Production and dissolution rates of earthworm-secreted calcium carbonate

Earthworms secrete granules of calcium carbonate. These are potentially important in soil biogeochemical cycles and are routinely recorded in archaeological studies of Quaternary soils. Production rates of calcium carbonate granules by the earthworm Lumbricus terrestris L. were determined over 27 days in a range of soils with differing chemical properties (pH, organic matter content, water holding capacity, bulk composition, cation exchange capacity and exchangeable cations). Production rate varied between soils, lay in the range $0–0.043{\text{mmol}}_{{\text{CaCO}}_3}$ (0–4.3 mg) earthworm^?1 d^?1 with an average rate of $8\times {10}^{?3}{\text{mmol}}_{{\text{CaCO}}_3}$ (0.8 mg) earthworm^?1 d^?1 and was significantly correlated (r = 0.68, P ≤ 0.01) with soil pH. In a second experiment lasting 315 days earthworms repeatedly (over periods of 39–57 days) produced comparable masses of granules. Converting individual earthworm granule production rates into fluxes expressed on a per hectare of land per year basis depends heavily on estimates of earthworm numbers. Using values of 10–20 L. terrestris m^?2 suggests a rate of $18–3139{\text{mol}}_{{\text{CaCO}}_3}{\text{ha}}^{?1}{\text{yr}}^{?1}$. Data obtained from flow-through dissolution experiments suggest that at near neutral pH, granule geometric surface area-normalised dissolution rates are similar to those for other biogenic and inorganic calcites. Fits of the data to the dissolution relationship r = k(1 ? Ω)ⁿ where r = dissolution rate, k = a rate constant, Ω = relative saturation and n = the reaction order gave values of k = 1.72 × 10^?10 mol cm^?2 s^?1 and n = 1.8 for the geometric surface area-normalised rates and k = 3.51 × 10^?13 mol cm^?2 s^?1 and n = 1.8 for the BET surface area-normalised rates. In 196 day leaching column experiments trends in granule dissolution rate referenced to soil chemistry corresponded to predictions made by the SLIM model for dissolution of limestone in soil. If soil solution approaches saturation with respect to calcium carbonate, granule dissolution will slow or even stop and granules be preserved indefinitely. Granules have the potential to be a small but significant component of the biogeochemical cycling of C and Ca in soil.     

A method for estimating coefficients of soil organic matter dynamics based on long-term experiments

The one-compartment C model $C_t=C_0{\text{e}}^{-k_{2}t}+k_{1}A/k_2(1-{\text{e}}^{-k_{2}t})$ is being long used to simulate soil organic C (SOC) stocks. C_t is the SOC stock at the time t; C₀, the initial SOC stock; k₂, the annual rate of SOC loss (mainly mineralization and erosion); k₁, the annual rate to which the added C is incorporated into SOC; and A, the annual C addition. The component $C_0{\text{e}}^{-k_{2}t}$ expresses the decay of C₀ and, for a time t, corresponds to the remains of C₀ (C_0 remains). The component $k_{1}A/k_2(1-{\text{e}}^{-k_{2}t})$ refers, at time t, to the stock of SOC derived from C crops (C_crop). We herein propose a simple method to estimate k₁ and k₂ coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions (A) and SOC stocks. We estimated k₁ and k₂ for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N-urea rates (0 kg N ha⁻¹ −0 N and 180 kg N ha⁻¹ −180 N) in a long-term experiment established in a subtropical Acrisol with C₀ = 32.55 Mg C ha⁻¹ in the 0–17.5 cm layer. A linear equation (C_t = a + bA) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model $C_t=C_0{\text{e}}^{-k_{2}t}+k_{1}A/k_2(1-{\text{e}}^{-k_{2}t})$, so that $a=C_0{\text{e}}^{-k_{2}t}$ and $bA=k_{1}A/k_2(1-{\text{e}}^{-k_{2}t})$. Such equivalences thus allow the calculation of k₁ and k₂. NT soil had a lower rate of C loss (k₂ = 0.019 year⁻¹) than CT soil (k₂ = 0.040 year⁻¹), while k₁ was not affected by tillage (0.148 year⁻¹ under CT and 0.146 year⁻¹ under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state (C_e) in the 0–17.5 cm layer ranged from 15.65 Mg ha⁻¹ in CT O/M 0 N to 60.17 Mg ha⁻¹ in NT OV/MC 180 N. The SOC half-life (t_1/2 = ln 2/k₂) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C_0 remais), while increments in C_crop are imparted mainly by crop additions.     

Earthworm ecosystem service and dis-service in an N-enriched agroecosystem: Increase of plant production leads to no effects on yield-scaled N2O emissions

Earthworms can enhance plant productivity by promoting nitrogen (N) mineralization in N-limited agroecosystems and may also enhance the risk of N₂O emissions and ${{\text{NO}}_3}^{-}–\text{N}$ leaching in N-enriched agroecosystems. However, direct evidence demonstrating the enhancement by earthworms of N₂O emissions and ${{\text{NO}}_3}^{-}–\text{N}$ leaching in the field is scarce, particularly in intensively managed systems. In addition, the interaction of earthworm feeding strategies and organic amendment may profoundly modulate N cycling. We examined these impacts using two earthworm species with distinct ecological strategies (epigeic Eisenia foetida and endogeic Metaphire guillemi) in combination with two manure application methods (surface mulch and incorporation into the soil) in a field experiment. Our results demonstrated that earthworm addition significantly increased the crop yield by 18%–47% and cumulative N₂O emissions by 19%–25% largely regardless of earthworm species and manure application methods, respectively. However, earthworms did not significantly increase the leachate ${{\text{NO}}_3}^{-}–\text{N}$ concentration. Earthworm-induced N₂O emissions were primarily attributed to increased soil N availability (${{\text{NO}}_3}^{-}–\text{N}$ and microbial biomass N) and carbon (C) availability (dissolved organic C). In contrast, a stepwise regression revealed that an earthworm-promoted soil macroaggregation exerted negative effects on N₂O emissions. Irrespective of earthworm species and manure application methods, earthworms had no stimulatory effects on the yield-scaled N₂O–N because the promotion of crop productivity counteracted the extent of N₂O increase. In conclusion, understanding the trade-off between earthworm services and dis-services will contribute to the development of environmentally justified soil management by allowing the full utilization of biological resources.     

Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola

The role of soil organisms as possible driver of flowering has never been investigated. We hypothesized that Collembola (microarthropods) will change plant allocation to reproductive modes by changing soil nutrient availability. Individual seedlings of Poa annua were planted in microcosms, in the presence or absence of Collembola. Collembola affected biotic (fungal biomass) and abiotic ($\text{N}-{{\text{NO}}_3}^{-}$, P₂O₅) soil properties and some morphological (number of leaves, root biomass) and chemical (C:N, K, Mg, N) traits of P. annua. As a result, flowering of P. annua was promoted by the presence of Collembola. This provides experimental evidence that soil microarthropods can affect the reproduction strategy and phenology of a plant.     

Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers

Competition for nutrients between plants and microbes is an important determinant for plant growth, biodiversity and carbon cycling. Perturbations such as drought affect the availability of nitrogen (N) and phosphorus (P), and may cause shifts in uptake of N and P between plants and microbes. Competitiveness for these nutrients may depend on how flexible plants and microbes are in taking up N and P. We used a novel dual isotope labelling technique (¹⁵N and ³²P) to assess short-term uptake of N and P by plants and microbes affected by drought in two different plant–soil systems. Mesocosms were extracted from two grassland sites differing in soil nutrient availability and plant species. Half of the mesocosms were subjected to drought one week prior to injection of ¹⁵N (as KNO₃) and ³²P (as H₃PO₄) tracers. Uptake rates of ${{\text{NO}}_3}^{-}$ and P in plants and microbes were estimated based on average source pool enrichment during the labelling period and on plant and microbial recovery of ¹⁵N and ³²P measured after 4 days of labelling. Overall competition for N and P was reduced with drought as less ${{\text{NO}}_3}^{-}$ and P was taken up in plants and microbes. However, plant uptake of ${{\text{NO}}_3}^{-}$ was more sensitive to drought than microbial ${{\text{NO}}_3}^{-}$ uptake, while microbial P uptake was more sensitive than plant P uptake. These different sensitivities to drought by plants and microbes may decouple the N and P cycle with increased drought conditions.