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Estimating soil nitrogen (N) availability is important for developing effective N fertilizer recommendations for plant growth. We examined the ability of various soil biological properties to predict soil N availability, and plant dry matter and N uptake in two greenhouse trials with sorghum-sudangrass (Sorghum bicolor (L.) Moench ssp. Drummondii) as test crop using soils (0–10 cm) from 57 tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) pastures mostly in North Carolina and Virginia. Soil biological properties, the flush of CO2, cumulative carbon (C) mineralization, soil microbial biomass C, basal soil respiration, and plant available N were positively correlated with net N mineralization, plant dry matter, and N uptake in both the greenhouse trials. The flush of CO2, which is the measure of carbon dioxide released from dried and rewetted soil during the first 3 days of aerobic incubation, explained 77% and 72% of the variation in the plant N uptake in Greenhouse Trial 1 and 2, respectively. Total soil N and residual inorganic N in both trials and manganese in Trial 1 and particulate organic C in Trial 2 added minor contributions to explaining variation in plant N uptake. The flush of CO2 was reflective of management history and soil texture. We conclude that the flush of CO2 is a reliable predictor of soil N availability across diverse pasture soils because (a) it was highly correlated with net N mineralization, plant dry matter, and N uptake, and (b) it was less laborious and more cost-effective than other direct indicators of soil N availability. [ABSTRACT FROM AUTHOR]

Corn (Zea mays L.) is an important cereal grain in many states and typically receives large N fertilizer inputs, irrespective of historical management. Tailoring N inputs to soil-specific conditions would help to increase efficiency of N use and avoid environmental contamination. A total of 47 trials were conducted on research stations and private farms in four different regions of North Carolina and Virginia from 2014 to 2016 to associate soil N availability with yield response to sidedress N application. Corn grain yield was 10.6 ± 3.4 Mg ha–1 on 36 sites and silage yield was 44.6 ± 8.2 Mg ha–1 on 11 sites. There was positive association between relative yield (i.e., yield without sidedress N application divided by yield with full fertilization) and levels of both plant available N (residual inorganic N + net N mineralization during 24 d) and the flush of CO2 following rewetting of dried soil during 3 d. Economically optimum N fertilizer requirement (EONR) at sidedress declined with increasing level of plant available N and soil-test biological activity (i.e., the flush of CO2). The scalable N factor for production at EONR declined from 20 kg N Mg–1 of grain (i.e., 1.1 lb N bu–1) with no soil biological activity to no N required with soil-test biological activity of 600 mg CO2–C kg–1 in a 3-d period (depth of 0–10 cm). The flush of CO2 when determined in spring at or prior to planting corn was considered an ideal soil-test indicator of soil biological activity due to its simple, rapid, and reliable characteristics related to potential soil N mineralization and corn yield responses to applied N fertilizer [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) mineralization is variably affected by management and edaphic conditions. A routine soil test that reflects both soil biological activity and N mineralization could improve predictions for N fertilizer recommendations to cereal grains on different soil types and landscape settings. We collected soils from 47 corn production fields in North Carolina and Virginia at depths of 0 to 10, 10 to 20, and 20 to 30 cm and evaluated soil C and N characteristics in association with sorghum-sudangrass [Sorghum bicolor (L.) Moench ssp. Drummondii] dry matter production and N uptake during 6 to 8 wk of growth in the greenhouse. Plant dry matter and N uptake were strongly associated, as expected. Plant available N (sum of net N mineralization during 24 d of aerobic incubation + residual inorganic N) had the strongest association with plant dry matter production (r² = 0.76) and N uptake (r² = 0.85). However, the flush of CO2 during a 0- to 3-d period following rewetting of dried soil was nearly equally effective at r² = 0.74 and r² = 0.76, respectively. Multiple regression models with 4 ± 2 additional variables led to r² = 0.88 ± 0.10 among different separations of data based on depth, region, and soil textural class. We suggest the optimum combination of variables to predict soil N availability would be the flush of CO2, residual inorganic N, and total soil N concentration, as they balance relevant scientific information with limited soil-testing resources (time and labor). We demonstrated that the flush of CO2 was a rapid and reliable indicator of soil N availability. [ABSTRACT FROM AUTHOR]

Nitrogen limits crop production when insufficient and harms the environment when excessive. Tailoring N inputs to cropping systems remains a high priority to achieve production and environmental goals. We collected soils from 47 corn (Zea mays L.) production fields in North Carolina and Virginia at depths of 0 to 10, 10 to 20, and 20 to 30 cm and evaluated soil C and N characteristics in association with soil N mineralization. Soil organic C at a depth of 0 to 10 cm varied among sites from ~10 to 80 g kg–1, and generally declined with depth because of many sites with no-tillage management. Net N mineralization during 24 d of aerobic incubation (25°C, 50% waterfilled pore space) was 54 to 114 mg N kg–1 (24 d)–1 at 0 to 10 cm, 22 to 41 mg N kg–1 (24 d)–1 at 10 to 20 cm, and 12 to 22 mg N kg–1 (24 d)–1 at 20 to 30 cm (middle 50% of observations at each depth). Total soil N was positively associated with net N mineralization (r² = 0.58), but the flush of CO2 during 3 d was even more closely associated with net N mineralization (r² = 0.77). Association between the flush of CO2 and net N mineralization did not change significantly when data were sorted by different regions or soil textural classes. The flush of CO2 is a rapid, reliable, and robust indicator of soil-test biological activity. The strong association of the flush of CO2 with net N mineralization also corroborated use of the flush of CO2 as a rapid and reliable indicator of soil N availability. [ABSTRACT FROM AUTHOR]