A series of soil N mineralization indices were evaluated using 153 samples chosen from arable fields representing a wide range of soil types, management practices, and climatic zones. These indices were compared against potentially mineralizable N (N0) determined by aerobic incubation at 25°C for 24 wk. Three different pools of mineralizable N were recognized: Pool I, the mineralization flush on rewetting in the first 2 wk; Pool II, gross N mineralization in the next 22 wk; and Pool III, the potentially mineralizable N, predicted from the fitted curve, that did not mineralize during the incubation period. Pool I was highly correlated with CaCl2–N, KCl‐NH4, and KCl‐NO3, which extract soil mineral N. Pool III was significantly correlated with ultraviolet absorbance of NaHCO3 extract at 205 and 260 nm (NaHCO3–205 and −260), Illinois soil N test, NaOH direct‐distillation N, and hot KCl‐NH4, which mostly extract hydrolyzable organic N. All indices except the mineral N based methods, phosphate‐borate buffer method, and microbial biomass C were significantly related to N0, which includes both Pools II and III. The NaHCO3–260, NaOH direct‐distillation N, and Illinois soil N test had the highest correlations with N0 (r2 = 0.74, 0.61, and 0. 51, respectively). Total organic C and N represent long‐term changes in N0 and were almost as effective in predicting N0 as the other indices (r2 = 0.60 and 0.67, respectively); however, they would be expected to be less sensitive to short‐term changes in N0 due to changes in soil management practices and history.

Accurate estimation of soil nitrogen (N) supply in the field is required to optimize fertilizer N management and to minimize environmental N losses in humid environments. Laboratory-based measures of N availability were evaluated as predictors of field-based indices of soil N supply within potato production systems. Pre-plant soil samples (0-15 cm) were collected from a series of forty treatments in established potato trials located in Atlantic Canada and Maine, USA. Total plant N uptake at topkill with no fertilizer N applied (PNU 0N ), PNU 0N plus soil mineral N to 30 cm depth at harvest and relative yield were considered as fieldbased indices of soil N supply. The potentially mineralizable N (N 0 ) was determined by aerobic Plant Soil (incubation at 25°C and periodic leaching for 24 weeks. A series of laboratory-based measures of soil N supply were measured in soil samples. Pre-plant soil nitrate or total mineral N at 0-30 cm depth was the best single predictor of PNU 0N (r=0.67 and 0.71, respectively) and relative yield (r=0.58 and 0.61). The ultraviolet absorbance of 0.01 M NaHCO 3 extract at 205 nm (NaHCO 3 -205) was suitable as a predictor of PNU 0N and relative yield in each growing season, however, the relationship between this parameter and PNU 0N or relative yield varied somewhat among years. A combination of pre-plant mineral N plus N mineralized in the first 2 weeks period of incubation after rewetting is proposed as a more robust measure of N availability compared with use of mineral N alone.

A 2-year study in Nova Scotia examined the effectiveness of thickly applied organic mulches as a method of weed control in highbush blueberries (Vaccinium corymbosum L.), and assessed weed and mulch impact on crop growth, leaf nutrient concentration, yield, and quality under organic production management. Mulches, applied in-row at 20-cm depth, included pine needles (PN), manure–sawdust compost (MC), and seafood waste compost (SC). Competition from weeds negatively affected crop growth and productivity, reducing canopy volume (16% to 38%), leaf nitrogen concentration and berry yields (up to 92%), number (up to 91%), and specific weight (up to 21%). Among mulches, PN proved to be the most effective in suppressing weed growth with 55% less and 73% less aboveground weed biomass compared with the control in 2005 and 2006, respectively, although PN productivity effects were much more modest. One year after application, PN lost some efficacy at suppressing weeds but was still superior to both composts. Distribution of weed species was substantially altered by mulch treatment. Both composts prevented some weed emergence (i.e., sheep sorrel), but weed seeds germinating in composts, especially SC, experienced prolific growth likely as a result of available nutrients in composts. No detrimental effects on short-term plant productivity were noted despite high C:N ratios of PN and MC (72:1 and 48:1, respectively). Plant vigor and yield were typically higher for compost mulch treatments, especially in weed-free subplots, and composts provided more complete fertilization reflected in increased leaf tissue elemental (NPK) composition. Fruit soluble solid (sugar) content was found to be significantly lower in PN and MC compared with SC, whereas total phenolic content was unaffected by mulches. Mulch application can improve organic highbush blueberry productivity by improving soil properties, nutrient availability, and weed suppression; however, precautions should be taken to avoid excess nutrient loading and weed seed contamination of mulches.

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I Assessment of tbe soil N supply capacity is essential to optimize N fertilizer use. Tbe soil N supply capacity of 102 soil samples (0-15 cm) from 25 sites collected from 2004 to 2007 across four Canadian provinces was evaluated by comparing a group of chemical N availability indices witb soil mineralizable N pools and a field-based measure of soil N supply. Soil N supply was estimated by corn {Zea mays I.) N uptake corrected for starter fertilizer N. Two subgroups were created based on tbe soil texture and were compared to tbe whole data set. Grouping soils provided limited benefits in predicting soil potentially mineralizable nitrogen {Ng), but improved tbe prediction of soil N supply. Tbe Ng was weakly related to soil N supply for tbe wbole data set (r = 0.09) and in fine-textured soils (r = 0.37) but tbe relationsbip was improved (r = 0.68) in medium-to coarse-textured soils. Tbe Ng was not necessarily a good predictor of soil N supply under field conditions wbicb emphasizes tbe need to also consider environmental conditions. Tbe UV absorbance of a 0.01 M NaHCOj extract at 205 nm (NaHCO3-205), tbe bot KCl extractable NH4-N (HotKCI-N) and Pool I (a labile mineralizable N pool) plus NO3-N were tbe most promising N availability indices because tbey are easy to perform and tbey were positively and significantly related to soil N supply in tbe wbole data set as well as tbe soil texture subgroups (0.28 < r < 0.62). Tbis study demonstrated tbat grouping soils based on texture can increase tbe proportion of variation in soil N supply explained by N availability indices wben data from contrasting environmental conditions, soil types, and years are used.Abbreviations: HotKCI-N, hot KCl extractable NH.-N; HotKCI-N HYDRO, hotKCI-N minus initial NH^-N; ISNT, Illinois soil nitrogen test for amino sugar nitrogen content; ISNT|_|Y[-"5, Illinois soil nitrogen test minus initial NH^-N; k, mineralization rate constant; Ng, potentially mineralizable nitrogen estimated by a first-order kinetic model after a 24-wk aerobic incubation period, excluding N mineralized during the first 2 wk; NaHCO3-205, the ultraviolet absorbance of a 0.01 M NaHCOj extract at 205 nm; NaHCO3-260, the ultraviolet absorbance of a 0.01 M NaHCO, extract at 260 nm; NaOH-DD, sodium hydroxide direct distillable NH^-N; NaOH-DD^YDRO' NaOH-DD minus initial NH4-N; OC, organic carbon; Pool 1, labile organic N mineralized in the first 2-wk period following rewetting under controlled conditions; Pool II, nitrogen mineralized between Weeks 2 and 24 under controlled conditions; Pool III, the amount of nitrogen that is potentially mineralizable but was not released during the 24-wk incubation. It is estimated by difference between Ng and the cumulative amount of mineralized nitrogen between Weeks 2 and 24; TN, total nitrogen.

Soil N mineralization is an important N contributor to crop uptake; however, the soil and climatic controls on soil mineralizable N are poorly understood. Soil samples from 56 sites across Canada were used to determine the potential to predict the size of mineralizable N pools through simple soil properties and through simple climatic indices and the re_clim indices. Mineralizable N was determined using a 24‐wk aerobic incubation at 25°C. Potentially mineralizable N (N0) was estimated by curve fitting using N mineralized from 2 to 24 wk, and Pool I, a labile mineralizable N pool, was determined as the N mineralized in the first 2‐wk period. Soil properties were relatively effective predictors of N0 with soil organic N (SON) and sand explaining 40 and 34% of the variability, respectively. Particulate organic matter N (POM‐N) and pH explained 18 and 25%, respectively, of the variability in Pool I. Simple climate normals were generally poor predictors of pool size except for potential evapotranspiration (PET), which predicted 24% of the variability in Pool I. The re_clim indices, normally applied to the activity of soil decomposers and applied here for the first time to explain soil mineralizable N pool size variability, performed better than simple climate indices and explained up to 26% of the variation in N0 By including soil and climatic parameters in a multiple regression model, it was possible to explain about 63 and 40% of the variability in N0 and Pool I, respectively, across a wide range of arable soils in Canada.

Tillage practices may affect the active fraction of soil organic N. As part of a national project to examine soil management and environmental controls on the active fraction of organic N, this study examined the effects of no‐till (NT) and conventional tillage (CT) systems on the quantity of potentially mineralizable soil N (N0) and mineralizable N pools, and the potential to detect changes in these pools using N availability indices. Preplant soil samples from the top 15 cm were collected from four long‐term tillage experiments at Swift Current, SK; Woodslee, ON; L'Acadie, QC; and Agassiz, BC. Potentially mineralizable N was determined by aerobic incubation at 25°C and periodic leaching for 24 wk. The N0 was greater under NT than under CT, but only at Swift Current. The labile and intermediate mineralizable N pools were significantly higher under NT than under CT at three of the four sites. The stable mineralizable N pool and the mineralization rate coefficient (k) were greater under NT than under CT at only one of the four sites. Adoption of NT influenced the quality of the active organic N fraction at three sites, as indicated by an increased proportion of mineralizable N in the more labile N pools. Among tested indices of N availability, KCl‐extractable NH4–N, NaOH‐extractable N, Illinois Soil N Test, phosphate‐borate buffer extractable N, and particulate organic C were most sensitive to tillage‐induced changes in the active organic N fraction. Tillage‐induced changes in the size and quality of the active organic N fraction may influence soil N supply and should be considered in optimizing fertilizer N management.