into freshwater spawning substrates is thought to be contributing to the declining success of salmonids reported over recent years for many rivers. Accordingly, the need for reliable information on the key sources of the sediment problem has progressed up the management agenda. Whilst previous work has focussed on apportioning the sources of minerogenic fine sediment degrading spawning habitats, there remains a need to develop procedures for generating corresponding information for the potentially harmful sediment-bound organic matter that represents an overlooked component of interstitial sediment. A source tracing procedure based on composite signatures combining bulk stable (13)C and (15)N isotope values with organic molecular structures detected using near infrared (NIR) reflectance spectroscopy was therefore used to assess the primary sources of sediment-bound organic matter sampled from artificial spawning redds. Composite signatures were selected using a combination of the Kruskal-Wallis H-test, principal component analysis and GA-driven discriminant function analysis. Interstitial sediment samples were collected using time-integrating basket traps which were inserted at the start of the salmonid spawning season and extracted in conjunction with critical phases of fish development (eyeing, hatch, emergence, late spawning). Over the duration of these four basket extractions, the overall relative frequency-weighted average median (±95% confidence limits) source contributions to the interstitial sediment-bound organic matter were estimated to be in the order: instream decaying vegetation (39±<1%; full range 0-77%); damaged road verges (28±<1%; full range 0-77%); septic tanks (22±<1%; full range 0-50%), and; farm yard manures/slurries (11±<1%; full range 0-61%). The reported procedure provides a promising basis for understanding the key sources of interstitial sediment-bound organic matter and can be applied alongside apportionment for the minerogenic component of fine-grained sediment ingressing the benthos. The findings suggest that human septic waste contributes to the interstitial fines ingressing salmonid spawning habitat in the study area.

Applications of stable isotope ratio mass spectrometry in cattle dung carbon cycling studies

Understanding the fate of dung carbon (C) in soils is challenging due to the ubiquitous presence of the plant-derived organic matter (OM), the source material from which both dung-derived OM and soil organic matter (SOM) predominantly originate. A better understanding of the fate of specific components of this substantial source of OM, and thereby its contribution to C cycling in terrestrial ecosystems, can only be achieved through the use of labelled dung treatments. In this short review, we consider analytical approaches using bulk and compound-specific stable carbon isotope analysis that have been utilised to explore the fate of dung-derived C in soils. Bulk stable carbon isotope analyses are now used routinely to explore OM matter cycling in soils, and have shown that up to 20% of applied dung C may be incorporated into the surface soil horizons several weeks after application, with up to 8% remaining in the soil profile after one year. However, whole soil delta(13)C values represent the average of a wide range of organic components with varying delta(13)C values and mean residence times in soils. Several stable (13)C isotope ratio mass spectrometric methods have been developed to qualify and quantify different fractions of OM in soils and other complex matrices. In particular, thermogravimetry-differential scanning calorimetry-isotope ratio mass spectrometry (TG-DSC-IRMS) and gas chromatography-combustion-IRMS (GC-C-IRMS) analyses have been applied to determine the incorporation and turnover of polymeric plant cell wall materials from C(4) dung into C(3) grassland soils using natural abundance (13)C isotope labelling. Both approaches showed that fluxes of C derived from polysaccharides, i.e. as cellulose or monosaccharide components, were more similar to the behaviour of bulk dung C in soil than lignin. However, lignin and its 4-hydroxypropanoid monomers were unexpectedly dynamic in soil. These findings provide further evidence for emerging themes in biogeochemical investigations of soil OM dynamics that challenge perceived concepts of recalcitrance of C pools in soils, which may have profound implications for the assessment of the potential of agricultural soils to influence terrestrial C sinks.

Seasonal variations in bulk tissue, fatty acid and monosaccharide δ13C values of leaves from mesotrophic grassland plant communities under different grazing managements.

Leaves of 26 grass, herb, shrub and tree species were collected from mesotrophic grasslands to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values under different grazing management (cattle- or deer-grazed) on three sample dates (May, July and October) such that interspecific and spatiotemporal variations in whole leaf tissues and compound-specific delta(13)C values could be determined. The total mean leaf bulk delta(13)C value for plants was -28.9 per thousand with a range of values spanning 7.5 per thousand. Significant interspecific variation between bulk leaf delta(13)C values was only determined in October (P=<0.001) when delta(13)C values of the leaf tissues from both sites was on average 1.5 per thousand depleted compared to during July and May. Samples from May were significantly different between fields (P=0.03) indicating an effect from deer- or cattle-grazing in young leaves. The average individual monosaccharide delta(13)C value was 0.8 per thousand higher compared with whole leaf tissues. Monosaccharides were the most abundant components of leaf biomass, i.e. arabinose, xylose, mannose, galactose and glucose, and therefore, fluctuations in their individual delta(13)C values had a major influence on bulk delta(13)C values. An average depletion of ca. 1 per thousand in the bulk delta(13)C values of leaves from the deer-grazed field compared to the cattle-grazed field could be explained by a general depletion of 1.1 per thousand in glucose delta(13)C values, as glucose constituted >50% total leaf monosaccharides. In October, delta(13)C values of all monosaccharides varied between species, with significant variation in delta(13)C values of mannose and glucose in July, and mannose in May. This provided an explanation for the noted variability in the tissue bulk delta(13)C values observed in October 1999. The fatty acids C(16:0), C(18:2) and C(18:3) were highly abundant in all plant species. Fatty acid delta(13)C values were lower than those of bulk leaf tissues; average values of -37.4 per thousand (C(16:0)), -37.0 per thousand (C(18:2)) and -36.5 per thousand (C(18:3)) were determined. There was significant interspecific variation in the delta(13)C values of all individual fatty acids during October and July, but only for C(18:2) in May (P=<0.05). This indicated that seasonal trends observed in the delta(13)C values of individual fatty acids were inherited from the isotopic composition of primary photosynthate. However, although wide diversity in delta(13)C values of grassland plants ascribed to grazing management, interspecific and spatiotemporal influences was revealed, significant trends (P=<0.0001) for fatty acid and monosaccharide delta(13)C values: delta(13)C(16:0)delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), respectively, previously described, appear consistent across a wide range of species at different times of the year in fields under different grazing regimes.

Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid δ13C values determined by gas chromatography-combustion-isotope ratio mass spectrometry†

Litter-dwelling (epigeic) Lumbricus rubellus and soil-dwelling (endogeic) Allolobophora chlorotica earthworms were observed aggregating under C(3) (delta(13)C = -31.3 per thousand; delta(15)N = 10.7 per thousand) and C(4) (delta(13)C = -12.6 per thousand; delta(15)N = 7.5 per thousand) synthetic dung pats applied to a temperate grassland (delta(13)C = -30.3 per thousand; delta(15)N = 5.7 per thousand) in an experiment carried out for 372 days. Bulk delta(13)C values of earthworms collected from beneath either C(3) or C(4) dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C(4) dung were significantly (13)C-enriched after 56 days (delta(13)C = -23.8 per thousand) and 112 days (delta(13)C = -22.4 per thousand) compared with those from C(3) dung treatments (56 days, delta(13)C = -26.5 per thousand; 112 days, delta(13)C = -27.0 per thousand), and (ii) A. chlorotica were 2.1 per thousand (13)C-enriched (delta(13)C = -24.2 per thousand) relative to those from C(3) dung (delta(13)C = -26.3 per thousand) treatments after 372 days. Bulk delta(15)N values did not suggest significant uptake of dung N by either species beneath C(3) or C(4) dung, but showed that the endogeic species (total mean delta(15)N = 3.3 per thousand) had higher delta(15)N values than the epigeic species (total mean delta(15)N = 5.4 per thousand). Although the two species exhibited similar fatty acid profiles, individual fatty acid delta(13)C values revealed extensive routing of dietary C into body tissue of L. rubellus, but minor incorporation into A. chlorotica. In particular, the direct incorporation of microbial biomarker fatty acids (iC(17:0), aC(17:0)) from (13)C-labelled dung in situ, the routing of dung C into de novo synthesised compounds (iC(20:4)(omega)(6),C(20:5)(omega)(3), and the assimilation of essential fatty acids ((C(18:1)(omega)(9), C(18:1)(omega(7), C(18:2)(omega(6), C(18:3)(omega)(3)) derived from dung, were determined.