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Hot water is believed to extract bio-available soil organic matter (SOM), including organic compounds from the biomass of soil microbes. The role of soil physico-chemical factors in relation to extractability of SOM in hot water is not well understood. We evaluated the influence of exchangeable sodium (Na) on the quantity and quality of organic matter extracted in hot water from soils with a range of total and extractable C (total C 19-60 g [kg.sup.-1]; hot water extractable C (HWC) 659-3292 mg [kg.sup.-1]). The soils were pre-treated with different rates of Na (0-156 [cmol.sub.c] [kg.sup.-1], as NaCl) to establish a range of exchangeable Na percentages (ESP), and then extracted with hot water (80[degrees]C) for 16 h. Hot water extractable C increased linearly as ESP increased, but the rate of increase differed between soils (the increase in HWC per unit increase in ESP ranged within 19-71 mg [kg.sup.-1]). At ESP 15, a threshold used to separate sodic and nonsodic soils, HWC was 30-60% greater than that measured without added Na. Ultraviolet absorbance (260 nm) data indicated that aromatic organic matter was preferentially released following Na pre-treatment. The proportion of HWC in phenolic form was generally little affected by Na treatment but there was a consistent increase in protein in response to Na (hot water extractable organic N in protein form increased from an average of 5.5 [+ or -] 2.2% without added Na to 11.0 [+ or -] 3.6% at the highest Na rate). The Na-induced increases in UV absorbance may be largely attributable to release of proteins containing aromatic amino acids (tryptophan and tyrosine). Our results suggest that organic matter desorbed from mineral surfaces may be an important contributor to hot water extractable C and N, and factors that affect the adsorption-desorption process may significantly influence organic matter extractability in hot water. Additional keywords: chemical controls, dissolved protein and phenols, extractable organic matter, ultraviolet absorbance. Received 13 August 2019, accepted 18 February 2020, published online 31 March 2020, Introduction Dissolved organic matter is a labile component of soil organic matter (SOM) and a key driver of soil biological processes. The quantity of dissolved organic matter in soil is [...]

Managed grasslands are a predominant land use in northern temperate regions. They are often on poorly drained soils and contain large stocks of soil organic carbon (SOC). It is important to understand C dynamics in grasslands to better assess their role in regional C budgets. Two adjacent grassland plots, one amended with 100 [m.sup.3][ha.sup.-1] of liquid swine manure (LSM) annually since 1978, and another unamended were killed by glyphosate in the autumn or (i) left with vegetation intact. Those killed were either (ii) left as an undisturbed chemical fallow, (iii) plowed by full-inversion tillage (FIT) in the autumn, or (iv) in the spring. Following the autumn plowing, we monitored C[O.sub.2] emissions from the fallow soil surface, C[O.sub.2] concentrations in the soil profile, soil temperature, and soil water content for 1 yr. Changes in soil aggregation, the light fraction organic matter, and microbial biomass were also monitored. Plowing decreased the total microbial biomass on average by 27 g C [m.sup.-2], the quantity of water-stable aggregates by 7 to 12% and with it the concentration of light fraction organic matter. Respiration was also reduced by 142 g C[O.sub.2]-C [m.sup.-2] by autumn-FIT and 175 g C[O.sub.2]-C [m.sup.-2] by spring-FIT on unamended soils, and 90 g C[O.sub.2]-C [m.sup.-2] by autumn-FIT and 98 g C[O.sub.2]-C [m.sup.-2] by spring-FIT on amended soils. Regression analyses suggested that reductions in C[O.sub.2] emissions were due to the placement of surface SOC at depth where soil temperature and oxygen availability were attenuated. In these soils, the cool humid conditions at depth in the soil profile may act to counter-balance the physical effects of tillage thereby preserving C deep in the soil profile. Effective calculation of C budgets and changes in SOC stocks depends on the ability to reproduce the interaction between climate, soil type, land use, and management action. A more complete understanding of the effect of management actions that modify the vertical distribution of biogeochemical (particularly organic C) and environmental parameters on soil respiration in poorly drained soils is required. doi: 10.2136/sssaj2010.0059

Cold (room temperature) and hot water (70-80[degrees]C) extraction methods have been used to assess the availability of organic matter and N in soil; however, the influence of temperature on their extractability is largely unknown. Twenty-one mineral soils from New Zealand and eastern Canada were incubated for 16 h in water (1:6 soil/water ratio) at temperatures ranging from 20 to 80[degrees]C. After centrifugation and removal of the supernatant solution, the soil residue was extracted with 2 tool [L.sup.-1] KCl to recover N[H.sub.4]-N released during incubation. In all cases, water-extractable organic C (WEOC) and N (WEON) increased exponentially with temperature. The temperature dependence of organic matter solubility differed considerably among the soils (estimated increase in WEOC and WEON: 1.1-5.3% [degrees][C.sup.-1]). These differences were not explained by management history or soil physicochemical properties, although there was evidence that organic matter extractability in some clay soils was less sensitive to temperature. Ammonium N was released in significant amounts at all temperatures. Peak N[H.sub.4]-N release typically occurred at 50[degrees]C, where N[H.sub.4]-N comprised an average of 49% of the total recovered N (WEON plus N[H.sub.4]-N). Factors that may account for N[H.sub.4]-N production during the 16-h incubation include mineralization at the lower temperatures and abiotic processes (e.g., release of clay-fixed N[H.sub.4] and thermal degradation of organic N) at higher temperatures. The possibility that mineralization made a significant contribution to N[H.sub.4]-N production at temperatures as high as 50[degrees]C warrants further investigation. Abbreviations: WEOC, water-extractable organic carbon; WEOM, water-extractable organic matter; WEON, water-extractable organic nitrogen. doi:10.2136/sssaj2008.0347

Identification of soil attributes most determinant to crop yield is still a matter of debate. The main objective of the present study was to relate the variations in corn (Zea mays L.) yield and N uptake to 16 soil attributes. Samples were collected in 2005 and 2006 from a long-term experiment. Soil organic C (SOC), total N (TN), potential mineralizable N (PMN), N[O.sub.3] extractable with KC1 (N[O.sub.3]-KC1) and Ca[Cl.sub.2] (N[O.sub.3]-Ca[Cl.sub.2]), N[O.sub.3] adsorbed on anion exchange membranes (N[O.sub.3]-AEM), N extracted with NaHC[O.sub.3] read at 205 nm (N-NaHC[O.sub.3]-205) and 220 nm (N-NaHC[O.sub.3]-220), N present in fulvic acid (FA-N), humic acid (HA-N) and non-humified fractions (NHF-N), mean weight diameter of aggregates (MWD), total, macro- and microporosity, and bulk density ([D.sub.b]) were measured. Principal component analysis (PCA) was conducted with the measured soil attributes, and the principal components (PCs) were used in a stepwise regression with corn yield and N uptake. In both years, a maximum of 88% of the total variance was explained. The stepwise regression analysis indicated that the first two PCs explained 78 to 91% of the variability in corn yield and N uptake. Based on the PCA, TN, HA-N, N[O.sub.3]-KC1, N[O.sub.3]-Ca[Cl.sub.2], N[O.sub.3]--AEM, and PMN appeared as primary indicators of corn yield and N uptake, whereas MWD, FA-N, and NHF-N appeared as secondary indicators. When the variability in corn yield and N uptake explained by each N availability index was assessed, N[O.sub.3]-KC1 and N[O.sub.3]Ca[Cl.sub.2] appeared as the best predictors of corn yield because of their ease of measurement and reliability across years. Abbreviations: CV, coefficient of variation; [D.sub.b], bulk density; FA-N, N in fulvic acids; HA-N, N in humic acids; MSA, measure of sampling adequacy; MWD, Mean weight diameter of water-stable soil aggregates; NHF-N, N in the non-humified fraction; N-NaHC[O.sub.3]--205, N extracted by 0.01 M NaHC[O.sub.3] and absorbance read at 205 nm; N-NaHC[O.sub.3]--220, the absorbance of a 0.01 M NaHC[O.sub.3]--extract at 220 nm; N[O.sub.3]--AEM, cumulative N[O.sub.3]--N adsorbed on anion exchange membranes; N[O.sub.3]Ca[Cl.sub.2], N[O.sub.3]--N extracted by Ca[Cl.sub.2]; N[O.sub.3]--KCl, N[O.sub.3]--N extracted by 2 M KC1; PCA, Principal Component Analysis; PCs, Principle Components; PMN, potentially mineralizable nitrogen; SOC, soil organic carbon; TN, total Nitrogen; ***, significant at the 0.001 probability level.

Both tillage and fertilizer management influence soil organic C (SOC) storage, but their interactive effects remain to be determined for various soil and climatic conditions. We evaluated the long-term effects of tillage (no-till, NT, and moldboard plowing, MP), and N and P fertilization on SOC stocks and concentrations in profiles of a clay loam soil (clayey, mixed, mesic Typic Humaquept). Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown in a yearly rotation for 14 yr. Our results showed that NT enhanced the SOC content in the soil surface layer, but MP resulted in greater SOC content near the bottom of the plow layer. When the entire soil profile (0-60 cm) was considered, both effects compensated each other, which resulted in statistically equivalent SOC stocks for both tillage practices. Nitrogen and P fertilization with MP increased the estimated crop C inputs to the soil but did not significantly influence SOC stocks in the whole soil profile. At the 0- to 20-cm depth, however, lower C stocks were measured in the plowed soil with the highest N fertilizer level than in any other treatment, which was probably caused by a greater decomposition of crop residues and soil organic matter. Conversely, the highest SOC stocks of the 0- to 20-cm soil layer were observed in the NT treatment with the highest N rates, reflecting a greater residue accumulation at the soil surface. When accounting for the whole soil profile, the variations in surface SOC due to tillage and fertilizer interactions were masked by tillage-induced differences in the 20- to 30-cm soil layer. Abbreviations: MP, moldboard plowing; NT, no-till; SOC, soil organic carbon.

Nitrate (N[O.sub.3.sup.-]) leached from golf greens has the potential to impair water quality. Dissolved organic N (DON) is also increasingly recognized as a form leached from fertilized soils. A controlled experiment was conducted to determine (i) the significance of DON in total N leaching losses under simulated golf-green profiles and (ii) the short-term contribution of fertilizer to leaching of inorganic and organic N forms. Various turfgrasses were grown in lysimeter columns designed to simulate a golf-green profile. Fertilizer was applied at 25 kg N [ha.sup.-1] every 14 d for 55 d, and the last application was labeled with [.sup.15]N. Leachates were analyzed for N[O.sub.3.sup.-]-N, N[H.sub.4.sup.+]-N, and DON. The [.sup.15]N recovery was assessed in plant, soil, and leachates. In the presence of plants, 10 to 70% (average 40%) of total N leached was accounted for by DON. Application of [.sup.15]N revealed that one-half to two-thirds of N[O.sub.3]-N leached in the following 14 d was derived from the fertilizer, whereas the majority of leached DON was derived from soil residual N. Nevertheless, D[O.sup.15]N was present in most leachate samples collected for 14 d after fertilizer application, indicating that only a few days were required to convert mineral fertilizer to leachable organic forms. We conclude that DON may be a significant component of total N leaching losses from putting greens and would account for part of the N losses traditionally attributed to volatilization and denitrification.

The anticipated benefits of increased soil C stocks on net soil-surface greenhouse gas (GHG) emissions after adoption of soil conservation practices can be offset by increases in soil [N.sub.2]O emissions. The objective of this study was to assess the short-term impacts of no-till (NT) on soil [N.sub.2]O emissions. The study was conducted in eastern Canada in the 3rd, 4th, and 5th yr after initiation of NT and fall moldboard plowing (MP) on heavy clay and gravelly loam soils. Annual emissions of [N.sub.2]O were exceptionally high in the heavy clay soil, varying from 12 to 45 kg [N.sub.2]O-N [ha.sup.-1] during the 3 yr of the study. Such high emissions were probably not associated with fertilizer N inputs but rather with denitrification sustained by the decomposition of large soil organic matter stocks (192 Mg C [ha.sub.-1] in the top 0.5 m). On average, NT more than doubled [N.sub.2]O emissions compared with MP in the heavy clay soil. The influence of plowing on [N.sub.2]O flux in the heavy clay soil was probably the result of increased soil porosity that maintained soil aeration and water content at levels restricting denitrification and [N.sub.2]O production in the top 0.20 m. In the loam soil, average emissions during the 3 yr were similar in the NT and MP plots. The results of this study indicate that the potential of NT for decreasing net GHG emissions may be limited in fine-textured soils rich in organic matter that are prone to high water content and reduced aeration. Abbreviations: GHG, greenhouse gas; MP, moldboard plowing; NT, no-till; WFPS, water-filled pore space.

Decomposition rates of organic amendments in agricultural soils provide information on nutrient release and on changes in soil C stocks. They are usually determined using incubation of soil samples under controlled conditions. Our objective was to study decomposition of liquid and solid dairy cattle manures under field conditions following application to a loamy and a clay soil. Manures were applied in two consecutive years to silage corn at rates equivalent to 150 kg [ha.sup.-1] total N providing 119 to 202 kg C [ha.sup.-1]. Soil-surface C[O.sub.2] fluxes, temperature and water content were monitored weekly in manured and control (mineral fertilizer) plots. Initial decomposition rates following manure application were much slower in the first than in the second year. This lag phase was attributed to cool soil temperatures anti to the time required by the soil microflora to adapt to a first addition of this exogenous organic substrate. During the first year, mineralization of manure organic C was lower in the clay (52% of manure C) than in the loamy (75%) soil, presumably as a result of less favorable aeration conditions and greater physical protection of organic substrates in the clay. Equal mineralization rates of liquid and solid manures during both years in the loamy soil indicated that the two manure types had similar decomposability. Slower decomposition of the solid than the liquid manure in the clay soil was attributed to poorer mixing with the soil (presence of clods) resulting in reduced access to the solid manure for soil decomposers. The results of this study evidenced that under field conditions manure C decomposition is not determined solely by manure characteristics but also by interactions between the manure type and soil physical properties such as texture.

In Quebec, most pig slurry is applied to agricultural soils in the spring and fall. A study was initiated to compare the impact of the contrasting spring and fall weather conditions on C[O.sub.2] and [N.sub.2]O emissions, and on the transformation of pig slurry C and N in a loamy soil cropped to maize (Zea mays L.). Treatments were approximately 200 kg total N [ha.sup.-1] either as a spring (SPRING) or fall (FALL) application of pig slurry, and 150 kg N [ha.sup.-1] as N[H.sub.4]N[O.sub.3] (control). Fluxes of C[O.sub.2] and [N.sub.2]O, and soil [O.sub.2], C[O.sub.2], [N.sub.2]O, N[H.sup.+.sub.4], N[O.sub.3], extractable C and microbial biomass C (MBC) contents were measured 50 times over a 1-yr period. Fluxes of [N.sub.2]O were generally low during the experiment but were greatly increased in recently manured soils when soil [O.sub.2] concentration fell below 0.20 mol [mol.sup-1]. Soil was warm and well-aerated following spring slurry application. Under these conditions, slurry N[H.sub.4]-N was rapidly nitrified and high [N.sub.]O emissions attributed to denitrification occurred when soil was rewetted by abundant rainfall. For the fall applied slurry, wet and cool conditions limited net nitrification and resulted in little accumulation of N[O.sub.3]-N, thus limiting potential for subsequent denitrification and [N.sub.2]O emissions. Cumulated [N.sub.2]O emissions during the experiment represented 1.74, 2.73, and 1.14% of added N in the FALL, SPRING, and N[H.sub.4]N[O.sub.3] plots, respectively. Fluxes of C[O.sub.2] and cumulated C[O.sub.2]-C losses were also greater for SPRING than for FALL application. Our results clearly show that the impacts of the timing of animal manure application on [N.sub.2]O emissions cannot be generalized, but will vary between years in response to interactions between crop, climatic, and soil factors.

The fate of pig (Sus scrofa) slurry labeled with [sup.15]N was investigated when applied on a clay soil (fine, mixed, frigid, Typic Humaquept) and a sandy loam (loamy, mixed, frigid, Typic Dystrochrept) cropped to maize (Zea mays L.) in 2000 and to barley (Hordeum vulgare L.) in 2001. The slurry was applied in spring 2000, and plant and soil samples were collected at 6 h (Day 1) and at Days 14, 42, 96, and 413 after application. The samples were analyzed for [sup.15]N content in plant, in whole soil, and in soil N[H.sup.+.sub.4]-N, N[O.sup.-.sub.3]-N, organic N, and clay-fixed N pools. Percentage [sup.15]N recovery was >93% in both soils at Day 1 and decreased slowly thereafter. Rapid clay fixation of slurry [sup.15]N occurred at Day 1, and was greater in the clay soil (34% of applied [sup.15]N) than in the sandy loam (11%). At Day 96, less of the applied slurry [sup.15]N was recovered in maize grown on the clay soil (29%), as compared with the sandy loam (50%). At the same period, the residual soil [sup.15]N was mostly present as organic N and N[O.sup.-.sub.3]-N in the sandy loam, and as organic and clay-fixed N in the clay soil. At Day 413, [sup.15]N recovery in barley was about 3% of the initially applied N in both soils. The [sup.15]N recovery was generally higher in the clay soil than in the sandy loam, but total [sup.15]N recovery from the soil-plant system was similar for both soils. We conclude that soil type had little influence on the total [sup.15]N recovery from the soil-plant system, but significantly influenced the fate of slurry N in the various soil pools and plant N uptake on the year of application.

There is considerable uncertainty relative to the emissions of [N.sub.2]O from legume crops. A study was initiated to quantify [N.sub.2]O fluxes from soils cropped to alfalfa (Medicago sativa L.) and soybean (Glycine max L.), and to improve our understanding of soil and climatic factors controlling [N.sub.2]O emissions from these crops. Measurements were made on three soils cropped to alfalfa, soybean, or timothy (Phleumpratense L.), a perennial grass used as a control. In situ soil-surface [N.sub.2]O emissions ([F.sub.N2O]) were measured 47 times during the 2001 and 2002 growing seasons. Soil water, N[H.sub.4]-N, N[O.sub.3]-N, and [N.sub.2]O contents, and soil temperature were also determined to explain the variation in gas fluxes. Emissions of [N.sub.2]O were small under the grass where very low soil mineral N content probably limited denitrification and [N.sub.2]O production. Soil mineral N contents under legumes were lip to 10 times greater than under timothy. However, soil mineral N contents and [F.sub.N2O] were not closely related, thus suggesting that the soil mineral N pool alone was a poor indicator of the intensity of [N.sub.2]O production processes. Higher [F.sub.N2O] were measured under legume than under timothy in only 6 out of 10 field comparisons (site-years). Moreover, the emissions associated with alfalfa (0.67-1.45 kg N [ha.sup.-1]) and soybean (0.46-3.08 kg N [ha.sup.-1]) production were smaller than those predicted using the emission coefficient proposed for the national inventory of greenhouse gases (alfalfa = 1.60-5.21 kg N [ha.sup.-1] ; soybean = 2.76-4.97 kg N [ha.sup.-1]). We conclude that the use of the current emission coefficient may overestimate the [N.sub.2]O emissions associated with soybean and alfalfa production in eastern Canada.

Ammonia volatilization occurs shortly following land application of pig slurry. Several slurry and soil characteristics modulate the intensity of this process, and their net effect on volatization is still hard to predict. Our aim was to compare volatilization following application of anaerobically stored (ASPS) and anaerobically digested (ADPS) pig slurry to a bare loamy soil (loamy, mixed, frigid, Aeric Haplaquept). Ammonia volatilization was measured using wind tunnels. Soil pH and water, N[H.sup.+.sub.4], N[O.sup.-.sub.3], and volatile fatty acid (VFA) contents were monitored in the 0- to 0.5-, 0.5- to 2-, 2- to 5-, and 5- to 10-cm soil layers to explain volatilization rates. Following slurry application, pH increased by 1 to 3 units in the top 2 cm of soft, resulting in high volatilization rates in the first 6 h of experiment. Thereafter, pH decreased more slowly in ASPS than ADPS plots, possibly due to the degradation of VFAs present in ASPS. After 2 d, 35% of slurry-added N[H.sup.+.sub.4]-N was lost as N[H.sub.3]-N for both slurries, corresponding well to the net decrease found in soil N[H.sup.+.sub.4]-N content. After 9 d, net soil N[H.sup.+.sub.4]-N disappearance accounted for about 60% of slurry-added N[H.sup.+.sub.4]-N for both slurries, whereas N[H.sup.+.sub.3]N losses represented only 40%. Therefore, for the first 2 d of the experiment N[H.sub.3] volatilization explained most of the decline in soil N[H.sup.+.sub.4]. Afterwards, biological processes, such as immobilization and nitrification, were assumed to play a significant role in slurry N[H.sup.+.sub.4] disappearance. Despite marked changes in slurry properties, anaerobic digestion did not significantly modify the proportion of slurry N that was lost as N[H.sub.3]. Ammonia volatilization was related mostly to soil pH and N[H.sup.+.sub.4] content in the top 2 cm of soil. Below 5-cm depth, slurry application had little effect on soil pH, water, VFA, or mineral N content. This finding stresses the importance of stratified soil sampling when studying the short-term effects of pig slurry on N[H.sub.3] volatilization and associated soil properties.