Long-term management impacts on soil C,N and physical fertility: Part III: Tamworth crop rotation experiment

Degradation of soil structure can lead to increased risk of run-off and soil erosion, and therefore, it is necessary to implement management practices that are more sustainable and will enhance and rehabilitate soils while increasing food production. The impact of small-grain rotations grown with legumes, fallow and continuously on total C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), aggregation expressed as mean weight diameter (MWD) and infiltration determined as unsaturated hydraulic conductivity (K_unsat) were examined in a long-term rotation trial established in 1966 on a Black Earth (Pellic Vertisol) and a Red Clay (Chromic Vertisol) soil near Tamworth, in New South Wales, Australia. The results were compared with an adjacent uncropped pasture on each soil type. Cropping reduced all C fractions, N_T, MWD and K_unsat on both soils, which were further degraded when long fallowing was included in the rotation. C_L decreased by 70% with long fallow in the Red Clay and by 78% in the Black Earth compared with the adjacent pasture, while MWD decreased by 61% in the Red Clay and 91% in the Black Earth. Rotation of cereals with legumes resulted in smaller decreases in C fractions, N_T, MWD and K_unsat when compared with pasture. Rotation with lucerne (Medicago sativa) resulted in 41% higher C_L, 45% higher MWD and 87% higher K_unsat (10 mm tension) than long fallow on the Red Clay soil and 65, 126 and 43% higher on the Black Earth soil. There were strong positive correlations of soil C fractions and N_T with MWD for both soil types. Similar significant relationships were found for all C fractions and N_T with K_unsat (10 mm tension) for the Red Clay soil, but not for the Black Earth. Rotations with forage legumes can limit declines in C fractions, N_T, MWD and K_unsat when cropping these soils and has potential to increase soil sustainability.     

Soil microbiological and biochemical properties affected by plant growth and different long-term fertilisation

The purpose of this study was to measure the effects of plant growth on soil microbial biomass C (C_mic) and soil enzyme activities. In a pot experiment using spring barley and sugar beet, we investigated the response of C_mic, hot water extractable C and N fractions (C_hwe, N_hwe), and enzyme activities involved in C, N and P cycling in a loess-derived Chernozem from Bad Lauchstädt (Central Germany). The study site has been receiving the same fertilisation treatments for 100 years. The soil originated from plots fertilised with 15 t ha⁻¹ farmyard manure (FYM) year⁻¹ + mineral fertiliser (NPK), or 15 t ha⁻¹ FYM year⁻¹, or NPK or from an unfertilised control. Pots were sampled monthly, and alkaline phosphatase- (AP), β-glucosidase- and protease-activities were analysed. At the beginning and the end of study, levels of C_mic, C_hwe and N_hwe were also measured. All three enzyme activities and C_mic were significantly and positively correlated with C_org and C_hwe. Results suggest that the enzyme activities measured originated mostly from microorganisms and that C_hwe is an important C source for soil microorganisms. β-Glucosidase and AP activities were higher in summer months than at other times. In contrast, protease activity changed only slightly during the growing period.     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

How do environmental factors and different fertilizer strategies affect soil CO2 emission and carbon sequestration in the upland soils of southern China?

Upland soils have been identified as a major CO₂ source induced by human activities, such as fertilizer applications. The aim of this study is to identify the characteristics of soil CO₂ emission and carbon balance in cropland ecosystems after continuous fertilizer applications over decades. The measurements of soil surface CO₂ fluxes throughout the years of 2009 and 2010 were carried out based on a fertilization experiment (from 1990) in a double cropping system rotated with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) in upland soil in southern China. Four treatments were chosen from the experiment for this study: no-fertilizer application (SR), nitrogen–phosphorus–potassium chemical fertilizers (NPK), NPK plus pig manure (NPKM) and pig manure alone (M). Results showed that the mean value of soil CO₂ fluxes from 08:00 to 10:00 am could represent its daily mean value in summer period (June–August) and that from 09:00 am to 12:00 pm for the rest season of a year. Soil temperature and moisture combined together could explain 70–83% of variations of CO₂ emission. Annual cumulative soil CO₂ fluxes in the treatments with manure applications (8.2 ± 0.8 and 11.0 ± 1.2 t C ha⁻¹ in 2009, and 7.9 ± 0.9 and 11.1 ± 1.2 t C ha⁻¹ in 2010 in NPKM and M, respectively) were significantly higher than those in the treatments with non-manure addition (2.5 ± 0.2 and 3.4 ± 0.2 t C ha⁻¹ in 2009, and 2.1 ± 0.2 and 3.7 ± 0.3 t C ha⁻¹ in 2010 in SR and NPK, respectively). However, the treatments with manure applications represented a carbon sink in the soil (carbon output/input ratio < 1.0), which demonstrated potential for carbon sequestration.     

The temperature dependence of dicyandiamide (DCD) degradation in soils: A data synthesis

Dicyandiamide (DCD, C₂H₄N₄) is a nitrification inhibitor that has been studied for more than 80 years. However, there are few papers that have examined the use of DCD on dairy farms where cattle graze pasture and where urine is the primary form of nitrogen (N) deposited onto soils. After DCD was applied (10 kg DCD ha^?1) with bovine urine (700–1200 kg N ha^?1) to five soils throughout New Zealand, the reduction in direct nitrous oxide (N₂O) emissions was significant and remarkably consistent (71 ± 8%, average ± standard error). The application of DCD to these soils occurred in autumn and winter; daily average soil temperature (T) was reported but these data were not further analysed. Perusal of the literature suggested no consensus on the temperature dependence of DCD degradation in soils. Based on published data from controlled-environment studies of soils sampled in four countries, we quantified the relation between T and the time for DCD concentration in soils to decline to half its application value (t_½) as t_½ (T) = 168e^?0.084T with parameter standard errors of ±16 d and ±0.011 d^?1, respectively (n = 16). For example, at 5 °C a 1 °C increase in T reduced t_½ from 110 to 101 d whereas at 25 °C the reduction was 20–19 d. Analysing T data from the New Zealand trials using our t_½ (T) function, over 43–89 d when direct N₂O emissions from treated plots became indistinguishable from the controls, the estimated percentage of applied DCD remaining in the soil averaged 43 ± 10%. These calculations suggested the apparently remaining DCD was ineffective with respect to direct N₂O emissions. In the absence of measurements, explanations for this interpretation included vertical displacement of the DCD and sorption onto organic matter in soils. The consistent DCD efficacy from these trials corresponded with T generally <10 °C, so it is suggested as an application criteria for the reduction of direct N₂O emissions from pastoral soils subjected to urine excretion by grazing cattle.     

Implications of input estimation,residue quality and carbon saturation on the predictive power of the Rothamsted Carbon Model

Effects of fertilisation and cropland management on soil organic carbon (SOC) dynamics can be assessed best in long-term experiments. Using data from the long-term fertilisation experiment in Puch, Germany (part of the series “Internationale Organische Stickstoff Dauerversuche”, IOSDV), we tested the performance of the Rothamsted Carbon Model 26.3 (RothC). The objectives of this work were: (i) quantify the C-input and the efficiency of SOC stabilisation, (ii) test the performance of different input estimates on predictive power of the RothC and (iii) test implementations of residue quality and C-saturation on model predictions. The experiment is a full-factorial strip design, the factors being “organic amendment” and “level of N-fertiliser”. Each treatment was replicated three times. The crop rotation is silage maize–winter wheat–winter barley. Five levels of the factor “organic amendment” were considered: (i) CON: no organic amendment; (ii) SLU: slurry application (on average 0.8 Mg C ha^? 1 year^? 1); (iii) FYM: application of farmyard manure (30 to 40 Mg ha^? 1 fresh mass every third year to maize, on average 1.0 Mg C ha^? 1 year^? 1); (iv) STR: straw incorporation after harvest of wheat and barley (depending on straw yield on average 0.7 to 2.2 Mg C ha^? 1 year^? 1); (v) STSL: slurry application plus straw incorporation (on average 1.1 to 2.4 Mg C ha^? 1 year^? 1). All treatments (including CON) were combined with five different levels of N-fertilisation (N0 to N4), whereas N0 was nil N application and N4 averaged 177 kg N ha^? 1 year^? 1. N-rates increased gradually and differed depending on the crop. Starting values for SOC stocks (Mg ha^? 1) were measured in 1983 as a mean among N-rates for organic amendment treatments (CON: 42; SLU: 39.8; FYM: 40.5; STR 39.8; STSL: 40.5). SOC stocks (0–25 cm) in 2004 (35.5 to 46.6 Mg C ha^? 1) were in the order STSL > FYM = SLU > STR = CON (p ≤ 0.001). However, slightly different starting values indicated a higher loss of SOC after 21 years in the CON (11–14%) compared to the STR treatments (1–10%). Effect of N-rate was not significant. The observed relation between change of SOC and C-input was quadratic (Y_O = ? 13.4 + 7.5x ? 0.9x²; R² = 0.74, p ≤ 0.001), which contrasted the linear relationship predicted by RothC (Y_P = ? 12.9 + 5.5x; R² = 0.97, p ≤ 0.0001). Serious deviation between observed and predicted relationship occurred above C-inputs of 2.5 Mg C ha^? 1 year^? 1. Mechanistic explanation (e.g. C-saturation or increased mineralisation by N-fertilisation) for the observation needs further exploration, but implication on regional estimates for C-accumulation for different cropland management scenarios is obvious: potential gain in SOC storage by increasing C-inputs may be overestimated, at least under conditions of the Puch site. Independent model predictions (i.e. no parameter adjustment and independent estimation and measurement of C-input) were successful for treatments without straw incorporation (CON, SLU, FYM). Using a regression between crop yields and crop residue input yielded better results than using a constant belowground-to-aboveground biomass ratio. SOC stocks of treatments STR and STSL were seriously overestimated by the model. Using a higher decomposability of crop residue improved result only marginally and required the change of a standard parameter. Using a simple implementation of C-saturation improved predictions for STR and STSL but failed to simulate dynamics in all other treatments. Overall, our results showed that it is important to recognise that relation between SOC change and C-input is not necessarily linear. However, the RothC model predicted SOC dynamics well at lower input levels. Observation that a regression equation for input estimation is superior to a constant biomass ratio for modelling purposes has to be tested further. An implementation of residue quality or saturation capacity in the RothC model may be promising for a better mechanistic understanding of SOC dynamics. However, this requires careful calibration and will increase the number of parameters to be fitted.     

Comparing net ecosystem exchange of carbon dioxide between an old-growth and mature forest in the upper Midwest,USA

Old-growth forests are often assumed to exhibit no net carbon assimilation over time periods of several years. This generalization has not been typically supported by the few whole-ecosystem, stand-scale eddy-covariance measurements of carbon dioxide exchange in old-growth forests. An eddy-flux tower installed in a >300-year-old hemlock–hardwood forest near the Sylvania Wilderness, Ottawa National Forest, MI, USA, observed a small annual carbon sink of CO₂ of −72 ± 36 g C m⁻² year⁻¹ in 2002 and −147 ± 42 g C m⁻² year⁻¹ in 2003. This carbon sink was much smaller than carbon sinks of −438 ± 49 g C m⁻² year⁻¹ in 2002 and −490 ± 48 g C m⁻² year⁻¹ in 2003 observed by a nearby flux tower in a 70-year-old mature hardwood forest (Willow Creek, WI). The mature forest had vegetation similar to the old-growth site prior to European settlement. Both sites had slightly larger carbon sinks in 2003, which was a drier and cooler year than 2002. However, the difference in sink strength between the two years was smaller than the uncertainty in the results arising from missing and screened data. Both sites also had significant systematic errors due to non-representative fluxes during certain micrometeorological conditions, which required careful screening. The difference in sink strength between the two sites was driven mainly by greater ER at the old-growth site (965 ± 35 g C m⁻² year⁻¹ in 2002 and 883 ± 69 g C m⁻² year⁻¹ in 2003) compared to the mature site (668 ± 21 g C m⁻² year⁻¹ in 2002 and 703 ± 17 g C m⁻² year⁻¹ in 2003). GEP was lower at the old-growth site (1037 ± 47 g C m⁻² year⁻¹ in 2002 and 1030 ± 41 g C m⁻² year⁻¹ in 2003) compared to the mature site (1106 ± 47 g C m⁻² year⁻¹ in 2002 and 1192 ± 51 g C m⁻² year⁻¹ in 2003), especially in 2003. Observations also suggested that growing season ER had greater interannual variability at the old-growth site. These results imply that old-growth forests in the region may be carbon sinks, though these sinks are smaller than mature forests, mostly likely due to greater ER.     

Recycling gamma irradiated sewage sludge as fertilizer: A case study using onion (Alium cepa)

Recycling sewage sludge into fertilizer for agricultural purposes may improve soil fertility by influencing the physical, chemical, and biological properties of the land. However, there is concern regarding elevated levels of heavy metals and pathogenic microorganisms, which may result from the use of untreated sewage sludge. Gamma radiation is found to be an efficient tool in the hygienization of municipal sewage sludge. In order to evaluate the agricultural potential of gamma irradiated sewage sludge and to assess the safety of this fertilizer, field experiments were performed in a root crop, onion (Alium cepa), during the 2003–2004 and 2004–2005 winter months. The influence over major nutrients, metallic micronutrients, and heavy metals in soil and crop plant were key factors to be analyzed. Treatments consisted of three source of fertilizers {S₁: farmyard manure (FYM); S₂: gamma irradiated sewage sludge (GISS); and S₃: non-irradiated sewage sludge (NISS)}, each at three separate levels (L₁: 5 t ha^?1; L₂: 10 t ha^?1; and L₃: 15 t ha^?1), which were evaluated and compared. The growth parameters and onion yield were not significantly influenced by the different sources of fertilizer, or the different application levels. Values of pH, organic carbon, organic N, available P and K, metallic micronutrients (Zn, Mn, Fe, Cu) and heavy metals (Ni, Cd, Pb, Co) indicate no negative effects on either soil or plant properties. Concentrations of heavy metals in soil and plant were slightly higher in NISS treatment in compare to GISS; however, the concentrations did remain within the prescribed limit, and no significant increase was consistently noted. The results prove that the gamma irradiated sludge material was of equal quality compared to the conventional FYM.     

Nitrogen fertilization and cropping systems effects on soil organic carbon and total nitrogen pools under chisel-plow tillage in Illinois

Agricultural soils can be a major sink for atmospheric carbon (C) with adoption of recommended management practices (RMPs). Our objectives were to evaluate the effects of nitrogen (N) fertilization and cropping systems on soil organic carbon (SOC) and total N (TN) concentrations and pools. Replicated soil samples were collected in May 2004 to 90 cm depth from a 23-year-old experiment at the Northwestern Illinois Agricultural Research and Demonstration Center, Monmouth, IL. The SOC and TN concentrations and pools, soil bulk density (ρ_b) and soil C:N ratio were measured for five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)]. Long-term N fertilization and cropping systems significantly influenced SOC concentrations and pools to 30 cm depth. The SOC pool in 0–30 cm depth ranged from 68.4 Mg ha⁻¹ for N₀ to 75.8 Mg ha⁻¹ for N₄. Across all N treatments, the SOC pool in 0–30 cm depth for CC was 4.7 Mg ha⁻¹ greater than for CS. Similarly, TN concentrations and pools were also significantly affected by N rates. The TN pool for 0–30 cm depth ranged from 5.36 Mg ha⁻¹ for N₀ to 6.14 Mg ha⁻¹ for N₄. In relation to cropping systems, the TN pool for 0–20 cm depth for CC was 0.4 Mg ha⁻¹ greater than for CS. The increase in SOC and TN pools with higher N rates is attributed to the increased amount of biomass production in CC and CS systems. Increasing N rates significantly decreased ρ_b for 0–30 cm and decreased the soil C:N ratio for 0–10 cm soil depth. However, none of the measured soil properties were significantly correlated with N rates and cropping systems below 30 cm soil depth. We conclude that in the context of developing productive and environmentally sustainable agricultural systems on a site and soil specific basis, the results from this study is helpful to strengthening the database of management effects on SOC storage in the Mollisols of Midwestern U.S.     

Soil losses due to cassava and sweet potato harvesting: A case study from low input traditional agriculture

Soil loss due to crop harvesting (SLCH) has been established as an important soil erosion process that has significantly contributed to soil degradation in highly mechanised agriculture. This has stimulated the need to investigate the importance of this process of erosion under low input agriculture where, until now, only water and tillage erosion are known as important phenomena causing soil degradation. This study was conducted in Eastern Uganda with the following objectives: (1) to assess the amount of soil lost due to the harvesting of cassava roots and sweet potato tubers under low input agriculture, (2) to look into the factors that influence variations in these soil losses, and (3) to estimate the amount of plant nutrients lost due to SLCH for cassava and sweet potato. Soil sticking to roots and tubers was washed and the soil suspension oven dried to estimate the amount of soil lost after harvesting. Mean annual soil loss for cassava was 3.4 tonnes ha⁻¹ and for sweet potato was 0.2 tonnes ha⁻¹. Ammonium acetate lactate extractable soil nutrient losses for cassava were N = 1.71 kg ha⁻¹ harvest⁻¹, P = 0.16 kg ha⁻¹ harvest⁻¹, K = 1.08 kg ha⁻¹ harvest⁻¹ and for sweet potato were N = 0.14, P = 0.01 kg ha⁻¹ harvest⁻¹, K = 0.15 kg ha⁻¹ harvest⁻¹. Difference in soil loss due to crop harvesting for cassava and sweet potato could be due to: (1) smaller yields of sweet potato leading to smaller soil losses on an area basis, (2) smoother skin and less kinked morphology of sweet potato that allowed less soil to adhere, and (3) the fact that sweet potato is planted in mounds which dry out faster compared to the soil under cassava. Soil moisture content at harvesting time and crop age were significant factors that explained the variations in the soil lost at cassava harvesting. Soil loss under cassava justifies the need to conduct further investigations on this process of soil erosion under low input agriculture.     

Biochar soil amendment increased bacterial but decreased fungal gene abundance with shifts in community structure in a slightly acid rice paddy from Southwest China

Biochar’s role on greenhouse gas emission and plant growth has been well addressed. However, there have been few studies on changes in soil microbial community and activities with biochar soil amendment (BSA) in croplands. In a field experiment, biochar was amended at rates of 0, 20 and 40 t ha⁻¹ (C0, C1 and C2, respectively) in May 2010 before rice transplantation in a rice paddy from Sichuan, China. Topsoil (0–15 cm) was collected from the rice paddy while rice harvest in late October 2011. Soil physico-chemical properties and microbial biomass carbon (MBC) and nitrogen (MBN) as well as selected soil enzyme activities were determined. Based on 16S rRNA and 18S rRNA gene, bacterial and fungal community structure and abundance were characterized using terminal-restriction fragment length polymorphism (T-RFLP) combined with clone library analysis, denaturing gradient gel electrophoresis (DGGE) and quantitative real-time PCR assay (qPCR). Contents of SOC and total N and soil pH were increased but bulk density decreased significantly. While no changes in MBC and MBN, gene copy numbers of bacterial 16S rRNA was shown significantly increased by 28% and 64% and that of fungal 18S rRNA significantly decreased by 35% and 46% under BSA at 20 and 40 t ha⁻¹ respectively over control. Moreover, there was a significant decrease by 70% in abundance of Methylophilaceae and of Hydrogenophilaceae with an increase by 45% in Anaerolineae abundance under BSA at 40 t ha⁻¹ over control. Whereas, using sequencing DGGE bands of fungal 18S rRNA gene, some bands affiliated with Ascomycota and Glomeromycota were shown inhibited by BSA at rate of 40 t ha⁻¹. Significant increases in activities of dehydrogenase, alkaline phosphatases while decreased β-glucosidase were also observed under BSA. The results here indicated a shift toward a bacterial dominated microbial community in the rice paddy with BSA.     

Long-term effects of land use and fertiliser treatments on sulphur transformations in soils from the Broadbalk experiment

Sulphur transformations were monitored in a unique set of arable, grassland and woodland soils from the Broadbalk Classical Experiment, which started in 1843. In an open incubation experiment with periodic leaching, 14–35 mg SO₄²⁻-S kg⁻¹ was mineralised in 28 weeks at 25°C, equivalent to 4.4–8.3% soil organic S. Cumulative amounts of S mineralised increased linearly during the 28 weeks, indicating constant rates of mineralisation. The rate of mineralisation was the greatest in the woodland soil (170 μg SO₄-S kg⁻¹ day⁻¹), followed by the grassland (120 μg SO₄-S kg⁻¹ day⁻¹) and the arable soil from the farmyard manure (FYM) plot (110 μg SO₄-S kg⁻¹ day⁻¹). Three soils from arable plots receiving different inorganic fertiliser treatments but no FYM had similar rates of S mineralisation (~70 μg SO₄-S kg⁻¹ day⁻¹). In an incubation experiment with ³⁵SO₄²⁻, addition of glucose greatly enhanced S immobilisation. In 132 days, the woodland and grassland soils immobilised more S than the arable soils, with or without glucose amendment. Immobilisation and mineralisation of S occurred concurrently, and both were stimulated by glucose addition. The results show that S mineralisation and immobilisation were influenced strongly by the type of land-use and long-term organic manuring, whereas annual application of sulphate-containing fertilisers for over 150 years had few effects on short-term S transformations.     

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.     

The role of biological activity (roots,earthworms) in medium-term C dynamics in vertisol under a Digitaria decumbens (Gramineae) pasture

The natural abundance of ¹³C was used to estimate the turnover of the soil organic matter in a vertisol re-grassed with Digitaria decumbens (C4 plant) following intensive market gardening (C3 plants). In addition, the experimental design allowed us to determine the respective roles of roots and earthworms (Polypheretima elongata) in soil C stock restoration in D. decumbens pasture.The C stock increased from 31 to 37 Mg C ha⁻¹ in 5 years and the δ¹³C increased from −18.1‰ in market gardening soil to −15.5‰ in the 5-year-old pasture soil in the upper 20 cm. Below the 20 cm soil layer, the C stock and the δ¹³C did not change significantly in 5 years. The net gain of 6 Mg C ha⁻¹ was the balance of a loss of 5 Mg C ha⁻¹ derived from market gardening and a gain of 11 Mg C ha⁻¹ derived from D. decumbens. Effects of earthworms on the C dynamics were not discernible.     

Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use

Minimum tillage practices are known for increasing soil organic carbon (SOC). However, not all environmental situations may manifest this potential change. The SOC and N stocks were assessed on a Mollisol in central Ohio in an 8-year-old tillage experiment as well as under two relatively undisturbed land uses; a secondary forest and a pasture on the same soil type. Cropped systems had 51±4 (equiv. mass) Mg ha⁻¹ lower SOC and lower 3.5±0.3 (equiv. mass) Mg ha⁻¹ N in the top 30 cm soil layer than under forest. Being a secondary forest, the loss in SOC and N stocks by cultivation may have been even more than these reported herein. No differences among systems were detected below this depth. The SOC stock in the pasture treatment was 29±3 Mg ha⁻¹ greater in the top 10 cm layer than in cultivated soils, but was similar to those under forest and no-till (NT). Among tillage practices (plow, chisel and NT) only the 0–5 cm soil layer under NT exhibited higher SOC and N concentrations. An analysis of the literature of NT effect on SOC stocks, using meta-analysis, suggested that NT would have an overall positive effect on SOC sequestration rate but with a greater variability of what was previously reported. The average sequestration rate of NT was 330 kg SOC ha⁻¹ year⁻¹ with a 95% confidence interval ranging from 47 to 620 kg SOC ha⁻¹ year⁻¹. There was no effect of soil texture or crop rotation on the SOC sequestration rate that could explain this variability. The conversion factor for SOC stock changes from plow to NT was equal to 1.04. This suggests that the complex mechanisms and pathways of SOC accrual warrant a cautious approach when generalizing the beneficial changes of NT on SOC stocks.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Organic soil amendments: Impacts on snap bean common root rot (Aphanomyes euteiches) and soil quality

Common root rot (causal agent Aphanomyes euteiches) is a major disease of commercially grown snap bean (Phaseolus vulgaris L.). Organic amendments hold potential to suppress plant diseases, which may be due to changes in soil biology and other soil properties. The objective of this study was to determine the potential of paper-mill residual by-products to suppress common root rot of snap bean in relation to soil properties. The study was done on soil (Plainfield sandy loam, Hancock, WI) from a field trial comparing annual applications of fresh paper-mill residuals (0, 22 or 33 dry Mg ha⁻¹) or composted paper-mill residuals (0, 38 or 78 dry Mg ha⁻¹). Soil was removed from each treatment that had been in place 3 years in April 2001 (1 year after last amendment) and on September 2001 (4 months after last amendment) and brought to the laboratory. Soils were incubated at field moisture content (25 °C) and periodically bioassayed with bean seedlings (9, 44, 84, 106, 137, 225 or 270 days after removal from the field) for snap bean root rot. Soils were sampled on the same day as the root rot bioassay and assayed for β-glucosidase, arylsulfatase and fluorescein diacetate hydrolysis activities (FDA), microbial biomass-C (MB_C) (by chloroform fumigation), water stable aggregation, and total C. There were large differences in snap bean root rot incidence between the field amendment treatments. The unamended field soil had high levels of disease incidence throughout the experiment but disease incidence tended to decrease over time in amended soils. The disease was suppressed by both fresh and composted paper-mill residuals, but the composted residuals at high rates had the lowest disease incidence (<40%) and produced healthiest plants. Root rot severity was strongly negatively correlated with total C (0.001 ≤ p) and arylsulfatase activity (0.001 ≤ p). β-Glucosidase activity was negatively correlated (0.05 ≤ p) with disease severity while soil MB_C showed inconsistent negative correlations with disease severity over the incubation sampling periods. Arylsulfatase activity was the best indicator for reflecting disease suppression. The amendments improved soil quality, which was exemplified by improved aggregation.     

Accounting for variability in soil microbial communities of temperate upland grassland ecosystems

This study aimed to determine the factors which regulate soil microbial community organisation and function in temperate upland grassland ecosystems. Soil microbial biomass (C_mic), activity (respiration and potential carbon utilisation) and community structure (phospholipid fatty acid (PLFA) analysis, culturing and community level physiological profiles (CLPP) (Biolog^®)) were measured across a gradient of three upland grassland types; Festuca–Agrostis–Galium grassland (unimproved grassland, National Vegetation Classification (NVC) — U4a); Festuca–Agrostis–Galium grassland, Holcus–Trifolium sub-community (semi-improved grassland, NVC — U4b); Lolium–Cynosurus grassland (improved grassland, NVC — MG6) at three sites in different biogeographic areas of the UK over a period of 1 year. Variation in C_mic was mainly due to grassland type and site (accounting for 55% variance, v, in the data). C_mic was significantly (P<0.001) high in the unimproved grassland at Torridon (237.4 g C m⁻² cf. 81.2 g C m⁻² in semi- and 63.8 g C m⁻² in improved grasslands) and Sourhope (114.6 g C m⁻² cf. in 44.8 g C m⁻² semi- and 68.3 g C m⁻² in improved grasslands) and semi-improved grassland at Abergwyngregyn (76.0 g C m⁻² cf. 41.7 g C m⁻² in un- and 58.3 g C m⁻² in improved grasslands). C_mic showed little temporal variation (v=3.7%). Soil microbial activity, measured as basal respiration was also mainly affected by grassland type and site (n=32%). In contrast to C_mic, respiration was significantly (P<0.001) high in the improved grassland at Sourhope (263.4 l h⁻¹m⁻² cf. 79.6 l h⁻¹m⁻² in semi- and 203.9 l h⁻¹m⁻² unimproved grasslands) and Abergwyngregyn (198.8 l h⁻¹m⁻² cf. 173.7 l h⁻¹m⁻² in semi- and 88.2 l h⁻¹m⁻² unimproved grasslands). Microbial activity, measured as potential carbon utilisation, agreed with the respiration measurements and was significantly (P<0.001) high in the improved grassland at all three sites (A₅₉₀ 0.14 cf. 0.09 in semi- and 0.07 in unimproved grassland). However, date of sampling also had a significant (P<0.001) impact on C utilisation potential (v=24.7%) with samples from April 1997 having highest activity at all three sites. Variation in microbial community structure was due, predominantly, to grassland type (average v=23.6% for bacterial and fungal numbers and PLFA) and date of sampling (average v=39.7% for bacterial and fungal numbers and PLFA). Numbers of culturable bacteria and bacterial PLFA were significantly (P<0.001) high in the improved grassland at all three sites. Fungal populations were significantly (P<0.01) high in the unimproved grassland at Sourhope and Abergwyngregyn. The results demonstrate a shift in soil microbial community structure from one favouring fungi to one favouring bacteria as grassland improvement increased. Numbers of bacteria and fungi were also significantly (P<0.001) higher in August than any other sampling date. Canonical variate analysis (CVA) of the carbon utilisation data significantly (P<0.05) differentiated microbial communities from the three grassland types, mainly due to greater utilisation of sugars and citric acid in the improved grasslands compared to greater utilisation of carboxylic acids, phenolics and neutral amino acids in the unimproved grasslands, possibly reflecting substrate availability in these grasslands. Differences in C_mic, activity and community structure between grassland types were robust over time. In addition, broad scale measures of microbial growth and activity (C_mic and respiration) showed little temporal variation compared to measures of soil microbial community structure, which varied quantitatively with respect to environmental variables (temperature, moisture) and plant productivity, hence substrate supply.     

Assessing the tolerance to heavy metals of arbuscular mycorrhizal fungi isolated from sewage sludge-contaminated soils

Different fungal ecotypes were isolated from soils which had received long-term applications of metal-contaminated sewage sludge with the aim of studying the degree of tolerance and adaptation to heavy metals of arbuscular mycorrhizal (AM) fungi. The development and structural aspects of AM colonization produced by the different fungal isolates were studied using two host plants, Allium porrum and Sorghum bicolor, which were grown in either contaminated or non-contaminated soils. Four different AM fungi were successfully isolated from the experimental field plots: (i) Glomus claroideum, isolated from plots receiving only inorganic fertilizer; (ii) another apparently similar ecotype of Glomus claroideum, but isolated from plots with 300 m³ ha⁻¹ year⁻¹ of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m³ ha⁻¹ year⁻¹ of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m³ ha⁻¹ year⁻¹ of amended or unamended sludge (intermediate rates of contamination). There were consistent differences in behaviour among the four AM fungi tested with regard to the colonization levels they produced in non-contaminated and contaminated soils. Both total and arbuscular colonization were affected by heavy metal contamination. The main conclusions of this study are that Glomus sp. and G. mosseae isolates are strongly inhibited by heavy metals, which acted mainly by interfering with the growth of the external mycelium, and also by limiting the production of arbuscules. Our results suggest that G. claroideum isolates, particularly the ecotype which was isolated from the plots receiving the highest dose of metal-contaminated sludge, shows a potential adaptation to increased metal concentration in soil.     

Detection of water stress in an olive orchard with thermal remote sensing imagery

An investigation of the detection of water stress in non-homogeneous crop canopies such as orchards using high-spatial resolution remote sensing thermal imagery is presented. An airborne campaign was conducted with the Airborne Hyperspectral Scanner (AHS) acquiring imagery in 38 spectral bands in the 0.43–12.5 μm spectral range at 2.5 m spatial resolution. The AHS sensor was flown at 7:30, 9:30 and 12:30 GMT in 25 July 2004 over an olive orchard with three different water-deficit irrigation treatments to study the spatial and diurnal variability of temperature as a function of water stress. A total of 10 AHS bands located within the thermal-infrared region were assessed for the retrieval of the land surface temperature using the split-window algorithm, separating pure crowns from shadows and sunlit soil pixels using the reflectance bands. Ground truth validation was conducted with infrared thermal sensors placed on top of the trees for continuous thermal data acquisition. Crown temperature (T_c), crown minus air temperature (T_c − T_a), and relative temperature difference to well-irrigated trees (T_c − T_R, where T_R is the mean temperature of the well-irrigated trees) were calculated from the ground sensors and from the AHS imagery at the crown spatial resolution. Correlation coefficients for T_c − T_R between ground IRT sensors and airborne image-based AHS estimations were R² = 0.50 (7:30 GMT), R² = 0.45 (9:30 GMT) and R² = 0.57 (12:30 GMT). Relationships between leaf water potential and crown T_c − T_a measured with the airborne sensor obtained determination coefficients of R² = 0.62 (7:30 GMT), R² = 0.35 (9:30 GMT) and R² = 0.25 (12:30 GMT). Images of T_c − T_a and T_c − T_R for the entire field were obtained at the three times during the day of the overflight, showing the spatial and temporal distribution of the thermal variability as a function of the water deficit irrigation schemes.