Long-term Management Regimes Research Articles

Long-term management is needed for conserving plant diversity in a Wadden Sea salt marsh

Evaluation of long-term management regimes is important for guiding biodiversity conservation in salt marshes. However, such long-term experiments are sparse. Using a 46-year experiment in a salt marsh, we evaluated long-term effects of eight different management regimes (treatments; control, grazing, mowing, and their combinations) on the expansion of a late successional plant species (Elytrigia atherica), plant species richness and diversity, and community composition (species identities and dominance structure). Results show that E. atherica expanded strongly over time in the control treatment (without grazing or mowing) while plant species richness and diversity declined substantially. By contrast, E. atherica was greatly suppressed while plant species richness and diversity remained relatively unchanged in all other treatments except for the mowing, where species richness declined in the late season mowing treatment and plant diversity declined after 17 years in the both early and late season mowing treatment. Therefore, all management types except for the mowing were effective in conserving plant diversity. The trends for change in species identities reversed: change in species identities accumulated in the control treatment and exceeded that of other treatments 15 years after the start of the experiment. This suggests that results based on shorter-term (< 15 years) experiments may provide misleading conservation recommendations. Also, trends for change in dominance structure (taking abundance into account) were substantially different from those for species identities. Our results highlight the importance of long-term monitoring for guiding conservation management, and that monitoring should not only focus on the number of species but also community composition, to fully identify critical changes.

Differences in substrate use efficiency: impacts of microbial community composition, land use management, and substrate complexity

Microbial substrate use efficiency is an important property in process-based soil organic matter models, but is often assumed to be constant in mechanistic models. However, previous studies question if a constant efficiency is appropriate, in particular when evaluating carbon (C) cycling across temperatures and various substrates. In the present study, we evaluated the relation between substrate use efficiency, microbial community composition and substrate complexity in contrasting long-term management regimes (47–49 years of either arable, ley farming, grassland, or forest systems). Microbial community composition was assessed by phospholipid fatty acid analysis and three indices of substrate use efficiencies were considered: (i) thermodynamic efficiency, (ii) calorespirometric ratio, and (iii) metabolic quotient. Three substrates, d-glucose, l-alanine, or glycogen, varying in complexity, were added separately to soils, and heat production as well as C mineralization was determined over a 32-h incubation period at 12.5 °C. Microbial communities from forest systems were most efficient in utilizing substrates, supporting our hypothesis that maturing ecosystems become more efficient. These changes in efficiency were linked to microbial community composition with fungi and Gram-negative bacteria being important biomarkers. Despite our initial hypothesis, complex substrate such as glycogen was utilized most efficiently. Our findings emphasize that differences in land use management systems as well as the composition of soil organic matter need to be considered when modelling C dynamics in soils. Further research is required to establish and evaluate appropriate proxies for substrate use efficiencies in various ecosystems.

Isothermal Microcalorimetry Provides New Insight into Terrestrial Carbon Cycling

Energy is continuously transformed in environmental systems through the metabolic activities of living organisms, but little is known about the relationship between the two. In this study, we tested the hypothesis that microbial energetics are controlled by microbial community composition in terrestrial ecosystems. We determined the functional diversity profiles of the soil biota (i.e., multiple substrate-induced respiration and microbial energetics) in soils from an arable ecosystem with contrasting long-term management regimes (54 years). These two functional profiling methods were then related to the soils' microbial community composition. Using isothermal microcalorimetry, we show that direct measures of energetics provide a functional link between energy flows and the composition of below-ground microbial communities at a high taxonomic level (Mantel R = 0.4602, P = 0.006). In contrast, this link was not apparent when carbon dioxide (CO2) was used as an aggregate measure of microbial metabolism (Mantel R = 0.2291, P = 0.11). Our work advocates that the microbial energetics approach provides complementary information to soil respiration for investigating the involvement of microbial communities in below-ground carbon dynamics. Empirical data of our proposed microbial energetics approach can feed into carbon-climate based ecosystem feedback modeling with the suggested conceptual ecological model as a base.

The thermodynamic efficiency of soil microbial communities subject to long-term stress is lower than those under conventional input regimes

Isothermal microcalorimetry measures the thermal flows occurring in systems with very high precision and may be used to quantify carbon (C) assimilation and resource-use efficiencies in soils. We determined the thermodynamic efficiency of soil microbial communities located in soils which had received contrasting long-term management regimes (53 y) with respect to organic matter and nitrogen (N) inputs, viz. farmyard manure, sewage sludge, straw and calcium nitrate, calcium nitrate only, or ammonium sulphate. Two thermodynamic efficiency indices were considered: (i) total thermodynamic efficiency of soil microbial communities (ηeff), i.e. general heat production released following substrate addition, per unit heat energy input to the soil system, and (ii) a specific thermodynamic efficiency index of energy retained in the soil system (ηsoil). The latter index provides quantitative data on how much C is assimilated and energy retained in the soil system. Further, we derived a ‘substrate-induced heat production’ (SIHP) index, which adjusts for size of the microbial biomass. Optimised concentrations of water or glucose plus water were added to the soil samples and resultant thermal signatures and C mineralisation were determined over a 48-h incubation period at 25°C. The thermal signatures were further related to the microbial community profiles of the soils. The phenotypic structural and functional diversity profiles of the microbial communities in soils were assessed by phospholipid fatty acid and multi-substrate induced respiration methods at the start of the experiment, confirming significant differences between all five treatments in community composition and functional capabilities. Both the total and specific thermodynamic efficiency indices of the soil microbial communities exposed to long-term stress by heavy metal toxicity (sewage sludge) and low pH ((NH4)2SO4) were significantly smaller in magnitude than those under the three conventional (i.e. Ca(NO3)2, Straw+Ca(NO3)2, farmyard manure) input regimes (P<0.05). The SIHP index however, was highest in the treatments receiving long-term inorganic inputs, indicating more heat production per unit biomass, than that found in all three organic input regimes. These differences in efficiencies were reflected in both the phenotypic and functional profiles of the communities. These indices may provide quantification of C assimilation and resource-use efficiency under different land-use and management scenarios, and potentially allow evaluation of the role of soils in governing the terrestrial C balance by studying the fate and regulation of C in soil systems.

Change in soil carbon following afforestation

Quantifying changes in soil C may be an important consideration under large-scale afforestation or reforestation. We reviewed global data on changes in soil C following afforestation, available from 43 published or unpublished studies, encompassing 204 sites. Data were highly variable, with soil C either increasing or decreasing, particularly in young (<10-year) forest stands. Because studies varied in the number of years since forest establishment and the initial soil C content, we calculated change in soil C as a weighted-average (i.e. sum of C change divided by sum of years since forest establishment) relative to the soil C content under previous agricultural systems at <10, >10 and <30 cm sampling depths. On average, soil C in the <10 cm (or <30 cm) layers generally decreased by 3.46% per year (or 0.63% per year) relative to the initial soil C content during the first 5 years of afforestation, followed by a decrease in the rate of decline and eventually recovery to C contents found in agricultural soils at about age 30. In plantations older than 30 years, C content was similar to that under the previous agricultural systems within the surface 10 cm of soil, yet at other sampling depths, soil C had increased by between 0.50 and 0.86% per year. Amounts of C lost or gained by soil are generally small compared with accumulation of C in tree biomass. The most important factors affecting change in soil C were previous land use, climate and the type of forest established. Results suggest that most soil C was lost when softwoods, particularly Pinus radiata plantations, were established on ex-improved pastoral land in temperate regions. Accumulation of soil C was greatest when deciduous hardwoods, or N 2-fixing species (either as an understorey or as a plantation), were established on ex-cropped land in tropical or subtropical regions. Long-term management regimes (e.g. stocking, weed control, thinning, fertiliser application and fire management) may also influence accumulation of soil C. Accumulation is maximised by maintaining longer (20–50 years) forest rotations. Furthermore, inclusion of litter in calculations reversed the observed average decrease in soil C, so that amount of C in soil and litter layer was greater than under preceding pasture.

Short-term nitrogen fluxes in grassland soils under different long-term nitrogen management regimes

Short-term N fluxes were measured during spring in perennial grasslands in S.W. England to examine processes controlling plant N availability. Three long-term management regimes were used, being grass plus white clover or grass receiving N fertilizer at 0 or 200 kg N ha −1 y −1. 15N-labelled NH 4 + solution was injected into microplot cores in the field and after 1 and 3 d the cores were removed for analysis of total N and 15N in soil inorganic N fractions, microbial biomass and plant shoots and roots. Gross N mineralization, gross N immobilization and N 2 fixation were estimated using 15N isotope dilution or pool enrichment calculations. Plant uptake was the dominant process of removal of inorganic N from soil, with short-term rates being similar for all three treatments. Rapid and large N fluxes through plant uptake and mineralization–immobilization reactions maintained a small soil inorganic N pool which turned over approximately daily. In the clover treatment, all fluxes of N (except N 2 fixation at 2.4 kg N ha −1 d −1) were intermediate between the 0 and 200 N treatments. Thus, treatment differences in N fluxes reflected differences in their long-term N inputs. Measurements of net N mineralization using several techniques showed a 2-fold difference between the 0 and 200 N treatments, with the clover treatment being intermediate. However, isotope dilution calculations revealed that rates of gross N mineralization were 3–12 times larger than net N mineralization rates, and that treatments differed in gross N mineralization rate by only 35% (3.0–4.1 kg N ha −1 d −1). An important factor influencing net N fluxes was the marked differences in gross N immobilization rate, which varied between 1.0 and 0.1 kg N ha −1 d −1 in the 0 and 200 N treatments, respectively.