Nitrogen (N) release from legume cover crops is a key N source for subsequent crops in rotation. In this study, chopped fresh shoots or roots (<5 mm) of crimson clover (CC), hairy vetch (HV), and red clover (RC) were incorporated into a 50:50 mixture of air-dried sandy loam soil (<2 mm) and washed builders sand at a rate of 300 mg N kg^-1. The mixtures were packed in leaching tubes (four replicates), leached with 100 mL of 5 mmol L^-1 CaCl₂, and then incubated for 10 wk (22 °C, 0.33 bar matric potential) with weekly leaching. Total N and inorganic N (NH₄⁺ plus NO₃^-) in leachate were quantified and organic N was determined as the difference between total N and inorganic N. More N was released from shoots (63.4%–70.0% of initial N) than from roots (27.3%–50.7% of initial N). Mineralized organic N and inorganic N followed the first order, single N-pool mineralization model [N_t = N₀(1 – e^-kt); R² = 0.94-0.99]. Potentially mineralizable N (N₀, as % of initial N) was similar for shoots (CC = 75.1%, HV = 74.2%, and RC = 71.3%), but varied for roots (CC = 36.2%, HV = 52.6%, and RC = 53.0%). The N₀ pool in shoots had a half-life (t_1/2 = ln 2/k) of 11.0, 9.8, and 15.1 d for CC, HV, and RC, respectively; and a half-life in roots of 23.9, 8.5, and 25.7 d, respectively. Hence, HV released its stored N in both roots and shoots faster than CC and RC. The results in this study would help farmers optimize their choice in legume cover crops and termination times to better synchronize N release with crop uptake.

Silver nanoparticles (AgNPs), a component of many consumer products, are considered an environmental risk due to the broad-spectrum toxicity of Ag⁺ to non-target organisms. Most AgNPs released from consumer products will end up as biosolids in wastewater treatment plants, which are often applied as a fertilizer to agriculture. Land application of biosolids may add AgNPs to the soil–plant system, with unknown consequences. This study investigated the growth of Hordeum vulgare seedlings, Ag bioconcentration and distribution in shoot and root tissues of barley exposed to biosolid-amended Delacour and Organization for Economic Co-operation and Development (OECD) soils spiked with AgNPs (up to 366 mg Ag kg^-1 dry soil). In both soils, root and shoot growth declined linearly as the concentration of AgNPs increased. Barley had higher Ag bioconcentration values when grown in the OECD soil than in the Delacour soil. Silver bioavailability was greater in the OECD soil due to its physicochemical properties, such as low calcium concentration and acidic pH, relative to the Delacour soil. Barley seedlings exhibited morphological changes, including smaller shoots and shorter, thick roots after 14 d exposure to AgNPs. We conclude that plant structural responses, particularly changes in root biomass, could be an early diagnostic of seedling exposure to AgNPs in biosolid-amended soils.

Urease inhibitors have been successfully used to reduce ammonia (NH₃) emission from urea-based fertilizers. However, studies on its effectiveness with manures have produced inconclusive results. Field and greenhouse studies were conducted to investigate the effectiveness of different rates of urease [N-(n-butyl) thiophosphoric triamide; NBPT] with and without nitrification inhibitor (NI) in reducing NH₃ emission from surface-applied liquid pig manure (LPM) and solid beef manure (SBM). Ammonia emission was measured with acid-charged discs at seven dates for 28 d. Total NH₃ emission (% of applied N) ranged from 4.3% to 8.2% in untreated LPM and 8.2% in untreated SBM. The corresponding NH₃ emission was 6.8%–7.4% in LPM treated with NBPT, 5.0%–12.3% in LPM treated with NBPT + NI (double inhibitor; DI), and 6.0%–10.8% in SBM treated with DI. In the field study, NH₃ emission was not significantly different between either LPM or SBM treated with and without DI. In the greenhouse, NBPT did not significantly reduce NH₃ emission from LPM, whereas DI applied at a lower rate significantly increased NH₃ emission from LPM. In conclusion, addition of NBPT to manure did not have any significant environmental benefit, whereas a combination of NBPT and NI increased NH₃ emission from manure.

Aphanomyces euteiches is a soil-borne pathogen that causes root rot of pea and can significantly affect pea production in western Canada. This study aimed to isolate and identify soil bacteria with antagonistic activity towards A. euteiches mycelial and zoospore developmental stages under in vitro conditions and assess their potential as biocontrol agents against aphanomyces root rot in field pea under growth chamber conditions. In vitro screening of soil bacteria identified 184 antagonistic isolates, including 22 from an existing culture collection. Mean mycelial growth inhibition zones ranged from 1 to 12 mm, and mean zoospore germination inhibition ranged from 0% to 100%. Use of 16S rDNA sequence analysis placed isolates into 18 different bacterial genera. Screening of 47 bacteria that inhibited both infective stages identified 29 potential biocontrol strains, including Rhizobium spp. that significantly (α = 0.05) suppressed aphanomyces root rot in field pea grown in vermiculite, suggesting the intriguing possibility of using N-fixing Rhizobium inoculants as biocontrol agents for aphanomyces control. Further screening of 20 isolates as soil inoculants identified K-Hf-L9 (Pseudomonas fluorescens), PSV1-7 (Pantoea agglomerans), and K-Hf-H2 (Lysobacter capsici) isolates as having the highest biocontrol activity, significantly (α = 0.05) suppressing aphanomyces root rot in field pea in growth chamber trials. This study demonstrates the possibility of aphanomyces root rot management using biocontrol agents.

Mine tailings are a potential source of heavy metals (HM) that can be toxic to microbes, plants, and animals in aquatic and terrestrial ecosystems. Bacteria have evolved several mechanisms to tolerate the uptake of HM ions. This study aimed to assess the physicochemical properties, concentrations of selected HM and metalloids [arsenic (As), nickel (Ni), lead (Pb), zinc (Zn), cadmium (Cd), and cobalt (Co)], and isolate potential metal-tolerant bacteria present at three abandoned gold mining sites with a view of understanding how tailings characteristics vary and the implications on microbial activities in tailings dumps. Heavy-metal-tolerant bacteria were isolated from the samples using minimum inhibitory and maximum tolerable concentrations of the Ni, Pb, Zn, Cd, and Co. The substrates of the studied sites were acidic and deficient in nutrients. High metals and metalloid concentrations in the order Zn > Ni > Co > As > Pb > Cd were recorded in some of the studied sites and its adjacent soil which exceeded South African recommended values for soil and sediments. Heavy-metal-tolerant bacteria that showed multiple tolerances to Ni, Pb, and Zn were isolated and putatively identified using biochemical tests as belonging to the phyla Proteobacteria, Actinobacteria, and Firmicutes. Gold mine tailings enriched the soil with HM and also affect soil physicochemical properties. Proper management of mine wastes must be ensured to prevent their adverse effects on the diversity, composition, and activity of soil microorganisms that help in maintenance of the ecosystem.

Long-term (58 yr) cropping and fertilization effects on soil water repellency were determined for a clay loam soil in southwestern Ontario, Canada by measuring soil organic carbon (SOC), soil water repellency index (RI), and soil hydrophobicity (SH). The 12 treatments (non-replicated) included fertilized and non-fertilized legume-based crop rotation (ROT) with four phases (corn–oat–alfalfa–alfalfa), continuous corn (CC), and continuous Kentucky bluegrass (KBG). We hypothesized that SOC, RI, and SH would be greater for each phase of the ROT versus CC, KBG versus CC and ROT, and fertilized versus non-fertilized treatments. Surface (0–10 cm) soil samples were collected in the spring of 2017. Laboratory measurements were conducted to determine SOC, RI (ratio of soil sorptivity to ethanol and water), and SH (ratio of hydrophobic CH– to hydrophilic CO– functional groups). Mean SOC and SH were greater (P ≤ 0.05) for each phase of the ROT versus CC (33% to 2.4 times), KBG versus CC (3.2–6 times) and each phase of ROT (2.2–2.8 times), and fertilized versus non-fertilized rotation oats and KBG (15%–30%). Mean RI was greater for KBG versus CC (4.8 times) and KBG versus each phase of the ROT (3.0–5.5 times) under fertilization only, greater for fertilized versus non-fertilized KBG (6.8 times), but similar for each phase of ROT versus CC. In general, legume-based rotations, perennial grass, and fertilizer enhanced SOC and SH, and to a lesser extent soil RI.

Short-rotation forestry relies on frequent harvests of fast-growing trees, which could deplete soil fertility and soil organic carbon (SOC) reserves. Our objective was to measure the accumulation of SOC fractions, namely the dissolved organic carbon, microbial biomass carbon, particulate organic carbon, permanganate-oxidizable carbon, and non-oxidizable organic carbon, in the soil profile of a Chinese fir plantation. Chronosequences of Chinese fir aged 7, 12, and 33 yr were sampled at depths of 0–20 cm, 20–40 cm, and 40–60 cm. The SOC stock (0–60 cm) was unchanged in the first 12 yr, but after 33 yr, there was a 41%–56% increase in the SOC stock, which reached 81.2 Mg ha^-1 (P < 0.05). Permanganate-oxidizable carbon increased with time in the 0–20 cm layer but not in deeper soil depths, whereas non-oxidizable organic carbon accumulated preferentially in the 40–60 cm layer of the soil profile. Inputs of chemically complex plant litter in the soil profile may be important to maintain the oxidizable and non-oxidizable organic carbon in Chinese fir plantations.

Field spectroscopy and other efficient hyperspectral techniques have been widely used to measure soil properties, including soil organic carbon (SOC) content. However, reflectance measurements based on field spectroscopy are quite sensitive to uncontrolled variations in surface soil conditions, such as moisture content; hence, such variations lead to drastically reduced prediction accuracy. The goals of this work are to (i) explore the moisture effect on soil spectra with different SOC levels, (ii) evaluate the selection of optimal parameter for external parameter othogonalization (EPO) in reducing moisture effect, and (iii) improve SOC prediction accuracy for semi-arid soils with various moisture levels by combing the EPO with machine learning method. Soil samples were collected from grassland regions of Inner Mongolia in North China. Rewetting laboratory experiments were conducted to make samples moisturized at five levels. Visible and near-infrared spectra (350–2500 nm) of soil samples rewetted were observed using a hand-held SVC HR-1024 spectroradiometer. Our results show that moisture influences the correlation between SOC content and soil reflectance spectra and that moisture has a greater impact on the spectra of samples with low SOC. An EPO algorithm can quantitatively extract information of the affected spectra from the spectra of moist soil samples by an optimal singular value. A SOC model that effectively couples EPO with random forest (RF) outperforms partial least-square regression (PLSR)-based models. The EPO–RF model generates better results with R² of 0.86 and root-mean squared error (RMSE) of 3.82 g kg^-1, whereas a PLSR model gives R² of 0.79 and RMSE of 4.68 g kg^-1.

The impact of hillslope vegetation restoration on the distribution and variability of carbon and water storage was studied across two catenary sequences of soils in the Liudaogou watershed of China’s Loess Plateau. Soil organic carbon storage (SOCS) under different land uses in the two catenas decreased significantly in the upper soil layers (<50 cm) but was relatively stable in the deeper soil layers (>50 cm). However, soil inorganic carbon storage (SICS) in the two catenas fluctuated (two maxima) with increasing soil depth. There was no significant difference of SOCS within the 200 cm soil profile between forestlands (FO) and grasslands (GR) at the catenary scale (p > 0.05). However, SICS in the 0–200 cm soil profile differed markedly between FO and GR (p < 0.05) in both catenas due to different degrees of root-facilitated CaCO₃ redistribution. Based on the coefficient of variance (CV), soil water storage (SWS) was divided into three layers: active layer (0–100 cm, CV = 20%–30%), subactive layer (100–200 cm, CV = 10%–20%), and stable layer (200–500 cm, CV < 10%). The SWS in the 0–500 cm soil profile was slightly higher in GR than in FO on the two slopes because of the higher water consumption under tree plantation than native grasses. SOCS, SICS, and SWS can be predicted by multiple regression equations using different soil properties. The study demonstrated that SOCS, SICS, and SWS respond differently to vegetation restoration at the catenary scale, which must be taken into account for improving ecosystem model predictions of soil carbon and water fluxes in sloping lands.

In Canada, most lettuce (Lactuca sativa L.) is produced on cultivated organic soils, which can be very productive but are also very sensitive to degradation and compaction. The objective of this work was to evaluate the effect of soil compaction, irrigation thresholds, and transplant type on the growth and water-use characteristics of Romaine lettuce that is grown in organic soil. The experiments were conducted in greenhouses at Laval University. Tensiometers and time-domain reflectometer probes were used to characterize the water-use characteristics of the Romaine lettuce. Most of the growth characteristics of the Romaine lettuce, with the exception of the dry weight, were significantly influenced by the available rooting depth (soil column height) and by the irrigation threshold used. Lettuce water uptake decreased significantly as the depth increased. In addition, in drier conditions, the deeper soil layers contributed more to the total water uptake than the surface soil layers. The water productivity was lower in the presence of a compacted layer combined with a direct seeding treatment, compared with all of the other treatments. First, it is concluded that the irrigation method should allow a certain degree of dryness by use of a lower irrigation threshold (ideally between -20 and -30 kPa) to stimulate deep rooting. Second, the use of small lettuce plant preseeded in small block of peat substrate instead of direct seeding in the field can compensate for a possible compaction effect.

Although compost is widely used as an organic soil amendment or conditioner, little is known of how it affects the characteristics or interactions among soil constituents. To address this, mixture theory was used to describe the mass–volume–density–porosity attributes and interactions among bulk soil, the mineral constituent, and the organic matter constituent of a sandy loam soil in a no-till corn field that had received one-time additions of yard waste compost at rates of 0 (control), 64, 154, and 380 dry t ha^-1. Bulk density (BD, 0–10 cm depth) decreased consistently and near-linearly with increasing soil organic matter (SOM) mass fraction (F_OM) for all six growing seasons (2012–2017) after compost addition. Fitting mixture theory expressions to BD vs. F_OM data and to soil particle density vs. F_OM data for 2013–2017 yielded constant mineral and SOM self-packing densities of D_M = 1.673 Mg m^-3 and D_O = 0.335 Mg m^-3, respectively, and constant mineral and SOM particle densities of ρ_M = 2.760 Mg m^-3 and ρ_O = 1.409 Mg m^-3, respectively. On a self-packing basis, soil mineral and SOM domain porosities were constants at n_M = 0.39 and n_O = 0.76, respectively. On a bulk soil volume basis, soil mineral and SOM porosities and volume ratios were linear functions of F_OM. The porosity and volume characteristics of the SOM domain differed substantially from those of bulk soil and the mineral domain, and may therefore control the agri-environmental performance of soil, given that organic matter influences soil functioning more than mineral matter.

To evaluate the potential of soil water recovery after thinning, in situ soil water content in the 0–500 cm soil profile under thinned (50%–100%) and unthinned peashrub and alfalfa plots and a nearby natural grassland in the Liudaogou watershed in China’s Loess Plateau (CLP) was measured monthly during 2015–2017 growing season using a neutron probe. At the start of experiment, the profile soil water storage (SWS_{0–500 cm}) under introduced peashrub and alfalfa was, respectively, 18.8% and 12.2% lower than that under natural grassland. This showed that there was higher water consumption by planted vegetation, compared with native grass. After thinning, SWS_{0–500 cm} in thinned peashrub and alfalfa plots was significantly higher than that in unthinned plots due to decrease in both interception and transpiration. The increase in SWS_{0–500 cm} in the 100% thinned peashrub plot (159.9–216.1 mm) was much higher than that in 50% thinned peashrub (39.1–169.8 mm) and 100% thinned alfalfa (20.3–118.1 mm) plots. This indicated that the extent of soil water recovery varied with thinning intensity and vegetation type. At the end of the third growing season, soil water restoration frontier in the thinned peashrub and alfalfa plots (>300 cm) was much greater than that in the unthinned plots (<180 cm). It also indicated that with thinning, soil water (<300 cm) can recover rapidly following two successive wet years. The results suggested that concerns about soil desiccation and the potential impact on long-term sustainability of restored ecosystems on CLP were resolvable.

Black medic (Medicago lupulina L.) is a self-regenerating cover crop which was tested for its ability to improve soil physical properties. Soil aggregate stability was assessed in plots that included a black medic cover crop in a no-till grain rotation, which was fertilized with two levels of nitrogen (N), for 15 yr. In the wheat phase of the rotation, the medic cover crop increased mean weight diameter by 21% in the reduced N fertilizer treatment but not in the recommended N treatment. Generally, the addition of medic reduced the proportion of small aggregates and increased the proportion of large aggregates. This pattern was stronger in reduced N compared with recommended N fertilizer levels. This study provided evidence for medic to increase aggregate stability under low external N input grain production.